Ascorbate function and associated transport systems in plants

Ascorbate is present in different cell compartments of higher plant cells. At a physiological level, the best-studied phenomena involving ascorbate is its participation in an oxygen scavenging pathway in the chloroplast known as the ascorbate-glutathione cycle. In addition, evidence is emerging that ascorbate fulfils essential roles in growth, development and defence outside the chloroplast. Despite its importance in plant biology, the pathway of ascorbate biosynthesis has only recently been elucidated. From the site of synthesis in the mitochondria, ascorbate must be transported to other cellular compartments where it accumulates to high concentrations. Translocation of ascorbate through the plasmalemma and chloroplast membrane is mediated by specific carriers. Initial observations indicate that carriers for both ascorbate and its oxidised form dehydroascorbate are present in plant membranes. Regulation of ascorbate transport systems may be central in the regulation of different physiological processes including progression through the cell cycle, expansion of the cell wall and defence against abiotic and biotic threats.