Pyridoxine is required for post-embryonic root development and tolerance to osmotic and oxidative stresses.

Pyridoxine (vitamin B6) is a cofactor required by numerous enzymes in all cellular organisms. Plants are the major source of vitamin B6 for animals, yet the biosynthesis pathway and the function of vitamin B6 in plants are not well elucidated. In this study, an Arabidopsis pyridoxine synthase gene PDX1 was characterized and its in vivo functions were investigated. The PDX1 gene was expressed in all plant parts examined and its expression level was not significantly regulated by abiotic stress or the phytohormone abscisic acid. In roots, PDX1 was highly expressed in a defined region behind the root tips that undergoes rapid cell division. The PDX1 protein was mainly associated with the plasma membrane and endomembranes, implying a potential involvement of vitamin B6 in membrane function. To reveal the in vivo role of PDX1, Arabidopsis insertional mutants were isolated. Strikingly, the pdx1 knockout mutants were impaired in root growth and early seedling development. The stunted roots resulted from both reduced cell division and elongation. Supplementation of the growth media with pyridoxine or reintroduction of the wild-type PDX1 gene into the mutants completely restored the mutant growth, demonstrating that PDX1 is required for pyridoxine biosynthesis in planta. In addition to the developmental defects, pdx1 mutants are hypersensitive to osmotic stress and oxidative stress. These mutant seedlings had increased peroxidation of membrane lipids following UV treatment. Our study establishes a critical role of vitamin B6 in plant development and stress tolerance and suggests that vitamin B6 may represent a new class of antioxidant in plants.