CHALCONE SYNTHASE MESSENGER-RNA AND ACTIVITY ARE REDUCED IN YELLOW SOYBEAN SEED COATS WITH DOMINANT I-ALLELES

The seed of all wild Glycine accessions have black or brown pigments b ecause of the homozygous recessive i allele in combination with allele s at the R and T loci. In contrast, nearly all commercial soybean (Gly cine max) varieties are yellow due to the presence of a dominant allel e of the I locus (either I or i(i)) that inhibits pigmentation in the seed coats. Spontaneous mutations to the recessive i allele occur in t hese varieties and result in pigmented seed coats. We have isolated a clone for a soybean dihydroflavonol reductase (DFR) gene using polymer ase chain reaction. We examined expression of DFR and two other genes of the flavonoid pathway during soybean seed coat development in a ser ies of near-isogenic isolines that vary in pigmentation as specified b y combinations of alleles of the I, R, and T loci. The expression of p henylalanine ammonia-lyase and DFR mRNAs was similar in all of the gen e combinations at each stage of seed coat development. In contrast, ch alcone synthase (CHS) mRNA was barely detectable at all stages of deve lopment in seed coats that carry the dominant I allele that results in yellow seed coats. CHS activity in yellow seed coats (I) was also 7- to 10-fold less than in the pigmented seed coats that have the homozyg ous recessive i allele. It appears that the dominant I allele results in reduction of CHS mRNA, leading to reduction of CHS activity as the basis for inhibition of anthocyanin and proanthocyanin synthesis in so ybean seed coats. A further connection between CHS and the I locus is indicated by the occurrence of multiple restriction site polymorphisms in genomic DNA blots of the CHS gene family in near-isogenic lines co ntaining alleles of the I locus.