FIG. 1.

Biosynthetic pathways to L-ascorbate in higher plants. The major ascorbate biosynthetic pathway is the D-Man/L-Gal or Smirnoff-Wheeler pathway (yellow). The minor L-gulose pathway is branched at GDP-Man 3′,5′-epimerase (purple). The last step of this alternative route is the oxidation of L-gulono-1,4-lactone to ascorbate. The myo-Inositol branch (blue); also provides L-gulono-1,4-lactone for the biosynthesis of ascorbate via the formation of UDP-glucuronate by myo-Inositol oxidase. The D-galacturonic acid route (green), uses D-galacturonic acid, a principal component of cell wall and similar to the classical D-Man/L-Gal pathway provides galactono-1,4-lactone via the formation of L-galactonate by D-galacturonate reductase. Question marks indicate reactions where the gene and the specific enzyme have not been identified yet. The conversion of L-galactono-1,4-lactone to ascorbate by GLDH coupled to mitochondrial ETC is shown in red (mitochondria-like shape) circle. ETC, electron transfer chain; GLDH, L-galactono-1,4-lactone dehydrogenase. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars