Transcriptomic Analysis of the Response of the Dioryctria abietella Larva Midgut to Bacillus thuringiensis 2913 Infection

<i>Dioryctria abietella</i> Denis Schiffermuller (Lepidoptera: Pyralidae) is an oligophagous pest that mainly damages Pinaceae plants. Here, we investigated the effects of the <i>Bacillus thuringiensis</i> 2913 strain (<i>Bt</i> 2913), which carries the <i>Cry</i>1Ac, <i>Cry</i>2Ab, and <i>Vip</i>3Aa genes, on the <i>D. abietella</i> midgut transcriptome at 6, 12, and 24 h after infection. In total, 7497 differentially expressed genes (DEGs) were identified from the midgut transcriptome of <i>D. abietella</i> larvae infected with <i>Bt</i> 2913. Among these DEGs, we identified genes possibly involved in <i>Bt</i> 2913-induced perforation of the larval midgut. For example, the DEGs included 67 genes encoding midgut proteases involved in <i>Cry</i>/<i>Vip</i> toxin activation, 74 genes encoding potential receptor proteins that bind to insecticidal proteins, and 19 genes encoding receptor NADH dehydrogenases that may bind to <i>Cry</i>1Ac. Among the three transcriptomes, 88 genes related to metabolic detoxification and 98 genes related to immune defense against <i>Bt</i> 2913 infection were identified. Interestingly, 145 genes related to the 60S ribosomal protein were among the DEGs identified in the three transcriptomes. Furthermore, we performed bioinformatic analysis of zonadhesin, GST, CYP450, and CarE in the <i>D. abietella</i> midgut to determine their possible associations with <i>Bt</i> 2913. On the basis of the results of this analysis, we speculated that trypsin and other serine proteases in the <i>D. abietella</i> larval midgut began to activate <i>Cry</i>/<i>Vip</i> prototoxin at 6 h to 12 h after <i>Bt</i> 2913 ingestion. At 12 h after <i>Bt</i> 2913 ingestion, chymotrypsin was potentially involved in degrading the active core fragment of <i>Vip</i>3Aa toxin, and the detoxification enzymes in the larvae contributed to the metabolic detoxification of the <i>Bt</i> toxin. The ABC transporter and several other receptor-protein-related genes were also downregulated to increase resistance to <i>Bt</i> 2913. However, the upregulation of 60S ribosomal protein and heat shock protein expression weakened the resistance of larvae to <i>Bt</i> 2913, thereby enhancing the expression of NADH dehydrogenase and other receptor proteins that are highly expressed in the larval midgut and bind to activating toxins, including <i>Cry</i>1Ac. At 24 h after <i>Bt</i> 2913 ingestion, many activated toxins were bound to receptor proteins such as APN in the larval midgut, resulting in membrane perforation. Here, we clarified the mechanism of <i>Bt</i> 2913 infection in <i>D. abietella</i> larvae, as well as the larval immune defense response to <i>Bt</i> 2913, which provides a theoretical basis for the subsequent control of <i>D. abietella</i> using <i>B. thuringiensis</i>.