Dual Resistance to Bacillus thuringiensis Cry1Ac and Cry2Aa Toxins in Heliothis virescens Suggests Multiple Mechanisms of Resistance

ABSTRACT One strategy for delaying evolution of resistance to Bacillus thuringiensis crystal (Cry) endotoxins is the production of multiple Cry toxins in each transgenic plant (gene stacking). This strategy relies upon the assumption that simultaneous evolution of resistance to toxins that have different modes of action will be difficult for insect pests. In B. thuringiensis-transgenic (Bt) cotton, production of both Cry1Ac and Cry2Ab has been proposed to delay resistance of Heliothis virescens (tobacco budworm). After previous laboratory selection with Cry1Ac, H. virescens strains CXC and KCBhyb developed high levels of cross-resistance not only to toxins similar to Cry1Ac but also to Cry2Aa. We studied the role of toxin binding alteration in resistance and cross-resistance with the CXC and KCBhyb strains. In toxin binding experiments, Cry1A and Cry2Aa toxins bound to brush border membrane vesicles from CXC, but binding of Cry1Aa was reduced for the KCBhyb strain compared to susceptible insects. Since Cry1Aa and Cry2Aa do not share binding proteins in H. virescens, our results suggest occurrence of at least two mechanisms of resistance in KCBhyb insects, one of them related to reduction of Cry1Aa toxin binding. Cry1Ac bound irreversibly to brush border membrane vesicles (BBMV) from YDK, CXC, and KCBhyb larvae, suggesting that Cry1Ac insertion was unaffected. These results highlight the genetic potential of H. virescens to become resistant to distinct Cry toxins simultaneously and may question the effectiveness of gene stacking in delaying evolution of resistance.

Download Full-text

Altered Glycosylation of 63- and 68-Kilodalton Microvillar Proteins in Heliothis virescens Correlates with Reduced Cry1 Toxin Binding, Decreased Pore Formation, and Increased Resistance to Bacillus thuringiensis Cry1 Toxins

Applied and Environmental Microbiology ◽

10.1128/aem.68.11.5711-5717.2002 ◽

2002 ◽

Vol 68 (11) ◽

pp. 5711-5717 ◽

Cited By ~ 56

Author(s):

Juan Luis Jurat-Fuentes ◽

Fred L. Gould ◽

Michael J. Adang

Keyword(s):

Light Scattering ◽

Bacillus Thuringiensis ◽

Brush Border ◽

Heliothis Virescens ◽

Pore Formation ◽

Brush Border Membrane ◽

Membrane Vesicles ◽

Binding Assays ◽

Toxin Binding ◽

Altered Glycosylation

ABSTRACT The binding and pore formation abilities of Cry1A and Cry1Fa Bacillus thuringiensis toxins were analyzed by using brush border membrane vesicles (BBMV) prepared from sensitive (YDK) and resistant (YHD2) strains of Heliothis virescens. 125I-labeled Cry1Aa, Cry1Ab, and Cry1Ac toxins did not bind to BBMV from the resistant YHD2 strain, while specific binding to sensitive YDK vesicles was observed. Binding assays revealed a reduction in Cry1Fa binding to BBMV from resistant larvae compared to Cry1Fa binding to BBMV from sensitive larvae. In agreement with this reduction in binding, neither Cry1A nor Cry1Fa toxin altered the permeability of membrane vesicles from resistant larvae, as measured by a light-scattering assay. Ligand blotting experiments performed with BBMV and 125I-Cry1Ac did not differentiate sensitive larvae from resistant larvae. Iodination of BBMV surface proteins suggested that putative toxin-binding proteins were exposed on the surface of the BBMV from resistant insects. BBMV protein blots probed with the N-acetylgalactosamine-specific lectin soybean agglutinin (SBA) revealed altered glycosylation of 63- and 68-kDa glycoproteins but not altered glycosylation of known Cry1 toxin-binding proteins in YHD2 BBMV. The F1 progeny of crosses between sensitive and resistant insects were similar to the sensitive strain when they were tested by toxin-binding assays, light-scattering assays, and lectin blotting with SBA. These results are evidence that a dramatic reduction in toxin binding is responsible for the increased resistance and cross-resistance to Cry1 toxins observed in the YHD2 strain of H. virescens and that this trait correlates with altered glycosylation of specific brush border membrane glycoproteins.

Download Full-text

Importance of Cry1 δ-Endotoxin Domain II Loops for Binding Specificity in Heliothis virescens(L.)

Applied and Environmental Microbiology ◽

10.1128/aem.67.1.323-329.2001 ◽

2001 ◽

Vol 67 (1) ◽

pp. 323-329 ◽

Cited By ~ 78

Author(s):

Juan Luis Jurat-Fuentes ◽

Michael J. Adang

Keyword(s):

Binding Sites ◽

Heliothis Virescens ◽

Brush Border Membrane ◽

Membrane Vesicle ◽

Membrane Vesicles ◽

Cross Resistance ◽

Binding Assays ◽

Binding Properties ◽

Toxin Binding ◽

Cry Toxin

ABSTRACT We constructed a model for Bacillus thuringiensis Cry1 toxin binding to midgut membrane vesicles from Heliothis virescens. Brush border membrane vesicle binding assays were performed with five Cry1 toxins that share homologies in domain II loops. Cry1Ab, Cry1Ac, Cry1Ja, and Cry1Fa competed with 125I-Cry1Aa, evidence that each toxin binds to the Cry1Aa binding site in H. virescens. Cry1Ac competed with high affinity (competition constant [K com] = 1.1 nM) for 125I-Cry1Ab binding sites. Cry1Aa, Cry1Fa, and Cry1Ja also competed for125I-Cry1Ab binding sites, though theK com values ranged from 179 to 304 nM. Cry1Ab competed for 125I-Cry1Ac binding sites (K com = 73.6 nM) with higher affinity than Cry1Aa, Cry1Fa, or Cry1Ja. Neither Cry1Ea nor Cry2Aa competed with any of the 125I-Cry1A toxins. Ligand blots prepared from membrane vesicles were probed with Cry1 toxins to expand the model of Cry1 receptors in H. virescens. Three Cry1A toxins, Cry1Fa, and Cry1Ja recognized 170- and 110-kDa proteins that are probably aminopeptidases. Cry1Ab and Cry1Ac, and to some extent Cry1Fa, also recognized a 130-kDa molecule. Our vesicle binding and ligand blotting results support a determinant role for domain II loops in Cry toxin specificity for H. virescens. The shared binding properties for these Cry1 toxins correlate with observed cross-resistance in H. virescens.

Download Full-text