# Vitamin C in Plants: From Functions to Biofortification
## 4 Vitamin C Biosynthesis in Plants
### Other Vitamin C biosynthetic Pathways  
As reported above, theulose pathway starts from the 3' epimerization of the GDP-p-mannose catalyzed by GME, with the formation of GDP-l-glucose (Figure 2) [122]. In this pathway, GDP-l-glucose is successively converted into l-guolse-1P, l-guolse, and l-gulono-1,4-lactone. l-gulono-1,4-lactone has been detected in plant extracts [161]. Moreover, external supplementation of l-gulono-1,4-lactone causes an increase in vitamin C content in _Arabidopsis_ cells and tobacco leaves but is less efficient than feeding with l-galactono-1,4-lactone [162; 163; 164]. Activity of gulonolactone oxidase (GuILO) has been found in potato [163], and more recently, two genes coding for GuILOs have been identified in _Arabidopsis_. These enzymes are dehydrogenases with specificity for l-gulono-1,4-lactone and differ from plant GLDHs and mammalian GuILOs. The _Arabidopsis_ GuILOs seem to be regulated post-transcriptionally and the limited enzyme availability could explain the slow utilization of the substrate [165].  
l-Gulono-1,4-lactone is also the last precursor of vitamin C biosynthesis in the myoinositol pathway (Figure 2). In this pathway, myoinositol is converted into p-glucuronate by myoinositol oxygenase (MIOX). The other two steps producing l-gulonic acid and l-gulono-1,4-lactone are respectively catalyzed by glucuronate reductase and aldono lactonase [166]. A myoinositol oxygenase (MIOX4) has  
Figure 2: Schematic representation of vitamin C biosynthetic pathways. Different colors indicate different pathways. In grey, the myoinositol pathway; in green, the gulose pathway; in yellow, the b-mannose/l-galactose pathway; in orange, the galacturonate pathway. Represented in blue are the initial steps leading to GDP-d-mannose, which is a common precursor to the d-mannose/l-galactose and gulose pathways. A question mark indicates enzymes not identified in plants (more details are provided in the text). Abbreviations: AL, aldono lactonase; GalUR, d-galacturonate reductase; GDH, l-galactose dehydrogenase; GGP, GDP-l-galactose-phosphorylase; GGIuP, GDP-l-glucose pyrophosphatase; GLDH, l-galactono-1,4-lactone dehydrogenase; GluUR, glucuronate reductase; GME, GDP-p-mannose epimerase; GMP, GDP-p-mannose pyrophosphylase; GPP, l-galactose-1-phosphate phosphatase; GuLDH, l-gulose dehydrogenase; GuIP, l-gulose-1-phosphate phosphatase; GuILO, gulonolactone oxidase; MIOX, myoinositol oxygenase; PGI, phosphoglucose isomerase; PME, pectin methyl esterase; PMI, phosphomannose isomerase; PMM, phosphomannose mutase.  
been identified in _Arabidopsis_[123]. However, the effective contribution of myoinositol to vitamin C synthesis in vivo is strongly debated [167; 168; 169].  
The galacturonate pathway is also known as the salvage pathway, since it utilizes sugars provided by the breakdown of cell walls [124]. The degradation of pectin releases methyl-galacturonate, which is converted into n-galacturonate by methyl esterase and successively into l-galactonate by p-galacturonate reductase (GalUR). An aldono lactonase converts l-galactonate into l-galactono-1,4-lactone, which is the last precursor of vitamin C in the Smirnoff-Wheeler pathway (Figure 2) [27]. A gene encoding GalUR was initially identified in strawberry [124; 170]. This pathway seems to be active mainly during fruit ripening in some species [124; 170; 171; 172; 173]. In tomato, vitamin C synthesis in immature green fruit is enhanced only by the supply of l-galactose, whereas in red ripened fruits by feeding with both l-galactose and p-galacturonate [170]. Moreover, the high vitamin C content found in tomato introgression lines IL12-4 compared with the parental M82 seems to be due to a higher expression of a pectinesterase and two polygalacturonases [171].