Cloning of a Ca2+-ATPase gene and the role of cytosolic Ca2+ in the gibberellin-dependent signaling pathway in aleurone cells

The ultimate goal of this investigation was to identify intermediary steps in the gibberellin (GA)-dependent signaling pathway in rice aleurone cells. By using a differential display approach, a number of putative GA-responsive genes were isolated. One of them, a GA-responsive Ca2+-ATPase gene, was identified and partially characterized. A genomic clone and a cDNA clone were isolated and sequenced. The deduced amino acid sequence showed that this protein resembles an endoplasmic reticulum membrane Ca2+-ATPase. In a transient assay in rice aleurone cells, expression of the introduced Ca2+-ATPase cDNA bypassed the GA requirement for stimulating the expression of a major target gene, the alpha-amylase c gene (Osamy-c). This result suggests that GA-dependent expression of this Ca2+-ATPase gene (OsCa-atpase) plays an important role in the GA-dependent signal-transduction pathway, To investigate the possible involvement of other proteins and genes that may affect the intracellular Ca2+ level, compounds which can block different putative steps in the signal-transduction pathway were introduced into rice aleurone cells, and then the level of the OsCa-atpase transcript or the Osamy-c transcript was monitored. In the presence of GA, the rice Ca2+-ATPase and the Ca2+ channels appeared to co-regulate the local concentration of cytosolic Ca2+. The release of Ca2+ from the internal stores to the cytoplasm was presumably initiated by inositol-1,4,5-triphosphate which reached a peak level within 25 min after GA induction. As a second messenger, Ca2+ binds to calmodulin (CaM), and the Ca2+/CaM complex regulates the cytosolic Ca2+ by affecting expression of the OsCa-atpase. Finally, a working model is proposed for the GA-dependent signaling pathway in aleurone cells.