The maize opaque2 (o2) mutant has a high nutritional value but it develops a chalky endosperm that limits its practical use. Genetic selection for o2 modifiers can convert the normally chalky endosperm of the mutant into a hard, vitreous phenotype, yielding what is known as quality protein maize (QPM). Previous studies have shown that enhanced expression of 27-kDa γ-zein in QPM is essential for endosperm modification. Taking advantage of genome-wide association study analysis of a natural population, linkage mapping analysis of a recombinant inbred line population, and map-based cloning, we identified a quantitative trait locus (qγ27) affecting expression of 27-kDa γ-zein. qγ27 was mapped to the same region as the major o2 modifier (o2 modifier1) on chromosome 7 near the 27-kDa γ-zein locus. qγ27 resulted from a 15.26-kb duplication at the 27-kDa γ-zein locus, which increases the level of gene expression. This duplication occurred before maize domestication; however, the gene structure of qγ27 appears to be unstable and the DNA rearrangement frequently occurs at this locus. Because enhanced expression of 27-kDa γ-zein is critical for endosperm modification in QPM, qγ27 is expected to be under artificial selection. This discovery provides a useful molecular marker that can be used to accelerate QPM breeding.B y 2030, the world population is predicted to reach 8.5 billion people. As a consequence, food production will need to be increased by more than 50% (1). However, the rate of food production has not kept pace with explosive population growth. Enhancing the nutritional quality of staple crops is one strategy for addressing the emerging food crisis (2, 3).Maize (Zea mays) is the highest-yielding crop in the world, but it cannot be used as the sole protein source for humans and monogastric livestock because its main storage proteins, zeins, are deficient in the essential amino acids, lysine and tryptophan (4). The poor protein quality of maize can be improved by the opaque2 (o2) mutation, which increases the lysine and tryptophan levels by decreasing the synthesis of zeins and compensatorily increasing other (nonzein) seed proteins. However, unfortunately, the chalky and soft texture of o2 kernels limits utilization of this mutant (5). The creation of quality protein maize (QPM) was based on modification of o2 by accumulating quantitative trait loci (QTLs), called o2 modifiers, that lead to a hard, vitreous endosperm (5, 6). The development of QPM has greatly improved the lives of people who suffer from malnutrition in the developing countries (7).Although QPM breeding has gone on more than 50 y, neither the mechanism nor the genetic components controlling endosperm modification are well understood. Seven o2 modifiers have been located on six chromosomes (8), including one, designated o2 modifier1 in bin 7.02 near the 27-kDa γ-zein locus, which has a major effect on endosperm modification; the other six loci contribute smaller effects (8). Coincidently, the RNA transcript and protein levels of 27-kDa γ-zein accu...
