Fig. 1. Ligand-receptor pairs in plant peptide signaling. Systemin is produced by post-translational processing of a precursor, prosystemin, and recognized by SR160 receptor-like kinase, which induces defense gene activation. PSK is produced from the precursor proPSK by sulfation and post-translational processing. Secreted PSK interacts with receptor-like kinase PSKR and activates a set of genes responsible for cellular dedifferentiation and redifferentiation. CLV3 is translated, secreted and binds a CLV1/CLV2 receptor-like kinase complex, which regulates the balance between meristem cell proliferation and differentiation. SCR/SP11 is translated, secreted and interacts with the S locus receptor-like kinase SRK, which triggers self-incompatibility responses.