Abstract
In plants, the shikimate pathway provides aromatic amino acids that are used to generate numerous secondary metabolites, including phenolic compounds. In this pathway, shikimate dehydrogenases (SDH) 'classically' catalyse the reversible dehydrogenation of 3-dehydroshikimate to shikimate. The capacity of SDH to produce gallic acid from shikimate pathway metabolites has not been studied in depth. In grapevine berries, gallic acid mainly accumulates as galloylated flavan-3-ols. The four grapevine SDH proteins have been produced in Escherichia coli In vitro, VvSDH1 exhibited the highest 'classical' SDH activity. Two genes, VvSDH3 and VvSDH4, mainly expressed in immature berry tissues in which galloylated flavan-3-ols are accumulated, encoded enzymes with lower 'classical' activity but were able to produce gallic acid in vitro The over-expression of VvSDH3 in hairy-roots increased the content of aromatic amino acids and hydroxycinnamates, but had little or no effect on molecules more distant from the shikimate pathway (stilbenoids and flavan-3-ols). In parallel, the contents of gallic acid, β-glucogallin, and galloylated flavan-3-ols were increased, attesting to the influence of this gene on gallic acid metabolism. Phylogenetic analysis from dicotyledon SDHs opens the way for the examination of genes from other plants which accumulate gallic acid-based metabolites.
Highlights
Grapevine (Vitis vinifera L.) is one of the most widespread fruit plants in the world
According to the 12X version of the Vitis vinifera genome and the V2 annotation, VvSDH1 is located on chromosome 5 whereas the other three genes are clustered on chromosome 14 (Supplementary Fig. S1)
VvSDH2 clustered in group II with NtSDH2 and the characterized poplar DQD/shikimate dehydrogenases (SDH) that did not exhibit ‘classical’ SDH activity, but rather quinate dehydrogenase activity from quinate and NAD+ (Poptr2 and -3, Guo et al, 2014)
Summary
Grapevine (Vitis vinifera L.) is one of the most widespread fruit plants in the world. Cultivated for winemaking, it is a crop of economic interest found in numerous world regions with a temperate climate. Proanthocyanidins (PAs) are oligomers and polymers of flavan-3-ols. They are the major flavonoids in grape berry and wine and influence astringency, bitterness, and colour stability (Arnold et al, 1980). Their biosynthesis starts before flowering and their accumulation peak occurs around véraison in seeds and skin (Bogs et al, 2005). The main PA subunit, (-)-epicatechin, is partly acylated with gallic acid (GA), a trihydroxybenzoic acid, to form (-)-epicatechin 3-O-gallate. The percentage of (-)-epicatechin 3-O-gallate in a PA chain defines the galloylation rate (%G). The GA glucose ester, named β-glucogallin (β-G), is a precursor for the biosynthesis of both hydrolysable tannins (Haslam and Cai, 1994; Niemetz and Gross, 2005) and galloylated PAs (Liu et al, 2012)
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