Involvement of calcium signalling in dormancy release of grape buds

Artificial induction of grape bud dormancy release by hydrogen cyanamide (HC) serves as a reliable model system to explore the events occurring shortly after the induction of dormancy release. Recently, a group of genes with remarkable differences in expression level between HC-treated and control buds was identified. The identification of several calcium signalling-related genes within that group raised the hypothesis of the involvement of Ca²⁺ signalling in grape bud dormancy release. Therefore, the effects of HC treatment on the expression profiles of several calcium sensors, the effect of the plasma membrane calcium channel blocker LaCl3 and the calcium chelator EGTA on HC-induced and chilling-induced bud-break, and the effect of HC application on calcium-dependent protein phosphorylation activities in the bud tissue were studied. Here the HC-induced expression of Ca²⁺-ATPase is described, indicating that this treatment might evoke an increase in [Ca²⁺]cyt. Similar induction was confirmed for calmodulin, calmodulin-binding protein, and calcium-dependent protein kinase (CDPK). Both LaCl3 and EGTA blocked the inducing effect of HC on bud-break, and their inhibitory effects were removed by supplying exogenous Ca²⁺. Calcium-dependent histone phosphorylation was up to 70% higher in HC-treated buds. Endogenous protein phosphorylation assays detected four proteins exhibiting increased phosphorylation following HC treatment, of which two were phosphorylated in a calcium-dependent manner. One of these, a 47 kDa protein, presented strong and Ca²⁺-dependent phosphorylation only in HC-treated buds. The potential role of CDPK in the phosphorylation of this protein was supported by an immunoprecipitation assay. The data suggest, for the first time, that calcium signalling is involved in the mechanism of bud dormancy release.