Molybdenum cofactor biosynthesis in two barley (Hordeum vulgare L.) genotypes as affected by nitrate in the tissue and in the growth medium

Two barley genotypes, the wild type and the nar 1a, nar7w mutant, impaired in two structural genes of the nitrate reductase (NR) apoproteins. exhibited a considerable level of molybdenum cofactor (MoCo) when grown in the absence of NO₃^-. Increasing concentrations of nitrate in the nutrient solutions did not affect MoCo content in shoots and roots of barley plants during the first 7 h of incubation. The MoCo and pterin content of the plants remained also unchanged during 24 h induction in 0.1 mM nitrate. Removal of NO₃^- from the nutrient medium caused a rapid loss of NR activity in the shoots of wild type plants while MoCo and pterin content remained unaffected by the presence or absence of nitrate in the medium. Previous observations of MoCo induction in higher plants by NO₃^- by a number of researchers may have been due to an overestimation of the actual MoCo content due to NO^3- accumulation in the tissue and its reduction by NADPH during the NR complementation process which resulted in nitrite accumulation. Exclusion of NADPH from the complementation medium prevented nitrite formation and allowed to estimate MoCo content in plant tissues containing a wide range of nitrate concentrations. The genotype nar 1a;nar7w, when grown in nitrate, exhibited MoCo levels similar to that of wild type plants.