Overexpression of ornithine-δ-aminotransferase increases proline biosynthesis and confers osmotolerance in transgenic plants

Accumulation of proline is a way to increase tolerance to water stress in plants. Therefore, considerable attention has been devoted to optimise proline biosynthesis in transgenic plants. Glutamate and ornithine are both precursors of proline but while genes of the glutamate pathway were overexpressed in transgenic plants, no gene encoding an enzyme of the ornithine pathway was considered until now. The present study aims to establish if the overexpression of ornithine-delta-aminotransferase (delta-OAT) represents an additional way to increase proline content. To achieve this goal, an Arabidopsis delta-OAT cDNA was fused to the CaMV35S promoter and introduced via Agrobacterium transformation into Nicotiana plumbaginifolia. Overexpression of the delta-OAT cDNA in the analysed transgenic lines was linked to an increase in delta-OAT enzyme activity. The transgenic lines presenting high enzymatic activity synthesized more proline than the control plants and showed a higher biomass and a higher germination rate under osmotic stress conditions. These studies reveal a new and efficient way to increase proline content in plants and to enhance crop tolerance.