Abstract
Six-day-old rice plants (Oryza sativa L., cv Bahia) were grown for 5 days more in nutrient solution culture containing 10−5 M abscisic acid (ABA) or gibberellic acid (GA3) (treated plants). Plasma membrane (PM) vesicles were isolated from the shoots of treated or control plants, and ATPase hydrolytic and proton-pumping activity, fluidity, and free sterol and phospholipid composition were determined. Both treatments resulted in modified plant growth and increases in PM fluidity. The ATPase hydrolytic activity was decreased by 25% of control values with ABA treatment and by 35% with GA3. Both treatments reduced proton-pumping by 23%. GA3 treatment reduced the relative amount (%) of stigmasterol in the PM from 38–34%, and increased Δ5-avenasterol from 4–7%. ABA and GA3 did affect the percentage composition of the fatty acyl chains of phosphatidylinositol (PI). GA3 treatment also resulted in a reduction of the total amount of PI from 73–37 mg mg−1 protein, as well as a decrease in the overall PM C16/C18 ratio and an increase in the degree of unsaturation. Our results show that there is no common link between a specific change in lipid composition and fluidity in these membranes. On the other hand, the reduction in Mg2+-ATPase activity found with both hormones suggests that there is no relationship between membrane-bound enzyme activity and membrane fluidity.
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