Peptide signalling during angiosperm seed development

REVIEW PAPER

Peptide signalling during angiosperm seed development

Gwyneth Ingram1,* and Jose Gutierrez-Marcos2,*1 Laboratoire Reproduction et Développement des Plantes, UMR 5667 CNRS/UMR 0879 INRA, ENS de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France2 School of Life Sciences, University of Warwick, Coventry CV4 7AL, UK

* To whom correspondence should be addressed. E-mail: [email protected] or [email protected]

Received 28 April 2015; Revised 9 June 2015; Accepted 11 June 2015

Editor: Thomas Dresselhaus

Abstract

Cell–cell communication is pivotal for the coordination of various features of plant development. Recent studies in plants have revealed that, as in animals, secreted signal peptides play critical roles during reproduction. However, the precise signalling mechanisms in plants are not well understood. In this review, we discuss the known and putative roles of secreted peptides present in the seeds of angiosperms as key signalling factors involved in coordinating dif-ferent aspects of seed development.

Key words: Cysteine-rich protein, fertilization, receptor-like kinase, seed development.

Introduction

The angiosperm seed is a complex structure composed of three genetically distinct components: the maternally derived testa, and the zygotic endosperm and embryo. These three tis-sues are arranged like Russian dolls, one inside the other, and develop synchronously post-fertilization to allow the forma-tion of a viable seed, which, through a wide variety of strate-gies, including environmentally controlled dormancy and the presence of specific morphological/physiological features, ensures the dissemination and survival of the next genera-tion. How the developmental coordination of seed develop-ment is achieved, however, remains poorly understood.

Plant cells communicate chemically with their neighbours either symplastically (via plasmodesmata) or apoplastically, by means of the transfer of information through the cell wall and across membranes. In order to consider inter-compart-mental communication in the seed, it is therefore necessary first to consider the origins and structure of the boundaries between each of the relevant compartments. This question has recently been reviewed in some detail (Bencivenga et al., 2011; Colombo et al., 2008; Ingram, 2010; Kelley and Gasser,

2009, amongst many), but is worth summarizing. The unfer-tilized ovule is a complete structure (Kelley and Gasser, 2009) composed of organs and tissues of sporophytic origin (the nucellus and the enveloping integuments), which enclose the female gametophyte. In angiosperms, the female gametophyte arises from the megaspore, one of four meiotic descendants of the megasporocyte, which differentiates from an archespo-rial cell within the nucellus. In Arabidopsis and many other angiosperms, after the degeneration of its three siblings, the megaspore undergoes three rounds of mitosis to give the embryo sac, containing eight nuclei repartitioned into seven cells. Two of these cells, the haploid egg cell and the double haploid central cell, are fertilization competent, and will give rise to the embryo and the endosperm, respectively, upon fusion with the two sperm cells delivered by the pollen tube.

During female gametophyte development, the initiation and outgrowth of the integuments from the base of the nucel-lus leads to a partial or complete sheathing of the nucellus in further layers of maternally derived tissue. In Arabidopsis, this process is succeeded by the progressive degeneration of

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Journal of Experimental Botany, Vol. 66, No. 17 pp. 5151–5159, 2015doi:10.1093/jxb/erv336 Advance Access publication 20 July 2015

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the nucellus cells surrounding the distal regions of the female gametophyte, so that by the time the ovule reaches maturity much of the female gametophyte is surrounded by a com-posite wall deposited by both the nucellus and the inner-most cell layer of the inner integument (the endothelium). Unsurprisingly, there is no evidence for the presence of sym-plastic connections, in the form of plasmodesmata, cross-ing this complex wall. Furthermore, even in more proximal zones where the antipodal cells of the female gametophyte meet the surviving nucellar tissue, symplastic connections that may have persisted during much of ovule development are progressively lost. Although evidence is fragmentary, it seems likely that even in this distal zone symplastic isolation occurs at or soon after ovule maturity, and is subsequently assured by the death of the antipodal cells (Song et al., 2014). The situation in maize is more complex, since the nucellus is highly proliferated and persists during early post-fertilization development, and the antipodal cells can also proliferate and persist. Nonetheless, there is little or any evidence for sym-plastic communication either between the nucellus and the endosperm, or between endosperm cells and residual antipo-dal cells post-fertilization in maize (Diboll and Larson, 1966).

Interestingly, although symplastic connections between the gametophyte and surrounding maternal tissues are progres-sively broken during ovule development, they appear to be maintained between the egg cell and surrounding cells of the female gametophyte until fertilization. Thereafter, a rapid loss of symplastic connections occurs between the zygote and sur-rounding cells (Han et al., 2000; Stadler et al., 2005), although residual plasmodesmata-like structures of unknown function-ality have been observed between the suspensor and surround-ing endosperm in a few species (Kozieradzka-Kiszkurno and Bohdanowicz, 2010). Thus, from an early point in post-fertiliza-tion development, the three genetically distinct components of the seed also represent three distinct symplastic fields, meaning that all movement of water, solutes, and signalling molecules between compartments must involve crossing the apoplast.

Given the symplastic isolation of the seed compartments, it would be logical to assume that peptide-mediated (apoplas-tic) signalling might play a key role in inter-compartmental communication during seed development. Any such com-munication is likely to involve the endosperm because of its central position. Before developing this idea further, it may therefore also be useful to consider the likely evolutionary origin of the process of seed development, with a particu-lar emphasis on the endosperm. Again, this subject has been widely reviewed (Baroux et al., 2002; Becraft and Gutierrez-Marcos, 2012; Friedman, 2001). The endosperm plays a key role as a conduit, transferring nutrients from maternal tissues to the developing embryo (Berger et al., 2006). This function is thought to be analogous to that of nutritive tissues formed in gymnosperm seeds through the proliferation of the unfer-tilized female gametophyte, and into which gymnosperm the embryo grows invasively, absorbing nutrients. However, whilst in gymnosperms the transformation of the gametophyte into a nutrient sink occurs independently of fertilization (and thus independently of the presence of an embryo), in angiosperms this transformation has become linked to fertilization by the

sexualization of the central cell. This innovation, and associ-ated reductions in energy wastage, may have provided a key evolutionary advantage to the angiosperms. Furthermore, by rendering both a maternal and a paternal genome necessary for endosperm development (and thus nutrient allocation), it has transformed the endosperm into a zone of intense paren-tal conflict (Baroux et al., 2002).

