Plants respond to wounding or pathogen attack by a variety of biochemical reactions, involving in some instances gene activation in tissues far apart from the actual site of wounding or pathogen invasion. One of the best analyzed examples for such a systemic reaction is the wound-induced expression of proteinase inhibitor genes in tomato and potato leaves. Local wounding of potato or tomato plants results in the accumulation of proteinase inhibitors I and II throughout the aerial part of the plant. In contrast to wild-type plants, abscisic acid-deficient mutants of potato (droopy) and tomato (sit) show a drastically reduced induction of these genes in response to plant wounding. High levels of proteinase inhibitor II gene expression are obtained in mutant and wild-type plants upon exogenous application of abscisic acid. Measurements of the endogenous abscisic acid levels in wild-type plants show that wounding results in increased levels of this phytohormone in wounded and nonwounded systemically induced leaves. Thus these results show that the plant hormone abscisic acid is involved in the wound-induced activation of the proteinase inhibitor II gene. Furthermore, they are compatible with a model assuming this hormone to be the actual mediator of the systemic wound response.
