Production of secondary metabolites from cell and organ cultures: strategies and approaches for biomass improvement and metabolite accumulation

Pyrroline-5-carboxylate reductase (P5CR) lies at the converging point of the glutamate and ornithine pathways and is the last and critical enzyme in proline biosynthesis. In the present study, a P5CR gene, named IbP5CR, was isolated from salt-tolerant sweetpotato line ND98. Expression of IbP5CR was up-regulated in sweetpotato under salt stress. The IbP5CR-overexpressing sweetpotato (cv. Kokei No. 14) plants exhibited significantly higher salt tolerance compared with the wild-type. Proline content and superoxide dismutase and photosynthetic activities were significantly increased, whereas malonaldehyde content was significantly decreased in the transgenic plants. H₂O₂ was also found to be significantly less accumulated in the transgenic plants than in the wild-type. Overexpression of IbP5CR up-regulated pyrroline-5-carboxylate synthase gene and down-regulated proline dehydrogenase and P5C dehydrogenase genes under salt stress. The systemic up-regulation of reactive oxygen species (ROS) scavenging genes was found in the transgenic plants under salt stress. These findings suggest that overexpression of IbP5CR increases proline accumulation, which enhances salt tolerance of the transgenic sweetpotato plants by regulating osmotic balance, protecting membrane integrity and photosynthesis and activating ROS scavenging system. This study indicates that IbP5CR gene has the potential to be used for improving salt tolerance of plants.