Pathogen-induced MAP kinases in tobacco.
The activation of two tobacco MAP kinases, SIPK and WIPK, by a variety of pathogen-associated stimuli and other stresses have been analyzed (Table 1). SIPK was activated by SA, a CWD carbohydrate elicitor and two elicitins from Phytophthora spp, bacterial harpin, TMV, and Avr9 from Cladosporium fulvum. In addition to these pathogen-associated stimuli, wounding also activated SIPK, suggesting that this enzyme is involved in multiple signal transduction pathways. In all cases tested, SIPK activation was exclusively post-translational via tyrosine and threonine/serine phosphorylation. WIPK was activated by only a subset of these stimuli, including infection by TMV or harpin-producing Pseudomonas syringae (preliminary unpubl. result) and treatment with the CWD elicitor, elicitins or Avr9. In contrast to SIPK, WIPK was activated at multiple levels. Low level activation (e.g. by the CWD elicitor) appeared to be primarily post-translational whereas dramatic increases in kinase activity (e.g. by TMV or elicitins) required not only post-translational phosphorylation, but also preceding rises in mRNA levels and de novo synthesis of WIPK protein. Interestingly, under conditions where the same stimulus activated both of these kinases, their kinetics of activation appeared to be distinct. SIPK was the first to be activated. Activation of the low basal level of WIPK protein present before treatment exhibited similar kinetics to that of SIPK; however, the appearance of high levels of WIPK enzyme activity was delayed, perhaps reflecting the need for WIPK transcription and de novo protein synthesis.