CLE Peptides in Vascular Development

Yi Qiang,Jinbin Wu,Huibin Han and Guodong Wang

Key Laboratory of Medicinal Resources and Natural Pharmaceutical Chemistry,Ministry of Education,National Engineering Laboratory for Resource Developing of Endangered Chinese Crude Drugs in Northwest of China,College of Life Sciences,Shaanxi Normal University,Xi’an 710062,China

Introduction

Cell-cell communication via peptides is essential for plant growth,development and responses to environmental cues.To date,only a few secreted peptides have been characterized as extracellular ligands that functionally interact with cell surface receptors to trigger signaling cascades leading to proper cellular functions(Murphy et al.2012).A number of Arabidopsis CLAVATA3/Embryo Surrounding Region-related(CLE)peptides,normally derived from the conserved C-terminal CLE motif of their precursors,coordinate stem cell fate in different types of plant meristems(Fiers et al.2007;Hirakawa et al.2010a,2011;Betsuyaku et al.2011).The mature CLV3 peptides are 12-AA hydroxylated peptides or 13-AA arabinosylated peptides(Gao and Guo 2012;Matsubayashi 2012).It has been recognized that leucine-rich repeat receptor-like kinases(LRR-RLK)are commonly receptors for CLE peptides,and WUSCHEL-related HOMEOBOX(WOX)transcription factors are downstream targets in CLE signaling pathways(Betsuyaku et al.2012;Murphy et al.2012).Accumulating evidence suggests that the CLE-RLK-WOX module appears to be conserved in different types of meristems including the shoot apical meristem(SAM),the root apical meristem(RAM),and the vascular meristem(Betsuyaku et al.2012;Murphy et al.2012).

The CLV3 peptide,a key player in stem cell homoeostasis at the SAM,is perceived by three parallel receptor complexes:CLV1,CLV2-CORYNE(CRN)/SUPPRESSOR OF LLP1–2(SOL2),and RECEPTOR-LIKE PROTEIN KINASE 2(RPK2)(Bleckmann et al.2010;Guo et al.2010;Kinoshita et al.2010;Zhu et al.2010).A negative feedback loop maintains a balanced stem cell population where CLV3 restricts expression of the stem cell-promoting transcription factor WUSCHEL(WUS),and WUS activates CLV3 expression(Brand et al.2000;Schoof et al.2000).

Overexpression of many CLE genes including CLV3,CLE19 and CLE40,or exogenously applied chemically synthesized peptides leads to an arrest of root growth,while clv2,crn/sol2,and rpk2 mutations can suppress the arrested short-root phenotype(Fiers et al.2005;Strabala et al.2006;Miwa et al.2008;Müller et al.2008;Kinoshita et al.2007,2010).The cle40 mutant exhibits abnormal cell patterning and a short-root phenotype with an enlarged WOX5 expression domain(Stahl et al.2009).The CLE40 peptide acts through the receptor-like kinase ACR4 to repress the expression of WOX5(Stahl et al.2009).Similar to the CLV3-CLV-WUS signaling pathway in the SAM,a CLE40-ACR4-WOX5 pathway exists to regulate stem cell homeostasis in the RAM(Stahl et al.2009).

Recently,CLE8 has been shown to play crucial roles in embryo and endosperm development in Arabidopsis(Fiume and Fletcher 2012).CLE8 is specifically expressed in the endosperm and in the apical portion of young embryos.The cle8 mutant produces smaller and defective seeds/embryos,while transgenic lines overexpressing CLE8 produce slightly larger seeds/embryos.CLE8 promotes the expression of WOX8,which suggests that CLE8-WOX8 could form a signaling module to regulate seed growth and seed size(Fiume and Fletcher 2012).Currently,little is known about the potential receptor(s)for CLE8 regarding its role in embryogenesis.Specific RLKs have been identified as key regulators for normal embryo and seed development(Nodine et al.2011).Thus,it is likely that CLE8 signaling is perceived by one or more RLKs.This hypothesis is further supported by the fact that there is a strong similarity of binding affinities among different CLE peptides(Matsubayashi 2012;Shinohara et al.2012).