It is in this complex morphological, genetic, and evo-lutionary context that we wish to consider the role of pep-tide-mediated signalling in regulating post-fertilization seed development. In this review we have specifically chosen to place a strong emphasis on peptide-mediated signalling, which we consider to be involved in controlling seed-specific, inter-compartmental communication and, importantly—to avoid in-depth discussion of signalling pathways whose roles are maintained (and thus studied)—during post-embryonic plant development. We would particularly like to investigate two predictions, which we feel could emerge from considera-tion of the situation outlined above. First, we propose that the fact that the concomitant development of the embryo and endosperm evolved from a situation in which their develop-ment was effectively independent (as in gymnosperms) may mean that any apoplastic signalling pathways coordinating the development of these compartments in angiosperms may either have evolved de novo and be angiosperm seed specific, or have, at the very least, involved the requisitioning/adapta-tion of existing signalling pathways with the potential incor-poration of seed-specific components. Secondly, we propose that because communication between all three compart-ments of the seed should involve components expressed in the endosperm, and could thus fundamentally affect nutri-ent allocation, these pathways may be specifically targeted for parental control during seed development.

It is becoming increasingly evident that the coordination of plant development is largely mediated by peptide signal-ling. Consistent with this, the genome of Arabidopsis contains genes coding for over 600 receptor-like proteins, more than 400 peptides, and a plethora of secreted proteases that may have the capacity to participate in the processing of bioactive peptides (De Smet et al., 2009; Huang et al., 2015). In addi-tion, plant genomes encode large numbers of genes encoding enzymes potentially involved in the post-translational modifi-cation of receptors and peptide precursors. However, despite the recent advances in genomic analysis, we still remain igno-rant of the detailed transcriptomic complexity of these genes during reproductive development, and only fragmentary functional data are available for a handful of signalling pep-tides involved in seed development. We review this evidence and discuss its implications in the context of the structural, functional, and evolutionary peculiarities of the developing angiosperm seed.

The role of signalling peptides in seed development

Secreted peptides are highly abundant during seed develop-ment. This highlights their potential role in novel or modified signalling pathways; however, we have only sparse information

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about their precise functions. Intriguingly, most of these pep-tides are exclusively expressed in specific gametophyte cells and/or seed compartments and potentially play a role in the coordination of seed development (Fig. 1A). The earliest-acting peptides in seed development belong to the cysteine-rich group and are expressed both in the gametes within the female gametophyte and in the early products of fertilization (Costa et al., 2014; Sprunck et al., 2012). For instance, in Arabidopsis, ESF1 peptides are expressed in the central cell and endosperm embryo-surrounding region (ESR) and act to positively regulate zygote elongation and the development of the suspensor (Costa et al., 2014). Intriguingly, these peptides are related to maize MATERNAL EXPRESSED GENE1 (MEG1) peptides, which are expressed from a small gene family that is exclusively expressed in basal endosperm trans-fer layer (BETL) cells (Gutierrez-Marcos et al., 2004). MEG1 peptides act as positive developmental regulators and, because they control transfer cell development, they play a major role in nutrient translocation in seeds (Costa et al., 2012). In maize, several secreted peptides are expressed in the BETL (Balandin et al., 2005; Magnard et al., 2000; Magnard et al., 2003; Serna et al., 2001). Although their functions remain unknown, they have been associated with pathogen defence and nutrient translocation. In addition, it is possible that they play regulatory roles to coordinate seed growth and develop-ment. Similarly, the ESR cells express cysteine-rich proteins (CRPs) related to those expressed in transfer cells. Although

the function of these peptides remains elusive, some of them are actively expressed during microspore embryogenesis (Balandin et al., 2005; Magnard et al., 2000). These findings support the view that ESR peptides form part of an as yet unknown signalling pathway (or pathways) that regulates ESR cell fate, differentiation, and/or communication between endosperm and embryo.

One distinct class of peptides found in maize ESR cells has been used to define the clavata/esr-like (CLE) class of pep-tides. These peptides are widely expressed in different organs (Jun et al., 2010) but some of them are restricted to repro-ductive tissues (Fiume et al., 2011). One of these peptides in Arabidopsis, termed CLE8, is confined to the early developing embryo and endosperm and plays a key role in the develop-mental regulation of both through the modulation of WOX8 expression (Fiume and Fletcher, 2012). The link between CLE8 peptides and the WOX8 homeobox gene is particularly relevant because it reveals that a peptide-transcriptional regu-latory module has been co-opted for the regulation not only of apical–basal embryo formation (Breuninger et al., 2008) but also for later stages of embryo development.

Another class of secreted peptides expressed in seeds is KISS OF DEATH (KOD) (Blanvillain et al., 2011). These peptides are expressed in embryos and are involved in the positive regulation of programmed cell death of suspensor cells. Although the precise function of KOD peptides is not fully known, it is likely that they form part of a signalling

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Fig. 1. Schematic representation showing the contribution of cysteine-rich proteins (CRPs) to early stages of seed development. (A) Peptides derived from different cell types are potentially implicated in the regulation of seed development. (B) Discrete expression of secreted peptides during three critical stages of sexual reproduction (pre-fertilization, fertilization, and post-fertilization) and evidence for the maternal cytoplasmic contribution of a small group of CRPs (data re-analysed from: Costa et al., 2014; Huang et al., 2015). The x axis indicates samples analysed; the y axis indicates expression frequency. n: Number of peptides identified for each expression group; I: immature ovules; M: mature ovules; 6: 6 hours after fertilization; 12: 12 hours after fertilization; 24: 24 hours after fertilization.

Peptide signalling in seeds | 5153


cascade that regulates the activation of cell death of the sus-pensor cell lineage during seed maturation (Zhao et al., 2013).

