A FUNCTIONAL HOMOLOG OF MAMMALIAN PROTEIN-KINASE-C PARTICIPATES IN THE ELICITOR-INDUCED DEFENSE RESPONSE IN POTATO

The elicitor-induced activation of the potato pathogenesis-related gen e PR-10a is positively controlled by a protein kinase(s) that affects the binding of the nuclear factors PBF-1 (for PR-10a binding factor-1) and PBF-2 to an elicitor response element (ERE). In this study, we ha ve identified a kinase that has properties similar to the conventional isoenzymes of the mammalian protein kinase C (PKC) family. The treatm ent of potato tuber discs with specific inhibitors of PKC abolished th e elicitor-induced binding of the nuclear factor PBF-2 to the ERE. Thi s correlated with a reduction in the accumulation of the PR-10a protei n. In contrast, treatment of the tuber discs with 12-O-tetradecanoylph orbol 13-acetate (TPA), an activator of PKC, led to an increase in bin ding of PBF-2 to the ERE and the corresponding increase in the level o f the PR-10a protein, mimicking the effect seen with the elicitor arac hidonic acid. Biochemical characterization of proteins extracted from the particulate fraction of potato tubers demonstrated that a kinase b elonging to the conventional isoforms of PKC is present. This was conf irmed by immunoprecipitation with antibodies specific to the conventio nal isoforms of human PKC and in-gel kinase assays. The ability of the immunoprecipitates to phosphorylate the alpha-peptide (a PKC specific substrate) in the presence of the coactivators calcium, phosphatidyls erine, and TPA strongly suggested that the immunoprecipitated kinase i s similar to the kinase characterized biochemically. Finally, the simi lar effects of the various modulators of PKC activity on the elicitor- induced resistance against a compatible race of Phytophthora infestans implicate this kinase in the overall defense response in potato.