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

自然土壤中蕴藏着丰富的微生物多样性，其中的一小部分可作为健康植物根部定植的播种源。虽然植物通过根分泌物的过程影响与根相关的细菌的生长，但有助于建立分类结构的细菌组合的宿主机制(S)仍然不清楚。植物通过细胞表面和细胞内的两类免疫受体检测到微生物衍生的分子而激活免疫反应，从而限制病原体的入侵。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居尚不清楚。假单胞菌科代表了核心根微生物区系的一个分类谱系。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定植方面的作用。通过产生这些共生假单胞菌的III型分泌系统(T3SS)的突变体，我们已经获得了T3SS是有效的根内圈定殖所必需的初步证据。我们推测，某些根微生物区系成员减弱了植物的免疫力，是影响根微生物区系结构的关键物种。我们建议使用日益复杂的合成细菌群落的微生物区系重建实验来阐明植物免疫系统在根微生物区系建立中的作用。此外，我们还将对部分免疫受损的拟南芥进行正向遗传实验，以确定根部微生物区系的生物多样性突变体。因此，该项目将测试是否需要植物的先天免疫系统来建立和维持健康植物的微生物动态平衡。

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.