Divergence of Bowman-Birk Protease Inhibitor Family into seed-specific and environmentally responsive subfamilies in Legume and Soybean: Implication for Legume Seed Composition Improvement.
Bowman-Birk inhibitors (BBI) are an ancient class of serine protease inhibitors originating prior to the emergence of the angiosperms. While BBIs have been preserved in the legume (Fabaceae) and cereal (Poaceae) families, they have been lost in many other divergent lineages. However, their underlying molecular evolution and regulation of BBI remain largely uncharacterized. Our study shows that BBIs in legumes and cereals are encoded by two large and divergent gene families. BBI genes in legumes have further diversified into two subfamilies with distinct gene expression patterns. Genes in one legume BBI subfamily are specifically expressed in seeds while BBI genes in the other legume subfamily and cereal do not have significant expressions in any examined tissues including seed, root, leaf and flower. The soybean BBI gene family shows evidence of expansion via whole genome, segmental and tandem duplication. Protein sequence and structural analysis predicts that functional domains for double-headed inhibitory loops and binding abilities to trypsin and chymotrypsin are largely preserved within the soybean BBI family. The seed-specific subfamily genes are specifically expressed at maturation stages and not at embryogenesis stages. The other, non-seed BBI subfamily genes are highly responsive to a distinct spectrum of signals related to abiotic and biotic stresses. Their specific expression under non-essential biological processes for plant growth and development suggests that, although BBIs have been retained in both cereals and legumes, likely due to their role in enhancing plant fitness under natural selection pressures, they are not involved in core developmental processes. This may explain why BBIs were lost in many divergent plant lineages and support their well-established roles in plant adaptation to environmental stress. Having knocked out the seed-specific BBIs through a CRISPR/Cas9 approach, we have successfully generated soybeans which exhibited 69.4 - 73.7% reduction of trypsin inhibitor activity and 76.4 - 79.4% reduced chymotrypsin inhibitor activity. The edited soybean did not show significant changes in key agronomic traits, supporting that the functions of BBIs are not essential. While BBIs in soybean seeds may have a desirable function in natural selection, they are antinutrients from an applied perspective for their use in feed and food. It provides an opportunity to reduce BBIs in seeds for quality improvement. Our findings provide insights into molecular evolution, regulation, and function of BBI in plants, and successfully demonstrate engineering BBI in seeds to result in production of food and feed of higher nutritional value with minimal impacts on the agronomic performance of the plant.