Shattering fruits: variations on a dehiscent theme

Available online at www.sciencedirect.com

ScienceDirect Shattering fruits: variations on a dehiscent theme Patricia Ballester and Cristina Ferra´ndiz Fruits are seed dispersal units, and for that they have evolved different strategies to facilitate separation and dispersal of the progeny from the mother plant. A great proportion of fruits from different clades are dry and dehiscent, opening upon maturity to disperse the seeds. In the last two decades, intense research mainly in Arabidopsis has uncovered the basic network that controls the differentiation of the Arabidopsis fruit dehiscence zone. This review focuses on recent discoveries that have helped to complete the picture, as well as the insights from evodevo and crop domestication studies that show how the conservation/variation of the elements of this network across species accounts for its evolutionary plasticity and the origin of evolutionary innovations. Address Instituto de Biologı´a Molecular y Celular de Plantas, Consejo Superior de Investigaciones Cientı´ficas, Universidad Polite´cnica de Valencia, Valencia 46022, Spain Corresponding author: Ferra´ndiz, Cristina ([email protected])

Current Opinion in Plant Biology 2017, 35:68–75 This review comes from a themed issue on Growth and development Edited by Ji Hoon Ahn and Marcus Schmid

http://dx.doi.org/10.1016/j.pbi.2016.11.008 1369-5266/# 2016 Published by Elsevier Ltd.

themselves, or they can open (dehisce) and free the seeds, which have to separate from the mother tissues as well. These processes, fruit abscission, fruit dehiscence and seed abscission, have some common elements: a layer with small cells held together by the extracellular matrix formed at the breaking point, and lignification of cells at surrounding tissues. For separation to occur, physical forces have to trigger the detachment of cells at the separation layer, in an interplay that involves weakening of cell adhesion with tensions provided by the surrounding tissues or external agents. In fruit dehiscence (also known as pod shattering), these tensions usually come from pod walls, mediated by the differential mechanical properties of lignified and non-lignified tissues and changes in turgor associated to fruit maturation (Figure 1). Fruit dehiscence and abscission have been extensively studied, both for their biological importance and as immediate targets for crop improvement. Here we review the last contributions to our understanding of dehiscence in dry fruits, and how comparative studies of natural variation or man-driven selection in crop domestication can help to complete a picture where still we can find important gaps.

Genetic networks directing fruit dehiscence in Arabidopsis The major players, the influencers, the newcomers and a few open questions

Fruits are a major evolutionary innovation of angiosperms, which serve two main purposes: to protect the developing seeds and to facilitate offspring dispersal. This last function has great ecological importance and explains the high adaptive forces that have driven the huge diversity in morphology and function of fruits, which usually co-evolve with dispersal vectors [1]. The most abundant fruit types found are dry dehiscent, dry indehiscent and fleshy, and transitions between the different types are very frequent within clades, indicating their huge adaptive value and the likely plasticity of a common basic genetic ground that can easily accommodate changes to evolve new morphologies and dispersal strategies.

The basic components of the genetic network directing the morphogenesis of the dehiscence zone (DZ) in the Arabidopsis fruit have been known for several years. In brief, four transcription factors are expressed at the valve margin to direct DZ formation. SHATTERPROOF1 (SHP1) and SHP2 redundantly upregulate INDEHISCENT (IND) and ALCATRAZ (ALC) (Figure 2). In shp1 shp2 and ind mutants, fruits lack lignified and separation layers and are fully indehiscent, while alc mutants show only defects in the separation layer, which no longer shows the typical small cells and the separation plane [2– 4]. Two additional transcription factors, FRUITFULL (FUL) and REPLUMLESS (RPL), act in the valves and in the replum, respectively, to confine the expression of the DZ genes to the valve margin [3,5,6]. This relatively simple network and the regulatory interactions established among these components are largely sufficient to explain DZ formation, building the scaffold on which the basic blocks are defined.

According to definition, a fruit is a seed dispersal unit, thus implying that a separation process is involved. To disperse the seeds, fruits can abscise from the plant

More recently other players have been identified, which appear to modulate the size and position of the DZ or to contribute in partially redundant manner to the functions

Introduction

Current Opinion in Plant Biology 2017, 35:68–75

www.sciencedirect.com

Update in fruit dehiscence Ballester and Ferra´ndiz 69

Figure 1

(a)

(b)

Arabidopsis thaliana

(c)

Nicotiana benthamiana Brassica napus

Current Opinion in Plant Biology

Morphology of the dehiscence zone of fruits from different species. Arabidopsis thaliana (a), Brassica napus (b) and Nicotiana benthamiana (c), both at the valve margin (also known as septicidal, top) and at the middle of the pod wall (a.k.a. loculicidal, bottom). Lignified cells are depicted in light blue, and cells at the separation layer in grey. In all cases, despite different morphologies, cells at the separation layer are small and define a fracture plane, and are adjacent to lignified tissue.

