Ascorbate (L-ascorbic acid, L-AA, Vitamin C) is essential for both plants and animals. It is an important enzymatic cofactor, as well as a primary antioxidant, that is thought to be important in preventing various oxidative stress-related conditions (Davey et al. 2000; De Tullio and Arrigoni 2004; Li and Schellhorn 2007). Because humans, and a few other animal species have lost the capacity to synthesize it, they are dependent on diet to ensure adequate ascorbate levels (Linster and Van Schaftingen 2007). Plants, especially leaves and fruits, are a good source of ascorbate; it is therefore important to consume substantial amounts of fruits and vegetables (Poiroux-Gonord et al. 2010).

In plants and animals, ascorbate detoxifies reactive oxygen species (ROS). In plants, ROS are formed during the process of lignification, cell division and the hypersensitive response (Davey et al. 2000). Ascorbate is the predominant form of vitamin C; its main oxidation product L-Dehydroascorbic acid (DHA) is usually less than 10% of total vitamin C. Only one report indicates an increase of the DHA/AA ratio during storage of crops(Lee Seung and A Kade 2000).

Ascorbate is proposed to be synthesised in plants through four biosynthetic pathways: Smirnoff-Wheeler (SW), galacturonate, myo-inositol, and gulose pathways (Ntagkas et al. 2018). The predominant one is the SW pathway, and because the others are not contributing significantly, they will not be considered here. In the SW pathway, ascorbate is synthesized from D-Glucose-6P via D-Mannose and L-Galactose to L-galactono-1,4-lactone (G14L) (Figure 3.6) (Wheeler et al. 1998; Ishikawa et al. 2006). During ROS scavenging ascorbate is firstly oxidized into monodehydroascorbate (MDHA), catalysed by ascorbate oxidase and ascorbate peroxidase (Nakano and Adada 1981). MDHA can be turned back into ascorbate by monodehydroascorbate reductase (MDHAR) (Hossain et al. 1984)or further oxidized into dehydroascorbate (DHA) in a non-enzymatic reaction. DHA can be reduced back to ascorbate by the enzyme dehydroascorbate reductase (DHAR) at the expense of reduced glutathione. These reactions occur in most subcellular components, with the exception of the apoplast (Dalton et al. 1993; Koshiba 1993; Yamaguchi et al. 1995)

Figure 3.6. Smirnoff Wheeler pathway for Ascorbate biosynthesis. Based on Ntagkas et al., (2018). Abbreviations: DHA: dehydro ascorbate; DHAR: DHA reductase; MDHA: monodehydroascorbate; MDHAR: MDHA reductase; *: conversion of MDHA to DHA is non-enzymatic.


Dalton DA, Langeberg L, Treneman NC. 1993. Correlations between the ascorbate‐glutathione pathway and effectiveness in legume root nodules. Physiologia Plantarum 87: 365–370. DOI: 10.1111/j.1399-3054.1993.tb01743.x.

Davey MW, Montagu M Van, Inze D, et al. 2000. Review Plant L -ascorbic acid : chemistry , function , metabolism , bioavailability and effects of processing. Journal of the Science of Food and Agriculture 860: 825–860.

De Tullio MC, Arrigoni O. 2004. Hopes, disillusions and more hopes from vitamin C. Cellular and Molecular Life Sciences 61: 209–219. DOI: 10.1007/s00018-003-3203-8.

Hossain MA, Nakano Y, Asada K. 1984. Monodehydroascorbate Reductase in Spinach Chloroplasts and Its Participation in Regeneration of Ascorbate for Scavenging Hydrogen Peroxide. Plant and Cell Physiology 25: 385–395. DOI: 10.1093/oxfordjournals.pcp.a076726.

Ishikawa T, Dowdle J, Smirnoff N. 2006. Progress in manipulating ascorbic acid biosynthesis and accumulation in plants. Physiologia Plantarum 126: 343–355. DOI: 10.1111/j.1399-3054.2006.00640.x.

Koshiba T. 1993. Cytosolic Ascorbate Peroxidase in Seedlings and Leaves of Maize (Zea mays). Plant and Cell Physiology 34: 713–721. DOI: 10.1093/oxfordjournals.pcp.a078474.

Lee Seung K, A Kade A. 2000. Pre-harvest and postharvest factors influencing vitamin C content of horticultural crops. Postharvest biology and technology 20: 207–220. DOI: 10.1016/S0925-5214(00)00133-2.

Li Y, Schellhorn HE. 2007. New Developments and Novel Therapeutic Perspectives for Vitamin C. The Journal of Nutrition 137: 2171–2184. DOI: 10.1093/jn/137.10.2171.

Linster CL, Van Schaftingen E. 2007. Vitamin C: Biosynthesis, recycling and degradation in mammals. FEBS Journal 274: 1–22. DOI: 10.1111/j.1742-4658.2006.05607.x.

Nakano Y, Adada K. 1981. Hydrogen Peroxide is Scavenged by Ascorbate-specific Peroxidase in Spinach Chloroplasts. Plant and Cell Physiology 22: 867–880. DOI: 10.1093/oxfordjournals.pcp.a076232.

Ntagkas N, Woltering EJ, Marcelis LFM. 2018. Light regulates ascorbate in plants: An integrated view on physiology and biochemistry. Environmental and Experimental Botany 147: 271–280. DOI: 10.1016/j.envexpbot.2017.10.009.

Poiroux-Gonord F, Bidel LPR, Fanciullino A-L, Gautier H, Lauri-Lopez F, Urban L. 2010. Health Benefits of Vitamins and Secondary Metabolites of Fruits and Vegetables and Prospects To Increase Their Concentrations by Agronomic Approaches. Journal of Agricultural and Food Chemistry 58: 12065–12082. DOI: 10.1021/jf1037745.

Wheeler GL, Jones MA, Smirnoff N. 1998. The biosynthetic pathway of vitamin C in higher plants. Nature 393: 365–369. DOI: 10.1038/30728.

Yamaguchi K, Mori H, Nishimura M. 1995. A Novel Isoenzyme of Ascorbate Peroxidase Localized on Glyoxysomal and Leaf Peroxisomal Membranes in Pumpkin. Plant and Cell Physiology 36: 1157–1162. DOI: 10.1093/oxfordjournals.pcp.a078862.