CONTRIBUTION OF ORNITHINE AMINOTRANSFERASE TO PROLINE ACCUMULATION IN NACL-TREATED RADISH COTYLEDONS

Light-grown cotyledons of radish (Raphanus sativus L. cv National) subjected to increasing NaCl concentrations displayed a dose-dependent accumulation of proline at the beginning of their differentiation, Neither erythromycin nor DCMU significantly affected the salt-induced proline accumulation, which suggests that plastids may not be involved in proline synthesis, Gabaculine has been shown previously to be a powerful and irreversible inhibitor of ornithine aminotransferase [Hervieu et al. (1993) Phytochemistry 34, 231-234]. This inhibitor applied in vivo at a very low concentration (1 mmol m(-3)) reduced considerably the salt-induced proline accumulation of radish cotyledons, These results indicate that the ornithine pathway contributes, via an increase of ornithine aminotransferase activity, to proline synthesis, as well as the glutamate pathway, in sail-treated cotyledons, The use of transcription and translation inhibitors revealed that the salt-induced increase in ornithine aminotransferase activity may proceed via an activation of the translation of preformed mRNAs. The pre-existing ornithine aminotransferase mRNAs in radish seeds may be an important adaptation to NaCl, leading to proline accumulation at the beginning of seedling growth.