of the FUL/SHP/IND/ALC/RPL factors, but are not essential to DZ formation. Among them, factors like BREVIPEDICELLUS (BP), NO TRANSMITTING TRACT (NTT) or WUSCHEL-RELATED HOMEOBOX 13 (WOX13) [7–9], generally associated to meristem-related functions, act in the replum and control replum width. Conversely, genes related to lateral organ development, like FILAMENTOUS FLOWER, ASYMMETRIC LEAVES1 or ASYMMETRIC LEAVES2, act in the valves [7,10]. Interestingly, boundary genes, which in the shoot apex mark the division between meristematic and lateral domains, are also expressed in the valve margins and show similar regulatory interactions with the rest of the network [11], which might suggest their possible role in the determination of DZ position (Figure 2). That meristem-related factors are expressed in the replum and the factors associated to organ differentiation are expressed in the valves has suggested that this network was coopted to establish medial (replum) versus www.sciencedirect.com

lateral (valve) fate in the gynoecium [12–14]. It is likely that its major role would be the specification of the medial meristematic ridge that forms the placenta and ovules, as strongly supported by the characterization of the transcriptome of the medial domain in early stages of gynoecium development, which extensively overlaps with that of other meristems in the plant [15]. However, they might be also working upstream the FUL/SHP/IND/ ALC/RPL network to regulate their spatial distribution. Recently, is has been uncovered that APETALA2 (AP2), best known because it controls perianth organ identity, negatively regulates both the DZ factors, SHP and IND, and the replum factors, RPL and BP, to ensure proper expression levels of all these genes and to restrict DZ and replum expansion to the valves [16]. Furthermore, FUL, together with ARF6 and ARF8, repress AP2 activity in the valves through activation of miR172 in this domain, thus uncovering the participation of posttranscriptional regulation in Arabidopsis fruit development (Figure 2). When AP2 activity is allowed in the valves, cell growth is Current Opinion in Plant Biology 2017, 35:68–75

70 Growth and development

Figure 2

m iR 17 2

Lateral FIL/YAB3/AS1/AS2

ARF6/8

Medial BP/WOX13/NTT

Boundary CUC/KNAT2/6

AP2

FUL

SHP

RPL

IND GA PID WAG2

ALC

Auxin influx

CK

Cell division WAG2

NST1/3

ABA Changes in turgor

Lignification

lignified layer

valve

ADPG1/2

Auxin efflux

Cell separation

separation layer

dehiscence zone

replum Current Opinion in Plant Biology

Updated model of dehiscence zone development in Arabidopsis. The transcription factors and enzymatic functions with major roles in DZ formation are included, as well as their regulatory interactions. The role of hormones at different levels and how hormone signaling is regulated by the transcriptional network is described. Discontinuous grey arrows note hypothetical relationships not well supported by experimental data. Negative regulation represented by brownish lines indicate a modulating effect on expression levels, but not complete repression. FRUITFULL (FUL), APETALA2 (AP2), SHATTERPROOF (SHP), INDEHISCENT (IND), REPLUMLESS (RPL), ALCATRAZ (ALC), NAC SECONDARY WALL THICKENING PROMOTING FACTOR1/3 (NST1/3), ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1), PINOID (PID), FILAMENTOUS FLOWER (FIL), YABBY3 (YAB3), ASYMMETRIC LEAVES1/2 (AS1/2), CUP-SHAPED COTYLEDON (CUC), BREVIPEDICELLUS (BP), WUSCHEL-RELATED HOMEOBOX 13 (WOX13), NO TRANSMITTING TRACT (NTT), AUXIN RESPONSE FACTOR6/8 (ARF6/8).

restricted, resembling the phenotypes of ful mutants [17]. Intriguingly, this role of AP2 in restricting cell size is somehow opposed, or at least act independently, of its function as negative regulator of SHP or IND, factors that also promote small cell size at the DZ, since in the ful ap2 double mutants, the small cell size and the ectopic lignification of the valves caused by ful mutations are further enhanced and not suppressed, as could be expected if AP2 ectopic activity caused the restricted cell growth in ful valves. The downstream effectors: hormones at work

In spite of its major role in directing DZ formation, it is somehow surprising that only a small number of IND direct targets have been described so far, which mainly include genes involved in hormone signaling pathways [18–20]. This has led to active research to elucidate the role of hormones in both DZ differentiation and/or as triggers of pod shattering. Current Opinion in Plant Biology 2017, 35:68–75

