FEEDBACK-REGULATION OF NITRATE INFLUX IN BARLEY ROOTS BY NITRATE, NITRITE, AND AMMONIUM

The short-lived radiotracer N-13 was used to study feedback regulation of nitrate influx through the inducible high-affinity transport syste m of barley (Hordeum vulgare L. cv Steptoe) roots. Both wild-type plan ts and the mutant line Az12:Az70 (genotype nar1a;nar7w), which is defi cient in the NADH-specific and NAD(P)H-bispecific nitrate reductases ( R.L. Warner, R.C. Huffaker [1989] Plant Physiol 91: 947-953) showed st rong feedback inhibition of nitrate influx within approximately 5 d of exposure to 100 muM nitrate. The result with the mutant, in which the flux of nitrogen into reduced products is greatly reduced, indicated that nitrate itself was capable of exercising feedback regulation upon its own influx. This conclusion was supported by the observation that feedback in wild-type plants occurred in both the presence and absenc e Of L-methionine sulfoximine, an inhibitor of ammonium assimilation. Nitrite and ammonium were also found to be capable of exerting feedbac k inhibition upon nitrate influx, although it was not determined wheth er these ions themselves or subsequent metabolites were responsible fo r the effect. It is suggested that feedback regulation of nitrate infl ux is potentially mediated through several nitrogen pools, including t hat of nitrate itself.