Proanthocyanidin Biosynthesis in Plants

Using the PA-specific stain dimethylaminocinnamaldehyde as a screen, we have recently described six novel Tannin-deficient seed Arabidopsis mutants

Gregory J. Tanner; Kathy T. Francki; Sharon Abrahams; John M. Watson; Philip J. Larkin; Anthony R. Ashton

2003

Scholarcy highlights

  • Leucoanthocyanidin reductase catalyzes the synthesis of catechin, an initiating monomer of condensed tannin or proanthocyanidin synthesis, from 3,4-cis-leucocyanidin and is the first committed step in PA biosynthesis
  • The first committed step in PA biosynthesis that diverges from the pathway common with anthocyanins is believed to be the synthesis of an initiating flavan-3-ol such as catechin or epicatechin directly or indirectly from flavan-3,4diols
  • Polymerization continues by sequential addition of flavan-3,4-diols to the growing chain. This makes the pathway unusual because during the synthesis of a polymer of ten units only 10% of the monomer flux need proceed the full-length of the pathway while the 90% contributing to extension units does not pass through the catechin or epicatechin steps The only biochemical steps that have been demonstrated in vitro are the NADPH-dependent reduction of3,4cis-leucocyanidin to catechin catalyzed by leucoanthocyanidin reductase that removes the C4 hydroxyl group and an anthocyanidin reductase that converts anthocyanidin to epicatechin
  • The extracted PA remained in the 30% PEG/pH 5.8 supernatant while about 1% of the PA was present in the protein precipitated by 30% PEG and a further 1% in the LAR fraction precipitated at pH 5.8
  • Incubation of LAR and NADPH with unlabeled 2,3-trans-3,4-cis-leucocyanidin transformed the leucocyanidin into an A280 absorbing peak that eluted at the same retention value as authentic catechin
  • In the presence of excess LAR more than 95% of the 3,4-cis-leucocyanidin could be converted tocatechin product
  • BANYULS has recently been shown to catalyze the conversion of anthocyanidin to epicatechin and the TDS4 gene has been identified as the Arabidopsis Leucocyanidin DiOXygenase gene2 indicating that there are two pathways from leucocyanidin to catechin-like molecules, a single step reaction catalyzed by LAR and a two step reaction catalyzed by LDOX and BANYULS
  • We have obtained similar phenotypes with tobacco containing ectopically expressed Arabidopsis BANYULS and while we can detect a range of molecules that react with DMACA and butyl alcohol-HCl, we can detect no polymers by thin layer chromatography.3 Our observations suggest that the claim that the expression of BANYULS alone results in the production of condensed tannin based on the specificity of DMACA and butyl alcohol-HCl reagents is premature

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