CLE Peptides Research Articles

Separate and Equal?

Signaling by the CLE small peptide family member CLAVATA 3 (CLV3) restricts stem cell proliferation in the apical meristem of Arabidopsis . CLV3 signals through the transmembrane receptors CLV1 and CLV2 to repress transcription of wuschel ( wus ), which encodes a homeodomain transcription factor that promotes stem cell proliferation. Müller et al. have identified a new member of this signaling pathway, CORYNE (CRN). A loss-of-function mutation in crn was isolated from a screen for suppressors of a weak meristem-arrest phenotype induced by clv3 overexpression. crn mutants displayed a set of phenotypes shared with clv1 and clv2 loss-of-function mutants, including meristem enlargement due to overproliferation of stem cells, extra carpels, and an increase in the number of meristem cells expressing clv3 and wus . The phenotype of plants carrying loss-of-function mutations in both crn and clv2 was indistinguishable from that of plants carrying either mutation alone. In contrast, plants carrying both crn and clv1 mutations had an exacerbated phenotype that was more severe than with either mutation alone. These genetic interaction experiments indicated that CRN and CLV2 functioned together, whereas CRN and CLV1 acted in parallel. The authors identified the gene affected in the crn mutant and found that it encoded a membrane-associated Ser-Thr kinase expressed in meristems and young organs but not in fully differentiated tissues. The authors propose a model in which CLV signaling is transduced separately through two receptor complexes, one of CLV1 homodimers and one of CRN-CLV2 heterodimers. Whereas CLV1 has both ligand-binding and kinase domains, CLV2 has only a ligand-binding domain, and CRN has only a kinase domain. Together, CRN and CLV2 could thus create a fully functional transmembrane receptor kinase. Consistent with this hypothesis, the crn allele identified through the screen for suppressors of the clv overexpression phenotype had a point mutation that could potentially disrupt dimerization in the transmembrane domain. Both CLV1 and CLV2/CRN transduced the CLV3 signal to WUS through the phosphatases POLTERGEIST (POL) and POLTERGEIST-LIKE 1 (PLL1). Although CLV3 signaling through CLV1 and CLV2/CRN appears to be equivalent in the apical meristem, it is possible that these receptors might differ in their ability to transduce signals from other CLE peptides or confer tissue- or stage-specificity to signaling. R. Müller, A. Bleckmann, R. Simon, The receptor kinase CORYNE of Arabidopsis transmits the stem cell-limiting signal CLAVATA3 independently of CLAVATA1. Plant Cell 20 , 934-946 (2008). [Abstract] [Full Text]

A large family of genes that share homology with CLE domain in Arabidopsis and rice

Thirty-one CLAVATA3/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) proteins are encoded in the Arabidopsis genome, and they are supposed to function as dodecapeptides with two hydroxyproline residues. Twenty-six synthetic CLE peptides, corresponding to the predicted products of the 31 CLE genes, were examined in Arabidopsis and rice. Nineteen CLE peptides induced root meristem consumption, resulting in the short root phenotype in Arabidopsis and rice, whereas no CLE peptides affected the shoot apical meristem in rice. Database searches revealed 47 putative CLE genes in the rice genome. Three of the rice CLE genes, OsCLE502, OsCLE504 and OsCLE506, encode CLE proteins with multiple CLE domains, which are not found in the Arabidopsis genome, and polyproline region was found between these CLE domains. These results indicate conserved and/or diverse CLE functions in each plant species.

Arabidopsis CLV3 Peptide Directly Binds CLV1 Ectodomain

CLV1, which encodes a leucine-rich repeat receptor kinase, and CLV3, which encodes a secreted peptide, function in the same genetic pathway to maintain stem cell populations in Arabidopsis shoot apical meristem. Here, we show biochemical evidence, by ligand binding assay and photoaffinity labeling, that the CLV3 peptide directly binds the CLV1 ectodomain with a dissociation constant of 17.5 nM. The CLV1 ectodomain also interacts with the structurally related CLE peptides, with distinct affinities depending on the specific amino acid sequence. Our results provide direct evidence that CLV3 and CLV1 function as a ligand-receptor pair involved in stem cell maintenance.

CLE peptides that regulate plant cell differentiation

CLE peptides that regulate plant cell differentiation

Diverse and conserved roles of CLE peptides

The function of plant CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR) (CLE) peptides in shoot meristem differentiation has been expanded in recent years to implicate roles in root growth and vascular development among different CLE family members. Recent evidence suggests that nematode pathogens within plant roots secrete ligand mimics of plant CLE peptides to modify selected host cells into multinucleate feeding sites. This discovery demonstrated an unprecedented adaptation of an animal gene product to functionally mimic a plant peptide involved in cellular signaling for parasitic benefit. This review highlights the diverse and conserved role of CLE peptides in these different contexts.

