opaque-15, a maize mutation with properties of a defective opaque-2 modifier.

Abstract

An opaque mutation was identified that reduces 'y-zein synthesis in maize endosperm. The mutation, opaque-15, causes a 2-to 3-fold reduction in y-zein mRNA and protein synthesis and reduces the proportion of the 27-kDa y-zein A gene transcript. Although the protein bodies in opaque-15 are similar in size and morphology compared to wild type, there are fewer of them in developing endosperm cells. The opaque-15 mutation maps near the telomere of chromosome 7L, coincident with an opaque-2 modifier locus. Based on its phenotype, opaque-15 appears to be a mutation of an opaque-2 modifier gene.Mutations that reduce the synthesis of maize seed storage proteins, zeins, typically result in endosperm with a soft, starchy phenotype. Some of these mutations, notably opaque-2 (o2) and floury-2 (fl2), increase the synthesis of other endosperm proteins (1, 2), resulting in a higher lysine content (3,4). Many years of effort were devoted to introgressing o2 into agronomically important maize genotypes to create high-lysine corn. However, the soft, starchy endosperm of the mutant kernel caused it to be susceptible to mechanical damage and insect pests and ultimately prevented wide utilization of o2 (5).In recent years, the introgression of genes known as o2 modifiers has solved many of the problems associated with the soft phenotype of o2 mutants (6). The modifiers increase seed density, vitreousness, and y-zein content of o2 mutants (7). Activity of the modifier genes is associated with a 2-to 3-fold increase in the synthesis of the 27-kDa y-zein mRNA and protein (8, 9) and appears to involve posttranscriptional regulation of y-zein gene expression (9). Several breeding programs have used the modifier genes to develop vitreous genotypes with a high lysine content that are referred to as Quality Protein Maize (6). Although o2 modification behaves as a complex quantitative trait (10), recent genetic evidence suggests that as few as two genes may be involved (11). In addition, two loci involved in o2 modification have been identified by restriction fragment length polymorphism (RFLP) mapping (11); one locus maps near the centromere of chromosome 7, and the second maps near the telomere on the long arm of chromosome 7.To isolate o2 modifier genes and determine their mode of action, we screened a group of mutant seeds for reduced levels of the 27-kDa y-zein protein. Here we report an opaque mutation, designated opaque-15 (o15), that significantly reduces y-zein mRNA and protein without significantly affecting the accumulation of other zeins. The mutation maps near an o2 modifier and, like an o2 modifier, alters the ratio of mRNAs encoded by the A and B y-zein genes. Understanding the biochemical basis of olS will help clarify the regulation of ,y-zein gene expression and the factors responsible for development of vitreous endosperm in maize. Although the terms vitreous and opaque describe mature kernel phenotypes, developing kernels that are wild type (i.e., 015) will be called vitreous, whereas mutant kernels (i.e., olS) wi...

Deletion Mutagenesis Identifies a Haploinsufficient Role for γ-Zein in opaque2 Endosperm Modification

“…While protein body number is reduced to about 40% of the wild-type number by 27-kD g-zein RNAi , the knockout mutant reduced the protein body number to about 12% of the wild-type and about 7% of the QPM protein body number. Vitreous endosperm formation in modified o2 is thought to be driven, at least in part, by the initiation of a larger number of protein bodies as a result of increased 27-kD g-zein accumulation (Dannenhoffer et al, 1995;Figure 7. Exon-seq and genomic and RT-PCR verification of deletion on chromosome 7.02.…”

Maize Opaque Endosperm Mutations Create Extensive Changes in Patterns of Gene Expression[W]

“…Mutations that alter zein synthesis can lead to protein bodies with abnormal morphology, size, or number, and they result in kernels with a soft, starchy texture. Mutations that reduce ␣ -zein synthesis, such as opaque2 ( o2 ), result in small, unexpanded protein bodies (Geetha et al, 1991), whereas those that reduce ␥ -zein synthesis, such as o15 (Dannenhoffer et al, 1995), lead to a smaller number of protein bodies. Conversely, the overproduction of ␥ -zein appears to enhance protein body number and result in the formation of more vitreous endosperm Moro et al, 1995).…”

γ-Zeins are essential for endosperm modification in quality protein maize

“…Two different QTLs, which are candidates for o2 modifier genes, affect 27-kDa γ-zein gene expression. The first of these is associated with increased expression (24) and the other is linked to o15, a mutation at a different chromosome 7 location (25), which causes decreased 27-kDa γ-zein expression. The first QTL could be a cis-acting mutation of the 27-kDa zein gene, and the latter a trans-acting factor.…”