Proteomic profiling of maize opaque endosperm mutants reveals selective accumulation of lysine-enriched proteins.

Kyla J Morton,David R Holding,Chi Zhang,Shangang Jia

doi:10.1093/jxb/erv532

Kyla J Morton, David R Holding + Show 2 more

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https://doi.org/10.1093/jxb/erv532

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Abstract

Reduced prolamin (zein) accumulation and defective endoplasmic reticulum (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. A liquid chromatography approach coupled with tandem mass spectrometry (LC-MS/MS) proteomics was used to compare non-zein proteins of nearly isogenic opaque endosperm mutants. In total, 2762 proteins were identified that were enriched for biological processes such as protein transport and folding, amino acid biosynthesis, and proteolysis. Principal component analysis and pathway enrichment suggested that the mutants partitioned into three groups: (i) Mc, DeB30, fl2 and o2; (ii) o1; and (iii) fl1. Indicator species analysis revealed mutant-specific proteins, and highlighted ER secretory pathway components that were enriched in selected groups of mutants. The most significantly changed proteins were related to stress or defense and zein partitioning into the soluble fraction for Mc, DeB30, o1, and fl1 specifically. In silico dissection of the most significantly changed proteins revealed novel qualitative changes in lysine abundance contributing to the overall lysine increase and the nutritional rebalancing of the o2 and fl2 endosperm.

Highlights

Maize endosperm differentiates into six distinct regions including the aleurone, subaleurone, embryo surrounding region, endosperm transfer cells, and central starchy region (Olsen et al, 1999)
The non-zein fraction from each opaque mutant and wild type (WT) was separated by liquid chromatography coupled to tandem mass spectrophotometry
WT had the highest number of proteins identified and all mutants had a smaller nonzein proteome. fl2, Mc, and o2 had over 90% of the WT number of proteins identified, while fl1 and defective endosperm*B30 (DeB30) had just less than 90%. o1 had the lowest number of proteins identified at 84% of WT

Summary

Introduction

Maize endosperm differentiates into six distinct regions including the aleurone, subaleurone, embryo surrounding region, endosperm transfer cells, and central starchy region (Olsen et al, 1999). The main endosperm storage tissues are the aleurone, subaleurone, and central starchy endosperm region. Aleurone cells accumulate mostly lipid bodies and protein storage vacuoles, while the starchy endosperm contains starch and endoplasmic reticulum (ER) protein bodies (Brouns et al, 2012; Xiong et al, 2013). The protein bodies are composed of zein polypeptides classified as α-, β-, γ-, and δ-zein (Lending and Larkins, 1989). ER-localized proteins bodies have an α- and δ-zein core surrounded by crosslinked γ-zein (Lending and Larkins, 1989; Woo et al, 2001).

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