Phosphate signaling in Arabidopsis and Oryza sativa

Plant Science 176 (2009) 170–180

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Review

Phosphate signaling in Arabidopsis and Oryza sativa Zhaoyuan Fang a,c,1, Chuan Shao b,1, Yijun Meng a, Ping Wu a, Ming Chen a,b,* a

State Key Laboratory of Plant Physiology and Biochemistry, College of Life Sciences, Zhejiang University, Hangzhou 310058, China Department of Bioinformatics, College of Life Sciences, Zhejiang University, Hangzhou 310058, China c Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Graduate University of the Chinese Academy of Sciences, 320 Yue Yang Road, Shanghai 200031, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 February 2008 Received in revised form 30 July 2008 Accepted 5 September 2008 Available online 20 September 2008

Phosphate signaling allows unicellular organisms and higher plants to respond and adapt to phosphate starvation efficiently. Four major adaptive processes include root system development, phosphate mobilization, phosphate transport and metabolism. Phosphite, a non-metabolic analog of phosphate, specifically attenuates several phosphate starvation responses, supporting the hypothesis that plants have a phosphate-sensing machinery comparable to that of unicellular organisms. Biochemical and molecular approaches have characterized acid phosphatases, RNases, high-affinity phosphate transporters, and metabolic enzymes as executive proteins in phosphate signaling. Mutant screening in Arabidopsis thaliana have identified several phosphate signaling regulators, such as PHR1, PHO2 and PHF1. PHO2, microRNA399 family and AtIPS1/At4 family represent a novel circuit in phosphate signaling. Microarray studies in A. thaliana and Oryza sativa suggest regulation at the mRNA level can be an important mechanism of phosphate signaling, and that the leaf and the root may occupy two separate phosphate signaling programs. In addition, phosphate signaling is suggested to interact with hormone and sugar signaling pathways. Finally, an integrated model summarizing the contemporary understanding of phosphate signaling, mainly in A. thaliana and O. sativa, is presented. ß 2008 Elsevier Ireland Ltd. All rights reserved.

Keywords: Phosphate signaling Arabidopsis Oryza sativa

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adaptation 1: the root systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Regulation of the root architecture . . . . . . . . . . . . . . . . . . . . . 2.2. Increased initiation and elongation of root hairs . . . . . . . . . . Adaptation 2: phosphate mobilization and utilization. . . . . . . . . . . . 3.1. Induced production of acid phosphatases and ribonucleases. 3.2. Induced synthesis and secretion of organic acids . . . . . . . . . . Adaptation 3: phosphate transport . . . . . . . . . . . . . . . . . . . . . . . . . . . Adaptation 4: metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Adjustment of cellular respiratory pathways . . . . . . . . . . . . . 5.2. Alteration of membrane lipid composition . . . . . . . . . . . . . . . 5.3. Accumulation of anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . Interacting with hormone signaling . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphate sensing and signaling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphate transporters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Phosphate transporter families . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Regulation of Pi transporters by phosphate starvation. . . . . .

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* Corresponding author at: State Key Laboratory of Plant Physiology and Biochemistry, Department of Bioinformatics, College of Life Sciences, Zhejiang University, Zijingang Campus, Hangzhou 310058, China. Tel.: +86 571 88206499; fax: +86 571 88206133. E-mail address: [email protected] (M. Chen). 1 These authors contribute equally to this work. 0168-9452/$ – see front matter ß 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2008.09.007

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The first regulator: PHR1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The first circuit: PHO2, microRNA399s and AtIPS1/At4 family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Putative regulators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. PHO1 and PHO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. SUMO ligase SIZ1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microarrays and proteomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toward a phosphate signaling network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Plants have gained series of evolutionarily adapted strategies to actively interact with various environmental and internal conditions. One of these critical conditions is the phosphorus (P) nutrition. In soil, P is dilute and less diffusible because it mainly exists as an integral component in Ca, Fe or Al salts, or in the form of organic molecules, while the uptake form by plants is orthophosphate (mainly H2PO4 and HPO42), designated as Pi (inorganic phosphate). For a typical plant, Pi concentrations are at the ranges of: 1 mM in the soil, 10,000 mM in the cells, and 400 mM in the xylem. Despite difficulties in its absorption, Pi is involved in many fundamental processes in plant life, including photosynthesis, respiration, biosynthesis, membrane construction, signal transduction and genetic information transmission and expression, as it is a critical element for phosphate esters (NTP and NDP) and phospholipids. Thus Pi nutrition and its availability are critical

to plants, forcing them to evolve an efficient signaling system to regulate Pi acquisition, utilization and homeostasis to adapt to diverse environmental Pi conditions, which is especially evident upon Pi starvation (Fig. 1). Current research focusing on the genetic regulation of Pistarvation responses has identified a number of genes likely to control these evolutionarily conserved processes. For example, PHR1, a conserved MYB transcription factor, was shown to regulate microRNA399 (miR399) family members and a number of other Pi-starvation response genes [1–5]. Meanwhile, understanding of molecular mechanisms underlying the Pi-starvation responses grow quickly. For instance, biochemical characterization along with genetic analysis have identified several acid phosphatases (APases) involved in Pi mobilization and their regulation during Pi deficiency [6–11]. Moreover, genome-wide profilings provide a global inspection of the Pi signaling system in plants [12–17]. Such fast progresses in the various aspects of

Fig. 1. A concise model of the Pi signaling network in Arabidopsis and Oryza.

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nutritional regulation of Pi signaling motivates us to provide a wide review of related work to gain a global knowledge of Pi signaling. Below we summarize the current opinion on Pi signaling in plants, with emphasis on the two model plants, Arabidopsis thaliana and Oryza sativa (rice). 2. Adaptation 1: the root systems 2.1. Regulation of the root architecture During Pi starvation, in Arabidopsis, primary root growth is inhibited [18,19] whereas lateral root growth alterations are more complex, with both positive [18,20,21] and negative [22,23] reports. Primary root inhibition by Pi starvation is controlled by local Pi status at the root tip [23,24] while lateral root alterations are regulated by systemic Pi status [25]. Auxin is believed to play a most central role in root development among the hormones [26], and during Pi starvation, auxin accumulation in the primary root apex is promoted [21]. However, treating mutants involved in auxin transport or response with low Pi obtained both positive and negative results [18–21]. Manipulation of ethylene also affects root tip growth under Pi-starvation condition [27], though ethylene signaling mutants show inconsistent effects on primary root growth and little effect on lateral root promotion under Pistarvation conditions, suggesting lateral root alterations during Pi starvation is largely independent of ethylene [18]. But it is still possible that ethylene may be involved in primary root inhibition during Pi starvation. Taken together, auxin signaling is likely to be involved in regulation of primary and lateral root development under Pi starvation. Several relevant genes/loci have been identified through genetic approaches [24,28,29]. Two multicopper oxidase encoding genes, low phosphate root 1 (LPR1) and low phosphate root 2 (LPR2), are essential and sufficient to establish the primary root inhibition effect by low phosphate [24]. The pdr2 mutant has a stronger primary root growth inhibition during Pi starvation, due to a greater inhibition of cell division under low Pi conditions, which can be attenuated by phosphite application [28]. The lpi1 and lpi2 mutants do not have a typical Pi starvation-induced primary root inhibition, and both genes are suggested to be required in regulation of root development under Pi starvation [29]. 2.2. Increased initiation and elongation of root hairs Root hair development begins with the specification of epidermal cells into trichoblasts (hair-forming cells), followed by the initiation and elongation of root hairs. Genetic studies have revealed a lot about the regulatory mechanisms of these processes [30,31]. Plant hormones also regulate these developmental processes, especially ethylene and auxin. Several mutants defective in ethylene or auxin signaling were found to have abnormal or even no root hairs [32–34]. And part of these abnormal phenotypes can be rescued by exogenous auxin application or increased endogenous auxin synthesis [35]. Manipulation of endogenous hormone production also affects root hair development [36]. During Pi starvation, both the length and density of root hairs are highly increased in Arabidopsis [37,38], which has a significant contribution to Pi acquisition [39–41]. However, negative evidences also exist. Some experiments show that auxin and ethylene response have little impact on the increase in root hair density and length induced by Pi starvation [38,42]. Taken together, Pi signaling is at least partially independent of auxin and ethylene signaling in root hair development.

