土壤根际促生菌—嗜根寡养单胞菌DSF群体感应信号分子的调控机理

Plant growth-promoting rhizobacteria (PGPR) are a group of free-living bacteria that benefit the plant growth and play a major role in the biocontrol of plant pathogens. It is important not only for nutrient-use efficiency in soil, but also for the crop yield and quality. In the rhizosphere, an extensive communication takes place between plants and root-associated microorganisms, involving diversity of released compounds and signals. These chemical signals play important roles in determining the assembly and functioning of rhizosphere communities. Various PGPR have been found to produce quorum sensing (QS) signals, which are known as cell-density-dependent phenomenon mediated by cell-to-cell communication. The identification of QS signals involved in the interaction of PGPR with each others, and the metabolic regulation of soil microorganism, are discovered recently. However, in the soil ecosystem, the mechanism and processes of growth-promoting regulated by QS in PGPR remain poorly understood. The representative PGPR strain of Stenotrophomonas rhizophila, with homologous components to diffusible signal factor (DSF) QS system, is selected in this project. Based on the methods of systems biology and microbial ecology, this project is designed to answer the following scientific questions: 1) whether DSF QS system in S. rhizophila has effects on the key functions of plant growth-promoting; 2) to discuss the mechanisms of control and regulation of the PGPR community structure and function by DSF QS system in S. rhizophila. This project will enhance understanding of the survival process and ecological function of PGPR in soil.

土壤中的根际促生菌在促进植物生长或降低植物病害方面发挥着积极的作用，它对改良土壤养分利用率和提高作物产量有着重要意义。这种相互作用的介导过程是通过小分子信号调控完成的，其中群体感应（QS）信号分子发挥着重要作用。群体感应本是细菌种群内的交流方式，近几年发现它在微生物种群间的互作与代谢调控中具有关键作用。然而根际促生菌的QS信号对促生作用的调控机理还是一个有待进一步揭示的科学问题。本申请拟选取土壤中代表性的根际促生菌（嗜根寡养单胞菌），生物信息学分析表明其具有可扩散性信号分子（diffusible signal factor, DSF）QS系统同源基因，利用系统生物学及微生物生态学方法，来回答以下科学问题：嗜根寡养单胞菌的DSF QS调控系统是否是促进植物生长的关键因素？其QS调控系统如何影响植物根际微生物的种群关系和功能？这些问题的揭示将提升对根际促生菌在土壤中生存方式及发挥功效的认识。

嗜根寡养单胞菌是土壤生态系统中重要的根际促生菌，对改良土壤养分利用率和提高作物产量有着重要意义。然而，这种根际促生菌自身关键的群体感应（quorum sensing, QS）调控作用却没有一个完整的科学认识。因此，我们迫切需要回答的科学问题是：嗜根寡养单胞菌是否含有功能性的QS调控体系？是否产生DSF（diffusible signal factor）信号分子？嗜根寡养单胞菌的QS系统是否对其生物膜的形成产生调控作用？嗜根寡养单胞菌的QS调控系统是否是促进植物生长的关键因素？基于以上科学问题，本项目研究发现：1）在嗜根寡养单胞菌中存在顺式-2-不饱和脂肪酸类DSF信号分子合成酶，其信号传导通路包括RpfC/RpfG双组分感应系统、二级信使环二鸟苷酸（cyclic di-GMP）及其受体全局性转录因子Clp等；2）通过同源重组的方法，构建了一株嗜根寡养单胞菌△rpfF突变菌株，发现嗜根寡养单胞菌中信号分子DSF达到一定的浓度阈值后，DSF胞内的受体蛋白（RpfC）发生磷酸化反应，激活下游c-di-GMP系统，调控生物膜的形成；3）在植物根际接种的嗜根寡养单胞菌能够招募土壤中的Cyanobacteria和Actinobacteria等固氮菌来重塑根际细菌群落，同时提高了土壤中氨化基因ureC的丰度，有助于改善植物对氮肥的利用效率；4）嗜根寡养单胞菌野生型和△rpfF突变型两者均能有效缓解盐碱胁迫，但受QS调控的成膜能力以及在植物根部的定植能力，可能会影响嗜根寡养单胞菌原位的抗盐碱水平；5）嗜根寡养单胞菌具有较好的Cr(VI)还原能力，为Cr(VI)污染的原位植物-微生物联合修复提供了潜在的菌剂材料。通过揭示嗜根寡养单胞菌生物膜的形成、抗盐碱胁迫、促生作用等QS直接或间接调控效应及机制，初步阐明了嗜根寡养单胞菌作为重要的根际促生菌在群体环境中的生存方式及其应用潜力。