Plant Genome IV Conference Plant Genome IV Conference
The opaque2 (o2) mutation in maize raises the percentage of lysine in the grain by decreasing zein synthesis and increasing the synthesis of a number of endosperm proteins. Unfortunately, the nutritional improvement of o2 is offset by its soft texture, which results in damaged kernels, an increased susceptibility to pests, and inferior food processing. Discovery of genes that convert soft endosperm o2 mutants to a normal phenotype made it possible to develop a new type of o2 mutant called Quality Protein Maize (QPM). QPM has the phenotypic features of normal maize kernels, but the enhanced lysine content of o2. The development of QPM requires the simultaneous selection of multiple loci: o2, o2 modifiers, and genes governing the synthesis of lysine-rich proteins. Our research has focused on the characterization of o2 modifier genes and the genes responsible for the increased lysine content of o2 endosperm. We identified two loci linked with endosperm modification, and they are associated with enhanced synthesis of g-zein. It appears this leads to a vitreous phenotype by causing the formation of a larger number of protein bodies. Despite knowledge of the molecular mechanism of the o2 mutation, we still do not understand the basis of the pleiotropic increase in the lysine content. We found elongation factor 1-a (EF-1a) to be one of several proteins significantly increased in o2 endosperm, and this protein has a remarkable correlation (r2= 0.9) with the lysine content. This relationship is also true for other cereals. By selecting for QPM with high concentrations of EF-1a, it should be possible to develop maize with the protein quality of milk.