MOLYBDENUM COFACTOR BIOSYNTHESIS IN 2 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 nar1a, nar7w mutant, impai red in two structural genes of the nitrate reductase (NR) apoproteins, exhibited a considerable level of molybdenum cofactor (MoCo) when gro wn 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 i n 0.1 mM nitrate. Removal of NO3- from the nutrient medium caused a ra pid loss of NR activity in the shoots of wild type plants while MoCo a nd pterin content remained unaffected by the presence or absence of ni trate in the medium. Previous observations of MoCo induction in higher plants by NO3- by a number of researchers may have been due to an ove restimation of the actual MoCo content due to NO3- accumulation in the tissue and its reduction by NADPH during the NR complementation proce ss which resulted in nitrite accumulation. Exclusion of NADPH from the complementation medium prevented nitrite formation and allowed to est imate MoCo content in plant tissues containing a wide range of nitrate concentrations. The genotype nar1a;nar7w, when grown in nitrate, exhi bited MoCo levels similar to that of wild type plants. Copyright (C) 1 997 Elsevier Science Ireland Ltd.