Chemical Priming by Neonicotinoids Unveils CaNEN4 as a Susceptibility Gene Against Phytophthora capsici in Pepper

The oomycete Phytophthora capsici causes Phytophthora blight, a major constraint on global pepper production. Our previous observations indicated that pretreating plants with thiamethoxam (TMX) and imidacloprid (IMI) could reduce the incidence of pepper blight, but the underlying mechanisms remained unclear. Here, we investigated how TMX and IMI induced resistance in pepper (Capsicum frutescens) against P. capsici. Both in vitro and in vivo assays demonstrated that TMX and IMI suppressed disease, not by directly impairing pathogen virulence but by inducing systemic resistance in susceptible (Cusheng L09) and resistant (Cusheng 356) pepper cultivars. Split-plant systemic resistance assays showed that TMX/IMI-primed plants developed smaller lesions in both treated and untreated leaves following P. capsici infection. Foliar application of TMX and IMI effectively alleviated disease severity, with IMI showing superior efficacy in attenuating reactive oxygen species (ROS) accumulation, and TMX/IMI priming concomitantly altering the activities of ROS-scavenging enzymes under pathogen challenge. Reverse transcription-quantitative PCR analysis revealed time-dependent changes in defence gene expression, and whole-genome transcriptome profiling highlighted temporal reprogramming of pathogenesis-related genes. Further functional validation identified CaNEN4 as a susceptibility factor. Collectively, our findings reveal that IMI/TMX primes pepper plants with systemic resistance by modulating ROS homeostasis, defence gene expression, and susceptibility gene function, offering novel insights into chemical-induced plant immunity and genetic targets for durable blight resistance in crops.