Not all peptides expressed in seeds are confined to embryo and/or endosperm tissues. For instance, JEKYLL peptides in barley are expressed primarily in the ovule and in seed integu-ments, and are involved in the regulation of nucellar develop-ment (Radchuk et al., 2006). JEKYLL is required not only for regulating the cell fate of the nucellar tissue but also for endosperm development, which highlights the importance of peptide signalling in maternal/filial interactions during seed development. The integuments and nucellus of potato ovules accumulate rapid alkalinization factor (RALF)-like peptides that regulate multiple aspects of ovule and seed development (Chevalier et al., 2013). Although the exact role of these pep-tides is currently unknown, their expression coincides with an increase of auxin in developing ovules, thus implying a cross-talk between phytohormones and signalling peptides in the regulation of the initial stages of seed development.

A major limitation in the analysis of seed-specific secreted peptides is the incomplete transcriptional information cur-rently available. Although recent work has revealed much of the transcriptome of different seed compartments in Arabidopsis by coupling laser-capture microdissection (LCM) and microarray analysis (Le et al., 2010), the expression of signalling peptide encoding genes is rather incomplete as they are not represented in the current expression platforms. This problem is compounded for seed-specific signalling peptide encoding genes, which are particularly poorly represented. To overcome this caveat, Tesfaye et al. (2013) generated an array including most known CRPs for Arabidopsis and Medicago; however, due to the large number of duplications in some CRP-encoding families, not all individual gene mem-bers were represented in this platform. Two recent studies in Arabidopsis have shown that coupling manual dissection and RNA sequencing is a suitable choice for the transcriptomic analysis of signalling peptides in reproductive tissues (Huang et al., 2015) (Fig. 1B). Perhaps the most suitable strategy is the coupling of LCM to next-generation transcriptome sequencing, as has been accomplished for maize seeds (Zhan et al., 2015). Although these technologies should aid deter-mination of the transcriptional profile of all signalling pep-tides in seeds, the computational prediction of these peptides in recently sequenced plant genomes continues to be limited (Zhou et al., 2013).

The role of receptors and potential downstream components in seed development

Because very few peptide/receptor pairs have been conclu-sively identified in plants, particularly in the context of inter-compartmental communication in seeds, almost as much can be learnt from considering developmentally important receptors as from considering the potential peptide signals themselves. Several receptors and/or receptor like molecules, as well as components potentially acting in downstream signalling cascades, have been shown to be involved in seed

development. Perhaps one of the earliest identified is the receptor kinase HAIKU2, mutants in which show a strongly reduced seed size (Garcia et al., 2003). HAIKU2 (IKU2) is a member of the LRR XI subfamily of Arabidopsis LRR receptor protein kinases (Luo et al., 2005), a subfamily con-taining several developmentally important proteins, includ-ing receptors involved in meristem maintenance (CLV1 and BAM1, 2 and 3) (Clark et al., 1997; DeYoung et al., 2006), vascular development (PXY/TDR) (Fisher and Turner, 2007), and organ abscission (HAESA and HSL1 and 2) (Jinn et al., 2000; Stenvik et al., 2008), as well as proteins involved in innate immunity (PEPR1 and 2) (Yamaguchi et al., 2010). The putative protein ligands of receptors from this family are diverse and fall into at least three different categories. IKU2 is expressed during early seed development specifically in the endosperm, and loss of iku2 function leads to the produc-tion of seeds of reduced size, due to a reduced growth of the early endosperm and reduced integument elongation (Garcia et al., 2003; Luo et al., 2005). IKU acts together with several other proteins, including the VQ-domain-containing IKU1 protein (Wang et al., 2010) and the WRKY transcription factor MINISEED3/WRKY10 (Luo et al., 2005), in a path-way that has recently been shown to regulate seed growth by affecting cytokinin homeostasis though the regulation of the CYTOKININ OXIDASE 2 (CKX2)-encoding gene (Li et al., 2013). Because IKU2 is expressed early in seed development, at a stage when the endosperm is composed of a single, large, coenocytic cell, it seems likely that if, as expected, IKU2 is inserted in the plasma membrane, it should be involved in the perception of molecules present either in the apoplast between the endosperm and the testa, or between the endosperm and the embryo. Although potential IKU ligands have not yet been identified, genetic studies suggest that the IKU pathway could be involved in cross-talk between the sporophytic integ-uments and the developing endosperm (Garcia et al., 2005), suggesting the possibility that maternal sporophytic tissues could be involved in the IKU signalling pathway. Studies of the subcellular localization of IKU2 protein might help to resolve this question, but are likely to be hampered by its very low levels of endogenous expression.

Interestingly, IKU2 is not the only LRR XI subfamily LRR receptor protein kinase to play a key, and most likely seed-specific, role in post-fertilization seed development. Two other members of the same family, GASSHO1 and GASSHO2 (GSO1 and GSO2) (Tsuwamoto et al., 2008), play a critical role in cross-talk between the two zygotic com-partments: the embryo and the endosperm (Xing et al., 2013). The two closely related GSO proteins have been shown to be expressed within the developing embryo and to act redun-dantly to ensure the formation of a functional embryonic cuticle (Tsuwamoto et al., 2008). Double gso1/gso2 mutants show a dramatic reduction in seedling viability owing to the fact that their cuticles are defective, leading to increased per-meability (and thus seedling desiccation sensitivity), as well as the adhesion of cotyledons to each other and/or to the seedling hypocotyl. The cotyledons of gso1/gso2 mutants also adhere to the endosperm during seed development, leading to an inversion in the direction of embryo bending



in the seed. This phenotype has also been described in the seeds of plants lacking an endosperm-expressed subtilisin-like serine protease called ABNORMAL LEAF SHAPE1 (ALE1) (Tanaka et al., 2001; Xing et al., 2013). Consistent with the similarity in their phenotypes, genetic analysis has confirmed that this protein likely acts in the same pathway as GSO1 and GSO2, strongly suggesting that these two receptor-like kinases (RLKs) are involved in an inter-organ-ismal signalling pathway which reinforces the formation of a functional apoplastic barrier between the embryo and the endosperm (Xing et al., 2013). Unfortunately, as is the case for IKU2, the ligands of GSO1/GSO2 have not yet been identified, making it difficult to comment on the regulation of this process. Unlike IKU2, the expression of GSO1 and GSO2 is not restricted to the developing embryo, with both showing expression during post-germination development (Tsuwamoto et al., 2008). Furthermore, GSO1 plays a role in the formation of another apoplastic barrier, the Casparian strip, during root development (Pfister et al., 2014). Since it is rather difficult to draw parallels between embryonic cuticle reinforcement (deposition of cutin polymers on the surface of the embryonic epidermis) and Casparian strip formation (deposition of lignin and then suberin in a band around the root endodermis) (Geldner, 2013), it seems likely that the role of GSO1/GSO2 in seeds is distinct from that in roots, and that the signalling pathway involved may involve seed-specific components (such as ALE1).

