The calcium-dependent protein kinase HvCDPK1 mediates the gibberellic acid response of the barley aleurone through regulation of vacuolar function

In the aleurone cells of the cereal grain, gibberellic acid (GA) induces the secretion of hydrolases that mobilize endosperm reserves to fuel early seedling growth. GA is known to trigger a range of cellular responses, including increases in cytoplasmic calcium, vacuolar reserve mobilization, gene transcription, and the synthesis and secretion of hydrolases. To further define elements of the Ca2+-dependent GA response machinery, we have cloned a Ca2+-dependent protein kinase (HvCDPK1) from these cells. Although expression of an inactivated (D140N) version of this kinase did not affect GA-induced gene expression or changes in cytosolic Ca2+, it did inhibit secretion, cell vacuolation, and vacuolar acidification, all responses linked to the GA response. Additionally, recombinant wild-type HvCDPK1 activated the V-type H+-ATPase present in isolated aleurone vacuoles. These results suggest HvCDPK1 may mediate Ca2+-dependent events of the GA response, such as control of vacuolar function, that lie downstream of transcriptional regulation.