Diverse Roles of ERECTA Family Genes in Plant Development

Invited Expert Review

Diverse Roles of ERECTA Family Genesin Plant DevelopmentElena D. Shpak*Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee 37996, USA�Corresponding authorTel: þ1 865 974 8383; Fax: þ1 865 974 6306; E‐mail: [email protected]

Articles can be viewed online without a subscription.Available online on 10 September 2013 at www.jipb.net and www.wileyonlinelibrary.com/journal/jipbdoi: 10.1111/jipb.12108

Abstract

Multiple receptor‐like kinases (RLKs) enable intercellular communication thatcoordinates growth and development of plant tissues. ERECTA family receptors(ERfs) are an ancient family of leucine‐rich repeat RLKs that in Arabidopsisconsists of three genes: ERECTA, ERL1, and ERL2. ERfs sense secretedcysteine‐rich peptides from the EPF/EPFL family and transmit the signal througha MAP kinase cascade. This review discusses the functions of ERfs in stomatadevelopment, in regulation of longitudinal growth of aboveground organs, duringreproductive development, and in the shoot apical meristem. In addition the roleof ERECTA in plant responses to biotic and abiotic factors is examined.

Keywords: Arabidopsis; ERECTA; leaf primordium initiation; receptor‐like kinase; stomata; vasculature.

Shpak ED (2013) Diverse roles of ERECTA family genes in plant development. J. Integr. Plant Biol. 55(12), 1238–1250.

Introduction

While all organisms have mechanisms with which to sense theirenvironment, multicellular organisms require additional signalingpathways to enable cellular cooperation. In metazoans a smallnumber of core signaling pathwaysare repeatedly used to regulatedifferent aspects of morphogenesis (Pires‐daSilva and Sommer2003). Traditionally, classical plant hormones have been per-ceivedas themain regulators of plantmorphogenesis.However, inrecent years it has become clear that plant development alsodepends heavily upon signaling pathways mediated by receptor‐like kinases. In plants, just as in animals, the central signalingpathways seem to be highly flexible and are used repeatedlyduring morphogenesis to generate distinct responses in differenttissues and organs. One of those central pathways is ERECTAfamily (ERf) receptor signaling. This family first appeared in earlyland plants (Villagarcia et al. 2012) and since then has evolved toregulate multiple aspects of plant development.

Themutation in theERECTAgenehasbeenknownsince 1957whenGeorge Rédei isolated it from an X‐ray irradiated Landsberg

accession of Arabidopsis (Rédei 1992). Due to its compact formLandsberg erecta became one of the favorite accessions ofArabidopsis researchers. A 1996 analysis of the er‐104 mutantlocated a T‐DNA insertion in a geneencoding a leucine‐rich repeat(LRR) receptor‐like kinase (Torii et al. 1996). This gene contains 26introns, many of which are necessary for efficient ERECTAexpression (Karve et al. 2011). Originally the ERECTA gene waslinked with regulation of plant architecture, but over time manyother functions of this receptor‐like kinase have emerged. Onediscovery that has enabled a more complete understanding ofERECTA’s role during plant growth was the realization that inmany developmental processes ERECTA functions synergistical-ly with its paralogs ERL1 and ERL2 (Shpak et al. 2004, 2005).

Inhibition of Stomata Developmentby the ERf Signaling Pathway

The function of ERfs and the mechanism of their action are mostwell understood in stomata development, which has recently

Elena D. Shpak(Corresponding author)

Journal of Integrative Plant Biology 2013, 55 (12): 1238–1250

© 2013 Institute of Botany, Chinese Academy of Sciences

been extensively reviewed (Peterson et al. 2010; Shimadaet al. 2011; Facette and Smith 2012; Lau and Bergmann 2012;Pillitteri and Torii 2012; Torii 2012; Wengier andBergmann 2013). Therefore they will be discussed here onlybriefly. Early in development protodermal cells undergosymmetric proliferative divisions (Figure 1). Then a subset ofcells called meristemoid mother cells (MMC) switches toasymmetric divisions, which give rise to small triangularmeristemoid cells and larger sister cells also known as stomatallineage ground cells (SLGCs). A SLGC may divide asymmetri-cally to produce a new meristemoid or may differentiate into apavement cell. A meristemoid typically undergoes severalrounds of proliferative asymmetric divisions, which occur in aninward spiral pattern and progressively generate smaller andsmaller cells. Ultimately, triangular meristemoids become roundguard mother cells (GMCs), which divide symmetrically togenerate a pair of guard cells. Throughout stomata develop-ment, cell‐to‐cell communication prevents the formation ofadjacent GMCs, and as a result stomata are separated by atleast one pavement cell.

Analyses of various combinations of ERf mutants haverevealed that ERf receptors have two major functions duringstomata development: they negatively regulate cell fatetransitions and they contribute to the orientation of asymmetriccell divisions (Shpak et al. 2005). At first, ERfs synergisticallyinhibit differentiation of protodermal cells into MMCs. Compari-son of epidermis development in the er, erl1, and erl2 mutantssuggested that ERECTA plays a major role during this process,as only in the er single mutant has an increased number ofasymmetric cell divisions been observed (Shpak et al. 2005).

ERL1 and ERL2 are important later on for maintaining theproliferative potential of meristemoids, preventing their differen-tiation into guard mother cells. In the erl1 erl2 mutant,meristemoids prematurely differentiate into GMCs (Shpaket al. 2005). Besides inhibiting cell fate transitions, all threereceptors are also important for the orientation of asymmetric celldivisions in MMCs and SLGCs, ensuring that stomata do notform right next to each other. The three ERfs seem to functionredundantly in regulation of cell polarity, as formation of stomataclusters is observed only in the triple er erl1 erl2 mutant (Shpaket al. 2005).

Activity of ERf receptors is regulated by the EPF/EPFL familyof secreted cysteine‐rich peptides, which can function asagonists or antagonists. In Arabidopsis the EPF/EPFL familyconsists of 11 genes, six of which have been shown to be part ofthe ERf signaling pathway and to interact with ERf receptors(Hara et al. 2007, 2009; Hunt and Gray 2009; Hunt et al. 2010;Kondo et al. 2010; Sugano et al. 2010; Abrash et al. 2011; Leeet al. 2012; Uchida et al. 2012a). The peptides EPF1 and EPF2have been demonstrated to have direct interactions with ERfs(Lee et al. 2012). EPF1 and EPF2 are expressed in theepidermis and, based on the phenotypes of mutants andoverexpression lines, they activate ERfs (Hara et al. 2007; Huntand Gray 2009). EPF2 is expressed during early epidermisdevelopment, where it inhibits differentiation of MMCs (Haraet al. 2009; Hunt and Gray 2009). EPF1 is expressed slightlylater, in meristemoids, GMCs, and young guard cells. EPF1inhibits differentiation of meristemoids into GMCs and orientsasymmetric cell division (Hara et al. 2007, 2009). The differentroles of EPF1 and EPF2 in stomata development cannot be

Figure 1. Cell‐fate transitions of stomatal lineage cells in Arabidopsis.

Early in development protodermal cells (salmon) undergo symmetric divisions. Some protodermal cells differentiate into meristemoid mother

cells (purple) that divide asymmetrically giving rise to meristemoid cells (red) and bigger stomatal linage ground cells (pink). Typically, after

several rounds of asymmetric divisions ameristemoid becomes a round guardmother cell (green), which then divides symmetrically to generate

a pair of guard cells (blue). Pavement cells are grey.

Diverse Roles of ERECTA Family Genes 1239

attributed to their distinct expression patterns, as promoterswapping experiments suggest that these ligands are function-ally different (Hara et al. 2009).

The density of stomata varies in aboveground plant organs.Photosynthetically active leaves require a high rate of gasexchange and therefore require an increased density of stomata.Mesophyll cells, the major sites of photosynthesis, promotestomata development by producing STOMAGEN/EPFL9 (Huntet al. 2010; Kondo et al. 2010; Sugano et al. 2010). While thethree‐dimensional structures of EPF2 and STOMAGEN aresimilar, these peptides contain unique loops that providefunctional specificity (Ohki et al. 2011). STOMAGEN is likelyan antagonist of EPF1/EPF2, as increasing its concentration inthe epf1 epf2 background does not lead to changes in stomatadevelopment in leaves (Kondo et al. 2010).

