Calcium and calcium sensors in fruit development and ripening

Scientia Horticulturae 253 (2019) 412–421

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Review

Calcium and calcium sensors in fruit development and ripening Qiyang Gao

a,1

, Tiantian Xiong

b,1

a

a

, Xueping Li , Weixin Chen , Xiaoyang Zhu

a,⁎

T

a

State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources; Guangdong Provincial Key Laboratory of Postharvest Science of Fruits and Vegetables; Engineering Research Center for Postharvest Technology of Horticultural Crops in South China, Ministry of Education; College of Horticulture, South China Agricultural University, Guangzhou, 510642, China b School of Life Science, South China Normal University, No. 55 Zhongshan Avenue West, Tianhe District, Guangzhou, 510631, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Calcium Calcium sensors CaM CML CDPK CBL Fruit development Phytohormone signal pathways Fruit ripening

Calcium ions (Ca2+) is an important element for plant cells and serve as second messengers, playing an important role in the regulation of plant physiology, specifically in postharvest fruits and vegetables. The roles of calcium and calcium-sensing receptors have been exhaustively studied in plants, and some of these receptors have been found to play a role in fruit ripening as well. However, a comprehensive overview of the manner in which calcium participates in regulating fruit development, ripening and the molecular mechanisms underlying fruit physiology and ripening have not been conducted. In this study, we reviewed the comprehensive roles of calcium in fruit development and ripening, attempting to elucidate the possible physiological and molecular mechanisms in fruit ripening. The roles of calcium sensors, including calmodulin (CaM), calmodulin-like (CML) proteins, calcineurin B-like proteins (CBL), and calcium-dependent protein kinases (CDPK), are discussed in fruit development and ripening. The interaction between the calcium signal and phytohormone signal pathways in fruit development and fruit ripening was also reviewed. The integral associations among calcium, calcium sensors, and fruit ripening were highlighted. This study provides a comprehensive overview of calcium signals in fruit development and ripening and may contribute to possible areas of interest for future research.

1. Introduction As essential nutrients for plants, calcium ions (Ca2+) play an important role in the cell wall and membrane as counteracting ions for inorganic and organic anions in the vacuole. These ions regulate physiological processes, including root hair elongation, pollen tube growth, and stomatal guard cell movement, and serve as an intracellular messenger in the cell (Kirkby and Pilbeam, 1984; Pauly et al., 2000). Calcium is considered an important element for plant and animal cells, as it has been well documented that calcium deficiency or excessive calcium concentration leads to disorders in plants and humans (White and Broadley, 2003). Another important role calcium plays is acting as a crucial second messenger, representing one of the most versatile signaling molecules in all eukaryotic organisms (Chigri et al., 2012; Ranty et al., 2016). Calcium plays a very important role in signal transduction pathways, and these calcium messages must be decoded and amplified by calcium-binding proteins (calcium sensors) to carry out the appropriate responses, including the expression of calmodulin (CaM) and calmodulin-like (CML) proteins (Kamthan et al., 2015; Reddy et al., 2011), calcineurin B-like proteins (CBL), and calcium-dependent

protein kinases (CDPK) (McCormack et al., 2005). Plant growth and development are adversely affected by various environmental stressors, including both abiotic and biotic stressors, such as pathogen and insect attacks (Vert and Chory, 2011). Unsuitable growth and storage conditions also affect fruit production and storability (Martínez-Romero et al., 2007). Fruits of all kinds are subject to different environmental or man-made stressors during their postharvest handling, storage, and transportation period, which usually results in product loss and subsequently reduces their economic value. To address these challenges, it is important to expand our knowledge of how plants adapt to environmental stressors and to improve our understanding of stress tolerance in plants of agricultural interest. It is well known that calcium plays an important role in plant development and adaption, and previous studies have summarized the comprehensive roles of calcium in these processes, as well as a possible mechanism (Aghdam et al., 2012; And and Wayne, 2003; Hepler, 2005; Ranty et al., 2016). Calcium was also reported to play a critical role in fruit development and ripening (Aghdam et al., 2012; Ferguson, 1984; Hocking et al., 2016). Several studies have shown that calcium treatment could effectively delay fruit ripening and maintain fruit quality

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Corresponding author. E-mail address: [email protected] (X. Zhu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.scienta.2019.04.069 Received 10 February 2019; Received in revised form 23 April 2019; Accepted 24 April 2019 Available online 01 May 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved.

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calcium pectate, which constitutes the skeletal structure of the cell wall, preventing the disintegration of the gel layer in the cell and increasing the structural strength of the cell wall (Zhang and Wang, 2019b). In the young fruit stage of apples, fruits are treated with EGTA, which distorts and deforms the cell wall, indicating that calcium deficiency affected the stability of the cell wall (Zhou, 1999). CaCl2 treatment increased the electron density of the cell wall, cell gap, plastid, and enhanced the stability of the cell wall structure (Zhou, 1999). CaCl2 treatment also maintained jujube quality by promoted calcium cross-linked with pectin and inhibited pectin degradation (Zhang and Wang, 2019b). It is speculated that the loss of calcium will affect the formation of calcium pectin, resulting in a decrease in cell wall strength; therefore, sufficient calcium is important for stabilizing the cell structure. In general, it is clear that Ca2+ plays an important role in fruit development.

(Aghdam et al., 2012; Ferguson, 1984; Irfan et al., 2013; Madani et al., 2016; Mahmud et al., 2008; Zhang and Wang, 2019a). For example, postharvest calcium treatment can effectively reduce physiological diseases and delay the senescence of apples, which greatly maintains fruit quality (Conway et al., 1993; Zhao and Wang, 2015). It has also been reported that exogenous calcium application could affect protein and chlorophyll content, cell membrane fluidity, and respiration rates, which are important parameters of senescence (Poovaiah et al., 1988; He et al., 2019). Calcium plays an important role in maintaining the structure of cell walls in fruits; thus, calcium treatment could effectively maintain fruit firmness and delay fruit softening and ripening (Zhang and Wang, 2019b). Calcium can also reduce the accessibility of cell wall-degrading enzymes to their substrates by binding to the cell wall components (Shewfelt and Prussia, 2009), reinforcing the cell wall structure by crosslinking pectic acid residues, and stabilizing the cell membrane (Hui et al., 2009; Poovaiah et al., 1988). However, a comprehensive overview of the manner in which calcium participates in regulating fruit ripening, and the molecular mechanisms underlying calcium’s role in fruit physiology and ripening, has not been conducted. In this study, we review the existing literature and discuss the roles of calcium in fruit development and ripening, especially the molecular mechanism underlying calcium’s role in delaying fruit ripening and improving fruit quality.

2.2. Calcium’s role in fruit ripening Calcium is a major regulatory ion in horticultural crops, playing an important role in fruit ripening through physical and biochemical mechanisms (Aghdam et al., 2012; Ferguson, 1984). Calcium treatment is a safe and non-toxic physical treatment method that can effectively delay fruit ripening and maintain fruit quality. For example, high calcium concentration in fruit could reduce fruit respiration rates, ethylene production, and delay fruit ripening (Ferguson, 1984). In a previous study, the combined treatment of gum arabic coating combined with calcium chloride effectively alleviated the fruit decay and delayed fruit ripening (Khaliq et al., 2015). The combination of chitosan and calcium chloride treatment synergistically extended the shelf-life of fresh-cut honeydew melon by maintaining the integrity of sodium carbonate-soluble pectin (SSP) via interactions either between SSP and calcium ions or protonated chitosan groups (Chong et al., 2015). Postharvest calcium treatment can effectively reduce the physiological disease of the fruit and delay the senescence of the apple fruit (Conway et al., 1993; Zhao and Wang, 2015). The combination of calcium and hot water treatment preserved the antioxidant properties of persimmon and prolong the postharvest life of persimmon (Naser and Rabiei, 2018). In papaya, preharvest or postharvest calcium treatment can delay fruit ripening and senescence and inhibit postharvest disease (Madani et al., 2016; Mahmud et al., 2008). There is a close correlation between the calcium content of stored fruits and pathological disorders. The ratio of soluble magnesium to calcium in the pericarp tissue of the fruit is considered to be a reliable basis for the diagnosis of apple bitter disease (Amarante et al., 2013). Pretreatment with calcium chloride effectively inhibited the growth of mesophilic aerobic bacteria, yeast, and molds in storage at low temperatures, delaying the ripening and senescence of figs (Irfan et al., 2013). Preharvest calcium chloride spray treatment significantly enhanced fruit calcium content and improved fruit quality, and reduced fruit weight loss and decay incidence during storage and shelf-life of kiwifruit (Shiri et al., 2016). Pre- and postharvest application of calcium prevented postharvest disorders, retarded fruit ripening, decreased postharvest fruit weight loss and decay, and maintained strawberry fruit firmness (Shafiee et al., 2010). The application of CaCl2 in combination with salicylic acid (SA) showed the strongest effect on the induction of disease resistance and maintenance of postharvest freshness of apples (Zhao and Wang, 2015). In papaya, calcium treatment increased the calcium content in fruit and reduced the ethylene production and anthracnose lesion diameter (Madani et al., 2016). Plenty of studies have shown that calcium plays an important role in the storage and preservation of fruits, vegetables, and flowers. As shown in Fig. 1, we summarized the role of calcium in fruit development and fruit ripening based on various literatures. However, how calcium regulates these physiological and metabolic processes, especially the material metabolism of the cell wall, the specific relationship between the activities of the related enzymes, the change in fruit firmness, and the molecular mechanism of fruit ripening, will discuss

