Dynamic Changes Of RRNA Gene And VP1Gene Associated Histone Modifications During Maize Seed Germination

Seed germination commences from a low metabolic state to a bioactive state and is regulated by many intracellular and extracellular factors. The N-terminal amino acid residuces of histones protruding from the nucleosomes can be acetylated, methylated, phosphorylated, ubiquitinated and ADP-ribosylated, which is called histone modifications. These modifications regulate gene transcription activity by affecting chromatin structure or recruiting specific chromatin-associated proteins. Recent studies have revealed that histone modifications are involved in regulating seed germination processes. The ribosomal RNA (rRNA) gene, which forms the nucleolus at interphase and is transcribed for ribosome production and protein synthesis, has an important role during seed germination. VIVIPAROUS1(VP1) is a B3domain-containing transcription factor that is central to the regulation of seed maturation in maize. Vpl mutant shows reduced sensitivity to ABA and fails to undergo normal maturation and germination viviparously. In this research, we use maize seed as materials and study dynamic changes of rRNA gene and VP1gene associated histone modifications during maize seed germination. The results are as follows:1. In this study, we showed that the histone H3K9ac, H44Kac and H3K9mel levels were increased gradually during seed germination. The histone H3K9me2level kept steady during seed germination. Histone acetylation is regulated by two kinds of enzymes:the histone acetyltransferases (HATs) and histone deacetylases (HDACs). The expression of both histone acetyltransferases (HATs) and histone deacetylases (HDACs) was increased gradually during seed germination. Application of exogenous ABA repressed the expression of HATs as well as HDACs and delayed histone acetylation. Suppressing histone deacetylase by treatment with a histone deacetylase inhibitor, trichostatin A (TSA), led to an increase in global histone acetylation and inhibited seed germination and growth. These results suggest that HATs and HDACs are important for seed germination.2. We reported that there was a decondensation of ribosomal DNA (rDNA) chromatin during seed germination accompanied with increased rRNA gene expression. Analysis of the rRNA gene promoter region by using Immuno-FISH and chromatin immunoprecipitation (ChIP) shows that there is an increase in acetylation levels at the rRNA gene promoter region. Application of seed germination inhibitor abscisic acid (ABA) suppresses rDNA chromatin decondensation, the expression of rRNA genes and global genomic acetylation. The further ChIP experiments show that ABA treatment hinders the elevation of acetyiation levels in the promoter region of the rRNA gene. The data together indicate that ABA treatment inhibits seed germination, which is associated with rDNA chromatin condensation, decreased transcription and rRNA gene hypoacetylation.3. ABA and TSA both delayed the downregulation of the embryogenesis related gene VP1during seed germination. The further chromatin immunoprecipitation experiments showed that the promoter region of the VP1gene was deacetylated during seed gemiination and this deacetylation event was inhibited by both ABA and TSA. These results suggested that a balance of the two enzymes HATs and HDACs affected the acetylation status of the VP1gene and ABA selectively activated its transcription by an accumulation of acetylated histone H3associated with the promoter region during seed germination.