| As the growing area of Bt transgenic cotton has been increasing rapidly, the resistance problem of cotton bollworm Helicoverpa armigera (Hübner) to Bt has become the key factor in continuable using of Bt cotton. Vip3A protein, secreted by Bacillus spp. during the vegetative stage of growth, represents a new family of insecticidal proteins. One of the interesting features of the Vip3A protein is that it shares no sequence homology with the known endotoxins, and it has different mode of action compared with other Bt insecticide crystal proteins. Vip3A protein not only provides a new control measure for caterpillar pests but also has been considered as a major protein in"New toxin"strategy for resistance management. One of requirements for this strategy to work is that the Vips must have low probability of cross-resistance with Cry1Ac. Here, in order to determine whether there have cross-resistance and clarify the potential mechanisms, by using several near-isogenic lines of Bt resistant strain of H. armigera, the cross-resistance of Bt-resistant H. armigera to Vip3Aa were tested, the binding kinetics and kinetics of mid-gut proteases were compared among different strains. The results were as follows:1. The bioassay results showed that the purified Vip3Aa protoxin had effective activity against H. armigera in all strains. There had no significant differences among strains in LC50s, but the resistant strains representd more tolerant to Vip3Aa toxin.2. By analysing the properties of gut proteases in H. armigera midgut, the process of activation in diffetent strains were compared. Vip3Aa full-length protoxin was processed to a 62 kDa toxin core with the gut juice extracts which collected from either susceptible or resistant strains. However, the three strains showed small differences in the process of activation, the gut juice extracted from susceptible insects has more effective and faster toxin activation than extracts from resistant strains.3 The activities of mid-gut protease in the susceptible and resistant strains after they ingested Vip3Aa toxin in different times were determined. The activities of total mid-gut protease had no significant changes in spite of the larvae feeding artifical diet containing Vip3Aa or not. The activities of active trypsin-like enzyme and chymotrypsin-like enzyme in different strains increased after the larvae feeding on toxin-diet 12 hours, but the activities of protease lowed down to the original level along with the treatment time.4 Binding abilities between BBMV and toxins were conducted using SPR technology (surface plasmon resonance). The results indicated that all BBMVs from susceptible and resistant strains could binded and had affinity with activated Vip3Aa toxin. The 96S susceptible strain had highest response units (186±16 RU), had more affinity, whereas the response units of BtR and BtI were 162±12 RU and 149±9 RU respectively. The competition binging analysis of Vip3Aa and Cry1Ac in BBMV from Cry1Ac-susceptible and–resistance strains showed that there is no competition binging between Vip3Aa and Cry1Ac to different strains. In a conclusion, althogh the tolerances of Bt-resistant strains of H. armigera appreciably incresed, but they have no cross-resistance to Vip3Aa, Vip3Aa and Cry1Ac had different binding sites. The results suggested Vip3Aa could be used as a novel insecticidal protoxin in resistance management strategy.
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