Growth inhibition by exogenous proline and its metabolism in saltgrass (Distichlis spicata) suspension cultures

The growth of Distichlis spicata suspension cultures in LS medium without NaCl was inhibited 54% by 2 mM proline. In medium containing 260 mM NaCl, 10 mM proline inhibited growth by only 22%. The uptake and metabolism of 10 mM L-1-¹³C] proline was followed by ¹³C NMR and ninhydrin analyses of suspensions cultured in the presence of 0 or 260 mM NaCl. Uptake of 85 to 92% of the exogenous proline occurred within 72 h in all media. In 10 mM proline and no NaCl, cellular proline reached a maximm of 51.5 moles/g FW compared to 1.9 moles/g FW in suspensions not grown on proline. In medium containing 260 mM NaCl and proline, cellular proline reached 59–65 moles/g FW compared to 30–40 moles/g FW in controls grown without proline. The ¹³C-label in the proline-C₁ was either retained in proline or disappeared, presumably released as carbon dioxide, by catabolism through the TCA cycle. Since no metabolite of ¹³C-proline was detected by NMR, proline was considered to be the molecule which inhibited the suspension culture growth.Abbreviations LS Linsmaier and Skoog medium - FW fresh weight - DW dry weight - P5C 1-pyrroline-5-carboxylate - TCA tricarboxylic acid cycle - FID free-induction-decay - NMR nuclear magnetic resonance spectroscopy - T₁ spin-lattice relaxation time - NOE Nuclear Overhauser Effect.