Rapid changes of protein phosphorylation are involved in transduction of the elicitor signal in plant cells.

Plant cells have an acute sense for pathogen-derived chemical stimuli, so-called elicitors, which induce the plant's defense response. To investigate the molecular basis of chemosensory transduction, elicitor-treated tomato cells were labeled with 1-min pulses of [32P] phosphate. This technique revealed drastic changes in protein phosphorylation in vivo within minutes of stimulation. The protein kinase inhibitors K-252a and staurosporine completely prevented these elicitor-induced changes in protein phosphorylation. They also blocked two early biochemical responses to elicitors, extracellular alkalinization and biosynthesis of ethylene. The ability of K-252a, staurosporine, and benzoylated staurosporine derivatives to inhibit elicitor responses in vivo correlated with their ability to inhibit tomato microsomal protein kinase in vitro. When K-252a was given to elicited cells 1 min after the[32] phosphate, the radioactivity in certain newly labeled phosphoprotein bands disappeared again within minutes. This correlated with an arrest of alkalinization within minutes when K-252a was applied in midcourse of elicitation. These data show that phosphorylation of protein substrates by K-252a-sensitive protein kinases is essential for transduction of elicitor signals in plant cells and that continuous phosphorylation of these proteins is required to maintain the elicited state.