Epigenetic Regulation Mechanism Of Rice Flowering Time, Gene Expression And Heterosis Through Histon Acetylation

Chromatin modification and remodelling are important components of epigenetic regulations of gene expression. Chromatin modification consists of covalent modifications of the N-terminal tails of the nucleosomal histones and DNA cytosine methylation. Histone modifications including acetylation, methylation, phosphorylation, ubiquitinylation and others provide mechanisms to regulate gene expression through changes in chromatin states and by recruiting protein complexes that regulate transcription. Histone lysine acetylation that is generally associated with gene activation is reversible, dynamic and regulated by histone acetyltransferases (HAT) and histone deacetylases (HDAC). Plant HAT and HDAC have been shown to play important roles in plant gene expression. Plant genome contains more than20genes encoding HDAC, which can be grouped into3classes. Among them the HD2class is found only in plants. HD2members have been shown to be involved in plant developmental and epigenetic pathways in arabidopsis. However, a few investigations about this protein family have been reportted in rice.Heterosis is a ubiquitous phenomenon that the hybrid exhibits greater biomass, growth rate, reproductive success and yield than the better of the two parents. Some hypotheses, including dominance complementation hypothesis, overdominance hypothesis and epistasis hypothesis, have partially explained the cause of formation. Because heterosis is an environmentally modified quantitative phenotype and results from gene regulation, epigenetic analyses will be an appropriate method to elucidate the mechanism.To study the function of histone acetylation in rice development and hybrid gene expression, the histone deacetylase gene OsHDT1was deeply researched. OsHDT1displayed a circadian rhythm expression and the protein may be mostly localized in euchromatic regions. Increased OsHDT1expression did not affect plant phenotype in the parent MH63but led to early flowering in the hybrid SY63. Over-expression of OsHDT1in the hybrid repressed the nonadditive expression of the key flowering repressors (i.e. OsGI and Hdl) inducing early flowering. Analysis of histone acetylation suggested that the acetylation state of flowering genes also displayed a circadian rhythm like their expression and OsHDT1might promote deacetylation on OsGI and Hdl chromatin only during the peak expression phase.High throughput digital gene expression profile analysis revealed that altered OsHDTl levels affected many nonadditive genes expression in the hybrid and resultted in different expression variation of a large number of genes in hybrid SY63than in parent MH63These data demonstrate that OsHDTl level was important for the nonadditive or differential expression of many genes including the flowering time genes, suggesting that OsHDTl may be involved in epigenetic control of nonadditive gene expression in the hybrid rice and parental genome interaction for differential gene expression. In addition, these data not only provide evidence of overdominance effectting in hybrid, but also demonstrate that variation in the levels of single trans-acting regulatory proteins such as chromatin regulation factors is important to establish differential gene expression pattern in the hybrid, supporting the hypothesis that subtle changes in single regulatory gene dosage may lead to overdominance in heterosis.