How plants deal with beneficial and pathogenic microorganisms and how they can tolerate beneficial ones and face pathogens at the same time are questions that remain puzzling to plant biologists. Legume plants are good models to explore those issues, as their interactions with nitrogen-fixing bacteria called rhizobia results in a drastic and easy-to-follow phenotype of nodulation. Intriguingly, despite massive and chronic infection, legume defense reactions are essentially suppressed during the whole symbiotic process, raising a question about a potential negative effect of plant immune responses on the establishment of nodulation. In the present study, we used the model legume, Medicago truncatula, coinoculated with mutualistic and phytopathogenic bacteria, Sinorhizobium medicae and Ralstonia solanacearum, respectively. We show that the presence of R. solanacearum drastically inhibits the nodulation process. The type III secretion system of R. solanacearum, which is important for the inhibition of pathogen-associated molecular pattern–triggered immunity (PTI), strongly contributes to inhibit nodulation. Thus, our results question the negative effect of PTI on nodulation. By including a pathogenic bacterium in the interaction system, our study provides a new angle to address the influence of the biotic environment on the nodulation process.

Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

植物如何处理有益微生物和病原微生物，以及它们如何在耐受有益微生物的同时面对病原体，这些问题仍然困扰着植物生物学家。豆科植物是探索这些问题的良好模型，因为它们与称为根瘤菌的固氮细菌的相互作用会导致显着且易于遵循的结瘤表型。有趣的是，尽管存在大规模和慢性感染，但豆科植物防御反应在整个共生过程中基本上受到抑制，这引发了一个问题，即植物免疫反应对根瘤形成的潜在负面影响。在本研究中，我们使用了模型豆科植物苜蓿，与共生菌和植物病原菌、中华根瘤菌（Sinorhizobium medicae）和分别为青枯病菌。我们表明青枯菌的存在极大地抑制了结瘤过程。R. solanacearum 的III 型分泌系统对抑制病原体相关分子模式触发免疫 (PTI) 很重要，强烈有助于抑制结瘤。因此，我们的结果质疑 PTI 对结节的负面影响。通过在相互作用系统中包括致病菌，我们的研究提供了一个新的角度来解决生物环境对结瘤过程的影响。

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