Involvement of laccase2 and yellow-e genes in antifungal host defense of the model beetle, Tribolium castaneum

Antifungal roles of adult-specific cuticular protein genes of the red flour beetle, Tribolium castaneum

The insect cuticle is a composite structure that can further be divided into a few sub-structural layers. Its large moiety comprises a lattice of chitin fibrils and structural proteins, both of which are stabilized by covalent bonding among them. The cuticle covers the whole surface of insect body, and thus has long been suggested for the involvement in defense against entomopathogens, especially entomopathogenic fungi that infect percutaneously. We have been addressing this issue in the past few years and have so far demonstrated experimentally that chitin synthase 1, laccase2 as well as benzoquinone synthesis-related genes of Tribolium castaneum have indispensable roles in the antifungal host defense. In the present study we focused on another major component of the insect cuticular integument, structural cuticular proteins. We chose three genes coding for adult-specific cuticular proteins, namely CPR4, CPR18 and CPR27, and examined their roles in forming immunologically sound adult cuticular integuments. Analyses of developmental expression revealed that the three genes showed high level expression in the pupal stage. These results are consistent with their proposed roles in constructing cuticle of adult beetles. The RNA interference-mediated gene knockdown was employed to silence these genes, and the administration of double strand RNAs in pupae resulted in the adults with malformed elytra. The single knockdown of the three genes attenuated somewhat the defense of the resulting adult beetles against Beauveria bassiana and Metarhizium anisopliae, but statistical analyses indicated no significant differences from controls. In contrast, the double or triple knockdown mutant beetles displayed a drastic disruption of the host defense against the two entomopathogenic fungal species irrespective of the combination of targeted cuticular protein genes, demonstrating the important roles of the three cuticular protein genes in conferring robust antifungal properties on the adult cuticle. Scanning electron microscopic observation revealed that the germination of conidia attached on the adult body surface was still suppressed after the gene knockdown as in the case of wild-type beetles, suggesting that the weakened antifungal phenotypes resulted from the combined knockdown of the adult-specific cuticular protein genes could not be accounted for by the disfunction of secretion/retention of fungistatic benzoquinone derivatives.

Immune properties of invertebrate phenoloxidases

Melanin production from different types of phenoloxidases (POs) confers immunity from a variety of pathogens ranging from viruses and microorganisms to parasites. The arthropod proPO expresses a variety of activities including cytokine, opsonin and microbiocidal activities independent of and even without melanin production. Proteolytic processing of proPO and its activating enzyme gives rise to several peptide fragments with a variety of separate activities in a process reminiscent of vertebrate complement system activation although proPO bears no sequence similarity to vertebrate complement factors. Pathogens influence proPO activation and thereby what types of immune effects that will be produced. An increasing number of specialised pathogens - from parasites to viruses - have been identified who can synthesise compounds specifically aimed at the proPO-system. In invertebrates outside the arthropods phylogenetically unrelated POs are participating in melanization reactions obviously aimed at intruders and/or aberrant tissues.

RNAi-Mediated Suppression of Laccase2 Impairs Cuticle Tanning and Molting in the Cotton Boll Weevil (Anthonomus grandis)

The cotton boll weevil, Anthonomus grandis, is the most economically important pest of cotton in Brazil. Pest management programs focused on A. grandis are based mostly on the use of chemical insecticides, which may cause serious ecological impacts. Furthermore, A. grandis has developed resistance to some insecticides after their long-term use. Therefore, alternative control approaches that are more sustainable and have reduced environmental impacts are highly desirable to protect cotton crops from this destructive pest. RNA interference (RNAi) is a valuable reverse genetics tool for the investigation of gene function and has been explored for the development of strategies to control agricultural insect pests. This study aimed to evaluate the biological role of the Laccase2 (AgraLac2) gene in A. grandis and its potential as an RNAi target for the control of this insect pest. We found that AgraLac2 is expressed throughout the development of A. grandis with significantly higher expression in pupal and adult developmental stages. In addition, the immunolocalization of the AgraLac2 protein in third-instar larvae using specific antibodies revealed that AgraLac2 is distributed throughout the epithelial tissue, the cuticle and the tracheal system. We also verified that the knockdown of AgraLac2 in A. grandis resulted in an altered cuticle tanning process, molting defects and arrested development. Remarkably, insects injected with dsAgraLac2 exhibited defects in cuticle hardening and pigmentation. As a consequence, the development of dsAgraLac2-treated insects was compromised, and in cases of severe phenotypic defects, the insects subsequently died. On the contrary, insects subjected to control treatments did not show any visible phenotypic defects in cuticle formation and successfully molted to the pupal and adult stages. Taken together, our data indicate that AgraLac2 is involved in the cuticle tanning process in A. grandis and may be a promising target for the development of RNAi-based technologies.

