SELECTIVE-INHIBITION OF AUXIN-STIMULATED NADH OXIDASE ACTIVITY AND ELONGATION GROWTH OF SOYBEAN HYPOCOTYLS BY THIOL REAGENTS

The NADH oxidase activity of isolated vesicles of soybean (Glycine max cv Williams 82) plasma membranes and elongation growth of 1-cm-long h ypocotyl segments were stimulated by auxins (indole-3-acetic acid or 2 ,4-dichlorophenoxyacetic acid [2,4-D]). The auxin-induced stimulations of both NADH oxidase and growth were prevented by the thiol reagents N-ethylmaleimide, p-chloromercuribenzoate, 5,5'-dithiobis(2-nitropheny lbenzoic acid), dithiothreitol, and reduced glutathione. These same re agents largely were without effect on or stimulated slightly the basal levels of NADH oxidase and growth when assayed in the absence of auxi ns. In the presence of dithiothreitol or reduced glutathione, both 2,4 -D and indole-3-acetic acid either failed to stimulate or inhibited th e NADH oxidase activity. The rapidity of the response at a given conce ntration of thiol reagent and the degree of inhibition of the 2,4-D-in duced NADH oxidase activity were dependent on order of reagent additio n. If the thiol reagents were added first, auxin stimulations were pre vented. If auxins were added first, the inhibitions by the thiol reage nts were delayed or higher concentrations of thiol reagents were requi red to achieve inhibition. The results demonstrate a fundamental diffe rence between the auxin-stimulated and the constitutive NADH oxidase a ctivities of soybean plasma membranes that suggest an involvement of a ctive-site thiols in the auxin-stimulated but not in the constitutive activity.