The Bacillus thuringiensis vegetative insecticidal protein, Vip3A, represents a new family of Bt toxin and is currently applied to commercial transgenic cotton. To determine whether the Cry1Ac-resistant Helicoverpa armigera is cross-resistant to Vip3Aa protein, insecticidal activities, proteolytic activations and binding properties of Vip3Aa toxin were investigated using Cry1Ac-susceptible (96S) and Cry1Ac-resistant H. armigera strain (Cry1Ac-R). The toxicity of Vip3Aa in Cry1Ac-R slightly reduced compared with 96S, the resistance ratio was only 1.7-fold. The digestion rate of full-length Vip3Aa by gut juice extracts from 96S was little faster than that from Cry1Ac-R. Surface plasmon resonance (SPR) showed there was no significant difference between the binding affinity of Vip3Aa and BBMVs between 96S and Cry1Ac-R strains, and there was no significant competitive binding between Vip3Aa and Cry1Ac in susceptible or resistant strains. So there had little cross-resistance between Vip3Aa and Cry1Ac,Vip3A+Cry proteins maybe the suitable pyramid strategy to control H. armigera in China in the future.

Cry toxins produced by Bacillus thuringiensis (Bt) are effective biological insecticides against certain insect species. However, there are potential risks of the evolved resistance of insects to Cry toxin owing to decreased binding of toxins to target sites in the brush border membranes of the larva midgut. The Cry toxins with different binding sites in the larval midgut have been considered to be a good combination to deploy in delaying resistance evolution. Bioassay results demonstrated that the toxicity of different Cry toxins ranked differently for each species. The toxicity ranking was Cry1Ac>Cry1Ab>Cry2Ab for Helicoverpa armigera, Cry1B>Cry1C>Cry2Ab for Spodoptera exigua, and Cry2Ab>Cry1B> Cry1C for S. litura. Only Cry2Ab was toxic to Agrotis ipsilon. Binding experiments were performed with 125I-Cry1Ab, 125ICry1Ac, 125I-Cry1B, 125I-Cry1C, 125I-Cry2Ab and the brush border membranes vesicles (BBMV) from H. armigera, S. exigua, S. litura and A. ipsilon. The binding of Cry1Ab and Cry1Ac was shown to be saturable by incubating with increasing concentrations of H. armigera BBMV (Kd=(45.00±2.01) nmol L-1 and (12.80±0.18) nmol L-1, respectively; Bmax=(54.95±1.79) ng and (55.44±0.91) ng, separately). The binding of Cry1B was shown to be saturable by incubating with increasing concentrations of S. exigua BBMV (Kd=(23.26±1.66) nmol L-1; Bmax=(65.37±1.87) ng). The binding of 125ICry toxins was shown to be non-saturable by incubating with increasing concentrations of S. litura and A. ipsilon BBMV. In contrast, Cry1B and Cry1C showed some combination with the BBMV of S. litura, and a certain amount of Cry2Ab could bind to the BBMV of A. ipsilon. These observations suggest that a future strategy could be devised for the focused combination of specific cry genes in transgenic crops to control target pests, widen the spectrum of insecticide effectiveness and postpone insect resistance evolution.

Cry toxins produced by Bacillus thuringiensis (Bt) are effective biological insecticides against certain insect species. In this study, bioassay results indicated that Cry1B and Cry1C were toxic to Spodoptera exigua. We also identified a cadherin-like gene in S. exigua that could enhance the toxicity of Cry1B and Cry1C. The cadherin-like gene identified from the larvae midgut tissue was cloned by reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The full-length cDNA of the gene consisted of 5 220 bp encoding 1 740 amino acid with a predicted molecular mass of 196 kD. BLAST search analysis showed that the predicted amino acid sequence had a high sequence identity to the published sequences of cadherin-like proteins from other Lepidoptera insects. Spatial expression of the cadherin-like gene detected by qRT-PCR analysis revealed that the cadherin-like gene was mainly present in the gut of 4th instar larvae and during different life stages. The results suggested that the commercial development of this synergist has the potential to enhance Cry1B and Cry1C toxicity against Lepidoptera insects.