Regulation of Mo-cofactor, NADH- and NAD(P)H-specific nitrate reductase activities in the wild type and two nar-mutant lines of barley (Hordeum vulgare L.)

Seedlings of three genotypes of barley, Hordeum vulgare L., cv. Winer, were grown in nutrient solutions for 12 d: (a) Wt, the wild type; (b) Chlo19 and (c) Chlo29, two nitrate reductase (NR) deficient nar-mutants. Nar-mutant plants grown in nitrate developed about 5-24% of NADH-NR (EC 1.6.6.1.) activity level characteristic of the Wt. The NR in vitro assays in which NADH or NADPH were used as electron donors showed that the two mutant lines contained a mixture of NADH-specific and NAD(P)H-bispecific (EC 1.6.6.2.) NRs. Chlo19 had a very low level of MoCo activity as compared to Chlo29 and Wt. Chlo19 appeared to be mutated in a MoCo gene rather than in the genes coding for the nitrate NR apoenzyme. NAD(P)H-NR was found in the shoots and roots of both mutants but only in the roots of Wt. Several aspects of the regulation of NADH and NAD(P)H specific NRs in plants of the barley cv. Winer genotypes are discussed. MoCo was a strong limiting factor for NR biosynthesis in nitrate-fed plants of Chlo19, but less limited in N-starved and ammonium-fed plants. Biomass production by the three genotypes was similar during first 12 d after germination, regardless of the level of NR detected in vitro. Mutant plants may be able to supply the nitrogen required for growth with only 5-24% of the NR level of the WT.