CLE peptide ligands and their roles in establishing meristems

Research in the past decade revealed that peptide ligands, also called peptide hormones, play a crucial role in intercellular communication and defense response in plants. Recent studies demonstrated that a family of plant-specific genes, CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR) (CLE), which has at least 31 members in Arabidopsis genome, are able to generate extracellular peptides to regulate cell division and differentiation. A hydroxyl 12-amino acid peptide derived from the conserved CLE motif of CLV3 promotes cell differentiation, whereas another CLE-derived peptide suppresses the differentiation. These peptides probably interact with membrane-bound, leucine-rich repeat receptor-like kinases (LRR-RLKs) to execute the decision between cell proliferation and differentiation.

Maintaining the Shoot Meristem Stem Cell Population

Stem cells in plant shoot meristems give rise to stems, leaves, and flowers. One arm of the meristem stem cell maintenance pathway involves a negative feedback loop between the stem cells and the cells of the organizing center (OC) beneath the stem cells. WUSHCEL (WUS), a transcription factor present in the OC, is essential for maintenance of the stem cells, and CLAVATA3 (CLV3), which is a peptide ligand produced by the stem cells, feeds back on the OC to inhibit WUS transcription. Thus, CLV3 promotes stem cell differentiation and WUS promotes stem cell maintenance. Two reports investigate meristem stem cell signaling. Würschum et al. identified a dominant-negative mutation in the gene encoding APETALA2 (AP2) in a screen for mutations that inhibited shoot meristem development in Arabidopsis . The mutation was mapped to the first of two AP2 domains in the protein, and the plants were called l28 mutants. When l28 plants with the dominant-negative mutation in AP2 were crossed with plants with null mutations in the gene encoding AP2, the frequency of premature shoot meristem termination was increased. l28 plants also exhibited reduced expression of WUS and CLV3 . Genetic crosses indicated that termination of the shoot meristem in the l28 plants required functional CLV3, because CLV3 loss of function rescued shoot meristem development. Analysis of l28 plants crossed with wus -null plants indicated that WUS and AP2 were in the same pathway controlling shoot meristem maintenance. Because l28 mutants did not exhibit an increase in CLV3 expression, the authors propose a model in which AP2 inhibits CLV3 signaling downstream of CLV3 transcription. Ni and Clark suggest that CLV3 may be cleaved extracellularly to release the CLE domain, which is the active ligand. The Arabidopsis genome encodes several proteins with a CLE domain. Nine of thirteen intronless CLE genes rescued or partially rescued the phenotype of supernumerary flower organs in clv3-1 transgenic plants. Replacement of the CLE domain of the inactive CLE8 with the CLE domain of CLV3 resulted in complete rescue. Furthermore, replacement of all but the CLE domain of CLV3 with sequences of unrelated protein ERECTA also rescued the clv3-1 phenotype, suggesting that the CLE domain is the essential portion. When glutathione S -transferase (GST)-tagged CLV3 or CLE1, which was one of the CLE proteins that fully rescued the clv3-1 phenotype, was incubated with cauliflower protein extracts, proteolytic processing that released the CLE domain was observed. The CLE domain of CLV3 had been previously shown to alter root development, thus suggesting that this portion of the molecule may be the active region. Thus, the authors propose that CLV3 may be cleaved to release the active CLE peptide. However, it remains to be determined if CLV3 or the related CLE proteins encoded by the other members of the CLE gene family are cleaved in vivo and whether this cleavage is mediated by a specific protease involved in CLV3 signaling. T. Würschum, R. Groβ-Hardt, T. Laux, APETALA2 regulates the stem cell niche in the Arabidopsis shoot meristem. Plant Cell 18 , 295-307 (2006). [Abstract] [Full Text] J. Ni, S. E. Clark, Evidence for functional conservation, sufficiency, and proteolytic processing of the CLAVATA3 CLE domain. Plant. Physiol. 140 , 726-733 (2006). [Abstract] [Full Text]

The 14–Amino Acid CLV3, CLE19, and CLE40 Peptides Trigger Consumption of the Root Meristem inArabidopsisthrough aCLAVATA2-Dependent Pathway

CLAVATA3 (CLV3), CLV3/ESR19 (CLE19), and CLE40 belong to a family of 26 genes in Arabidopsis thaliana that encode putative peptide ligands with unknown identity. It has been shown previously that ectopic expression of any of these three genes leads to a consumption of the root meristem. Here, we show that in vitro application of synthetic 14-amino acid peptides, CLV3p, CLE19p, and CLE40p, corresponding to the conserved CLE motif, mimics the overexpression phenotype. The same result was observed when CLE19 protein was applied externally. Interestingly, clv2 failed to respond to the peptide treatment, suggesting that CLV2 is involved in the CLE peptide signaling. Crossing of the CLE19 overexpression line with clv mutants confirms the involvement of CLV2. Analyses using tissue-specific marker lines revealed that the peptide treatments led to a premature differentiation of the ground tissue daughter cells and misspecification of cell identity in the pericycle and endodermis layers. We propose that these 14-amino acid peptides represent the major active domain of the corresponding CLE proteins, which interact with or saturate an unknown cell identity-maintaining CLV2 receptor complex in roots, leading to consumption of the root meristem.