Toxicity and Characterization of Cotton Expressing Bacillus thuringiensis Cry1Ac and Cry2Ab2 Proteins for Control of Lepidopteran Pests

Abstract

Cry1Ac protoxin (the active insecticidal toxin in both Bollgard and Bollgard II cotton [Gossypium hirsutum L.]), and Cry2Ab2 toxin (the second insecticidal toxin in Bollgard II cotton) were bioassayed against five of the primary lepidopteran pests of cotton by using diet incorporation. Cry1Ac was the most toxic to Heliothis virescens (F.) and Pectinophora gossypiella (Saunders), demonstrated good activity against Helicoverpa zea (Boddie), and had negligible toxicity against Spodoptera exigua (Hübner) and Spodoptera frugiperda (J. E. Smith). Cry2Ab2 was the most toxic to P. gossypiella and least toxic to S. frugiperda. Cry2Ab2 was more toxic to S. exigua and S. frugiperda than Cry1Ac. Of the three insect species most sensitive to both Bacillus thuringiensis (Bt) proteins (including H. zea), P. gossypiella was only three-fold less sensitive to Cry2Ab2 than Cry1Ac, whereas H. virescens was 40-fold less sensitive to Cry2Ab2 compared with CrylAc. Cotton plants expressing Cry1Ac only and both Cry1Ac and Cry2Ab2 proteins were characterized for toxicity against H. zea and S.frugiperda larvae in the laboratory and H. zea larvae in an environmental chamber. In no-choice assays on excised squares from plants of different ages, second instar H. zea larvae were controlled by Cry1Ac/Cry2Ab2 cotton with mortality levels of 90% and greater at 5 d compared with 30-80% mortality for Cry1Ac-only cotton, depending on plant age. Similarly, feeding on leaf discs from Cry1Ac/Cry2Ab2 cotton resulted in mortality of second instars of S.frugiperda ranging from 69 to 93%, whereas exposure to Cry1Ac-only cotton yielded 20-69% mortality, depending on plant age. When cotton blooms were infested in situ in an environmental chamber with neonate H. zea larvae previously fed on synthetic diet for 0, 24, or 48 h, 7-d flower abortion levels for Cry1Ac-only cotton were 15, 41, and 63%, respectively, whereas for Cry1Ac/Cry2Ab2 cotton, flower abortion levels were 0, 0, and 5%, respectively. Cry1Ac and Cry2Ab2 concentrations were measured within various cotton tissues of Cry1Ac-only and Cry1Ac/Cry2Ab2 plants, respectively, by using enzyme-linked immunosorbent assay. Terminal leaves significantly expressed the highest, and large leaves, calyx, and bracts expressed significantly the lowest concentrations of Cry1Ac, respectively. Ovules expressed significantly the highest, and terminal leaves, large leaves, bracts, and calyx expressed significantly (P < 0.05) the lowest concentrations of Cry2Ab2. These results help explain the observed differences between Bollgard and Bollgard II mortality against the primary lepidopteran cotton pests, and they may lead to improved scouting and resistance management practices, and to more effective control of these pests with Bt transgenic crops in the future.

Asymmetrical cross-resistance betweenBacillus thuringiensistoxins Cry1Ac and Cry2Ab in pink bollworm

“…Third, pink bollworm larvae from the BX-R1 strain selected with Cry2Ab did not survive on bolls of Bt cotton producing Cry1Ac and Cry2Ab, despite their high levels of resistance to both toxins in diet bioassays and survival on Bt cotton bolls producing only Cry1Ac. This pattern probably reflects the 160-fold higher toxin concentration in bolls for Cry2Ab (792 g toxin/g) versus Cry1Ac (4.95 g toxin/g in bolls producing Cry1Ac only and bolls producing both toxins) (36). The concentration of Cry2Ab in bolls was 8 to 29 times higher than the LC 50 values of Cry2Ab for BX-R1 (Table 1).…”

“…However, because cotton producing both Cry1Ac and Cry2Ab was relatively rare from 2002 to 2004, the positive correlation between LC 50 values for Cry1Ac and Cry2Ab detected during this period probably resulted from crossresistance, rather than sequential resistance to the 2 toxins. Cross-resistance is more problematic in H. zea than in pink bollworm or H. virescens, because H. zea showed nonrecessive inheritance of resistance to Cry1Ac and Cry2Aa (43,46), relatively low inherent susceptibility to Cry1Ac and Cry2Ab (36), and survival in the field on cotton plants producing Cry1Ac and Cry2Ab (47).…”

Histopathology and the lethal effect of Cry proteins and strains of Bacillus thuringiensis Berliner in Spodoptera frugiperda J.E. Smith Caterpillars (Lepidoptera, Noctuidae)

“…In laboratory and field studies with eight corn hybrids evaluated for resistance to S. frugiperda and Diatraea grandiosella Dyar, 1911, Williams et al (1997 concluded that the expression of the Cry1Ab toxin produced characteristics of high resistance to the S. frugiperda and practically conferred immunity of the D. grandiosella. In tests utilising cotton expressing the Cry1Ac protein, Sivasupramaniam et al (2008) found mortality rates between 20-69% for caterpillars of the 2 nd instar of S. frugiperda.…”

“…Lima et al (2008), utilizing the Cry2Ab protein, found a CL 50 of 3.45 µg.mL -1 for larvae of the 2º ínstar of S. frugiperda. Sivasupramaniam et al (2008), testing transgenic cotton and expressing the Cry1Ac and Cry2Ab2 proteins, found that cotton plants expressing Cry1Ac isolation and those expressing both Cry1Ac and Cry2Ab2 were toxic for Heliothis virescens Fabicius, 1777, and S. frugiperda. The no-choice tests resulted in mortality levels of 20-69% in S. frugiperda.…”

Biological activity of Bt proteins expressed in different structures of transgenic corn against Spodoptera frugiperda

“…In this context, the long-term efficacy of YieldGard VT PRO™ and PowerCore™ corn technologies is partly dependent of the Cry2Ab2 protein, which does not exhibit crossresistance with Cry1 proteins (FERRÉ & VAN RIE, 2002;CACCIA et al, 2010). However, Cry2Ab2 protein showed low toxicity against S. frugiperda (SIVASUPRAMANIAM et al, 2008).…”