The third signalling component that we will consider in this context is the SHORT SUSPENSOR (SSP) protein, a member of the RLCKII family of receptor-like cytoplasmic kinases (RLCKs) (Bayer et al., 2009). Mutant alleles of SSP cause strong reductions in the length of the embryonic sus-pensor, a structure required for nutrient uptake by the early embryo from the endosperm (Kawashima and Goldberg, 2010). Intriguingly, the progeny of plants heterozygous for ssp mutant alleles segregate embryos with short suspensors and normal embryos in a 1:1 ratio, and this has been shown to be due to a parent-of-origin effect. Unusually, the pheno-type of the embryo is dependent upon the genotype of the pollen; this phenomenon has been shown to be due to the fact that the SSP protein, although expressed in the embry-onic suspensor after fertilization, is in fact translated from RNA delivered to the zygote from the sperm cell at fertiliza-tion (Bayer et al., 2009; Lin et al., 2013).

Unlike RLK, RLCKs are not transmembrane proteins but can be anchored to the cytoplasmic faces of membranes via N-terminal modifications such as palmitoylation and/or mirostylation. This type of stable association is predicted for SSP, and indeed, mutation of the domains predicted to mediate membrane association renders the protein cytoplas-mic and non-functional. In plants, RLCKs have commonly been shown to act in association with RLKs, to modify sig-nalling (reviewed recently in Lin et al., 2013). Although SSP contains a kinase domain, this is thought to be inactive due to the lack of critical residues in the kinase catalytic sites. Despite this, considerable genetic evidence has been accrued to suggest that SSP acts at the head of a signalling cascade involving the mitogen-activated protein kinase kinase kinase

(MAPKKK) YODA, and the MAPK3 and MAPK6 pro-teins, to regulate suspensor length (Lukowitz et al., 2004). These observations suggest that SSP most likely functions via interaction with an active protein kinase at the plasma mem-brane, although the identity of this molecule, and whether it is an RLK, remains for the moment unknown. Interestingly, SSP is thought to have evolved relatively recently, specifically in the Brassicaceae, after duplication of the ancestor of the BRASSINOSTEROID KINASE1 (BSK1) gene. This dupli-cation is thought to have permitted rapid neo-functionaliza-tion of SSP1, associated with loss of kinase activity (Bayer et al., 2009; Liu and Adams, 2010).

Intriguingly, the SSP protein, which is translated in the embryo from paternally expressed mRNAs, has recently been shown to act synergistically with maternally produced ESF1 peptides present in the central cell before fertilization and the embryo-surrounding endosperm immediately post-fertiliza-tion, suggesting that the regulation of suspensor development may be a key site for parental control of nutrient allocation (Costa et al., 2014).

Potential modifying enzymes acting upstream of signalling pathways

Apoplastic signalling pathways depend not only upon the presence of peptides and ligands, but potentially are also sub-ject to regulation at the level of peptide processing and modifi-cation. Peptide precursors from several classes are extensively modified by protein cleavage (mediated by proteases present either in the secretory system or in the apoplast) and by post-translational modification of specific amino acids by sulfa-tion, hydroxylation, glycosylation, and potentially by other as yet unidentified modifications (reviewed extensively, includ-ing Matsubayashi, 2011, 2012). Again, the enzymes involved in these modifications may be present in either the secretory apparatus or the apoplast. Furthermore, it is entirely pos-sible that the apoplastic domains of receptor molecules are extensively modified either during or after their insertion in the plasma membrane. In the context of inter-compartmental communication, it could be envisaged that peptides secreted by one compartment are post-translationally modified by enzymes secreted by an adjacent compartment. However, no concrete examples of such signalling control mechanisms have yet been uncovered in plants, possibly because func-tional studies of the enzymes involved in peptide modification remain technically challenging and thus are few in number. Furthermore, although candidate proteins for involvement in pro-peptide cleavage, peptide sulfation, and peptide hydroxy-lation have been identified, the enzymes involved in glycosyla-tion remain unknown.

In this context, it is perhaps unsurprising that little if anything is known about the mechanisms via which post-translational modifications can affect the early stages of post-fertilization seed development. Nonetheless, at least one credible candidate for pro-peptide cleavage has a confirmed role in inter-com-partmental communication in seeds. The subtilisin serine pro-tease ALE1 has been implicated in the GSO1/GSO2 signalling



pathway described in the previous section (Tanaka et al., 2001; Xing et al., 2013). Unlike GSO1 and GSO2, the expression of ALE1 appears to be entirely seed-specific, and specifically endosperm-specific, suggesting that this protease could impart a tissue specificity to this pathway. However, until the substrate of ALE1 is unambiguously identified, this question will remain unresolved. Interestingly, another subtilisin-like serine pro-tease, SBT1.1, has also been shown to play a key role in regu-lating seed size in both Medicago truncatula and Pisum sativum (D’Erfurth et al., 2012). In Medicago, the SBT1.1 gene shows strong expression at the endosperm/testa interface and, fur-thermore, the most closely related Arabidopsis gene, AtSBT1.1, shows an endosperm-specific expression pattern. Intriguingly, AtSBT1.1 cleaves the AtPSK4 phytosulfokine (PSK) precursor in vitro. Phytosulfokines are known to regulate growth and have been shown to stimulate somatic embryogenesis (Hanai et al., 2000), suggesting that this subtilase could potentially play a conserved role in coordinating growth within developing seeds (Srivastava et al., 2008). Interestingly, although AtSBT1.1 has unambiguous orthologues in most eudicots, the same is not true for ALE1, suggesting that the function of ALE1 in seed development may have emerged later during the course of angiosperm diversification (Rautengarten et al., 2005).