The plant vascular system,which connects different plant organs,typically consists of two conductive tissues named phloem and xylem,and a meristematic tissue named the(pro-)cambium.The(pro-)cambium is located between phloem and xylem,and is able to differentiate into phloem and xylem(Lucas 2010;Hirakawa et al.2011).A picture of the complex regulatory network that controls vascular development is emerging(Fukuda 2012).The progress is highlighted by the identification of many CLE genes that are involved in vascular development(Fiers et al.2004;Ito et al.2006;Hirakawa et al.2008;Whitford et al.2008;Kondo et al.2011).In particular,the characterization of the Tracheary Element Differentiation Inhibitory Factor(TDIF)/CLE41/CLE44-TDR/PXY-WOX4 signaling module has greatly advanced our understanding of vascular regulation(Hirakawa et al.2008;Hirakawa et al.2010a,2010b).Here,we review the recent advances in cell-cell communication via CLE peptides and their receptors in aspects of vascular development.

TDIF/CLE41/CLE44 Regulates Vascular Stem Cell Maintenance

Tracheary Element Differentiation Inhibitory Factor(TDIF),a CLE-like peptide,was originally isolated as a suppressor of tracheary element differentiation from Zinnia elegans mesophyll cell culture medium(Ito et al.2006).The functional TDIF is a 12-AA peptide with two hydroxyproline residues.TDIF is identical to the CLE domains of CLE41 and CLE44,and is highly homologous to those of CLE42 and CLE46(Ito et al.2006).The CLE family can be classified into two groups of A-type and B-type CLE peptides.The majority of CLE peptides,grouped as A-type CLE peptides,affect SAM and RAM development but show no TDIF-like activity(Kinoshita et al.2007;Whitford et al.2008).B-type CLE peptides such as TDIF,CLE41,CLE42,and CLE44 were able to inhibit xylem vessel differentiation,but had no effect on SAM and RAM.Consistently,transgenic plants overexpressing CLE41 and CLE44 exhibit a xylem vessel strand-discontinuous phenotype,which has also been observed in peptide-treated plants(Hirakawa et al.2008;Whitford et al.2008).In addition,both overexpression and exogenous treatments with TDIF-like peptides promote procambial cell proliferation,suggesting a dual role of TDIF-like peptides in vascular development(Hirakawa et al.2008;Whitford et al.2008).

TDR/PXY has been identified as the receptor for TDIF/CLE41/CLE44 by screening T-DNA insertion mutants for LRR-RLK genes(Fisher and Turner 2007;Hirakawa et al.2008).T-DNA insertion lines of the TDR/PXY gene were insensitive to TDIF/CLE41/CLE44(Hirakawa et al.2008).Biochemical analysis further confirmed that TDIF/CLE41/CLE44 bind specifically to TDR/PXY in vitro.However,the A-type peptides CLV3,CLE2,CLE9,and CLE19 do not bind to TDR/PXY(Hirakawa et al.2008).In addition,the tdr/pxy mutant exhibited formation of xylem vessels adjacent to phloem cells and reduced procambial cell proliferation,while no defects were observed in the SAM and the RAM(Fisher and Turner 2007;Hirakawa et al.2008;Etchells and Turner 2010).Collectively,these results confirm that TDR/PXY functions specifically as a receptor for TDIF.TDR/PXY is expressed primarily in the(pro-)cambium and CLE41 is expressed preferentially in phloem cells.Thus,CLE41,which is produced in the phloem,is perceived by TDR/PXY to fine-tune proliferation and xylem differentiation of procambial cells(Figure 1;Hirakawa et al.2008;Etchells and Turner 2010).

