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.