Tribolium castaneum immune defense genes are differentially expressed in response to Bacillus thuringiensis toxins sharing common receptor molecules and exhibiting disparate toxicity

• Cadherin-like and sodium solute symporter are T. castaneum Cry3Aa toxin receptors.
• Apolipoprotein-III induction upon B. thuringiensis intoxication is strain specific.
• T. castaneum Defensins 3 and 2 are involved in B. thuringiensis immune response.
• B. thuringiensis susceptibility is associated to T. castaneum immune response.
• A T. castaneum Defensin 3 fragment kills E. coli, S. aureus and C. albicans.

In Tribolium castaneum larvae we have demonstrated by RNA interference knockdown that the Bacillus thuringiensis Cry3Ba toxin receptors Cadherin-like and Sodium solute symporter proteins are also functional receptors of the less active Cry3Aa toxin. Differences in susceptibility to B. thuringiensis infection might not only rely on toxin–receptor interaction but also on host defense mechanisms. We compared the expression of the immune related genes encoding Apolipophorin-III and two antimicrobial peptides, Defensin3 and Defensin2 after B. thuringiensis challenge. All three genes were up-regulated following Cry3Ba spore–crystal intoxication whereas only Defensins gene expression was induced upon Cry3Aa spore–crystal treatment, evidencing a possible association between host immune response and larval susceptibility to B. thuringiensis. We assessed the antimicrobial activity spectra of T. castaneum defensins peptide fragments and found that a peptide fragment of Defensin3 was effective against the human microbial pathogens, Escherichia coli, Staphylococcus aureus and Candida albicans, being S. aureus the most susceptible one.

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