3. Adaptation 2: phosphate mobilization and utilization 3.1. Induced production of acid phosphatases and ribonucleases Pi is accessible into plant cells, since various high and low affinity Pi transporters are presented in the cellular membrane systems. In contrast, organic P, such as phosphomonoesters and nucleic acids, is difficult for direct translocation. Hydrolases such as phosphatases and ribonucleases, which are involved in releasing Pi from such organic sources for efficient transport and subsequent utility, are likely to be important in these processes: mobilization of the organic P in soil for root absorption, remobilization of organic P in senescing organs, storage tissues, and intracellular compartments. During Pi starvation, these processes might be enhanced to reduce the Pi-starvation stress, since numerous reports suggest members of phosphatases and ribonucleases are induced by Pi starvation, as indicated below. Acid phosphatases are greatly induced by Pi deficiency. They release Pi from phosphomonoesters and have optimum activity in acidic conditions [43]. In tomato (Lycopersicon esculentum), three APases purified from suspension cell culture systems all display both acid phosphatase and alkaline peroxidase activities [44,45]. In white lupin (Lupinus albus L.), one secreted APase has been well characterized [6,7,46,47], and here it is designated as LaSAP (secreted APase). LaSAP is strongly induced in cluster roots of Pi-starved white lupin and shows high homology with an Arabidopsis APase-coding gene AtPAP12 (At2g27190). AtPAP12 was proposed to encode a secreted APase [8]. The signal peptide of AtPAP12 could direct the secretion of its GFP-fusion protein [8]. AtPAP12 mRNA accumulated during Pi starvation in both the leaf and the root [7,12–14,48]. AtPAP12 and its promoter show high homology with LaSAP [6,7]. APase activity staining on native PAGE gels resolves several isoforms of APases in Arabidopsis tissue extracts and root exudates, some of which are Pi starvation-inducible [49,50]. One Pi starvation-inducible APase isoform that has been purified and characterized from Pi-starved seedlings is a 34 kD monomeric protein AtACP5/AtPAP17 [48,10]. AtACP5 (At3g17790) shows expression in flowers and senescent leaves when Pi is sufficient, and is transcriptionally upregulated in root and leaf by Pi starvation [12–15,10]. Its promoter contains binding sites for AtPHR1, a transcriptional regulator involved in Pi-starvation response [1]. The presence of an N-terminal signal peptide, and that no APases were extracted from apoplastic fluid, raises the possibility that AtACP5 is linked to the cell wall or plasma membrane [10]. Thus, AtACP5 is likely to function in Pi starvationinducible Pi mobilization, possibly from extracellular sources. In addition, it can also be induced by salt stress, H2O2 and ABA, and display both acid phosphatase and alkaline peroxidase in vitro [10], suggesting its potential role in linking Pi-starvation response and other stress-related processes. A second purified Arabidopsis APase from Pi-starved suspension cell cultures is a 100 kD dimeric vacuolar protein AtPAP26 (encoded by At5g34850). It is upregulated by Pi starvation probably at the protein level [11], suggesting its role in remobilizing intracellular phosphomonoesters in a Pi starvation-induced manner. AtPAP26 also displays alkaline peroxidase activity [11] and can be transcriptionally upregulated by PCD/senescence [51]. AtPAP12, AtACP5/AtPAP17, AtPAP26 are all purple APases (PAPs) which contain a conserved motif forming a binuclear metal ion complex and thus display a characteristic purple color [52,53]. In the Arabidopsis genome, 29 PAP genes have been identified [48]. Their response to Pi starvation has been examined at the transcriptional level, showing that at least 11 of them are upregulated [14], indicating their potential roles in Pi nutrition.

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However, regulation at the protein level should not be ignored and might be even dominant as in the case of AtPAP26. Several phosphatase under-producer (pup) mutants displaying reduced APase activities at root surface or in root exudates have been isolated and two of them have been biochemically and physiologically characterized: pup1 and pup3 [50,54]. pup1 and pup3 are mapped to chromosome 2 (between 40.2  6.2 and 44.9  9.9 cM) and 5 (between At5g29584 and At5g36210), respectively. Two additional Pi starvation-inducible genes with PHR1binding sequence in their promoters are AtVSP2 [55] and LePS2 [56]. And they are also suggested to be APases. LePS2 responds to Pi starvation more specifically [56] than AtVSP2, which is induced by several other stimuli such as wounding and insect feeding [55]. Although bacterial expression showed both protein products display maximum phosphatase activity within the acidic pH range, AtVSP2 was indicated to be more likely an anti-insect protein [55] whereas LePS2 still requires further dissection of its role in Pistarvation response. Taken together, the three best studied Pi starvation-inducible Arabidopsis APases are AtPAP12 likely to be secreted into the soil to mobilize organic P. AtACP5/AtPAP17 likely to function in internal P remobilization. And AtPAP26 likely to be involved in intracellular P remobilization. However, more solid evidences and the reasons why many APases characterized to date display double activities and/or are regulated by several factors besides Pi starvation are expected for further researches. Other members of the 29 PAPs, especially those induced during Pi starvation, may be shown to function in Pi mobilization in the future. Mutants such as pup1 and pup3 are of interest and a final characterization would be valuable. Certain types of ribonucleases (RNases) are hypothesized to participate in Pi mobilization from RNA during Pi starvation. In Arabidopsis, three genes encoding S-like RNases that could be involved in Pi mobilization have been identified, i.e. AtRNS1 (At2g02990), AtRNS2 (At2g39780) and AtRNS3 (At1g26820) [57,58]. AtRNS3 is expressed in various tissues, but during Pi starvation and senescence, it is not obviously or only modestly regulated at the mRNA level [58]. And further exploration of its role may need to examine its regulation at the protein level. AtRNS1 encodes a secreted protein [59] expressed mainly in flowers [58]. It is upregulated during Pi starvation at both mRNA [12–15,58] and protein [59] levels and is also induced by wounding [60]. AtRNS2 encodes an intracellular protein [59] expressed in various organs [61]. It is strongly induced at both the mRNA level in leaf [12,13,61] and the protein level [59] during Pi starvation, and it is also induced at the mRNA level during senescence [61]. Antisense silencing of either AtRNS1 or AtRNS2 leads to accumulation of anthocyanins [59]. It seems possible that AtRNS1 and AtRNS2 are involved in Pi mobilization from extracellular and intracellular RNA sources during Pi starvation as well as senescence or wounding. 3.2. Induced synthesis and secretion of organic acids Organic acids exuded from plant roots, such as citrate and malate, are involved in several physiological activities including liberation of Pi from Al–, Fe– and Ca–P complexes and reducing toxic cations like Al3+, and are thus stimulated by Pi starvation or Al3+ accumulation [62]. Organic acid exudation induced by Pi starvation in Arabidopsis [63] or rice [64] has been reported. Organic acid exudation induction can be largely attributed to increased synthesis and secretion in the root systems. Increased synthesis of organic acids upon Pi starvation are likely due to

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enhanced activity of enzymes including phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), and citrate synthase (CS), as well as reduced activity in aconitase and reduced respiration rates, based on evidences mainly from cluster roots of white lupin [65]. In Arabidopsis, among the four PPC encoding genes, i.e. ATPPC1 (At1g53310), ATPPC2 (At2g42600), ATPPC3 (At3g14940) and ATPPC4 (At1g68750), ATPPC1 and ATPPC3 but not ATPPC2 are found to be upregulated under Pi starvation in the leaf by microarray [13]. In addition, several PPC kinase (PPCK) genes are also found to be upregulated [13,15]. Overexpression of a maize PEPC gene in rice increased organic acid synthesis and exudation [64]. Overexpression of a citrate synthase gene in Arabidopsis increases citrate synthesis and secretion [66], whereas in tobacco there is discrepancy in reports [67,68]. Mechanisms of secretion may involve anion transporters since organic acids exist in their anion forms in cytoplasm pH conditions, though the molecular basis is not clear [65]. There is a correlation between proton secretion and organic acid anion exudation from white lupin cluster roots, though their temporal manners are different during Pi starvation [69]. Regulation of some plasma membrane H+-ATPase may explain proton extrusion enhancement by Pi starvation [70]. 4. Adaptation 3: phosphate transport Pi transport is involved in three important processes: Pi acquisition, Pi homeostasis and Pi redistribution. Regulation of these Pi transport processes is an important adaptive strategies. Pi acquisition can be divided into several steps [71]. For example, below is a five-step model: (1) Pi uptake by root hairs and epidermal cells from external medium; (2) Pi transport across the cortical cells, or through an apoplast pathway before reaching the Casparian strip in the endodermis; (3) Pi loading into the root xylem and diffusion along the xylem vessels to the shoot and leaf regions; (4) Pi unloading from the xylem into shoot and leaf cells and (5) intracellular Pi translocation. In contrast to Arabidopsis, the mycorrhizae between some plants and their symbiosed fungi markedly extend the root rhizosphere and improves Pi acquisition [72–74]. During Pi starvation, it had been found that Pi uptake ability was increased in various root systems [75–77]. The underlying mechanisms of this phenomena are the induction of high-affinity Pi transporters, which are discussed in detail below. Pi homeostasis occurs in two different scales. In the whole plant level, it requires a flexible regulation of Pi acquisition in response to in vivo Pi status and a sophisticated tissue Pi supply coordination; whereas in the scale of one cell, it mainly depends on the homeostatic function of vacuole. The vacuole stores up to 85– 95% of the total Pi when the supply is sufficient and, when undergoing Pi deficiency, this Pi storage is remobilized and exported for use, constituting an intracellular homeostasis mechanism [78–81]. Pi redistribution is the remobilization of Pi in old tissues and retranslocation to young and/or active growing tissues. Although it occurs during the normal developmental process, it is markedly activated in response to Pi starvation. This adaptation accelerates the remobilization of Pi in old tissues and protects the newly growing tissues from an immediate damage by Pi deficiency since they are most sensitive to Pi declining. Regulators of this adaptation probably include miR399s, PHO2 and the AtIPS1/At4 family [82–86], which might function by inducing APases (e.g. AtACP5 and AtPAP26) and RNases (e.g. RNS1 and RNS2) to accelerate Pi remobilization, as well as some unknown molecules to enhance Pi retranslocation.