2. Calcium’s role in fruit development and fruit ripening 2.1. Calcium’s role in fruit development Calcium acts as a central regulator during plant development. High calcium concentration in plant tissue can result in cellular toxicity or developmental abnormalities (Hocking et al., 2016). Calcium deficiencies can lead to membrane breakdown and result in fruit disorders, such as blossom-end rot (Brown and Ho, 1993; Freitas et al., 2012). Reviews conducted by Hepler and Wayne (1985) demonstrated that calcium is involved in nearly all aspects of plant development (And and Wayne, 2003; Hepler, 2005; Feijó and Wudick, 2018). Additionally, a review conducted by Hocking et al. (2016) confirmed the role of calcium in fruit development (Hocking et al., 2016). Calcium deficiencies usually occur in the orchard, which can easily result in numerous disorders that affect fruit development and reduce crop quality. Low levels of Ca2+ also cause poor root development, leaf necrosis and curling, bitter pit, blossom-end rot, fruit cracking, poor fruit storage, water soaking, and other physiological disorders during fruit development (White and Broadley, 2003). Moreover, calcium supply and transport is important for fruit development. Calcium is absorbed from the root system; however, only a small portion of calcium can be transported to the actual fruit. Calcium accumulation in fruit is dependent on xylem delivery, due to the low phloem mobility of Ca2+ (Dražeta et al., 2004). The low phloem mobility of Ca2+ can also lead to calcium deficiencies in fruit. Calcium accumulation during the early period of fruit development leaves younger fruit vulnerable to calcium deficiency (Bemadac et al., 1996). Thus, it appears that calcium application may be more effective during the early fruit development stage. For example, Ca2+ deficiency during tomato development causes blossom-end rot (Freitas et al., 2012). And ethephon combined with calcium chloride spray treatments increase fruit tissue Ca2+ levels and effectively reduce bitter pit in Honeycrisp apple (Cline J.A. 2019). Calcium absorption during the early stages of young fruit development is involved mainly in cell division and metabolism, specifically during initial expansion. The absorption of Ca2+ during the later stages of fruit development is mainly involved in cell-to-cell junction (Hocking et al., 2016). For example, pre-veraison Ca2+ applications greatly enhanced Ca2+ concentrations in the skin tissues of grapes, and Ca2+ applications significantly improved flesh firmness and reduced the occurrence of Botrytis cinerea rots when in storage (Ciccarese et al., 2013). Pectic acid in the cell wall can be combined with calcium to form 413

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Fig. 1. Diagram depicting the role of calcium in fruit development and fruit ripening (Hepler, 2005; Hocking et al., 2016).

by inhibiting some cell-wall degrading enzymes and enhanced pectin cross links. The activities of β-galactosidase (β-gal), a-L-arabinofuranosidase, β-xylosidase (β-Xyl), the soluble fraction of phenol-acetic acid, and water in fruit were significantly and positively correlated with the soluble fraction of the cell wall but negatively correlated with fruit firmness (Ortiz et al., 2011). It was reported that the pectin methylesterase (PMEs) expression impacted the cellular calcium distribution, which further caused the blossom-end rot in tomatoes. High expression of PME increases Ca2+ bound to the cell wall and, subsequently, leads to the decreasing Ca2+ content available for other cellular functions causing the fruit to be susceptible to blossom-end rot (Freitas et al., 2012). Overexpression of the Arabidopsis calcium transporter CAX in tomatoes can effectively maintain the integrity of the fruit’s cell structure and firmness and prolong the shelf life of the fruit (Park et al., 2005). In general, calcium application can increase the density of the intercellular layer of the cell wall, prevent the entry of hydrolase and the disintegration of the gelatinous layer, and affect the changes of the cell wall pectin component. Thereby, cell wall stability and fruit firmness are maintained.

following. 3. The physiological roles of calcium in delaying fruit senescence It is known that calcium plays important physiological roles in fruit senescence. Specifically, calcium delays fruit ripening, mainly because calcium maintains cell membrane integrity as a cellular structural component, cross-links in cell wall pectin polymerization to maintain fruit firmness, reduces the occurrence of physiological diseases, and maintains normal physiological metabolic activities of fruits (Shewfelt and Prussia, 2009). 3.1. Maintaining the fruit cell wall structure and function The plant cell wall is composed of the intercellular layer, primary wall, and secondary wall. The intercellular layer, or the middle layer, is shared by adjacent cells and is mainly composed of pectin polymer. The cell wall contains 60―75% of the total tissue Ca2+ content, serving as the largest pool of Ca2+ in plant tissue (Aghdam et al., 2012). Calcium also participates in the composition of the intercellular layer in the form of pectin-calcium. The pectin-calcium cross-links are important in determining the physical and structural properties of fruit. Moreover, Ca2+ plays a key role in cross-linking acidic pectin residues in the cell wall, acting as the bridge of antiparallel homogalacturonan chains with negatively charged carboxyl groups to form structures called “eggboxes” (Lionetti and Francocci, 2010). Postharvest calcium treatment maintains cell shape, cell membrane integrity, tissue hardness, and catabolism of cell membrane lipids to prolong fruit storage (Picchioni et al., 1998). It has been reported that cell wall changes and calcium-pectin crosslink formation affected fruit structural properties and pathogen susceptibility (Hocking et al., 2016; Zhang and Wang, 2019b). In hand with calcium treatment, low-temperature storage effectively reduces fruit cell wall degradation and extends the shelf life of apricot (Liu et al., 2017). Calcium treatment delayed fruit ripening by preventing the normal degradation of the cell wall during cold storage of the Chilean strawberry (Figueroa et al., 2012). Preharvest CaCl2 treatment effectively delayed apple softening

3.2. Maintaining the cell membrane structure and function Senescence, or ripening, is a prerequisite to fruit softening. As a structural component of the cell membrane, calcium forms a bridge connecting phospholipids and proteins to the plasma membrane, which affects the phase transition and fluidity of the membrane, thereby changing membrane permeability and maintaining membrane integrity (Lionetti and Francocci, 2010). Exogenous calcium application mainly increases the Ca2+ content in the extracellular calcium pool (Silveira et al., 2011), which prevents protoplasmic viscosity, reduces cell wall rigidity and cell membrane stability caused by calcium deficiency and fruit senescence caused by impaired cell membrane system structure and function (Ferguson, 1984). CaCl2 treatment also alleviated the peel browning of pears fruit after refrigerated storage by regulating the cellular membrane metabolism (Zhang and Wang, 2019a). Calcium treatment can reduce membrane permeability by reducing the 414

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related enzyme activities, including β-Galactosidase, α-L-arabinofuranosidase, and pectate lyase (Ortiz et al., 2011). Calcium also affects the pH level in the cell by regulating enzyme activities (Denès et al., 2000). Lipid peroxidation initiated by lipoxygenase (LOX) is a major factor in the ripening and softening of fruits (Carrington et al., 1993). Low Ca2+ content was found to result in higher plasma membrane leakage, which in turn affected lipid metabolism through its effects on LOX activity (Sharma et al., 2012). The accumulation of reactive oxygen species (ROS) can induce membrane lipid peroxidation and enhance the destruction of the membrane structure, thus promoting fruit senescence (Cheng et al., 2008). Calcium is also closely related to the activities of active oxygen scavenging enzymes, such as superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT), and the AsA-GSH cycle. Calcium pretreatment enhanced the activities of antioxidant enzymes, including SOD, APX, and CAT, improving thermotolerance in maize seedlings (Ming et al., 1998). Another important player in regulating abiotic stress response in plants is γ-aminobutyric acid (GABA) (Aghdam et al., 2012). Exogenous GABA treatment can mitigate chilling injury symptoms in peaches by enhancing the activities of antioxidant enzymes and maintaining higher levels of adenylate energy charge (AEC) and adenosine triphosphate (ATP) content to protect membranes from chilling injury (Shang et al., 2011). Glutamate decarboxylase (GAD), the key enzyme in the GABA biosynthesis pathway, is regulated by pH and calcium/calmodulin activation (Arazi et al., 1995). Exogenous CaCl2 significantly promoted the activities of GAD, GABA transaminase (GABA-T), polyamine oxidase (PAO) and diamine oxidase (DAO) (Wang and Xu, 2019). Thus, it is clear that calcium can modulate the GAD activity to regulate GABA content for improving fruit stress adaption and ripening. Anthocyanins play an important role in the stress and disease resistance of fruits and vegetables, as well as fruit quality, thereby prolonging their shelf life. By regulating the expression of related genes, calcium treatment can enhance the accumulation of anthocyanins and total phenolics in the fruit, improving and maintaining fruit quality of strawberry (Xu et al., 2014). Further more, calcium/calmodulin could bind to FvUGT1 and enhance anthocyanin accumulation via alleviation of the substrate inhibition in strawberry fruit (Peng and Yang, 2016). It is apparent that calcium is involved in regulating the activities of various enzymes and the metabolism of important biomolecules related to fruit senescence.

