Multifunctional role of a fungal pathogen‐secreted laccase 2 in evasion of insect immune defense

Abstract

Summary Laccases are widely present in bacteria, fungi, plants and invertebrates and involved in a variety of physiological functions. Here, we report that Beauveria bassiana, an economic important entomopathogenic fungus, secretes a laccase 2 (BbLac2) during infection that detoxifies insect immune response‐generated reactive oxygen species (ROS) and interferes with host immune phenoloxidase (PO) activation. BbLac2 is expressed in fungal cells during proliferation in the insect haemocoel and can be found to distribute on the surface of haemolymph‐derived in vivo fungal hyphal bodies or be secreted. Targeted gene‐knockout of BbLac2 increased fungal sensitivity to oxidative stress, decreased virulence to insect, and increased host PO activity. Strains overexpressing BbLac2 showed increased virulence, with reduced host PO activity and lowered ROS levels in infected insects. In vitro assays revealed that BbLac2 could eliminate ROS and oxidize PO substrates (phenols), verifying the enzymatic functioning of the protein in detoxification of cytotoxic ROS and interference with the PO cascade. Moreover, BbLac2 acted as a cell surface protein that masked pathogen associated molecular patterns (PAMPs), enabling the pathogen to evade immune recognition. Our data suggest a multifunctional role for fungal pathogen‐secreted laccase 2 in evasion of insect immune defenses.

Melanization is an important antifungal defense mechanism in Galleria mellonella hosts

“…This implies that some of the trigger for melanization comes from laccase-catalyzed initiation of melanin formation using host-derived catecholamines in the hemolymph. This is consistent with the observation that for B. bassiana infection of G. mellonella, that laccases play a role in virulence by oxidizing the hemolymph catecholamines and preventing them from producing antifungal melanization and reducing the oxidative burden on the fungus 24 . It is also worth noting that the lac1∆ mutant is less virulent in G. mellonella infections compared to the parental 19 .…”

“…The lysis of oenocytoid cells results in the release of antimicrobial peptides, signaling molecules, and enzymes important to immune function [9][10][11] . One class of host enzymes that are often released and activated during oenocytoid lysis and nodulation are laccase, a fungal enzyme that oxidizes mammalian and insect catecholamines, is an important virulence factor in both hosts but, seemingly by distinct and diverse mechanisms 19,24,25 . In insects, fungal laccase appears to oxidize and deplete host catecholamines required for encapsulating the fungus in melanin, thus weakening the host immune response.…”

“…In insects, fungal laccase appears to oxidize and deplete host catecholamines required for encapsulating the fungus in melanin, thus weakening the host immune response. Fungal laccases also help detoxify reactive oxygen species that form during insect immune processes 24 . In contrast, during mammalian infection, fungal laccase enhances production of fungal melanin to evade key mammalian immune defenses 26 .…”

“…This suggests that fungal melanins can activate the immune system which could help promote fungal clearance. This is interesting in the context of naturally-occurring fungal pathogens of insects, which tend to have a white color and/or do not naturally produce melanin pigment, such as Beauveria bassiana and Metarhizium anisophilae 24,25 . This may be because of evolutionary pressure that selects for entomopathogenic fungi that produce less fungal melanin and thus are better at evading the insect's melanin-based immune response.…”

Nanomaterials and nanotechnology for the delivery of agrochemicals: strategies towards sustainable agriculture

“…NMs can easily penetrate cell membranes due to their nanostructure and small size, and the surface properties of small materials allow them to attach to cell organelles, influencing normal cell organ function and directly or indirectly generating abnormal reactive oxygen species (ROS) [ 48 ]. In important physiological processes such as cell signaling, gene expression, and protein redox regulation, a normal ROS level is essential [ 49 ]; however, high ROS levels in the above cellular processes cause anomalies and interrupt normal functioning, resulting in cell death [ 50 ].…”