Control of photosynthesis in barley mutants with reduced activities of glutamine synthetase and glutamate synthase |
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Authors: | Rainer E Häusler Karen J Bailey Peter J Lea Richard C Leegood |
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Institution: | (1) Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, S10 2TN Sheffield, UK;(2) Department of Biological Sciences, University of Lancaster, Bailrigg, LA1 4YQ Lancaster, UK;(3) Present address: Botanisches Institut, Lehrstuhl II Universität zu Köln, Gyrhofstrasse 15, D-50934 Köln, Germany |
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Abstract: | Heterozygous mutants of barley (Hordeum vulgare L. cv. Maris Mink) with decreased activities of chloroplastic glutamine synthetase (GS) between 97 and 47% of the wild type and ferredoxin dependent glutamate synthase (Fd-GOGAT) down to 64% of the wild type have been used to study aspects of glyoxylate metabolism and the effect of glyoxylate on the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in vivo. In the leaf, the extractable activities of serine:glyoxylate aminotransferase decreased with a decrease in GS whereas activities of glutamate and alanine:glyoxylate aminotransferase increased, pointing to a re direction of amino donors from serine to glutamate and alanine. Under ambient conditions, the leaf contents of glutamate and alanine declined continuously with a decrease in GS, in parallel with the decrease in total amino acids. Glycine, serine and asparagine contents decreased with a decrease in GS to approximately 70% of the wild type, but increased again with a further decrease in GS. At high irradiances and at low CO2 concentrations, glyoxylate contents exhibited a pronounced minimum between 60% and 80% GS. With a further decrease in GS, glyoxylate contents recovered and approached values similar to the wild type. The activation state of Rubisco showed a negative correlation with glyoxylate contents, indicating that a decrease in GS feeds back on the first step of carbon assimilation and photorespiration. The activation state of stromal fructose-1,6-bisphosphatase was unaffected by a decrease in GS or Fd-GOGAT, whereas the activation state of NADP dependent malate dehydrogenase changed in a complex manner. The CO2photocompensation point, *, was appreciably increased in mutants with 47% GS. Mitochondrial respiration in the light (Rd) was reduced with a decrease in GS. Relative rates of CO2 release into CO2-free air between the wild type and the 47%-GS mutant correlated with determinations of *. These data are consistent with the view that when GS is decreased there is an increased oxidative decarboxylation of glyoxylate resulting from a decreased availability of amino donors for the transamination of glyoxylate to glycine, and that when GS activities are lower than 70% of the wild type an additional mechanism operates to reduce the photorespiratory loss of ammonia.Abbreviations AGAT
nine:glyoxylate aminotransferase
- FBPase
fructose-1,6-bisphosphatase
- Fd-GOGAT
ferredoxin dependent glutamate synthase
- GGAT
glutamate:glyoxylate aminotransferase
- GS
glutamine synthetase
- MDH
malate dehydrogenase
- PFD
photon flux density
- Rubisco
ribulose-1,5-bisphosphate carboxylase-oxygenase
- SGAT
serine:glyoxylate aminotransferase
This research was supported by the Biotechnology and Biological Sciences Research Council initiative on the Biochemistry of Metabolic Regulation in Plants (PG 50/555). |
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Keywords: | Hordeum Glutamine synthetase Glyoxylate Glyoxylate metabolism Photorespiration Ribulose-1 5-bisphosphate carboxylase-oxygenase |
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