A defective seed coat pattern (Net) is correlated with the post-transcriptional abundance of soluble proline-rich cell wall proteins

The pigmented seed coats of several soybean (Glycine max (L.) Merr.) plant introductions and isolines have unusual defects that result in cracking of the mature seed coat exposing the endosperm and cotyledons. It has previous ly been shown that the T (tawny) locus that controls the color of trichomes on stems and leaves also has an effect on both the structure and pigmentat ion of the seed coat. Distribution of pigmentation on the seed coat is cont rolled by alleles of the I (inhibitor) locus. It was also found that total seed coat proteins were difficult to extract from pigmented seed coats with i T genotypes because they have procyanidins that exhibit tannin propertie s. We report that the inclusion of poly-L-proline in the extraction buffer out-competes proteins for binding to procyanidins. Once this problem was so lved, we examined expression of the proline-rich cell wall proteins PRP1 an d PRP2 in pigmented genotypes with the dominant T allele. We found that bot h homozygous i T and i t genotypes have reduced soluble PRP1 levels. The ep istatic interaction of the double recessive genotype at both loci is necess ary to produce the pigmented, defective seed coat phenotype characteristic of seed coats with the double recessive i and t alleles. This implies a nov el effect of an enzyme in the flavonoid pathway on seed coat structure in a ddition to its effect on flavonoids, anthocyanidins, and proanthocyanidins. No soluble PRP1 polypeptides were detectable in pigmented seed coats (i T genotypes) of isolines that also display a net-like pattern of seed coat cr acking, known as the Net defect. PRP2 was also absent in one of the these l ines. However, both PRP1 and PRP2 cytoplasmic mRNAs were found in the Net-d efective seed coats. Together with in vitro translation studies, these resu lts suggest that the absence of soluble PRP polypeptides in the defective N et lines is post-translational and could be due to a more rapid or prematur e insolubilization of PRP polypeptides within the cell wall matrix.