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Complex signals for simple cells:the expanding ranks of signals and receptors guiding stomatal development圆融时代广场
Matthew H Rowe and Dominique C Bergmann
digIn development,pattern formation requires that cell
龙洞小学proliferation and differentiation be precily coordinated.
Stomatal development has rved as a uful model system for understanding how this is accomplished in plants.Although it has been known for some time that stomatal development is regulated by a family of receptor-like kinas (RLKs)and an accompanying receptor-like protein (RLP),only recently have putative ligands been identified.Despite the structural
homology demonstrated by the genes that encode the small,creted peptides,they convey differe
nt information,vary with one another in their relationship to common signaling components,control distinct aspects of stomatal
development,and do so antagonistically.Their discovery has revealed the intricate network of interactions required upstream of RLK signal transduction for the patterning of complex tissues.However,at issue still is whether specific ligand –receptor combinations are responsible for the activation of discrete signaling pathways or spatiotemporal modulation of a common pathway.This review integrates the latest findings regarding RLK-mediated signaling in stomatal development with emerging paradigms in the field.
Address
Department of Biology,Stanford University,Stanford,CA 94305-5020,USA
Corresponding author:Bergmann,Dominique C (dbergmann@stanford.edu )
Current Opinion in Plant Biology 2010,13:548–555This review comes from a themed issue on Cell signalling and gene regulation
Edited by Zhiyong Wang and Giltsu Choi Available online 16th July 2010
1369-5266/$–e front matter
#2010Elvier Ltd.All rights rerved.DOI 10.1016/j.pbi.2010.06.002
Introduction
For many multicellular organisms,the coordination of cell proliferation and differentiation during pattern for-mation is accomplished in part through intercellular communication.Ligand –receptor-bad signaling is amenable to short-range,position-dependent forms of communication in which cells preside over the fates of their immediate neighbors.The extent to which signal transduction is activated within target cells can be con-tingent on signal strength (i.e.amount of ligand pro-duced)and at least two distinct positional relationships:proximity of the signal source and orientation of the source relative to the target(s).While proximity be-tween the signal source and target cells ties into signal strength and is a general requirement for ligand –recep-tor-bad systems,orientation becomes important in situations where the output involves a directional respon such as polarized cell growth or orientation of division planes.
In plants,the importance of communication via cell-sur-face localized receptors is reflected by their considerable reprentation in the genome and their involvement in a wide array of biological proces
s.Recent genetic approaches have led to the characterization of receptor-like kinas (RLKs)involved in diver developmental phenomena,including organ abscission (HAESA [1]and EVERSHED [2]),lateral root initiation (ARABIDOPSIS CRINKLY4(ACR4)[3]),root hair and ovule patterning (SCRAMBLED [4]/STRUBBELIG [5]),embryogenesis (RECEPTOR-LIKE PROTEIN KINASE1/TOAD-STOOL2[6]),vascular patterning (PHLOEM INTER-CALATED WITH XYLEM (PXY)[7,8 ]),and shoot meristem maintenance (CLAVATA (CLV)[9]and BARELY ANY MERISTEM (BAM)[10]families).Despite the breakthroughs,little is known about how the receptors mediate signal transduction,and genetic relationships with other signaling components remain to be verified biochemically.In many instances the identi-ties of ligands,signaling partners,and substrates remain a mystery.
In stomatal development,both the proximity and orientation of positional cues are important.The fre-quency and orientation of asymmetric cell divisions and the differentiation of stomata and precursors depend on the identities of neighboring cells (Figure 1,[11]).It has been known for some time that the ERECTA (ER),ERECTA-LIKE1(ERL1)and ERECTA-LIKE2(ERL2)(collectively referred to as the ER-family (ERf))leucine-rich repeat (LRR)RLKs and the TOO MANY MOUTHS (TMM)LRR-RLP coordinate the process in Arabidopsis to ensure that stomata are produced at an appropriate de
nsity and that they rarely form in contact [12,13].However,the relationship between the TMM and ERf receptors is complicated,and while it is conceivable that they dimerize and activate signal transduction in tandem (yet to be demonstrated biochemically),they also function antagonistically in a tissue-specific manner [12].
The recent identification of four members of the EPI-DERMAL PATTERNING FACTOR (EPF)family of cysteine-rich creted peptides has helped to define the roles of the TMM and ERf receptors.Genetic analys demonstrate that EPF1[14],EPF2[15 ,16 ],CHAL-LAH (CHAL,also EPFL6)[17 ],and STOMAGEN (EPFL9)[18 ,19 ,20 ]affect distinct phas of stomatal development,can function antagonistically,and diverge in their relationship to TMM and the ERf.In addition,the unique expression patterns of the putative ligands in the epidermis (EPF1/2)and underlying tissues (CHAL and STOMAGEN),suggest that they may con-vey different types of information to cells of the stomatal lineage.Elements of the signaling logic and mechanisms derived from the cas,including how specificity may be conferred on broadly functioning RLKs,are likely to illustrate general principles at work in many other devel-opmental contexts.
