1. The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, School of Biological Sciences, The University of Edinburgh, Daniel Rutherford Building, The King's Buildings, Edinburgh EH9 3JH, UK

Correspondence to: Stephen C. Fry¹ Correspondence and requests for materials should be addressed to S.C.F. (Email: S.Fry@Ed.Ac.UK).

Abstract

Increasing the l-ascorbate (vitamin C) content of crops could in principle involve promoting its biosynthesis or inhibiting its degradation. Recent progress has revealed biosynthetic pathways for ascorbate^{1, 2, 3}, but the degradative pathways remain unclear. The elucidation of such pathways could promote an understanding of the roles of ascorbate in plants⁴, and especially of the intriguing positive correlation between growth rate and ascorbate oxidase^{5, 6} (or its products⁷). In some plants (Vitaceae), ascorbate is degraded via l-idonate to l-threarate (l-tartrate), with the latter arising from carbons 1–4 of ascorbate^{3, 8, 9, 10, 11}. In most plants, however (including Vitaceae)¹¹, ascorbate degradation can occur via dehydroascorbate, yielding oxalate¹² plus l-threonate, with the latter from carbons 3–6 of ascorbate^{3, 10, 13}. The metabolic steps between ascorbate and oxalate/l-threonate, and their subcellular location, were unknown. Here we show that this pathway operates extracellularly in cultured Rosa cells, proceeds via several novel intermediates including 4-O-oxalyl-l-threonate, and involves at least one new enzyme activity. The pathway can also operate non-enzymatically, potentially accounting for vitamin losses during cooking. Several steps in the pathway may generate peroxide; this may contribute to the role of ascorbate as a pro-oxidant^{14, 15} that is potentially capable of loosening the plant cell wall and/or triggering an oxidative burst.

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