Ascorbate metabolism and the developmental demand for tartaric and oxalic acids in ripening grape berries.

Vanessa J Melino,Kathleen L Soole,Christopher M Ford

doi:10.1186/1471-2229-9-145

Vanessa J Melino, Kathleen L Soole + Show 1 more

Open Access

https://doi.org/10.1186/1471-2229-9-145

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Abstract

BackgroundFresh fruits are well accepted as a good source of the dietary antioxidant ascorbic acid (Asc, Vitamin C). However, fruits such as grapes do not accumulate exceptionally high quantities of Asc. Grapes, unlike most other cultivated fruits do however use Asc as a precursor for the synthesis of both oxalic (OA) and tartaric acids (TA). TA is a commercially important product in the wine industry and due to its acidifying effect on crushed juice it can influence the organoleptic properties of the wine. Despite the interest in Asc accumulation in fruits, little is known about the mechanisms whereby Asc concentration is regulated. The purpose of this study was to gain insights into Asc metabolism in wine grapes (Vitis vinifera c.v. Shiraz.) and thus ascertain whether the developmental demand for TA and OA synthesis influences Asc accumulation in the berry.ResultsWe provide evidence for developmentally differentiated up-regulation of Asc biosynthetic pathways and subsequent fluctuations in Asc, TA and OA accumulation. Rapid accumulation of Asc and a low Asc to dehydroascorbate (DHA) ratio in young berries was co-ordinated with up-regulation of three of the primary Asc biosynthetic (Smirnoff-Wheeler) pathway genes. Immature berries synthesised Asc in-situ from the primary pathway precursors D-mannose and L-galactose. Immature berries also accumulated TA in early berry development in co-ordination with up-regulation of a TA biosynthetic gene. In contrast, ripe berries have up-regulated expression of the alternative Asc biosynthetic pathway gene D-galacturonic acid reductase with only residual expression of Smirnoff-Wheeler Asc biosynthetic pathway genes and of the TA biosynthetic gene. The ripening phase was further associated with up-regulation of Asc recycling genes, a secondary phase of increased accumulation of Asc and an increase in the Asc to DHA ratio.ConclusionWe demonstrate strong developmental regulation of Asc biosynthetic, recycling and catabolic genes in grape berries. Integration of the transcript, radiotracer and metabolite data demonstrates that Asc and TA metabolism are developmentally regulated in grapevines; resulting in low accumulated levels of the biosynthetic intermediate Asc, and high accumulated levels of the metabolic end-product TA.

Highlights

Fresh fruits are well accepted as a good source of the dietary antioxidant ascorbic acid (Asc, Vitamin C)
Biochemical and metabolite approaches were taken to study the various facets of Asc metabolism including Asc biosynthesis, Asc recycling and Asc turnover. We demonstrate that both grapevine fruit and vegetative tissue can use D-mannose and Lgalactose for the synthesis of Asc and for further metabolism to tartaric acids (TA) and OA
The developmental stage of veraison is indicated by a grey dotted box

Summary

Introduction

Fresh fruits are well accepted as a good source of the dietary antioxidant ascorbic acid (Asc, Vitamin C). Fruits such as grapes do not accumulate exceptionally high quantities of Asc. Grapes, unlike most other cultivated fruits do use Asc as a precursor for the synthesis of both oxalic (OA) and tartaric acids (TA). The enzyme L-galactono-1,4-lactone dehydrogenase, which is capable of synthesising Asc from L-galactono-1,4-lactone, is bound to the inner mitochondrial membrane, in association with Complex I [6,7] This enzyme is part of the Smirnoff-Wheeler Asc biosynthetic pathway, which is widely accepted as the major pathway contributing to Asc accumulation in plants (Figure 1)

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