The Histone Modification Research In Maize Root Cells

Epigenetics has become one of the hottest topics of research in plant functional genomics since it appears promising in deciphering and imparting stressadaptive potential in crops and other plant species. Abiotic stress responses are important for plants because they cannot survive unless they are able to cope with environmental changes. The term "abiotic stress" includes numerous stresses caused by complex environmental conditions, e.g. strong light, UV, high and low temperatures, freezing, drought, salinity, heavy metals and hypoxia. These stresses will increase in the near future because of global climate change, according to reports from the Intergovernmental Panel of Climate Change?In the European heatwave of 2003, crop production was reduced by around 30%(Ciais et al.,2005). Therefore, understanding abiotic stress responses in plants is an important and challenging topic in plant research. Physiological and molecular biological analyses have allowed us to draw a picture of abiotic stress responses in various plants.Different differentiated cells in roots responds differently to various abiotic stresses, suggesting that different cell types respond differently to abiotic stresses. Our understanding of the whole-plant stress response mechanism is very limited. To dissect this system, we need to investigate stress responses in differentiated cells, tissues and organs, and to connect the data relevantly. Systems biological and mathematic biological approaches will be required to integrate the data and to draw a complete overall picture of the abiotic stress response in plants.Maize is one of the most extensively cultivated crop plants, for both staple food and industrial usage, in tropical and temperate soils worldwide. Maize is sensitive to diverse environmental stresses, such as high salinity, phytohormone or epigenetic modification-related inhibitor. So, in this paper, we use maize (Zea mays L.) as materials to systematically analyze the epigenetic regulation from two aspects:epigenetic modification patterns and cell cycle arrest. The results of this research are as follows:1. Comparison of chromatin epigenetic modification patterns among root meristem, elongation and maturation zones in maize(Zea mays L.)Plant roots mainly consist of division, elongation and maturation regions. Histone modifications of chromatin play a vital role in plant cell growth and differentiation. However, there has been no systematic attempt to investigate the distribution patterns of histone modifications in the different plant root zones. In this study, histone H3 acetylation (H3K9ac), histone H4 acetylation (H4K5ac), and histone H3 methylation (H3K4me2, H3K4me3, H3K9me1, H3K9me2, and H3K27me2) levels and distribution patterns were examined in the root meristem, elongation and maturation zones of maize primary roots. Overall, the cells of the maturation zone displayed the highest level of multiple histone modifications. The lowest level of histone modification was detected in the root meristem. H3K9ac was enriched in the euchromatin and nucleoli of most nuclei from the elongation and maturation zones. The nucleoli of more than 60% of cells from all root regions were labeled by H4K5ac. In only a small proportion of cells (less than 7%), knobs showed H4K5ac signals. H3K4me2 and H3K4me3 were specifically detected in euchromatin. H3K9me1, H3K9me2 and H3K27me2 labeled heterochromatin and euchromatin in all the root tissues analyzed. Over 30% of elongation and maturation cells exhibited H3K9me1 signals around knobs, approximately 5% of maturation cells showed signals of H3K9me2 around knobs, and H3K27me2 was stained weakly in approximately 95% of maturation cells in knobs. Analysis of the genomic patterns of histone modifications across functionally distinct regions of maize roots reveals a root zone-specific chromatin distribution.2. Histone Acetylation and Reactive Oxygen Species are Involved in the Preprophase Arrest Induced by Sodium Butyrate in Maize RootsHistone acetylation plays a critical role in controlling chromatin structure and Reactive Oxygen Species (ROS) is involved in cell cycle progression. To study the relationship between histone acetylation and cell cycle progression in plants, Sodium butyrate (NaB), a histone deacetylase (HDAC) inhibitor which can cause a significant increase in histone acetylation in both mammal and plant genome, was applied to treat maize seedlings. The results showed that NaB had significant inhibition effect on different root zones at the tissue level and caused cell cycle arrest at preprophase in the root meristem zones. This effect was accompanied by a dramatical increase in the total level of H3K9ac and H4K5ac. Exposure of maize roots in NaB led to a continuous rise of intracellular ROS concentration, accompanied by higher electrolyte leakage ratio and Malondialdehyde (MDA) relative value. The NaB-treated group displayed negative results in both TdT-mediated dUTP nick end labeling (TUNEL) and ?-H2AX immunostaining assays. The expression of topoisomerase genes was reduced after treatment with NaB. These results suggested that NaB increased levels of H3K9ac and H4K5ac and could cause preprophase arrest accompanied with ROS formation leading to inhibition of DNA topoisomerase.