Methionine and Sulfate Increase a Bowman-Birk-Type Protease Inhibitor and Its Messenger RNA in Soybeans

A soybean cotyledon cDNA library was screened for clones representing RNAs that were more abundant in cotyledons grown on methionine-supplemented medium than in those grown on unsupplemented (basal) medium. Three isolated clones encoded the Bowman-Birk protease inhibitor (BBPI). RNA gel blot hybridization confirmed that the BBPI mRNA level is increased by methionine (Met). Met and/or sulfate supplementation increased the amount of protease inhibitor activity in cultured cotyledons. In seeds of greenhouse-grown plants, protease inhibitor activity was increased by the addition of exogenous sulfate to the nutrient solution. Since BBPI and related low molecular weight proteinase inhibitors have an unusually high cyst(e)ine content, these compounds have been hypothesized to function in the storage of reduced sulfur. The data presented here support this hypothesis by providing evidence that additional sulfur as Met or sulfate increases the quantity of these proteins in seeds by increasing their mRNA levels.