Auxin signaling has been included in all models of gynoecium morphogenesis for almost two decades, including DZ differentiation [21,22,23–25]. IND directly targets several genes related to auxin transport at the valve margin, including two ACG kinases, PINOID (PID) and WAG2, which control the subcellular distribution of the PIN-FORMED3 (PIN3) auxin efflux carrier [19,20]. As a consequence of this IND-mediated regulation of PID, WAG2 and PIN3, a dynamic pattern of auxin accumulation is established at the valve margin, which appears to be crucial for dehiscence. At early stages of fruit development, DZ differentiation would depend on auxin influx into the valve margin for which PID and WAG2 genes need to be repressed at this domain [26]. These results are somehow contradictory to earlier reports of an auxin minimum in the DZ of mature fruits, just before pod shattering, which together with the indehiscent phenotype caused by driving auxin synthesis to the valve margin with an IND::IND:IaaM transgene, led to propose www.sciencedirect.com

Update in fruit dehiscence Ballester and Ferra´ndiz 71

the requirement of auxin depletion at the valve margin to allow DZ formation and pod shattering [20]. This study also reports that IND downregulates PID, but upregulates WAG2, and this likely induces PIN3 relocalization and auxin efflux out the DZ [20]. Although apparently opposite, it is possible to reconcile both models if a clear temporal distinction of auxin requirement is made at the valve margin: auxin accumulation in early stages would be required for DZ differentiation, while auxin depletion at later stages could be important for triggering cell separation (Figure 2). On the other hand, the proposed morphogenetic roles of either auxin influx or efflux to the valve margin are based on correlative evidence of the effect of inducing auxin synthesis or depletion in the IND expression domain (at levels maybe far from physiological) with the pattern of auxin distribution deduced from reporters, and does not take into account possible temporal changes of IND activity on its targets, like for example the possible switch from repressor to activator of WAG2, or the participation of other IND partners, targets, or independent factors yet to be identified. High cytokinin (CK) responses are detected at the valve margins when using a TCS::GFP reporter, but absent in shp1 shp2 or ind mutants. Moreover, when CK are applied on shp or ind mutant fruits, pod shattering is restored. It is not clear how CK can induce DZ formation downstream of IND [27,28]. A possible scenario is that, as described for other cellular contexts, it has an impact on PID or PIN expression and thus affects auxin dynamics at the valve margin [29,30]. It would be interesting to test if CK treatment of ind mutants restores also the auxin accumulation pattern at the valve margins and vice versa, to place them upstream or downstream auxin signaling. Finally, another direct target of IND is the gibberellin (GA) biosynthetic enzyme GA3ox1. IND-mediated GA accumulation in the DZ directs de-repression of ALC by DELLA degradation to direct separation layer development and to feedback negatively on IND expression levels [18,31] (Figure 2). It is possible that this interplay of IND, DELLAs and ALC delimits the differentiation of separation and lignified layers in the DZ, another big question that is not fully solved yet. The downstream effectors: enzymatic functions in cell wall metabolism

At the other end of the differentiation process, several genes involved in lignification and remodeling of the cell wall have been shown to function in silique dehiscence. NAC SECONDARY WALL THICKENING PROMOTING FACTOR 1 (NST1) is expressed in the lignified layer of the DZ of Arabidopsis fruits and, together with the related NST3 factor, in the endocarpb and the lignified cells of the replum [32]. NST1/NST3 regulate a suite of genes involved in lignin and cellulose synthesis, and nst1 mutants, which are moderately indehiscent, fail to form www.sciencedirect.com

the lignified layer in the DZ, while the separation layer is not affected [32]. Likewise, ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1) and ADPG2 are specifically expressed in the separation layer, where they are required to trigger cell separation at maturity [33]. Pectin methylesterases are also associated to the DZ and likely contribute to the degradation of the middle lamella at valve separation, although their precise requirement in the dehiscence process has not been characterized yet [34] (Figure 2). NST1 and ADPG1/2 expression in the DZ is absent in ind mutants, suggesting that they could be direct or indirect targets of IND regulation [32,33]. Again, the question of how the upregulation of the lignification pathway and of the cell wall remodeling pathway could be spatially separated into adjacent domains if both were regulated by IND remains to be explained, but it is possible that it is mediated by other unknown factors and/or modulated by hormonal cues.

