Metabolite Control of Nitrate Reductase Activity in Soybean Seedlings

Abstract

Summary Nitrogen nutrition was investigated as an affector of nitrate reductase activity in soybean seedlings ( Glycine max var. Williams 82). The activity of the pH 6.5 NADPH-linked, the pH 6.5 NADH-linked, and the pH 7.5 NADH-linked isoforms of nitrate reductase were measured in plants treated with nitrate, glutamine, nitrate and glutamine, and no nitrogen. Nitrate supplied to 10 day old soybeans greatly enhanced primary leaf nitrate reductase activities over a 48 hour period. Plants irrigated with Hoagland's solution without nitrogen had approximately 25 % of the pH 6.5 NADH and NADPH-linked activities of plants supplied with nitrate. The NADH-linked activity at pH 7.5 showed virtually no activity without the addition of nitrate. Plants treated with nitrate and glutamine had nitrate reductase activities intermediate to those of plants treated with nitrate alone and those lacking any nitrate treatment. The concentration of nitrate in these tissues was reduced significantly over those tissues provided only with nitrate. Glutamine supplied as a sole nitrogen source slightly enhanced the pH 6.5 linked nitrate reductase activities compared with plants given no nitrogen. These results support the concept that nitrogen nutrition and nitrogenous metabolites play key roles in the regulation of nitrate reductase activity in soybeans. In addition, they demonstrate that the pH 6.5 linked activities appear less sensitive to the flux of nitrate than the pH 7.5 linked activity.