membrane unsaturated fatty acid content and lipid peroxidation, preventing extracellular permeability of intracellular substances (Zhang et al., 2006). These results indicate that calcium treatment decreases hydraulic permeability in fruits, and calcium not only plays an important role in cell wall structure but also plays a critical role in membrane-associated changes during fruit senescence. 3.3. The effects on fruit respiration and ethylene synthesis Calcium content in climacteric fruit is negatively correlated with its respiratory intensity. Calcium treatment can delay and lower the fruit respiration peak. For example, calcium salt treatment reduced the respiration rate of fresh-cut ‘Galia’ melon, increased the total Ca2+ tissue content, and maintained fruit firmness (Silveira et al., 2011). Calcium treatment also reduced microbial growth, which reduced the respiration rate (Silveira et al., 2011). Exogenous calcium treatment could reduce the rate of ethylene production and respiration rate in apples during storage (Poovaiah et al., 1988). Fruit calcium content and storage disorders showed a close relationship in apples, in which Ca2+ concentrations were significantly higher in disorder-free fruit than in fruit with different disorders; the ethylene production and respiration rate were also significantly lower in disorder-free fruit than in symptomatic fruit (Sharma et al., 2012). Generally, calcium within a certain concentration range can effectively reduce the metabolism activities of endogenous substances, inhibit or delay physiological metabolism disorder, reduce fungal infection and growth of microorganisms, and inhibit the respiration rate of fruits (Shafiee et al., 2010). However, excessive calcium may change the cytoplasmic calcium concentration, which may cause membrane damage and enhance respiratory intensity. The effects of calcium treatment on the ripening of fruits and vegetables also extends to ethylene synthesis and signal transduction (Aghdam et al., 2012), as calcium signaling is involved in ethylene synthesis and ethylene signal transduction (Ludwig et al., 2005). In plants, the ethylene-mediated responses strongly rely upon calciumdependent protein phosphorylation (Raz and Fluhr, 1992). In senescent pear and apple tissues, high calcium content reduced the ethylene production rate and delayed the ethylene peak by protecting the cell membrane structure and inhibiting 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO) activity (Friend and Rhodes, 1981). Preharvest calcium spraying treatment ameliorated aroma weakening in pears after long-term refrigerated storage, and CaCl2 treatment enhanced the respiration rate, ethylene production and during fruit ripening in shelf life at room temperature (Zhang and Wang, 2019c). Ethylene production was significantly and negatively correlated with calcium concentration; namely, calcium treatment inhibited ethylene production by reducing ACC content and ethylene-forming enzyme (EFE) activity (Cheverry et al., 2010). Ethylene also regulates calcium signaling and ethylene signaling interactions, which in turn regulates various physiological responses in plants (Yang and Poovaiah, 2001).

4. The signaling roles of calcium in delaying fruit senescence The calcium signal must be decoded and amplified by calcium sensors to carry out the appropriate responses. Most Ca2+ sensors recognize Ca2+ signals via the EF-hands (named after the E and F regions of the parvalbumin) (Nakayama and Kretsinger, 1994), characterized by a conserved helix-loop-helix structure that binds one Ca2+ ion, inducing conformational changes to interact with downstream effectors. In the Arabidopsis genome, roughly 250 proteins possess at least one EF-hand domain, which is a structure known to bind calcium with high affinity (Day et al., 2002). However, among these proteins, only some can be considered true calcium sensors because a characteristic of calcium sensors is that these sensors not only bind to Ca2+ but also relay this information to downstream targets. This interaction is often necessary for the regulation of protein activity and signaling events, which then elicits a specific physiological response (Dodd et al., 2010; McCormack et al., 2005). There are two major groups of calcium sensors in plants: 1) the sensor relays and 2) the sensor responders (Kudla et al., 2010; Sanders et al., 2002). The Ca2+ sensor relay proteins include CaMs, CMLs, and CBLs, which only possess EF-hand structures without any other functional domain. When binding to calcium, they relay the Ca2+ signal through their interaction with other proteins to regulate their activities or localization (Kleist et al., 2014; Reddy et al., 2011). With the

3.4. Regulating activities of enzymes and metabolism of substances related to senescence Calcium is an important activator of enzymes and coenzymes in fruit ripening and softening (Ferguson, 1984). It is involved in regulating cell wall-degrading enzyme activity by affecting membranebound enzyme activity, inhibiting membrane lipid peroxidation, and improving tissue antioxidant capacity (Poovaiah et al., 1988). Fruit softening is the result of cell wall degradation. Calcium is involved in regulating cell wall-degrading enzymes, including cellulase, pectinesterase, polygalacturonase, β-galactosidase, and pectin lyase. Pectin degradation results in the increase of free Ca2+, the loss of cell-to-cell polymerization, and fruit softening (Veau et al., 2010). Calcium treatment effectively maintains fruit firmness by decreasing the softening 415

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exception of CDPKs that As for Ca2+ sensor responder proteins, including CDPKs, which contain a kinase domain, their catalytic activities are regulated by Ca2+ binding to EF-hand structures (Valmonte et al., 2014). These CDPKs can then translate Ca2+ information directly to target proteins through the phosphorylation process. Developmental and environmental cues are perceived by the plant cell’s surface, and these cues are responsible for calcium increases in the cytosol and nucleus (Pauly et al., 2000). These calcium variations can then be decoded by the Ca2+ sensor proteins, which will be relayed and amplified by various downstream targets, including transporters, transcription factors, and metabolic enzymes. This illustrates that the calcium signaling network is a complex pathway involving the generation of several specific calcium signatures and the activation of multiples calcium sensors and downstream target proteins that ultimately allow plants to develop specific, adapted responses to the initial stimulus (DeFalco et al., 2010).

regulated proteins are also involved in tomato ripening, suggesting that calcium signaling is involved in the fruit development and ripening process through calcium/CaM/SlSR interactions (Yang et al., 2012). For CMLs, 52 SlCMLs genes were found in the tomato genome, which showed different expression patterns in the developmental stages and tissues of different fruits. These genes were able to respond to different abiotic stressors and hormone treatments (including ethylene), indicating that they may be involved in the growth and development of tomatoes (Munir et al., 2016). During tomato fruit ripening, calciumbinding EF (CBEF), CaM-like protein 1 (CLP1), CaM-like protein (CLP), CBL-interacting protein kinase 18 (CBLPK18), CDPK3, and CaMbinding heat-shock proteins (CBHSP) were significantly up-regulated during the fruit pink stage and fruit color change, indicating that these molecular components may be involved in fruit ripening (Arhondakis et al., 2016). In litchi fruit, LcCaM1 interact with transcription factors LcNAC13 and LcWRKY1 to regulating fruit senescence (Jiang and Xiao, 2017). In papaya, CaM/CMLs showed diverse expression profiles during fruit development and ripening, and various CaM/CML genes were regulated by ethophon and 1-MCP treatment (Ding et al., 2018). A group of CaM/CMLs genes were positively regulated by ethylene and were up-regulated by ethephon but down-regulated by 1-MCP treatment during storage and nother group of CaM/CML genes were downregulated by ethephon but up-regulated by 1-MCP treatment (Ding et al., 2018). Other studies have shown that in 1-MCP-treated papaya, the concentration of CaM proteins was significantly higher than that of the control group (Huerta-Ocampo et al., 2012). These results suggest that there is a close relationship between ethylene and CaM/CMLs gene expression and indicate that CaM/CMLs may be involved in the fruit ripening process via calcium and ethylene interactions (Ding et al., 2018). As further evidence, in bananas, ethylene treatment inhibits the expression of CaM proteins in banana peels (Du, 2016). The expression of MaCaM1 was up-regulated during banana ripening and senescence; this gene also interacted with MaCAT1 and MaHY5-1 during the ripening and senescence of bananas (Jiang et al., 2018). Lastly, SIGNAL RESPONSIVE1 (SR1) encodes CaM-binding transcriptional activators. In Arabidopsis, SR1 regulates ethylene-induced senescence by directly binding to the ethylene insensitive-3 (EIN3) promoter region in vivo (Nie and Tang, 2012). In tobacco, an early ethylene-up-regulated gene, NtER1 was found to bind to CaM in a calcium-dependent manner. This gene was rapidly induced by ethylene and rapidly increased in leaves and flowers that began to age, indicating that NtER1 is a regulator of plant development and triggers aging and death (Yang and Poovaiah, 2001). These results indicate that CaM/CMLs are involved in the fruit ripening process and jointly regulate fruit ripening with ethylene.

4.1. CaM/CML in fruit ripening Calmodulin is a central component of the Ca2+ signaling machinery and is the most conserved calcium sensor in eukaryotes (Cheval et al., 2013; Yang and Poovaiah, 2003), as well as one of the best characterized Ca2+ sensors. CaMs are highly conserved calcium sensors and are highly similar to one another with only minor differences (more than 99% of CAMs), which may contribute to the specificity of interactions with different target proteins (Bhattacharya et al., 2004; McCormack and Braam, 2003). All CaMs contain 149 amino acid residues, are composed of four EF-hands, and have a maximum of four amino acid differences that separate the most diverse isoforms (McCormack and Braam, 2003). There have been CaM genes identified in numerous plant species, such as Arabidopsis, rice, and tomatoes, which have 7, 5, and 6 CaM genes, respectively (McCormack and Braam, 2003; Zhao et al., 2013). In addition to the conserved CaMs, plants possess a unique expanded family of CMLs. They share 16%―75% of amino acid identity with CaMs (McCormack et al., 2005) and have more sequence diversity than CaMs overall. The CML sequence ranges from 80 to 318 amino acids, and the number of EF-hands is also diverse ranging from 1 to 6 (Zhu et al., 2015). There are 50 genes coding for CaM-like proteins in the Arabidopsis genome and 32 CML genes reported in rice and other plant species ranging from algae to land plants (McCormack and Braam, 2003; Zhu et al., 2015). As the major Ca2+ sensors, CaMs have been shown to be multifunctional in plants, which regulate many cellular processes, including gene regulation, protein synthesis, and ion homeostasis. They also play an important role in regulation of growth, development, and abiotic stress resistance in plants (Parvin et al., 2012; Wang et al., 2009). To date, more and more information has provided valuable evidence that CaMs and CMLs are involved in different plant development processes and have different abiotic and biotic stress responses (Ranty et al., 2016). Additionally, increasing evidence has suggested that CaMs and CMLs may play important roles in fruit ripening. The gene for soluble intercellular adhesion molecule (SlCaMs) has different expression patterns during fruit development and ripening. Among them, SlCaM-1 was highly expressed during fruit development, while SlCaM-2 was highly expressed during fruit ripening. Transient overexpression of SlCaM-2 in green ripe fruits could delay fruit ripening while inhibiting the expression that accelerates fruit ripening (Yang et al., 2013). Six CaM genes were identified with double-peak expression patterns during fruit development (Yang et al., 2013). Moreover, SlCaM-2 in tomato participated in ethylene-coordinated fruit ripening (Yang et al., 2013). During the development and ripening of tomato fruit, calcium regulates fruit ripening through the interaction of calcium, CaM, and SlSR (Yang et al., 2012). The SR/CAMTA protein gene families and the calcium/CaM-

