A soybean cell wall protein is affected by seed color genotype

The dominant I gene inhibits accumulation of anthocyanin pigments in epidermal cells of the soybean seed coat. We compared saline-soluble proteins extracted from developing seed coats and identified a 35-kilodalton protein that was abundant in Richland (genotype I/I, yellow) and much reduced in an isogenic mutant line T157 (genotype i/i, imperfect black seed coats). We purified the 35-kilodalton protein by a novel procedure using chromatography on insoluble polyvinylpolypyrrolidone. The 35-kilodalton protein was composed primarily of proline, hydroxyproline, valine, tyrosine, and lysine. Three criteria (N-terminal amino acid sequence, amino acid composition, and sequence of a cDNA) proved that the seed coat 35-kilodalton protein was PRP1, a member of a proline-rich gene family expressed in hypocotyls and other soybean tissues. The levels of soluble PRP1 polypeptides and PRP1 mRNA were reduced in young seed coats with the recessive i/i genotype. These data demonstrated an unexpected and novel correlation between an anthocyanin gene and the quantitative levels of a specific, developmentally regulated cell wall protein. In contrast, PRP2, a closely related cell wall protein, was synthesized later in seed coat development and was not affected by the genotype of the I locus.