CLE peptide signaling during plant development

Peptide signaling in plants is a rapid developing area of research which focuses on so called peptide hormones. These signaling molecules are utilized for inter-cellular communication in different developmental processes, beside the usage of the more well-known phytohormones. Probably the best studied peptide ligands in plants are the CLAVATA3 (CLV3)/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) proteins. This family of signaling polypeptides is comprised of 32 members in Arabidopsis and, with the exception of the presence of related proteins in some parasitic worms, is restricted to the plant kingdom. CLV3 is one of the founding CLE genes and is involved in stem cell niche maintenance in apical meristems during plant development. While the CLV signaling pathway is well characterized with the identification of three receptors and a stem-cell-promoting transcription factor as target, the functioning of other family members is not or poorly understood. The recent discoveries of a new type of receptor involved in CLV signaling and a functional pathway for CLE40 in root development mark the rapid progress that is made in the area of CLE peptide signaling. This review gives an overview how CLE peptides are used as signaling molecules, and how they are involved in cell-to-cell communication in concert with different known and unknown receptors in a range of developmental processes during plant development.