Transcriptional regulation of post-germination peptide signalling pathways

Our starting hypothesis—that apoplastic signalling path-ways involved in the post-fertilization coordination of angi-osperm seed development may have evolved through the de novo acquisition of seed-specific components—implies the presence of transcription factors potentially involved in the regulation of the seed-specific expression of pathway com-ponents. Such transcription factors do undoubtedly exist. A prime example is the ZHOUPI (ZOU)/RGE (Kondou et al., 2008; Yang et al., 2008) transcription factor, a mem-ber of the bHLH family, which is expressed exclusively in the Arabidopsis endosperm. ZOU function is required for the expression of ALE1, and thus for inter-compartmental com-munication leading to the formation of an intact embryonic cuticle. We have proposed that the apoplastic separation of the embryo and endosperm became a major developmental problem only after endosperm development was rendered fertilization dependent, due to the concurrent development of the two organisms (Moussu et al., 2013). The fact that the ALE1 protein does not appear to be a particularly ancient gene supports this view. However, ZOU plays a second role in the endosperm, which leads to endosperm degradation. Given the apparently ancient origins of the ZOU protein, this function (or at least a function in mediating female gameto-phyte degradation) may well date back to gymnosperms and even lycophytes (Yang et al., 2008). It is therefore possible that ZOU acquired novel functions in regulating inter-com-partmental apoplastic communication pathways during the angiosperm radiation. The functional characterization of ZOU in angiosperms other than Arabidopsis may help to

elucidate to what extent these functions differ in different angiosperm groups.

More recently, a key regulator necessary for seed develop-ment has been described in the legume M. truncatula. DOF Acting in Seed embryogenesis and Hormone accumulation (DASH), a member of the DOF transcription factor family, appears to be necessary for the establishment of post-fertiliza-tion seed development in M. truncatula (Noguero et al., 2015). DASH, like ZOU, shows strictly endosperm-specific expres-sion, but this expression is restricted to a subpopulation of cells in the chalazal zone adjacent to the point of attachment of the seed to the placental tissues of the pod. Loss of DASH function leads to markedly reduced seed size and, in some alleles, to early seed abortion. These phenotypes are accom-panied by an apparent defect in auxin distribution, since the zygotic tissues of dash mutants show a dramatic over-accu-mulation of auxin and embryo defects consistent with defects in auxin-mediated morphogenesis, whereas the surrounding pod tissues show reduced expression of auxin-induced genes compared to wild-type tissues. A possible explanation for this complex phenotype is that DASH is necessary to establish a flow of auxin from the zygotic to the maternal tissues, which is, in turn, necessary to stimulate the stable establishment of the seed as a nutrient sink. Interestingly, amongst the most strongly down-regulated genes in dash mutants are two genes encoding CRPs. These genes show an identical expression pattern to DASH, suggesting that they could be direct tar-gets of DASH. It is entirely possible that these peptides play a role in establishing the contact zone between the chalazal endosperm and the maternal seed tissues, but this possibility has not yet been investigated.

A more concrete example of CRP function in establishing the zone of nutrient transfer between maternal and zygotic tissues is, of course, provided by the MEG1/CRPs described above. At the level of transcription, these peptides appear to be necessary for the expression of a master transcriptional regu-lator of the BETL, Myb-Related Protein-1 (MRP1) (Gómez et al., 2002; Gómez et al., 2009) which in turn appears to feed back on the expression of MEG-encoding genes. It is inter-esting to note that MRP1 and DASH, despite belonging to totally different families of transcription factors, both act in the endosperm transfer layer and appear to control very simi-lar processes, permitting the establishment of the endosperm as a nutrient sink. It is tempting to speculate that the rapid establishment of nutrient transfer from maternal to endosperm tissues became necessary only upon the sexualization of the endosperm, and that the regulation of this process could have evolved separately in the monocot and legume lineages. To date, similar regulatory modules have not been uncovered in other species, although a transfer-cell-expressed CRP has been identified in barley and wheat (Kovalchuk et al., 2009).

Parental control of seed development is mediated by peptide signalling pathways

Over the past three decades, numerous studies have revealed that the development of seeds is under firm parent-of-origin regulation (Haig, 2013). A key component of this regulation



is the imbalanced contribution of transcripts to the two prod-ucts of fertilization—the embryo and the endosperm. There are several mechanisms implicated in this phenomenon, but the best studied is associated with differences in the DNA methylation of male and female genomes in seed tissues. The asymmetry in DNA methylation associates with the unequal expression of parental transcripts after fertilization, a phenomenon known as genomic imprinting. Intriguingly, genomic imprinting in angiosperms primarily affects the endosperm. This phenomenon mostly involves an extensive loss of DNA methylation, which takes place in the central cell gamete, and this information is later transmitted, after fertili-zation, to the endosperm (Gehring, 2013). There are currently over 200 imprinted genes identified in the endosperm, most of which encode putative transcriptional regulators and met-abolic enzymes, with only a few encoding signalling peptides (Jiang and Kohler, 2012). One of these imprinted peptides, MEG1, is expressed exclusively through the maternal alleles during the early stages of endosperm development (Gutierrez-Marcos et al., 2004). MEG1 acts in a dose-dependent man-ner to regulate the development of transfer cells and therefore acts as a regulatory step in the translocation of nutrients to the embryo (Costa et al., 2012). Although these studies have identified imprinting as one potential means of regulating peptide dosage and signalling pathways in seeds, the precise components of this pathway (or pathways) remain unknown.

