Abstract
The use of hybrids is widespread in agriculture, yet the molecular basis for hybrid vigor (heterosis) remains obscure. To identify molecular components that may contribute to trait heterosis, we analyzed paired proteomic and transcriptomic data from seedling leaf and mature leaf blade tissues of maize hybrids and their inbred parents. Nuclear- and plastid-encoded subunits of complexes required for protein synthesis in the chloroplast and for the light reactions of photosynthesis were expressed above midparent and high-parent levels, respectively. Consistent with previous reports in Arabidopsis, ethylene biosynthetic enzymes were expressed below midparent levels in the hybrids, suggesting a conserved mechanism for heterosis between monocots and dicots. The ethylene biosynthesis mutant, acs2/acs6, largely phenocopied the hybrid proteome, indicating that a reduction in ethylene biosynthesis may mediate the differences between inbreds and their hybrids. To rank the relevance of expression differences to trait heterosis, we compared seedling leaf protein levels to the adult plant height of 15 hybrids. Hybrid/midparent expression ratios were most positively correlated with hybrid/midparent plant height ratios for the chloroplast ribosomal proteins. Our results show that increased expression of chloroplast ribosomal proteins in hybrid seedling leaves is mediated by reduced expression of ethylene biosynthetic enzymes and that the degree of their overexpression in seedlings can quantitatively predict adult trait heterosis.
Highlights
IntroductionThe use of hybrids is widespread in agriculture, yet the molecular basis for hybrid vigor (heterosis) remains obscure
The use of hybrids is widespread in agriculture, yet the molecular basis for hybrid vigor remains obscure
Multiplexed proteomic analyses were performed by using tandem mass tag (TMT) peptide tags and high-resolution mass spectrometry to quantify differences in the levels of proteins extracted from seedling leaves of hybrids and their inbred parents
Summary
The use of hybrids is widespread in agriculture, yet the molecular basis for hybrid vigor (heterosis) remains obscure. A minority of messenger RNAs (mRNAs) and proteins are expressed above or below midparent (MP) levels, and most of these gene products are not obviously related to each other in function or to hybrid phenotypes, making their unusual levels difficult to interpret [8] Despite these ambiguities, progress has been made in understanding hybrid vigor. Maize hybrids have greater photosynthetic capacity than their inbred parents [10], which presumably contributes to their increased biomass and yield In both maize and Arabidopsis, various genes for photosynthesis have been observed to be expressed above MP levels [10,11,12]. It is unclear whether substantial improvements in heterosis are possible and yet have lagged because breeders lack seedling biomarkers for adult plant heterosis
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