Inhibition of NaCI-Induced Proline Biosynthesis by Exogenous Proline in HalophilicDistichlis spicataSuspension Cultures

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Abstract
Cell suspensions of the halophilic grass, Distichlis spicata L., accumulate up to 50 μmol g−1 fresh weight proline when transferred to medium containing 260 mol m−3 NaCl. The metabolism of L-[5-13C]-glutamic acid to L-[5-13C]-proline was followed by 13C-NMR analysis to detect 13C-enrichment of carbon-5 of proline. Chemical and enzymatic analysis of cell extracts and media were used to determine L-proline and L-glutamic acid concentrations, respectively. 13C-Glutamic acid metabolism to proline was greater in cells adapting to 260 mol m-3 NaCl compared with cells not adapted to NaCl, or grown long-term on NaCl medium. Metabolism of 13C-glutamic acid (5.0 mol m−3) to proline was inhibited when 5.0 mol m−3 proline was also included in the growth medium. Exogenous proline did not sufficiently reduce glutamic acid transport to account for this inhibition. The inhibition was thus either regulated by feedback-inhibition of existing enzymes, or by repression of transcription and/or translation of genes encoding enzymes involved in proline biosynthesis.