Like most mammalian receptor kinases, ERfs form homo‐and heterodimers (Figure 2) (Lee et al. 2012). Whether ERfdimerization is dependent on ligand binding and is necessary foractivation of the intracellular kinase domain by cross phosphor-ylation has yet to be addressed. Activity of ERfs is regulated bythe receptor‐like protein TOO MANY MOUTHS (TMM) which isable to heterodimerize with ERfs (Lee et al. 2012). TMM isabsolutely essential for ERf function in the epidermis ofcotyledons and leaves, where the tmm‐1 phenotype isindistinguishable from the er erl1 erl2 phenotype (Geisler et al.1998; Shpak et al. 2005 and personal observations). Incotyledons and leaves, overexpression of EPF1, EPF2, andSTOMAGEN causes changes in stomata development only inplants with a functional TMM gene, but not in tmm‐1 (Haraet al. 2007, 2009; Kondo et al. 2010; Sugano et al. 2010).Therefore, TMM is hypothetically an indispensible part of an ERfreceptor complex that can be activated by EPF1 and EPF2 andinactivated by STOMAGEN (Figure 2A–D). TMM might benecessary for productive conformational changes of ERfreceptors in response to EPF1 and EPF2 binding. Thecontribution of TMM to the formation of functional complexesincludes direct binding of EPF2 but not of EPF1 (Lee et al. 2012).Thus, TMM might not only be essential for the formation offunctional ERf complexes, but it may also enable the differentialresponse of ERfs to EPF1 versus EPF2 (Figure 2A–C).

However, if TMM is necessary for the perception of EPF1 andEPF2 by ERfs, why are the phenotypes of tmm‐1 and er erl1 erl2different in stems and pedicels? The ERf receptors inhibitstomata development in these organs. At the same time there areno stomata in tmm‐1 stems. While meristemoids form in tmm‐1stems, they differentiate into pavement cells instead of GMCs(Geisler et al. 1998). In tmm‐1 pedicels, stomata donot format theproximal endwhile at the distal end they develop in small clusters(Geisler et al. 1998). Thus it seems that TMM promotes GMCdifferentiation in stems and pedicels and is antagonistic to ERfs.

To understand TMM’s role in those organs, one needs toremember that EPF1, EPF2, and STOMAGEN are not the only

ligands of ERfs and that ERfs function in other tissues besides theepidermis. Recent work suggests that ERfs are also active in thevasculature where they perceive a different set of ligands (Uchidaet al. 2012a). EPFL4/CLL2 and EPFL6/CHALLAH are expressedby endodermal cells and they are primarily sensed by ERfslocalized in the vasculature (Uchida et al. 2012a). ERf signaling inthe vasculature regulates inflorescence architecture and isimportant for elongation of stems and pedicels (Uchida et al.2012a). TMM, however, is not expressed in vasculature (Nadeauand Sack 2002 and personal observations). Interestingly, neitherTMM, EPF1, EPF2 nor STOMAGEN have any impact on theshape of aboveground organs (Hara et al. 2007; Abrashet al. 2011). While one might suggest that EPF1 and EPF2 donot reach vascular tissue, this explanation cannot be valid forSTOMAGEN. On the other hand mutations in CHALLAH familygenes do not have any impact on stomata development unlessthey are in the tmm‐1 background (Abrash et al. 2011). Over-expression of EPFL4 and EPFL6 in wild type inhibits stomatadevelopment, and this inhibition is even stronger in tmm‐1.

All of these data can be reconciled by the following model:EPF1 and EPF2 activate ERf complexes containing TMM,STOMAGEN deactivates ERf complexes containing TMM, andEPFL4 and EPFL6 activate ERf receptor complexes withoutTMM (Figure 2). In the epidermis the majority of ERf receptorsnormally form complexes with TMM, but TMM is absent in thevasculature. In cotyledons and leaves, very little EPFL4 orEPFL6 reaches the epidermis, and thus in tmm‐1 epidermalERfs are nonfunctional. In stems that have a higher density ofvasculature some EPFL4 and EPFL6 may diffuse to theepidermis, and while in the wild type there are no suitable ERfcomplexes for them to activate, in the tmm‐1 mutant suchcomplexes exist and as a result GMC differentiation is inhibited.This model suggests that TMM brings specificity to ERf‐mediated responses, enabling these receptors to perceivedifferent sets of ligands in the epidermis and vasculature.

Stomata development is positively regulated by the basic helix‐loop‐helix (bHLH) transcription factors SPEECHLESS (SPCH),MUTE, and FAMA which correspondingly promote differentiationofMMCs,GMCs, andguard cells (Ohashi‐Ito andBergmann2006;MacAlister et al. 2007; Pillitteri et al. 2007b). Two other bHLHtranscription factors SCREAM/ICE1 and SCREAM2 are requiredfor all three differentiation steps as SPCH, MUTE, and FAMA arefunctional only as heterodimers with those proteins (Kanaokaet al. 2008). Once activated, ERfs use a MAPK signaling cascadeconsisting of YODA (YDA), MKK4, MKK5, MPK3, and MPK6 toinhibit SPCHandMUTE (Bergmann et al. 2004;Wang et al. 2007;Meng et al. 2012). MPK3 and MPK6 can directly phosphorylateSPCH, presumably decreasing its stability and/or changing itsinteractions with partners (Lampard et al. 2008).

While the exact mechanism of MAPK cascade activation byERfs is still unknown, some potential candidates that couldtransmit the signal downstream from the receptors are starting to

1240 Journal of Integrative Plant Biology Vol. 55 No. 12 2013

emerge (Figure 3). Intriguingly, the first steps of signaltransduction from ERf receptors and from the brassinosteroidreceptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) appearto be similar. The signal from the BRI1 receptor is transmitted byBrassinosteroid‐Signaling Kinases (BSKs), CONSTITUTIVEDIFFERENTIAL GROWTH 1 (CDG1) kinase, and bri1 SUP-PRESSOR1 (BSU1) phosphatases (Wang et al. 2012; Linet al. 2013). BSK and CDG1 kinases promote the interaction ofBSU1 with the GSK3/SHAGGY‐like kinase BRASSINOSTE-ROID‐INSENSITIVE2 (BIN2), which leads to BIN2 dephosphor-ylation and inactivation. In Arabidopsis the BSK family consistsof 12 genes (Tang et al. 2008). SHORT SUSPENSOR (SSP)/BSK12 has been proposed to function upstream of YDA during

embryogenesis and its overexpression suppresses stomatadevelopment only in plants with functional YDA (Bayeret al. 2009). Thus, hypothetically, the transduction of signalfrom ERfs to YDA might involve one or more kinases from theBSK family. The BSU1 family of four phosphatases has beenlinked with ERf signaling based on the fact that knockout of allBSU1 family members (bsu‐q) leads to dramatic stomataclustering in a YDA and SPCH dependent manner (Kimet al. 2012). Finally, GSK3/SHAGGY‐like kinases (also calledArabidopsis SHAGGY kinases or AtSKs) are also likely to beinvolved in the ERf signal transduction pathway. There are 10AtSKs in Arabidopsis. Seven of the nine AtSKs that have beentested are inhibited by bikinin, a chemical that impedes ATP

Figure 2. A hypothetical model of ERECTA family receptor/TOO MANY MOUTHS (ERf/TMM) receptor complexes in the plasmamembrane.

(A) During early epidermis development a receptor complexmade of an ERf dimer and TMM is activated by EPF2, which leads to suppression of

meristemoid mother cells (MMC) fate and negative regulation of SPEECHLESS (SPCH).

(B) and (C) Later on a receptor complexmade of anERf dimer and TMM is activated byEPF1which leads to inhibition of guardmother cell (GMC)

fate and orientation of asymmetric cell divisions.

(D) STOMAGEN binds to and inactivates receptor complexes made of ERfs and TMM.

(E) EPF1 and EPF2 cannot activate ERf receptor complexes that lack TMM.

(F) CHALf peptides activate only ERf complexes without TMM.


binding (DeRybel et al. 2009). Bikinin is a highly specific inhibitorof AtSKs as it does not affect the activity of structurally similarkinases such as MPKs. Bikinin is very effective in rescuingstomata clustering in er erl1 erl2, tmm, and bsu‐q, but not in yda,which suggests that an AtSK functions downstream of ERfs andTMM and upstream of YDA (Kim et al. 2012). One AtSK (BIN2)phosphorylates YDA, MKK4, and MKK5 in vitro and interactswith YDA in vivo when both proteins are transiently co‐expressed in Nicotiana benthamiana leaves (Kim et al. 2012;Khan et al. 2013). Since stomata development is only marginallyaffected in the BIN2 knockout the identities of AtSKs functioningdownstream of ERfs are still an open question (Gudesblat

et al. 2012). The ability of bikinin and not brassinolide to rescue ererl1 erl2 and tmm mutants (Shpak and Aksenova unpubl. data,2012) suggests that ERfs might inhibit AtSKs independently ofbrassinosteroids.