4.2. CDPK in fruit ripening As unique Ca2+ sensors, CDPKs fuse to a CaM domain due to the presence of a catalytic domain, which enables these proteins to function as Ca2+ sensor-responders. The number of EF-hand domains ranges from 1 to 5, but most CDPKs contain 4 domains (Valmonte et al., 2014). Additionally, CDPKs are present in various organisms, including protists (Billker et al., 2009), oomycetes (Valmonte et al., 2014), green algae, and other plants (Zhu et al., 2015), but they are not found in animal and fungal genomes (Hrabak et al., 2003; Zhang and Choi, 2001). The number of CDPK genes ranged from 3 to 40 in green algae and other green plants (Zhu et al., 2015). In model plants, including Arabidopsis thaliana, Oryza sativa (rice), and Populus trichocarpa (poplar), there are 34, 29, and 30 CDPK genes that have been identified, respectively (Asano et al., 2005; Cheng et al., 2002; Zuo et al., 2013). Increasing evidence has shown that CDPKs may be involved in plant development and stress responses, and recent reviews have reported the evolution and functional importance of plant CDPKs (Hamel et al., 2014; Shi et al., 2018; Valmonte et al., 2014). The complexity of CDPKs 416

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4.4. Crosstalk of calcium signals and hormone signals in fruit ripening

identified in different species and their contributions to different functions, such as plant development (e.g., pollen, seed, and flower formation), abiotic (e.g., temperature and drought), and biotic stressors (e.g., virus, bacterial, and fungal elicitors), has been thoroughly discussed in several reviews. Compared to plant development and stress response, there has been less attention focused on fruit ripening and CDPK. However, several pieces of evidence have shown that CDPK may act as an important player in fruit ripening. Specifically, MaCDPK7 was induced by ethylene treatment in bananas. Additionally, the calcium signaling blockers, EGTA and LaCl3, inhibited fruit ripening and the expression of MaCDPK7, which indicated that MaCDPK7 was involved in the ripening of bananas (Wang et al., 2017). Another gene, FaCDPK1, in strawberries was not expressed in young fruit but was up-regulated during fruit ripening, indicating that it may be involved in this process (Llop‐Tous et al., 2002). In tomatoes, a total of 29 CDPKs have been identified to date, and the transcription of most CDPK genes was organ dependent (Hu et al., 2016). The expression levels of most CDPK genes were downregulated in senescing leaves compared to mature leaves, while the expression levels of most of these genes were up-regulated in the red fruits, indicating that these genes may be involved in fruit ripening as well (Arhondakis et al., 2016). The CDPK-related kinase, LeCRK1, which is involved in fruit ripening, is found in tomatoes and regulates fruit development and environmental adaptability (Jauneau and Latché, 2004). The transcription level of LeCRK1 increases in the late stages of fruit ripening and is induced by ethylene, but its expression is not detected in the Nr, Nor, or Rin mutants of tomato. This suggests that it may be involved in the ripening of tomatoes (Jauneau and Latché, 2004). The rate-limiting enzyme, ACS, is involved in ethylene biosynthesis, and ACS proteins in Arabidopsis were divided into three main groups. One group contains the conserved Ser residue, which is a phosphorylation site for CDPK, indicating that CDPK plays an important role in ethylene synthesis (Argueso et al., 2007). The aforementioned evidence indicates that CDPK may be an important player in fruit ripening.

Calcium serves as a secondary messenger interplaying with hormone signaling in plants. It is known to participate in ethylene, auxin, gibberellin (GAs), and abscisic acid (ABA) signaling to regulate fruit set, cell division, initiation of ripening, cell expansion, and fruit softening (Aghdam et al., 2012; Ferguson, 1984; Hocking et al., 2016). Ethylene plays a central role in fruit ripening. Both ethylene and calcium have been reported to play important roles in ripening and senescence (Aghdam et al., 2012), such that there is crosstalk between ethylene and calcium signals in plants and fruit ripening (Aghdam et al., 2012; Raz and Fluhr, 1992). Specifically, calcium is involved in the ethylene signal transduction pathway where SR1 encodes CaM-binding transcriptional activators. In Arabidopsis, SR1 regulates ethylene-induced senescence by directly binding to the EIN3 promoter region in vivo (Nie and Tang, 2012). An ethylene up-regulated gene, NtER1, interacts with CaM and is involved in plant senescence and death, which indicates that Ca2+/ CaM-mediated signaling is involved in ethylene action (Yang and Poovaiah, 2001). Manipulation of the expression of the NtER1 gene can delay senescence and may be useful in prolonging storage life of horticultural crops. An encoding CaM-binding protein, ER66, is an ethylene responsive and ripening-regulated gene in tomato (Zegzouti et al., 2010). Calcium also plays an important role in ethylene biosynthesis as it impacts the expression of ACO (Kwak and Lee, 1997), and is an essential component for the ethylene-dependent biosynthesis and accumulation of pathogenesis-related proteins (PRs) (Raz and Fluhr, 1992). Another enzyme, ACC synthase (ACS), is a key rate-limited enzyme in ethylene synthesis. An isoform of this enzyme, LeACS2, in tomatoes was found to be phosphorylated by CDPK, and its phosphorylation site is conserved among other ACS family members (Sebastià et al., 2004; Tatsuki and Mori, 2001). Therefore, CDPK plays an important role in the ethylene synthesis pathway. Calcium and protein phosphorylation and dephosphorylation are prerequisites for ethylene-induced ACC oxidase and are involved in the signaling pathway of ethylene (Gallardo et al., 1999). As for other calcium-binding proteins (CaBPs), they were up-regulated by ethephon treatments and down-regulated by 1-MCP in peaches during ripening (Zhang et al., 2012). It was reported that EIN2, a central element in the ethylene signaling pathway, binds to several calcium ions in its C-terminus domain, which is closely related to EIN2 structural changes and affects ethylene signal transduction (Allekotte et al., 2008), indicating that Ca2+ may play a critical role in signaling to and/or from EIN2. Phospholipase D (PLD) is the enzyme catalyzing the formation of phosphatidic acid, a key component in the signal transduction pathway, and a key enzyme that integrates the hormone receptor action and increases cytosolic calcium (Aghdam et al., 2012). It is well documented that PLD plays an important role in membrane lipid catabolism during fruit ripening, flowers senescence, plant stress, membrane destabilization, loss of homeostasis, and compartmentalization (Tiwari and Paliyath, 2011). These enzymes autocatalytically accumulate during fruit ripening, leading to massive membrane destabilization and degradation, resulting in fruit senescence (Aghdam et al., 2012). It has been proposed that ethylene promoted the increase in cytosolic Ca2+, then stimulated PLD expression and membrane deterioration, resulting in fruit senescence (Aghdam et al., 2012). This indicated that the crosstalk between Ca2+ and ethylene signals is mainly mediated by the interaction between calcium sensors and the components in ethylene synthesis and signal pathways. Calcium signals are also involved in other hormone signal pathways. For example, SlCaM/CML in tomatoes showed different expression patterns after various hormones treatments, such that SlCML18, SlCML38, and SlCML51 exhibited higher expression after GA treatments, SlCML31 and SlCML51 were induced by ABA, and most SlCML genes were induced by ethylene treatments (Munir et al., 2016). It has been reported that CaM interacts with ZmSAUR1, a protein in the auxin

4.3. CBL in fruit ripening One family of calcium sensors, CBLs, is plant specific. CBLs contain four conserved EF-hand domains that can bind to Ca2+ and elicit a subcellular localization signal at the N-terminus. The CBL-interacting protein kinase (CIPKs) separate the Ca2+ binding function (sensor relay function) and kinase activity (response activity) into two flexible combinable modules. Additionally, CBL and CIPK physically and functionally interact with each other and modulate their kinase activity (Luan, 2009; Shi et al., 1999). The CBL-CIPK network decodes calcium signals and transmits these signals through reversible protein phosphorylation. These proteins were originally identified in the model plant, Arabidopsis (Kudla et al., 1999; Shi et al., 1999). Around 10 CBLs and 26 CIPKs have been identified in Arabidopsis (Albrecht et al., 2001), and 10 CBLs and 30 CIPKs have been identified in the rice (Oryza sativa) genome (Kolukisaoglu et al., 2004). More and more CBL-CIPKs have been identified from the green adage of plants in order to analyze evolutionary processes, as well as physiological functions (Kleist et al., 2014). Moreover, it has been reported that CBLs are involved in all kinds of stress responses and development processes, including salt, drought, wound, cold, and seed germination (Mahajan et al., 2006). Up until now, few studies have focused on the relationship between CBLs and fruit ripening. Several recent reviews have discussed the structural features, regulation mechanisms, gene expression patterns, and functions analysis of the CBL-CIPK network, which has been found to be mainly related to abiotic stress responses (Kleist et al., 2014; Kolukisaoglu et al., 2004; Luan, 2009). However, it is not clear if any CBLs are involved in fruit ripening. 417

