Chemosensory protein BmCSP4 enhances silkworm adaptation to mulberry by suppressing plant defense responses.

The silkworm (Bombyx mori) feeds exclusively on mulberry leaves; however, the genetic basis underlying its adaptation to mulberry defenses remains unclear. In this study, we identified a silkworm chemosensory protein (CSP), BmCSP4, which is highly expressed in the salivary glands of larvae and secreted onto mulberry leaf wounds during feeding. Biochemical and molecular docking assays revealed that BmCSP4 binds to various mulberry volatiles, particularly cis-3-hexenyl acetate, cis-jasmone, and trans-3-hexen-1-ol. Mulberry leaves chewed by silkworms with exogenous BmCSP4 application or transgenic overexpression showed reduced expression of defense-related genes. Feeding assays showed that while transgenic silkworms overexpressing BmCSP4 exhibited no significant change in food consumption, they displayed significant increases in larval weight, pupal weight, cocoon weight, and fecundity. Transcriptomic analyses showed that mulberry leaves bitten by BmCSP4-expressing silkworms exhibited downregulated transcription of genes involved in defense hormone signaling and anti-insect secondary metabolism. In conclusion, as a salivary effector, BmCSP4 enhances silkworm growth fitness and performance by binding and blocking plant volatile defense signals to suppress host immunity.