ERfs and BRI1 appear to transmit signals using the samefamilies of kinases and phosphatases, so it is not surprising thatcrosstalk between these two pathways has recently beenproposed (Gudesblat et al. 2012; Kim et al. 2012). The role ofbrassinosteroids (BRs) in stomata development is complicated;in cotyledons they inhibit stomata development, while inhypocotyls and leaves they promote it (Gudesblat et al. 2012;Kim et al. 2012; Khan et al. 2013). Several mechanisms havebeen proposed for the regulation of stomata development byBRs (Gudesblat et al. 2012; Kim et al. 2012: Khan et al. 2013).However, a better understanding of BR’s role in stomatadevelopment might depend on a better knowledge of the ERfsignal transduction mechanism, as these two pathways likelyshare and compete for the same components.

Regulation of Longitudinal PlantGrowth by ERfs

In addition to their role in stomata development, ERfs areimportant for plant morphogenesis. The involvement of ERfs inregulation of growth along the apical‐basal and proximodistalaxes in aboveground organs is evident from analyses of theirmutants. The single er mutation confers a short stature andcompact inflorescence due to reduced elongation of internodesand pedicels (Torii et al. 1996). The elongation of otheraboveground plant organs such as sepals, petals, stamens,pistils, siliques, petioles, and leaf blades is also reduced in er(Torii et al. 1996, 2003). Temporal analysis of pedicel develop-ment in the er mutant suggests that ERECTA promotes organelongation over a large fraction of the growth period, but it doesnot change the total duration of growth (Bundy et al. 2012).Whilethe erl1 and erl2mutations do not affect plant morphogenesis ontheir own, they do enhance the elongation defects of er (Shpaket al. 2004). The er erl1 erl2mutant has the strongest phenotypeand is a severe dwarf. The growth defects ofERfmutants cannotbe attributed purely to their role in stomata differentiation as anincrease in stomata density does not necessarily cause adecrease in the size of organs as evident from analysis of thetmm and sdd mutants (Yang and Sack 1995; Berger andAltmann 2000). In addition to the epidermis, ERf genes areexpressed in the xylem and phloem (Uchida et al. 2012a). Theexpression of ERECTA under a series of promoters demon-strated that the elongation defects of stems, pedicels, siliques,and leaves can be rescued by ERECTA expression in thephloem but not in xylem or epidermis (Uchida et al. 2012a).Activity of ERfs in the vasculature is controlled by EPFL4 andEPFL6 ligands expressed in the endodermis. While the epfl4

Figure 3. Predicted ERECTA family receptor/TOO MANYMOUTHS (ERf/TMM) signal transduction cascade controllingstomata development.

In the plasma membrane ERfs form complexes with TMM and

perceive EPF1 and EPF2 ligands. Once activated, ERfs hypotheti-

cally transmit the signal through selected Brassinosteroid‐Signaling

Kinases (BSKs), BSUs, and AtSKs to the mitogen‐activated protein

(MAP) kinase cascade. The cascade consists of YODA, MKK4/

MKK5, and MPK3/MPK6 and it inhibits SPEECHLESS (SPCH) and

MUTE transcription factors.


epfl6 mutant does not have any obvious epidermal defects it issignificantly reduced in stature and has compact inflorescences(Abrash et al. 2011; Uchida et al. 2012a). These defects can berescued by expression of either ligand under the endodermis‐specific promoter SCARECROW (Uchida et al. 2012a). Invasculature, just as in epidermis, the signal from ERfs istransmitted by a MAPK cascade consisting of YDA, MKK4/MKK5, and MPK3/MPK6 (Meng et al. 2012). The targets of ERfsignaling pathways in the vasculature are not clear. NeitherSPCH nor MUTE are expressed at a significant level in internaltissues (MacAlister et al. 2007; Pillitteri et al. 2008). Abioinformatic examination of gene expression profiles in theLandsberg erecta/Cape Verde Islands (Ler/Cvi) recombinantinbred population has predicted WRKY transcription factors tobe transcriptionally upregulated by the ERECTA signalingpathway (Terpstra et al. 2010). In addition, microarray analysisidentified WRKY15 as one of the genes that is significantlydownregulated in the epfl4 epfl6 and er mutants (Uchidaet al. 2012a). Many WRKYs have been shown to be MPKsubstrates in vitro and in some cases in vivo (Ishihama andYoshioka 2012). However, WRKYs have been functionallyconnected almost exclusively with responses to biotic andabiotic stresses and their involvement in regulation of plant sizehas not been demonstrated (Chen et al. 2012).

The data discussed here suggest that the ERf signalingpathway promotes intercellular communication between theendodermis and vasculature, and that this communication isessential for longitudinal growth of aboveground organs.However, the question of how vasculature responds to ERfsignaling is not yet answered. It has been shown that the ermutation causes premature differentiation of vascular bundles inpedicels and radial expansion of xylem in the inflorescence stem(Douglas and Riggs 2005; Ragni et al. 2011). In the er erl1 erl2mutant the vascular bundles of stems and hypocotyls consist of adecreased number of cells and their diameter is reduced, whileno major changes in the overall organization of the vasculaturewere detected (Etchells et al. 2013). Surprisingly, the effect of erand er erl1 erl2 on xylemsize and on the number of its constituentcells seems to be opposite. In addition, a recent study of vascularstructure in stems and hypocotyl of the er erl1mutant uncovereda reduced number of procambial cells, thus linking ERfs withmaintenance of procambium (Uchida and Tasaka 2013).Analysis of procambium cell size at earlier stages of develop-ment, in forming leaf primordia of er erl1 erl2mutants, suggestedthat ERfs might be involved in regulation of vascular cellelongation (Chen et al. 2013). A detailed temporal analysis ofvasculature development in the mutants utilizing tissue‐specificmarkers might be helpful in deciphering the role of ERfs in thatprocess. The enhancement of cellular growth by ERfs is specificto aboveground organs. Whether this is related to some uniqueaspect of vasculature differentiation in shoots is an interestingfuture question to answer.

The regulation of plant size by ERfs is not limited to themodification of growth and development of vasculature. Inmature pedicels of er and er erl1 erl2 mutants the number ofcortex cells is reduced and they are radially expanded (Shpaket al. 2003, 2005). A reduced number of cells has been observedin the epidermis of er and er erl1 erl2 petals (Shpak et al. 2005;Abraham et al. 2013). The mesophyll tissue of er leaves is madeup of fewer cells that are more loosely packed (Masleet al. 2005). A temporal study of pedicel development in wildtype and the ermutant revealed that during early stages of organgrowth ERECTA promotes elongation of epidermal and cortexcells along the proximodistal axis and thus impinges on theduration of cell cycle (Bundy et al. 2012). At the same time, cellcycle progression in the er cortex is still coordinated with cellgrowth and the proliferative window is not affected, suggestingthat ERECTA‐mediated signaling does not control the cortex cellcyclemachinery (Bundy et al. 2012). The regulation of cortex cellelongation is likely to be indirect, as expression of ERECTA inthe phloem is sufficient to rescue cortical defects in the ermutant(Uchida et al. 2012a). The sensitivity of cortex cell elongation toprocesses occurring in the vasculature is a fascinating exampleof mechanisms coordinating the growth of tissues in an organ.During early epidermis development ERECTA does not affectcell cycle progression but it does extend the proliferative windowof symmetric cell divisions and prevents premature celldifferentiation (Bundy et al. 2012). Whether ERECTA controlselongation of protodermal cells directly or indirectly has not beentested. However, ERECTA expression in that tissue at thatdevelopmental stage suggests that the regulation could bedirect. Interestingly, after epidermal and cortex cells differentiateand switch frommitosis to endoreduplication the role of ERECTAin regulation of cell elongation is greatly reduced (Bundyet al. 2012).

