Ascorbate biosynthesis in higher plants. (a) Chemical structure of L-ascorbate. (b) Two alternative pathways, the D-galacturonate and the myo-inositol pathways, may contribute to L-ascorbate biosynthesis via L-galactonate and L-glucuronate, respectively, entering the L-galactose pathway. However, because hard proof for the existence of either pathway is missing for plants, the D-galacturonate and myo-inositol pathways are shown in light grey. L-GulLO, L-gulono-1,4-lactone oxidase; L-GalDH, L-galactose dehydrogenase; GDP-mannose epimerase (GME). Dashed arrows indicate multiple steps in between two points.