Polypeptide signalling for plant defence genes.

The synthesis of proteinase inhibitor proteins in response to wounding is a defensive response against pest and pathogen attacks. Wounding of leaves results in the release of a mobile wound signal which induces proteinase inhibitor synthesis throughout the plant. The signal transduction pathway regulating this response is not fully understood, but several compounds have been identified which are capable of inducing proteinase inhibitor synthesis in tomato and potato leaves. These compounds include cell wall fragments of both plant and pathogen origin [1-3], abscisic acid [4], jasmonic acid [5] and, most recently, an 18-amino-acid polypeptide, called systemin, isolated from tomato leaves [6]. In this chapter, we describe the properties of systemin and its precursor prosystemin, and we summarize the evidence supporting a role for systemin as an initial signal that regulates proteinase inhibitor synthesis in response to wounding.