Oxidation of NADH by hypocotyl segments from soybean is stimulated by 2,4-D

Sections cut from regions of cell elongation of hypocotyls of dark-grown so ybean seedlings oxidized externally supplied NADH as estimated from the dec rease in A,,, measured spectrophotometrically. The oxidation of NADH by l-c m sections was stimulated 1.5- to 2-fold by 1 muM of the synthetic auxin, 2 ,4-dichlorophenoxyacetic acid (2,4-D). 2,4-D-Stimulated oxidation of NADH w as resistant to cyanide. Stimulations were also given by the naturally occu rring auxin, indole-3-acetic acid (IAA) but not by the growth inactive 2,4- D analog 2,3-dichlorophenoxyacetic acid (2,3-D) and the growth inactive bet a -naphthaleneacetic acid (beta -NAA). Since NADH is a membrane impermeant substrate, the findings confirm studies with inside-out and right-side-out vesicles that show the 2,4-D-stimulated NADH oxidase to be located at the e xternal cell surface. Cut surfaces are not responsible for the activity as shown by experiments with lanolin-sealed sections. The external NADH oxidas e measurements do not require special equipment and exhibit characteristics normally associated with enzyme-catalyzed reactions. (C) 2000 Editions sci entifiques et medicales Elsevier SAS.