Stomatal development requires carefully coordinated cell proliferation and differentiation steps
Examples of most major developmental decisions are found in the stomatal lineage.Entry into the lineage is initiated in the plant epidermis when meristemoid mother cells (MMCs)divide asymmetrically to generate
可上可下打一字daughter cells of divergent size and fate (Figure 1,[11]).While the larger daughter cells (referred to as ‘stomatal lineage ground cells’or ‘SLGCs’)are capable of further asymmetric divisions,they often expand in size and terminally differentiate.In contrast,the smaller,triangu-lar-shaped daughters (referred to as ‘meristemoids’)demonstrate the proliferative capacity of stem cells and typically engage in additional asymmetric divisions that amplify the number of cells in the lineage.Stomatal development ceas when meristemoids differentiate into guard mother cells (GMCs)and divide symmetrically to yield stomata:two guard cells flanking a pore.The frequency and orientation of asymmetric divisions that occur next to meristemoids,GMCs,or stomata are regulated in such a manner that stomata rarely form in contact with one another.This so-called ‘one-cell spacing rule’is violated in tmm and er erl1erl2mutant plants [12,21],suggesting that intercellular communication is required for the proper coordination of stomatal devel-opment.
Stomatal development is regulated by the ERECTA family of RLKs
The ERf of LRR-RLKs functions broadly in aerial plant tissues to promote organ growth [22].Plants carrying mutations in the founding member (ER)exhibit reduced
Signaling in stomatal development Rowe and Bergmann 549
Figure
1
Progression through the stomatal lineage and expression patterns of EPF,ERf,and TMM genes.Depiction of the origin and progression of cells in the stomatal lineage of the leaf epidermis.Examples of divisions creating specific precursor cell types are shown in bulk on the growing leaf.Above each leaf stage are depicted two cells,one stomatal lineage and one not,and their development over time.The expression pattern of ligands (color-coded shading as indicated in the key)follows their published transcriptional reporter expression.Receptors (depicted with the extracellular portion as a V-shape)are placed with their intracellular domains in the cell type corresponding to published transcriptional reporter expression.MMC,meristemoid mother cell;M,meristemoid;GC,guard cell.
stature and organ size due to diminished cell proliferation [23].Like other families of RLKs that regulate plant development(ACR4[3],PXY[7],and HAESA[1]), the ERf exhibits considerable functional redundancy. In this ca,distinct and measurable growth defects of erl1and erl2are revealed only in an er background[22]. Promoter-swap experiments suggest that functional differences between ERf members in promoting growth are primarily due to differential expression rather than variation in protein structure[22].
The ERf also maintains a delicate balance between proliferation and differentiation in the developing stoma-tal lineage.The er erl1erl2phenotype involves the formation of massive clusters of guard cells and elevated stomatal densities in all stomata-producing tissues,and its verity(relative to single mutant phenotypes)implies that the ERf may regulate stomatal development in a redundant manner as well[12].However,in this context ERL1and ERL2posss distinct functions not encapsu-lated by ER.Whereas ER is responsible for restricting entry into the stomatal lineage,ERL1and ERL2main-tain the proliferative capacity of meristemoids by inhibit-ing the acquisition of GMC identity[12].Each ERf member is expresd uniformly in protodermal cells of emerging leaf primordia,but ER expression ceas before differentiation while expression of ERL1and ERL2is maintained specifically in cells of the stomatal lineage (Figure1)[12].Thefindings indicate that ERL1and ERL2are likely responsible for perceiving the positional cues that dictate cell fate decisions within the stomatal lineage.It has not yet been demonstrated whether the contrasting roles of ERf members in stomatal develop-ment are due in part to divergent biochemical activities. The TOO MANY MOUTHS RLP is an entry point for cell-type and tissue-specific modulation of ERECTA signaling
Given that receptor-like proteins(RLPs)such as TMM lack functional intracellular kina domains,their participation in signal transduction pathways likely hinges on interactions(physical or o
therwi)with proteins that posss such domains.Perhaps the best-characterized example of such a partnership in plant development involves the RLK CLAVATA1(CLV1) and the RLP CLAVATA2(CLV2),which,along with CLV2’s partner kina CORYNE[24]function coopera-tively to restrict stem cell proliferation in the shoot apical meristem of Arabidopsis[25].However,the relationship between TMM and the ERf differs from that of CLV1 and CLV2in key ways.Pertinent to this discussion is genetic evidence that TMM is both a potentiator and an inhibitor of ERf signaling[12].In leaves,tmm phenotypes are consistent with tho of ERf mutants:MMCs and SLGCs divide asymmetrically more frequently,meriste-moids differentiate into GMCs more readily,and stomata form in clusters[13,26].However,in tmm stems and hypocotyls,asymmetric divisions occur rarely and the production of stomata is eliminated[13,27].Mutations in specific ERf members are epistatic to tmm in stems and in leaves[12],suggesting that the positive and negative roles of TMM in stomatal development may both involve modulation of ER-bad signal transduction.The recent characterization of EPF genes involved in TMM-and ER-dependent regulation of stomatal development has provided support for this hypothesis.
