Gibberellic acid and abscisic acid coordinately regulate cytoplasmic calcium and secretory activity in barley aleurone protoplasts

The effects of gibberellic acid (GA3) and abscisic acid (ABA) on the temporal and spatial dynamics of cytosolic calcium concentration ([Ca(2+)])i in aleurone protoplasts of barley (Hordeum vulgare L. cv. Himalaya) were measured by using fluorescence ratio analysis and confocal microscopy. After 4-6 h of treatment, GA3 induced a sustained increase in [Ca2+]i from 50 to 150 nM in aleurone protoplasts. The increase in [Ca(2+)]i preceded the GA3-induced increase in alpha-amylase synthesis and secretion by 4 h. The elevation of [Ca(2+)]i was highest in the peripheral cytoplasm and may play a role in coordinating the secretory events there. Reducing Ca(2+) levels in the incubation medium to below 500 micromolar inhibited the increase in [Ca(2+)]i and the GA3-induced stimulation of alpha-amylase synthesis and secretion. These data suggest that GA3 may increase [Ca(2+)]i by increasing the influx of Ca(2+) at the plasma membrane. ABA reversed the effect of GA3 on [Ca(2+)]i within 3 h, 2 h before its effect on alpha-amylase production could be detected. Thus changes in [Ca(2+)]i may play a role in mediating the effects of GA3 and ABA on the synthesis and secretion of alpha-amylase in the aleurone cell