Two UGT84A Family Glycosyltransferases Regulate Phenol, Flavonoid, and Tannin Metabolism in Juglans regia (English Walnut).

Houston J Saxe,Timothy S Butterfield,Abhaya M Dandekar,Takanori Horibe,Bipin Balan,Christopher M Zabaneh,Noah G Feinberg,Aaron E Jacobson

doi:10.3389/fpls.2021.626483

Abstract

We showed previously that gallic acid is produced in walnut from 3-dehydroshikimate by a shikimate dehydrogenase (JrSkDH). This study focuses on the next step in the hydrolysable tannin pathway, the formation of 1-O-galloyl-β-D-glucose from the phenolic gallic acid and UDP glucose by a glycosyltransferase. JrGGT1 (UGT84A73) and JrGGT2 (UGT84A74) are predicted to be two such glycosyltransferases, which we expressed in tobacco plants. GC-MS analysis of the transgenic tobacco revealed moderate, yet significant alterations in plant secondary metabolism, such as depleted phenolic acids, including gallic acid. We postulate that these effects are due to JrGGT1 and JrGGT2 activity, as JrGGT orthologs glycosylate these phenolic compounds in vitro. Moreover, JrGGT expression in tobacco caused upregulation of shikimic acid pathway metabolites and differing responses in phenylpropanoids, such as phenolic acids and flavonoids. In transcriptome analysis of walnut pellicle tissues, both JrGGTs showed substantial and significant expression correlations with the gallic acid-producing JrSkDHs and were highly coexpressed with the genetic circuits constituting the shikimic acid and phenylpropanoid biosynthetic pathways. Verification of JrGGT gene expression by transcriptome analysis of 20 walnut tissues revealed striking similarities with that of the pellicle data, with the greatest expression in roots, wood, buds, and leaves of Juglans regia cv. Chandler: tissues that typically accumulate hydrolysable tannins. Like the transgenic tobacco, pellicle metabolomic analyses revealed that many phenylpropanoids correlated negatively with JrGGT expression, while shikimic acid pathway metabolites correlated positively with JrGGT expression. This research supports the hypothesis that JrGGT1 and JrGGT2 play non-trivial roles in metabolism of phenolic acids, flavonoids, and ostensibly, tannins.

Highlights

Phenolic compounds are a large class of plant secondary metabolites that include phenolic acids, flavonoids, and tannins (Abbas et al, 2017)
We identified two genes, JrGGT1 and JrGGT2, based on their significant homology to QrUGT84A13 and postulated that these two genes were likely candidates involved in the synthesis of 1-O-galloyl-β-D-glucose, an essential precursor for synthesis of hydrolysable tannins (HTs) (MartinezGarcia et al, 2016)
We cloned full-length protein-encoding regions corresponding to JrGGT1 and JrGGT2 directly from DNA extracted from Juglans regia cv

Summary

Introduction

Phenolic compounds are a large class of plant secondary metabolites that include phenolic acids, flavonoids, and tannins (Abbas et al, 2017). JrSkDH facilitates a two-step reaction in which 3-DHQ is converted to 3dehydroshikimate (3-DHS) by the dehydratase subunit and is subsequently acted on by the NADP+/(H)-dependent dehydrogenase subunit to produce shikimate (Muir et al, 2011). This assembly of two enzymes is different than in bacteria, where dehydratase and dehydrogenase are separate enzymes (Michel et al, 2003; Muir et al, 2011; Maeda and Dudareva, 2012)

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Discussion

Conclusion