EXPRESSION OF NADH-SPECIFIC AND NAD(P)H-BISPECIFIC NITRATE REDUCTASE GENES IN RESPONSE TO NITRATE IN BARLEY

Barley (Hordeum vulgare L.) has two, differentially regulated, nitrate reductase (NR) genes, one encoding the NADH-specific NR (Nar1) and th e other encoding the NAD(P)H-bispecific NR (Nar7). Regulation of the t wo NR genes by nitrate was investigated in wild-type Steptoe and in an NADH-specific NR structural gene mutant (Az12). Gene-specific probes were used to estimate NADH and NAD(P)H NR mRNAs. The kinetics of induc tion by nitrate were similar for the two NR genes; expression was gene rally below the limits of detection prior to induction, reached maximu m levels after 1 to 2 h of induction in roots and 4 to 8 h of inductio n in leaves, and then declined to steady-state levels. Derepression of the NAD(P)H NR gene in leaves of the NADH-specific NR gene mutant Az1 2 did not appear to be associated with changes in nitrate assimilation products or nitrate flux. Nitrate deprivation resulted in rapid decre ases in NADH and NAD(P)H NR mRNAs in seedling roots and leaves and equ ally rapid decreases in the concentration of nitrate in the xylem sap. These results indicate that factors affecting nitrate uptake and tran sport could have a direct influence on NR expression in barley leaves.