Phenotypic effects of opaque2 modifier genes in normal maize endosperm
The opaque2 modifier genes convert the soft, chalky endosperm, as found in maize opaque2 mutants, to a hard, vitreous phenotype. Modified opaque2 genotypes, also known as Quality Protein Maize (QPM), have increased levels of the essential amino acid lysine and a normal appearing kernel. The development of vitreous endosperm in QPM is associated with a two- to three-fold increase in the gamma-zein storage protein. To determine whether an increased concentration of gamma-zein can influence hardness and the formation of vitreous endosperm in normal genotypes, we derived 13 modified normal inbreds from a cross of the normal line W64A with Pool 34 QPM. Analysis of protein content, kernel density and hardness revealed variation among the 13 inbreds. The modified normal inbreds show an increased content of gamma-zein in the endosperm, similar to that of QPM. But the high level of gamma-zein did not increase hardness and density above a threshold level. The modified normal inbreds did not have an increased lysine content, suggesting modifier genes, per se, do not improve the nutritional quality of maize endosperm.
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