REVERSIBLE PROTEIN-PHOSPHORYLATION REGULATES JASMONIC ACID-DEPENDENT AND ACID-INDEPENDENT WOUND SIGNAL-TRANSDUCTION PATHWAYS IN ARABIDOPSIS-THALIANA

Plants responses to mechanical injury are complex and include the indu ced expression of defence-related genes. The phytohormone JA has been reported to mediate some of these responses. To elucidate further the signal transduction processes involved, the action of specific agonist s and antagonists of known signalling effecters on the response of Ara bidopsis thaliana plantlets to JA and wounding was investigated. The i dentification and characterization of a reversible protein phosphoryla tion step in a transduction pathway leading to JA-induced gene transcr iption is reported. This phosphorylation event involved the opposing a ctivities of a saturosporine-sensitive protein kinase, negatively regu lating the pathway, and a protein phosphatase, most probably of type 2 A, which activated JA-responsive gene expression. JA activation via t his pathway was blocked in the A. thaliana JA-insensitive mutants jin1 , jin4 and col1, and by exogenous application of cycloheximide or auxi ns. Wound-induced activation of JA-responsive genes was also regulated by this protein phosphorylation step. An alternative wound signalling pathway, independent of JA, was also identified, leading to the trans criptional activation of a different set of genes. This JA-independent pathway was also regulated by a protein phosphorylation switch, in wh ich the protein kinase positively regulated the pathway while the prot ein phosphatase negatively regulated it. Moreover, a labile protein ap parently repressed the expression of these genes. One of the genes ana lysed, JR3, had a complex pattern of expression, possibly because it w as regulated via both of the wound signalling pathways identified, Acc ording to the function of an homologous gene, JR3 may be involved in f eedback inhibition of the JA response.