ERfs and Reproductive OrganDevelopment

ERf genes are critical for reproductive organ development, withthe er erl1 erl2mutant being bothmale and female sterile (Shpaket al. 2004). The role of ERfs in ovule development has beenstudied using er erl1 erl2/þ, where ERL2 is haploinsufficient forfemale fertility (Pillitteri et al. 2007a). Ovule development beginswith the formation of finger‐like primordia on the internal surfaceof carpels. Protodermal cells on each side of a primordium giverise to the inner and outer integuments, protective layers thatlater form the seed coat (Jenik and Irish 2000). An analysis ofearly ovule development in er erl1 erl2/þ did not detect anyabnormalities in the initiation and radial patterning of finger‐likeprimordia or in differentiation of megaspore mother cells (Pillitteriet al. 2007a). The first defects were detected at a later stage; inthe mutant both inner and outer integuments were able to initiate


but the cells in those tissues had limited proliferative ability,which led to formation of small ovules with exposed nucelli(Pillitteri et al. 2007a). In addition, the outer integument cells of ererl1 erl2/þ ovules are less organized but are more uniform inlength than in the wild type. Proper development of integumentsis essential for embryo sac formation (Gasser et al. 1998).Consistent with this, megagametogenesis in er erl1 erl2/þ isarrested. The lack of significant difference in the integument cellsize suggests that ERfsmight promote the rate of integument cellgrowth, which would accelerate the cell cycle. Thus, the role ofERfs appears to be similar in cells of protodermal origin whetherthey are in integuments or pedicels. While the er erl1 erl2/þmutant is female infertile, carpel patterning defects in thisgenotype are not as strong as in the er erl1 erl2 mutant,suggesting that the role of ERfs in carpels might not be limited tointegument growth (Shpak et al. 2004).

In integuments, ERfs transmit a signal using the same MAPkinase cascade as in the epidermis or vasculature. In the mpk6background MPK3 is haploinsufficient for female fertility and theovule phenotype of the mpk3/þ mpk6 mutant is identical to ererl1 erl2/þ (Wang et al. 2008). Indirect evidence also implicatesYDA and MKK4/MKK5 in regulation of integument growth.Female fertility of the yda mutant can be rescued by a gain‐of‐function mutant of MEK2 (GVG‐NtMEK2DD), the Nicotianatabacum ortholog of Arabidopsis MKK4 and MKK5 (Wanget al. 2007). In the transgenic GVG‐NtMEK2DD/yda lines whererescue is partial, integuments are short and the nucellus isexposed (Wang et al. 2008).

The impact of ERfs on stamen development is wide‐ranging.Given that stamen number in er erl1 erl2 flowers is significantlyreduced, ERfs must be needed for their initiation (Shpaket al. 2004; Hord et al. 2008). The stamens that do form in ererl1 erl2 either lack anthers or contain under‐differentiatedanthers with a reduced number of locules (Hord et al. 2008).During locule differentiation an archesporial cell undergoes aseries of asymmetric periclinal cell divisions to give rise to fourdifferent layers: endothecium, middle layer, tapetum and pollenmother cells (Sanders et al. 1999). All layers are able to form in ererl1erl2, but the middle layer and tapetum are larger and aredisorganized (Hord et al. 2008). Of the two layers, tapetum ismore strongly affectedwith a considerably higher number of cellsthat are radially larger in size. In rare circumstances er erl1 erl2can form viable pollen, but anthers never dehisce and pollengrains adhere to the anther walls. Similar to other plant organs,the ability of anthers to grow longitudinally in er erl1erl2 isseverely affected. Whether some of the observed defects ofstamen differentiation are related to ERfs’ role in the regulation ofproximodistal growth and inhibition of asymmetric cell divisionsis still an open question. Similar albeit slightly weaker stamendefects of the mpk3/þ mpk6 mutant suggest that MPK3 andMPK6 function downstream of ERfs in stamens (Hord et al.2008).

The described findings suggest that ERfs play prominentroles during development of reproductive tissues. However,multiple questions are still unanswered, such as the nature ofcell‐to‐cell communication enabled by ERfs and the identity ofthe downstream targets in those organs. The reduced fertilityof the epfl4 epfl5 epfl6 mutant suggests that at least some ofthese ligandsmight be sensed by ERfs in developing ovules andstamens (Abrash et al. 2011; Uchida et al. 2012a). In the future,analyses of their expression patterns in reproductive organsmight be helpful in determining the type of cell‐to‐cell communi-cation facilitated by ERfs.

ERfs Regulate Size of the ShootApical Meristem and Initiation ofLeaf Primordia

The shoot apical meristem (SAM) serves two major functions: itis a long‐term stem cell reservoir and a progenitor of almost allaboveground plant organs. This dual nature of the meristemdepends on tight coordination of cell behavior with a variety ofcell‐to‐cell communications. The CLAVATA‐WUSCHEL (CLV‐WUS) regulatory loop and cytokinin signaling regulate themaintenance of the central zone containing undifferentiatedslow‐dividing cells (Perales and Reddy 2012). In the peripheralzone of the SAM, the emergence of organ primordia dependsupon auxin signaling (Braybrook and Kuhlemeier 2010). Brassi-nosteroids are important for organ boundary formation (Gendronet al. 2012). ERf signaling is yet another pathway regulating SAMorganization.

ERfs are strongly expressed in the vegetative and reproduc-tive SAMand in developing leaf and flower primordia (Yokoyamaet al. 1998; Shpak et al. 2004; Chen et al. 2013; Uchidaet al. 2013). As these receptors are redundant in the SAM, theirfunction there has become evident only recently with analysis ofthe er erl1 erl2mutant. The meristem of the triple mutant is muchbroader and flatter compared to the wild type (Chen et al. 2013;Uchida et al. 2013). In addition, in er erl1 erl2 leaf initiation isreduced and phyllotaxy is irregular (Uchida et al. 2012b; Chenet al. 2013). In agreement with this last observation, inhibition ofERf signaling using truncated ERECTA protein decreases thenumber of leaves formed in tomatoes (Villagarcia et al. 2012).While increased meristem size in the mutant is associated withincreased expression of CLV3 and WUS, ERfs are not likely tobe part of the CLV‐WUS regulatory loop but might affect thissignaling pathway indirectly (Chen et al. 2013; Uchida et al.2013). While CLV‐WUS regulates cell division in the centralzone, the increased size of the SAM in er erl1 erl2 is caused byboth an increased number of cells and an increase in radial cellexpansion (Chen et al. 2013). Moreover, in contrast to er erl1 erl2the boost in meristem size of the clv3 mutant is associated withan increase in leaf initiation (Vernoux et al. 2000).


The initiation of leaves and the establishment of phyllotaxyare dependent upon the formation of auxin maxima in theperipheral zone of the SAM (Braybrook and Kuhlemeier 2010).Auxin accumulates in discrete areas of the L1 layer at themaximal distance from existing primordia and then moves intothe inner tissues, promoting the formation of the midvein andinducing the appearance of the primordium bulge. Auxintransport in the SAM relies heavily on the auxin efflux carrierPIN1 (Benková et al. 2003; Reinhardt et al. 2003). Recentevidence suggests that the reduced leaf initiation and abnormalphyllotaxy in er erl1 erl2might be caused by deficiencies in auxintransport (Chen et al. 2013). Analysis of DR5rev::GFP expres-sion in the mutant implied that auxin accumulates uniformly andat high levels in the L1 layer of the SAM, but does not move intointernal layers. While in wild type seedlings DR5rev::GFP isexpressed most strongly in the vasculature of hypocotyls, in ererl1 erl2 the expression there is practically absent (Chenet al. 2013). The changes in auxin distribution in er erl1 erl2are associated with altered expression of PIN1. PIN1 expressionis increased in the L1 layer of the er er1 erl2 SAM, with some ofthe PIN1 being in the plasma membrane and some in theputative vacuoles. Simultaneously, PIN1 expression is dramati-cally reduced in the procambium of mutant leaf primordia,suggesting that ERfs induce PIN1 expression there. Asexpression of PIN1 mRNA is not changed in the SAM of ererl1 erl2, ERfs appear to control PIN1 expression at theposttranscriptional level; the exact mechanism by which thisoccurs is not yet clear (Chen et al. 2013). The importance of ERfsfor auxin transport is further supported by an observedinefficiency in er erl1 erl2 phototropic bending, a developmentalresponse that depends on auxin transport (Chen et al. 2013).The altered structure of the er erl1 erl2meristem might also be aconsequence of deficient auxin transport, as evident by thesimilar phenotypes of er erl1 erl2 and pin1‐6 meristems; in bothcases meristems are bigger and cells in the L1 layer becomewider (Vernoux et al. 2000; Chen et al. 2013).

The ability of constitutively active YDA and bikinin to rescueleaf initiation in er erl1 erl2 suggests that a similar signalingcascade functions downstream of ERfs in the meristem as inother tissues (Chen et al. 2013). But the identity and expressionlocations of ligands sensed by ERfs in the SAM are not known.Future progress in this areamight help to determine the nature ofcell‐to‐cell communication enabled by ERfs in the meristem.Circumstantial evidence suggests that ERfs may regulate auxintransport outside of the SAM: in er mutants stem and pedicelelongation have enhanced sensitivity to auxin, and the expres-sion of multiple auxin‐inducible genes promoting abovegroundgrowth is reduced in er and in er erl1 erl2 (Woodward et al. 2005;Uchida et al. 2012a; Chen et al. 2013). However, more directdata are needed to answer this question convincingly.

