Genetic length polymorphisms create size variation in proline‐rich proteins of the cell wall

Summary Two genes, Prp1 and Prp2 , encode proline‐rich proteins that are found in different stages of developing seed coats, hypocotyls, and roots of soybeans ( Glycine max (L.) Merr.). PRP1 is found in young seed coats and PRP2 is found later during seed dessication. In some soybean varieties, both proteins are smaller as determined by immunoblotting seed coat proteins separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. It was found that Prp1 and Prp2 genes are linked to each other by testing seed coat protein extracts from an F 2 population of a cross between the cultivar Richland, which exhibits the larger PRP proteins, and Blackhawk, which has the smaller PRP proteins. The Prp1 and Prp2 genes were separated by approximately 13% recombination. Simultaneous expression of soluble PRP2 polypeptides in the maternal seed coat and underlying aleurone layer of the embryo was found. The molecular basis for the size difference between the two varieties was examined using polymerase chain reaction to isolate the Prp genes from Blackhawk, the variety that exhibited the smaller proteins. Both of the genes from Blackhawk contained length polymorphisms that result in omission of some of the repeat units (pro pro val tyr lys) from the proteins. In Prp1 , there were two separate deletions in different parts of the gene, each being two tandem repeats in length. In Prp2 , there was only one deletion of two tandem repeats. These deletions occur within the coding regions in a manner that conserves the reading frame. The results are the first description of genetic variation in cell wall proteins and its molecular basis. The nature of these genetic length polymorphisms indicates that the basic repeat unit structure is conserved and that the proline‐rich proteins of the cell wall tolerate loss or gain of integral numbers of repeat units.