WOUNDING INDUCES THE RAPID AND TRANSIENT ACTIVATION OF A SPECIFIC MAPKINASE PATHWAY

Mechanical injury in plants induces responses that are involved not on ly in healing but also in defense against a potential pathogen, To und erstand the intracellular signaling mechanism of wounding, we have inv estigated the involvement of protein kinases. Using specific antibodie s, we showed that wounding alfalfa leaves specifically induces the tra nsient activation of the p44(MMK4) kinase, which belongs to the family of mitogen-activated protein kinases. Whereas activation of the MMK4 pathway is a post-translational process and was not blocked by alpha-a manitin and cycloheximide, inactivation depends on de novo transcripti on and translation of a protein factor(s), After wound-induced activat ion, the MMK4 pathway was subject to a refractory period of 25 min, du ring which time restimulation was not possible, indicating that the in activation mechanism is only transiently active, After activation of t he p44(MMK4) kinase by wounding, transcript levels of the MMK4 gene in creased, suggesting that the MMK4 gene may be a direct target of the M MK4 pathway. In contrast, transcripts of the wound-inducible MsWIP gen e, encoding a putative proteinase inhibitor, were detected only severa l hours after wounding, Abscisic acid, methyl jasmonic acid, and elect rical activity are known to mediate wound signaling in plants. However , none of these factors was able to activate the p44(MMK4) kinase in t he absence of wounding, suggesting that the MMK4 pathway acts independ ently of these signals.