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Fig. 2. Model illustrating the role of calcium in fruit ripening. Exogenous calcium treatment increased the cytosol calcium, which helped to maintain the cell wall structure, rigidity, and integrity. This, in turn, greatly maintained fruit firmness and enhanced the activities of anti-aging enzymes, increased the metabolism of substances related to fruit ripening, and reduced fruit respiration and ethylene production. As a signal ion, Ca2+ participates in hormone synthesis directly, which could affect the fruit ripening process. Most importantly, the Ca2+ signal is perceived, decoded, amplified, and relayed by calcium sensors to the downstream targets and triggers specific responses. During signal transduction, calcium sensors, including CaM/CML and CDPK, can interact with components in the hormone signal pathway and affect the physiological response.

dependent growth phenotype (Zhu et al., 2007), which indicates that loss-of-function mutations resulted in pleiotropic ABA-insensitive phenotypes in seed germination, seedling growth, and stomatal movement. Moreover, CDPKs are involved in ABA signal transduction and affect the activities of ABA-responsive element-binding factors. For instance, AtCPK12 interacted with ABI2, phosphorylate AtABF1, AtABF4, and two ABA-responsive transcription factors in vitro (Zhao et al., 2011). The loss-of-function mutant of atcpk12 showed multi-effect ABA-insensitive phenotypes in seed germination and seedling growth (Zhao et al., 2011). It has been reported that the expression and activities of CDPKs can be induced by auxins, and CDPKs also influence auxin levels to regulate plant development. In potatoes, StCDPK1 is regulated by miR390 at the posttranscriptional level and phosphorylates the auxin efflux carrier StPIN in vitro (Santin et al., 2016). Other studies demonstrate StCDPK1regulated tuberization via the involvement of the crosstalk between GA synthesis and auxin transport (Efstathios et al., 2013). In cucumbers, CDPKs have induced expression by IAA and nitric oxide (NO) and are involved in NO- and auxin-induced adventitious root formation (Lanteri et al., 2006). It has also been demonstrated in tabacum that ethylene can modulate the crosstalk between CDPK and MAPK signaling pathways to regulate plant stress response (Ludwig et al., 2005). In general, a wide range of crosstalk between Ca2+ signals and hormones regulate plant development and fruit ripening.

signal pathway, in a calcium-dependent manner, indicating calcium/ CaM-mediated signaling is involved in auxin-mediated signal transduction (Yang and Poovaiah, 2000). Calcium interacts with auxin to regulate root growth and development by regulating PIN1-mediated auxin transport in Arabidopsis (Goh et al., 2012). Indole-3-acetic acid (IAA) is involved in calcium uptake and distribution in tomatoes, and the chlorofluorenolmethyl ester (CME) treatment (an auxin transport inhibitor) reduced the calcium uptake and caused blossom end rot (Brown and Ho, 1993). The expression CML12 (TCH3) is up-regulated by auxin, interacts with the PID protein kinase, regulates its activity in response to calcium signals, and is involved in auxin-regulated plant development (Benjamins et al., 2003). Calcium concentration during fruit development was affected by the GA level, which is vital for preventing fruit physiological disorders (Saure, 2005). In strawberries, the expression of three Ca2+-binding proteins annexins genes were closely related to fruit ripening and regulated by ABA and IAA treatments, which may be involved in hormone regulation in fruit development and ripening via calcium signal transduction (Chen et al., 2016). A comprehensive overview of the interaction between CDPK and phytohormone signaling has been reported recently (Xu and Huang, 2017). It was demonstrated that CDPKs are involved in the interaction with plant hormone signaling pathways, including auxins, GAs, ethylene, jasmonic acid (JAs), and ABA to regulate plant development (Xu and Huang, 2017). These enzymes participate in GA synthesis, such that AtCDPK28, NtCDPK1, and NaCDPK5 are involved in GA3 homeostasis and the development phenotype correlated with GA3 (Ishida et al., 2008; Matschi et al., 2013). It was suspected that AtCDPK28 and its orthologs form a branching point between GA3 and JA-Ile to balance plant growth and defense (Schulz et al., 2013). As an example, consider the atcpk28 mutant that showed reduced GA biosynthesis. The expression of GA3ox1, an enzyme catalyzing the final GA biosynthesis step, was reduced in the atcpk28 mutant compared to the wild type. An exogenous GA treatment restored the growth of the mutation compared to the wild type (Matschi et al., 2013). In Arabidopsis, CDPK4 and CDPK11 act as positive regulators in CDPK/ calcium-mediated ABA signaling pathways to regulate the ABA-

5. Discussion and perspective Calcium serves as the second messenger in plants, playing an important role in development, environmental adaption, stress response, as well as in the ripening and senescence of fruits and vegetables. Various studies have shown that exogenous calcium treatment can effectively improve the quality, delay ripening, and reduce the postharvest decay of numerous fruits and vegetables. In the model plant Arabidopsis, the mechanism of calcium and calcium sensors on plant development and stress responses are well documented. Based on published studies, we aimed to draw a scheme and attempted to illustrate the roles of calcium and calcium sensors on fruit 418

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ripening (Fig. 2). However, how calcium and calcium sensors work in fruit are still largely uncharacterized. This review discussed much of the published work that explores the interplay among calcium, calcium sensors, and fruit development and ripening. As a nontoxic and environment-friendly treatment, calcium treatment has shown a profound effect on fruit ripening. However, the ways in which calcium and the ripening hormone ethylene work during fruit ripening, and how calcium and other hormones interact during fruit ripening, requires further investigation. The calcium signal is decoded, amplified, and relayed by calcium sensors. However, different calcium sensors have distinct affinities for Ca2+, and their binding affinities are often changed by their interaction with target proteins. Identifying these target proteins is crucial for understanding the mechanism of Ca2+ signal transduction. The repertoire of calcium sensor targets in plants includes diverse proteins that are involved in various biological processes, such as morphogenesis, cell division, cell elongation, ion transport, and stress tolerance (Yang and Poovaiah, 2003). Ever increasing evidence has shown that calcium and calcium sensors play important roles in fruit ripening. However, the targets of these crucial calcium sensors and the possible mechanism(s) remain unclear. Future studies should aim to identify the targets and increase our comprehension of the interactions between calcium signals and fruit ripening. Finally, the CBL-CIPK networks are mainly related to abiotic stress responses and plant development. However, little information on their function in fruit ripening is available. Whether or not the CBL-CIPK complex is involved in fruit development and ripening remains unclear. Future investigations should explore different calcium sensors in fruit ripening, which will allow for the development of better strategies that avoid fruit physiological disorders and improve fruit physical traits after harvest.

