根瘤菌－豆科植物共生体系具有显著的固氮能力，在提高农作物产量及改善生态环境方面潜力巨大，但是宿主特异性选择极大地限制了其应用。宿主类黄酮及根瘤菌三型分泌系统（T3SS）是决定宿主特异性的重要因素，其影响机制还不清楚。本课题组分离到一株刺槐中慢生根瘤菌，包含两套T3SS且都与共生相关。本申请即以该菌株－刺槐共生体系为模式，从土壤生态学和根系代谢组学入手进行研究：利用已构建的突变体接种植物，收集根及根系分泌物，分别进行代谢组学分析，明确两套T3SS对根系类黄酮合成和分泌的影响，并考察根系类黄酮对低氮胁迫的响应；在此基础上，通过高通量测序方法解析不同接种处理下根际微生物群落组成和功能，分析根瘤菌T3SS—根系类黄酮代谢—根际微生物组之间的关系，揭示T3SS调控共生过程的生态学机制，为最终阐明根瘤菌宿主选择与适应性机理提供重要信息，同时为定向调控根际生物学过程并实现非豆科作物的生物固氮提供参考。

The symbiotic system of rhizobia and legumes has significant nitrogen-fixation capacity, and has great application potential in improving crop yield and ecological environment. However, host-specific selection limits greatly the application of this symbiotic system. The flavonoids of host and the type Ⅲ secretory system (T3SS) of rhizobium are the important factors determining host specificity, but the mechanisms are unclear. Mesorhizobium amorphae CCNWGS0123 was isolated from the nodules of Robinia pseudoacacia by our research group, which contains two sets of T3SS and both are all related to the symbiotic nodulation. In this study, M. amorphae CCNWGS0123 - R. pseudoacacia will be used as a model system for the symbiosis investigations from the view of soil ecology and root metabonomics. The mutants of T3SS have been constructed and they will be used to inoculate the host roots, respectively. To clear influence of two sets of T3SS on the synthesis and secretion of flavonoids in the roots, the roots and secretions of root system will be collected for the analyses of their metabonomics. In addition, the response of flavonoids on low nitrogen stress will be investigated. On this basis, the rhizosphere microbiome assembly will be analyzed by high-throughput sequencing method. The ecological mechanisms of T3SS will be elucidated during the nodule formation. This will provide important information to unlock the mechanisms of rhizobia selectivity on the host and theoretical foundation to achieve the biological nitrogen fixation in non-leguminous crops by directed regulation of rhizosphere biological processes.