JASMONIC ACID IN WOUND SIGNAL-TRANSDUCTION PATHWAYS

Wounding induces expression of genes encoding defense-related proteins involved in wound healing. An intensive survey has been carried out t o clarify the initial signal transduction pathways that mediate this s tress to expression of genes. Ln this context, signal molecules that i ntermediate in the wound signal to cellular response have been activel y searched for. Jasmonic acid (JA) has been considered to be a key sig nal molecule in this pathway. Systemin, ABA, ethylene, and electrical current have been suggested to function by transmitting the wound sign al to JA. A mitogen-activated protein kinase has been shown to respond rapidly to wounding, and proposed to function as one of the key enzym es involved in JA biosynthesis. Transgenic plants overexpressing a gen e encoding a Rab-type, small GTP-binding protein contained 6-fold high er levels of cytokinins than wild-type plants, and responded to woundi ng by rapidly producing JA and, uncommonly, accumulating salicylic aci d (SA), a pathogenic signal. These phenomena observed in the transgeni c plants were reproduced when wild-type plants were wounded in the pre sence of the synthetic cytokinin, benzylaminopurine, suggesting that c ytokinins are indispensable in the control of endogenous levels of JA and SA.