Over-expression of FaGalLDH Increases Ascorbic Acid Concentrations and Enhances Salt Stress Tolerance in Arabidopsis thaliana

The strawberry (Fragaria x ananassa) is an economically important perennial crop plant, and its fruits are rich in vitamin C (l-ascorbic acid [AsA]) and other nutrients. l-galactono-1,4-lactone dehydrogenase (GalLDH) is a key enzyme in the terminal step of AsA biosynthesis pathway in plants. Here, the GalLDH gene (FaGalLDH) was cloned from 'Benihoppe' strawberries. AsA content increased during fruit development and peaked at the red-ripening stage, and AsA concentrations in different tissues were correlated with enzyme activity and transcription level of FaGalLDH. Transient over-expression of FaGalLDH in strawberry fruit increased its overall expression and AsA production significantly, whereas transient RNAi of FaGalLDH decreased its expression and AsA content. Furthermore, the optimum pH and temperature for FaGalLDH activity were 8.0 and 25 degrees C, respectively. Ectopic expression of the FaGalLDH gene in Arabidopsis resulted in higher AsA content and enzyme activity in transgenic lines than in wild-type plants. FaGalLDH over-expression resulted in enhanced tolerance to salt stress due to reduced accumulation of malondialdehyde, H2O2, and O-2(.-), as well as higher survival rates, root length, proline content, and superoxide dismutase, peroxidase, and catalase activities. These results provide useful information regarding AsA biosynthesis and salt tolerance, which may help to improve strawberry fruit quality and productivity.