Induction of chitinase activity by exogenous cytokinins in excised dark-grown cucumber cotyledons: involvement of Ca2+ and staurosporine-sensitive protein kinase(s) in cytokinin signaling

Cotyledons were excised from 7-day-old dark-grown cucumber seedlings and treated with water, benzyladenine (BA), kinetin (K), zeatin (Z), or zeatin riboside (ZR) in dark after endogenous cytokinin depletion. We have compared changes in chitinase (EC. 3.2.1.14) activity induced by these cytokinins. We find that the activities of chitinase and its isoforms increase by approximately 3- to 6-fold following BA, Z, and ZR treatments. Among these treatments, Z was more effective. K was totally ineffective in inducing chitinase activity. Immunoblot analysis suggests that the cytokinin Z-induction of enzyme activity is due to the induction of higher chitinase protein levels and not the activation of existing enzyme. Furthermore, the Z-induced chitinase activity and its protein accumulation were completely inhibited by the protein kinase inhibitor staurosporine, whereas the protein phosphatase inhibitor sodium fluoride was not effective in such inhibitions. Treatment of cotyledons with extemal $CaCl_2$ and calcium ionophore increased the basal chitinase activity by 6- and 5-fold, respectively. Moreover, the effects of staurosporine, sodium fluoride, and $Ca^{2+}$ on Z-induced chitinase activity correlate with their effects on chitinase protein levels. Taken together, our data suggests $Ca^{2+}$ and staurosporine-sensitive protein kinase(s) as components of the cytokinin transduction machinery involving induction of chitinase in cucumber.