Ligand-deprivation activates receptor signaling.

Signaling peptides,together with their receptor kinases and proteins,are involved in a wide range of processes related to plant growth,development,and stress responses.In Arabi-dopsis,over 1000 different peptides and 251 leucine-rich repeat receptor kinases(LRR-RKs)were identified,but knowl-edge of peptide-receptor-kinase/protein-mediated signaling in plants is very limited.PSY1,a tyrosine-sulfated peptide with hydroxyproline arabinosylation modification,is known to play a role in root growth modulation(Amano et al.,2007).