Roles of secreted peptides in intercellular communication and root development.

Intercellular signaling networks control cell identity and activity in all multicellular organisms. Secreted peptides that function as extracellular ligands play essential roles in local communication between adjacent plant cells. The extracellular domain of receptor kinases bind to secreted peptides and initiate downstream cellular responses, resulting in cell proliferation, growth, or differentiation in multicellular organisms. Root growth and development are highly organized processes involving cell division, expansion, and differentiation; these processes depend on the establishment and maintenance of root apical meristem. The regulatory networks controlling root growth and development are tightly controlled by various signal transduction pathways, feedback loops, and crosstalk among signaling pathways. This review demonstrates the remarkable diversity and importance of secreted peptides in cell signaling and summarizes the current understanding of the molecular mechanisms underlying the peptide signaling cascades with particular emphasis on pathways involved in regulating root apical meristem and vascular tissue development and those involved in rhizobium-legume symbiosis. Furthermore, this review provides an integrated view of the regulatory networks that control root development, including transcription factors, phytohormones and peptide signalings. Future perspectives in peptide signaling are also discussed.