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5. Adaptation 4: metabolism

5.3. Accumulation of anthocyanins

5.1. Adjustment of cellular respiratory pathways

Accumulation of anthocyanins, a subclass of flavonoids, can occur in various tissues in response to a number of developmental and environmental signals, including Pi deficiency [98]. The physiological significance of its accumulation is not clear, though photoprotection has been suggested [98]. A great deal of the biosynthetic pathways and regulatory genes have been understood [99,100]. Many genes involved in anthocyanin biogenesis pathways have been found to be induced in mRNA level in microarray profiling [14,15]. PHR1 is required for Pi starvation-induced anthocyanin accumulation [1].

Long-term Pi starvation would reduce cytoplasmic Pi and nucleoside phosphates such as ATP and ADP to a low level, whereas the level of pyrophosphate (PPi) is more constant [87]. Thus, the normal glycolysis and oxidative phosphorylation pathways would be severely impaired, since they involve several ADP/ATP/Pidependent reactions. This would have a rather unfavorable impact on the systemic metabolism. However, in fact, plants seem to have evolved an adaptive strategy to change the respiratory pathways to bypass the ADP/ATP/Pi-dependent reactions [88,89]. Several alterations of the glycolytic pathways have been proposed according to enzyme activity data from experimental systems such as Pi-starved Brassica nigra suspension cells. UDPglucose pyrophosphorylase, non-phosphorylating NADP-dependent glyceraldehyde 3-phosphate dehydrogenase (non-phosphorylating NADP-dependent G3PDH), phosphoenolpyruvate (PEP) phosphatases and PEPC activities are induced during Pi starvation. The metabolic byproduct PPi is used as an alternative energy donor to cope with largely reduced ATP concentration. These bypasses serve to maintain essential carbon flow, and to recycle phosphate esters to Pi. New pathways have also been suggested to be activated to bypass the normal oxidative phosphorylation pathways in the mitochondria: the rotenone-insensitive NADH dehydrogenase pathway and the cyanide-resistant alternative oxidase pathway. These bypasses confer the limited ATP production upon severe Pi starvation. 5.2. Alteration of membrane lipid composition Pi starvation induces drastic changes in lipid composition of plant membranes, including a decrease of phospholipids and an increase of non-phosphorous lipids in several species, including photosynthetic bacteria [90] and Arabidopsis [91]. In Pi-starved Arabidopsis, sulfolipids increase in thylokoid membranes [14,91], whereas galactolipids increase in both the thylokoid membranes [91] and extraplastidic membranes [92,93]. Phospholipid degradation enhancement during Pi starvation is consistent with microarray results that several genes encoding phospholipase C (e.g. At3g03530 [94], At3g03540 [14]) and D (e.g. At3g05630 [14]), are upregulated in the leaf and root. Consistently, two genes encoding phosphoethanolamine N-methyltransferases involved in phospholipid biosynthesis, NMT1 and NMT3, are downregulated during Pi starvation [13]. Sulfolipid sulfoquinovosyl diacylglycerol (SQDG) synthesis involves two steps catalyzed by two genes, SQD1 (At4g33030) [92] and SQD2 (At5g01220) [95], respectively. SQD1 is upregulated during Pi starvation in both the leaf and the root [12–15]. Galactolipid digalactosyldiacylglycerol (DGDG) synthesis, under Pi-replete conditions, involves MGD1 (At4g31780) and DGD1 (At3g11670) in the leaf and MGD2/3 (At5g20410/At2g11810) and DGD2 (At4g00550) in non-photosynthetic tissues [96]. During Pi starvation, the pathway involving MGD2/3 and DGD2 is strongly activated, producing galactoglycerolipids to be transported to extraplastidic membranes [14,96]. Phosphite application attenuates Pi starvation-induced MGD2/3 expression and lipid alteration. And in AtMGD2/3::GUS Arabidopsis, auxin and cytokinin also impact Pi starvation-induced MGD2/3 expression and auxin signaling is essential for this induction [97]. These results suggest Pi signaling interacts with auxin signaling, and perhaps cytokinin signaling as well, to control membrane lipid alterations during Pi starvation.

6. Interacting with hormone signaling Auxin is believed to have a most central role in root development among the hormones [26]. Focusing on whether root system alteration by Pi starvation is mediated by auxin, discrepancies exist, but the important role of auxin is generally accepted here [18–23]. The mechanisms of this regulation process are still not clear; however, the involvement of auxin redistribution is suggested [20,21]. Besides the role in phosphate starvationinduced root development, auxin signaling, as well as cytokinin, have also been suggested to have interactions with Pi signaling at membrane lipid composition regulation [97]. Cytokinins, as several evidences supported, are likely to interact with Pi signaling. Their levels in plants are downregulated during Pi starvation [25], and exogenous application of cytokinins represses many Pi starvation-inducible genes in Arabidopsis [83] and rice [101], including AtIPS1, At4, AtPT1/AtPht1;1 and AtACP5/ AtPAP17 [83], partially through increasing in vivo Pi concentrations [101]. Furthermore, the repressive effects of cytokinins on Pi signaling are attenuated by mutations at genes encoding cytokinin receptors, including CRE1/WOL/AHK4 and AHK3 [102,103]. However, increased root hair number and length during Pi starvation, which are controlled by local Pi status, are not affected by cytokinin application, suggesting the possible role of cytokinins in sensing systemic Pi status [83]. 7. Phosphate sensing and signaling The idea that Pi, or Pi concentration, can be perceived as a signal by plants with a Pi-status sensing system is inspired by discoveries of Pi sensing and signaling systems in unicelluar organisms. In Escherichia coli [104,105] and Saccharomyces cerevisiae [106,107], PhoR and PHO81 are the possible Pi sensors, respectively, which sense Pi status and regulate downstream signaling networks. The existence of similar Pi sensing systems in higher plants is supported by the following phenomena. Phosphite (H2PO3), an analog of Pi that cannot be metabolized by Arabidopsis, is able to repress several characteristic responses to Pi difeciency, such as the promotion of lateral roots and root hairs, membrane lipid alteration, anthocyanin accumulation and upregulation of several Pi starvation-inducible genes in Arabidopsis [28,97,108,109] as well as proteoid root formation in white lupin [110]. However, in higher plants, the mechanism of Pi sensing is almost unknown. Even which organ(s) or tissues of the plant is (are) responsible for Pi sensing is still in controversy. Two modes are involved in Pi sensing/signaling: systemic Pi sensing/long distance signaling and external Pi sensing/local signaling [111]. Whole plant Pi status can regulate root growth and Pi uptake, and most of the responses discussed above are controlled by such systemic Pi level. In contrast, some responses, e.g. root hair modifications during Pi starvation are regulated by local external Pi status. Root development modifications regulated

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by systemic Pi status may involve long-distance signals, and the suggested signals include Pi [28], cytokinins [102], miRNAs [2,112], short open-reading frame mRNAs (sORF-mRNAs) [82] and peptides [82]. 8. Phosphate transporters 8.1. Phosphate transporter families Discovery of Pi transporters was a milestone in the molecular physiology of phosphate nutrition. Pi transporters that have been fully or poorly characterized can be classified into several groups. The first group, named Pht1 family, is composed of high-affinity Pi transporters, which belongs to the Pi:H+ symporters. This family has been identified in many species. In Arabidopsis, among the nine AtPht1 family members, AtPht1;1 (other names: AtPT1, PHT1 or APT1) and AtPht1;4 (other names: AtPT2 or PHT4) have been shown to be two major Pi transporters involved in Pi uptake [113–115], while the other members are speculated to function in Pi transport in different tissues [116]. Some members of this family have also been suggested to function in Pi transport in root–fungi interface, as mycorrhiza-specific Pht1 transporters have been found in many species [72,73], including OsPT11 in rice [74]. Two groups of low-affinity Pi transporters, characterized in Arabidopsis, have been mainly implicated in transport across intracellular membranes. One group is AtPht3 family proteins involved in mitochondrial Pi transport [117]. The other group has one member in Arabidopsis, AtPht2;1, which encodes a plastidic protein upregulated by light but not Pi starvation. AtPht2;1 involved in Pi distribution in leaves, is required for Pi transport into the chloroplast, as well as cell wall metabolism in young tissues [118,119]. In conclusion, Pi transporters involved in Pi acquisition and, Pi translocation in mitochondrial and chroloplast, are better understood, whereas those involved in Pi uploading and distribution, vacuolar import and export [81,120], and Pi retranslocation are still poorly known.

