Cry1C Toxins Research Articles

Determination of Binding of Bacillus thuringiensis (delta)-Endotoxin Receptors to Rice Stem Borer Midguts

Insecticidal activity and receptor binding properties of Bacillus thuringiensis toxins to yellow and striped rice stem borers (Sciropophaga incertulas and Chilo suppresalis, respectively) were investigated. Yellow stem borer (YSB) was susceptible to Cry1Aa, Cry1Ac, Cry2A, and Cry1C toxins with similar toxicities. To striped stem borer (SSB), Cry1Ac, Cry2A, and Cry1C were more toxic than Cry1Aa toxin. Binding assays were performed with (sup125)I-labeled toxins (Cry1Aa, Cry1Ac, Cry2A, and Cry1C) and brush border membrane vesicles (BBMV) prepared from YSB and SSB midguts. Both Cry1Aa and Cry1Ac toxins showed saturable, high-affinity binding to YSB BBMV. Cry2A and Cry1C toxins bound to YSB BBMV with relatively low binding affinity but with high binding site concentration. To SSB, both Cry1Aa and Cry1Ac exhibited high binding affinity, although these toxins are less toxic than Cry1C and Cry2A. Cry1C and Cry2A toxins bound to SSB BBMV with relatively low binding affinity but with high binding site concentration. Heterologous competition binding assays were performed to investigate the binding site cross-reactivity. The results showed that Cry1Aa and Cry1Ac recognize the same binding site, which is different from the Cry2A or Cry1C binding site in YSB and SSB. These data suggest that development of multitoxin systems in transgenic rice with toxin combinations which recognize different binding sites may be useful in implementing deployment strategies that decrease the rate of pest adaptation to B. thuringiensis toxin-expressing rice varieties.

Contribution of Bacillus thuringiensis Spores to Toxicity of Purified Cry Proteins Towards Indianmeal Moth Larvae

The influence of Bacillus thuringiensis subsp. kurstaki HD-1 spores upon the toxicity of purified Cry1Ab and Cry1C crystal proteins toward susceptible and BT-resistant Indianmeal moth (IMM, Plodia interpunctella) larvae was investigated. With susceptible larvae, HD-1 spores were toxic in the absence of crystal protein and highly synergistic (approximately 35- to 50-fold) with either Cry1Ab or Cry1C protein. With BT-resistant IMM larvae, HD-1 spores were synergistic with Cry1Ab and Cry1C protein in all three resistant strains examined. Synergism was highest (approximately 25- to 44-fold) in insects with primary resistance toward Cry1C (IMM larvae with resistance to B. thuringiensis subsp. aizawai or entomocidus). However, HD-1 spores also synergized either Cry1Ab or Cry1C toxicity toward larvae resistant to B. thuringiensis subsp. kurstaki at a lower level (approximately five- to sixfold). With susceptible larvae, the presence of spores reduced the time of death when combined with each of the purified Cry proteins. Without spores, the speed of intoxication and eventual death for larvae treated with Cry1C and Cry1Ab proteins was much slower than for the HD-1 preparation containing both spores and crystals together. Neither spores nor toxin dose affected the mean time of death of resistant larvae treated with either Cry1Ab or Cry1C toxins. Both Cry1Ab and Cry1C toxins appeared to reduce feeding and consequently toxin consumption.