Genome-Wide Identification of PGRP Gene Family and Its Role in Dendrolimus kikuchii Immune Response Against Bacillus thuringiensis Infection

Peptidoglycan recognition proteins (PGRPs) are conserved pattern recognition receptors (PRRs) that play key roles in insect innate immunity by binding bacterial peptidoglycan (PGN) and activating downstream signaling pathways. The <i>Dendrolimus kikuchii</i>, a major defoliator of coniferous forests in southern China, has incompletely characterized immune defenses. This study systematically identified the PGRP gene family in <i>D. kikuchii</i> based on genome-wide data, identifying 10 PGRP genes with typical PGRP/Amidase_2 conserved domains, including 6 PGRP-S proteins and 4 PGRP-L proteins. Additionally, to further investigate the evolutionary relationships of these PGRP genes, a maximum likelihood (ML) phylogenetic tree was constructed using PGRP amino acid sequences from 6 different insect species, along with the 10 PGRP amino acid sequences from <i>D. kikuchii</i>. Phylogenetic analysis revealed that the DkikPGRP genes of <i>D. kikuchii</i> are distributed across distinct evolutionary branches and share high homology with PGRP genes from other insects, suggesting a close evolutionary relationship between the PGRP genes of <i>D. kikuchii</i> and those of other insect species. Transcriptome profiling revealed that <i>DkikPGRP-S1</i>, <i>-S2</i>, <i>-S3</i>, <i>-S4</i>, and <i>-S5</i> were upregulated in the midgut, fat body, and hemolymph after Bt infection, showing tissue- and time-specific immune responses. Functional assays using siRNA knockdown demonstrated distinct roles of <i>DkikPGRP-S4</i> and <i>DkikPGRP-S5</i>: <i>DkikPGRP-S5</i> mainly promoted antimicrobial peptide (AMP) expression, including attacin, lebocin, lysozyme, and cecropin, whereas <i>DkikPGRP-S4</i> showed a complex regulatory pattern, enhancing lebocin and lysozyme but suppressing attacin without affecting gloverin or cecropin. Silencing either gene significantly increased larval mortality upon Bt challenge. These results highlight the specialized immune regulatory functions of PGRPs in <i>D. kikuchii</i>, provide new insights into host-pathogen interactions, and suggest potential molecular targets for sustainable pest management strategies.