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tion in higher plants [1]. The functional ortholog of PHR1 in Chlamydomonas reinhardtii is CrPSR1 (phosphorus starvation response 1) [3]. Thus PHR1 probably represents a conserved transcription factor of phosphate starvation response. PHR1 contains a MYB DNA binding domain and belongs to the GARP family, one of the families in the MYB superfamily [1]. The Chlamydomonas mutant psr1 is impaired in several Pistarvation responses, including development of high-affinity Pi uptake system and secretion of alkaline phosphatase [3]. Pi responses affected in phr1 mutant include Pi starvation-induced, but not N starvation-induced, anthocyanin accumulation and Pi transport activation, as well as reduced induction of many Pi starvation-inducible genes [1]. Pi responses such as root hair induction are not affected in phr1 mutant [1]. During Pi starvation, PSR1 expression is upregulated, whereas PHR1 is not. Dimerized PHR1 are likely to target a number of Pi starvation-inducible genes, indicated by the presence of the PHR1-binding sequence (GNATATNC) in the promoters. However, few of these genes are directly or dominantly regulated by PHR1, since few of them show a remarkable impairment in the induction by Pi starvation in phr1 [2]. Recently, the OsPHR1 and OsPHR2 genes in rice have been cloned and both are involved in Pi-starvation response, while overexpression of OsPHR2 induces Pi-starvation responses to execute, suggesting a conserved upstream role in Pi signaling regulation [5]. Two additional transcription factors, OsPTF1 and WRKY75, have been identified later. OsPTF1 is transcriptionally upregulated in the root in response to Pi starvation and its overexpressing can confer greater Pi-starvation tolerance to transgenic rice [124]. OsPTF1 are likely to target genes involved in root growth and Pi utilization, based on microarray analysis of genes differentially regulated in OsPTF1 overexpressing lines [124]. WRKY75 is another Pi starvation-induced transcription factor [125]. Upon RNAi knockdown of WRKY75, induction of AT4 and AtIPS1 by Pi starvation was greatly impaired whereas induction of AtACP5 was nearly unaffected [125]. 10. The first circuit: PHO2, microRNA399s and AtIPS1/At4 family

8.2. Regulation of Pi transporters by phosphate starvation Both positive and negative transcriptional regulation of Pi transporters have been reported. Pi uptake enhancement by Pi starvation is consistent with the induction of AtPht1;1, AtPht1;4 [14,15], AtPht1;2 and AtPht1;3 [14,15,116]. PHR1, the first characterized transcription factor in Pi signaling as discussed below, may partially account for this positive regulation of Pi transporters. The PHR1-binding sequence (GNATATNC) can be found in the promoters of several AtPht1 genes, including AtPht1;1, AtPht1;2, AtPht1;3 and AtPht1;4. On the other hand, unknown negative regulatory factors, which are removed upon Pi deficiency, were found to be bound to the AtPht1;4 promoter during normal conditions [121]. At the protein trafficking level, a Pi starvation-inducible gene encoding the endoplasmic reticulum (ER) protein, PHF1 (phosphate transporter traffic facilitator 1) was shown to be involved in controlling the ER exit of AtPht1;1, and possibly other members of this family [122]. Expression of PHF1 is also increased in senescing leaves, suggesting its additional regulatory roles [122]. Several members of the PHO1 family are upregulated [13,15,123], though their roles in Pi transport regulation are still obscure. 9. The first regulator: PHR1 PHR1 (phosphate starvation response 1) is the first and best characterized transcription factor in phosphate starvation regula-

AtPHO2 (At2g33770), defined by the pho2 mutant with a Pi overloading, Pi redistribution and recycling impairment phenotype, encodes an E2 ubiquitin-conjugating enzyme (UBC) [126,127]. For the Pi overloading phenotype, shoot removal resulted similar Pi uptake rate between pho2 mutant and wildtype seedlings [71], whereas grafting a wild-type shoot to a pho2 root retains the phenotype [2], indicating the Pi loading regulation by PHO2 occurs in the root, though the shoot is essential for enhanced Pi uptake in the PHO2. The microRNA399 (miR399) family is one of the 24 conserved microRNA families in plants [128], containing six members in Arabidopsis and 11 in rice [126,129]. MiR399s are predicted to target the 50 UTR of the AtPHO2 mRNA [2,4,130,131]. Expression analysis showed miR399s and PHO2 were colocalized in the vascular cylinder [131]. And during Pi starvation, miR399s are strongly upregulated while the PHO2 is downregulated [2,4,130]. Further more, miR399s overexpression and pho2 loss result in almost identical phenotypes, including increased Pi loading to shoot, decreased Pi recycling to root and reduced Pi redistribution from old leaves to young tissues [4,71,131]. Collectively, these evidences indicate miR399s target PHO2 and regulate several Pi transport processes including Pi loading, redistribution and recycling. The AtIPS1/At4 family, including At4, AtIPS1 and two At4-like genes in Arabidopsis [82,83,85] and OsPI1 and OsPI2 in rice [84,86], are not similar in the overall sequences but with a conserved 22-nt

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segment in the open-reading frames. Genetic evidences suggest At4 is involved in Pi redistribution and hybridizes a unknown small RNA [82]. A recent report shows that the AtIPS1 mRNA imperfectly pairs with and sequesters miR399s, blocking their silencing of PHO2 [132]. This novel mechanism is called ‘target mimicry’ [132]. Another recent work proposes that miR399s can move from the shoot to root upon Pi starvation to improve Pi acquisition [112]. This is the first direct evidence that miRNAs function as a longdistance Pi-starvation signal that not characterized before. Thus, PHO2, miR399s and AtIPS1/At4 family constitute a novel circuit in Pi signaling. It is possible that during the normal Pi conditions, PHO2 plays a role of limiting Pi loading in the root and maintaining Pi recycling and redistribution in the leaves. During initial Pi starvation, both miR399s and AtIPS1 are induced and PHO2 is protected and not downregulated. However, as the Pi starvation gets severe and the shoot Pi concentration becomes intolerably decreased, shoot miR399s would be strongly induced and transported into the root, exceeding the antagonizing effect of AtIPS1, tuning down the PHO2 mRNA level in the root, releasing its limit on Pi shoot loading to slow down Pi reduction in the shoot.

number of these genes will be mentioned section by section below. It has been proposed that there are distinct stages, or potential programes, of transcriptional responses to Pi starvation [12,14]. The so-called ‘early’ genes are those that are induced rapidly by Pi deficiency and often just represent a general-stress response, rather than Pi starvation-specific. On the other hand, ‘late’ genes are suggested to be more tightly related to the morphological, metabolic or physiological responses to Pi starvation. Promoter dissection of ‘late’ genes may provide clues of their potential upstream transcription factors [145]. Regulation at the mRNA level is only one component of expression regulation, and although its role in Pi response is better known than other mechanisms, this does not indicate other regulations can be ignored. For instance, induction of AtPAP26 activity upon Pi starvation is due to protein level, rather than mRNA level, regulations, as suggested recently, indicating that proteomic analysis should be an essential complement to the microarray approach [11]. Proteomics of Pi-starved plants and mutants involved in Pi-starvation response may shed new light on Pi signaling mechanisms [146]. 13. Toward a phosphate signaling network

11. Putative regulators 11.1. PHO1 and PHO3 The pho1 mutant of Arabidopsis is deficient in loading root Pi into the xylem vessels, resulting in strong Pi deficiency in all the above-ground tissues. Cloning, sequencing and expression profiling [133,134] reveal that the PHO1 family, characterized by a SPX tripartite domain and an EXS domain, contains 11 members in Arabidopsis [133]. However, only PHO1;H1 is functionally equivalent to PHO1. PHO1;H1 is downstream of PHR1 and responsive to phosphite, whereas PHO1 is under a distinct control [123]. Another mutant pho3, likely due to a SUC2 gene mutation, has lower Pi content and root acid phosphatase activity [135,136]. Pi acquisition seems to have interactions with sucrose signaling [137,138]. 11.2. SUMO ligase SIZ1 Peptide modification has been shown to be a broad mechanism of biochemical pathways in both animal and plants. Besides ubiquitin, other ubiquitin-like peptides are also identified, such as the small ubiquitin-related modifier (SUMO) family proteins, which are also added to target proteins through three enzymes [139]. Sumoylation has different roles compared with ubiquitination [140]. SIZ1 is the only SUMO E3 ligase in Arabidopsis, and has been shown to be involved in stress responses [141–143]. It is likely that SIZ1 is not a regulator that specifically regulates Pistarvation responses. 12. Microarrays and proteomics Many works analyzing transcriptomic changes during Pi starvation by microarrays have been reported, such as in the leaf [12,13], both the leaf and the root [14,144], or whole seedling [15] in Arabidopsis and in the leaf [16], the root [17] or whole plant [101] in rice. Many genes are significantly regulated by Pi deprivation. Based on an initial estimation with reported data [14,15], near 5% of the genome expression has a twofold or more-fold induction or inhibition in Arabidopsis, and only about 20% of expression changes are shared by the root and the leaf, supporting the notion that phosphate signaling programs differ between the two organs. A

