Toward Nutritional Enhancement of Sorghum for Africa.

Grain sorghum (Sorghum bicolor L. Moench) is the fifth most important cereal worldwide. Sorghum is used primarily for animal feed in the developed world but, particularly in semi-arid regions of Africa and Asia, it is a principal source of energy and protein for millions of the poorest people. While having greater tolerance than maize to abiotic stresses, like low water availability, sorghum has undesirable nutritional qualities that hinder its wider use. These include poor digestibility, low protein content and quality, deficiency of Vitamins A and E and low bioavailability of zinc and iron. These deficiencies are being addressed through the Africa Biofortified Sorghum (ABS) project funded by the Gates Foundation. With regard to protein, storage proteins form the major fraction of starchy endosperm with kafirins representing on average ~50% of that protein. Since these proteins have low contents of certain essential amino acids, notably lysine, two lysine-rich proteins, barley hordothionin, HT12, and another barley high lysine gene, BHL9, were introduced. An improved Agrobacterium-based transformation procedure was developed to increase the efficiency of transgene introduction. Poor nutritional grain quality is compounded by the fact that grain protein is difficult to digest and digestibility decreases with cooking. The basis for protein and starch indigestibility in sorghum is being investigated in several ways. Using comparative analyses of two U.S. varieties from the same cross that differ markedly in digestibility, several factors appear to account for the poor protein and starch digestibility in these varieties. These features should be considered when improving sorghum for food and feed, as well as biofuel use.