Tribolium castaneum Apolipophorin-III acts as an immune response protein against Bacillus thuringiensis Cry3Ba toxic activity

• Bt challenge induced ApoLp-III mRNA up-regulation in Tc larvae.
• ApoLp-III is involved in Tc defense against Bt.
• ApoLp-III activates the prophenoloxidase cascade in Tc immune response to Bt.

In this study, a 2.1-fold Apolipophorin-III mRNA up-regulation was found in Tribolium castaneum larvae challenged with Bacillus thuringiensis Cry3Ba spore–crystal mixture. Knockdown of Apolipophorin-III by RNAi resulted in increased T. castaneum larvae susceptibility following Cry3Ba spore–crystal treatment, demonstrating Apolipophorin-III involvement in insect defense against B. thuringiensis. We showed that Apolipophorin-III participates in T. castaneum immune response to B. thuringiensis activating the prophenoloxidase cascade since: (i) phenoloxidase activity significantly increased after Cry3Ba spore–crystal treatment compared to untreated or Cry1Ac spore–crystal treated larvae and (ii) phenoloxidase activity in Cry3Ba spore–crystal treated Apolipophorin-III silenced larvae was 71 ± 14% lower than that of non-silenced intoxicated larvae.

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