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

The enhancement of nitrate reductase activity (NRA) and metabolic NO3- 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 NO3- with or without 0.25 mM MSO and assayed in vitro using NADH as the electron donor. Metabolic NO3- 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 NO3-, even though root NO3- accumulation continued at higher external NO3- concentrations. MSO enhanced the induction of NRA in roots exposed to NO3- concentrations greater than 0.1 mM, although total NO3- accumulation in MSO-treated roots decreased. In the absence of MSO, induction of NRA reached a plateau after 6 h exposure to 100 mM NO3- whereas in the presence of MSO the enzyme activity increased gradually up to 24 h. Metabolic NO3- concentrations were similar in roots induced with 0.1 and 100 mM NO3-. MSO had no effect on the metabolic NO3- concentration when roots were induced with 0.1 mM NO3- but it increased the metabolic NO3- concentration in roots induced with 100 mM NO3- by 2-fold between 6 and 24 h. The results indicate that in the absence of MSO the metabolic NO3- pool is saturated at 0.1 mM exogenous NO3- and that enhancement of NRA by MSO at higher NO3- levels may be due to increased accumulation of NO3- in that pool. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.