Enhancement of nitrate reductase activity and metabolic nitrate concentration by methionine sulfoximine in barley roots

Abstract

The enhancement of nitrate reductase activity (NRA) and metabolic NO 3 − concentration by methionine sulfoximine (MSO) in roots of 8-day-old intact barley ( Hordeum vulgare L.) seedlings was studied. NRA was induced with 0.05–100 mM NO 3 − with or without 0.25 mM MSO and assayed in vitro using NADH as the electron donor. Metabolic NO 3 − concentration was estimated by the anaerobic in vivo NRA assay method. In the absence of MSO, induction of NRA reached a plateau at 0.1 mM external NO 3 −, even though root NO 3 − accumulation continued at higher external NO 3 − concentrations. MSO enhanced the induction of NRA in roots exposed to NO 3 − concentrations greater than 0.1 mM, although total NO 3 − accumulation in MSO-treated roots decreased. In the absence of MSO, induction of NRA reached a plateau after 6 h exposure to 100 mM NO 3 −, whereas in the presence of MSO the enzyme activity increased gradually up to 24 h. Metabolic NO 3 − concentrations were similar in roots induced with 0.1 and 100 mM NO 3 −. MSO had no effect on the metabolic NO 3 − concentration when roots were induced with 0.1 mM NO 3 −, but it increased the metabolic NO 3 − concentration in roots induced with 100 mM NO 3 − by 2-fold between 6 and 24 h. The results indicate that in the absence of MSO the metabolic NO 3 − pool is saturated at 0.1 mM exogenous NO 3 − and that enhancement of NRA by MSO at higher NO 3 − levels may be due to increased accumulation of NO 3 − in that pool.