# A C-Type Lectin, RfCTL27, Activates the Immune Defense in the Red Palm Weevil Rhynchophorus ferrugineus (A.G. Olivier, 1791) (Coleoptera: Curculionidae: Dryophthorinae) by the Recognition of Gram-Negative Bacteria

## TL;DR

This study identifies RfCTL27, a protein in red palm weevils, as a key player in immune defense against bacteria by activating antimicrobial peptides.

## Contribution

The novel finding is that RfCTL27 activates both systemic and gut immunity in red palm weevils by controlling antimicrobial peptide genes.

## Key findings

- RfCTL27 expression increases in gut and fat body after exposure to Gram-negative bacteria.
- Silencing RfCTL27 reduces antimicrobial peptide gene expression in gut and fat body.
- RfCTL27 helps clear bacterial pathogens in red palm weevil larvae.

## Abstract

Red palm weevil (RPW), Rhynchophorus ferrugineus (Olivier), is an invasive and highly destructive insect pest that poses a significant threat to palm trees. Currently, employing the biological agents are the important alternative method to fight against the infestation and alleviate the pesticide resistance of this pest. However, the control effect is unsatisfactory. It has been well known that the innate immunity plays the vital role for the protection of insect pests upon the attack of pathogens. In this research, we aimed to investigate the role of a C-type lectin, RfCTL27, in the immune response of RPWs. RT-qPCR showed that Escherichia coli induced a significant increase in the expression of RfCTL27 in the gut and fat body. After RfCTL27 was silenced, the expression level of antimicrobial peptide (AMP) genes in the gut and fat body was significantly decreased. Therefore, our findings indicate that RfCTL27 is involved in both systemic and gut local immunity by controlling the expression of AMP genes upon the exposure of Gram-negative bacteria.

Red palm weevil, Rhynchophorus ferrugineus (Olivier), is a palm tree insect pest that causes significant damage in the many countries from the Indian sub-continent and southeast Asia into date palm-growing countries of Africa, the Middle East, and the Mediterranean Basin. This study is aimed at determining the role of a C-type lectin, RfCTL27, in the immune defense of RPW larvae. RfCTL27 is a secreted protein that possesses a QPD motif, being integral for the discrimination of Gram-negative bacteria. The abundance of RfCTL27 transcripts in the gut and fat body was significantly higher than that in other tissues. Six hours after injection of Escherichia coli, the expression level of RfCTL27 in the gut of RPW larvae was significantly elevated compared with other groups. At 12 h after injection of E. coli, the expression of RfCTL27 in fat body was dramatically induced in contrast with other treatments. More interestingly, the ability of RPW larvae to clear the pathogenic bacteria in the body cavity and gut was markedly impaired by the silencing of RfCTL27. Additionally, the expression levels of two antimicrobial peptide genes, RfCecropin in the gut and RfDefensin in fat body of RPW larvae, were significantly decreased. Taken together, these data suggested that RfCTL27 can recognize the Gram-negative bacterium and activate the expression of antimicrobial peptides to remove the invaded bacterial pathogens. This study provides a new scientific basis for improving the control efficiency of pathogenic microorganisms against red palm weevils in production practice.