The Biochemical Basis and Implications of Grain Strength in Sorghum and Maize

This review deals with the biochemical basis and implications of hardness and grain strength in sorghum and maize. Grain hardness affects various aspects of the growth and processing of cereal grain from resistance to fungal infection to cooking quality. It is clear that the prolamins play an important role with the γ-prolamins being the most important. It would appear that these prolamins help shape the protein bodies and form disulphide bonds within themselves or with other proteins. The γ-prolamins form the cement while the α-prolamins are the bricks. Both prolamins are present in greater proportions in hard grains and in the vitreous portion of hard grains. Genetic and environmental effects on the amounts of the different prolamins and on their distributions within the protein body and in different parts of the endosperm also determine grain hardness. Grains that will be hard appear to deposit prolamins and antifungal proteins earlier and in greater amount than do soft grains. The cell wall composition is also different between the two types of grains while there is a higher proportions of amylose in the starch of hard than in that of soft grains. Most of the differences that exist between hard and soft grains also exist between the outer and inner portion of the grain. It is postulated that there might be a master gene controlling the onset of strength in grains by simultaneously altering the levels of various apparently unrelated biochemical events. It is also suggested that solute availability may play an important role in the regulation of expression of genes for hardness-related proteins.