PGRP-S1 maintains gut microbiota homeostasis of Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae) by negatively regulating

Red palm weevil (RPW), Rhynchophorus ferrugineus Olivier, is a non-native stem-boring insect pest in China. Previous studies have confirmed that RPW establishes symbiotic interactions with gut bacteria, which profoundly affect its physiological traits. However, the molecular mechanisms by which RPW maintain immune homeostasis to tolerate a beneficial microbiome and protect against enterophathogens remain poorly understood. Peptidoglycan recognition proteins (PGRPs) are a family of immune molecules conserved across species from insects to mammals. In the present study, we found that RPW PGRP-S1 is a secreted protein with amidase activity. RT-qPCR revealed that the expression levels of RfPGRP-S1 in the gut were significantly higher than those of other tissues. Furthermore, RfPGRP-S1 could be drastically induced to be expressed upon bacterial challenge, suggesting that RfPGRP-S1 may mediate RPW immunity. In vitro bioassays revealed that rRfPGRP-S1 caused significant agglutination of Escherichia coli and Staphylococcus aureus. Moreover, rRfPGRP-S1 could significantly retard the growth of S. aureus, indicating that RfPGRP-S1 is a pathogen recognition receptor with bactericidal activity. RfPGRP-S1 knockdown dramatically reduced the persistence of introduced E. coli in the gut and hemolymph. More interestingly, the load of the indigenous gut microbiota was significantly decreased, and its community structure was also intensively altered by RfPGRP-S1 knockdown. Furthermore, higher expression levels of the antimicrobial peptide, RfDefensin, were detected in guts after RfPGRP-S1 silencing. Collectively, these results demonstrate that RfPGRP-S1 can prevent excessive activation of intestinal immunity through its amidase activity, thereby maintaining gut bacterial homeostasis.