Trends in Biotechnology

Trends in Biotechnology

Volume 26, Issue 10, October 2008, Pages 573-579
Journal home page for Trends in Biotechnology

Review
How to cope with insect resistance to Bt toxins?

https://doi.org/10.1016/j.tibtech.2008.06.005Get rights and content

Transgenic Bt crops producing insecticidal crystalline proteins from Bacillus thuringiensis, so-called Cry toxins, have proved useful in controlling insect pests. However, the future of Bt crops is threatened by the evolution of insect resistance. Understanding how Bt toxins work and how insects become resistant will provide the basis for taking measures to counter resistance. Here we review possible mechanisms of resistance and different strategies to cope with resistance, such as expression of several toxins with different modes of action in the same plant, modified Cry toxins active against resistant insects, and the potential use of Cyt toxins or a fragment of cadherin receptor. These approaches should provide the means to assure the successful use of Bt crops for an extended period of time.

Section snippets

Transgenic crops: an environmentally friendly alternative for insect control

Insect pests are one of the major problems in agriculture. Although chemical insecticides have been able to control these pests, their intensive use has created severe problems. Some chemical insecticides are recalcitrant and pollute the environment, and kill not only insect pests, but also beneficial insects and vertebrates, including people. Moreover, many insects have evolved resistance to chemicals, which has resulted in increased pesticide use. Since 1996, insect-protected transgenic

Is insect resistance a major threat for the long-term use of Bt crops?

Evolution of resistance is a genetically based decrease in a population's susceptibility to a toxin [5]. Insects are able to evolve resistance to Cry toxins 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 (Table 1). Resistance is evaluated with laboratory bioassays estimating the resistance ratio, which is the LC50 (concentration of toxin killing 50%) of a field-derived strain divided by the LC50 of the susceptible strain 5, 7. Because of concerns that insects would evolve

Mechanisms for insect resistance to Bt toxins

To kill insect larvae, 3D-Cry toxins undergo a multi-step process that results in insect midgut cells bursting. Two different hypotheses for the mode of action of these toxins have been proposed, one relies on pore formation and the other on signal transduction (Figure 2). The first steps in both models are similar: the toxin crystals are ingested by the larvae and solubilized in the gut into protoxins. These are cleaved by midgut proteases to give rise to a 60-kDa 3D-Cry toxin that includes a

Bt plants with novel Cry toxins

Besides the 3D-Cry toxins, several Bt strains produce other Cry-toxins that have no sequence similarity with 3D-Cry proteins, including the Bin-like-Cry, Mtx-like-Cry and Vip toxins (Figure 1) 3, 30. It has been proposed that these toxins have a different mode of action to that of the 3D-Cry toxins and hence might be able to control insects that are resistant to 3D-Cry. For example, the Bin-like Cry34/Cry35 binary toxins, which are toxic to the coleopteran Diabrotica virgifera, have been

Pyramiding or cry gene stacking

The concept of a ‘pyramid’ applies when two or more toxins with different modes of action, for example, they bind to different receptor molecules, are produced in the same plant. In this case, the possibility of generating resistant insects is diminished exponentially because multiple mutations would be required to lose susceptibility to both toxins [33]. In 2003, the first transgenic cotton plants expressing two 3D-Cry toxins, Cry1Ac and Cry2Ab, were tested: the dual-toxin was shown to be

Modified Cry toxins that bypass primary receptor interaction

As mentioned above, binding of Cry1A to cadherin facilitates the proteolytic removal of the helix α-1 of the toxin, thereby inducing toxin oligomerization and pore formation (Figure 2) [24]. In accordance with this observation, modified Cry1Ab and Cry1Ac toxins, which lacked helix α-1 (i.e. Cry1AbMod and Cry1AcMod) formed oligomers in the absence of cadherin [38]. Interestingly, these modified toxins killed M. sexta insects in which the cadherin protein was silenced by RNA interference (RNAi)

Bacillus thuringiensis subsp. israelensis (Bti): a natural and efficient way to counter resistance to Cry toxins

Bacillus thuringiensis subsp. israelensis (Bti) strain is active against mosquitoes and produces four 3D-Cry toxins (Cry4Aa, Cry4Ba, Cry10Aa and Cry11Aa) and one Cyt1Aa protein. Each of these toxins shows low toxicity by itself, but using them in combination greatly increases their effectiveness in killing mosquito larvae and Cyt1Aa has been shown to be responsible for the synergistic activity of the other Cry toxins in Bti 4, 39. Despite the widespread application of Bti for more than 30

Potential use of cadherin fragments to counteract insect resistance

Insect cadherins are modular proteins composed of three domains. The ectodomain contains the signaling peptide, 11 to 12 cadherin repeats (CR1 to CR12) and the membrane proximal ectodomain. The other domains are the transmembrane domain and the intracellular domain (Figure 3) [23]. Interestingly, a CR12-fragment expressed in E. coli that contained a Cry1A binding site enhanced Cry1Ab activity in different lepidopteran larvae [48]. It has been suggested that the CR12-fragment provided additional

Conclusions and perspectives

In this Review, we discussed the different strategies that have been used to cope with insect resistance to Bt-toxins. Insect resistance is expected to occur in the near future based on the observation that resistant populations have been selected for in laboratory settings, and because lepidopteran insects have been identified that have already become resistant to Bt-toxins in the field 7, 14, 21. The most frequently found mechanism of resistance involves mutations in toxin receptors, and

Acknowledgements

The research work of our group was supported in part by DGAPA/UNAM IN218608, IN210208-N, CONACyT 46829-Q, 46176-Q, U48631-Q, USDA 2007-35607-17780 and NIH 1R01 AI066014.

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