Suppression of INF-1 and BAX-Induced Hypersensitive Response (HR) by Pseudoperonospora Cubensis Effector Proteins

Pseudoperonospora cubensis is a pathogenic oomycete responsible for causing downy mildew disease in cucurbit crops. This pathogen secretes various effector proteins, including RXLR effectors. Our hypothesis suggests that during infection, the pathogenic RXLR effector of P. cubensis is secreted into host plant cells to manipulate and suppress the plant’s immune system. We investigated to determine whether the RXLR effector of P. cubensis effectively inhibits the cell death process induced by Bcl-associated X (BAX) and Phytophthora infestans elicitin (INF-1). We identified 45 RXLR effectors through whole-genome sequencing of P. cubensis. Following RXLR effector selection criteria, we selected 39 of them for a cell death suppression assay. Transient gene expression experiments carried out in Nicotiana benthamiana revealed that most of the P. cubensis RXLR (PcRXLR) effectors could suppress programmed cell death (PCD) triggered by BAX and INF1. For the BAX-induced cell death suppression assay, 21 PcRXLR effectors partially suppressed cell death, and 7 PcRXLR effectors fully suppressed it. In contrast, 18 PcRXLR effectors fully suppressed PCD triggered by INF1 elicitin, while 7 PcRXLR effectors only partially suppressed it. However, 14 PcRXLR effectors were unable to inhibit necrosis induced by INF1 elicitin. Notably, PcRXLR 6, PcRXLR 13, and PcRXLR 14 effectors demonstrated the ability to suppress hypersensitive response (HR) triggered by both BAX and INF-1. Understanding the virulence mechanisms of P. cubensis is essential for unraveling how RXLR effectors inhibit PCD. Our study provides valuable insights for developing innovative strategies to control downy mildew disease in cucurbit crops.