铝胁迫诱导玉米DNA甲基化变异及其与铝毒性耐性基因型间差异的可能关系研究

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英文题名：Aluminum Stress-Induced Alteration in Cytosine DNA Methylation and Its Possible Association with the Genotypic Difference in Tolerance to Aluminum Toxicity in Maize 作者：Josphert ; Ngui ; Kimatu 论文级别：博士 学科专业名称：遗传学 中文关键词：铝毒性 ; 表观遗传学 ; 土壤酸度 ; 甲基化 ; 玉米 英文关键词：Aluminium toxicity ; epigenetics ; soil acidity ; methylation ; maize 学位年度：2010 导师：刘宝 学科代码：071007 学位授予单位：东北师范大学 论文提交日期：2010-05-01

摘要

上一个世纪,植物育种专家利用杂种优势这一重要生物学机制培育了很多新品种,可以提高作物产量15-60%,但是杂种优势在分子水平的认识还很缺乏。世界上40%以上的耕地因为铝离子毒性和土壤酸性的综合作用而限制了农作物的生长。在发展中国家,农作物比如玉米由于这种非生物胁迫——铝(Al)离子毒性作用而减产约30-40%。铝离子严重地阻碍了根部吸收水和营养的能力并削弱了作物的生长和其它机能。然而,玉米显示了非常高水平的表型和遗传多样性。而且,其他研究表明,在植物种内和种间都有一种自然变异的耐铝性机制。作物正常生长发育需要稳定的DNA胞嘧啶甲基化模式和染色体结构,但表观基因组对环境因素表现异常不稳定,并可以影响胚的正常发育。
     我们选取了12个玉米自交系,分别由两种铝毒耐受性和两种铝毒敏感性相互杂交的得到,分析它们的遗传学和表观基因学的变化,用于耐铝筛选以及DNA胞嘧啶甲基化和杂种优势的研究。我们利用扩增片段长度多态性(AFLP)和甲基敏感扩增片段多态性(MSAP)的分析方法进一步研究了基于表观遗传观点的杂种优势有关的铝筛选的可能性。这个研究同时也包括了利用标准的CIMMYT铝敏感玉米系和相互杂交系进行铝毒筛选的对比评估。最后,我们用铝离子毒性和低pH胁迫筛选研究了与植物生理学、杂种优势和DNA胞嘧啶甲基化相关的玉米表型和产量的农艺性状。这种方法可用于研究玉米铝毒性的杂种优势作用和解释表观遗传关键机制——DNA胞嘧啶甲基化----在杂种优势分子水平的研究。我们配制了150μMAl和pH 4.5的无机1/2 MS营养液,分别在4、24、48小时的记录杂交种的生长变化。
     通过对根进行苏木素染色,和耐受指数(RTi)的生理分析表明,耐铝玉米表现出了很强的对铝毒的适应和迅速的调整响应。MSAP分析了CCGG超甲基化和低甲基化水平,显示了总甲基化水平的变化范围为1.5-15%,其中,与铝毒相关的CG水平上有4.75%-5.30%的变化,CNG水平上有6.76%-7.31%的变化。这种CG和CNG表观遗传学的超甲基化和低甲基化变化可以通过由净种子根长度(NSRL) %分析和根生长反应具体表现出来。对于胚阶段的铝毒耐受性的选择性筛选导致了耐性自交系和敏感自交系的产量分别有15.07%和8.17%的增长,而低pH筛选导致了耐性自交系和敏感自交系的产量分别有3.03%和3.63%的增长。这个结果证实了以前的研究——不同品种间存在铝毒敏感性的区别,同时也显示了低pH在较低水平上对作物有独立的影响作用。由于亲本起源的杂种优势影响是杂交育种中值得考虑的一个重要因素,比如,铝毒导致的N5xN1和N1xN5的产量增长分别是23.90%和3.12%。但是低pH(pH4.0)的影响却很小(对N5xN1和N1xN5分别是2.62%和1.75)。由聚丙烯酰胺凝胶(PAGE)分析演化来的MSAP分析表明,与细胞壁和细胞膜相关的基因CSLD3 (CELLULOSE SYNTHASE-LIKE)、激酶KINASES、植物生长素auxins、ABC转运蛋白等都与玉米的铝毒耐受性和敏感性相关。铝耐受筛选在杂交育种中是有效的,并且玉米铝胁迫的基因组印记可能发生在胚阶段。
     这些研究从表观遗传学角度对玉米耐铝性的复杂机制提供了可能的解释。尤其是本研究中提出的如胞嘧啶甲基化和在玉米基因组中基因间区的亲源现象等的表观遗传调控机制加上逆转录转座子的插入和DNA转座子的积累催化了快速的基因顺序和基因内容的重组。这有可能决定着玉米杂种优势的分子基础的重要组成部分。我们也利用铝胁迫耐受水平和“铝胁迫回想记忆”比较了热带玉米和温带玉米对土壤中酸度和铝离子水平提高的适应能力。此外,这个研究结果对于其他热带作物如高粱、大米和小麦的改善和育种也是有益的,以培育能表达耐铝基因的杂种优势的基因型,并通过分子水平的研究,从表观遗传学角度揭示了酸性土壤耐铝育种计划的开展的理论基础。
Heterosis in crops has been one of the major mechanisms harnessed by breeders to increase grain yield by 15-60% over the last century, yet the molecular basis of heterosis still remains poorly understood. The combination of Aluminium toxicity with soil acidity is a major crop growth limiting factor in over 40% of the arable land in the world. Aluminium (Al) toxicity is particularly one of the most critical abiotic stresses which limit crop production in acid soils reducing maize yield by about 30-40% in developing countries. The Al ions dramatically damage the ability of roots to take water and nutrients therefore impairing crop growth and functions. However, maize and other plants exhibit considerable genotypic and phenotypic natural variation in Al tolerance both within and between species. The epigenome is particularly vulnerable to environmental factors and may become dysregulated during early seedling growth when DNA cytosine methylation patterning and chromatin structure are required for normal development.
