Pathway Engineering of the Plant Vitamin C Metabolic Network

Abstract

Vitamin C (ascorbic acid, AsA) is an important primary metabolite of plants that functions as an antioxidant, an enzyme cofactor, and a cell-signalling modulator in a wide array of crucial physiological processes, including biosynthesis of the cell wall, secondary metabolites and phytohormones, stress resistance, photoprotection, cell division, senescence, and growth. Humans and related primates have lost the ability to synthesize AsA and therefore must obtain it in the diet – primarily from plants. Despite its importance, our understanding of plant vitamin C biosynthesis remains incomplete. Several routes leading to AsA formation have been proposed: from d-glucose via d-mannose and l-galactose; from myo-inositol; from galacturonic acid, and from l-gulose. It is unclear whether these are independent pathways or whether they interlink, possibly via enzymes with nonspecific activity. Several enzymes in the vitamin C network have yzet to be characterized, either biochemically or genetically, and the relative contribution of each branch to total AsA in plant tissues, and the mechanisms behind AsA homeostasis are largely unknown. Mutant analysis and transgenic studies in Arabidopsis thaliana and other model systems have provided important insight into the regulation, activities, integration, and evolution of individual enzymes and are already providing a knowledge base for breeding and transgenic approaches to modify the level of vitamin C in agricultural crops