RNA interference of tyrosine hydroxylase caused rapid mortality by impairing cuticle formation in Nilaparvata lugens (Hemiptera: Delphacidae)

BACKGROUND

The application of RNA interference (RNAi) technique in controlling agricultural insect pests has been receiving much attention since the discovery of RNAi. The brown planthopper (BPH) Nilaparvata lugens, a notorious pest of rice, has evolved a high level of resistance to many kinds of insecticides. Tyrosine hydroxylase (Th) is an indispensable survival gene in holometabolous insects, playing key roles in cuticle tanning and immunity. In this study, we investigated whether Th could be used as a potential target in controlling N. lugens.

RESULTS

Here, we demonstrated that NlTh had a periodical expression pattern during molting with the highest level observed in epidermis. Dysfunction of NlTH by dsNlTh microinjection or 3-IT feeding similarly caused rapid death of N. lugens. Compared with dsGFP control BPHs, dsNlTh injected BPHs (i) had cuticle pigmentation and sclerotizaton defects; (ii) had less endocuticle lamella in tergum integument; (iii) showed higher mortality during the molting process as a result of defective cuticle shedding; (iv) showed feeding disorders indicated by a low number of probe wound dots on rice; (v) had more vulnerable cuticle.

CONCLUSION

This study demonstrated that TH orthologues play a conservative and crucial role for exocuticle tanning in both holometabolous and hemimetabolous insects, and NlTh could be targeted for RNAi-mediated BPH control. The rapid lethal phenotype of NlTH dysfunction BPHs partly induced by cuticle formation defects. © 2020 Society of Chemical Industry.

Benzoquinone synthesis-related genes of Tribolium castaneum confer the robust antifungal host defense to the adult beetles through the inhibition of conidial germination on the body surface

Insects fight against invading microbial pathogens through various immune-related measures that comprise 'internal', 'external' as well as 'social' immunities. The defenses by external immunity associated with the cuticular integument are supposed to be of particular importance in repelling entomopathogenic fungi that infect host insects transcutaneously. Among such integument-related defenses, external secretions of benzoquinone derivatives typical of tenebrionid beetles have been suggested to play important roles in the antimicrobial defenses. In the present study, by utilizing the experimental infection system composed of the red flour beetle Tribolium castaneum and generalist ascomycete entomopathogens Beauveria bassiana and Metarhizium anisopliae, we performed the functional assays of the three T. castaneum genes whose involvement in benzoquinone synthesis in the adults has been reported, namely GT39, GT62 and GT63. Observations by scanning electron microcopy (SEM) revealed that the conidia of the two fungal species did not germinate on the wild-type adult body surface but did on the pupae. The expression analyses demonstrated that the levels of GT39 and GT62 mRNA increased from middle pupae and reached high in early adults while GT63 did not show a clear adult-biased expression pattern. The RNA interference-based knockdown of any of the three genes in pupae resulted in the adults compromised to the infection of the both fungal species. SEM observations revealed that the gene silencing allowed the conidial germination on the body surface of the knockdown beetles, thereby impairing the robust antifungal defense of adult beetles. Thus, we have provided direct experimental evidence for the functional importance in vivo of these benzoquinone synthesis-related genes that support the antifungal defense of tenebrionid beetles.