Proanthocyanidin Biosynthesis in the Developing Wheat Seed Coat Investigated by Chemical and RNA-seq Analysis

The composition of proanthocyanidins in the testa (seed coat) of bread wheat was analysed by thiolysis of PA oligomers from developing grain and found to consist of (+)-catechin monomers, with a small amount of (+)-gallocatechin. The average chain length of soluble PA stayed relatively constant between 10 and 20 days post-anthesis, whereas that of unextractable PA increased over the same period, suggesting that increases in chain length might account for the insolubility of PAs from mature wheat grain. We carried out RNAseq followed by differential expression analysis from dissected tissues of developing grain from red- and white-grained near-isogenic lines differing in the presence of an active R gene that encodes a MYB transcription factor involved in control of PA biosynthesis. In addition to genes already identified encoding chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase and dihydroxyflavonoid 4-reductase, we showed that wheat genes encoding phenylalanine ammonia lyase, flavonoid 3’5’-hydroxylase, leucoanthocyanidin reductase and a glutathione S-transferase (the orthologue of maize Bronze-2) were more highly expressed in the red NIL. We also identified candidate orthologues of other catalytic and regulatory components of flavonoid biosynthesis in wheat.