The VTC2 cycle and the de novo biosynthesis pathways for vitamin C in plants: An opinion

The VTC2 cycle originally proposed by [Liang et al. Proc. Natl. Acad. Sci. USA 104, 2007, 9534-9539] comprises the plant GDP-l-galactose/GDP-d-glucose: hexose 1-phosphate guanylyltransferase (VTC2) cycle that involves a long-known, although still undentified GDP-d-mannose 2″-epimerase in addition to the VTC2 guanylyltransferase and GDP-mannose 3″,5″-epimerase, and couples the synthesis of vitamin C and the formation of GDP-sugars for polymers with the photosynthetic carbon flow and salvage pathways for the utilization of hexose in plants.