Transcriptomic Screening and Functional Characterization of Immune-Related Genes in Tetranychus urticae Responding to Isaria cateniannulata Infection

Clarifying the immune response mechanisms of Tetranychus urticae against Isaria cateniannulata is critical for developing sustainable pest control strategies. In this study, we employed transcriptome sequencing to identify immune-related genes responsive to Isaria cateniannulata infection, followed by integrative analysis using qRT-PCR, bioinformatics analysis and RNA interference (RNAi) to systematically study the expression patterns and functions of key candidate genes. The results indicated that a total of 342 genes were significantly differentially expressed in Tetranychus urticae following Isaria cateniannulata infection, including 322 up-regulated genes and 20 down-regulated genes. These differentially expressed genes were mainly enriched in immune-related pathways, including lysosomes, autophagy, phagosomes, apoptosis and NOD-like receptor signaling. Isaria cateniannulata infection resulted in different expression levels of candidate genes across various developmental stages of Tetranychus urticae. Among these, TuAPOD and TuPLA2 were highly expressed across all developmental stages, and their encoded proteins—comprising α-helices, extended strands, and random coils—shared high sequence similarity with homologs from other arthropods. RNAi-mediated silencing of these two genes achieved over 75% efficiency at 36 h. Combining RNAi with Isaria cateniannulata treatment significantly increased mortality and suppressed oviposition, with the highest 72 h mortality rate observed in the TuPLA2 + H13 treatment group (98.33%). This study provides preliminary insight into the molecular mechanism underlying Tetranychus urticae’s response to Isaria cateniannulata infection, clarifies the key role of lipid metabolism-related genes in antifungal immunity, and further confirms that combining RNA interference with Isaria cateniannulata treatment exerts a significant synergistic effect. These findings offer new insights and a theoretical basis for developing environmentally friendly strategies for controlling Tetranychus urticae.