To better understand the role ERfs play in leaf initiation andgrowth it might be valuable to explore their connection with the

AS1/AS2 transcription factor complex. AS1 and AS2 specify theadaxial polarity and promote leaf primordia identity by repressingmeristematic and boundary genes (Ori et al. 2000; Semiartiet al. 2001; Xu et al. 2003, 2008). AS2 has been shown topromote PIN1 expression in embryos and roots; if it has a similarrole in the SAM is not known (Rast and Simon 2012). Variousdata suggest that ERfs impinge on the AS1/AS2 pathway. The ermutation enhances developmental defects of as1 and as2mutants (Qi et al. 2004; Xu et al. 2003). During stamendevelopment ERf function is more critical for formation of adaxiallobes than abaxial, suggesting their involvement in promotion ofadaxial identity (Hord et al. 2008). Finally, the expression of AS1is dramatically reduced in er erl1 erl2mutants (Chen et al. 2013).However, whether ERfs regulate the activity/expression of AS1and AS2 directly or indirectly through promotion of leaf primordiainitiation and growth is currently not clear.

In addition to regulation of auxin transport, ERfs altersensitivity of the SAM to cytokinins. Involvement of ERfs inresponse to cytokinins first became evident from analysis of theuni‐1D mutant (Uchida et al. 2011). Uni‐1D is a gain‐of‐functionmutation in a gene encoding amember of the nucleotide‐bindingleucine‐rich repeat (NB‐LRR) protein family, which leads toincreased cytokinin accumulation (Igari et al. 2008). ERfmutations are able to rescue some of the uni‐1D phenotypes,which can be partially explained by a reduction of cytokininresponses (Uchida et al. 2012b). The reduced expression ofenzymes involved in cytokinin biosynthesis (CYP735A2 in ererl1/þ erl2 uni‐1D/þ versus uni‐1D/þ and LOG4 in er erl1 erl2 vs.wild type) indirectly indicates that ERfs might promote cytokininbiosynthesis (Uchida et al. 2012b, 2013). However, thisconclusion is not consistent with the increased size of the ererl1 erl2 meristem and elevated expression of WUS, both ofwhich are characteristics of mutants with increased cytokininaccumulation (Giulini et al. 2004; Gordon et al. 2009; Bartrinaet al. 2011; Chen et al. 2013; Uchida et al. 2013). A more directassessment of cytokinin levels in er erl1 erl2 is needed todetermine the impact of ERfs on cytokinin accumulation.

Cytokinins are important for the rate of aboveground organinitiation and for phyllotaxy although their impact on thoseprocesses is complex. Increased cytokinin levels in the amp1mutant are associated with a faster rate of leaf initiation andinduction of leaf initiation by light is strictly cytokinin dependent(Chaudhury et al. 1993; Yoshida et al. 2011). Decreasedexpression of cytokinin oxidases/dehydrogenases (CKXs),enzymes that degrade cytokinins, results in elevated levels ofcytokinin and increased rate of flower formation in both rice andArabidopsis (Ashikari et al. 2005; Bartrina et al. 2011).Decreased cytokinin levels in the atipt1 3 5 7 and atipt3 5 7mutants are associated with a decreased rate of leaf initiation(Miyawaki et al. 2006). In agreement with this positive role ofcytokinins, the treatment of er erl1 erl2with 6‐benzylaminopurinepromotes leaf initiation (Uchida et al. 2013). At the same time,


cytokinins have a negative impact on auxin transport; theydownregulate PIN1 expression in roots (Rûzicka et al. 2009;Marhavý et al. 2011; Zhang et al. 2011) and ABPH1, a negativeregulator of cytokinin signaling, is necessary forPIN1 expressionin leaf primordia (Lee et al. 2009). Further analysis of cytokininhomeostasis in er erl1erl2 might be helpful in establishingwhether ERfs regulate auxin transport through cytokininsignaling or by some independent pathway.

The Role of ERECTA in Responsesto Biotic and Abiotic Factors

ERfs regulate multiple aspects of plant morphology that areimportant for efficient plant responses to environmentalchanges. In light of this, it is not surprising that studies of naturalgenetic variation between Arabidopsis ecotypes have linkedERECTA to several environmental responses. The er mutationhas been found to alter circadian leaf movements, hyponasticpetiole growth, and shade avoidance response (Swarupet al. 1999; van Zanten et al. 2010a; Kasulin et al. 2013; Patelet al. 2013). Petioles in er are shorter than in the wild type andhave a reduced capacity for growth in response to environmentalsignals (Yokoyama et al. 1998; van Zanten et al. 2010a, 2010b).The effect of er on circadian leaf movements has been proposedto be due to the alteration in petiole growth (Swarup et al. 1999).In response to flooding, low light and shade, plants exhibithyponasty or upward bending of leaves caused by differentialelongation of petioles. Ethylene is an important trigger ofhyponastic responses in shade and during flooding, whilephytochrome B and cryptochrome 2 are required for hyponastyin low light (Millenaar et al. 2005, 2009). ERECTA promotes allhyponastic responses, whether induced by ethylene or triggeredby light conditions, but it does not change the production ofethylene or alter sensitivity to it, and ERECTA functionsindependently from phytochrome B and cryptochrome 2 (vanZanten et al. 2010a,b; Patel et al. 2013). In shade, ERECTA notonly regulates petiole angle but also the extent of its elongation,particularly at cool temperatures (Patel et al. 2013). ERECTAmost likely promotes hyponastic growth and petiole elongationdue to its effect on petiole morphology and/or the capacity ofpetiole cells to expand. A study of shade avoidance also showedthat ERECTA is important for environmental modulation offlowering time and the growth of hypocotyls and leaf blades(Kasulin et al. 2013). However, the effect of ermutation on thoseparameters is subtle and depends on the ecotype.

ERECTA not only modulates plant responses to abioticfactors but it is also involved in responses to pathogens.Arabidopsis plants carrying the ermutation aremore susceptibleto the bacterium Ralstonia solanacearum (Godiard et al. 2003),the necrotrophic fungus Plectosphaerella cucumerina (Llorenteet al. 2005; Sánchez‐Rodríguez et al. 2009), and the pathogenic

oomycete Pythium irregulare (Adie et al. 2007). The mechanismbywhichERECTAenhances plant disease resistance is not veryclear. Because of its impact on plant morphology and specificallyon structure of the epidermis and vasculature, ERECTA mightcontribute to disease resistance by reducing pathogen invasionand spread. Experimental evidence supporting this idea is,however, currently lacking. It has been demonstrated thatERECTA contributes to other aspects of disease resistance.When wild type and er plants were inoculated with Ralstoniasolanacearum by cutting and dipping their roots in bacterialsuspensions, root invasion and progression towards aerialorgans were not affected, suggesting that ERECTA is importantin later stages of colonization (Godiard et al. 2003). Duringinfection by Plectosphaerella cucumerina, ERECTA is neces-sary for pathogen‐induced callose deposition but not for otherdefense responses such as induction of PR1 and PDF1.2 geneexpression or production of reactive oxygen species (Llorente etal. 2005). Two mutations, ser 1 and ser 2, restore diseaseresistance of er to Plectosphaerella cucumerina (Sánchez‐Rodríguez et al. 2009). These mutations modify er cell wallcomposition, making it more similar to the wild type, but they donot suppress the developmental defects of er. Thus, the majorcause of reduced disease resistance of er mutants toPlectosphaerella cucumerina was proposed to be due tochanges in the cell wall composition instead of changes in plantmorphology (Sánchez‐Rodríguez et al. 2009). While ERECTApromotes resistance to a broad range of pathogens includingbacteria, fungi and oomycetes, it affects susceptibility only toselected species within each group (Sánchez‐Rodríguez et al.2009). In the future it would be interesting to see whether someaspect of pathogenicity is common between all of these species.

Future Perspectives

During the last decade remarkable progress has been made inunderstanding ERf signaling and functions. Many ligands anddownstream effectors of ERfs have been identified. Since itsappearance in early land plants, ERf‐mediated signaling hasbeen used to enable communication between a variety of celltypes in different aboveground tissues. One universal role ERfsplay is to promote polarized cell growth. When ERfs are absent,cells become rounder and growmore slowly, which extends theircell cycle duration. This phenotype has been observed inepidermis, cortex, vasculature, and during integument develop-ment. The second role of ERfs is to control cell fate transitions inepidermis and possibly in other tissues.