A second mechanism involved in the parent-of-origin regulation of seed development is the direct contribution of transcripts present in gametes that are deposited in the two products of fertilization (Fig. 1). Several studies have revealed that in both the embryo and endosperm the activation of parental genomes is delayed, and consequently the earliest stages of seed development must rely on proteins or tran-scripts deposited from the gametes (Autran et al., 2011; Del Toro-De Leon et al., 2014; Grimanelli et al., 2005). As high-lighted above, one of these deposited transcripts corresponds to SSP. Mutants defective in SSP expression show parent-of-origin defects in embryo development because SSP is exclu-sively expressed in sperm cells and delivered via fertilization to the zygote, where the protein is translated and functionally relevant (Bayer et al., 2009). However, the greatest contribu-tion of transcripts and proteins to the early products of fer-tilization comes from maternal gametes, primarily because of their significant cytoplasmic contribution. Molecular and genetic analyses in Arabidopsis have shown that a significant number of transcripts in embryos and endosperms are mater-nally derived (Autran et al., 2011). However, it is not known whether they are derived from the female gametes and/or newly transcribed after fertilization from only the maternally inherited genome. Some of the genes involved encode signal-ling peptides, as is the case for ESF1, that are expressed in the central cell and early endosperm. Furthermore, central cell expression is required for ESF1 function and the correct development of the zygote and early embryo (Costa et al., 2014). Because the expression of CRP peptides is primarily confined to mature ovules and early seed development (Costa et al., 2014; Huang et al., 2015) (see Fig. 1B), it is likely that they play maternal regulatory roles in seeds. If this is the

case, the functional characterization of these peptides should reveal their exact roles and the complex relationship between gametes and all three of the seed components. To reduce the complexity of these functional analyses, studies could focus on those signalling peptides that are conserved across all angiosperms.

Conclusions

In recent years, molecular and genetic studies have collec-tively identified signalling peptides as critical components of seed development in plants. Surprisingly, we have precise functional information for only a handful of them. However, even this relatively restricted view has highlighted the fact that many of these peptides are highly seed-specific and, moreover, may act via signalling cascades containing seed-specific com-ponents. Furthermore, it is clear that seed-specific apoplastic signalling pathways are privileged targets for parental con-trol of seed development. Improved genome annotations and molecular techniques should reveal the precise expression pat-tern of these peptides in seeds, including basal angiosperms. Such analysis could aid not only their functional analysis in model species, but also contribute to an understanding of how these peptides, and their perception, evolved during and after the emergence of the angiosperm lineage. Secreted pep-tides act as ligand molecules in receptor-mediated complexes and have a major role in host recognition signalling mecha-nisms, pollen tube growth in female reproductive tissues, and plant development. Despite the large number of peptides and receptors encoded in plant genomes, only a few have been characterized, and future work will focus on addressing this gap in our understanding. It seems certain that novel tech-niques to uncover receptor–peptide interactions and novel genome editing technologies will further help to uncover the hidden secrets of seed signalling peptides.

AcknowledgementsWe kindly acknowledge funding from the Royal Society, ESF/RTD Framework COST action (FA0903), BBSRC grants (BB/E008585/1 and BB/F008082), and the ANR grant INASEED (ANR-13-BSV2-0002).

ReferencesAutran D, Baroux C, Raissig MT, et al. 2011. Maternal epigenetic pathways control parental contributions to Arabidopsis early embryogenesis. Cell 145, 707–719.

Balandin M, Royo J, Gomez E, Muniz L, Molina A, Hueros G. 2005. A protective role for the embryo surrounding region of the maize endosperm, as evidenced by the characterisation of ZmESR-6, a defensin gene specifically expressed in this region. Plant Molecular Biology 58, 269–282.

Baroux C, Spillane C, Grossniklaus U. 2002. Evolutionary origins of the endosperm in flowering plants. Genome Biology 3, reviews1026.1–1026.5.

Bayer M, Nawy T, Giglione C, Galli M, Meinnel T, Lukowitz W. 2009. Paternal control of embryonic patterning in Arabidopsis thaliana. Science 323, 1485–1488.

Becraft PW, Gutierrez-Marcos J. 2012. Endosperm development: dynamic processes and cellular innovations underlying sibling altruism. Wiley Interdisciplinary Reviews. Developmental Biology 1, 579–593.



Bencivenga S, Colombo L, Masiero S. 2011. Cross talk between the sporophyte and the megagametophyte during ovule development. Sexual Plant Reproduction 24, 113–121.

Berger F, Grini PE, Schnittger A. 2006. Endosperm: an integrator of seed growth and development. Current Opinion in Plant Biology 9, 664–670.

Blanvillain R, Young B, Cai YM, Hecht V, Varoquaux F, Delorme V, Lancelin JM, Delseny M, Gallois P. 2011. The Arabidopsis peptide kiss of death is an inducer of programmed cell death. The EMBO Journal 30, 1173–1183.

Breuninger H, Rikirsch E, Hermann M, Ueda M, Laux T. 2008. Differential expression of WOX genes mediates apical-basal axis formation in the Arabidopsis embryo. Developmental Cell 14, 867–876.

Chevalier E, Loubert-Hudon A, Matton DP. 2013. ScRALF3, a secreted RALF-like peptide involved in cell-cell communication between the sporophyte and the female gametophyte in a solanaceous species. The Plant Journal 73, 1019–1033.

Clark SE, Williams RW, Meyerowitz EM. 1997. The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis. Cell 89, 575–585.

Colombo L, Battaglia R, Kater MM. 2008. Arabidopsis ovule development and its evolutionary conservation. Trends in Plant Science 13, 444–450.

Costa LM, Marshall E, Tesfaye M, et al. 2014. Central cell-derived peptides regulate early embryo patterning in flowering plants. Science 344, 168–172.

Costa LM, Yuan J, Rouster J, Paul W, Dickinson H, Gutierrez-Marcos JF. 2012. Maternal control of nutrient allocation in plant seeds by genomic imprinting. Current Biology 22, 160–165.

D’Erfurth I, Le Signor C, Aubert G, et al. 2012. A role for an endosperm-localized subtilase in the control of seed size in legumes. The New Phytologist 196, 738–751.

De Smet I, Voss U, Jurgens G, Beeckman T. 2009. Receptor-like kinases shape the plant. Nature Cell Biology 11, 1166–1173.

Del Toro-De Leon G, Garcia-Aguilar M, Gillmor CS. 2014. Non-equivalent contributions of maternal and paternal genomes to early plant embryogenesis. Nature 514, 624–627.

DeYoung BJ, Bickle KL, Schrage KJ, Muskett P, Patel K, Clark SE. 2006. The CLAVATA1-related BAM1, BAM2 and BAM3 receptor kinase-like proteins are required for meristem function in Arabidopsis. The Plant Journal 45, 1–16.

Diboll AG, Larson DA. 1966. An electron microscopic study of the mature megagametophyte in Zea mays. American Journal of Botany 53, 391–402.

Fisher K, Turner S. 2007. PXY, a receptor-like kinase essential for maintaining polarity during plant vascular-tissue development. Current Biology 17, 1061–1066.

