Characterization of the Increased Lysophospholipase Activity in Gibberellic Acid-Treated Barley Aleurone Layers

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Abstract
A lysophospholipase (LPL) activity appears in the aleurone of barley half seeds during imbibition on moist agar. Secretion of LPL by half seeds is promoted by GA3; the increase in secretory rate is almost linear from 10-10 to 10-6 M GA3. LPL activity is likewise promoted in isolated aleurone layers by GA3. Its secretion into the incubation medium requires the continued presence of GA3 and commences after a 10-14 h lag period when 10 mM Ca2+ is present. In the absence of Ca2+, the lag period remains unchanged but attainment of the maximum secretory rate is delayed. Ca2+ alone has very little effect either on LPL activity accumulated in the aleurone layer or in the surrounding medium. However, 50 mM Ca2+ together with GA3 dramatically increase the level of secreted activity and of total (accumulated and secreted) activity. The metabolic inhibitors cycloheximide and actinomycin D inhibit the accumulation of LPL activity in the aleurone and also the secreted activity. Actinomycin D added after the lag period results in a much lower inhibition. The increase in LPL activity in response to GA3 occurs as a result of de novo synthesis; LPL activity from barley half seeds incubated in 80% D2O in the presence of GA3 undergoes a shift to higher density compared with the activity from similar controls incubated in H2O. The characteristics of the GA3 enhancement of LPL activity are compared specifically with .alpha.-amylase and generally with other GA3-controlled hydrolases.