However, many questions remain to be answered. Themechanism of signal transfer from ERfs to YDA is vague and allconjunctions presented here are hypothetical. Besides SPCH andMUTE there should be other targets of ERf signaling that arestill unknown. While ERfs have a strong impact on auxin and


cytokinin signaling, the precise mechanistic understanding of howthey modify those pathways is lacking. We are also only justbeginning to understand the role of ERfs in the shoot apicalmeristem, in vasculature, in reproductive tissues, and in responseto pathogens.

Acknowledgements

This work was supported by the National Science Foundation(IOS‐0843340 to E.S.).

Received 6 Jun. 2013 Accepted 3 Sept. 2013

References

Abraham MC, Metheetrairut C, Irish VF (2013) Natural variation

identifies multiple loci controlling petal shape and size in Arabidopsis

thaliana. PLoS One 8, e56743.

Abrash EB, Davies KA, Bergmann DC (2011) Generation of signaling

specificity in Arabidopsis by spatially restricted buffering of ligand‐

receptor interactions. Plant Cell 23, 2864–2879.

Adie BAT, Pérez‐Pérez J, Pérez‐Pérez MM, Godoy M, Sánchez‐

Serrano JJ, Schmelz EA, Solano R (2007) ABA is an essential

signal for plant resistance to pathogens affecting JA biosynthesis and

the activation of defenses in Arabidopsis. Plant Cell 19, 1665–1681.

Ashikari M, Sakakibara H, Lin S, Yamamoto T, Takashi T, Nishimura

A, Angeles ER, Qian Q, Kitano H, Matsuoka M (2005) Cytokinin

oxidase regulates rice grain production. Science 309, 741–745.

Bartrina I, Otto E, Strnad M, Werner T, Schmülling T (2011) Cytokinin

regulates the activity of reproductive meristems, flower organ size,

ovule formation, and thus seed yield in Arabidopsis thaliana. Plant

Cell 23, 69–80.

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.

Benková E, Michniewicz M, Sauer M, Teichmann T, Seifertová D,

Jürgens G, Friml J (2003) Local, efflux‐dependent auxin gradients

as a common module for plant organ formation. Cell 115, 591–602.

Berger D, Altmann T (2000) A subtilisin‐like serine protease involved in

the regulation of stomatal density and distribution in Arabidopsis

thaliana. Genes Dev. 14, 1119–1131.

Bergmann DC, Lukowitz W, Somerville CR (2004) Stomatal develop-

ment and pattern controlled by aMAPKKKinase.Science 304, 1494–

1497.

Braybrook SA, Kuhlemeier C (2010) How a plant builds leaves. Plant

Cell 22, 1006–1018.

Bundy MGR, Thompson OA, Sieger MT, Shpak ED (2012) Patterns of

cell division, cell differentiation and cell elongation in epidermis and

cortex of Arabidopsis pedicels in the wild type and in erecta. PLoS

One 7, e46262.

Chaudhury AM, LethamS, Craig S, Dennis ES (1993) amp 1 ‐ amutant

with high cytokinin levels and altered embryonic pattern, faster

vegetative growth, constitutive photomorphogenesis and precocious

flowering. Plant J. 4, 907–916.

Chen L, Song Y, Li S, Zhang L, Zou C, Yu D (2012) The role of WRKY

transcription factors in plant abiotic stresses. Biochim. Biophys. Acta

1819, 120–128.

Chen MK, Wilson RL, Palme K, Ditengou FA, Shpak ED (2013)

ERECTA family genes regulate auxin transport in the shoot apical

meristem and forming leaf primordia. Plant Physiol. 162, 1978–

1991.

De Rybel B, Audenaert D, Vert G, RozhonW, Mayerhofer J, Peelman

F, Coutuer S, Denayer T, Jansen L, Nguyen L, Vanhoutte I,

Beemster GTS, Vleminckx K, Jonak C, Chory J, Inzé D,

Russinova E, Beeckman T (2009) Chemical inhibition of a subset

of Arabidopsis thaliana GSK3‐like kinases activates brassinosteroid

signaling. Chem. Biol. 16, 594–604.

Douglas SJ, Riggs CD (2005) Pedicel development in Arabidopsis

thaliana: Contribution of vascular positioning and the role of the

BREVIPEDICELLUS and ERECTA genes. Dev. Biol. 284, 451–

463.

Etchells JP, Provost CM, Mishra L, Turner SR (2013) WOX4 and

WOX14 act downstream of the PXY receptor kinase to regulate plant

vascular proliferation independently of any role in vascular organiza-

tion. Development 140, 2224–2234.

Facette MR, Smith LG (2012) Division polarity in developing stomata.

Curr. Opin. Plant Biol. 15, 585–592.

Gasser CS, Broadhvest J, Hauser BA (1998) Genetic analysis of ovule

development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 1–24.

Geisler M, Yang M, Sack FD (1998) Divergent regulation of stomatal

initiation and patterning in organ and suborgan regions of the

Arabidopsismutants toomanymouths and four lips.Planta 205, 522–

530.

Gendron JM, Liu JS, Fan M, Bai MY, Wenkel S, Springer PS, Barton

MK, Wang ZY (2012) Brassinosteroids regulate organ boundary

formation in the shoot apical meristem of Arabidopsis. Proc. Natl.

Acad. Sci. USA 109, 21152–21157.

Giulini A, Wang J, Jackson D (2004) Control of phyllotaxy by the

cytokinin‐inducible response regulator homologueABPHYL1.Nature

430, 1031–1034.

Godiard L, Sauviac L, Torii KU, Grenon O, Mangin B, Grimsley NH,

Marco Y (2003) ERECTA, an LRR receptor‐like kinase protein

controlling development pleiotropically affects resistance to bacterial

wilt. Plant J. 36, 353–365.

GordonSP,ChickarmaneVS,OhnoC,Meyerowitz EM (2009)Multiple

feedback loops through cytokinin signaling control stem cell number

within the Arabidopsis shoot meristem. Proc. Natl. Acad. Sci. USA

106, 16529–16534.

Gudesblat GE, Schneider‐Pizon J, Betti C, Mayerhofer J, Vanhoutte

I, van Dongen W, Boeren S, Zhiponova M, de Vries S, Jonak C,

Russinova E (2012) SPEECHLESS integrates brassinosteroid and

stomata signalling pathways. Nat. Cell Biol. 14, 548–554.


Hara K, Kajita R, Torii KU, Bergmann DC, Kakimoto T (2007) The

secretory peptide gene EPF1 enforces the stomatal one‐cell‐spacing

rule. Genes Dev. 21, 1720–1725.

Hara K, Yokoo T, Kajita R, Onishi T, Yahata S, PetersonKM, Torii KU,

Kakimoto T (2009) Epidermal cell density is autoregulated via a

secretory peptide, EPIDERMAL PATTERNING FACTOR 2 in

Arabidopsis leaves. Plant Cell Physiol. 50, 1019–1031.

Hord CLH, Sun YJ, Pillitteri LJ, Torii KU, Wang H, Zhang S, Ma H

(2008) Regulation of Arabidopsis early anther development by the

mitogen‐activated protein kinases, MPK3 and MPK6, and the

ERECTA and related receptor‐like kinases. Mol. Plant 1, 645–658.

Hunt L,Gray JE (2009) TheSignaling peptide EPF2 controls asymmetric

cell divisions during stomatal development. Curr. Biol. 19, 864–

869.

Hunt L, Bailey KJ, Gray JE (2010) The signalling peptide EPFL9 is a

positive regulator of stomatal development. New Phytol. 186, 609–

614.

Igari K, Endo S, Hibara K, Aida M, Sakakibara H, Kawasaki T, Tasaka

M (2008) Constitutive activation of a CC‐NB‐LRR protein alters

morphogenesis through the cytokinin pathway in Arabidopsis. Plant

J. 55, 14–27.

Ishihama N, Yoshioka H (2012) Post‐translational regulation of WRKY

transcription factors in plant immunity.Curr. Opin. Plant Biol. 15, 431–

437.

Jenik PD, Irish VF (2000) Regulation of cell proliferation patterns by

homeotic genes duringArabidopsis floral development.Development

127, 1267–1276.

Kanaoka MM, Pillitteri LJ, Fujii H, Yoshida Y, Bogenschutz NL,

Takabayashi J, Zhu JK, Torii KU (2008) SCREAM/ICE1 and

SCREAM2 specify three cell‐state transitional steps leading to

Arabidopsis stomatal differentiation. Plant Cell 20, 1775–1785.

Karve R, Liu W,Willet SG, Torii KU, Shpak ED (2011) The presence of

multiple introns is essential for ERECTA expression in Arabidopsis.

RNA 17, 1907–1921.