Fiume E, Fletcher JC. 2012. Regulation of Arabidopsis embryo and endosperm development by the polypeptide signaling molecule CLE8. The Plant Cell 24, 1000–1012.

Fiume E, Monfared M, Jun J, Fletcher JC. 2011. CLE polypeptide signaling gene expression in Arabidopsis embryos. Plant Signaling & Behavior 6, 443–444.

Friedman WE. 2001. Developmental and evolutionary hypotheses for the origin of double fertilization and endosperm. Comptes Rendus de l’Académie des Sciences. Série III, Sciences de la Vie 324, 559–567.

Garcia D, Fitz Gerald JN, Berger F. 2005. Maternal control of integument cell elongation and zygotic control of endosperm growth are coordinated to determine seed size in Arabidopsis. The Plant Cell 17, 52–60.

Garcia D, Saingery V, Chambrier P, Mayer U, Jurgens G, Berger F. 2003. Arabidopsis haiku mutants reveal new controls of seed size by endosperm. Plant Physiology 131, 1661–1670.

Gehring M. 2013. Genomic imprinting: insights from plants. Annual Review of Genetics 47, 187–208.

Geldner N. 2013. Casparian strips. Current Biology 23, R1025–1026.

Gómez E, Royo J, Guo Y, Thompson R, Hueros G. 2002. Establishment of cereal endosperm expression domains: identification and properties of a maize transfer cell-specific transcription factor, ZmMRP-1. The Plant Cell 14, 599–610.

Gómez E, Royo J, Muñiz LM, Sellam O, Paul W, Gerentes D, Barrero C, López M, Perez P, Hueros G. 2009. The maize transcription factor myb-related protein-1 is a key regulator of the differentiation of transfer cells. The Plant Cell 21, 2022–2035.

Grimanelli D, Perotti E, Ramirez J, Leblanc O. 2005. Timing of the maternal-to-zygotic transition during early seed development in maize. The Plant Cell 17, 1061–1072.

Gutierrez-Marcos JF, Costa LM, Biderre-Petit C, Khbaya B, O’Sullivan DM, Wormald M, Perez P, Dickinson HG. 2004. maternally expressed gene1 is a novel maize endosperm transfer cell-specific gene with a maternal parent-of-origin pattern of expression. The Plant Cell 16, 1288–1301.

Haig D. 2013. Kin conflict in seed development: an interdependent but fractious collective. Annual Review of Cell and Developmental Biology 29, 189–211.

Han YZ, Huang BQ, Zee SY, Yuan M. 2000. Symplastic communication between the central cell and the egg apparatus cells in the embryo sac of Torenia fournieri Lind. before and during fertilization. Planta 211, 158–162.

Hanai H, Matsuno T, Yamamoto M, Matsubayashi Y, Kobayashi T, Kamada H, Sakagami Y. 2000. A secreted peptide growth factor, phytosulfokine, acting as a stimulatory factor of carrot somatic embryo formation. Plant Cell Physiology 41, 27–32.

Huang Q, Dresselhaus T, Gu H, Qu LJ. 2015. The active role of small peptides in Arabidopsis reproduction: Expression evidence. Journal of Integrative Plant Biology 57, 518–521.

Ingram GC. 2010. Family life at close quarters: communication and constraint in angiosperm seed development. Protoplasma 247, 195–214.

Jiang H, Kohler C. 2012. Evolution, function, and regulation of genomic imprinting in plant seed development. Journal of Experimental Botany 63, 4713–4722.

Jinn TL, Stone JM, Walker JC. 2000. HAESA, an Arabidopsis leucine-rich repeat receptor kinase, controls floral organ abscission. Genes & Development 14, 108–117.

Jun J, Fiume E, Roeder AH, et al. 2010. Comprehensive analysis of CLE polypeptide signaling gene expression and overexpression activity in Arabidopsis. Plant Physiology 154, 1721–1736.

Kawashima T, Goldberg RB. 2010. The suspensor: not just suspending the embryo. Trends in Plant Science 15, 23–30.

Kelley DR, Gasser CS. 2009. Ovule development: genetic trends and evolutionary considerations. Sexual Plant Reproduction 22, 229–234.

Kondou Y, Nakazawa M, Kawashima M, et al. 2008. RETARDED GROWTH OF EMBRYO1, a new basic helix-loop-helix protein, expresses in endosperm to control embryo growth. Plant Physiology 147, 1924–1935.

Kovalchuk N, Smith J, Pallotta M, et al. 2009. Characterization of the wheat endosperm transfer cell-specific protein TaPR60. Plant Molecular Biology 71, 81–98.

Kozieradzka-Kiszkurno M, Bohdanowicz J. 2010. Unusual electron-dense dome associates with compound plasmodesmata in the embryo-suspensor of genus Sedum (Crassulaceae). Protoplasma 247, 117–120.

Le BH, Cheng C, Bui AQ, et al. 2010. Global analysis of gene activity during Arabidopsis seed development and identification of seed-specific transcription factors. Proceedings of the National Academy of Sciences of the United States of America 107, 8063–8070.

Li J, Nie X, Tan JL, Berger F. 2013. Integration of epigenetic and genetic controls of seed size by cytokinin in Arabidopsis. Proceedings of the National Academy of Sciences of the United States of America 110, 15479–15484.

Lin W, Ma X, Shan L, He P. 2013. Big roles of small kinases: the complex functions of receptor-like cytoplasmic kinases in plant immunity and development. Journal of Integrative Plant Biology 55, 1188–1197.

Liu SL, Adams KL. 2010. Dramatic change in function and expression pattern of a gene duplicated by polyploidy created a paternal effect gene in the Brassicaceae. Molecular Biology and Evolution 27, 2817–2828.

Lukowitz W, Roeder A, Parmenter D, Somerville C. 2004. A MAPKK kinase gene regulates extra-embryonic cell fate in Arabidopsis. Cell 116, 109–119.

Luo M, Dennis ES, Berger F, Peacock WJ, Chaudhury A. 2005. MINISEED3 (MINI3), a WRKY family gene, and HAIKU2 (IKU2), a leucine-rich repeat (LRR) KINASE gene, are regulators of seed size in Arabidopsis.



