LC-MS/MS proteomic profiling of maize opaque endosperm mutants

Reduced prolamin (zein) accumulation and defective ER-body formation occurs in maize opaque endosperm mutants opaque2 (o2), floury2 (fl2), defective endosperm*B30 (DeB30), and mucronate (Mc) whereas other opaque mutants such as opaque1 (o1) and floury1 (fl1) are normal in these regards. This suggests that other factors contribute to kernel texture. We used a LC-MS/MS proteomics approach to compare non-zein proteins of nearly isogenic opaque mutants. In total, 2762 proteins were identified that were enriched for biological processes such as protein transport and folding, amino acid biosynthesis, and proteolysis. Amino acid analysis of top proteins revealed qualitative changes in lysine abundance contributing to the overall lysine increase. Principal component analysis and pathway enrichment suggested that the mutants partition into three groups: Mc, DeB30, fl2 and o2; o1; and fl1. Indicator species analysis revealed mutant-specific proteins, and highlighted ER secretory pathway components that are enriched in selected groups of mutants, albeit with no common proteins across all mutants. Most significantly changed proteins were related to stress or defense and zein partitioning into the soluble fraction for Mc, DeB30, o1 and fl1 specifically. We suggest that ER stress is a universal trigger of opacity regardless of qualitative or quantitative changes in zein accumulation.