Auxin-stimulated ATPase in membrane fractions from pumpkin hypocotyls (Cucurbita maxima L.)

Membrane fractions from Cucurbita maxima hypocotyls were isolated in a medium which inhibits the action of endogenous phospholipases. After removal of soluble phosphatases by Sepharose 2B-CL column chromatography, an auxin-stimulated ATPase activity was found in membrane fractions from linear sucrose gradients. In the presence of 10(-4) M phenylacetic acid (PAA), the stimulation by indol-3-acetic acid (IAA) exhibited a bimodal concentration dependence with maximal stimulation of about 50% at 10(-6) M IAA. Without PAA, only a high concentration of 10(-4) M IAA was stimulatory, whereas 10(-6) M IAA had no apparent effect and 10(-8) M IAA exhibited weak inhibition. PAA alone had only weak or no effects. The effects of IAA must be considered as hormone-specific. The ATPase activity in the presence of 10(-4) M PAA was activated only by 2,4-dichlorophenoxyacetic acid (2,4-D), an active auxin analogue, but not by the inactive stereoisomers, 2,3-D and 3,5-D. Comparison with marker enzyme profiles suggested that part of the auxin-stimulated ATPase was localized on plasma membranes as well as other compartments. Thus, the auxin-stimulated ATPase may become a useful tool in the investigation of the mechanism of action of auxin.