Genetic Basis of Protein Digestibility in Grain Sorghum

Sorghum [Sorghum bicolor (L.) Moench] plays a key role in food and nutritional security for over half-a-billion people in Africa and Asia. In industrialized nations, sorghum is mainly cultivated as animal feed and more recently as a feedstock for biofuel production. Despite its comparable nutritional profile with other cereals, which is even better in health promoting phytochemicals, sorghum is viewed as a low-value crop primarily due to the low digestibility of its proteins. Decades of research to improve the trait has not yet produced high-protein-digestible sorghum cultivar with acceptable agronomic characteristics. The efforts were frustrated by strong association between the high lysine and high protein digestibility traits and floury endosperm phenotype. Recently, several biotechnological strategies have been pursued to address the problem. Perturbation of sorghum protein bodies by downregulating alpha-kafirin and suppression of kafirin cross-linking by silencing gamma-kafirin synthesis have produced transgenic sorghum lines with substantially improved protein digestibility, but these too were, in most cases, associated with floury endosperm phenotype. However, there is an indication that careful tweaking of the expression of gamma-kafirin has potential to produce cultivars with high protein digestibility and vitreous endosperm texture. Moreover, natural variation for protein digestibility exists among sorghum germplasm and can be used in improving the traits through breeding. The objective of this article is to compile and document previous endeavors in improving sorghum nutritional quality through research targeted at increasing protein digestibility, to highlight recent developments, and to share perspectives on future research to address the challenge.