PARALLEL INHIBITION BY EXTERNAL OSMOTICA OF AUXIN-INDUCED ELONGATION IN STEM SECTIONS AND AUXIN-STIMULATED NADH OXIDASE ACTIVITY OF PLASMA-MEMBRANE VESICLES FROM SOYBEAN HYPOCOTYLS

NADH oxidase of the plasma membrane of etiolated soybean hypocotyls, a n activity that is stimulated by elongation growth-active plant regula tors of the auxin type, both natural (IAA) and synthetic (2,4-D), was inhibited when plasma membrane vesicles were incubated in various osmo tica including sucrose, mannitol, sorbitol and glucose. With all osmot ica tested, auxin-stimulated elongation of excised sections and the au xin-stimulated NADH oxidase activity of the isolated plasma membrane v esicles were inhibited in parallel. The effect was greater with 2,4-D- induced elongation and oxidase activity than with basal elongation and oxidase activity. Both auxin-induced elongation and the auxin-stimula ted component of NADH oxidase activity were reduced or eliminated when assayed at osmoticum concentrations of 0.2 to 0.3 M, concentrations o f osmotica sufficient to plasmolyze cells of soybean hypocotyl section s. If the vesicles were first equilibrated with sucrose but subsequent ly assayed in the absence of sucrose, activity was not inhibited. Espe cially with mannitol and sorbitol, NADH oxidase activity was stimulate d by high concentrations of osmotica. These stimulations were eliminat ed by addition of catalase and are assumed to somehow involve producti on of hydrogen peroxide. An intact membrane was required. When membran es were solubilized with Triton X-100 or if preparations of NADH oxida se were partially purified from membranes and assayed in the absence o f added detergent, NADH oxidase activity was unaffected by osmotica. W ith vesicles assayed with increasing concentrations of sodium chloride , NADH oxidase activity also was inhibited but activity was restored a s osmotic equilibrium was approached. A similar restoration of activit y was observed with osmotic equilibration of vesicles plasmolyzed with sucrose, mannitol, sorbitol or glucose. The findings suggest that NAD H oxidase was sensitive to vesicle volume or stretch and that activity was greatest when membrane stretch of vesicles was greatest.