PIGMENTED SOYBEAN (GLYCINE-MAX) SEED COATS ACCUMULATE PROANTHOCYANIDINS DURING DEVELOPMENT

The dominant I gene inhibits accumulation of anthocyanin pigments in t he epidermal layer of soybean (Glycine max) seed coats. Seed-coat colo r is also influenced by the R locus and by the pubescence color allele s (T, tawny; t, gray). Protein and RNA from cultivars with black (i,R, T) and brown (i,r,T) seed coats are difficult to extract. To determine the nature of the interfering plant products, we examined seed-coat e xtracts from Clark isogenic lines for flavonoids, anthocyanins, and po ssible proanthocyanidins by thin-layer chromatography. We show that ye llow seed-coat varieties (I) do not accumulate anthocyanins (anthocyan idin glycosides) or proanthocyanidins (polymeric anthocyanidins). Matu re, black (i,R,T) and imperfect-black (i,R,t) seed coats contained ant hocyanins, whereas mature, brown (i,r,T) and buff (i,r,t) seed coats d id not contain anthocyanins. In contrast, all colored (i) genotypes te sted positive for the presence of proanthocyanidins by butanol/HCl and 0.5% vanillin assays. Immature, black (i,R,T) and brown (i,r,T) seed coats contained significant amounts of procyanidin, a 3',4'-hydroxylat ed proanthocyanidin. Immature, black (i,R,T) or brown (i,r,T) seed-coa t extracts also tested positive for the ability to precipitate protein s in a radial diffusion assay and to bind RNA in vitro. Imperfect-blac k (i,R,t) or buff (i,r,t) seed coats contained lesser amounts of prope largonidin, a 4'-hydroxylated proanthocyanidin. Seed-coat extracts fro m these genotypes did not have the ability to precipitate protein or b ind to RNA. In summary, the dominant I gene controls inhibition of not only anthocyanins but also proanthocyanidins in soybean seed coats. I n homozygous recessive i genotypes, the T-t gene pair determines the t ypes of proanthocyanidins present, which is consistent with the hypoth esis that the T locus encodes a microsomal 3'-flavonoid hydroxylase.