Most animals harbor a gut microbiota that consists of potentially pathogenic, commensal, and mutualistic microorganisms. Dual oxidase (Duox) is a well described enzyme involved in gut mucosal immunity by the production of reactive oxygen species (ROS) that antagonizes pathogenic bacteria and maintains gut homeostasis in insects. However, despite its nonspecific harmful activity on microorganisms, little is known about the role of Duox in the maintenance of mutualistic gut symbionts. Here we show that, in the bean bug Riptortus pedestris, Duox-dependent ROS did not directly contribute to epithelial immunity in the midgut in response to its mutualistic gut symbiont, Burkholderia insecticola. Instead, we found that the expression of Duox is tracheae-specific and its down-regulation by RNAi results in the loss of dityrosine cross-links in the tracheal protein matrix and a collapse of the respiratory system. We further demonstrated that the establishment of symbiosis is a strong oxygen sink triggering the formation of an extensive network of tracheae enveloping the midgut symbiotic organ as well as other organs, and that tracheal breakdown by Duox RNAi provokes a disruption of the gut symbiosis. Down-regulation of the hypoxia-responsive transcription factor Sima or the regulators of tracheae formation Trachealess and Branchless produces similar phenotypes. Thus, in addition to known roles in immunity and in the formation of dityrosine networks in diverse extracellular matrices, Duox is also a crucial enzyme for tracheal integrity, which is crucial to sustain mutualistic symbionts and gut homeostasis. We expect that this is a conserved function in insects.

大多数动物的肠道微生物群由潜在的致病微生物、共生微生物和共生微生物组成。双氧化酶 (Duox) 是一种通过产生活性氧 (ROS) 来对抗病原菌并维持昆虫肠道稳态而参与肠道粘膜免疫的酶。然而，尽管它对微生物具有非特异性有害活性，但人们对 Duox 在维持共生肠道共生体中的作用知之甚少。在这里，我们表明，在豆虫Riptortus pedestris 中，Duox 依赖性 ROS 没有直接促进中肠的上皮免疫，以响应其共生的肠道共生体Burkholderia infanticola. 相反，我们发现 Duox 的表达是气管特异性的，它被 RNAi 下调导致气管蛋白质基质中二酪氨酸交联的丧失和呼吸系统的崩溃。我们进一步证明，共生的建立是一个强大的氧汇，触发了广泛的气管网络的形成，包围了中肠共生器官以及其他器官，并且 Duox RNAi 对气管的破坏会引发肠道共生的破坏。缺氧反应性转录因子 Sima 或气管形成调节因子的下调 Trachealess 和 Branchless 产生相似的表型。因此，除了在免疫和在不同细胞外基质中形成二酪氨酸网络中的已知作用外，Duox 还是气管完整性的关键酶，这对于维持共生体和肠道稳态​​至关重要。我们预计这是昆虫的保守功能。