     We investigated genetic and epigenetic variations in 12 maize inbred lines from which 2 Al tolerant and 2 Al sensitive inbreds were selected and used to make 12 reciprocal hybrids for Al screening, DNA cytosine methylation and heterotic studies. We further explored the possibility of heterotic links of such Al screening differences from an epigenetic perspective using Amplified Fragment Length Polymorphism (AFLP) and Methylation Sensitive Amplified Polymorphism (MSAP) analyses. These also included comparative evaluations involving Al toxicity screening using a standardized CIMMYT Al sensitive maize line and reciprocal hybrids. Finally, we used Al toxicity and low pH screening to study field traits on maize morphology and yield as related to their physiology, heterosis and DNA cytosine methylation. This approach was utilized to study the heterotic effect of toxicity of aluminum (Al) in maize in order to elucidate the molecular basis of heterosis using DNA cytosine methylation which is a key epigenetic mechanism. Heterotic morphological growth responses were measured after 4, 24 and 48 hrs exposure to 150μMAl and at pH 4.5 in inorganic half-strength MS nutrient solution.
     Root tolerance indexes (RTi) and physiological analysis via hematoxylin staining showed a high Al stress response and a rapid adjustment response to in tolerant lines. MSAP analysis of the CCGG hyper and hypo methylation patterns showed a range of 1.5-15% alteration in the total methylation patterns; the %CG level alteration (4.75-5.30) and % CNG (6.76-7.31) were as positively correlated to Al3+ toxicity. This dynamism of the CG and CNG epigenome hyper- and hypomethylation could be reflected in the Net Seminal Root Lengths (NSRL) % analyses and root growth responses. The selective screening of Al for tolerance even at embryonic stage increased the yield by 15.07% in Al tolerant inbred lines and also increased it by 8.17% in Al sensitive inbred lines; but, the screening for low pH was 3.03% in the Al sensitive inbred lines but 3.63% amongst the Al tolerant inbred lines. There results demonstrate and confirm past results that there are differences in Al sensitivity between cultivars but also show that low pH could be also influencing plants independently and at lower level. The influence of heterosis due to parent of origin was shown to be a significant factor to be considered in heterosis breeding, for example, the yield increase for N5xN1 in Al3+ was 23.90% while N1xN5 had 3.12% but, very little difference (2.62% vs 1.75 respectively) was observed due to pH 4.0. Polyacrylamide gel (PAGE) band variants from MSAP analysis found that cell wall and membrane related genes like CSLD3 (CELLULOSE SYNTHASE-LIKE), stress signals like KINASES, phytohormones such as auxins and protein transporters like the ABC transporters are involved in Al tolerance or sensitivity in maize. Al stress was effective in screening for heterosis and could be imprinted in maize at embryonic stage.
     Hence, these studies provide possible explanations on the complexity of Al tolerance mechanism from an epigenetic aspect in maize. Especially, this study proposes that the dynamisms in epigenetic regulation mechanisms like cytosine DNA methylation and parent of origin phenomenon in the intergenic regions in maize genome, plus the accumulation of retro-transposon insertions and DNA transposons which catalyze a rapid reshuffling of gene orders and gene contents are possible key components which determine the molecular basis of maize heterosis. It also uses the tropical maize Al tolerance level and“Al stress recall memory”to gauge the temperate maize ability to cope with increasing soil acidity and Al toxicity. Furthermore, the findings are profitable in the improvement and breeding of other tropical crops like sorghum, rice and wheat so as to produce genotypes which express Al tolerant genes heterotically and foster an epigenetic dimension in molecular Al tolerance breeding programs in acidic soils.

引文

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