CsRbohB Binds to CsCML35 and Regulates Reactive Oxygen Species Accumulation and Defense Against Fusarium Wilt Disease in Cucumber.

Fusarium wilt (FW) disease, caused by Fusarium oxysporum f. sp. cucumerinum (Foc), severely restricts cucumber (Cucumis sativus L.) growth and yield worldwide. Upon recognition of the invading pathogen, the host plant rapidly produces reactive oxygen species (ROS), which serve as a core component of the defense response. Here, we identified a plasma membrane-binding NADPH oxidase named CsRbohB and confirmed that it interacts with the calcium-binding protein CsCML35. Knockdown of CsRbohB in cucumber using an RNA interference (RNAi) method resulted in reduced ROS levels and increased susceptibility to FW. Comparative RNA sequencing analyses between the cucumber inbred line CCMC (wild type) and the CsRbohB-RNAi transgenic cucumber line inoculated with Foc revealed that hormone metabolism and defense-related genes were significantly down-regulated in the CsRbohB-RNAi line. Quantitative analyses showed that ethylene and salicylic acid levels were significantly reduced in the CsRbohB-RNAi line compared with wild type, and the activation of defense-related gene expression was also inhibited in the CsRbohB-RNAi line. Together, these results show that CsRbohB binds to CsCML35 and regulates ROS signals and mediates hormones and defense-related genes during the defense response to Foc in cucumber. This study has identified new targets for breeding disease-resistant cultivars.