Variations in a theme: adaptation of the network and new insights from studies in other species The conservation of the FUL/SHP/IND/ALC/RPL network has been explored mainly in other species of the Brassicaceae family, to which Arabidopsis also belongs. Several studies address the conservation of expression patterns [34,35,36], and some of them also describe how the FUL/SHP/IND/ALC/RPL network carries out similar functions in other Brassicaceae dehiscent fruits, even when fruit morphologies are diverse, confirming a high degree of conservation in the observed regulatory interactions, and that modifications of the network can account for morphological diversity and for transitions between different seed dispersal strategies [31,36–40]. In other groups of angiosperms with dry dehiscent fruits there is much less information available, although some evo-devo and crop domestication studies also address these questions. In addition to explore at a broader scope the conservation of the network, they provide new information on additional players in the dehiscent game that were not uncovered by the Arabidopsis studies and can help us to revisit some of the prevailing ideas. In Nicotiana benthamiana fruits, dehiscence takes place at four DZs formed in the apical part of the capsule, two running at the valve margins, and two forming at the center of the valves (Figure 1). Both NbFUL overexpression and NbSHP inactivation cause indehiscence, strongly supporting the functional conservation of FUL and SHP roles in DZ formation across eudicots [41,42]. Interestingly, the FUL-SHP module appears to act both at valve margins and at the center of the valves, arguing against the importance of the boundary between meristematic/medial and differentiated/lateral domains in the gynoecium at Current Opinion in Plant Biology 2017, 35:68–75

72 Growth and development

defining the position of the DZs. It also raises the question of how is the expression of DZ genes directed to this nonmarginal positions. Still, these ideas need to be further explored, since there are no reports on how these medial/ lateral genes are expressed in fruits with differently positioned DZs. In legumes, pod shattering reduction has been a major target for domestication. Dozens of QTLs related to dehiscence have been identified in different legume species [43], but the identification of the underlying genetic functions lags far behind. Only two genes in soybean have been identified related to domestication events to reduce pod dehiscence. SHAT1-5, which encodes a homolog of NST1, impacts on DZ lignification extent and thus shattering resistance, providing again proof of the conservation of the DZ network [44]. Interestingly, the second gene, PDH1, encodes a Dirigent-like protein, which is not homolog to any described gene related to dehiscence in Arabidopsis, and appears to control the composition of the lignified cell walls. Several shattering resistant soybean cultivars carry a null allele of PDH1 and show different mechanical properties of the lignified tissues in the pods [45]. Also in legumes, but unrelated to domestication, increased lignin deposition at the valve margin has been correlated to transition from straight dehiscent pods to coiled indehiscent pods of some species of the genus Medicago [46]. This correlation is further extended to a change in the protein sequence of SHP orthologs that may affect the regulation of downstream effectors in the lignification pathway [46]. An intriguing aspect remains on IND functional conservation. Phylogenetic studies have proven that IND orthologs are confined to the Brassicaceae, where its essential role in DZ formation appears to be conserved [31,38,47,48]. Outside Brassicaceae, attempts to assign dehiscence-related QTLs with IND closest sequence homologs (HECATE (HEC)-like genes, often mistaken in the literature by IND orthologs) have been unsuccessful [44,49]. This supports the idea of the neofunctionalization of IND to direct DZ formation in Brassicaceae, and it might suggest that in other species this function could depend on different genes, other than HEC-like factors. Interestingly, in Arabidopsis, HEC genes have been related to the regulation of lignin deposition and separation processes in seed abscission or in anther dehiscence, being involved in genetic routes with similar components to the DZ network [47,50]. Moreover, Arabidopsis HEC factors also have been shown to regulate PID, WAG2, PIN3 and other genes involved in CK signaling in different developmental contexts [29,30]. Altogether, this evidence suggests that HEC and IND ancestors could share similar biological roles and that HEC-like genes are still good candidates to direct DZ differentiation in non-brassicaceae dehiscent fruits. Current Opinion in Plant Biology 2017, 35:68–75

