Minisymposium 11: Secondary Metabolites

Abs # 23002: The BANYLUS gene encodes anthocyanidin reductase, a key enzyme for metabolic engineering of condensed tannins in plants.

Presenter:	Xie, De-Yu , dxie@noble.org
Authors	Xie, De-Yu  (A)   Sharma, Shashi B. (A)   Paiva, Nancy L. (A)   Ferreira, Daneel  (B)   Dixon, Richard A. (A)
Affiliations:	(A): The Samuel Roberts Noble Foundation, P.O.Box 2180, Ardmore, Oklahoma, 73401, USA (B): National Center for Natural Products, School of Pharmacy, University of Mississippi, MS 38677, USA

Condensed tannins (CTs, also known as proanthocyanidins) are oligomers or polymers of flavonoid (flavan-3-ol) units that are beneficial for animal and human health. CTs present in forage plant vegetative tissues protect ruminant animals against pasture bloat and improve their nitrogen nutrition by increasing the amount of dietary protein exiting the rumen. CTs and flavan-3-ol monomers are powerful antioxidants with beneficial effects on cardiac health, cancer development, immunity and urinary tract infections. The biosynthesis of CTs in higher plants shares the same upstream pathway as that of anthocyanins. However, the major monomer present in CTs, (-)-epicatechin, has different stereochemistry from the intermediates of anthocyanin biosynthesis, and the biosynthetic origin of this molecule has therefore puzzled scientists for over 30 years. The banyuls mutant of Arabidopsis lacks CTs and precociously accumulates anthocyanins in the seed coat, and it has been suggested that BAN may encode a leucoanthocyanidin reductase (LAR) involved in CT biosynthesis. However, efforts to demonstrate that Arabidopsis BAN (AtBAN) has LAR activity (conversion of leucoanthocyanidin into (+)-flavan-3-ol) have been unsuccessful. We cloned a Medicago truncatula BAN gene (MtBAN) from a developing seed library. Transferring MtBAN into tobacco resulted in CT accumulation in flowers. MtBAN and AtBAN were shown to encode a novel NADPH/ NADH-dependent anthocyanidin reductase (ANR) converting anthocyanidin into (-)-flavan-3-ol (Xie, D-Y et al. Science, 299, 396-399, 2003). This observation explains the biosynthetic origin of (-)-epicatechin. We also present kinetic properties of BAN and the characteristics of CTs in transgenic tobacco flowers. How plants biosynthesize CTs will be discussed in this presentation. Our results suggest a new approach for the engineering of CTs for reduction of pasture bloat and value added food enhancement.