GLUTAMINE SYNTHETASE AND AMMONIUM ASSIMILATION IN ROOTS OF ZINC‐TOLERANT AND NON‐TOLERANT CLONES OF DESCHAMPSIA CESPITOSA (L.) BEAUV. AND ANTHOXANTHUM ODORATUM L.

Abstract

SUMMARYThe influence of zinc on glutamine synthetase (GS) activity and ammonium assimilation in roots of zinc-tolerant and non-tolerant clones of Deschampsia cespitosa (L.) Beauv. and Anihoxanthum odoratum L. was studied. One isoform of GS was resolved by ion exchange chromatography in D. cespitosa roots. However, two isoforms were found in A. odoratum roots; one of these was attributed to fungal contamination and was not studied further. There was no difference in the sensitivity of partially purified GS from zinc-tolerant and non-tolerant clones to inhibition by zinc. Inhibition of GS by Zn was reversible and the kinetics of inhibition were studied. Methionine sulphoximine (an inhibitor of GS) induced ammonium accumulation in the roots of control and zinc-grown roots of Zn-tolerant D. cespitosa. Zinc inhibited incorporation of [^^N]ammonium into free amino acids (determined by combined gas chromatography-mass spectrometry) in both clones of D. cespitosa, but higher concentrations were required to achieve inhibition in the tolerant clone. The labelling pattern was unaffected by zinc in both clones. The results suggest that zinc has no effect on the pathway of ammonium assimilation in zinc-grown roots and that assimilation is via the glutamine synthetase-glutamate synthase pathway. There was no evidence to support the hypothesis that glutamate dehydrogenase functions preferentially in ammonium assimilation under conditions of environmental stress.

NITROGEN ASSIMILATION AND ZINC TOXICITY TO ZINC‐TOLERANT AND NON‐TOLERANT CLONES OF DESCHAMPSIA CESPITOSA (L.) BEAUV.

“…Nitrate reductase in vitro is extremely sensitive to inhibition by zinc (Mathys, 1975), much more so than glutamine synthetase (Smirnoff & Stewart, 1987). However, the in vivo root nitrate reductase was not inhibited by zinc.…”

“…Zinc toxicity could be greater in nitrate-grown plants if the processes of nitrate uptake and assimilation were more sensitive to inhibition by zinc than ammonium assimilation. Nitrate reductase in vitro is extremely sensitive to inhibition by zinc (Mathys, 1975), much more so than glutamine synthetase (Smirnoff & Stewart, 1987). However, the in vivo root nitrate reductase was not inhibited by zinc.…”

Regulation of Glutamate Dehydrogenase Activity in Relation to Carbon Limitation and Protein Catabolism in Carrot Cell Suspension Cultures

“…The enzyme GDH4 is reported to be present in all higher plants examined, and often high levels of activity are present in tissues such as roots and senescing leaves (15,16). Until recently, controversy existed as to its metabolic role, and two apparently conflicting functions have been proposed.…”

The Role of Glutamate Dehydrogenase in Plant Nitrogen Metabolism

“…However, controversy still exists as to the role of the enzyme GDH in higher plants. It is found in all higher plants examined and is often present at high levels in, for example, senescing and root tissues (1 1,22). The 'Abbreviations: GOGAT, glutamate synthase; GS, glutamine synthetase; GABA, y-aminobutyric acid; GDH, glutamate dehydrogenase; MSO, methionine sulfoximine; AOA, aminooxyacetic acid; ANCA, automated '5N/'3C analyzer; MTBSTFA, N-methyl-N-(tertbutyldimethylsilyl)trifluoroacetamide; HFIB, heptafluorbutyryl isobutyl; t.BDMS, tert-butyldimethylsilyl; INEPT, insensitive nuclei enhanced by polarization transfer.…”