Kasulin L, Agrofoglio Y, Botto JF (2013) The receptor‐like kinase

ERECTA contributes to the shade‐avoidance syndrome in a

background‐dependent manner. Ann. Bot. 111, 811–819.

Khan M, Rozhon W, Bigeard J, Pflieger D, Husar S, Pitzschke A,

Teige M, Jonak C, Hirt H, Poppenberger B (2013) Brassinosteroid‐

regulated GSK3/Shaggy‐like kinases phosphorylate mitogen‐acti-

vated protein (MAP) kinase kinases, which control stomata develop-

ment in Arabidopsis thaliana. J. Biol. Chem. 288, 7519–7527.

Kim TW, Michniewicz M, Bergmann DC, Wang ZY (2012) Brassinos-

teroid regulates stomatal development by GSK3‐mediated inhibition

of a MAPK pathway. Nature 482, 419–422.

Kondo T, Kajita R, Miyazaki A, Hokoyama M, Nakamura‐Miura T,

Mizuno S, Masuda Y, Irie K, Tanaka Y, Takada S, Kakimoto T,

Sakagami Y (2010) Stomatal density is controlled by a mesophyll‐

derived signaling molecule. Plant Cell Physiol. 51, 1–8.

Lampard GR, MacAlister CA, Bergmann DC (2008) Arabidopsis

stomatal initiation is controlled by MAPK‐mediated regulation of the

bHLH SPEECHLESS. Science 322, 1113–1116.

Lau OS, Bergmann DC (2012) Stomatal development: A plant’s

perspective on cell polarity, cell fate transitions and intercellular

communication. Development 139, 3683–3692.

Lee BH, Johnston R, Yang Y, Gallavotti A, Kojima M, Travençolo

BAN, Costa L, da F, Sakakibara H, Jackson D (2009) Studies of

aberrant phyllotaxy1 mutants of maize indicate complex interactions

between auxin and cytokinin signaling in the shoot apical meristem.

Plant Physiol. 150, 205–216.

Lee JS, Kuroha T, Hnilova M, Khatayevich D, Kanaoka MM, McAbee

JM, Sarikaya M, Tamerler C, Torii KU (2012) Direct interaction of

ligand–receptor pairs specifying stomatal patterning.Genes Dev. 26,

126–136.

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. J. Integr. Plant Biol. DOI: 10.1111/jipb.

12071.

Llorente F, Alonso‐Blanco C, Sánchez‐Rodriguez C, Jorda L, Molina

A (2005) ERECTA receptor‐like kinase and heterotrimeric G protein

from Arabidopsis are required for resistance to the necrotrophic

fungus Plectosphaerella cucumerina. Plant J. 43, 165–180.

MacAlister CA, Ohashi‐Ito K, Bergmann DC (2007) Transcription

factor control of asymmetric cell divisions that establish the stomatal

lineage. Nature 445, 537–540.

Marhavý P, Bielach A, Abas L, Abuzeineh A, Duclercq J, Tanaka H,

ParezováM, Petrásek J, Friml J, Kleine‐Vehn J, BenkováE (2011)

Cytokinin modulates endocytic trafficking of PIN1 auxin efflux carrier

to control plant organogenesis. Dev. Cell 21, 796–804.

Masle J, Gilmore SR, Farquhar GD (2005) The ERECTA gene

regulates plant transpiration efficiency in Arabidopsis. Nature 436,

866–870.

Meng X, Wang H, He Y, Liu Y, Walker JC, Torii KU, Zhang S (2012) A

MAPK cascade downstream of ERECTA receptor‐like protein kinase

regulates Arabidopsis inflorescence architecture by promoting

localized cell proliferation. Plant Cell 24, 4948–4960.

Millenaar FF, Cox MC, van Berkel YE, Welschen RA, Pierik R,

Voesenek LA, Peeters AJ (2005) Ethylene‐induced differential

growth of petioles in Arabidopsis. Analyzing natural variation,

response kinetics, and regulation. Plant Physiol. 137, 998–

1008.

Millenaar FF, van Zanten M, CoxMC, Pierik R, Voesenek LA, Peeters

AJ (2009) Differential petiole growth in Arabidopsis thaliana:

Photocontrol and hormonal regulation. New Phytol. 184, 141–152.

Miyawaki K, Tarkowski P, Matsumoto‐Kitano M, Kato T, Sato S,

Tarkowska D, Tabata S, Sandberg G, Kakimoto T (2006) Roles of

Arabidopsis ATP/ADP isopentenyltransferases and tRNA isopente-

nyltransferases in cytokinin biosynthesis. Proc. Natl. Acad. Sci. USA

103, 16598–16603.

Nadeau JA, Sack FD (2002) Control of stomatal distribution on the

Arabidopsis leaf surface. Science 296, 1697–1700.

Ohashi‐ItoK, BergmannDC (2006)Arabidopsis FAMAcontrols the final

proliferation/differentiation switch during stomatal development.

Plant Cell 18, 2493–2505.


Ohki S, Takeuchi M, Mori M (2011) The NMR structure of stomagen

reveals the basis of stomatal density regulation by plant peptide

hormones. Nat. Commun. 2, 512.

Ori N, EshedY, ChuckG, Bowman JL, HakeS (2000)Mechanisms that

control knox gene expression in the Arabidopsis shoot.Development

127, 5523–5532.

Patel D, Basu M, Hayes S, Majláth I, Hetherington FM, Tschaplinski

TJ, Franklin KA (2013) Temperature‐dependent shade avoidance

involves the receptor‐like kinase ERECTA. Plant J. 73, 980–992.

Perales M, Reddy GV (2012) Stem cell maintenance in shoot apical

meristems. Curr. Opin. Plant Biol. 15, 10–16.

Peterson KM, Rychel AL, Torii KU (2010) Out of the mouths of plants:

The molecular basis of the evolution and diversity of stomatal

development. Plant Cell 22, 296–306.

Pillitteri LJ, Bemis SM, Shpak ED, Torii KU (2007a) Haploinsufficiency

after successive loss of signaling reveals a role for ERECTA‐family

genes in Arabidopsis ovule development. Development 134, 3099–

3109.

Pillitteri LJ, Sloan DB, Bogenschutz NL, Torii KU (2007b) Termination

of asymmetric cell division and differentiation of stomata.Nature 445,

501–505.

Pillitteri LJ, Bogenschutz NL, Torii KU (2008) The bHLH protein,

MUTE, controls differentiation of stomata and the hydathode pore in

Arabidopsis. Plant Cell Physiol. 49, 934–943.

Pillitteri LJ, Torii KU (2012) Mechanisms of stomatal development.

Annu. Rev. Plant Biol. 63, 591–614.

Pires‐daSilva A, Sommer RJ (2003) The evolution of signalling

pathways in animal development. Nat. Rev. Genet. 4, 39–49.

Qi Y, Sun Y, Xu L, Xu Y, Huang H (2004) ERECTA is required for

protection against heat‐stress in the AS1/AS2 pathway to regulate

adaxial–abaxial leaf polarity in Arabidopsis. Planta 219, 270–276.

Ragni L, Nieminen K, Pacheco‐Villalobos D, Sibout R, Schwech-

heimer C, Hardtke CS (2011) Mobile gibberellin directly stimulates

Arabidopsis hypocotyl xylem expansion. Plant Cell 23, 1322–1336.

Rast MI, Simon R (2012) Arabidopsis JAGGED LATERAL ORGANS

acts with ASYMMETRIC LEAVES2 to coordinate KNOX and PIN

expression in shoot and root meristems. Plant Cell 24, 2917–2933.

Reinhardt D, Pesce ER, Stieger P, Mandel T, Baltensperger K,

Bennett M, Traas J, Friml J, Kuhlemeier C (2003) Regulation of

phyllotaxis by polar auxin transport. Nature 426, 255–260.

Rédei GP (1992) A heuristic glance at the past of Arabidopsis genetics.

In: Koncz C, Chua N. H, Schell J, eds. Methods in Arabidopsis

Research. World Scientific Press Inc., Singapore. pp. 1–15.

Rûzicka K, Simásková M, Duclercq J, Petrasek J, Zazímalová E,

Simon S, Friml J, Van Montagu MCE, Benková E (2009) Cytokinin

regulates root meristem activity via modulation of the polar auxin

transport. Proc. Natl. Acad. Sci. USA 106, 4284–4289.

Sánchez‐RodríguezC, Estévez JM, Llorente F, Hernández‐BlancoC,

Jordá L, Pagán I, Berrocal M, Marco Y, Somerville S, Molina A

(2009) The ERECTA receptor‐like kinase regulates cell wall‐

mediated resistance to pathogens in Arabidopsis thaliana. Mol.

