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The never-ending story: Bt vs. Bt receptor
Evolution of resistance to entomopathogenic bacterium, Bacillus thuringiensis (Bt), can potentially reduce the efficacy of insecticidal proteins from Bt to insect pests in fields. Bt resistance is involved in modification of the toxin binding to its specific midgut membrane receptors, such as cadherin, aminopeptidase N, alkaline phosphatase, and ABC transporters. The beet armyworm, Spodoptera exigua, is one of major lepidopteran insect pest in Korea and showed highly susceptible to Cry1Ca. We investigated the Cry1Ca toxicity with respect to its binding affinity to a Bt receptor, cadherin compared with Cry1Ac. RNA interference (RNAi) of a cadherin of S. exigua (SeCad1) significantly suppressed the Cry1Ca to the toxic level of Cry1Ac. Binding affinity of Cry1Ca to brush border membrane vesicle (BBMV) of S. exigua midgut was significantly lost after SeCad1 RNAi. Binding affinity of Cry1Ac to BBMV was much low compared to that of Cry1Ca and less sensitive to SeCad1 RNAi. Direct binding assay of Cry toxins to SeCad1 was assessed using a recombinant cadherin repeat 10-11 (rCR10-11) of SeCad1. The addition to rCR10-11 to Cry1Ca significantly enhanced the toxicity under SeCad1 RNAi. However, the synergistic effect of rCR10-11 on toxicity of Cry1Ac was not much significant with poor binding affinity of Cry1Ac compared to Cry1Ca. These results indicate that the differential toxicity of Cry toxins against S. exigua is caused by the different affinities to the Bt receptor, cadherin.
