Abstract
Suspension-cultured tobacco cells respond to fungal elicitor by activating the transcription of so-called defense genes. This response has been shown to be blocked by staurosporine, an inhibitor of protein kinases, and by Gd3+, which blocks Ca2+ channels. We report here that treating tobacco cells with the elicitor triggers the rapid and transient activation of a 47-kD protein kinase that phosphorylates serine and/or threonine residues of the myelin basic protein (MBP). Staurosporine and Gd3+ inhibited the elicitor-induced activation of the 47-kD MBP kinase, and staurosporine inhibited the activity of the MBP kinase itself. In the presence of either cycloheximide or calyculin A, the elicitor induced sustained activation of the MBP kinase. Immunoblot and immunoprecipitation analysis using a phosphotyrosine-specific antibody showed that tyrosine phosphorylation of the 47-kD MBP kinase was induced in tobacco cells that had been treated with the elicitor. The results suggest that the 47-kD MBP kinase is a component of the pathway for transduction of the elicitor signal in tobacco cells and that the activity of the MBP kinase is regulated by the post-translational phosphorylation of tyrosine residues.