Relationship Between Space Or Heavy Ion Radiation Environment Induced Mutagenic Effects And DNA Methylation Alterations

In order to study the mechanism of biological effects of space environment on plant seeds, in the present dissertation, we investigated the space flight induced epigenetic effects on rice plant with or without phenotypic mutations on many aspects including: level of polymorphic DNA methylation changes, character of genome sequences with DNA methylation changes, inheritance of polymorphic site and the relationship between DNA methylation changes with gene expression and genomic mutations. At the same time, we detected the epigenetic effects of on-ground radiation experiment to analysis the role of radiation in space environment induced biological effects. Results show that DNA methylation changes can be induced on rice by both space flight and low-dose (2Gy) heavy ion irradiation (P<0.05). The frequencies of DNA methylation changes on phenotypic mutants are obviously higher than that on normal plants (P<0.01). Moreover, DNA methylation changes are prone to happen on CNG sequences on rice genome. We also found the correlations of the polymorphic rate between DNA methylation and genomic sequence alterations (P<0.001). Higher level of DNA methylation changes and genomic sequence changes often appear on the same individuals. Sequencing of the polymorphic sequences show that space flight and heavy ion radiation induced DNA methylation changes are widely spread on rice genomes including coding regions, while changes of genomic sequence are more prone to appear on repeat regions. We also prove that the hyper-methylation or hypo-metylation changes on coding genes are corresponding to the increase and decrease of the expression of the gene. Both the altered DNA methylation patterns and the expression level changes are heritable. These results strongly suggest that epigenetic mechanisms are involved in the biological effects on rice seeds induced by space environment. Moreover, the radiation factor may be the main mutagen in the complex pressure in space. The heritable DNA methylation changes are related to gene expression, so we speculate that DNA methylation changes may be a reason of space radiation induced heritable phenotypic mutants in the first generation after treatment.In this study, we also compared the DNA methylation changes on different life stages of rice (seed and seedling) under the heavy ion radiation at same dose (2Gy) to explain the mechanism of the high level epigenetic changes on seeds of plant. The results show that frequencies of DNA methylation changes on rice seeds are higher than that on seedlings (P=0.011). Observations on chromosomal aberrations show that the frequencies of chromosomal aberrations were increased with the increasing dose in all of the three rice materials. More chromosomal damages were observed in seeds than seedlings at dose lower than 2Gy. Detect of malondialdehyde (MDA) content and the activities of superoxide dismutase (SOD) and catalase (CAT) also show that the activities of SOD and CAT in seeds are lower than that in seedlings at dose lower than 2Gy. The content of MDA in seeds is higher than in seedlings at dose lower than 2Gy. At dose higher than 2Gy, obvious increase of content of MDA and decrease of activity of SOD and CAT together with the high level of chromosomal damages and lethal effects are observed in seedlings. Our results prove that the biological effects of radiation are not only related to the kind of radiation, but also related to the ability of antioxidant systems of biological materials. We suggest that the higher radiosensitivity to IR induced chromosomal damages of non-growing seeds than growing seedlings at lower dose may be because of the inactive antioxidant systems in seeds.