Hongjun Liu, Junpeng Shi, Chuanlong Sun, Hao Gong, Xingming Fan, Fazhan Qiu, Xuehui Huang, Qi Feng, Xixi Zheng, Ningning Yuan, Changsheng Li, Zhiyong Zhang, Yiting Deng, Jiechen Wang, Guangtang Pan, Bin Han, Jinsheng Lai, and Yongrui Wu

Significance

More than half a century ago, Oliver Nelson and Edwin Mertz at Purdue University found the maize opaque2 (o2) mutation produces doubling of the endosperm lysine content, creating the foundation for quality protein maize (QPM) breeding. QPM has the potential to benefit millions of people in developing countries who consume maize as their sole protein source. However, breeding new QPM hybrids takes longer than regular hybrids, primarily because of the complex and unknown components of o2 endosperm modification; this has limited expansion of QPM worldwide. We identified a quantitative trait locus, a gene duplication at the 27-kDa γ-zein locus, which confers enhanced expression of this protein and leads to endosperm modification. This knowledge can effectively be applied in QPM breeding.

Abstract

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.

See http://www.pnas.org/content/113/18/4964.full

PNAS May 3 2016; vol.113; no.18: 4964–4969

Fig. 1.

Silencing of Pbf in QPM. (A) Ear phenotypes of K0326Y and K0326Y × o2;PbfRNAi/+. In K0326Y × o2;PbfRNAi/+, half of the progeny inherited the PbfRNAi transgene and expressed a reduced level of 27-kDa γ-zein and exhibited an opaque phenotype. Two representative vitreous and opaque seeds are indicated by asterisks and arrowheads. (B) Truncated seed phenotypes. The left two seeds not inheriting the PbfRNAi are vitreous, whereas the right two positive seeds are opaque. (C) The seeds inheriting the PbfRNAi expressed a decreased level of 27-kDa γ-zein. The 27-kDa γ-zein is indicated by the arrow.