Springs and triggers: the role of physical forces as adaptive mechanisms Pod dehiscence takes place when the adhesion of cells at the separation layer is weaker than the forces provided from pod walls to break them. In Arabidopsis, at fruit maturity pod walls senesce and dehydrate, probably creating tension between the rigid lignified layer and the shrinking mesocarp that helps to break the middle lamella at the separation layer [51]. In Brassica species, different susceptibility to shattering in B. juncea and B. rapa has been correlated with different patterns of pod dehydration at maturity [52] and, interestingly, with differential ABA responses in pods of both species [34] (Figure 2). That enhanced ABA responses and thus desiccation tolerance are greater in less dehiscent fruits may reflect a better adaptation to dry environments and a mechanism to ensure proper timing of pod shattering. Likewise, as seen in soybean landraces carrying the active PDH1 allele, lignin composition may have a high impact of mechanical properties of the pod wall and thus render pods with stronger forces to break up the DZ [45]. It would be interesting to explore whether lignin composition varies in related species with different shattering behavior and thus it could also be target of natural selection. In addition to the senescence-related dehiscence processes described so far, explosive fruits in several species have evolved to literally develop shooting mechanisms that are able to disperse the seeds at great distances. A recent work addresses the study of the molecular and cellular mechanisms underlying the explosive behavior of Cardamine hirsuta fruits with a comprehensive multifactorial approach that describes both the mechanical properties of the pod wall and the morphological innovations that allowed Cardamine species to evolve a novel highly efficient long-range dispersal strategy [53]. In Cardamine, the endocarp has a modified pattern of lignification, which deposits asymmetrically in these cells creating an elastic instead of a rigid layer at the inner pod. These hinges are able to accumulate tension and to respond to differential contraction of pod wall tissues mediated by local changes in cell wall properties and increased turgor. The release of this accumulated force breaks open the silique and produces extreme rapid coiling of the valves [53]. Moreover, this asymmetric deposition of lignin in the endocarp is found in all Cardamine species with explosive dehiscence and not in other non-explosive relatives, suggesting that this evolutionary innovation was mediated by a change in the lignification pathway, that again appears as a key element driving diversification [53]. Although tensions provided by tissues with different mechanical properties are the most frequent driving force triggering dehiscence, other agents could also play a role. Hydrostatic pressure appears to be the cause for explosive www.sciencedirect.com

Update in fruit dehiscence Ballester and Ferra´ndiz 73

discharge of the seed in dwarf mistletoes [54]. This study shows that there is an increase of around 2 8C in ripe fruits that precedes and probably triggers dehiscence, thus identifying thermogenesis as a novel shattering-promoting factor [54].

6.

Roeder AH, Ferrandiz C, Yanofsky MF: The role of the REPLUMLESS homeodomain protein in patterning the Arabidopsis fruit. Curr Biol 2003, 13:1630-1635.

7.

Alonso-Cantabrana H, Ripoll JJ, Ochando I, Vera A, Ferra´ndiz C, Martı´nez-Laborda A: Common regulatory networks in leaf and fruit patterning revealed by mutations in the Arabidopsis ASYMMETRIC LEAVES1 gene. Development 2007, 134:2663-2671.

8.

Marsch-Martinez N, Zuniga-Mayo VM, Herrera-Ubaldo H, Ouwerkerk PB, Pablo-Villa J, Lozano-Sotomayor P, Greco R, Ballester P, Balanza V, Kuijt SJ et al.: The NTT transcription factor promotes replum development in Arabidopsis fruits. Plant J 2014, 80:69-81.

9.

Romera-Branchat M, Ripoll JJ, Yanofsky MF, Pelaz S: The WOX13 homeobox gene promotes replum formation in the Arabidopsis thaliana fruit. Plant J 2013, 73:37-49.

Concluding remarks Although the major components of the genetic network that drives DZ formation in Arabidopsis have been known for several years, recent work has made important additions to the model. The role of post-translational and posttraductional regulation, of hormone signaling and of physical forces as triggers of shattering have been highlighted. Also, the conservation of the elements of the network across dicots and the evolutionary importance of variation to drive morphological and functional innovation is starting to be addressed and used for crop improvement. Still, important questions remain that should be the focus of future research. For example, it is unclear how SHP and IND, which are essential for DZ formation, direct the differentiation of lignified and separation layers in adjacent domains, as well as the identity of the factors carrying the role of IND in non-brassicaceae species. We need to better understand the exact interplay of hormones, whether they feedback or not in the initial upstream components, or if they also have a role in the cell wall modification events that mediate the later steps in DZ formation. Finally, the study of the physical forces that influence dehiscence can also help to understand the process and to identify targets for better adapted crops.

Acknowledgements We apologize to those whose publications could not be cited because of space limitations. The work in Cristina Ferrandiz lab is supported by Spanish MINECO grant BIO2015-64531-R.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Lorts C, Briggeman T, Sang T: Evolution of fruit types and seed dispersal: a phylogenetic and ecological snapshot. J System Evol 2008, 46:396-404.

2.

Liljegren SJ, Ditta GS, Eshed Y, Savidge B, Bowman JL, Yanofsky MF: SHATTERPROOF MADS-box genes control seed dispersal in Arabidopsis. Nature 2000, 404:766-770.

