GENETIC LENGTH POLYMORPHISMS CREATE SIZE VARIATION IN PROLINE-RICH PROTEINS OF THE CELL-WALL

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 varie ties, both proteins are smaller as determined by immunoblotting seed c oat proteins separated by sodium dodecyl sulfate polyacrylamide gel el ectrophoresis. It was found that Prp1 and Prp2 genes are linked to eac h other by testing seed coat protein extracts from an F-2 population o f 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. Sim ultaneous expression of soluble PRP2 polypeptides in the maternal seed coat and underlying aleurone layer of the embryo was found. The molec ular basis for the size difference between the two varieties was exami ned using polymerase chain reaction to isolate the Prp genes from Blac khawk, the variety that exhibited the smaller proteins. Both of the ge nes from Blackhawk contained length polymorphisms that result in omiss ion of some of the repeat units (pro pro val tyr lys) from the protein s. In Prp1, there were two separate deletions in different parts of th e gene, each being two tandem repeats in length. In Prp2, there was on ly one deletion of two tandem repeats. These deletions occur within th e coding regions in a manner that conserves the reading frame. The res ults are the first description of genetic variation in cell wall prote ins and its molecular basis. The nature of these genetic length polymo rphisms indicates that the basic repeat unit structure is conserved an d that the proline-rich proteins of the cell wall tolerate loss or gai n of integral numbers of repeat units.