Exogenous ornithine is an effective precursor and the δ-ornithine amino transferase pathway contributes to proline accumulation under high N recycling in salt-stressed cashew leaves

Abstract

The role of the δ-ornithine amino transferase (OAT) pathway in proline synthesis is still controversial and was assessed in leaves of cashew plants subjected to salinity. The activities of enzymes and the concentrations of metabolites involved in proline synthesis were examined in parallel with the capacity of exogenous ornithine and glutamate to induce proline accumulation. Proline accumulation was best correlated with OAT activity, which increased 4-fold and was paralleled by NADH oxidation coupled to the activities of OAT and Δ(1)-pyrroline-5-carboxylate reductase (P5CR), demonstrating the potential of proline synthesis via OAT/P5C. Overall, the activities of GS, GOGAT and aminating GDH remained practically unchanged under salinity. The activity of P5CR did not respond to NaCl whereas Δ(1)-pyrroline-5-carboxylate dehydrogenase was sharply repressed by salinity. We suggest that if the export of P5C from the mitochondria to the cytosol is possible, its subsequent conversion to proline by P5CR may be important. In a time-course experiment, proline accumulation was associated with disturbances in amino acid metabolism as indicated by large increases in the concentrations of ammonia, free amino acids, glutamine, arginine and ornithine. Conversely, glutamate concentrations increased moderately and only within the first 24h. Exogenous feeding of ornithine as a precursor was very effective in inducing proline accumulation in intact plants and leaf discs, in which proline concentrations were several times higher than glutamate-fed or salt-treated plants. Our data suggest that proline accumulation might be a consequence of salt-induced increase in N recycling, resulting in increased levels of ornithine and other metabolites involved with proline synthesis and OAT activity. Under these metabolic circumstances the OAT pathway might contribute significantly to proline accumulation in salt-stressed cashew leaves.