novel protein–protein interaction module conserved in Ca2+‐regulated kinases. EMBO J. 20, 1051–1063. Allekotte, S., van Vormizeele, J.V., van Vormizeele, N.V., Groth, G., 2008. Molecular characterization of EIN2, a central element in plant hormone signaling. The 8th International Symposium on the Plant Hormone Ethylene. Amarante, C.V.T.D., Miqueloto, A., Freitas, S.T.D., Steffens, C.A., Silveira, J.P.G., Corrêa, T.R., 2013. Fruit sampling methods to quantify calcium and magnesium contents to predict bitter pit development in ‘Fuji’ apple: a multivariate approach. Sci. Hortic. 157, 19–23. And, P.K.H., Wayne, R.O., 2003. Calcium and plant development. Annu. Rev. Plant Biol. 36, 397–439. Arazi, T., Baum, G., Snedden, W.A., Shelp, B.J., Fromm, H., 1995. Molecular and biochemical analysis of calmodulin interactions with the calmodulin-binding domain of plant glutamate decarboxylase. Plant Physiol. 108, 551–561. Argueso, C.T., Hansen, M., Kieber, J.J., 2007. Regulation of ethylene biosynthesis. J. Plant Growth Regul. 26, 92–105. Arhondakis, S., Bita, C.E., Perrakis, A., Manioudaki, M.E., Krokida, A., Kaloudas, D., Kalaitzis, P., 2016. In silicoTranscriptional regulatory networks involved in tomato fruit ripening. Front. Plant Sci. 7. Asano, T., Tanaka, N., Yang, G., Hayashi, N., Komatsu, S., 2005. Genome-wide identification of the rice calcium-dependent protein kinase and its closely related kinase gene families: comprehensive analysis of the CDPKs gene family in rice. Plant Cell Physiol. 46, 356–366. Bemadac, A., Jean-Baptiste, I., Bertoni, G., Morard, P., 1996. Changes in calcium contents during melon (Cucumis melo L.) fruit development. Sci. Hortic. 66, 181–189. Benjamins, R., Ampudia, C.S.G., Hooykaas, P.J., Offringa, R., 2003. PINOID-mediated signaling involves calcium-binding proteins. Plant Physiol. 132, 1623–1630. Bhattacharya, S., Bunick, C.G., Chazin, W.J., 2004. Target selectivity in EF-hand calcium binding proteins. Biochim. et Biophys. Acta (BBA)-Mol. Cell Res. 1742, 69–79. Billker, O., Lourido, S., Sibley, L.D., 2009. Calcium-dependent signaling and kinases in apicomplexan parasites. Cell Host Microbe 5, 612–622. Brown, M.M., Ho, L.C., 1993. Factors affecting calcium transport and basipetal IAA movement in tomato fruit in relation to blossom-end rot. J. Exp. Bot. 44, 1111–1117. Carrington, C., Greve, L.C., Labavitch, J.M., 1993. Cell wall metabolism in ripening fruit (VI. Effect of the antisense polygalacturonase gene on cell wall changes accompanying ripening in transgenic tomatoes). Plant Physiol. 103, 429–434. Chen, J., Mao, L., Mi, H., Lu, W., Ying, T., Luo, Z., 2016. Involvement of three annexin genes in the ripening of strawberry fruit regulated by phytohormone and calcium signal transduction. Plant Cell Rep. 35, 733–743. Cheng, G., Duan, X., Shi, J., Lu, W., Luo, Y., Jiang, W., Jiang, Y., 2008. Effects of reactive oxygen species on cellular wall disassembly of banana fruit during ripening. Food Chem. 109, 319–324. Cheval, C., Aldon, D., Galaud, J.-P., Ranty, B., 2013. Calcium/calmodulin-mediated regulation of plant immunity. Biochim. et Biophys. Acta (BBA) Mol. Cell Res. Cheverry, J.L., Pouliquen, J., Guyader, H.L., Marcellin, P., 2010. Calcium regulation of exogenous and endogenous 1-aminocylopropane-1-carboxylic acid bioconversion to ethylene. Physiol. Plant. 74, 53–57. Chigri, F., Flosdorff, S., Pilz, S., Kölle, E., Dolze, E., Gietl, C., Vothknecht, U.C., 2012. The Arabidopsis calmodulin-like proteins AtCML30 and AtCML3 are targeted to mitochondria and peroxisomes, respectively. Plant Mol. Biol. 78, 211–222. Chong, J.X., Lai, S., Yang, H., 2015. Chitosan combined with calcium chloride impacts fresh-cut honeydew melon by stabilising nanostructures of sodium-carbonate-soluble pectin. Food Control 53, 195–205. Ciccarese, A., Stellacci, A.M., Gentilesco, G., Rubino, P., 2013. Effectiveness of pre- and post-veraison calcium applications to control decay and maintain table grape fruit quality during storage. Postharvest Biol. Technol. 75, 135–141. Conway, W.S., Sams, C.E., Tobias, R.B., 1993. Reduction of storage decay in apples by postharvest calcium infiltration. Acta Hortic. 115–122. Day, I.S., Reddy, V.S., Ali, G.S., Reddy, A., 2002. Analysis of EF-hand-containing proteins in Arabidopsis. Genome Biol. 3 1-0056.0024. DeFalco, Thomas A., Bender, Kyle W., Snedden, Wayne A., 2010. Breaking the code: Ca2+ sensors in plant signalling. Biochem. J. 425, 27–40. Denès, J.-M., Baron, A., Renard, C.M.G.C., Péan, C., Drilleau, J.-F., 2000. Different action patterns for apple pectin methylesterase at pH 7.0 and 4.5. Carbohydr. Res. 327, 385–393. Ding, X., Zhang, L., Hao, Y., Xiao, S., Wu, Z., Chen, W., Li, X., Zhu, X., 2018. Genome-wide identification and expression analyses of the calmodulin and calmodulin-like proteins reveal their involvement in stress response and fruit ripening in papaya. Postharvest Biol. Technol. 143, 13–27. Dodd, A.N., Kudla, J., Sanders, D., 2010. The language of calcium signaling. Annu. Rev. Plant Biol. 61, 593–620. Dražeta, L., Lang, A., Hall, A.J., Volz, R.K., Jameson, P.E., 2004. Causes and effects of changes in xylem functionality in apple fruit. Ann. Bot. 93, 275–282. Du, L., 2016. Proteome changes in banana fruit peel tissue in response to ethylene and high-temperature treatments. Hortic. Res. 3, 16012. Efstathios, R., Bjorn, K., Marian, O., Visser, R.G.F., Bachem, C.W.B., 2013. The PIN family of proteins in potato and their putative role in tuberization. Front. Plant Sci. 4, 524. Feijó, J.A., Wudick, M.M., 2018. Calcium is life. J. Exp. Bot. 69 (17), 4147–4150. Ferguson, I.B., 1984. Calcium in plant senescence and fruit ripening. Plant Cell Environ. 7, 477–489. Figueroa, C.R., Opazo, M.C., Vera, P., Arriagada, O., Díaz, M., Moya-León, M.A., 2012. Effect of postharvest treatment of calcium and auxin on cell wall composition and expression of cell wall-modifying genes in the Chilean strawberry (Fragaria chiloensis) fruit. Food Chem. 132, 2014–2022. Freitas, S.T.D., Handa, A.K., Wu, Q., Park, S., Mitcham, E.J., 2012. Role of pectin methylesterases in cellular calcium distribution and blossom‐end rot development in

Author contributions Q.Y.G, T.T.X and X.Y.Z. compiled the materials and wrote the first draft. X.P.L. and W.X.C. coordinated the writing effort and revised this manuscript. Funding This work was supported by National Natural Science Foundation of China (grants no. 31701970), Pearl River Talent Program for Young Talent (grant no. 2017GC010321), The Characteristic Innovation Project of Guangdong Provincial Department of Education (grant no. 2017KTSCX017), College Students' Science and Technology Innovation Cultivation Special Funds Program of Guangdong Province (grant no. pdjhb0089). Conflict of interest statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Acknowledgements We would like to thank Jean-Philippe Galaud from Université Paul Sabatier (Toulouse, France) for the suggestion during the manuscript preparation. References Aghdam, M.S., Hassanpouraghdam, M.B., Paliyath, G., Farmani, B., 2012. The language of calcium in postharvest life of fruits, vegetables and flowers. Sci. Hortic. 144, 102–115. Albrecht, V., Ritz, O., Linder, S., Harter, K., Kudla, J., 2001. The NAF domain defines a

