Abstract
Insect defensins are effector components of the innate defense system. During infection, these peptides may play a role in the control of pathogens by providing protective antimicrobial barriers between epithelial cells and the hemocoel. The cDNAs encoding four defensins of the migratory locust, Locusta migratoria, designated LmDEF 1, 3–5, were identified for the first time by transcriptome-targeted analysis. Three of the members of this CSαβ defensin family, LmDEF 1, 3, and 5, were detected in locust tissues. The pro regions of their sequences have little-shared identities with other insect defensins, though the predicted mature peptides align well with other insect defensins. Phylogenetic analysis indicates a completely novel position of both LmDEF 1 and 3, compared to defensins from hymenopterans. The expression patterns of the genes encoding LmDEFs in the fat body and salivary glands were studied in response to immune-challenge by the microsporidian pathogen Nosema locustae and the fungus Metarhizium anisopliae after feeding or topical application, respectively. Focusing on Nosema-induced immunity, qRT-PCR was employed to quantify the transcript levels of LmDEFs. A higher transcript abundance of LmDEF5 was distributed more or less uniformly throughout the fat body along time. A very low baseline transcription of both LmDEFs 1 and 3 in naïve insects was indicated, and that transcription increases with time or is latent in the fat body or salivary glands of infected nymphs. In the salivary glands, expression of LmDEF3 was 20-40-times higher than in the fat body post-microbial infection. A very low expression of LmDEF3 could be detected in the fat body, but eventually increased with time up to a maximum at day 15. Delayed induction of transcription of these peptides in the fat body and salivary glands 5–15 days post-activation and the differential expression patterns suggest that the fat body/salivary glands of this species are active in the immune response against pathogens. The ability of N. locustae to induce salivary glands as well as fat body expression of defensins raises the possibility that these AMPs might play a key role in the development and/or tolerance of parasitic infections.
Highlights
Multicellular organisms continually defend themselves against infection through diverse, effective systems of immunity
The results showed that LmDEF3 was not induced in fat body at 1–10 days post-infection (p.i.) and its transcripts were nearly absent compared to their prominent levels detected in the salivary glands
It is noteworthy that Zhang et al [36] identified only one unigene corresponding to a defensin in the overall transcriptome library of M. acridum-infected L. migratoria
Summary
Multicellular organisms continually defend themselves against infection through diverse, effective systems of immunity. Unlike vertebrates, which possess both innate and adaptive arms of the immune system, insects rely on a highly-evolved system of innate immunity [1, 2]. When the latter is activated, it produces a broad spectrum of humoral effector molecules, endowed with a repertoire of defensive mechanisms, including the production of antimicrobial peptides (AMPs) [3]. Defensins are members of a large family of naturally occurring AMPs that contribute significantly to host defense against microbial invasion in animals, including insects and humans, and are widely produced in plants [6, 7]
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