Plant Microbe Interact. 22, 953–963.

Sanders PM, Bui AQ, Weterings K, McIntire KN, Hsu YC, Lee PY,

Truong MT, Beals TP, Goldberg RB (1999) Anther developmental

defects in Arabidopsis thaliana male‐sterile mutants. Sex. Plant

Reprod. 11, 297–322.

Semiarti E, Ueno Y, Tsukaya H, Iwakawa H, Machida C, Machida Y

(2001) The ASYMMETRIC LEAVES2 gene of Arabidopsis thaliana

regulates formation of a symmetric lamina, establishment of venation

and repression of meristem‐related homeobox genes in leaves.

Development 128, 1771–1783.

Shimada T, Sugano S, Hara‐Nishimura I (2011) Positive and negative

peptide signals control stomatal density.Cell. Mol. Life Sci. 68, 2081–

2088.

Shpak ED, Lakeman MB, Torii KU (2003) Dominant‐negative receptor

uncovers redundancy in the Arabidopsis ERECTA Leucine‐rich

repeat receptor‐like kinase signaling pathway that regulates organ

shape. Plant Cell 15, 1095–1110.

Shpak ED, Berthiaume CT, Hill EJ, Torii KU (2004) Synergistic

interaction of three ERECTA‐family receptor‐like kinases controls

Arabidopsis organ growth and flower development by promoting cell

proliferation. Development 131, 1491–1501.

Shpak ED, McAbee JM, Pillitteri LJ, Torii KU (2005) Stomatal

patterning and differentiation by synergistic interactions of receptor

kinases. Science 309, 290–293.

Sugano SS, Shimada T, Imai Y, Okawa K, Tamai A, Mori M, Hara‐

Nishimura I (2010) Stomagen positively regulates stomatal density

in Arabidopsis. Nature 463, 241–244.

Swarup K, Alonso‐Blanco C, Lynn JR, Michaels SD, Amasino RM,

Koornneef M, Millar AJ (1999) Natural allelic variation identifies new

genes in the Arabidopsis circadian system. Plant J. 20, 67–77.

Tang W, Kim TW, Oses‐Prieto JA, Sun Y, Deng Z, Zhu S, Wang R,

Burlingame AL, Wang ZY (2008) BSKs mediate signal transduction

from the receptor kinase BRI1 in Arabidopsis. Science 321, 557–

560.

Terpstra IR, Snoek LB, Keurentjes JJ, Peeters AJM, Van den

Ackerveken G (2010) Regulatory network identification by genetical

genomics: Signaling downstream of the Arabidopsis receptor‐like

kinase ERECTA. Plant Physiol. 154, 1067–1078.

Torii KU (2012) Mix‐and‐match: Ligand‐receptor pairs in stomatal

development and beyond. Trends Plant Sci. 17, 711–719.

Torii KU, Mitsukawa N,Oosumi T, Matsuura Y, YokoyamaR,Whittier

RF, Komeda Y (1996) The Arabidopsis ERECTA gene encodes a

putative receptor protein kinase with extracellular leucine‐rich

repeats. Plant Cell 8, 735–746.

Torii KU, Hanson LA, Josefsson CA, Shpak ED (2003) Regulation of

inflorescence architecture and organ shape by the ERECTA gene in

Arabidopsis. In: Sekimura T, ed. Morphogenesis and Patterning

in Biological Systems. Springer‐Verlag, Japan, Tokyo. pp. 153–

164.

Uchida N, Tasaka M (2013) Regulation of plant vascular stem cells by

endodermis‐derived EPFL‐family peptide hormones and phloem‐

expressed ERECTA‐family receptor kinases. J. Exp. Bot. DOI:

10.1093/jxb/ert196.


Uchida N, Igari K, Bogenschutz NL, Torii KU, Tasaka M (2011)

Arabidopsis ERECTA‐family receptor kinases mediate morphologi-

cal alterations stimulated by activation of NB‐LRR‐type UNI proteins.

Plant Cell Physiol. 52, 804–814.

Uchida N, Lee JS, Horst RJ, Lai HH, Kajita R, Kakimoto T, Tasaka M,

Torii KU (2012a) Regulation of inflorescence architecture by

intertissue layer ligand‐receptor communication between endoder-

mis and phloem. Proc. Natl. Acad. Sci. USA 109, 6337–6342.

Uchida N, Shimada M, Tasaka M (2012b) Modulation of the balance

between stem cell proliferation and consumption by ERECTA‐family

genes. Plant Signal. Behav. 7, 1506–1508.

Uchida N, Shimada M, Tasaka M (2013) ERECTA‐family receptor

kinases regulate stem cell homeostasis via buffering its cytokinin

responsiveness in the shoot apical meristem. Plant Cell Physiol. 54,

343–351.

van Zanten M, Basten Snoek L, Eck‐Stouten E, Proveniers MC, Torii

KU, Voesenek LA, Peeters AJ, Millenaar FF (2010a) Ethylene‐

induced hyponastic growth in Arabidopsis thaliana is controlled by

ERECTA. Plant J. 61, 83–95.

van Zanten M, Snoek LB, Eck‐Stouten E, Proveniers CG, Torii KU,

Voesenek LA, Millenaar FF, Peeters AJ (2010b) ERECTA controls

low light intensity‐induced differential petiole growth independent of

phytochrome B and cryptochrome 2 action in Arabidopsis thaliana.

Plant Signal. Behav. 5, 284–286.

Vernoux T, Kronenberger J, Grandjean O, Laufs P, Traas J (2000)

PIN‐FORMED 1 regulates cell fate at the periphery of the shoot apical

meristem. Development 127, 5157–5165.

Villagarcia H, Morin AC, Shpak ED, Khodakovskaya MV (2012)

Modification of tomato growth by expression of truncated

ERECTA protein from Arabidopsis thaliana. J. Exp. Bot. 63, 6493–

6504.

Wang H, Ngwenyama N, Liu Y, Walker JC, Zhang S (2007) Stomatal

development and patterning are regulated by environmentally

responsive mitogen‐activated protein kinases in Arabidopsis. Plant

Cell 19, 63–73.

WangH, Liu Y, Bruffett K, Lee J, Hause G,Walker JC, Zhang S (2008)

Haplo‐insufficiency ofMPK3 inMPK6mutant background uncovers a

novel function of these two MAPKs in Arabidopsis ovule develop-

ment. Plant Cell 20, 602–613.

Wang ZY, Bai MY, Oh E, Zhu JY (2012) Brassinosteroid signaling

network and regulation of photomorphogenesis. Annu. Rev. Genet.

46, 701–724.

Wengier DL, Bergmann DC (2012) On fate and flexibility in

stomatal development. Cold Spring Harb. Symp. Quant. Biol. 77,

53–62.

Woodward C, Bemis SM, Hill EJ, Sawa S, Koshiba T, Torii KU (2005)

Interaction of auxin and ERECTA in elaborating Arabidopsis

inflorescence architecture revealed by the activation tagging of a

new member of the YUCCA family putative flavin monooxygenases.

Plant Physiol. 139, 192–203.

Xu L, Xu Y, Dong A, Sun Y, Pi L, Xu Y, Huang H (2003) Novel as1 and

as2 defects in leaf adaxial‐abaxial polarity reveal the requirement for

ASYMMETRIC LEAVES1 and 2 andERECTA functions in specifying

leaf adaxial identity. Development 130, 4097–4107.

Xu B, Li Z, Zhu Y,Wang H, Ma H, Dong A, Huang H (2008) Arabidopsis

genes AS1, AS2, and JAG negatively regulate boundary‐specifying

genes to promote sepal and petal development. Plant Physiol. 146,

566–575.

Yang M, Sack FD (1995) The too many mouths and four lips mutations

affect stomatal production in Arabidopsis. Plant Cell 7, 2227–2239.

Yokoyama R, Takahashi T, Kato A, Torii KU, Komeda Y (1998) The

Arabidopsis ERECTA gene is expressed in the shoot apical meristem

and organ primordia. Plant J. 15, 301–310.

Yoshida S, Mandel T, Kuhlemeier C (2011) Stem cell activation by light

guides plant organogenesis. Genes Dev. 25, 1439–1450.

ZhangW, To JPC, Cheng CY, Eric Schaller G, Kieber JJ (2011) Type‐

A response regulators are required for proper root apical meristem

function through post‐transcriptional regulation of PIN auxin efflux

carriers. Plant J. 68, 1–10.

(Co‐Editor: Jia Li)


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Diverse Roles of ERECTA Family Genes in Plant Development