Systemin: A polypeptide signal for plant defensive genes

Damage to leaves of several plant species by herbivores or by other mechani cal wounding induces defense gene activation throughout the plants within h ours. An 18-amino acid polypeptide, called systemin, has been isolated from tomato leaves that is a powerful inducer of over 15 defensive genes when s upplied to the tomato plants at levels of fmol/plant. Systemin is readily t ransported from wound sites and is considered to be the primary systemic si gnal. The polypeptide is processed from a 200-amino acid precursor called p rosystemin, analogous to polypeptide hormones in animals. However, the plan t prohormone does not possess typical dibasic cleavage sites, nor does it c ontain a signal sequence or any typical membrane-spanning regions. The sign al transduction pathway that mediates systemin signaling involves linolenic acid release from membranes and subsequent conversion to jasmonic acid, a potent activator of defense gene transcription. The pathway exhibits analog ies to arachidonic acid/prostaglandin signaling in animals that leads to in flammatory and acute phase responses.