WUSCHEL-related HOMEOBOX4(WOX4),a gene controlling maintenance of the vascular cambium,was recently found to be a key target for proliferation but not for the differentiation of procambial cells into xylem in the TDIF/CLE41/CLE44 pathway(Hirakawa et al.2010b;Suer et al.2011).WOX4 expression is detected preferentially in procambium and cambium cells,which is similar to the expression pattern of TDR/PXY(Figure 1).Additionally,WOX4 expression is positively controlled by the TDIF/CLE41/CLE44 peptide in a TDR/PXY-dependent manner(Hirakawa et al.2010b).Overexpression of WOX4 does not affect expression levels of CLE41/44 or alter the phenotype mediated by CLE41/44(Hirakawa et al.2010b).Loss-of-function studies of WOX4 revealed that it is required for procambial/cambial cell proliferation,but not for inhibition of cambial cell differentiation into xylem cells.Thus,there are at least two TDIF/CLE41/CLE44-dependent pathways in the regulation of(pro-)cambial development.One of which,the TDIF/CLE41/CLE44-TDR/PXY-WOX4 pathway,promotes(pro-)cambial cell proliferation(Figure 1).Another pathway may target a yet unidentified component(s)to inhibit the differentiation of(pro-)cambial cells into xylem cells(Hirakawa et al.2010b).

Figure 1.The TDIF/CLE41/CLE44-TDR/PXY-WOX4 signaling pathway in vascular tissues.The TDIF/CLE41/CLE44 peptide secreted from phloem cells(lime),is perceived by TDR/PXY on(pro-)cambial cells(grey)to upregulate WOX4 expression.The signal then promotes(pro-)cambial cell proliferation and inhibits the differentiation of(pro-)cambial cells into xylem cells(green).The involvement of a xylem-derived signal(?)and its putative receptor(??)in controlling phloem development is an open question.The proposed TDIF/CLE41/CLE44-TDR/PXYWOX4 signaling module is shown on the right.There is no evidence for WOX4 controlling CLE41 expression(???).

Other CLE Peptides Control Vascular Development

The Brassica napus CLE19(BnCLE19)gene is expressed in organ primordia and in pericycle cells facing the protoxylem poles of the root hair region(Fiers et al.2004).Misexpression of BnCLE19 in Arabidopsis leads to the formation of disconnected xylem elements as well as vascular islands in flower buds,suggesting that CLE19 functions in early xylem development(Fiers et al.2004).Additionally,exogenously applied A-type CLE peptide CLE19,as well as CLE6 and CLV3,enhance the effects of B-type peptides TDIF/CLE41/CLE44 on vascular development(Whitford et al.2008).Consistently,transgenic plants simultaneously overexpressing CLE41 and CLE6 genes exhibit a massive proliferation of vascular cells(Whitford et al.2008).Exogenous application of an A-type CLE peptide alone normally results in suppression of stem cell proliferation in the SAM and RAM,while having no effect on vascular development(Kinoshita et al.2007;Whitford et al.2008).Thus,rather than an antagonistic relationship,A-type CLE peptides cooperate with TDIF/CLE41/CLE44 to enhance(pro-)cambium proliferation,suggesting a complex crosstalk of different CLE peptides in vascular development.

A number of A-type CLE peptides inhibit protoxylem element formation in Arabidopsis root when applied exogenously(Kondo et al.2011).One of these,CLE10,is preferentially expressed in root vascular tissues.In vitro application of CLE10 peptide or overexpression of CLE10 significantly represses protoxylem vessel formation in roots as well as root growth,which appears to be accomplished by repression of two Arabidopsis Response Regulator(ARR)genes,ARR5 and ARR6(Kondo et al.2011).Furthermore,it is thought that CLE10 acts through the CLV2 receptor,as the clv2 mutant exhibited insensitivity to CLE10 in the suppression of protoxylem vessel formation(Kondo et al.2011).

It has been shown that a number of CLE genes are preferentially expressed in vascular tissues by analysis of CLE promoter-driven reporters,suggesting that additional CLEs other than the aforementioned CLE peptides may play roles in vascular development(Jun et al.2010).It was reported previously that transcripts for a few CLE genes increase during xylogenesis,making them very good candidates for future investigation(Kubo et al.2005).Thus,the sophisticated synergistic and/or antagonistic interactions of various CLE peptides may orchestrate the well-organized formation of vascular tissues.Further studies to determine the functions of additional CLE genes that are expressed in vascular tissues will be crucial for our understanding of the roles of CLEs in vascular development.