Members of the EPIDERMAL PATTERNING FACTOR(EPF)family of small,creted peptides share common structural motifs Each of the11EPF genes encodes a small protein predicted to undergo
cretion and processing into a mature form 50amino acids in length,and to adopt a knot-like conformation by virtue of intramolecular dis-ulfide bridges.[16 ,19 ](Figure2).The predictions were confirmed in vivo by isolation of STOMAGEN from the apoplasm as a bioactive45aa C-terminal peptide fragment[18 ,19 ].The EPFs are structurally distinct from other known peptide ligands in Arabidopsis,though they share a requirement for conrved cysteines with the defensins[28]and the recently characterized LUREs [29].EPFs join the ranks of other proteinaceous signal families involved in plant development,such as the DVLs[30],CLEs,and IDAs(reviewed in[31]).While it was predicted previously that signaling peptides involved in stomatal development might be procesd into active form by the putative protea STOMATAL DENSITY AND DISTRIBUTION[32],genetic analys suggest that EPFs function independently of this factor.
EPF1and EPF2:conveying positional information through ERf-signaling
Genetic analys of EPF1and EPF2have uncoupled the roles of ERf-signaling in coordinating cell proliferation and orienting cell division at distinct phas of stomatal development.In epf2mutants,protodermal cells acquire MMC identity and divide asymmetrically more fre-quently,resulting in elevated stomatal densities and hyperproliferation of meristemoids,but minimal cluster-ing of stomata[15 ,16 ].In epf1mutants,asymmetric divisions within the stomatal lineage occ
ur despite the prence of neighboring stomata and precursors,and are often misoriented such that stomata form in contact[14]. EPF1and EPF2overexpression phenotypes are depend-ent on TMM and the ERf,and mutations in either do not discernibly affect stomatal phenotypes in the ERf triple mutant background,suggesting that both EPF1and EPF2function upstream of ERf signal transduction [14,16 ].
Despite their mutual involvement in ERf-signaling,it should be noted that EPF1and EPF2em to differ somewhat in their relationship to TMM.While epf1and爱国之情的诗句
恩施有什么好玩的
550Cell signalling and gene regulation
epf2have no effect on stomatal clustering or density,respectively,in a tmm background,detailed phenotypic analysis revealed that tmm epf2leaves exhibit the ectopic,arrested meristemoids of epf2single mutants and reduced stomatal clustering relative to tmm [14,15 ,16 ].Thus it is likely that EPF2activates ERf signal transduction both dependently and independently of TMM.
Consistent with their distinct roles in stomatal develop-ment,EPF2is expresd earlier in the lineage than EPF1(Figure 1,[15 ,16 ]).Promoter-swap experiments revealed that in the epf1mutant background,expression of pEPF1::EPF2but not pEPF2::EPF1is sufficient for partial rescue of elevat
ed stomatal clustering [16 ].Thus the specialization of EPF1in regulating stomatal cluster-ing is due in part to differential expression relative to EPF2.However in the epf2mutant background,expres-sion of pEPF1::EPF2but not pEPF2::EPF1is sufficient for partial rescue of elevated stomatal density [16 ].This suggests that differences in biochemical activity between EPF1and EPF2are integral to the role of EPF2in regulating stomatal density.One intriguing possibility is that the divergent functions of EPF1and EPF2involve varying affinities for different combinations of ERf RLKs and/or TMM.Given that ER expression ceas around the time when asymmetric entry divisions take place and that er and epf2mutants both demonstrate excessive entry
divisions,EPF2functionality may hinge upon interaction with ER specifically.
