Review of genetic basis of protein digestibility in grain sorghum

Authors: DURESSA, DECHASSA - Kansas State University; WEERASOORIYA, DILOOSHI - Kansas State University; Bean, Scott; Tilley, Michael - Mike; TESSO, TESFAYE - Kansas State University

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2018
Publication Date: 10/18/2018
Citation: Duressa, D., Weerasooriya, D., Bean, S.R., Tilley, M., Tesso, T. 2018. Review of genetic basis of protein digestibility in Grain sorghum. Crop Science. 58:2183-2199. https://doi.org/10.2135/cropsci2018.01.0038.
DOI: https://doi.org/10.2135/cropsci2018.01.0038

Interpretive Summary: Sorghum, an ancient crop of the semiarid tropics, plays a key role in food and nutritional security for over half-a-billion people in Africa and Asia. In industrialized nations, sorghum is cultivated as animal feed and more recently as a feedstock for biofuel production and as health food alternative. Despite that it is as nutritious as other major cereal grains and even better in health promoting phytochemicals, sorghum is viewed as ‘a low value crop’ primarily due to the low digestibility of its proteins/kafirins. Several grain physicochemical properties influence sorghum protein digestibility of which structure and composition of kafirin being the main factor. Kafirins deposit, aggregate and form dense endosperm protein bodies during grain filling which morph into increasingly disulfide cross-linked enzyme resistant structures during maturation and drying of the grain. The objective of this article is to compile and document previous endeavors in improving sorghum nutritional quality through research targeted at increasing protein digestibility, highlight recent developments and share perspectives on future directions of research aimed at sorghum improvement for human food and animal feed.

Technical Abstract: Sorghum, an ancient crop of the semiarid tropics, plays a key role in food and nutritional security for over half-a-billion people in Africa and Asia. In industrialized nations, sorghum is cultivated as animal feed and more recently as a feedstock for biofuel production and as health food alternative. Despite that it is as nutritious as other major cereal grains and even better in health promoting phytochemicals, sorghum is viewed as ‘a low value crop’ primarily due to the low digestibility of its proteins/kafirins. Several grain physicochemical properties influence sorghum protein digestibility of which structure and composition of kafirin being the main factor. Kafirins deposit, aggregate and form dense endosperm protein bodies during grain filling which morph into increasingly disulfide cross-linked enzyme resistant structures during maturation and drying of the grain. Research efforts to improve sorghum protein digestibility has spanned decades but has not yet produced high protein digestible sorghum germplasm with acceptable agronomic characteristics and been commercially adopted. The discovery of high lysine sorghum mutant line in the 70s has fueled interest among sorghum breeders and geneticists to develop high protein quality sorghum. However, the hope gradually faded as it proved difficult to dissociate the high lysine and high protein digestibility trait from the floury endosperm trait. Recently several biotechnological approaches have been pursued to improve the protein digestibility through transgenic modulation of the expression of kafirin proteins. Indeed, transgenic perturbation of digestion resistance sorghum protein bodies by down-regulating a-kafirin and suppression of kafirin cross-linking by silencing '-kafirin synthesis have produced transgenic sorghum lines with substantially improved protein digestibility, but these too, were in most cases associated with floury endosperm phenotype. Yet there is an indication that careful tweaking of the expression of '-kafirin has a potential to produce sorghum with high protein digestibility and vitreous endosperm texture. Also, there exists genetic variation in sorghum germplasm for protein digestibility and protein content which can be utilized in cross breeding to improve protein digestibility. The objective of this article is to compile and document previous endeavors in improving sorghum nutritional quality through research targeted at increasing protein digestibility, highlight recent developments and share perspectives on future directions of research aimed at sorghum improvement for human food and animal feed.