超积累植物根系分泌物与根际微生物的相互作用及其对重金属活化吸收的影响

The rhizosphere, the region of soil surrounding a plant root, is the site of highest microbial biomass and activity influenced by compounds exuded by the root. The interactions between root exudates and microorganisms in the rhizosphere of hyperaccumulator have not been studied extensively, major questions remain unanswered because of a reliance on cultivation-based techniques and an inability to determine the origin and fate of mediating organic compounds. Sedum alfredii (Crassulaceae), a species native to China, has been characterized as a Zn/Cd cohyperaccumulator and Pb accumulator. To date, studies on the mechanism of metal tolerance and hyperaccumulation of S. alfredii were mainly focused on the uptake and subcellular distribution characteristics, while no information is available about the interactions between root exudates and microorganisms in the rhizosphere of this plant. In this project, taking into account of the problems in the research of microecology in the rhizosphere of hyperaccumulator, both hydroponic, rhizobox and simulated experiments, as well as new techniques like RNA-SIP, T-RFLP, PCR-DGGE, synchrotron radiation techniques, stable isotope tracing, HPLC-MS and ICP-MS were used to (1) investigate the microorganisms communities and diversity trait in the rhizosphere of S. alfredii and tracking the assimilation process of root exudates by the microorganisms in the rhizosphere; (2) examine the effect of microorganisms on the constitute and content of root exudates secreted by hyperaccumulator S. alfredii; and then elucidate the mechanism of interactions between root exudates and microorganisms; (3) to investigate the influence of the interactions between root exudates and microorganisms on heavy metal speciation and uptake by the plant, and then assess the contribution of root exudates and microorganisms on Pb and Cd activation. Results of this project will help to understand the mechanisms by which S. alfredii mobilizes and takes up heavy metal from soil and will optimize the management practices for maximum Pb and Cd phytoextraction, and it is of great importance for protecting soil ecosystem health.

由于植物根系分泌作用的存在，使根际成为土壤中生命活动最活跃的区域，在污染土壤植物修复中发挥着极为重要的作用。目前，有关超积累植物根系分泌物与根际微生物相互作用的研究报道很少，很多现象和问题还不能从机理上进行阐明。本项目从超积累植物根际微生态效应入手，以镉超积累、铅富集植物东南景天为材料，综合应用RNA-SIP、T-RFLP 、PCR-DGGE、同步辐射、稳定同位素示踪、HPLC-MS、ICP-MS等技术，系统研究东南景天根际微生物群落组成和多样性特征，追踪根系分泌物被根际微生物利用的过程，分析东南景天根际微生物对根系分泌物组成和含量的影响，阐明东南景天根系分泌物与根际微生物相互作用的机制；研究根系分泌物与根际微生物交互作用对根际铅、镉形态转化与吸收的影响，分析二者对土壤铅镉活化、吸收的贡献。为揭示植物重金属超积累机理，提高重金属污染土壤植物修复效率、保障土壤生态系统健康提供科学依据。

项目以超积累植物东南景天为材料，系统研究东南景天根际微生物群落组成和多样性特征，分析东南景天根际微生物对根系分泌物组成和含量的影响，研究东南景天根系分泌物与根际微生物相互作用及其对根际重金属形态转化与吸收的影响。取得的主要研究成果如下：（1）初步阐明了东南景天根际微生物群落组成和多样性特征。发现东南景天根际土壤拥有大量具有ACC脱氨酶活性微生物，且以α变形菌纲和β变形菌纲为主；另一方面，东南景天不同根际空间（非根际，根际，根表和根内）门分类水平上微生物群落组成存在显著差异，其中根内放线菌门的丰度要显著高于根际、根表和非根际，根表中变形菌门和拟杆菌门的丰度显著地高于根内，东南景天根部关联的微生物群落受到基因型和土壤类型的影响。与膜转运相关的基因如转运蛋白和ABC转运蛋白基因的丰度从根际到根内逐渐增加；（2）明确了微生物的定殖模式及其对东南景天生长及镉吸收的影响。发现根际微生物SaZR4, SaMR12和SaNR1具有在东南景天根部形成生物被膜的能力，而SaZR4和SaMR12可以定殖在根内部组织。SaCS12促进了东南景天根系的发育及重金属的积累，同时促进东南景天草酸的分泌。（3）阐明了东南景天根系分泌物的组成特点及其在根际重金属活化吸收的作用。超积累生态型东南景天有机酸分泌总量远高于非超积累生态型，且以苹果酸、草酸、酒石酸、柠檬酸为主；东南景天根际可溶性有机质(DOM) 通过络合作用形成Cd-DOM复合物，实现根际锌镉的活化；（4）阐明了东南景天根尖草酸分泌对镉吸收转运的影响机制。首次发现PG抑制了草酸的分泌，降低镉的吸收，而再添加草酸后，镉吸收量逐渐恢复，明确根尖草酸分泌参与了东南景天根端镉的吸收和运输，（5）阐明了高浓度CO2对东南景天根系分泌物及根际重金属有效性的影响。发现大气CO2浓度升高加速土壤酸化，提高土壤重金属生物有效性。（6）项目共产出研究论文12篇，其中已发表SCI论文9篇。此外，参加国内学术会议13次， 国际学术交流及会议3次。

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