Abstract
External secretions play a vital role in external immune defense. However, the functions and components of these exudates are largely unknown in the red palm weevil, Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae). In order to determine their role in external immunity, the immunosuppressive efficacy of the secretions in vitro against microbes, including bacteria and fungi, was clarified. In the present study, we found that these secretions had antimicrobial activity in vitro, implying external immunizing potency against pathogens. Surprisingly, all liquid phases of secretions could not significantly inhibit the growth of microbes in vitro compared to solid phases. To explain this phenomenon, the composition and emission differentia of secretions from the exocrine glands associated with different developmental stages, secretory regions, and phases were identified and analyzed based on metabonomics techniques. A total of more than 200 compounds, including quinines, phenols, aldehydes, acids, alcohols, saccharides, ketones, esters, amines, salts, ureas, and heterocycles, were identified in the secretions of larvae and adults. The liquid phase shared a number of metabolites with the solid phase, but the emission types and amounts were significantly different in the two phases, resulting in differences in external immunological activity. Tyrosine and p-benzoquinone were the dominant metabolites in all of the secretions, accounting for approximately 11.29% of emissions, with the portion in the solid phase being generally higher than that in the liquid phase. Moreover, only p-benzoquinone was entirely significantly upregulated in the solid phase compared to the liquid phase. Therefore, metabolome analysis suggested that p-benzoquinone, which may potentially be developed to be a valuable marker for determining external immunity, was considered to be the main substance responsible for external immune functions. This hypothesis was further demonstrated by the antimicrobial activity of p-benzoquinone.
Highlights
The invasive red palm weevil (RPW), Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae), is considered to be one of the most devastating pests in a number of palm tree species
To further determine the minimum inhibitory concentration (MIC) of secretion stostes required for S. aureus, E. coli, B. thuringiensis, and M. anisopliae inhibition, the microbial suspension (104 colony-forming unit (CFU)/ml or 104 conidia/ml) and diluents of stoste diluted with methanol into 500.00, 250.00, 125.00, 62.50, and 31.25 μl/ml using the twofold dilution method were mixed with equal volumes and cultured for 2 h for bacteria or 24 h for fungi at the temperatures described above
1http://www.omicshare.com secretions, as well as control filter papers treated with tetracycline, successfully inhibited growth of bacteria and fungi in vitro, including S. aureus, E. coli, B. thuringiensis, and M. anisopliae
Summary
The invasive red palm weevil (RPW), Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae), is considered to be one of the most devastating pests in a number of palm tree species. External immune defense involves by definition any heritable trait acting outside an organism improving protection from pathogens, such as bacteria, fungi, viruses, and nematodes, or manipulating the composition of the microbial community in favor of the organism (Otti et al, 2014) This immune defensive system, which includes the cuticle and external secretions, constitutes an initial physical and chemical barrier to pathogens (Gołebiowski et al, 2008; Silva et al, 2016; Pu et al, 2017a). Previous research has shown that the external secretions of insects can inhibit the growth of a variety of pathogens in vitro, including gram-positive bacteria, gram-negative bacteria, and fungi (Li et al, 2009; Cotter et al, 2013; Pedrini et al, 2015; Ulrich et al, 2015) We speculate that these chemical mixtures may be able to initiate the insect’s immune functions to protect themselves by preventing the invasion of foreigner pathogens. It is hoped that they will provide the basis for a new concept and approach in the development of a pest inhibitor or behavioral interference agent which utilizes the immune system of the pest as a target
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