INHERITANCE OF RED-BUFF SEED COAT IN SOYBEAN

In soybean [Glycine max (L.) Merr.], five loci ((I/i-k/i-i/i, R/r-m/r, T/t, W1/w1, O/o) interact to produce seed coat and/or hilum color phenotypes of gray (G), yellow (Y), black (Bl), imperfect black (Ib), brown (Br), striped brown-black, red-brown (Rbr), or buff (Bf). The T/t and W1/w1 loci also condition tawny/gray pubescence and purple/white flowers, respectively. T236, a white-flowered gray-pubescence accession in the soybean genetic collection, has an unusual red-buff (Rbf) seed coat. The genetic relationship of Rbf to the other seed coat colors was not known. To evaluate this relationship, T236 was mated to six soybean lines of known genotype relative to the five loci. F2 individuals were classified as to flower, pubescence, seed coat, and hilum color. In all crosses, only F2 plants with white flowers, gray pubescence, and an R- genotype produced Rbf seed coats or hila. The F2 segregation ratios were not compatible with an inheritance model for Rbf that invoked allelic segregation at a sixth locus. Indeed, Rbf seed coat seemed to be conditioned by a new allele at the T/t locus, symbolized as t-r. While there was no perceptible difference in the gray pubescence phenotypes conditioned by the t and t-r alleles, the t-r allele was detectable in an R-w1w1 genetic background, where the allelic series T/t-r/t produced the seed coat color phenotypes of B1/Rbf/Bf (in left-to-right dominance-recessiveness order). In all other backgrounds (i.e., R-W1-, rrW1-, and rrw1w1), the seed coat colors produced by t and t-r were identical. The currently accepted model for the inheritance of anthocyanidin pigments in the soybean seed coat is that: (i) R is necessary for anthocyanidin production, (ii) T is needed for dihydroxylation of the B-ring, (iii) W1 is needed for trihydroxylation of the B-ring. Our postulation that the Rbf seed coat phenotype requires an R-t-r-w1w1 genotype is consistent with this theory. It is also consistent with the report that pelargonidin, a monohydroxylated B-ring anthocyanidin, is the predominant pigment in the Rbf seed coats of strain T236.