Control of photosynthesis in barley leaves with reduced activities of glutamine synthetase or glutamate synthase I. Plant characteristics and changes in nitrate, ammonium and amino acids

1994 
Wild-type and mutant plants of barley (Hordeum vulgate L. cv. Marls Mink) lacking activities of chloroplastic glutamine synthetase (GS) and of ferredox- in-dependent glutamate synthase (Fd-GOGAT) were crossed to generate heterozygous plants. Crosses of the F 2 generation containing GS activities between 47% and 97% of the wild-type and Fd-GOGAT activities down to 63% of the wild-type have been selected to study the control of both enzymes on photorespiratory carbon and nitrogen metabolism. There were no major pleiotropic effects. Decreased GS had a small impact on leaf protein and the total activity of ribulose-l,5-bisphosphate car- boxylase-oxygenase (Rubisco). The activation state of Rubisco was unaffected in air, but a decrease in GS influ- enced the activation state of Rubisco in low CO2. In illu- minated leaves, the amino-acid content decreased with decreasing GS, while the content of ammonium rose, showing that even small reductions in GS limit ammoni- um re-assimilation and may bring about a loss of nitro- gen from the plants, and hence a reduction in protein and Rubisco. Leaf amino-acid contents were restored, and ammonium and nitrate contents decreased, by leaving plants in the dark for 24 h. The ratios of serine to glycine decreased with a decrease in GS when plants were kept at moderate photon flux densities in air, suggesting a possi- ble feedback on glycine decarboxylation. This effect was absent in high light and low CO 2. Under these conditions ammonium contents exhibited an optimum and amino- acid contents a minimum at a GS activity of 65% of the wild-type, suggesting an inhibition of ammonium release in mutants with less than 65% GS. The leaf contents of glutamate, glutamine, aspartate, asparagine, and alanine largely followed changes in the total amino-acid contents Abbreviations: Chl = chlorophyll; FBPase = fructose-l,6-bisphos- phatase; Fd-GOGAT = ferredoxin-dependent glutamine: 2-oxo- glutarate aminotransferase; GS = glutamine synthetase; PEP- phosphoenolpyruvate; PFD = photon flux density; Rubisco = ribulose- 1,5-bisphosphate carboxylase-oxygenase
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