Studies of the physiological role of leghaemoglobin in soybean root nodules

Abstract

Evolution of H 2 by nitrogenase in intact soybean nodules was consistently inhibited by CO when the nodules were equilibrated with argon-CO mixtures for 1 h prior to adding O 2 to initiate the reaction. Evolution of H 2 by nitrogenase in bacteroid suspensions prepared from nodules, was not inhibited by CO. Dense, slowly shaken suspensions of bacteroids, with 12% O 2 in the gas phase maintained slow rates of H 2 evolution and acetylene reduction for up to 12 h. Addition of leghaemoglobin to these assays greatly enhanced the nitrogenase-mediated reactions. CO prevented stimulation by leghaemoglobin of H 2 evolution by bacteroids. Stimulation of acetylene reduction by bacteroid suspensions was dependent upon leghaemoglobin concentration up to about 1 mM. Increased shaking rates gave greater rates of acetylene reduction and O 2 uptake. Stimulation of nitrogenase activity in bacteroid suspensions by leghaemoglobin was much greater than stimulation of O 2 uptake. Increasing acetylene reduction in response to increasing agitation was accompanied by increasing oxygenation of the leghaemoglobin. The significance of these results in relation to the physiological role of leghaemoglobin in symbiotic N 2 fixation by legume root nodules is discussed.