Functional genomics of maize endosperm maturation and protein quality

Maize is one of the most important cereal crops and widely cultivated throughout the world. The study on maize kernel development including protein quality improvement is essential for removing dietary protein deficiency because of the lack of essential amino acids, especially lysine and tryptophan, in maize kernel. Quality Protein Maize (QPM) is a hard kernel variant of the high-lysine mutant, opaque-2. We created opaque QPM variants to identify opaque-2 modifier genes and to investigate deletion mutagenesis combined with Illumina sequencing as a maize functional genomics tool. A K0326Y-QPM deletion mutant, line 107, was null for the 27- and 50-kD γ-zeins and abolished vitreous endosperm formation. Characterization of line 107 identified 27-kD γ-zeins as an opaque-2 modifier gene within the largest QPM QTL, and may suggest the 50-kD γ-zeins also contributes to this QTL. It further demonstrates that genome-wide deletions in non-reference maize lines can be identified through a combination of assembly of Illumina reads against the B73 genome and integration of RNA-seq data. Characterization of a low α-zein mutant, line 198, mapped its mutation on chromosome 3.02 and I describe our efforts towards its molecular identification. Besides the K0326Y-QPM mutagenesis project, I made a significant contribution to the study of zein function and their inverse relationship to kernel protein quality by using RNA interference. I addressed the extent of functional non-redundancy within the γ- and α-zein subclasses. My involvement in this work was in-depth analysis of transcript and protein levels in endosperms of these lines. I was also involved in the characterization of a sorghum high digestibility high lysine opaque mutant hdhl. We found that the mutant results from the missense mutation results in the 21st conserved amino acid alanine (Ala) in the signal peptide being replaced with threonine (Thr), which renders the signal peptide resistant to processing, indirectly reducing the levels of lysine-poor kafirins and thereby increasing lysine-rich proteins and protein digestibility in the seeds