连作番茄根系分泌物促进土传青枯菌入侵的机制研究

Tomato bacterial wilt (TBW) caused by the soilborne pathogen Ralstonia solanacearum occurs frequently in a continuous crop system. It is believed that the accumulation effect of key components of tomato root exudates might play important roles in facilitating the invasion of R. solanacearum around or on the surface of tomato roots. However, the members of the key tomato root exudates and their potential mechanisms in mediating pathogen invasion have not been clearly investigated. The pathogen R. solanacearum finishes its root invasion in three steps：(1) move to the host roots, (2) colonize on the root surface, and (3) enter into tomato root. We hypothesize that at each stage there are certain tomato root exudates components facilitating R. solanacearum invasion. Based on their potential roles, the tomato root exudates were divided into three groups in the present study: Pathogen Preference Component (PPC), Allelochemical and Autotoxins. PPCs refer to two kinds of root exudates, one of which is the chemotacticum that can attract pathogen moving to the roots of host plant; it helps R. solanacearum invasion in step 1. Another one is the food source of pathogen, with which R. solanacearum can grow fast, and increase its population efficiently, and facilitate R. solanacearum invasion in step 2. Allelochemical refers to the root exudates components which have potential deleterious effect on soil microbiome. We hypothesize that the accumulation of allelochemicals in the soil after continuous monoculture might simply the microbial community structure and weaken its ecosystem functions, such as pathogen suppression via direct or indirect competitions, and finally benefit for pathogen colonization on host root surface. Autotoxins here refer to certain components of tomato root exudates which are toxic to the tomato root. The damage of root tissues by the accumulation of the autotoxins might play an important role in the third step of pathogen root invasion. To test our hypotheses, utilization of tomato root exudates by R. solanacearum, deleterious effect to soil microbiome and functions by allelochemicals, and damage of root tissues by the accumulation of the autotoxins will be detected in the present study, so that we can identify the substances PPCs, allelochemicals and autotoxins. Furthermore, pot experiment will be designed to study the effect of interaction among these three kinds of substances on the invasion by R. solanacearum. Based on the above studies, we will finally illuminate the facilitating mechanism to invasion of R. solanacearum showed by the critical substances, which can provide theoretical basis for revealing why continuous cropping obstacle could happen and how to control soilborne disease.

番茄连作易发生土传青枯病（致病菌为青枯菌），研究认为连作体系下某些番茄根系分泌物的累积效应在病菌入侵过程中可能起重要作用，但其组成及贡献机制尚未清楚阐明。本项目提出番茄根系分泌物中“利病菌”、“化感”和“自毒”三大类物质，分别通过促进青枯菌根际定殖（入侵第一阶段：病菌根际定殖竞争能力增强）、抑制土著（有益）微生物群落功能（入侵第二阶段：有益菌根表抵御病菌能力减弱）和破坏根系组织（入侵第三阶段：植物抵御病菌入侵根内能力降低）等三方面促进青枯菌入侵番茄根系的假设。通过研究青枯菌利用番茄根系分泌物的特征、化感物质调控下的土壤微生物群落结构和功能特征和自毒物质影响下根系组织受损情况，确定“利病菌”、“化感”和“自毒”三类物质组成，并通过盆栽试验研究三类物质交互作物下青枯菌侵染番茄根系的效率。最终阐明番茄根系分泌物的关键组分促进青枯菌入侵的机制，为揭示连作障碍发生机制和防控土传病害提供理论基础。

番茄连作易发生土传青枯病（致病菌为青枯菌），研究认为连作体系下某些番茄根系分泌物的累积效应在病菌入侵过程中可能起重要作用，但其组成及贡献机制尚未清楚阐明。本项目提出番茄根系分泌物中“利病菌”、“化感”和“自毒”三大类物质，分别通过促进青枯菌根际定殖（入侵第一阶段：病菌根际定殖竞争能力增强）、抑制土著（有益）微生物群落功能（入侵第二阶段：有益菌根表抵抵御病菌能力减弱）和破坏根系组织（入侵第三阶段：植物抵御病菌入侵根内能力降低）等三方面促进青枯菌入侵番茄根系的假设。通过研究青枯菌利用番茄根系分泌物的特征，结果表明，青枯菌能有效利用52种供试分泌物中的28种组分，其中糖类12种，有机酸10种，氨基酸3种以及酚酸3种。限菌微宇宙系统培苗试验结果表明，番茄青枯病的发病速率和发病强度与青枯菌对根系分泌物利用效率高低相关，在青枯菌能快速有效利用的分泌物种类处理中番茄最先出现发病症状，且发病率高于其它处理，经检测番茄根际青枯菌数量表明，在利病菌物质存在时，青枯菌数量均显著高于对照。微根际土培试验表明，外源持续添加不同根系分泌物组分2个月后，酚酸类物质对土壤细菌多样性的影响与糖类、有机酸类和氨基酸类存在显著差异，在门水平上，酚酸类物质中的邻苯二甲酸二丁酯、丁香酸、绿原酸、咖啡酸、阿魏酸和苯甲酸处理破坏了土壤细菌多样性，其中变化最显著的为Firmicutes门细菌，对照处理中该门的丰度为8.4%，而酚酸处理后最高可达90.4%，说明该门类的细菌在酚酸处理后成为土壤中的优势菌，导致其它门类的细菌种类和数量较对照急剧下降，土壤的细菌多样性受到了破坏。进一步研究表明，在细菌多样性受到破坏的土壤中接种青枯菌后，青枯菌的生长速率和繁殖数量均高于未受破坏的土壤，表明化感物质可通过破坏土壤细菌的多样性降低土壤微生物群落对青枯菌入侵的抵抗力，从而导致青枯病的发生。