Plasma membrane NADH oxidase is gravi-responsive

NADH oxidase activities measured with intact tissue sections and with isola ted plasma membrane vesicles from etiolated hypocotyls of soybean (Glycine max) respond to gravity and imposed centrifugal forces. The response is one of inhibition of activity with tissue sections lying flat for 20 min or le ss at 1 x g and one of stimulation with times of lying flat of 30 min or lo nger at 1 x g. Turning the tissue sections upside down resulted in stimulat ion of the activity with a lag of about 30 min. Returning the sections to t he normal upright position resulted in a return to initial rates with a lag of less than 20 min. Both the stimulated and non-stimulated activities osc illate with a period of 24 min, precluding a more precise analyses of lag t imes. The activity was stimulated reversibly to a maximum of about 2-fold b oth in tissue sections and in isolated plasma membrane vesicles when subjec ted to centrifugal forces of 10 to 400 x g for 0.5 to 4 min duration. The f indings are the first description of a gravi-responsive enzymatic activity related to the growth response in plants. (C) Elsevier, Paris.