10. Dinneny JR, Weigel D, Yanofsky MF: A genetic framework for fruit patterning in Arabidopsis thaliana. Development 2005, 132:4687-4696. 11. Hepworth SR, Pautot VA: Beyond the divide: boundaries for patterning and stem cell regulation in plants. Front Plant Sci 2015, 6:1052. 12. Balanza´ V, Navarrete M, Trigueros M, Ferra´ndiz C: Patterning the female side of Arabidopsis: the importance of hormones. J Exp Bot 2006, 57:3457-3469. 13. Girin T, Sorefan K, Ostergaard L: Meristematic sculpting in fruit development. J Exp Bot 2009, 60:1493-1502. 14. Sundberg E, Ferra´ndiz C: Gynoecium patterning in Arabidopsis: a basic plan behind a complex structure. Annual Plant Reviews Volume 38: Fruit Development and Seed Dispersal. WileyBlackwell; 2009: 35-69. 15. Villarino GH, Hu Q, Manrique S, Flores-Vergara M, Sehra B, Robles L, Brumos J, Stepanova AN, Colombo L, Sundberg E et al.:  Transcriptomic signature of the SHATTERPROOF2 expression domain reveals the meristematic nature of Arabidopsis gynoecial medial domain. Plant Physiol 2016, 171:42-61. In this work, the authors use a novel approach combining FACS-sorting and synchronized gynoecia at early stages of development to analyze the transcriptome of the medial domain with great precision. The results uncover new possible targets of SHP and confirm the meristematic nature of this domain. 16. Ripoll JJ, Roeder AH, Ditta GS, Yanofsky MF: A novel role for the floral homeotic gene APETALA2 during Arabidopsis fruit development. Development 2011, 138:5167-5176. 17. Ripoll JJ, Bailey LJ, Mai Q-A, Wu SL, Hon CT, Chapman EJ,  Ditta GS, Estelle M, Yanofsky MF: microRNA regulation of fruit growth. Nat Plants 2015, 1:15036. The authors uncover the role of miR172 in fruit valve growth. Regulatory interactions of FUL, ARF6, ARF8 and AP2 are also revealed, building up in a previous work that identified AP2 as a component of the DZ specification network. 18. Arnaud N, Girin T, Sorefan K, Fuentes S, Wood TA, Lawrenson T, Sablowski R, Østergaard L: Gibberellins control fruit patterning in Arabidopsis thaliana. Genes Dev 2010, 24:2127-2132. 19. Girin T, Paicu T, Stephenson P, Fuentes S, Ko¨rner E, O’Brien M, Sorefan K, Wood TA, Balanza´ V, Ferra´ndiz C et al.: INDEHISCENT and SPATULA interact to specify carpel and valve margin tissue and thus promote seed dispersal in Arabidopsis. Plant Cell 2011, 23:3641-3653. 20. Sorefan K, Girin T, Liljegren SJ, Ljung K, Robles P, Galva´nAmpudia CS, Offringa R, Friml J, Yanofsky MF, Ostergaard L: A regulated auxin minimum is required for seed dispersal in Arabidopsis. Nature 2009, 459:583-586.

3.

Liljegren SJ, Roeder AHK, Kempin SA, Gremski K, Østergaard L, Guimil S, Reyes DK, Yanofsky MF: Control of fruit patterning in Arabidopsis by INDEHISCENT. Cell 2004, 116:843-853.

4.

Rajani S, Sundaresan V: The Arabidopsis myc/bHLH gene ALCATRAZ enables cell separation in fruit dehiscence. Curr Biol 2001, 11:1914-1922.

21. Larsson E, Roberts CJ, Claes AR, Franks RG, Sundberg E: Polar auxin transport is essential for medial versus lateral tissue specification and vascular-mediated valve outgrowth in Arabidopsis gynoecia. Plant Physiol 2014, 166:1998-2012.

5.

Ferra´ndiz C, Liljegren SJ, Yanofsky MF: Negative regulation of the SHATTERPROOF genes by FRUITFULL during Arabidopsis fruit development. Science 2000, 289:436-438.

22. Marsch-Martinez N, de Folter S: Hormonal control of the development of the gynoecium. Curr Opin Plant Biol 2016,  29:104-114.