419

Scientia Horticulturae 253 (2019) 412–421

Q. Gao, et al.

II). J. Plant Nutr. 39 00-00. Mahajan, S., Sopory, S.K., Tuteja, N., 2006. Cloning and characterization of CBL‐CIPK signalling components from a legume (Pisum sativum). FEBS J. 273, 907–925. Mahmud, T.M.M., Al, E.R., Syed, O., Mohamed, Z., Al, A.R., Abdul-Rahman, E., 2008. Effects of different concentrations and applications of calcium on storage life and physicochemical characteristics of Papaya (Carica Papaya l.). Am. J. Agric. Biol. Sci. 3. Martínez-Romero, D., Bailén, G., Serrano, M., Guillén, F., Valverde, J.M., Zapata, P., Castillo, S., Valero, D., 2007. Tools to maintain postharvest fruit and vegetable quality through the inhibition of ethylene action: a review. Crit. Rev. Food Sci. Nutr. 47, 543. Matschi, S., Werner, S., Schulze, W.X., Legen, J., Hilger, H.H., Romeis, T., 2013. Function of calcium-dependent protein kinase CPK28 of Arabidopsis thaliana in plant stem elongation and vascular development. Plant J. 73, 883–896. McCormack, E., Braam, J., 2003. Calmodulins and related potential calcium sensors of Arabidopsis. New Phytol. 159, 585–598. McCormack, E., Tsai, Y.-C., Braam, J., 2005. Handling calcium signaling: arabidopsis cams and CMLs. Trends Plant Sci. 10, 383–389. Ming, G., Li, Y.J., Chen, S.Z., 1998. Abscisic acid-induced thermotolerance in maize seedlings is mediated by calcium and associated with antioxidant systems. J. Plant Physiol. 153, 488–496. Munir, S., Khan, M.R.G., Song, J., Munir, S., Zhang, Y., Ye, Z., Wang, T., 2016. Genomewide identification, characterization and expression analysis of calmodulin-like (CML) proteins in tomato (Solanum lycopersicum). Plant Physiol. Biochem. 102, 167–179. Nakayama, S., Kretsinger, R.H., 1994. Evolution of the EF-hand family of proteins. Annu. Rev. Biophys. Biomol. Struct. 23, 473–507. Naser, F., Rabiei, V., et al., 2018. Effect of calcium lactate in combination with hot water treatment on the nutritional quality of persimmon fruit during cold storage. Sci. Hortic. 233, 114–123. Nie, H., Tang, D., 2012. SR1, a calmodulin-binding transcription factor, modulates plant defense and ethylene-induced senescence by directly regulating NDR1 and EIN3. Plant Physiol. 158, 1847–1859. Ortiz, A., Graell, J., Lara, I., 2011. Cell wall-modifying enzymes and firmness loss in ripening ‘Golden Reinders’ apples: a comparison between calcium dips and ULO storage. Food Chem. 128, 1072–1079. Park, S., Cheng, N.H., Pittman, J.K., Yoo, K.S., Park, J., Smith, R.H., Hirschi, K.D., 2005. Increased calcium levels and prolonged shelf life in tomatoes expressing Arabidopsis H+/Ca2+ transporters. Plant Physiol. 139, 1194–1206. Parvin, S., Lee, O.R., Sathiyaraj, G., Khorolragchaa, A., Kim, Y.-J., Devi, B.S.R., Yang, D.C., 2012. Interrelationship between calmodulin (CaM) and H2O2 in abscisic acidinduced antioxidant defense in the seedlings of Panax ginseng. Mol. Biol. Rep. 39, 7327–7338. Pauly, N., Knight, M.R., Thuleau, P., van der Luit, A.H., Moreau, M., Trewavas, A.J., Ranjeva, R., Mazars, C., 2000. Cell signalling: control of free calcium in plant cell nuclei. Nature 405, 754–755. Peng, H., Yang, T., et al., 2016. Calcium/calmodulin alleviates substrate inhibition in a strawberry UDP-glucosyltransferase involved in fruit anthocyanin biosynthesis. BMC Plant Biol. 16 (1), 197. Picchioni, G.A., Watada, A.E., Conway, W.S., Whitaker, B.D., Sams, C.E., 1998. Postharvest calcium infiltration delays membrane lipid catabolism in apple fruit. J. Agric. Food Chem. 46, 2452–2457. Poovaiah, B.W., Glenn, G.M., Reddy, A.S.N., 1988. Calcium and fruit softening: physiology and biochemistry. Hortic. Rev. 10, 107–152. Ranty, B., Aldon, D., Cotelle, V., Galaud, J.P., Thuleau, P., Mazars, C., 2016. Calcium sensors as key hubs in plant responses to biotic and abiotic stresses. Front. Plant Sci. 7, 327. Raz, V., Fluhr, R., 1992. Calcium requirement for ethylene-dependent responses. Plant Cell 4, 1123–1130. Reddy, A.S.N., Ali, G.S., Celesnik, H., Day, I.S., 2011. Coping with stresses: roles of calcium-and calcium/calmodulin-regulated gene expression. Plant Cell Online 23, 2010–2032. Sanders, D., Pelloux, J., Brownlee, C., Harper, J.F., 2002. Calcium at the crossroads of signaling. Plant Cell Online 14, S401–S417. Santin, F., Bhogale, S., Fantino, E., Grandellis, C., Banerjee, A.K., Ulloa, R.M., 2016. Solanum tuberosum StCDPK1 is regulated by miR390 at the posttranscriptional level and phosphorylates the auxin efflux carrier StPIN4 in vitro, a potential downstream target in potato development. Physiol. Plant. 159, 244. Saure, M.C., 2005. Calcium translocation to fleshy fruit: its mechanism and endogenous control. Sci. Hortic. 105, 65–89. Schulz, P., Herde, M., Romeis, T., 2013. Calcium-dependent protein kinases: hubs in plant stress signaling and development. Plant Physiol. 163, 523–530. Sebastià, C.H., Hardin, S.C., Clouse, S.D., Kieber, J.J., Huber, S.C., 2004. Identification of a new motif for CDPK phosphorylation in vitro that suggests ACC synthase may be a CDPK substrate. Arch. Biochem. Biophys. 428, 81–91. Shafiee, M., Taghavi, T.S., Babalar, M., 2010. Addition of salicylic acid to nutrient solution combined with postharvest treatments (hot water, salicylic acid, and calcium dipping) improved postharvest fruit quality of strawberry. Sci. Hortic. 124, 40–45. Shang, H., Cao, S., Yang, Z., Cai, Y., Zheng, Y., 2011. Effect of exogenous γ-Aminobutyric acid treatment on proline accumulation and chilling injury in peach fruit after longterm cold storage. J. Agric. Food Chem. 59, 1264. Sharma, R.R., Pal, R.K., Singh, D., Singh, J., Dhiman, M.R., Rana, M.R., 2012. Relationships between storage disorders and fruit calcium contents, lipoxygenase activity, and rates of ethylene evolution and respiration in ‘Royal Delicious’ apple (Malus × domestica Borkh.). J. Pomol. Hortic. Sci. 87, 367–373. Shewfelt, R.L., Prussia, S.E., 2009. Postharvest Handling: a Systems Approach.

tomato fruit. Plant J. Cell Mol. Biol. 71, 824–835. Friend, J., Rhodes, M.J.C., 1981. Recent Advances in the Biochemistry of Fruit and Vegetables. Academic Pr. Gallardo, M., Gómezjiménez, M.C., Matilla, A., 1999. Involvement of calcium in ACCoxidase activity from Cicer arietinum seed embryonic axes. Phytochemistry 50, 373–376. Goh, C.S., Lee, Y., Kim, S.-H., 2012. Calcium could be involved in auxin-regulated maintenance of the quiescent center in the Arabidopsis root. J. Plant Biol. 55, 143–150. Hamel, L.-P., Sheen, J., Séguin, A., 2014. Ancient signals: comparative genomics of green plant CDPKs. Trends Plant Sci. 19, 79–89. Hepler, P.K., 2005. Calcium: a central regulator of plant growth and development. Plant Cell 17, 2142–2155. Hocking, B., Tyerman, S.D., Burton, R.A., Gilliham, M., 2016. Fruit calcium: transport and physiology. Front. Plant Sci. 7. Hrabak, E.M., Chan, C.W., Gribskov, M., Harper, J.F., Choi, J.H., Halford, N., Kudla, J., Luan, S., Nimmo, H.G., Sussman, M.R., 2003. The Arabidopsis CDPK-SnRK superfamily of protein kinases. Plant Physiol. 132, 666–680. Hu, Z., Lv, X., Xia, X., Zhou, J., Shi, K., Yu, J., Zhou, Y., 2016. Genome-wide identification and expression analysis of calcium-dependent protein kinase in tomato. Front. Plant Sci. 7. Huerta-Ocampo, J.Á., Osuna-Castro, J.A., Lino-López, G.J., Barrera-Pacheco, A., Mendoza-Hernández, G., De, L.-R.A., Ap, B.D.L.R., 2012. Proteomic analysis of differentially accumulated proteins during ripening and in response to 1-MCP in papaya fruit. J. Proteomics 75, 2160–2169. Hui, L., Chen, F., Yang, H., Yao, Y., Gong, X., Ying, X., Ding, C., 2009. Effect of calcium treatment on nanostructure of chelate-soluble pectin and physicochemical and textural properties of apricot fruits. Food Res. Int. 42, 1131–1140. Irfan, P.K., Vanjakshi, V., Prakash, M.N.K., Ravi, R., Kudachikar, V.B., 2013. Calcium chloride extends the keeping quality of fig fruit (Ficus carica L.) during storage and shelf-life. Postharvest Biol. Technol. 82, 70–75. Ishida, S., Yuasa, T., Nakata, M., Takahashi, Y., 2008. A tobacco calcium-dependent protein kinase, CDPK1, regulates the transcription factor REPRESSION OF SHOOT GROWTH in response to gibberellins. Plant Cell 20, 3273–3288. Jauneau, A., Latché, A., et al., 2004. Molecular and biochemical characterization of LeCRK1, a ripening-associated tomato CDPK-related kinase. J. Exp. Bot. 56 (409), 25–35. Jiang, G., Xiao, L., et al., 2017. Redox regulation of methionine in calmodulin affects the activity levels of senescence-related transcription factors in litchi. Biochim. et Biophys. Acta (BBA) Gen. Sub. 1861 (5, Part A), 1140–1151. Jiang, G., Wu, F., Li, Z., Li, T., Gupta, V.K., Duan, X., Jiang, Y., 2018. Sulfoxidation regulation of Musa acuminata calmodulin (MaCaM) influences the functions of MaCaM-Binding proteins. Plant Cell Physiol. 59. Kamthan, A., Kamthan, M., Kumar, A., Sharma, P., Ansari, S., Thakur, S.S., Chaudhuri, A., Datta, A., 2015. A calmodulin like EF hand protein positively regulates oxalate decarboxylase expression by interacting with E-box elements of the promoter. Sci. Rep. 5, 14578. Khaliq, G., Muda Mohamed, M.T., Ali, A., Ding, P., Ghazali, H.M., 2015. Effect of gum arabic coating combined with calcium chloride on physico-chemical and qualitative properties of mango (Mangifera indica L.) fruit during low temperature storage. Sci. Hortic. 190, 187–194. Kirkby, E., Pilbeam, D., 1984. Calcium as a plant nutrient. Plant Cell Environ. 7, 397–405. Kleist, T.J., Spencley, A.L., Luan, S., 2014. Comparative phylogenomics of the CBL-CIPK calcium-decoding network in the moss Physcomitrella, Arabidopsis, and other green lineages. Front. Plant Sci. 5. Kolukisaoglu, Ü., Weinl, S., Blazevic, D., Batistic, O., Kudla, J., 2004. Calcium sensors and their interacting protein kinases: genomics of the Arabidopsis and rice CBL-CIPK signaling networks. Plant Physiol. 134, 43–58. Kudla, J., Xu, Q., Harter, K., Gruissem, W., Luan, S., 1999. Genes for calcineurin B-like proteins in Arabidopsis are differentially regulated by stress signals. Proc. Natl. Acad. Sci. 96, 4718–4723. Kudla, J., Batistic, O., Hashimoto, K., 2010. Calcium signals: the lead currency of plant information processing. Plant Cell 22, 541–563. Kwak, S.H., Lee, S.H., 1997. The requirements for Ca2+, protein phosphorylation, and dephosphorylation for ethylene signal transduction in Pisum sativum L. Plant Cell Physiol. 38, 1142. Lanteri, M.L., Pagnussat, G.C., Lamattina, L., 2006. Calcium and calcium-dependent protein kinases are involved in nitric oxide-and auxin-induced adventitious root formation in cucumber. J. Exp. Bot. 57, 1341–1351. Lionetti, V., Francocci, F., et al., 2010. Engineering the cell wall by reducing de-methylesterified homogalacturonan improves saccharification of plant tissues for bioconversion. Proc. Natl. Acad. Sci. 107 (2), 616–621. Liu, H., Chen, F., Lai, S., Tao, J., Yang, H., Jiao, Z., 2017. Effects of calcium treatment and low temperature storage on cell wall polysaccharide nanostructures and quality of postharvest apricot (Prunus armeniaca). Food Chem. 225, 87–97. Llop‐Tous, I., Domínguez‐Puigjaner, E., Vendrell, M., 2002. Characterization of a strawberry cDNA clone homologous to calcium-dependent protein kinases that is expressed during fruit ripening and affected by low temperature. J. Exp. Bot. 53, 2283–2285. Luan, S., 2009. The CBL–CIPK network in plant calcium signaling. Trends Plant Sci. 14, 37–42. Ludwig, A.A., Saitoh, H., Felix, G., Freymark, G., Miersch, O., Wasternack, C., Boller, T., Jones, J.D., Romeis, T., 2005. Ethylene-mediated cross-talk between calcium-dependent protein kinase and MAPK signaling controls stress responses in plants. Proc. Natl. Acad. Sci. U. S. A. 102, 10736–10741. Madani, B., Mirshekari, A., Sofo, A., Mohamed, M.T.M., 2016. Preharvest calcium applications improve postharvest quality of papaya fruits (Carica papaya L. Cv. Eksotika

