水稻根际砷甲基化与降低大米中无机砷比例的机制研究

Rice (Oryza sativa) is a main source of inorganic arsenic (As) exposure for populations in south and south-east Asia. In rice grains, arsenite and DMA are the predominant arsenic species. DMA is low toxic to human, while arsenite is carcinogenic. Therefore, how to promote the process of arsenic methylation in the soil-plant system, and subsequently decrease the proportion of arsenite in rice gains have been the key points, also the difficulties for mitigation the health risk from rice arsenic contamination in south-east Asia. It has been demonstrated that rice plants lack the ability to methylate arsenic, and DMA in rice grain is derived from the soil through microorganism-mediated methylation. In addition, it has been found that the proportion of arsenite and DMA in rice grains is varied significantly among difference rice cultivars. However, the mechanism of this kind of difference has not been revealed so far. According to previous research, we hypothesize that rice root exudates are different among difference rice cultivars, and these root exudates may alter the composition of the microbial community and chemical properties of soil around roots, thus subsequently influence arsenic species in soil and rice grains. Recently, high-DMA cultivars (Padi Perak, Handao) and low-DMA cultivars (Koshihikari, CNT87059-3) have been identified, with the proportion of DMA in rice grain was 50.1% , 81% and 8.5%, 2%, respectively. In this study, we will use these rice cultivars to investigate the mechanism of arsenic methylation in the rice rhizosphere for a lower proportion of inorganic arsenic in rice grains. First, we will measure the root exudates of these cultivars, and analyze the relationship between arsenic methylation and root exudates, aiming to find the key factors regulating arsenic methylation in rice rhizosphere; second, we will conduct macro genomic analysis of microbial community in rhizosphere of these cultivars by high throughput sequencing (MiseqIllumina Sequencing Platform), aiming to identify the potential microbial communities involved in arsenic methylation; third, we will perform pot experiment to investigate the effect of supplying root exudates on the rhizosphere microbial community and arsenic speciation in rice grains, aiming to explore how to promote the process of arsenic methylation in the soil-plant system. Results of this study will provide theoretical and practical guidance for reducing the proportion of inorganic arsenic in rice grains, and subsequently for mitigation the health risk of rice arsenic contamination in south-east Asia.

大米食用是人体摄入无机砷的主要途径。大米中的砷主要有As(III)和DMA，其中As(III)对人体的毒性远大于DMA，因此，降低大米中As(III)的比例对于控制大米砷污染的健康风险至关重要。水稻自身没有甲基化砷的功能，但不同水稻品种的大米中无机砷/有机砷的比例差异显著，这种差异可能是水稻与根际微生物的相互作用导致的。本研究将利用高甲基化和低甲基化水稻品种为研究对象，解析高、低甲基化品种的根系分泌物差异，并阐明根系分泌物对大米中砷形态的影响，为筛选高甲基化水稻品种提出评价指标；表征砷甲基化的关键功能基因在不同品种根际的丰度与多样性，解析其对根系分泌物添加的响应；利用宏基因组技术鉴定根际砷甲基化功能微生物的分类信息及其可能的代谢通路；系统诠释大米中甲基砷含量的土壤-微生物-植物共同调控机制，提出促进稻田砷甲基化过程，降低大米砷污染健康风险的新途径，为砷污染土壤治理提供理论与实践指导。

大米食用是人体摄入无机砷的主要途径。大米中的砷主要以As(III)和DMA的形态存在，其中As(III)对人体的毒性远大于DMA，因此，降低大米中As(III)的比例对于控制大米砷污染的健康风险至关重要。水稻自身没有甲基化砷的功能，但不同水稻品种的大米中无机砷/有机砷的比例差异显著，这种差异可能是水稻与根际微生物的相互作用导致的。本研究利用高甲基化与低甲基化品种、不同基因型的野生水稻与栽培水稻开展盆栽试验，从根系形态结构、根表铁膜形成、根系分泌物、根际和根内微生物群落结构等多方面探索了水稻籽粒中砷含量和形态存在品种差异的机制。研究结果表明水稻籽粒中砷含量和形态受到一系列环境因子的影响，其中根系分泌物是关键因子。进一步通过添加腐殖酸和柠檬酸的微宇宙培养试验，发现小分子的有机质（酸）不仅可以通过竞争吸附、络合等界面过程影响砷迁移转化，更为重要的是小分子有机质（酸）还能显著改变土壤微生物群落结构，进而以微生物为纽带，将砷的迁移转化过程与铁锰等矿物质，以及有机质的代谢、转化过程进行耦合，进而影响水稻籽粒中的砷含量与形态。另外，土壤中砷甲基化功能微生物的丰度与和活性是影响籽粒中甲基砷比例的主控因子。本研究通过高通量测序与生物信息鉴定等技术，发现了稻田土壤中的携带砷甲基化arsM基因的产甲烷菌和硫酸盐还原菌（SRB）可能是砷甲基化的主要参与者。通过添加产甲烷抑制剂（BES）的微宇宙培养试验，进一步探明了稻田土壤中砷甲基化转化通路可能与产甲烷过程耦合。这些研究成果有助于揭示水稻籽粒中砷含量与形态的品种差异机制，并推进稻田土壤中砷甲基化过程的认识。本研究还系统比较分析了不同有机质添加对砷迁移转化，特别是对砷甲基化与挥发的影响，相关研究成果将为降低水稻籽粒中砷的积累，促进砷甲基化，最终为降低大米砷污染的健康风险提供理论与实践指导。