www.sciencedirect.com

Current Opinion in Plant Biology 2017, 35:68–75

74 Growth and development

Recent update on the latests advances regarding the role of auxin and cytokinin during gynoecium morphogenesis, and the interplay between these hormones. 23. Sehra B, Franks RG: Auxin and cytokinin act during gynoecial patterning and the development of ovules from the meristematic medial domain. Wiley Interdiscip Rev Dev Biol 2015, 4:555-571. 24. Moubayidin L, Ostergaard L: Dynamic control of auxin distribution imposes a bilateral-to-radial symmetry switch during gynoecium development. Curr Biol 2014, 24:2743-2748. 25. Nemhauser J, Feldman L, Zambryski P: Auxin and ETTIN in Arabidopsis gynoecium morphogenesis. Development 2000, 127:3877-3888. 26. van Gelderen K, van Rongen M, Liu A, Otten A, Offringa R: An  INDEHISCENT-controlled auxin response specifies the separation layer in early Arabidopsis fruit. Mol Plant 2016, 9:857-869. Interesting and very detailed study on the differentiation of the DZ and the correlation with auxin dynamics at the valve margin throughout fruit development. The role of IND in the regulation of auxin distribution is addressed and a model is proposed that challenges and complements the prevailing hypothesis on this matter. 27. Marsch-Martinez N, Ramos-Cruz D, Irepan Reyes-Olalde J, Lozano-Sotomayor P, Zuniga-Mayo VM, de Folter S: The role of cytokinin during Arabidopsis gynoecia and fruit morphogenesis and patterning. Plant J 2012, 72:222-234. 28. Zuniga-Mayo VM, Reyes-Olalde JI, Marsch-Martinez N, de Folter S: Cytokinin treatments affect the apical-basal patterning of the Arabidopsis gynoecium and resemble the effects of polar auxin transport inhibition. Front Plant Sci 2014, 5:191. 29. Schuster C, Gaillochet C, Lohmann JU: Arabidopsis HECATE genes function in phytohormone control during gynoecium  development. Development 2015, 142:3343-3350. The role of HEC and SPT in apical gynoecium development is explained by uncovering the interaction of a HEC–SPT complex with auxin synthesis and transport and with cytokinin signaling. 30. Schuster C, Gaillochet C, Medzihradszky A, Busch W, Daum G, Krebs M, Kehle A, Lohmann JU: A regulatory framework for shoot stem cell control integrating metabolic, transcriptional, and phytohormone signals. Dev Cell 2014, 28:438-449. 31. Lenser T, Theissen G: Conservation of fruit dehiscence pathways between Lepidium campestre and Arabidopsis thaliana sheds light on the regulation of INDEHISCENT. Plant J 2013, 76(4):545-556. 32. Mitsuda N, Ohme-Takagi M: NAC transcription factors NST1 and NST3 regulate pod shattering in a partially redundant manner by promoting secondary wall formation after the establishment of tissue identity. Plant J 2008, 6(5):768-778. 33. Ogawa M, Kay P, Wilson S, Swain SM: ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1), ADPG2, and QUARTET2 are polygalacturonases required for cell separation during reproductive development in Arabidopsis. Plant Cell 2009:18. 34. Jaradat MR, Ruegger M, Bowling A, Butler H, Cutler AJ: A  comprehensive transcriptome analysis of silique development and dehiscence in Arabidopsis and Brassica integrating genotypic, interspecies and developmental comparisons. GM Crops Food 2014, 5:302-320. This study compares at different stages of development the transcriptomes of Arabidopsis fruits from wt, shp, ful and alc backgrounds and of two species of Brassica with different shattering susceptibility. The study identifies correlations between different shattering behaviours and many developmental, hormonal and metabolic pathways that can help understand better the process and uncover new players in the process. 35. Avino M, Kramer EM, Donohue K, Hammel AJ, Hall JC: Understanding the basis of a novel fruit type in Brassicaceae: conservation and deviation in expression patterns of six genes. Evodevo 2012, 3:20. 36. Muhlhausen A, Lenser T, Mummenhoff K, Theissen G: Evidence that an evolutionary transition from dehiscent to indehiscent fruits in Lepidium (Brassicaceae) was caused by a change in Current Opinion in Plant Biology 2017, 35:68–75

