Maize mutant opaque2 and the improvement of protein quality through conventional and molecular approaches

Maize endosperm protein is deficient in two essential amino acids, lysine and tryptophan. Several spontaneous and induced mutations that affect amino acid composition in maize have been discovered amongst which the opaque2 gene has been used in association with endosperm and amino acid modifier genes for developing quality protein maize (QPM), which contains almost double the amount of lysine and tryptophan compared to normal maize. These increases have been shown to have dramatic impacts on human and animal nutrition, growth and performance, A range of hard endosperm QPM germplasm has been developed at the International Maize and Wheat Improvement Center (CIMMYT) mostly through conventional breeding approaches to meet the requirements of various maize growing regions across the world. Microsatellite markers located within the opaque2 gene provide opportunities for accelerating the pace of QPM conversion programs through marker-assisted selection (MAS). Thus, CIMMYT scientists are developing a package of reliable, easy to use markers for endosperm hardness and free amino acid content in the maize endosperm. Recent technological developments in molecular biology at CIMMYT such as single seed-based DNA extraction and low cost, high throughput SNP genotyping strategies promise enhanced efficiency and cost effectiveness of QPM molecular breeding programs. Here we present a summary of QPM research and breeding with respect to the history of conventional improvement methodologies, genetic and molecular basis of opaque2, epistasis between opaque2 and other high lysine mutant genes and recent advances in genomics technologies that could potentially enhance the efficiency of QPM molecular breeding in future. (author)