After examining the recent understanding of Pi signaling, an initial Pi signaling network is emerging (Fig. 2). The regulatory molecules in this network include plant hormones (auxin and cytokinin), transcriptional regulators (AtPHR1, OsPHR1, AtWRKY75 and OsPTF1), protein traffic facilitators (AtPHF1), miRNAs (miR399 family), UBC (AtPHO2) and possibly the SUMO ligase (AtSIZ1). There are five major downstream outputs of Pi signaling: regulation of the root system, regulation of Pi mobilization and remobilization, regulation of Pi transport processes, regulation of Pi utilization, and regulation of anthocyanin biosynthesis. In the context of Pi starvation, primary root inhibition involves auxin and several loci defined by Arabidopsis mutants (AtLPR1, AtLPR2, AtPDR2, AtLPI1 and AtLPI2), and lateral root promotion is also likely to involve auxin to some extent, whereas root hair induction and elongation stimulation are partially independent of auxin and ethylene. Pi mobilization and remobilization enhancement involves induction of extra- and/or intra-cellular APases (AtPAP12, AtPAP17/ACP5 and AtPAP26) and RNases (AtRNS1 and AtRNS2), and increased organic acids (malate and citrate) exudation, possibly via anion transporters and plasma membrane H+-ATPase, which is partially due to an induction of biosynthetic enzymes (PEPC, MDH and CS). Most Pi transport processes are enhanced: Pi uptake enhancement involves induction of AtPht1 family members (AtPht1;1, AtPht1;4; also AtPht1;2 and AtPht1;3) and a protein facilitating the ER exit of AtPht1 transporters (AtPHF1), transporters or regulators involved in Pi loading (AtPHO1 family, miR399 family, AtPHO2, AtIPS1/At4 family), tissue Pi acquisition (AtPht1 family, AtPHO1 and AtPHO1;H1) and Pi redistribution (miR399 family, AtPHO2 and AtIPS1/At4 family) are regulated to allow transport enhancement, vacuole P export is activated. In addition, in rice a mycorrhizaspecific Pi transporter (OsPT11) is induced to acquire more Pi from the fungi. Regulation of Pi utilization on the one hand relies on differential regulation of several respiratory enzymes, resulting in three major bypasses at ADP/ATP/Pi-dependent reactions to allow carbon flow in spite of gradually aggravated reduction in Pi/ATP/ ADP, and on the other hand upregulates enzymes involved in phospholipid degradation (phospholipases C and D), sulfolipid (AtSQD1 and AtSQD2) and galactolipid (AtMGD2/3 and AtDGD2) biosynthesis to conserve the limited Pi. Accumulated anthocyanins, requiring AtPHR1 and likely due to an upregulation of anthocyanin biosynthetic enzymes, may play a role in photoprotection.

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Fig. 2. An integrated model of the Pi signaling network in Arabidopsis and Oryza. This map was created using CellDesigner version 4.0 (http://www.systems-biology.org/cd/). A total number of 176 reactions and 203 species were included. Each icon in the diagram represents distinct chemical species. At4, Arabidopsis thaliana 4; CHA, CHABLIS; CPC, CAPRICE; CRE, cytokinin receptor; DAG, diacylglycerol; DAHP, 3-deoxy-D-arabinoheptulosonate-7-phosphate; DGDG, galactolipid digalactosyldiacylglycerol; DHQ, 3-dehydroquinate; E4P, erythrose-4-phosphate; EPSP, 5-enolpyruvylshikimate-3-phosphate; GL, GLABRA; IPS, induced by phosphate starvation; LPI, low phosphorus insensitive; MGDG, monogalactosyldiacylglycerol; NMP, N-methylpyrrolidinone; NMT, N-myristoyltransferase; PAP, purple acid phosphatase; PDR, phosphate deficiency response; PEP, phosphoenolpyruvate; PHO, phosphatase; PHR phosphate starvation response; Pht, phosphate transporter; PPC, 40 -phosphopantothenoylcysteine; PTF, plant transcription factor; PUP, purine permease; RNS, ribonuclease; S3P, shikimate-3-phosphate; SCM, SCRAMBLED; SIZ, SAP and Miz1; SQDG, sulfolipid sulfoquinovosyl diacylglycerol; UDP, uridine diphosphate; WER, WEREWOLF; WRKY, name given by WRKYGQK peptide in its N-terminus.

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Nonetheless, this Pi signaling network is still far from comprehensive. One key question is how the Pi-starvation responses are regulated. This may involve signaling pathways that underlying the Pi signal sensing and transduction at the cellular level and information communication between tissues at the systemic level. The cellular signal transduction is relatively clear, though far from sufficient to explain the drastic transcriptomic shift. For instance, although AtPHR1 is involved in the upregulation of many Pi starvation-inducible genes, its regulatory role seems to be redundant, or even subordinate. Moreover, Pi signaling is likely to interact with a number of signaling pathways or developmental programs, such as hormone signaling (see above), cell cycle regulation [147], sugar signaling [13,103, 135,136] and leaf senescence [4,10,51,61,131]. Characterization of the molecular links between these signaling programs would allow the construction of a highly integrated network. Acknowledgements This work was partially supported by the National Natural Science Foundation of China (No. 30771326 and No. 30500106), 973 project (No. 2005CB120901), 863 program (No. 2008AA10Z125), the Department of Science and Technology of Zhejiang Province, China (No. 2007C22025), NCET, and by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, China. References [1] V. Rubio, et al., A conserved MYB transcription factor involved in phosphate starvation signaling both in vascular plants and in unicellular algae, Genes Dev. 15 (16) (2001) 2122–2133. [2] R. Bari, et al., PHO2, microRNA399, and PHR1 define a phosphate-signaling pathway in plants, Plant Physiol. 141 (3) (2006) 988–999. [3] D.D. Wykoff, et al., Psr1, a nuclear localized protein that regulates phosphorus metabolism in Chlamydomonas, Proc. Natl. Acad. Sci. U.S.A. 96 (26) (1999) 15336–15341. [4] T.J. Chiou, et al., Regulation of phosphate homeostasis by microRNA in Arabidopsis, Plant Cell 18 (2) (2006) 412–421. [5] J. Zhou, et al., OsPHR2 is involved in phosphate-starvation signaling and excessive phosphate accumulation in shoots of plants, Plant Physiol. 146 (4) (2008) 1673–1686. [6] J. Wasaki, et al., Secreted acid phosphatase is expressed in cluster roots of lupin in response to phosphorus deficiency, Plant Soil 248 (1–2) (2003) 129–136. [7] S.S. Miller, et al., Molecular control of acid phosphatase secretion into the rhizosphere of proteoid roots from phosphorus-stressed white lupin, Plant Physiol. 127 (2) (2001) 594–606. [8] S. Haran, et al., Characterization of Arabidopsis acid phosphatase promoter and regulation of acid phosphatase expression, Plant Physiol. 124 (2) (2000) 615– 626. [9] D. Li, et al., Purple acid phosphatases of Arabidopsis thaliana. Comparative analysis and differential regulation by phosphate deprivation, J. Biol. Chem. 277 (31) (2002) 27772–27781. [10] J.C. del Pozo, et al., A type 5 acid phosphatase gene from Arabidopsis thaliana is induced by phosphate starvation and by some other types of phosphate mobilising/oxidative stress conditions, Plant J. 19 (5) (1999) 579–589. [11] V. Veljanovski, et al., Biochemical and molecular characterization of AtPAP26, a vacuolar purple acid phosphatase up-regulated in phosphate-deprived Arabidopsis suspension cells and seedlings, Plant Physiol. 142 (3) (2006) 1282–1293. [12] J.P. Hammond, et al., Changes in gene expression in Arabidopsis shoots during phosphate starvation and the potential for developing smart plants, Plant Physiol. 132 (2) (2003) 578–596. [13] R. Muller, et al., Genome-wide analysis of the Arabidopsis leaf transcriptome reveals interaction of phosphate and sugar metabolism, Plant Physiol. 143 (1) (2007) 156–171. [14] J. Misson, et al., A genome-wide transcriptional analysis using Arabidopsis thaliana affymetrix gene chips determined plant responses to phosphate deprivation, Proc. Natl. Acad. Sci. U.S.A. 102 (33) (2005) 11934–11939. [15] R. Morcuende, et al., Genome-wide reprogramming of metabolism and regulatory networks of Arabidopsis in response to phosphorus, Plant Cell Environ. 30 (1) (2007) 85–112. [16] J. Wasaki, et al., Transcriptomic analysis indicates putative metabolic changes caused by manipulation of phosphorus availability in rice leaves, J. Exp. Bot. 57 (9) (2006) 2049–2059. [17] J. Wasaki, et al., Transcriptomic analysis of metabolic changes by phosphorus stress in rice plant roots, Plant Cell Environ. 26 (9) (2003) 1515–1523.

