d-Glycerate Transport by the Pea Chloroplast Glycolate Carrier : Studies on [1-C]d-Glycerate Uptake and d-Glycerate Dependent O(2) Evolution |
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| Authors: | Howitz K T McCarty R E |
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| Affiliation: | The Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55108. |
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| Abstract: | Rapid direct conversion of exogenously supplied 14C]aspartate to 14C] asparagine and to tricarboxylic cycle acids was observed in alfalfa (Medicago sativa L.) nodules. Aspartate aminotransferase activity readily converted carbon from exogenously applied 14C]aspartate into the tricarboxylic acid cycle with subsequent conversion to the organic acids malate, succinate, and fumarate. Aminooxyacetate, an inhibitor of aminotransferase activity, reduced the flow of carbon from 14C]aspartate into tricarboxylic cycle acids and decreased 14CO2 evolution by 99%. Concurrently, maximum conversion of aspartate to asparagine was observed in aminooxyacetate treated nodules (30 nanomoles asparagine per gram fresh weight per hour. Metabolism of 14C]aspartate and distribution of nodulefixed 14CO2 suggest that two pools of aspartate occur in alfalfa nodules: (a) one involved in asparagine biosynthesis, and (b) another supplying a malate/aspartate shuttle. Conversion of 14C]aspartate to 14C]asparagine was not inhibited by methionine sulfoximine, a glutamine synthetase inhibitor, or azaserine, a glutmate synthetase, inhibitor. The data did not indicate that asparagine biosynthesis in alfalfa nodules has an absolute requirement for glutamine. Radioactivity in the xylem sap, derived from nodule 14CO2 fixation, was markedly decreased by treating nodulated roots with aminooxyacetate, methionine sulfoximine, and azaserine. Inhibitors decreased the 14C]aspartate and 14]asparagine content of xylem sap by greater than 80% and reduced the total amino nitrogen content of xylem sap (including nonradiolabeled amino acids) by 50 to 80%. Asparagine biosynthesis in alfalfa nodules and transport in xylem sap are dependent upon continued aminotransferase activity and an uninterrupted assimilation of ammonia via the glutamine synthetase/glutamate synthase pathway. Continued assimilation of ammonia apparently appears crucial to continued root nodule CO2 fixation in alfalfa. |
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