Crosstalk Between CLE Peptides and Phytohormones in Vascular Development

Phytohormones(e.g.,auxin,cytokinins,and brassinosteroids)are important in mediating intercellular communications that control the differentiation and proliferation of vascular cells(Fukuda 2004;Dettmer et al.2009).With the recent advances in our understanding of CLE peptide function,a picture is emerging that vascular development is regulated through the crosstalk between CLE peptide-mediated pathways and phytohormone-mediated pathways.

As discussed previously,A-type CLEs strengthen the effect of B-type CLEs on procambium proliferation.In addition,the synergistic effect on procambium cell proliferation depends on auxin because it is enhanced in the presence of NAA,and is suppressed by the auxin transport inhibitor NPA(Whitford et al.2008).This indicates that auxin is necessary for enhancement of procambium proliferation mediated by CLE peptides.Taken together,it is apparent that there is not only crosstalk among different types of CLE peptides,but also crosstalk between CLE peptides and auxin.A plausible explanation for these observations is that procambial cells are specified by overlapping expression domains of A-type and B-type CLE peptides that are regulated by auxin.

The majority of A-type CLE peptides,including CLE10,can inhibit protoxylem vessel formation in Arabidopsis roots by specifically repressing the expression of ARR5 and ARR6 which are two negative regulators of cytokinin signaling(Kondo et al.2011).Indeed,the protoxylem vessel formation of lateral roots was strongly inhibited in arr5 arr6 double mutants.However,the double mutant arr10 arr12 is resistant to the CLE10 peptide in terms of protoxylem vessel formation(Kondo et al.2011).ARR10 and ARR12 are members of the type-B ARR family,which are positive regulators of cytokinin signaling(Yokoyama et al.2007).Additionally,mutants of cytokininrelated genes and cytokinin receptors exhibit increased protoxylem cell files and loss of other cell types(M?h?nen et al.2000;M?h?nen et al.2006).CLE10 overexpression and exogenously applied cytokinin and CLE10 strongly inhibit the formation of protoxylem vessels(Kondo et al.2011).Collectively,these data suggest that CLE10 regulates protoxylem vessel formation by activating cytokinin signaling through the repression of type-A ARRs.

In addition to auxin and cytokinin,brassinosteroids positively regulate xylem differentiation,as BR-signaling mutants exhibit a reduced number of vascular bundles and xylem vessels(Ca?o-Delgado et al.2004;Fukuda 2004).Furthermore,it was recently shown that auxin transport coupled with brassinosteroid signaling is required to establish a proper pattern of vascular bundles,strengthening the crucial role of brassinosteroids in vascular development(Iba?es et al.2009).However,little is currently known about the crosstalk between brassinosteroidmediated and CLE peptide-mediated signaling pathways in vascular development.

Conclusions and Future Perspectives

Over the past couple of years,our understanding of the functions of CLE ligands in vascular development has made significant progress.The finding of the TDIF/CLE41/CLE44-TDR/PXY-WOX4 module provides valuable insights into the regulatory mechanism and intercellular communication in vascular tissues.The TDIF/CLE41/CLE44 peptide,secreted from phloem,is perceived by TDR/PXY located in the plasma membrane of(pro-)cambial cells to regulate WOX4 expression.This signal then suppresses xylem cell differentiation of procambial cells,and promotes their proliferation(Figure 1;Hirakawa et al.2008;Hirakawa et al.2010b).However,this proposed regulatory model is likely missing xylem-derived intercellular signals for phloem-fate commitment.The involvement of a xylem-derived signal and its putative receptor in controlling phloem development remain unknown(Figure 1).Thus,further investigation is essential to identify such signal molecules.In addition,two LRRRLKs,MORE LATERAL GROWTH(MOL1)and REDUCED IN LATERAL GROWTH(RUL1),were lately revealed to antagonistically control cambium activity,in which MOL1 is a repressor and RUL1 is an activator(Agusti et al.2011).MOL1 negatively regulates the expression of RUL1,PXY and WOX4,indicating the existence of unidentified and possibly parallel RLK-mediated signaling pathways that regulate vascular development.It is plausible that CLE genes,which are preferentially expressed in vasculature,may be perceived by MOL1 or RUL1,as they are close homologues of TDR/PXY.