[18] J. Lopez-Bucio, et al., Phosphate availability alters architecture and causes changes in hormone sensitivity in the Arabidopsis root system, Plant Physiol. 129 (1) (2002) 244–256. [19] L.C. Williamson, et al., Phosphate availability regulates root system architecture in Arabidopsis, Plant Physiol. 126 (2) (2001) 875–882. [20] J. Lopez-Bucio, et al., An auxin transport independent pathway is involved in phosphate stress-induced root architectural alterations in arabidopsis. Identification of BIG as a mediator of auxin in pericycle cell activation, Plant Physiol. 137 (2) (2005) 681–691. [21] P. Nacry, et al., A role for auxin redistribution in the responses of the root system architecture to phosphate starvation in Arabidopsis, Plant Physiol. 138 (4) (2005) 2061–2074. [22] Y. Al-Ghazi, et al., Temporal responses of Arabidopsis root architecture to phosphate starvation: evidence for the involvement of auxin signalling, Plant Cell Environ. 26 (7) (2003) 1053–1066. [23] B.I. Linkohr, et al., Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis, Plant J. 29 (6) (2002) 751–760. [24] S. Svistoonoff, et al., Root tip contact with low-phosphate media reprograms plant root architecture, Nat. Genet. 39 (6) (2007) 792–796. [25] J.M. Franco-Zorrilla, et al., The transcriptional control of plant responses to phosphate limitation, J. Exp. Bot. 55 (396) (2004) 285–293. [26] C.S. Hardtke, Root development—branching into novel spheres, Curr. Opin. Plant Biol. 9 (1) (2006) 66–71. [27] Z. Ma, et al., Regulation of root elongation under phosphorus stress involves changes in ethylene responsiveness, Plant Physiol. 131 (3) (2003) 1381–1390. [28] C.A. Ticconi, et al., Arabidopsis pdr2 reveals a phosphate-sensitive checkpoint in root development, Plant J. 37 (6) (2004) 801–814. [29] L. Sanchez-Calderon, et al., Characterization of low phosphorus insensitive mutants reveals a crosstalk between low phosphorus-induced determinate root development and the activation of genes involved in the adaptation of Arabidopsis to phosphorus deficiency, Plant Physiol. 140 (3) (2006) 879–889. [30] S. Guimil, C. Dunand, Patterning of Arabidopsis epidermal cells: epigenetic factors regulate the complex epidermal cell fate pathway, Trends Plant Sci. 11 (12) (2006) 601–609. [31] J.W. Schiefelbein, Constructing a plant cell. The genetic control of root hair development, Plant Physiol. 124 (4) (2000) 1525–1531. [32] L. Dolan, The role of ethylene in root hair growth in Arabidopsis, J. Plant Nutr. Soil Sci.-Zeitschrift fur Pflanzenernahrung und Bodenkunde 164 (2) (2001) 141–145. [33] H.M. Leyser, et al., Mutations in the AXR3 gene of Arabidopsis result in altered auxin response including ectopic expression from the SAUR-AC1 promoter, Plant J. 10 (3) (1996) 403–413. [34] M. Muller, W. Schmidt, Environmentally induced plasticity of root hair development in Arabidopsis, Plant Physiol. 134 (1) (2004) 409–419. [35] J.D. Masucci, J.W. Schiefelbein, The rhd6 mutation of Arabidopsis thaliana alters root-hair initiation through an auxin- and ethylene-associated process, Plant Physiol. 106 (4) (1994) 1335–1346. [36] R.J. Pitts, A. Cernac, M. Estelle, Auxin and ethylene promote root hair elongation in Arabidopsis, Plant J. 16 (5) (1998) 553–560. [37] T.R. Bates, J.P. Lynch, Stimulation of root hair elongation in Arabidposis thaliana by low phosphorus availability, Plant Cell Environ. 19 (5) (1996) 529–538. [38] Z. Ma, et al., Regulation of root hair density by phosphorus availability in Arabidopsis thaliana, Plant Cell Environ. 24 (4) (2001) 459–467. [39] T.R. Bates, J.P. Lynch, Plant growth and phosphorus accumulation of wild type and two root hair mutants of Arabidopsis thaliana (Brassicaceae), Am. J. Bot. 87 (7) (2000) 958–963. [40] T.R. Bates, J.P. Lynch, The efficiency of Arabidopsis thaliana (Brassicaceae) root hairs in phosphorus acquisition, Am. J. Bot. 87 (7) (2000) 964–970. [41] T.R. Bates, J.P. Lynch, Root hairs confer a competitive advantage under low phosphorus availability, Plant Soil 236 (2) (2001) 243–250. [42] W. Schmidt, A. Schikora, Different pathways are involved in phosphate and iron stress-induced alterations of root epidermal cell development, Plant Physiol. 125 (4) (2001) 2078–2084. [43] S.M.G. Duff, G. Sarath, W.C. Plaxton, The role of acid phosphatases in plant phosphorus metabolism, Physiologia Plantarum 90 (4) (1994) 791–800. [44] G.G. Bozzo, K.G. Raghothama, W.C. Plaxton, Structural and kinetic properties of a novel purple acid phosphatase from phosphate-starved tomato (Lycopersicon esculentum) cell cultures, Biochem. J. 377 (2004) 419–428. [45] G.G. Bozzo, K.G. Raghothama, W.C. Plaxton, Purification and characterization of two secreted purple acid phosphatase isozymes from phosphate-starved tomato (Lycopersicon esculentum) cell cultures, Eur. J. Biochem. 269 (24) (2002) 6278– 6286. [46] K. Ozawa, et al., Purification and properties of acid phosphatase secreted from lupin roots under phosphorus-deficiency conditions, Soil Sci. Plant Nutr. 41 (3) (1995) 461–469. [47] J. Wasaki, et al., Molecular cloning and root specific expression of secretory acid phosphatase from phosphate deficient lupin (Lupinus albus L.), Soil Sci. Plant Nutr. 46 (2) (2000) 427–437. [48] D. Li, et al., Purple acid phosphatases of Arabidopsis thaliana. Comparative analysis and differential regulation by phosphate deprivation, J. Biol. Chem. 277 (31) (2002) 27772–27781. [49] M.C. Trull, et al., The responses of wild-type and ABA mutant Arabidopsis thaliana plants to phosphorus starvation, Plant Cell Environ. 20 (1) (1997) 85–92. [50] J.L. Tomscha, et al., Phosphatase under-producer mutants have altered phosphorus relations, Plant Physiol. 135 (1) (2004) 334–345.

