Molecular Biology of the Flavonoid Biosynthetic Pathway of Theobroma cacao

by Yi Liu (former grad student)

The flavonoids quercetin and epicatechin, and their polymerized oligomers the proanthocyanidins, accumulate to levels of up to 15% of the total weight of dry cacao seeds.  These compounds have been associated with several health benefits in humans including antioxidant activity, improvement of cardiovascular health and reduction of cholesterol levels.  They also play important roles in pest and disease defense throughout the plant.  To molecularly dissect the flavonoid biosynthetic pathway of cacao, we have isolated and begun to characterize the key genes of this pathway; leucoanthocyanidin reductase (LAR), flavonol synthase (FLS), dihydroflavonol 4-reductase (DFR), anthocyanidin reductase (ANR), leucoanthocyanidin dioxygenase (ANS) and UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT).  We have used sequence conservation with other species, and a cacao EST database to generate PCR probes used to isolate putative cDNA and genomic orthologs of each of these genes.  We are using the model plants Arabidopsis and ectopic tobacco as expression platforms to accelerate functional analysis of the encoded enzymes.  The genes are expressed in tissue specific and developmentally regulated expression patterns consistent with the known distributions of flavonoids.  Studies currently underway include evaluating differences in expression among different varieties of cacao.  This research will contribute to the development of varieties of cacao with altered levels of flavonoids through applications of molecular markers for conventional breeding and potentially in the future, through transgenic modification.