Amino Acid Metabolism in Relation to Osmotic Adjustment in Plant Cells
Amino acid metabolism has been investigated in suspension cultures of tobacco (Nicotiana tabacum L. var Wisconsin 38) adapted to 0 mM and 428 mM Nacl, using [15N] to determine the fluxes via the major free amino acid pools and amino acid residues of total soluble protein. Adaptation to 428 mM Nacl is shown to be associated with an 80-fold accumulation of free proline. This involves at least an 11-fold increase in the rate of synthesis of proline from glutamate via negligible pools of intermediates. Of the 1000 nmol/h.gfw of proline synthesized by salt adapted cells only 296 nmol/h.gfw is required to maintain the free proline pool with growth, and only 9 nmol/h.gfw is required to sustain protein synthesis. As much as 695 nmol/h.gfw of proline may be catabolized in salt adapted cultures. A small fraction of the newly synthesized proline in salt adapted cells is used in the synthesis of a proline conjugate which copurifies with N-malonyl-1-aminocyclopropane-1-carboxylate. The major metabolic fate of proline is unknown, but may simply involve oxidation to glutamate. From measurements of the fluxes of [15N] into amino acid residues of total soluble protein, it has been possible to estimate protein synthesis and turnover rates. Both rates are substantially reduced in salt adapted relative to unadapted cultures.
KeywordsPool Size Amino Acid Metabolism Osmotic Adjustment Total Soluble Protein Proline Accumulation
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