Ascorbate as a biosynthetic precursor in plants

Abstract

Background and aims: l-Ascorbate (vitamin C) has well-documented roles in many aspects of redox control and anti-oxidant activity in plant cells. This Botanical Briefing highlights recent developments in another aspect of l-ascorbate metabolism: its function as a precursor for specific processes in the biosynthesis of organic acids.

Scope: The Briefing provides a summary of recent advances in our understanding of l-ascorbate metabolism, covering biosynthesis, translocation and functional aspects. The role of l-ascorbate as a biosynthetic precursor in the formation of oxalic acid, l-threonic acid and l-tartaric acid is described, and progress in elaborating the mechanisms of the formation of these acids is reviewed. The potential conflict between the two roles of l-ascorbate in plant cells, functional and biosynthetic, is highlighted.

Conclusions: Recent advances in the understanding of l-ascorbate catabolism and the formation of oxalic and l-tartaric acids provide compelling evidence for a major role of l-ascorbate in plant metabolism. Combined experimental approaches, using classic biochemical and emerging 'omics' technologies, have provided recent insight to previously under-investigated areas.

Fig. 1.

Proposed synthetic pathways of tartaric acid (A) and oxalic acid (B) in plants (adapted from Loewus, 1999). Intermediates and final products are (i) l-ascorbic acid (ASC); (ii) 2-keto-l-idonic acid; (iii) l-idonic acid; (iv) 5-keto-d-gluconic acid; (v) l-threo-tetruronic acid; (vi) glycoaldehyde; (vii) l-tartaric acid (TA); (viii) oxalic acid (OA); and (ix) l-threonic acid. The precise nature of the reaction steps indicated by the striped arrows at (A) and (B) is unknown.