the control of valve margin identity genes. Plant J 2013, 73:824-835. 37. Arnaud N, Lawrenson T, Ostergaard L, Sablowski R: The same regulatory point mutation changed seed-dispersal structures in evolution and domestication. Curr Biol 2011, 21(14):1215-1219. 38. Girin T, Stephenson P, Goldsack CMP, Kempin SA, Perez A, Pires N, Sparrow PA, Wood TA, Yanofsky MF, Østergaard L: Brassicaceae INDEHISCENT genes specify valve margin cell fate and repress replum formation. Plant J 2010, 63:329-338. 39. Langowski L, Stacey N, Ostergaard L: Diversification of fruit shape in the Brassicaceae family. Plant Reprod 2016, 29:149-163. 40. Ostergaard L, Kempin SA, Bies D, Klee HJ, Yanofsky MF: Pod shatter-resistant Brassica fruit produced by ectopic expression of the FRUITFULL gene. Plant Biotechnol J 2006, 4:45-51. 41. Ferrandiz C, Fourquin C: Role of the FUL-SHP network in the evolution of fruit morphology and function. J Exp Bot 2014. 42. Fourquin C, Ferrandiz C: Functional analyses of AGAMOUS family members in Nicotiana benthamiana clarify the evolution of early and late roles of C-function genes in eudicots. Plant J 2012:990-1001. 43. Li LF, Olsen KM: To have and to hold: selection for seed and  fruit retention during crop domestication. Curr Top Dev Biol 2016, 119:63-109. Comprehensive review of our knowledge on crop domestication events related to dehiscence, fruit abscission and seed shattering in cereals. The paralelism between these three processes are highlighted. 44. Dong Y, Yang X, Liu J, Wang BH, Liu BL, Wang YZ: Pod shattering  resistance associated with domestication is mediated by a NAC gene in soybean. Nat Commun 2014, 5:3352. Combining a candidate gene approach based on the previous knowledge generated in Arabidopsis and QTL mapping, the authors identified SHAT1-5, a major QTL for pod dehiscence in soybean, as the ortholog of NST1. Higher levels of SHAT1-5 expression lead to increased lignification of the valve margin and shattering resistance. 45. Funatsuki H, Suzuki M, Hirose A, Inaba H, Yamada T, Hajika M,  Komatsu K, Katayama T, Sayama T, Ishimoto M et al.: Molecular basis of a shattering resistance boosting global dissemination of soybean. Proc Natl Acad Sci U S A 2014, 111:17797-17802. Through map-based cloning, the authors identify PDH1, which encodes a dirigent-like protein, as a determinant of pod shattering resistance in soybean, a novel function not previously related to dehiscence. PDH1 affects lignin composition and the pod wall mechanical properties, which apply higher force to break open the pod at maturity. 46. Fourquin C, del Cerro C, Victoria FC, Vialette-Guiraud A, de  Oliveira AC, Ferrandiz C: A change in SHATTERPROOF protein lies at the origin of a fruit morphological novelty and a new strategy for seed dispersal in medicago genus. Plant Physiol 2013, 162:907-917. In this work, changes in the pattern of lignin deposition are correlated to a novel fruit morphology and dispersal strategy found in the genus Medicago. This differential lignification of the pod is also correlated with a change in SHP coding sequence that alters protein function. The results supports the conserved role of SHP in valve margin differentiation in legumes. 47. Kay P, Groszmann M, Ross JJ, Parish RW, Swain SM: Modifications of a conserved regulatory network involving INDEHISCENT controls multiple aspects of reproductive tissue development in Arabidopsis. New Phytol 2012, 197:73-87. 48. Pabon-Mora N, Wong GK, Ambrose BA: Evolution of fruit  development genes in flowering plants. Front Plant Sci 2014, 5:300. Comprehensive phylogenetic study of SHP, IND, ALC, SPT and RPL homologs across angiosperms. Useful framework to propose hypothesis on the evolution of the network. 49. Gioia T, Logozzo G, Kami J, Spagnoletti Zeuli P, Gepts P: Identification and characterization of a homologue to the Arabidopsis INDEHISCENT gene in common Bean. J Hered 2013, 104:273-286. 50. Balanza V, Roig-Villanova I, Di Marzo M, Masiero S, Colombo L:  Seed abscission and fruit dehiscence required for seed www.sciencedirect.com

Update in fruit dehiscence Ballester and Ferra´ndiz 75

dispersal rely on similar genetic networks. Development 2016, 143:3372-3381. The role oh HEC3 in seed absciccion in Arabidopsis is characterized. The parallelism between SHP/IND/lignification in the valve margin and STK/ HEC/lignification in the seed abscission zone is uncovered. Since SHP/ STK and IND/HEC are homolog proteins, this work suggests that developmental pathways controlling different processes of separation could have a common origin. 51. Spence J, Vercher Y, Gates P, Harris N: ‘Pod shatter’ in Arabidopsis thaliana, Brassica napus and B. juncea. J Microsc 1996, 181:195-203. 52. Squires TM, Gruwel MLH, Zhou R, Sokhansanj S, Abrams SR, Cutler AJ: Dehydration and dehiscence in siliques of Brassica napus and Brassica rapa. Can J Bot 2003, 81:248-254.

www.sciencedirect.com

53. Hofhuis H, Moulton D, Lessinnes T, Routier-Kierzkowska AL,  Bomphrey RJ, Mosca G, Reinhardt H, Sarchet P, Gan X, Tsiantis M et al.: Morphomechanical innovation drives explosive seed dispersal. Cell 2016, 166:222-233. In this work, the authors study the basis of explosive dehiscence of Cardamine fruits. Combining a multifaceted approach (genetics, modelling, determination of physical properties, so on) the authors show how morphological innovations linked to different patterns of lignin deposition and response to turgor in Cardamine pod wall allow to accumulate enough tension to produce explosive shattering. 54. deBruyn RA, Paetkau M, Ross KA, Godfrey DV, Friedman CR: Thermogenesis-triggered seed dispersal in dwarf mistletoe. Nat Commun 2015, 6:6262.

Current Opinion in Plant Biology 2017, 35:68–75