根瘤是根瘤菌在植物体中定殖和共生固氮的场所，在每个根瘤中含有多达成百上千个根瘤菌。然而，在豆科植物有效固氮根瘤形成过程中，根瘤菌侵染和定殖是如何逃逸寄主植物免疫防御反应的问题一直悬而未决。在前期研究工作中，我们从苜蓿中鉴定到一个激活根瘤防御反应的突变体（nad1）。根瘤菌在nad1突变体上形成不能固氮的无效根瘤，且根瘤中产生了强烈的植物免疫反应，导致细菌和共生细胞坏死。在此基础上，本项目拟通过酵母单杂、染色质共沉淀、蛋白质质谱分析及免疫电镜等生物和生化技术，开展对NAD1在根瘤中表达调控、蛋白相互作用、亚细胞定位及其功能和作用机制的研究。旨在阐明根瘤菌在植物细胞内侵染、定殖而不导致寄主植物超敏反应的分子机制，揭示根瘤中免疫抑制反应相关蛋白的作用和功能。为探索根瘤菌在非豆科植物细胞内定殖，建立根瘤菌与非豆科植物共生固氮体系提供理论依据和突破口。

Rhizobia invade and colonize in plant root cells without triggering obvious host defense response in the symbiotic relationship between legume plant and rhizobia. A question that how rhizobia evade or inhibit host defense has been proposed for a long time, but poorly understood till now. In our previous work, a Medicago truncatula mutant, referred to as nad1 for nodules with activated defense 1, was identified. In the nad1 plants, rhizobial infection and propagation in infection threads are normal, while rhizobia and their symbiotic plant cells become necrotic at the late stage when rhizobia were released from infection threads into symbiotic cells of nodules. Base on the characterization of the suppressing defense responses protein NAD1, the project will be investigated the mechanisms of NAD1 gene expression regulation, isolated transcription factors by yeast one-hybrid and EMSA methods. Using chromatin co-immunoprecipitation, protein mass spectrometry, immunoelectron microscopy as well as other biological and biochemical approaches，the investigation will be focused NAD1 interacted proteins, subcellular localization and other molecular functions. it will help to understand the complicate association between symbiotic response and host defense response in Rhizobium-legume symbiosis and the help for obtained results would provide a useful theoretical foundation and a new avenue for establish of Rhizobium-nonlegume symbiosis in the future.