Maintaining plant immune homeostasis in the rhizosphere microbiome

植物根际微生物群落（根际微生物组）为植物宿主提供多种益处，包括营养吸收和病原体保护。为了在宿主存在下生存，微生物必须逃避或抑制宿主免疫。同时，为了避免被微生物侵害，植物免疫系统必须尽量减少非适应微生物的生长。我的实验室研究的一个主要目标是了解植物免疫系统如何在允许有益微生物群增殖的同时排除一些微生物。为了鉴定抑制植物免疫所需的细菌基因和形成根际微生物组所需的植物基因，我们使用荧光假单胞菌和模式植物拟南芥作为易处理的根际微生物组模型。荧光假单胞菌提供了一个具有分子、遗传和生物化学工具的系统，以回答宿主-微生物组相互作用中的基本问题。该项目提案包括三个互补的目标，以研究最终影响植物微生物组免疫稳态的细菌和植物机制。 目标一：确定根际细菌酸化如何抑制植物免疫用荧光假单胞菌预处理拟南芥根将抑制随后用微生物相关分子模式（MAMP）如细菌鞭毛蛋白挑战的防御反应。通过正向遗传筛选，我们发现多种细菌酸化机制抑制植物免疫。本目标描述了改变根际pH值抑制植物免疫力的机制的表征。目标二：确定水杨酸信号传导的负调控因子如何塑造根际微生物组植物激素水杨酸（SA）在植物免疫中起着核心作用，也影响植物的代谢和发育。SA信号传导的正调节剂已被证明可以塑造根际微生物组的结构和功能。该目标的重点是表征SA信号传导的负调节剂在植物-微生物组相互作用中塑造植物代谢和免疫稳态的作用。 目标三：为了鉴定调节根际微生物组的植物基因，我们重新筛选了一系列植物“hsm”突变体，这些突变体最初被鉴定为在植物免疫串扰中受损的突变体。我们鉴定了6个hsm突变体，它们支持在无菌条件下荧光假单胞菌水平的改变。这一目标的重点是描述hsm基因在形成根际微生物组中的作用。这三个互补的目标将有助于确定微生物群及其宿主如何在免疫稳态中取得平衡，最终导致微生物群的建立，耐受性和维持。

Plant root-associated microbial communities (the rhizosphere microbiome) provide their plant hosts with diverse benefits including nutrient uptake and pathogen protection. In order to survive in the presence of a host, microbes must evade or suppress host immunity. At the same time, in order to avoid being overrun by microbes, the plant immune system must minimize the growth of non-adapted microbes. A major goal of my lab's research is to understand how the plant immune system excludes some microbes while allowing proliferation of beneficial microbiota. To identify bacterial genes required to suppress plant immunity and plant genes required to shape the rhizosphere microbiome, we use Pseudomonas fluorescens and the model plant Arabidopsis thaliana as a tractable rhizosphere microbiome model. Arabidopsis-P. fluorescens provides a system with molecular, genetic and biochemical tools to answer basic questions in host-microbiome interactions. This project proposal includes three complementary aims to study the bacterial and plant mechanisms that ultimately affect plant microbiome immune homeostasis. Objective 1: Determine how bacterial acidification of the rhizosphere suppresses plant immunity Pre-treatment of Arabidopsis roots with P. fluorescens will suppress a defense response upon subsequent challenge with a microbe-associated molecular pattern (MAMP) such as bacterial flagellin. Through forward genetic screens, we found that multiple bacterial acidification mechanisms work to suppress plant immunity. This objective describes characterization of the mechanisms by which altering the rhizosphere pH suppresses plant immunity. Objective 2: Determine how negative regulators of Salicylic Acid signaling shape the rhizosphere microbiome The plant hormone salicylic acid (SA) plays a central role in plant immunity, and also affects plant metabolism and development. Positive regulators of SA signaling have been shown to shape the rhizosphere microbiome structure and function. This objective is focused on characterizing the role of negative regulators of SA signaling in shaping plant metabolism and immune homeostasis in plant-microbiome interactions.  Objective 3: Characterize the role of HSM genes in the rhizosphere microbiome To identify plant genes that regulate the rhizosphere microbiome, we rescreened a collection of plant "hsm" mutants, which were originally identified as mutants impaired in hormone-immunity crosstalk. We identified 6 hsm mutants that support altered levels of P. fluorescens under gnotobiotic conditions. This aim focuses on characterizing the role of hsm genes in shaping the rhizosphere microbiome. These three complementary objectives will help determine how microbiota and their host strike a balance in immune homeostasis that ultimately results in establishment, tolerance and maintenance of a microbiome.