Control of photosynthesis in barley leaves with reduced activities of glutamine synthetase or glutamate synthase.

Wild-type and mutant plants of barley (Hordeum vulgare L. cv. Maris 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 F2 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-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The activation state of Rubisco was unaffected in air, but a decrease in GS influenced the activation state of Rubisco in low CO2. In illuminated leaves, the amino-acid content decreased with decreasing GS, while the content of ammonium rose, showing that even small reductions in GS limit ammonium re-assimilation and may bring about a loss of nitrogen 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 possible feedback on glycine decarboxylation. This effect was absent in high light and low CO2. 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 determined under different environmental conditions. Decreased Fd-GOGAT resulted in a decrease in leaf protein, chlorophyll, Rubisco and nitrate contents. Chlorophyll a/b ratios and specific leaf fresh weight were lower than in the wild-type. Leaf ammonium contents were similar to the wild-type and total leaf amino-acid contents were only affected in low CO2 at high photon flux densities, but mutants with decreased Fd-GOGAT accumulated glutamine and contained less glutamate

Metabolism of acyl‐lipids in Chlamydomonas reinhardtii

International audienceMicroalgae are emerging platforms for production of a suite of compounds targeting several markets, including food, nutraceuticals, green chemicals, and biofuels. Many of these products, such as biodiesel or polyunsaturated fatty acids (PUFAs), derive from lipid metabolism. A general picture of lipid metabolism in microalgae has been deduced from well characterized pathways of fungi and land plants, but recent advances in molecular and genetic analyses of microalgae have uncovered unique features, pointing out the necessity to study lipid metabolism in microalgae themselves. In the past 10 years, in addition to its traditional role as a model for photosynthetic and flagellar motility processes, Chlamydomonas reinhardtii has emerged as a model organism to study lipid metabolism in green microalgae. Here, after summarizing data on total fatty acid composition, distribution of acyl-lipid classes, and major acyl-lipid molecular species found in C. reinhardtii, we review the current knowledge on the known or putative steps for fatty acid synthesis, glycerolipid desaturation and assembly, membrane lipid turnover, and oil remobilization. A list of characterized or putative enzymes for the major steps of acyl-lipid metabolism in C. reinhardtii is included, and subcellular localizations and phenotypes of associated mutants are discussed. Biogenesis and composition of Chlamydomonas lipid droplets and the potential importance of lipolytic processes in increasing cellular oil content are also highlighted