Z. Fang et al. / Plant Science 176 (2009) 170–180 [51] S. Gepstein, et al., Large-scale identification of leaf senescence-associated genes, Plant J. 36 (5) (2003) 629–642. [52] G. Schenk, et al., Identification of mammalian-like purple acid phosphatases in a wide range of plants, Gene 250 (1–2) (2000) 117–125. [53] M. Olczak, B. Morawiecka, W. Watorek, Plant purple acid phosphatases—genes, structures and biological function, Acta Biochim. Pol. 50 (4) (2003) 1245–1256. [54] M.C. Trull, J. Deikman, An Arabidopsis mutant missing one acid phosphatase isoform, Planta 206 (4) (1998) 544–550. [55] Y.L. Liu, et al., Arabidopsis vegetative storage protein is an anti-insect acid phosphatase, Plant Physiol. 139 (3) (2005) 1545–1556. [56] J.C. Baldwin, A.S. Karthikeyan, K.G. Raghothama, LEPS2, a phosphorus starvationinduced novel acid phosphatase from tomato, Plant Physiol. 125 (2) (2001) 728– 737. [57] C.B. Taylor, P.J. Green, Genes with homology to fungal and S-gene RNases are expressed in Arabidopsis thaliana, Plant Physiol. 96 (3) (1991) 980–984. [58] P.A. Bariola, et al., The Arabidopsis ribonuclease gene RNS1 is tightly controlled in response to phosphate limitation, Plant J. 6 (5) (1994) 673–685. [59] P.A. Bariola, G.C. MacIntosh, P.J. Green, Regulation of S-like ribonuclease levels in Arabidopsis. Antisense inhibition of RNS1 or RNS2 elevates anthocyanin accumulation, Plant Physiol. 119 (1) (1999) 331–342. [60] N.D. LeBrasseur, et al., Local and systemic wound-induction of RNase and nuclease activities in Arabidopsis: RNS1 as a marker for a JA-independent systemic signaling pathway, Plant J. 29 (4) (2002) 393–403. [61] C.B. Taylor, et al., RNS2: a senescence-associated RNase of Arabidopsis that diverged from the S-RNases before speciation, Proc. Natl. Acad. Sci. U.S.A. 90 (11) (1993) 5118–5122. [62] P.R. Ryan, E. Delhaize, D.L. Jones, Function and mechanism of organic anion exudation from plant roots, Annu. Rev. Plant Physiol. Plant Mol. Biol. 52 (2001) 527–560. [63] R.A. Narang, A. Bruene, T. Altmann, Analysis of phosphate acquisition efficiency in different Arabidopsis accessions, Plant Physiol. 124 (4) (2000) 1786–1799. [64] H.H. Begum, et al., The function of a maize-derived phosphoenolpyruvate carboxylase (PEPC) in phosphorus-deficient transgenic rice, Soil Sci. Plant Nutr. 51 (4) (2005) 497–506. [65] C.P. Vance, C. Uhde-Stone, D.L. Allan, Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource, New Phytologist 157 (3) (2003) 423–447. [66] H. Koyama, et al., Overexpression of mitochondrial citrate synthase in Arabidopsis thaliana improved growth on a phosphorus-limited soil, Plant Cell Physiol. 41 (9) (2000) 1030–1037. [67] J. Lopez-Bucio, et al., Enhanced phosphorus uptake in transgenic tobacco plants that overproduce citrate, Nat. Biotechnol. 18 (4) (2000) 450–453. [68] E. Delhaize, D.M. Hebb, P.R. Ryan, Expression of a Pseudomonas aeruginosa citrate synthase gene in tobacco is not associated with either enhanced citrate accumulation or efflux, Plant Physiol. 125 (4) (2001) 2059–2067. [69] L. Sas, Z. Rengel, C. Tang, Excess cation uptake, and extrusion of protons and organic acid anions by Lupinus albus under phosphorus deficiency, Plant Sci. 160 (6) (2001) 1191–1198. [70] F. Yan, et al., Adaptation of H+-pumping and plasma membrane H+ ATPase activity in proteoid roots of white lupin under phosphate deficiency, Plant Physiol. 129 (1) (2002) 50–63. [71] B. Dong, Z. Rengel, E. Delhaize, Uptake and translocation of phosphate by pho2 mutant and wild-type seedlings of Arabidopsis thaliana, Planta 205 (2) (1998) 251–256. [72] M. Bucher, Functional biology of plant phosphate uptake at root and mycorrhiza interfaces, New Phytologist 173 (1) (2007) 11–26. [73] H. Javot, N. Pumplin, M.J. Harrison, Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles, Plant Cell Environ. 30 (3) (2007) 310–322. [74] U. Paszkowski, et al., Rice phosphate transporters include an evolutionarily divergent gene specifically activated in arbuscular mycorrhizal symbiosis, Proc. Natl. Acad. Sci. U.S.A. 99 (20) (2002) 13324–13329. [75] H. Marschner, Mineral Nutrition of Higher Plants, second ed., Academic Press, London, 1995. [76] R.B. Lee, Selectivity and kinetics of ion uptake by barley plants following nutrient deficiency, Ann. Bot. 50 (4) (1982) 429–449. [77] T. Mimura, R. Reid, F. Smith, Control of phosphate transport across the plasma membrane of Chara corallina, J. Exp. Bot. 49 (318) (1998) 13–19. [78] T. Mimura, K. Sakano, T. Shimmen, Studies on the distribution, re-translocation and homeostasis of inorganic phosphate in barley leaves, Plant Cell Environ. 19 (3) (1996) 311–320. [79] S.I. Tu, J.R. Cavanaugh, R.T. Boswell, Phosphate uptake by excised maize root tips studied by in vivo P nuclear magnetic resonance spectroscopy, Plant Physiol. 93 (2) (1990) 778–784. [80] K. Sakano, Y. Yazaki, T. Mimura, Cytoplasmic acidification induced by inorganic phosphate uptake in suspension cultured Catharanthus roseus cells: measurement with fluorescent pH indicator and P-nuclear magnetic resonance, Plant Physiol. 99 (2) (1992) 672–680. [81] M. Ohnishi, et al., Inorganic phosphate uptake in intact vacuoles isolated from suspension-cultured cells of Catharanthus roseus (L.) G. Don under varying Pi status, Planta 225 (3) (2007) 711–718. [82] H. Shin, et al., Loss of At4 function impacts phosphate distribution between the roots and the shoots during phosphate starvation, Plant J. 45 (5) (2006) 712–726. [83] A.C. Martin, et al., Influence of cytokinins on the expression of phosphate starvation responsive genes in Arabidopsis, Plant J. 24 (5) (2000) 559–567.

179

[84] J. Wasaki, et al., Expression of the OsPI1 gene, cloned from rice roots using cDNA microarray, rapidly responds to phosphorus status, New Phytologist 158 (2) (2003) 239–248. [85] S.H. Burleigh, M.J. Harrison, The down-regulation of Mt4-like genes by phosphate fertilization occurs systemically and involves phosphate translocation to the shoots, Plant Physiol. 119 (1) (1999) 241–248. [86] X.L. Hou, et al., Regulation of the expression of OsIPS1 and OsIPS2 in rice via systemic and local Pi signalling and hormones, Plant Cell Environ. 28 (3) (2005) 353–364. [87] S.M. Duff, et al., Phosphate starvation inducible;bypasses’ of adenylate and phosphate dependent glycolytic enzymes in Brassica nigra suspension cells, Plant Physiol. 90 (4) (1989) 1275–1278. [88] W.C. Plaxton, The organization and regulation of plant glycolysis, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996) 185–214. [89] M.E. Theodorou, W.C. Plaxton, Metabolic adaptations of plant respiration to nutritional phosphate deprivation, Plant Physiol. 101 (2) (1993) 339–344. [90] C. Benning, et al., The sulfolipid sulfoquinovosyldiacylglycerol is not required for photosynthetic electron transport in Rhodobacter sphaeroides but enhances growth under phosphate limitation, Proc. Natl. Acad. Sci. U.S.A. 90 (4) (1993) 1561–1565. [91] B. Essigmann, et al., Phosphate availability affects the thylakoid lipid composition and the expression of SQD1, a gene required for sulfolipid biosynthesis in Arabidopsis thaliana, Proc. Natl. Acad. Sci. U.S.A. 95 (4) (1998) 1950–1955. [92] H. Hartel, P. Dormann, C. Benning, DGD1-independent biosynthesis of extraplastidic galactolipids after phosphate deprivation in Arabidopsis, Proc. Natl. Acad. Sci. U.S.A. 97 (19) (2000) 10649–10654. [93] J. Jouhet, et al., Phosphate deprivation induces transfer of DGDG galactolipid from chloroplast to mitochondria, J. Cell Biol. 167 (5) (2004) 863–874. [94] Y. Nakamura, et al., A novel phosphatidylcholine-hydrolyzing phospholipase C induced by phosphate starvation in Arabidopsis, J. Biol. Chem. 280 (9) (2005) 7469–7476. [95] B. Yu, C.C. Xu, C. Benning, Arabidopsis disrupted in SQD2 encoding sulfolipid synthase is impaired in phosphate-limited growth, Proc. Natl. Acad. Sci. U.S.A. 99 (8) (2002) 5732–5737. [96] C. Benning, H. Ohta, Three enzyme systems for galactoglycerolipid biosynthesis are coordinately regulated in plants, J. Biol. Chem. 280 (4) (2005) 2397–2400. [97] K. Kobayashi, et al., Membrane lipid alteration during phosphate starvation is regulated by phosphate signaling and auxin/cytokinin cross-talk, Plant J. 47 (2) (2006) 238–248. [98] W.J. Steyn, et al., Anthocyanins in vegetative tissues: a proposed unified function in photoprotection, New Phytologist 155 (3) (2002) 349–361. [99] K. Springob, et al., Recent advances in the biosynthesis and accumulation of anthocyanins, Nat. Prod. Rep. 20 (3) (2003) 288–303. [100] B. Winkel-Shirley, Flavonoid biosynthesis. A colorful model for genetics, biochemistry, cell biology, and biotechnology, Plant Physiol. 126 (2) (2001) 485–493. [101] X.M. Wang, et al., Cytokinin represses phosphate-starvation response through increasing of intracellular phosphate level, Plant Cell Environ. 29 (10) (2006) 1924–1935. [102] J.M. Franco-Zorrilla, et al., Mutations at CRE1 impair cytokinin-induced repression of phosphate starvation responses in Arabidopsis, Plant J. 32 (3) (2002) 353–360. [103] J.M. Franco-Zorrilla, et al., Interaction between phosphate-starvation, sugar, and cytokinin signaling in Arabidopsis and the roles of cytokinin receptors CRE1/ AHK4 and AHK3, Plant Physiol. 138 (2) (2005) 847–857. [104] A. Torriani, From cell membrane to nucleotides: the phosphate regulon in Escherichia coli, Bioessays 12 (8) (1990) 371–376. [105] B.L. Wanner, Gene regulation by phosphate in enteric bacteria, J. Cell Biochem. 51 (1) (1993) 47–54. [106] J.M. Mouillon, B.L. Persson, New aspects on phosphate sensing and signalling in Saccharomyces cerevisiae, FEMS Yeast Res. 6 (2) (2006) 171–176. [107] B.L. Persson, et al., Regulation of phosphate acquisition in Saccharomyces cerevisiae, Curr. Genetics 43 (4) (2003) 225–244. [108] C.A. Ticconi, C.A. Delatorre, S. Abel, Attenuation of phosphate starvation responses by phosphite in Arabidopsis, Plant Physiol. 127 (3) (2001) 963–972. [109] D.K. Varadarajan, et al., Phosphite, an analog of phosphate, suppresses the coordinated expression of genes under phosphate starvation, Plant Physiol. 129 (3) (2002) 1232–1240. [110] G.A. Gilbert, et al., Proteoid root development of phosphorus deficient lupin is mimicked by auxin and phosphonate, Ann. Bot. 85 (6) (2000) 921–928. [111] C.A. Ticconi, S. Abel, Short on phosphate: plant surveillance and countermeasures, Trends Plant Sci. 9 (11) (2004) 548–555. [112] S.-I. Lin, et al., Regulatory network of microRNA399 and PHO2 by systemic signaling, Plant Physiol. 147 (2) (2008) 732–746. [113] J. Misson, et al., Transcriptional regulation and functional properties of Arabidopsis Pht1;4, a high affinity transporter contributing greatly to phosphate uptake in phosphate deprived plants, Plant Mol. Biol. 55 (5) (2004) 727–741. [114] N. Mitsukawa, et al., Overexpression of an Arabidopsis thaliana high-affinity phosphate transporter gene in tobacco cultured cells enhances cell growth under phosphate-limited conditions, Proc. Natl. Acad. Sci. U.S.A. 94 (13) (1997) 7098–7102. [115] H. Shin, et al., Phosphate transport in Arabidopsis: Pht1;1 and Pht1;4 play a major role in phosphate acquisition from both low- and high-phosphate environments, Plant J. 39 (4) (2004) 629–642. [116] S.R. Mudge, et al., Expression analysis suggests novel roles for members of the Pht1 family of phosphate transporters in Arabidopsis, Plant J. 31 (3) (2002) 341– 353.