A few CLE genes other than TDIF/CLE41/CLE44,for instance CLE10,have been found to regulate various aspects of vascular development,which suggests a complicated intercellular communication network is involved in forming well-organized vascular tissues.Futhermore,the finding that many CLE genes are preferentially expressed in vasculature implies that additional CLE genes may be involved in regulating vascular development.However,the exact function of these CLE genes in vascular development remains to be elucidated.It is very challenging to study the function of CLE genes because of functional redundancies and difficulties in obtaining loss-of-function mutants.An antagonistic peptide technology is currently being established for the functional dissection of CLE genes(Song et al.2013).It is expected that this technology will potentially be applied to elucidate the role(s)of CLE genes during various developmental processes.

Based on limited findings,a picture of crosstalk between CLE peptides and phytohormones is emerging.For instance,CLE peptide-mediated procambial proliferation is enhanced in the presence of auxin.Additionally,CLE10 inhibits protoxylem vessel formation through the activation of cytokinin signaling.Despite these new insights,the interactions between CLE peptides and phytohormones remain largely unknown.The challenge in the future lies in understanding how CLE signals integrate with phytohormone signals to build the regulatory network that controls vascular development.

Acknowledgements

We thank Dr.Susan Urbanus(Ludwig-Maximilians University)for her critical reading of the manuscript.Research in our lab was supported by the National Natural Science Foundation of China(31271575;31200902),by the Fundamental Research Funds for the Central Universities(GK201103005),and by the Specialized Research Fund for the Doctoral Program of Higher Education from the Ministry of Education of China(20120202120009).

Agusti J,Lichtenberger R,Schwarz M,Nehlin L,Greb T(2011)Characterization of transcriptome remodeling during cambium formation identifies MOL1 and RUL1 as opposing regulators of secondary growth.PLoS Genet.7,e1001312.

Betsuyaku S,Sawa S,Yamada M(2011)The function of the CLE peptides in plant development and plant-microbe interactions.The Arabidopsis Book 9,e0149.doi:10.1199:tab.0149.

Bleckmann A,Weidtkamp-Peters S,Seidel CA,Simon R(2010)Stem cell signaling in Arabidopsis requires CRN to localize CLV2 to the plasma membrane.Plant Physiol.152,166–176.

Brand U,Fletcher JC,Hobe M,Meyerowitz EM,Simon R(2000)Dependence of stem cell fate in Arabidopsis on a feedback loop regulated by CLV3 activity.Science 289,617–619.

Ca?o-Delgado A,Yin Y,Yu K,Vafeados D,Mora-García S,Cheng JC,Nam KH,Li J,Chory J(2004)BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis.Development 131,5341–5351.

Dettmer J,Elo A,Helariutta Y(2009)Hormone interactions during vascular development.Plant Mol.Biol.69,347–360.

Etchells JP,Turner SR(2010)The PXY-CLE41 receptor ligand pair defines a multifunctional pathway that controls the rate and orientation of vascular cell division.Development 137,767–774.

Fiers M,Hause G,Boutilier K,Casamitjana-Martinez E,Weijers D,Offringa R,van der Geest L,van Lookeren Campagne M,Liu CM(2004)Mis-expression of the CLV3/ESR-like gene CLE19 in Arabidopsis leads to a consumption of root meristem.Gene 327,37–49.

