重金属胁迫诱导水稻发生可遗传的表观遗传变异及其可能作用于水稻提高抗重金属能力的机制的研究
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摘要
重金属胁迫作为非生物胁迫的一种可引起植物的胁迫应答。这里我发现重金属胁迫后水稻表现出明显的生长抑制,并且呈现一个剂量依赖的抑制效应,即浓度越高,抑制作用越大。由于表观遗传修饰具有可应答于生物体内在胁迫和外部胁迫的特点,我相信重金属胁迫可诱导表观遗传修饰如DNA甲基化变异和基因表达变异的发生。在研究中我发现重金属胁迫可诱导水稻在一些特定的序列上发生DNA甲基化的变异和基因表达的变异。这些变异具有以下的特点:(1)变异主要为CNG位点的去甲基化变异;(2)改变的甲基化类型在自交的一代中发生新的去甲基化变异,在自交二代中基本可遗传一代的甲基化类型;(3)那些负责建立和维持胞嘧啶的甲基化类型和/或染色质结构的基因如DNA甲基转移酶基因,5-甲基胞嘧啶糖基化酶基因和染色质重构因子(DDM1)可被重金属胁迫诱导发生表达的变异,在一定程度上可能是导致DNA甲基化变异的原因之一;(4)特定序列(转座子和基因)的表达改变主要为表达的上调,偶尔的表达下调也存在,改变的基因表达可不同程度的遗传给自交一代和二代植株。基因表达的改变和自身序列的甲基化改变不存在一一对应的关系,但是大体上序列的去甲基化变异对应着基因表达的上调。此外,重金属胁迫后的植株表现出一定程度的剂量依赖的重金属抗性应答,这可能与水稻重金属转运蛋白OsHMAs基因的表达增强有关。根据以上几方面的研究我认为重金属胁迫诱导的表观遗传变异可为研究植物抗重金属胁迫的机制和培育抗重金属水稻新品种提供一定的理论依据和参考。
As a kind of abiotic stress, heavy metal stress may provoke stress response. In this thesis I found that heavy metal stress induced significant growth inhibition in rice, which showed a dose-dependent response, the extent of inhibition increased with increased metal concentrations. Given the inherent property of epigenetic modifications to response to intrinsic as well as external stress, it is conceivable that epigenetic markers like DNA methylation and gene expression may undergo alterations in response to heavy metal stress. In deed I found here that heavy metal stress could induce extensive alteration in both DNA methylation and gene expression in rice plants, revealed by a set of characterized sequences including transposons and cellular genes. I found that several features characterize the alteration: (1) Alterations are predominant CNG hypomethylation events; (2) Progeny analysis indicated demethylated patterns could be inherited to next generation in most plants, leading to more intensive hypomethylation, and subsequent generation could inherit the altered methylation patterns in most progeny plants; (3) Heavy metal stress induced the perturbation of the expression state of these genes encoding for the enzymatic machinery responsible for establishing and maintaining cytosine methylation patterns and/or chromatin structure such as DNA methytransferases, 5-methylcytosine glycosylase and SWI/SNF chromatin remodeller (DDM1), which may be a major cause for the high-incidence of DNA methylation alterations; (4) Alterations in expression occurred in transposons and cellular genes are predominant up-regulations and occasional down-regulations events, and the alteration state could inherit to progenies at variable frequencies. Alteration in expression did not show a general correlation with alterations in methylation, but I found that hypomethylation specifically occurred, and most elements exhibited up-regulated expression upon heavy metal stress, so the changes in DNA methylation may influence the expression level of a number of genes to some extent. Furthermore, I found that progenies of treated plants showed enhanced resistance in a dose-dependent manner, and OsHMAs showed significant up-regulated expression might help to metal efflux and contribute to enhanced resistance to heavy metal in progenies of treated rice plants. I discuss implications of heavy metal stress-induced epigenetic variations with regard to the resistance mechanism to heavy metal of plants and potentiality for mutagenesis in rice breeding.
As a kind of abiotic stress, heavy metal stress may provoke stress response. In this thesis I found that heavy metal stress induced significant growth inhibition in rice, which showed a dose-dependent response, the extent of inhibition increased with increased metal concentrations. Given the inherent property of epigenetic modifications to response to intrinsic as well as external stress, it is conceivable that epigenetic markers like DNA methylation and gene expression may undergo alterations in response to heavy metal stress. In deed I found here that heavy metal stress could induce extensive alteration in both DNA methylation and gene expression in rice plants, revealed by a set of characterized sequences including transposons and cellular genes. I found that several features characterize the alteration: (1) Alterations are predominant CNG hypomethylation events; (2) Progeny analysis indicated demethylated patterns could be inherited to next generation in most plants, leading to more intensive hypomethylation, and subsequent generation could inherit the altered methylation patterns in most progeny plants; (3) Heavy metal stress induced the perturbation of the expression state of these genes encoding for the enzymatic machinery responsible for establishing and maintaining cytosine methylation patterns and/or chromatin structure such as DNA methytransferases, 5-methylcytosine glycosylase and SWI/SNF chromatin remodeller (DDM1), which may be a major cause for the high-incidence of DNA methylation alterations; (4) Alterations in expression occurred in transposons and cellular genes are predominant up-regulations and occasional down-regulations events, and the alteration state could inherit to progenies at variable frequencies. Alteration in expression did not show a general correlation with alterations in methylation, but I found that hypomethylation specifically occurred, and most elements exhibited up-regulated expression upon heavy metal stress, so the changes in DNA methylation may influence the expression level of a number of genes to some extent. Furthermore, I found that progenies of treated plants showed enhanced resistance in a dose-dependent manner, and OsHMAs showed significant up-regulated expression might help to metal efflux and contribute to enhanced resistance to heavy metal in progenies of treated rice plants. I discuss implications of heavy metal stress-induced epigenetic variations with regard to the resistance mechanism to heavy metal of plants and potentiality for mutagenesis in rice breeding.
引文
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