The above results regarding EPF1and EPF2support the idea that the factors both convey positional information about neighboring cell identities among members of the stomatal lineage.However this information appears to be interpreted in distinct ways by various receptor combi-nations in the target cells.Whereas the level of EPF2determines whether target cells will divide asymmetri-cally,the orientation of the EPF1signal source may dictate the directionality of asymmetric divisions.The idea that RLKs enable cells to perceive and respond to signal intensity and orientation is gaining traction due to recent characterization of the PXY –CLE41receptor –ligand pair
in Arabidopsis [7,8 ].Vascular development is an elegant system for the investigation of how cell divisions are oriented becau entire files of meristematic initials must divide precily along the apical –basal axis in a coordinated manner to produce well-ordered xylem.Proper orientation of the divisions is dependent on the PXY family of LRR-RLKs expresd in the procambium and the CLV3/ESR-related (CLE)ligands CLE41and CLE44,expresd in the adjoining phloem [7,8 ].Ecto-pic expression of CLE41in opposing cells of the pro-cambium and xylem (but not overexpression in the phloem)misorients the divisions of the meristematic
扶摇直上的意思
Signaling in stomatal development Rowe and Bergmann 551
Figure
2
Structural conrvation and phylogenetic relationships within the 11-member EPF gene family.(a)Protein quence alignment of conrved C-termini generated using ClustalW2.The STOMAGEN propeptide is procesd in vivo to yield the bioactive 45-amino-acid C-terminal fragment depicted above [18 ,19 ].Among all EPFs,cysteine residues are strictly conrved at six positions.It was demonstrated with STOMAGEN that the residues form disulfide bridges and are esntial for function [19 ].(b)A neighbor-joining phylogeny [36]of EPF family members generated using Kalignvu [37].In contrast to CHALLAH and EPF1/2,which belong to distinct clades,STOMAGEN fails to cluster with any other EPF genes.
initials [33 ],suggesting that polarized activation of RLK signal transduction is required for proper cell division orientation in the incipient vasculature.Stomatal devel-opment involves a more modular pattern of cells that generate and perceive intercellular signals,but it is con-ceivable that similar mechanisms are responsible for orienting cell divisions in each ca.
STOMAGEN:antagonist of ERf-mediated signaling
STOMAGEN is the first positively acting signaling pep-tide to be identified in stomatal development.Overexpres-sion and knockdown experiments demonstrate that STOMAGEN expressi
on positively correlates with stoma-tal density in all stomata-producing tissues [15 ,18 ,19 ].STOMAGEN is expresd broadly in the mesophyll and not in stomata or precursors [18 ,19 ],suggesting that it does not convey positional information from within the stomatal lineage in the same manner as EPF1and EPF2.However,like EPF1and EPF2,STOMAGEN requires TMM for function,as STOMAGEN knockdown or over-expression has no effect on stomatal density in the leaves or stems of tmm mutants [18 ,19 ].
Given that STOMAGEN and EPF1/2function antagon-istically and require a common signaling component in
TMM,one plausible hypothesis is that STOMAGEN inhibits the function of EPF1and EPF2by competitive inhibition.This system could have evolved simply via the degeneration of an EPF family member such that it is capable of binding TMM/ER complexes without activat-ing signal transduction.In this manner,the requirement for TMM in EPF1/2-mediated ERf signaling could be exploited as a means for signal attenuation.However,if this hypothesis is correct,there must be other ligands involved becau knockdown and overexpression exper-iments in epf1and epf2mutant backgrounds have clearly demonstrated that STOMAGEN regulates stomatal de-velopment in the abnce of the factors [18 ,20 ].While it has also been reported that application of 10m M STOMA
GEN peptide has no effect on stomatal density in epf1epf2double mutants [19 ]this apparent discrepancy could be due to technical challenges associ-ated with synthetic STOMAGEN application.
Assuming that STOMAGEN functions independently of EPF1and EPF2,another possibility is that STOMAGEN and TMM are involved in the activation of a discrete signaling pathway that promotes stomatal development.The idea of structurally related peptides functioning antagonistically in such a manner is not as far-fetched as it might em.It has been demonstrated in Zinnia cell
552Cell signalling and gene regulation
Figure
3
Genetic model for EPF signaling in stomatal development.The EPF family peptides regulate distinct phas of stomatal development and diverge in their relationship to TMM and the ERf.EPF2(red)restricts acquisition of MMC identity in protodermal cells 1.and activates ERf-signaling in the prence and abnce of TMM.EPF2may also function synergistically with EPF1(orange)(and TMM-dependently)to inhibit amplifying asymmetric divisions 2.,the acquisition of MMC identity among SLGCs 3.,and the meristemoid to GMC transition 4.EPF1functions TMM-dependently to orient the production of satellite meristemoids such that they do not form in contact with stomata or precursors 5.STOMAGEN (green)may inhibit TMM-mediated signaling by binding TMM receptor complexes in competition with other EPFs.In a subt of stomata-producing tissues (such as stems and hypocotyls),TMM titrates CHAL (yellow)to prevent CHAL from activating ERf-signaling complexes.Ectopic activation of ER-signaling by CHALLAH inhibits entry and progression through the stomatal lineage 6.All depicted relationships are bad on genetic data.In the above figure,arrows or T-bars indicate activation and inhibition,respectively and are colored to correspond with the factors responsible.Black T-bars correspond with roles that may be fulfilled collectively by EPF1and EPF2.For the sake of simplicity,ERf members are depicted en mas ,but it is important to recognize that ER,ERL1,and ERL2function in distinct phas of stomatal development.
