Multiple levels of tobacco WIPK activation during the induction of cell death by fungal elicitins

Three protein kinases of 48, 44 and 40 kDa are activated at different stages in tobacco cells treated with fungal elicitins. Previously we demonstrated that the rapidly activated 48 kDa protein kinase is encoded by SIPK. Here we report that the elicitin-activated 44 kDa kinase is encoded by WIPK. Activation of this kinase occurred 2-4 h after elicitin treatment and was preceded by dramatic increases in WIPK mRNA and protein levels. Studies using actinomycin D and cycloheximide demonstrated that de novo transcription and translation were required for this activation of the kinase activity. Strikingly, the kinetics of WIPK activation following elicitin treatment correlated with the onset of hypersensitive response (HR)-like cell death. Moreover, staurosporine and K-252a, two Ser/Thr protein kinase inhibitors that blocked WIPK activation, suppressed cell death. The timing for elicitin-treated cells to commit to a death program correlated with the appearance of high levels of WIPK activity. These correlative data suggest that WIPK may play a role during HR development in tobacco. Interestingly, a fungal cell-wall elicitor that does not cause cell death induced WIPK mRNA and protein to similar levels as those observed with the elicitins. However, no corresponding increase in WIPK activity was detected. Thus WIPK appears to be controlled at multiple levels.