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

Abstract

A lysophospholipase (LPL) activity appears in the aleurone of barley (Hordeum vulgare L. cv Himalaya) half seeds during imbibition on moist agar. Secretion of LPL by half seeds is promoted by GA(3); the increase in secretory rate is almost linear from 10(-10) to 10(-6) molar GA(3). LPL activity is likewise promoted in isolated aleurone layers by GA(3). Its secretion into the incubation medium requires the continued presence of GA(3) and commences after a 10 to 14 hour lag period when 10 millimolar Ca(2+) is present. In the absence of Ca(2+), the lag period remains unchanged but attainment of the maximum secretory rate is delayed. Ca(2+) alone has very little effect either on LPL activity accumulated in the aleurone layer or in the surrounding medium. However, 50 millimolar Ca(2+) together with GA(3) 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 GA(3) occurs as a result of de novo synthesis; LPL activity from barley half seeds incubated in 80% D(2)O in the presence of GA(3) undergoes a shift to higher density compared with the activity from similar controls incubated in H(2)O. The characteristics of the GA(3) enhancement of LPL activity are compared specifically with alpha-amylase and generally with other GA(3)-controlled hydrolases.