Mode of action analyses of Cry1 toxins from Bacillus thuringiensis in susceptible and resistant Heliothis virescens
Jurat-Fuentes, Juan Luis
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Development of resistance is one of the concerns in the use of Bacillus thuringiensis Cry proteins as insecticides. After ingestion, Cry toxins are processed in the insect midgut to activated toxins that bind to receptors in the midgut cells and form pores that cause cell death by osmotic lysis. Alterations in any of these steps can lead to resistance. In this dissertation, I expanded a toxin-binding model to study the mechanisms of resistance in three Cry1Ac laboratory-selected strains of Heliothis virescens (tobacco budworm). In toxin binding assays Cry1A, Cry1Fa and Cry1Ja toxins shared a population of binding sites (receptor A) in midgut brush border membrane vesicles (BBMV) from susceptible H. virescens. Two populations of receptors were only recognized by Cry1Ab and Cry1Ac (receptor B), or Cry1Ac (receptor C). According to ligand blotting, aminopeptidase-N proteins of 170- and 110-kDa constituted receptor A, a 130-kDa protein constituted receptor B, while different proteins of less than 100-kDa in size constituted receptor C. After continuous selection with Cry1Ac, the YHD2 strain showed increased resistance to Cry1Ac when compared to previous reports. Using toxin- binding experiments and ligand blots, I demonstrated that increased resistance correlated with reduced toxin binding that was not due to absence of toxin binding molecules in YHD2 BBMV. Lectin blots with soybean agglutinin (SBA) demonstrated that resistance correlated with altered glycosylation of YHD2 BBMV proteins. In contrast to the YHD2 strain, CXC and KCBhyb Cry1Ac-resistant strains developed cross-resistance to Cry2Aa. These strains were also cross-resistant to Cry1Aa and Cry1Ab toxins. Toxin binding competition experiments demonstrated that only binding of Cry1Aa was reduced in both strains. Altered Cry1Aa but not Cry1Ab or Cry1Ac toxin binding had been previously reported for the YHD2 strain, suggesting that a similar resistance mechanism is occurring. Since Cry1Ac and Cry2Aa did not share binding proteins, resistance to Cry2Aa in CXC and KCBhyb is due to a different mechanism. I conclude that at least two mechanisms of resistance are present in each of the resistant strains studied. In the case of the CXC and KCBhyb strains different resistance mechanism have evolved to confer resistance to very different Cry toxins.