Plasma membrane NADH oxidase is gravi-responsive

NADH oxidase activities measured with intact tissue sections and with isolated 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 less at 1 × g and one of stimulation with times of lying flat of 30 min or longer at 1 × g. Turning the tissue sections upside down resulted in stimulation of the activity with a lag of about 30 min. Returning the sections to the 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 oscillate with a period of 24 min, precluding a more precise analyses of lag times. The activity was stimulated reversibly to a maximum of about 2-fold both in tissue sections and in isolated plasma membrane vesicles when subjected to centrifugal forces of 10 to 400 × g for 0.5 to 4 min duration. The findings are the first description of a gravi-responsive enzymatic activity related to the growth response in plants.