420

Scientia Horticulturae 253 (2019) 412–421

Q. Gao, et al.

Trends Plant Sci. 8, 505–512. Yang, T., Peng, H., Whitaker, B.D., Conway, W.S., 2012. Characterization of a calcium/ calmodulin-regulated SR/CAMTA gene family during tomato fruit development and ripening. BMC Plant Biol. 12, 19. Yang, T., Peng, H., Bauchan, G.R., 2013. Functional analysis of tomato calmodulin gene family during fruit development and ripening. Hortic. Res. 1. Zegzouti, H., Jones, B., Frasse, P., Marty, C., Maitre, B., Latché, A., Pech, J.C., Bouzayen, M., 2010. Ethylene-regulated gene expression in tomato fruit: characterization of novel ethylene-responsive and ripening-related genes isolated by differential display. Plant J. 18, 589–600. Zhang, X.S., Choi, J.H., 2001. Molecular evolution of calmodulin-like domain protein kinases (CDPKs) in plants and protists. J. Mol. Evol. 53, 214–224. Zhang, L., Wang, J.W., et al., 2019a. Calcium inhibited peel browning by regulating enzymes in membrane metabolism of ‘Nanguo’ pears during post-ripeness after refrigerated storage. Sci. Hortic. 244, 15–21. Zhang, L., Wang, J.W., et al., 2019b. Preharvest spraying calcium ameliorated aroma weakening and kept higher aroma-related genes expression level in postharvest’ Nanguo’ pears after long-term refrigerated storage. Sci. Hortic. 247, 287–295. Zhang, L., Wang, P., et al., 2019c. Effects of calcium and pectin methylesterase on quality attributes and pectin morphology of jujube fruit under vacuum impregnation during storage. Food Chem. 289, 40–48. Zhang, X.S., Zhou, W., Chen, H., You, F.U., 2006. Influence of applying calcium to apple young fruits on fatty acids composition of membrane and lipid peroxidation. J. Hebei Agric. Sci. Zhang, L., Jiang, L., Shi, Y., Luo, H., Kang, R., Yu, Z., 2012. Post-harvest 1-methylcyclopropene and ethephon treatments differently modify protein profiles of peach fruit during ripening. Food Res. Int. 48, 609–619. Zhao, Y., Wang, C., 2015. Effect of calcium chloride in combination with salicylic acid on post-harvest freshness of apples. Food Sci. Biotechnol. 24, 1139–1146. Zhao, R., Sun, H.L., Mei, C., Wang, X.J., Yan, L., Liu, R., Zhang, X.F., Wang, X.F., Zhang, D.P., 2011. The Arabidopsis Ca2+‐dependent protein kinase CPK12 negatively regulates abscisic acid signaling in seed germination and post‐germination growth. New Phytol. 192, 61–73. Zhao, Y., Liu, W., Xu, Y.P., Cao, J.Y., Braam, J., Cai, X.Z., 2013. Genome-wide identification and functional analyses of calmodulin genes in Solanaceous species. BMC Plant Biol. 13, 122–142. Zhou, 1999. Study on characteristics of calcium uptake by young fruit of apple (Malus pumila) and its regulation by hormone. Sci. Agric. Sin. 90–97. Zhu, S.-Y., Yu, X.-C., Wang, X.-J., Zhao, R., Li, Y., Fan, R.-C., Shang, Y., Du, S.-Y., Wang, X.-F., Wu, F.-Q., 2007. Two calcium-dependent protein kinases, CPK4 and CPK11, regulate abscisic acid signal transduction in Arabidopsis. Plant Cell 19, 3019–3036. Zhu, X., Dunand, C., Snedden, W., Galaud, J.-P., 2015. CaM and CML emergence in the green lineage. Trends Plant Sci. 20, 483–489. Zuo, R., Hu, R., Chai, G., Xu, M., Qi, G., Kong, Y., Zhou, G., 2013. Genome-wide identification, classification, and expression analysis of CDPK and its closely related gene families in poplar (Populus trichocarpa). Mol. Biol. Rep. 40, 2645–2662.

Postharvest Handling a Systems Approach. Shi, J., Kim, K.-N., Ritz, O., Albrecht, V., Gupta, R., Harter, K., Luan, S., Kudla, J., 1999. Novel protein kinases associated with calcineurin B–like calcium sensors in Arabidopsis. Plant Cell 11, 2393–2405. Shi, S., Li, S., Asim, M., Mao, J., Xu, D., Ullah, Z., Liu, G., Wang, Q., Liu, H., 2018. The Arabidopsis calcium-Dependent protein kinases (CDPKs) and their roles in plant growth regulation and abiotic stress responses. Int. J. Mol. Sci. 19. Shiri, M.A., Ghasemnezhad, M., Fatahi Moghadam, J., Ebrahimi, R., 2016. Effect of CaCl2 sprays at different fruit development stages on postharvest keeping quality of “Hayward” kiwifruit. J. Food Process. Preserv. 40, 624–635. Silveira, A.C., Aguayo, E., Chisari, M., Artés, F., 2011. Calcium salts and heat treatment for quality retention of fresh-cut ‘Galia’ melon. Postharvest Biol. Technol. 62, 77–84. Tatsuki, M., Mori, H., 2001. Phosphorylation of tomato 1-aminocyclopropane-1-carboxylic acid synthase, LE-ACS2, at the C-terminal region. J. Biol. Chem. 276, 28051–28057. Tiwari, K., Paliyath, G., 2011. Cloning, expression and functional characterization of the C2 domain from tomato phospholipase Dα. Plant Physiol. Biochem. 49, 18–32. Valmonte, G.R., Arthur, K., Higgins, C.M., MacDiarmid, R.M., 2014. Calcium-dependent protein kinases in plants: evolution, expression and function. Plant Cell Physiol. 55, 551–569. Veau, E.J.I.D., Gross, K.C., Huber, D.J., Watada, A.E., 2010. Degradation and solubilization of pectin by β-galactosidases purified from avocado mesocarp. Physiol. Plant. 87, 279–285. Vert, G., Chory, J., 2011. Crosstalk in cellular signaling: background noise or the real thing? Dev. Cell 21, 985–991. Wang, K.K., Xu, F., et al., 2019. Effects of exogenous calcium chloride (CaCl2) and ascorbic acid (AsA) on the gamma-aminobutyric acid (GABA) metabolism in shredded carrots. Postharvest Biol. Technol. 152, 111–117. Wang, L., Tsuda, K., Sato, M., Cohen, J.D., Katagiri, F., Glazebrook, J., 2009. Arabidopsis CaM binding protein CBP60g contributes to MAMP-induced SA accumulation and is involved in disease resistance against Pseudomonas syringae. PLoS Pathog. 5, e1000301. Wang, H., Gong, J., Su, X.G., Li, L., Pang, X., Zhang, Z., 2017. MaCDPK7, a calciumdependent protein kinase gene from banana is involved in fruit ripening and temperature stress responses. J. Pomol. Hortic. Sci. 92, 240–250. White, P.J., Broadley, M.R., 2003. Calcium in plants. Ann. Bot. 92, 487–511. Xu, W., Huang, W., 2017. Calcium-dependent protein kinases in phytohormone signaling pathways. Int. J. Mol. Sci. 18, 2436. Xu, W., Peng, H., Yang, T., Whitaker, B., Huang, L., Sun, J., Chen, P., 2014. Effect of calcium on strawberry fruit flavonoid pathway gene expression and anthocyanin accumulation. Plant Physiol. Biochem. 82, 289–298. Yang, T., Poovaiah, B.W., 2000. Molecular and biochemical evidence for the involvement of Calcium/Calmodulin in auxin action. J. Biol. Chem. 275, 3137. Yang, T., Poovaiah, B.W., 2001. An early ethylene up-regulated gene encoding a calmodulin-binding protein involved in plant senescence and death. J. Biol. Chem. 275, 38467–38473. Yang, T., Poovaiah, B., 2003. Calcium/calmodulin-mediated signal network in plants.

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