Histone Acetylation Mediated Expression Of The Cell Wall Related Genes Is Involved In Maize Salt Stress Response

Due to the sessile developmental style, plants have limited mechanisms to avoid the external stress. Over the aeons of evolutionary time, plants have developed many efficient strategies to adapt to the environmental stress. At present, the saline-alkali land are existed in more than100countries in the world, with a total area of about10x109hm2, and the area of the saline soil in China is about3.5x107hm2. Salt stress is the main limited factor of crop productivity around the world. Therefore, it is necessary to understand the effects of salt stress on plant in detail and the physiology, cytology and molecular genetics mechanism of how the plants adapt to salt stress, in order to improve the ability of plants resistance to salt stress, develop new salt resistant crop varieties, improve crop productivity. In recent years, the plant epigenetics research is developed rapidly. Epigenetic regulation is involved in plant responses to environmental stimuli. Epigenetic regulation modification including DNA methylation, histone acetylation, histone methylation, histone phosphorylation, histone ubiquitin, etc. Cell is the basic component unit of the plant, single cell division and enlargement to form the organs and tissues. The cell wall is thought to be the major control point for cell enlargement. Cell wall related proteins including xyloglucan endotransglucosylase (XET), endoglucanase (EGase), expansins (EXP), and the plasma membrane proton pump (MHA). Expansin usually can be divided into two categories, a-expansin (EXPA) and (3-expansin (EXPB), respectively. In this research, we used maize as materials and analyzed cell morphological alterations in maize roots as a consequence of excess salinity in relation to the transcriptional and epigenetic regulation of the cell wall related protein genes. The results are as follows:With Feulgen staining and semi-thin section, we analyzed the cell morphology changes of maize under salt stress. In this study, maize seedling roots got shorter and the length and weight of primary root were significantly reduced after exposure to200mM NaCl for48h and96h. By feulgen staining we can observed the primary root swell from the root tip, and the swelling region get longer as the increase of treatment time, while the swelling in the elongation zone is most obvious.Further crytological observation showed that the growth inhibition of maize roots was due to the reduction in meristematic zone cell division activity and elongation zone cell production. The enlargement of the stele tissue and cortex cells contributed to root swelling in the elongation zone.With quantitative real-time polymerase chain reaction (RT-PCR), we analysed the gene expression pattern of the cell wall related proteins under salt stress. RT-PCR results displayed an up-regulation of cell wall related ZmEXPA1, ZmEXPA3, ZmEXPAS, ZmEXPB1, ZmEXPB2and ZmXET1genes and the down-regulation of cell wall related ZmEXPB4and ZmMHA genes as the duration of exposure was increased. These data suggested that the up-regulation of some cell wall related genes mediated cell enlargement to possibly mitigate the salinity-induced ionic toxicity, and different genes had specific function in response to salt stress.By immunostaining and Western Blot, with H3K9Ac and H4K5Ac as antibodies,we analysed the histone acetylation modification changes under salt stress. The histones H3K9and H4K5acetylation levels were not significantly altered under normal growth conditions, in contrast, the histones H3K9and H4K5acetylation levels was increased after treatment with200mM NaCl. It is known that histone acetylation is catalyzed by histone acetyltransferase (HATs) and histone deacetylase (HDACs). Thus we analyzed HATs and HDACs expression pattern in maize roots treated with and without200mM NaCl using RT-PCR. Two HATs genes (ZmHATB and ZmGCNS) and two HDACs genes (ZmHD2and ZmHDA113) were selected. We found that mRNA levels of the ZmHATB and ZmGCN5genes were increased under salt stress. These results suggested that histone acetylation modification is involved in response to salt stress.To determine whether the change of the transcript levels of ZmEXPB2and ZmXET1at48h under salt stress was due to the alteration of histone modifications, we performed ChIP experiments using an antibody against at histone H3acetylated at K9(H3K9Ac), and found the up-regulation of the ZmEXPB2and ZmXET1genes was associated with the elevated H3K9acetylation levels on the promoter regions and coding regions of these two genes. These results indicated histone modification as a mediator may contribute to rapid regulation of cell wall related gene expression, which reduces the damage of excess salinity to plants.