Proceedings of the National Academy of Sciences of the United States of America 102, 17531–17536.

Magnard JL, Le Deunff E, Domenech J, Rogowsky PM, Testillano PS, Rougier M, Risueno MC, Vergne P, Dumas C. 2000. Genes normally expressed in the endosperm are expressed at early stages of microspore embryogenesis in maize. Plant Molecular Biology 44, 559–574.

Magnard JL, Lehouque G, Massonneau AE, Frangne N, Heckel T, Gutierrez-Marcos JF, Perez P, Dumas C, Rogowsky PM. 2003. ZmEBE genes show a novel, continuous expression pattern in the central cell before fertilization and in specific domains of the resulting endosperm after fertilization. Plant Molecular Biology 53, 821–836.

Matsubayashi Y. 2011. Small post-translationally modified peptide signals in Arabidopsis. The Arabidopsis Book 9, e0150.

Matsubayashi Y. 2012. MBSJ MCC Young Scientist Award 2010. Recent progress in research on small post-translationally modified peptide signals in plants. Genes to Cells 17, 1–10.

Moussu S, San-Bento R, Galletti R, Creff A, Farcot E, Ingram G. 2013. Embryonic cuticle establishment: the great (apoplastic) divide. Plant Signaling & Behavior 8, e27491.

Noguero M, Le Signor C, Vernoud V, et al. 2015. DASH transcription factor impacts Medicago truncatula seed size by its action on embryo morphogenesis and auxin homeostasis. The Plant Journal 81, 453–466.

Pfister A, Barberon M, Alassimone J, et al. 2014. A receptor-like kinase mutant with absent endodermal diffusion barrier displays selective nutrient homeostasis defects. eLife 3, e03115.

Radchuk V, Borisjuk L, Radchuk R, Steinbiss HH, Rolletschek H, Broeders S, Wobus U. 2006. Jekyll encodes a novel protein involved in the sexual reproduction of barley. The Plant Cell 18, 1652–1666.

Rautengarten C, Steinhauser D, Bussis D, Stintzi A, Schaller A, Kopka J, Altmann T. 2005. Inferring hypotheses on functional relationships of genes: Analysis of the Arabidopsis thaliana subtilase gene family. PLoS Computational Biology 1, e40.

Serna A, Maitz M, O’Connell T, et al. 2001. Maize endosperm secretes a novel antifungal protein into adjacent maternal tissue. The Plant Journal 25, 687–698.

Song X, Yuan L, Sundaresan V. 2014. Antipodal cells persist through fertilization in the female gametophyte of Arabidopsis. Plant Reproduction 27, 197–203.

Sprunck S, Rademacher S, Vogler F, Gheyselinck J, Grossniklaus U, Dresselhaus T. 2012. Egg cell-secreted EC1 triggers sperm cell activation during double fertilization. Science 338, 1093–1097.

Srivastava R, Liu JX, Howell SH. 2008. Proteolytic processing of a precursor protein for a growth-promoting peptide by a subtilisin serine protease in Arabidopsis. The Plant Journal 56, 219–227.

Stadler R, Lauterbach C, Sauer N. 2005. Cell-to-cell movement of green fluorescent protein reveals post-phloem transport in the outer integument and identifies symplastic domains in Arabidopsis seeds and embryos. Plant Physiology 139, 701–712.

Stenvik GE, Tandstad NM, Guo Y, Shi CL, Kristiansen W, Holmgren A, Clark SE, Aalen RB, Butenko MA. 2008. The EPIP peptide of INFLORESCENCE DEFICIENT IN ABSCISSION is sufficient to induce abscission in Arabidopsis through the receptor-like kinases HAESA and HAESA-LIKE2. The Plant Cell 20, 1805–1817.Tanaka H, Onouchi H, Kondo M, Hara-Nishimura I, Nishimura M, Machida C, Machida Y. 2001. A subtilisin-like serine protease is required for epidermal surface formation in Arabidopsis embryos and juvenile plants. Development 128, 4681–4689.Tesfaye M, Silverstein KA, Nallu S, et al. 2013. Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes. PLoS One 8, e58992.Tsuwamoto R, Fukuoka H, Takahata Y. 2008. GASSHO1 and GASSHO2 encoding a putative leucine-rich repeat transmembrane-type receptor kinase are essential for the normal development of the epidermal surface in Arabidopsis embryos. The Plant Journal 54, 30–42.Wang A, Garcia D, Zhang H, Feng K, Chaudhury A, Berger F, Peacock WJ, Dennis ES, Luo M. 2010. The VQ motif protein IKU1 regulates endosperm growth and seed size in Arabidopsis. The Plant Journal 63, 670–679.Xing Q, Creff A, Waters A, Tanaka H, Goodrich J, Ingram GC. 2013. ZHOUPI controls embryonic cuticle formation via a signalling pathway involving the subtilisin protease ABNORMAL LEAF-SHAPE1 and the receptor kinases GASSHO1 and GASSHO2. Development 140, 770–779.Yamaguchi Y, Huffaker A, Bryan AC, Tax FE, Ryan CA. 2010. PEPR2 is a second receptor for the Pep1 and Pep2 peptides and contributes to defense responses in Arabidopsis. The Plant Cell 22, 508–522.Yang S, Johnston N, Talideh E, Mitchell S, Jeffree C, Goodrich J, Ingram G. 2008. The endosperm-specific ZHOUPI gene of Arabidopsis thaliana regulates endosperm breakdown and embryonic epidermal development. Development 135, 3501–3509.Zhan J, Thakare D, Ma C, et al. 2015. RNA sequencing of laser-capture microdissected compartments of the maize kernel identifies regulatory modules associated with endosperm cell differentiation. The Plant Cell 27, 513–531.Zhao P, Zhou XM, Zhang LY, Wang W, Ma LG, Yang LB, Peng XB, Bozhkov PV, Sun MX. 2013. A bipartite molecular module controls cell death activation in the basal cell lineage of plant embryos. PLoS Biology 11, e1001655.Zhou P, Silverstein KA, Gao L, Walton JD, Nallu S, Guhlin J, Young ND. 2013. Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application). BMC Bioinformatics 14, 335.



Peptide signalling during angiosperm seed development

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