The biochemical genetics of quality protein maize

Abstract

Quality Protein Maize (QPM) genotypes are opaque2 (o2) mutants that, through the action of modifier genes, develop a hard, vitreous endosperm. QPM kernels have the increased level of lysine observed in o2 mutants, but the appearance and physical properties of normal genotypes. In this dissertation, two important aspects regarding the development of QPM genotypes are addressed: the process of o2 endosperm modification and the accumulation of lysine in the endosperm. The formation of vitreous endosperm in o2 kernels is associated with the increased accumulation of the 27-kD γ-zein protein. When modifier genes were backcrossed into a normal (non-o2) genotype, higher levels of γ-zein were observed in the endosperm, similar to what happens with o2 mutants. However, in this case the increase in γ-zein content did not translate into increased endosperm hardness or density. Thus, there appear to be limits in the extent to which γ-zein can affect the physical properties of the endosperm. It has been suggested that the AB allele of the γ-zein locus is necessary, but not sufficient, for o2 endosperm modification. However, a recombinant o2 plant homozygous for the γ-zein rA allele was identified, in which the kernels have a uniformly vitreous endosperm and increased level of γ-zein protein. This indicates that the formation of vitreous endosperm in o2 genotypes does not require the presence of the AB γ-zein allele and that the increased level of γ-zein protein in modified o2 endosperm is not dependent on the presence of two γ-zein coding sequences. Very little is known about the proteins that account for the majority of lysine in maize endosperm. A close relationship between the level of non-zein proteins and endosperm lysine content was determined. Additionally, a very high correlation between the concentration of the protein synthesis factor, EF-1α, and the lysine content of the endosperm was found. This is a significant step towards understanding the biochemical basis of lysine accumulation in the endosperm. EF-1α content can be used as an indicator of lysine levels for breeding programs, as well as a tool to investigate factors influencing the accumulation of lysine-rich protein in maize endosperm.