180

Z. Fang et al. / Plant Science 176 (2009) 170–180

[117] Y. Poirier, M. Bucher, Phosphate transport and homeostasis in Arabidopsis, in: C. Somerville, E. Meyerowitz (Eds.), The Arabidopsis Book, American Society of Plant Biologists, Rockville, MD, 2002, pp. 1–35. [118] C. Rausch, et al., Expression analysis suggests novel roles for the plastidic phosphate transporter Pht2;1 in auto- and heterotrophic tissues in potato and Arabidopsis, Plant J. 39 (1) (2004) 13–28. [119] W.K. Versaw, M.J. Harrison, A chloroplast phosphate transporter, PHT2;1, influences allocation of phosphate within the plant and phosphate-starvation responses, Plant Cell 14 (8) (2002) 1751–1766. [120] A. Massonneau, et al., Phosphate uptake across the tonoplast of intact vacuoles isolated from suspension-cultured cells of Catharanthus roseus (L.) G. Don, Planta 211 (3) (2000) 390–395. [121] U.T. Mukatira, et al., Negative regulation of phosphate starvation-induced genes, Plant Physiol. 127 (4) (2001) 1854–1862. [122] E. Gonzalez, et al., Phosphate transporter traffic facilitator1 is a plant-specific SEC12-related protein that enables the endoplasmic reticulum exit of a highaffinity phosphate transporter in Arabidopsis, Plant Cell 17 (12) (2005) 3500– 3512. [123] A. Stefanovic, et al., Members of the PHO1 gene family show limited functional redundancy in phosphate transfer to the shoot, and are regulated by phosphate deficiency via distinct pathways, Plant J. 50 (6) (2007) 982–994. [124] K.K. Yi, et al., OsPTF1, a novel transcription factor involved in tolerance to phosphate starvation in rice, Plant Physiol. 138 (4) (2005) 2087–2096. [125] B.N. Devaiah, A.S. Karthikeyan, K.G. Raghothama, WRKY75 transcription factor is a modulator of phosphate acquisition and root development in Arabidopsis, Plant Physiol. 143 (4) (2007) 1789–1801. [126] R. Sunkar, J.K. Zhu, Novel and stress-regulated microRNAs and other small RNAs from Arabidopsis, Plant Cell 16 (8) (2004) 2001–2019. [127] E. Allen, et al., MicroRNA-directed phasing during trans-acting siRNA biogenesis in plants, Cell 121 (2) (2005) 207–221. [128] M.W. Jones-Rhoades, D.P. Bartel, B. Bartel, MicroRNAs and their regulatory roles in plants, Annu. Rev. Plant Biol. 57 (2006) 19–53. [129] M.W. Jones-Rhoades, D.P. Bartel, Computational identification of plant MicroRNAs and their targets, including a stress-induced miRNA, Mol. Cell 14 (6) (2004) 787–799. [130] H. Fujii, et al., A miRNA involved in phosphate-starvation response in Arabidopsis, Curr. Biol. 15 (22) (2005) 2038–2043. [131] K. Aung, et al., Pho2, a phosphate overaccumulator, is caused by a nonsense mutation in a MicroRNA399 target gene, Plant Physiol. 141 (3) (2006) 1000–1011.

[132] J.M. Franco-Zorrilla, et al., Target mimicry provides a new mechanism for regulation of microRNA activity, Nat. Genet. 39 (8) (2007) 1033–1037. [133] Y. Wang, et al., Structure and expression profile of the Arabidopsis PHO1 gene family indicates a broad role in inorganic phosphate homeostasis, Plant Physiol. 135 (1) (2004) 400–411. [134] D. Hamburger, et al., Identification and characterization of the Arabidopsis PHO1 gene involved in phosphate loading to the xylem, Plant Cell 14 (4) (2002) 889– 902. [135] J.C. Lloyd, O.V. Zakhleniuk, Responses of primary and secondary metabolism to sugar accumulation revealed by microarray expression analysis of the Arabidopsis mutant, pho3, J. Exp. Bot. 55 (400) (2004) 1221–1230. [136] O.V. Zakhleniuk, C.A. Raines, J.C. Lloyd, Pho3: a phosphorus-deficient mutant of Arabidopsis thaliana (L.) Heynh, Planta 212 (4) (2001) 529–534. [137] A.S. Karthikeyan, et al., Phosphate starvation responses are mediated by sugar signaling in Arabidopsis, Planta 225 (4) (2007) 907–918. [138] R. Muller, et al., Interaction between phosphate starvation signalling and hexokinase-independent sugar sensing in Arabidopsis leaves, Physiologia Plantarum 124 (1) (2005) 81–90. [139] O. Kerscher, R. Felberbaum, M. Hochstrasser, Modification of proteins by ubiquitin and ubiquitin-like proteins, Annu. Rev. Cell Dev. Biol. 22 (2006) 159–180. [140] E.S. Johnson, Protein modification by SUMO, Annu. Rev. Biochem. 73 (2004) 355– 382. [141] T. Colby, et al., SUMO-conjugating and SUMO-deconjugating enzymes from Arabidopsis, Plant Physiol. 142 (1) (2006) 318–332. [142] K. Miura, et al., The Arabidopsis SUMO E3 ligase SIZ1 controls phosphate deficiency responses, Proc. Natl. Acad. Sci. U.S.A. 102 (21) (2005) 7760–7765. [143] C.Y. Yoo, et al., SIZ1 small ubiquitin-like modifier E3 ligase facilitates basal thermotolerance in Arabidopsis independent of salicylic acid, Plant Physiol. 142 (4) (2006) 1548–1558. [144] P. Wu, et al., Phosphate starvation triggers distinct alterations of genome expression in Arabidopsis roots and leaves, Plant Physiol. 132 (3) (2003) 1260–1271. [145] J.P. Hammond, M.R. Broadley, P.J. White, Genetic responses to phosphorus deficiency, Ann. Bot. 94 (3) (2004) 323–332. [146] K. Li, et al., Proteomic analysis of roots growth and metabolic changes under phosphorus deficit in maize (Zea mays L.) plants, Proteomics 7 (9) (2007) 1501– 1512. [147] F. Lai, et al., Cell division activity determines the magnitude of phosphate starvation responses in Arabidopsis, Plant J. 50 (3) (2007) 545–556.