Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings

The effect of exogenous NH4+ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3- in the presence of 0, 1 or 10 mM NH4+. Exogenous NH4+ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to or 10 mM NO3-. In contrast, the induction of NiRA was inhibited by NH4+ at all NO3- levels, Maximum inhibition of the enzyme activities occurred at 1.0 mM NH4+. Pretreatment with NH4+ had no effect on the subsequent induction of NRA in the absence of additional NH4+; whereas the induction of NiRA in NH4+-pretreated roots was inhibited in the absence of NH4+. At 10 mM NO3- L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH4+ supply. During the postinduction phase, exogenous NH4+ decreased NRA in roots supplied with 0.1 mM but not with 1 mM NO3-; whereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NRA in roots by limiting the availability of NO3-. Conversely, it has a direct effect, independent of the availability of NO3-, on the induction of NiRA. The lack of an NH4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.