CALCIUM-DEPENDENT PROTEIN-KINASE IN MAIZE AND SORGHUM INDUCED BY POLYETHYLENE-GLYCOL

Several environmental and hormonal stresses activate signal transducti on in plants. Various enzymes are involved in these mechanisms. Studyi ng one of these enzymes, we found osmotically-induced, calcium-depende nt protein kinases (CDPKs), and hypothesized a connection between osmo tic and hormonal signals. CDPKs were investigated in cultivars of rela tively drought-tolerant maize (Zea mays L.) and drought-tolerant sorgh um (Sorghum bicolor L.). Plants were grown hydroponically for 11 days and treated with abscisic acid (ABA, 0.1 mu M) starting from day 5, an d with polyethylene glycol 6000 starting from day 8 at 0, 100 and 200 mOsm concentrations. As a function of time, treatments lasted for 0, 1 5, 30, 60, 120 min and 72 h. For the determination of CDPK activity, t he 25 000 g supernatant of shoots and roots were used for SDS-PAGE and autoradiography. An increased CDPK activity was found after 1 h of os motic stress in sorghum roots and only low levels of phosphorylation c ould be measured in maize. Little or no activities were detected in th e shoots. The Ca2+-dependent phosphorylation appeared as 49- and 52-kD a bands. Calmodulin (CaM) added in vitro did not change the enzyme act ivity but inhibition by a CaM antagonist, trifluoroacetic acid, was si gnificant. We also found that pretreatment with ABA reverted the stres s-induced changes in sorghum roots. We concluded that this CDPK is inv olved in the early steps of the signal transduction pathway, and is co nnected with ABA-induced mechanisms.