Fiers M,Golemiec E,Xu J,van der Geest L,Heidstra R,Stiekema W,Liu CM(2005)The 14-Amino Acid CLV3,CLE19,and CLE40 Peptides trigger consumption of the root meristem in Arabidopsis through a CLAVATA2-dependent pathway.Plant Cell 17,2542–2553.

Fiers M,Ku KL,Liu CM(2007)CLE peptide ligands and their roles in establishing meristems.Curr.Opin.Plant Biol.10,39–43.

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

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

Fukuda H(2004)Signals that control plant vascular cell differentiation.Nat.Rev.Mol.Cell Biol.5,379–391.

Fukuda H(2012)Peptides regulating plant vascular development.In:Irving HR,Gehring C,eds.Plant Signaling Peptides,Signaling and Communication in Plants.Springer-Verlag,Berlin.pp.59–76.

Gao X,Guo Y(2012)CLE peptides in plants:Proteolytic processing,structure-activity relationship,and ligand-receptor interaction.J.Integr.Plant Biol.54,738–745.

Guo Y,Han L,Hymes M,Denver R,Clark SE(2010)CLAVATA2 forms a distinct CLE-binding receptor complex regulating Arabidopsis stem cell specification.Plant J.63,889–900.

Hirakawa Y,Shinohara H,Kondo Y,Inoue A,Nakanomyo I,Ogawa M,Sakagami Y(2008)Non-cell-autonomous control of vascular stem cell fate by a CLE peptide/receptor system.Proc.Natl.Acad.Sci.USA 105,15208–15213.

Hirakawa Y,Kondo Y,Fukuda H(2010a)Regulation of vascular development by CLE peptide-receptor systems.J.Integr.Plant Biol.52,8–16.

Hirakawa Y,Kondo Y,Fukuda H(2010b)TDIF peptide signaling regulates vascular stem cell proliferation via the WOX4 homeobox gene in Arabidopsis.Plant Cell Physiol.22,2618–2629.

Hirakawa Y,Kondo Y,Fukuda H(2011)Establishment and maintenance of vascular cell communities through local signaling.Curr.Opin.Plant Biol.14,17–23.

Iba?es M,F`abregas N,Chory J,Ca?o-Delgado AI(2009)Brassinosteroid signaling and auxin transport are required to establish the periodic pattern of Arabidopsis shoot vascular bundles.Proc.Natl.Acad.Sci.USA 106,13630–13635.

Ito Y,Nakanomyo I,Motose H,Iwamoto K,Sawa S,Dohmae N,Mimura T,Fukuda H,Demura T(2006)Dodeca-CLE peptides as suppressors of plant stem cell differentiation.Science 313,842–845.

Jun J,Fiume E,Roeder AHK,Meng L,Sharma VK,Osmont KS,Baker C,Ha CM,Meyerowitz EM,Feldman LJ,Fletcher JC(2010)Comprehensive analysis of CLE polypeptide signaling gene expression and overexpression activity in Arabidopsis.Plant Physiol.154,1721–1736.

Kinoshita A,Nakamura Y,Sasaki E,Kyozuka J,Fukuda H,Sawa S(2007)Gain-of-function phenotypes of chemically synthetic CLAVATA3/ESR-related(CLE)peptides in Arabidopsis thaliana and Oryza sativa.Plant Cell Physiol.48,1821–1825.

Kinoshita A,Betsuyaku S,Osakabe Y,Mizuno S,Nagawa S,Stahl Y,Simon R,Yamaguchi-Shinozaki K,Fukuda H,Sawa S(2010)RPK2 is an essential receptor-like kinase that transmits the CLV3 signal in Arabidopsis.Development 137,3911–3920.

Kondo Y,Hirakawa Y,Kieber JJ,Fukuda H(2011)CLE peptides can negatively regulate protoxylem vessel formation via cytokinin signaling.Plant Cell Physiol.52,37–48.

Kubo M,Udagawa M,Nishikubo N,Horiguchi G,Yamaguchi M,Ito J,Mimura T,Fukuda H,Demura T(2005)Transcription switches for protoxylem and metaxylem vessel formation.Genes Dev.19,1855–1860.

