Three sorghum serpin recombinant proteins inhibit midgut trypsin activity and growth of corn earworm

The sorghum (Sorghum bicolor) genome contains a number of putative serpins (serine protease inhibitors) that are expressed under varying conditions, but little is known about their biological function. One of the sorghum serpin genes encodes a protein that contains reactive center residues Leu-Arg-X (X = small residue), called a LR serpin. Similar plant-derived LR serpin proteins can inactivate mammalian trypsins and have activity against insect trypsins. In this study the sorghum LR serpin, and two non-LR sorghum serpins, which were expressed in Escherichia coli and purified using immobilized metal-affinity chromatography, were shown to inhibit in vitro trypsin activity from larval midgut extract of corn earworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda). Each serpin was added individually to sorghum leaf insect diet that was fed to corn earworm and fall armyworm larvae. Statistically significant reductions (30-53%) in the mean weight of corn earworm larvae, but not for fall armyworm larvae, were found in the larvae feeding on diet containing each of the serpins compared to the mean weight of those feeding on control diet. These studies suggest that the sorghum serpin genes could be utilized for corn earworm larvae resistance in sorghum breeding but fall armyworm larvae have compensatory mechanisms to counter the tested sorghum serpins.