Dual mode of action of Bt proteins: protoxin efficacy against resistant insects

Abstract

Transgenic crops that produce Bacillus thuringiensis (Bt) proteins for pest control are grown extensively, but insect adaptation can reduce their effectiveness. Established mode of action models assert that Bt proteins Cry1Ab and Cry1Ac are produced as inactive protoxins that require conversion to a smaller activated form to exert toxicity. However, contrary to this widely accepted paradigm, we report evidence from seven resistant strains of three major crop pests showing that Cry1Ab and Cry1Ac protoxins were generally more potent than the corresponding activated toxins. Moreover, resistance was higher to activated toxins than protoxins in eight of nine cases evaluated in this study. These data and previously reported results support a new model in which protoxins and activated toxins kill insects via different pathways. Recognizing that protoxins can be more potent than activated toxins against resistant insects may help to enhance and sustain the efficacy of transgenic Bt crops.

Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea

“…We infer that the reduced activation of Cry1Ac reported here increases resistance to activated toxin, but not to protoxin. This interpretation is consistent with the idea that decreased protoxin activation is a relatively weak mechanism of resistance …”

“…Decreased protoxin activation has been associated with resistance to Cry1Ac or Cry1Ab in some strains of H. virescens, 25 P. interpunctella, 17 O. nubilalis [21][22][23] and Mythimno unipuncta. 26 However, previously reported results from two independent research teams imply that both activated toxin and protoxin can kill larvae of H. zea, 8,39 with activated toxin effective via the primary pathway and protoxin via a putative secondary pathway. 39 We infer that the reduced activation of Cry1Ac reported here increases resistance to activated toxin, but not to protoxin.…”

“…26 However, previously reported results from two independent research teams imply that both activated toxin and protoxin can kill larvae of H. zea, 8,39 with activated toxin effective via the primary pathway and protoxin via a putative secondary pathway. 39 We infer that the reduced activation of Cry1Ac reported here increases resistance to activated toxin, but not to protoxin. This interpretation is consistent with the idea that decreased protoxin activation is a relatively weak mechanism of resistance.…”

Resistance to Bt Crops; Influence, mechanisms and management strategies

“…Nowadays, pyramided plants have been broadly used to delay the development of insect-resistance Jin et al, 2015;Moar and Anilkumar, 2007;Roush, 1998;Tabashnik et al, 2015;Zhao et al, 2002Zhao et al, , 2003.…”

Development of a sensitive monoclonal antibody-based sandwich ELISA to detect Vip3Aa in genetically modified crops

“…These insecticidal proteins form inclusion bodies (such as Cry and Cyt proteins) or are secreted into the cultural medium (such as Vip and Sip proteins). Cry proteins are widely used in agricultural pest control (Ashouri 2004;Clive 2007;de Maagd et al 2003;Estruch et al 1997;Shelton 2012;Tabashnik et al 2015). However, some important pests, such as Agrotis ipsilon and Diabrotica spp., exhibit high tolerance to Cry proteins and are seriously harmful to crops (Chakroun et al 2016;Chattopadhyay and Banerjee 2018).…”