Functions of T3SS-containing Proteobacteria and plant innate immunity in root microbiota establishment

天然土壤中蕴藏着丰富多样的微生物，其中一部分作为健康植物根系定植的种子来源。虽然已知植物通过根渗出影响根相关细菌的生长，但宿主机制有助于建立一个分类结构的细菌组合，称为根微生物群，仍然不明确。已知植物通过激活免疫反应来限制病原体的入侵，当检测到微生物来源的分子时，两类免疫受体存在于细胞表面和植物细胞内。然而，具有完整免疫系统的植物如何耐受根微生物群成员的亲密定植尚不清楚。假单胞菌科代表了核心根微生物群的分类谱系。我们的前期实验证明了拟南芥细胞表面驻留模式识别受体（PRRs）启动的免疫反应在限制共生假单胞菌在根内的定植中的作用。通过产生这些共生假单胞菌的III型分泌系统（T3SS）突变体，我们获得了T3SS是有效的根内球定植所必需的初步证据。我们假设一些根菌群成员削弱植物免疫力，并作为影响根菌群结构的关键物种。我们建议利用越来越复杂的合成细菌群落的微生物群重建实验来阐明植物免疫系统在根系微生物群建立中的作用。此外，我们将对部分免疫功能低下的拟南芥进行前瞻性遗传实验，以鉴定根部微生物群的生态失调突变体。因此，本项目将测试植物的先天免疫系统是否需要在健康植物中建立和维持微生物稳态。

Natural soil harbors a rich diversity of microbes and a subset of these serve as seeding source for colonization of healthy plant roots. Although plants are known to influence the growth of root-associated bacteria through the process of root exudation, host mechanism(s) contributing to the establishment of a taxonomically structured bacterial assemblage, called the root microbiota, remain ill-defined. Plants are known to limit pathogen invasion by the activation of immune responses upon the detection of microbe-derived molecules by two classes of immune receptors that reside on the cell surface and inside plant cells. However, how plants with an intact immune system tolerate intimate colonization by root microbiota members is unknown. The Pseudomonadaceae family represents a taxonomic lineage of the core root microbiota. Our pilot experiments demonstrated a role of immune responses initiated by Arabidopsis thaliana cell surface-resident pattern recognition receptors (PRRs) in restricting the colonization of Pseudomonas commensals inside roots. By generating mutants of the Type III secretion system (T3SS) of these Pseudomonas commensals we have obtained preliminary evidence that the T3SS is needed for efficient root endosphere colonization. We hypothesize that some root microbiota members attenuate plant immunity and serve as keystone species that influence the structure of the root microbiota. We propose to use microbiota reconstitution experiments with increasingly complex synthetic bacterial communities to elucidate the role of the plant immune system in root microbiota establishment. In addition, we will conduct forward genetic experiments with partially immunocompromised A. thaliana to identify dysbiosis mutants of the root microbiota. Thus, this project will test whether the innate immune system of plants is needed for the establishment and maintenance of microbial homeostasis in healthy plants.