Lucas WJ(2010)Plant vascular biology and agriculture.J.Integr.Plant Biol.52,4–7.

M?h?nen AP,Bonke M,Kauppinen L,Riikonen M,Benfey PN,Helariutta Y(2000)A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root.Gene.Dev.14,2938–2943.

M?h?nen AP,Bishopp A,Higuchi M,Nieminen KM,Kinoshita K,T?orm?kangas K,Ikeda Y,Oka A,Kakimoto T,Helariutta Y(2006)Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development.Science 311,94–98.

Matsubayashi Y(2011)Small post-translationally modified peptide signals in Arabidopsis.The Arabidopsis Book 9,e0150.doi:10.1199:tab.0150.

Miwa H,Betsuyaku S,Iwamoto K,Kinoshita A,Fukuda H,Sawa S(2008)The receptor-like kinase SOL2 mediates CLE signaling in Arabidopsis.Plant Cell Physiol.49,1752–1757.

Müller R,Bleckmann A,Simon R(2008)The receptor kinase CORYNE of Arabidopsis transmits the stem cell-limiting signal CLAVATA3 independently of CLAVATA1.Plant Cell 20,934–946.

Murphy E,Smith S,De Smet I(2012)Small signaling peptides in Arabidopsis development:How cells communicate over a short distance.Plant Cell 24,3198–3217.

Nodine MD,Bryan AC,Racolta A,Jerosky KV,Tax FE(2011)A few standing for many:Embryo receptor-like kinases.Trends Plant Sci.16,211–217.

Shinohara H,Moriyama Y,Ohyama K,Matsubayashi Y(2012)Biochemical mapping of a ligand-binding domain within Arabidopsis BAM1 reveals diversified ligand recognition mechanisms of plant LRR-RKs.Plant J.70,845–854.

Schoof H,Lenhard M,Haecker A,Mayer KF,Jürgens G,Laux T(2000)The stem cell population of Arabidopsis shoot meristems in maintained by a regulatory loop between the CLAVATA and WUSCHEL genes.Cell 100,635–644.

Song XF,Guo P,Ren SC,Xu TT,Liu CM(2013)Antagonistic peptide technology for functional dissection of CLE genes in Arabidopsis.Plant Physiol.doi:dx.doi.org/10.1104/pp.112.211029

Stahl Y,Wink RH,Ingram GC,Simon R(2009)A signaling module controlling the stem cell niche in Arabidopsis root meristems.Curr.Biol.19,909–914.

Strabala TJ,O’donnell PJ,Smit AM,Ampomah-Dwamena C,Martin EJ,Netzler N,Nieuwenhuizen NJ,Quinn BD,Foote HCC,Hudson KR(2006)Gain-of-function phenotypes of many CLAVATA3/ESR genes,including four new family members,correlate with tandem variations in the conserved CLAVATA3/ESR domain.Plant Physiol.140,1331–1344.

Suer S,Agusti J,Sanchez P,Schwarz M,Greb T(2011)WOX4 imparts auxin responsiveness to cambium cells in Arabidopsis.Plant Cell 23,3247–3259.

Whitford R,Fernandez A,De Groodt R,Ortega E,Hilson P(2008)Plant CLE peptides from two distinct functional classes synergistically induce division of vascular cells.Proc.Natl.Acad.Sci.USA 105,18625–18630.

Yokoyama A,Yamashino T,Amano Y,Tajima Y,Imamura A,Sakakibara H,Mizuno T(2007)Type-B ARR transcription factors,ARR10 and ARR12,are implicated in cytokinin-mediated regulation of protoxylem differentiation in roots of Arabidopsis thaliana.Plant Cell Physiol.48,84–96.

Zhu Y,Wang Y,Li R,Song X,Wang Q,Huang S,Jin JB,Liu CM,Lin J(2010)Analysis of interactions among the CLAVATA3 receptors reveals a direct interaction between CLAVATA2 and CORYNE in Arabidopsis.Plant J.61,223–233.