Contribution of the ornithine pathway to proline accumulation in radish cotyledons exposed to salt stress

Proline content and ornithine aminotransferase (OAT) activity in light-grown cotyledons of radish (Raphanus sativus L,., cv. National) increased when they were transferred to salt conditions. Gabaculine, an irreversible inhibitor of OAT, considerably reduced the salt-induced proline accumulation. These results indicate that, in salt-treated cotyledons, the ornithine pathway contributes, via an increase in OAT activity, to proline synthesis, in addition to the glutamate pathway. Experiments with transcription and translation inhibitors revealed that the salt-induced increase in OAT activity can proceed via an activation of the translation of preformed mRNAs. The pre-existing OAT mRNAs in radish seeds provide an adaptation to NaCl, leading to proline accumulation at the beginning of seedling growth.