Intercellular signaling is essential for the coordination of growth and development in higher plants. Although hundreds of putative receptors have been identified in Arabidopsis (Arabidopsis thaliana), only a few families of extracellular signaling molecules have been discovered, and their biological roles are largely unknown. To expand our insight into the developmental processes potentially regulated by ligand-mediated signal transduction pathways, we undertook a systematic expression analysis of the members of the Arabidopsis CLAVATA3/ESR-RELATED (CLE) small signaling polypeptide family. Using reporter constructs, we show that the CLE genes have distinct and specific patterns of promoter activity. We find that each Arabidopsis tissue expresses at least one CLE gene, indicating that CLE-mediated signaling pathways are likely to play roles in many biological processes during the plant life cycle. Some CLE genes that are closely related in sequence have dissimilar expression profiles, yet in many tissues multiple CLE genes have overlapping patterns of promoter-driven reporter activity. This observation, plus the general absence of detectable morphological phenotypes in cle null mutants, suggest that a high degree of functional redundancy exists among CLE gene family members. Our work establishes a community resource of CLE-related biological materials and provides a platform for understanding and ultimately manipulating many different plant signaling systems.

CLE peptides, named for the CLV3/ESR-related peptide family, participate in intercellular-signaling pathways. Here we investigated members of the CLE-like (CLEL) gene family that encode peptide precursors recently designated as root growth factors [Matsuzaki Y et al. (2010) Science 329:1065-1067]. CLEL precursors share a similar domain structure with CLE precursors (i.e., they contain a putative Nterminal signal peptide and a C-terminal conserved 13-amino-acid CLEL motif with a variable middle portion). Our evidence shows that, unlike root growth factor, CLEL peptides are (i) unmodified and (ii) function in the regulation of the direction of root growth and lateral root development. Overexpression of several CLEL genes in Arabidopsis resulted in either long roots or long and wavy roots that also showed altered lateral root patterning. Exogenous application of unmodified synthetic 13-amino-acid peptides derived from two CLEL motifs resulted in similar phenotypic changes in roots of wild-type plants. In CLEL peptide-induced long roots, the root apical meristem (RAM) was enlarged and consisted of an increased number of cells, compared with wild-type root apical meristems. The wavy-root phenotype appeared to be independent of other responses of the roots to the environment (e.g., gravitropism, phototropism, and thigmotropism). Results also showed that the inhibition of lateral initiation by CLEL overexpression was not overcome by the application of auxin. These findings establish CLEL as a peptide family with previously unrecognized regulatory functions controlling the pattern of root growth and lateral root development in plants.peptide hormone | root waving | cell division | signal transduction | synthetic peptides S ecreted peptides are important signals in intercellular communication in plants. They elicit signaling pathways that determine the identity and activity of adjacent cells and, in so doing, regulate the structure and function of plant tissues and organs. Secreted peptides are typically derived by proteolytic processing of precursor proteins. They act as signaling ligands, triggering downstream cellular signaling cascade reactions upon binding to a receptor-like kinase (RLK) and/or a receptor-like protein at the plasma membrane (1).The largest group of such peptides identified so far, CLE, named for the CLV3/ESR-related gene family, is thought to function as a 12-or 13-amino-acid (aa) peptide ligand that regulates meristem activity in shoots and roots as well as in vascular tissues (2-6). In several studies, the 12-and 13-aa peptides encoded by specific CLE genes have been detected in plants, and synthetic forms have been shown to functionally mimic the overexpression of corresponding genes (3-6). Accumulated evidence also indicates that secreted peptides are commonly subjected to a variety of posttranslational modifications, such as proline hydroxylation, hydroxyproline arabinosylation, and tyrosine sulfation (3-7).All CLE precursors contain an N-terminal signal peptide and a 14-aa conserved C-terminal...