Farmers in dozens of countries have embraced crops genetically engineered to produce proteins from Bacillus thuringiensis (Bt) bacteria that kill some key pests yet are safe for people and wildlife. Although this biotech approach reduces reliance on insecticide sprays thereby providing economic and environmental benefits, resistance to Bt crops has evolved in at least 11 species of pests. Thus, effective ways to combat such pest resistance are urgently needed.
A new study published in the Proceedings of the National Academy of Sciences identifies a natural strategy for thwarting pest resistance to Bt proteins. The researchers at the University of Arizona and Nanjing Agricultural University discovered that a Bt protein kills one of the world's most damaging crop pests via two different pathways. "So, the protein's efficacy is more durable because even if the pest blocks one pathway, the other pathway remains lethal and the pest is not resistant unless both pathways are disarmed," said Bruce Tabashnik, one of the study's authors and head of the Department of Entomology at the University of Arizona.
Insights from disabling Bt receptors in the Asian corn borer
To kill insect pests, Bt proteins must be ingested and bind to specific receptors in the lining of the gut. Because humans and other animals lack such receptors, they are not harmed by Bt proteins. But as with disease-causing germs and antibiotics, pests can evolve resistance to Bt proteins. The most common and most potent mechanism of Bt resistance entails changes in the receptors that reduce or eliminate their binding of Bt proteins. Three of the receptors implicated in many cases of Bt resistance are gut proteins called ABCC2, ABCC3, and cadherin.
The team of scientists used gene editing to disable ABCC2, ABCC3, and cadherin in caterpillars of the Asian corn borer (Ostrinia furnacalis), the major pest of corn in China and elsewhere in Asia. They determined how disabling the three receptors singly and in pairs affects the pest's responses to Bt proteins Cry1Ab and Cry1Fa, which are used widely in Bt corn that targets corn borers and other lepidopteran pests.
The researchers discovered that Cry1Ab kills the caterpillars via two different toxic pathways. One pathway requires ABCC2, while the other requires cadherin and ABCC3. This means that if a mutation in the pest blocks one pathway, the other pathway can still deliver a lethal blow. Only when both pathways are knocked out does the pest become resistant. This "backup system" for Cry1Ab makes it much harder for resistance to evolve, because the pest needs mutations simultaneously inactivating two separate pathways to survive.
Cry1Fa, on the other hand, uses only one pathway, the one with ABCC2. If that's blocked, the pest survives exposure to Cry1Fa. Thus, a single mutation in the pest disrupting ABCC2 can make it highly resistant to Cry1Fa.
To check the predictions from the results summarized above, the scientists did the reverse experiment by modifying a cell line from another lepidopteran pest (the fall armyworm) to produce the receptors from the Asian corn borer. The results from the modified cells support the conclusions from the caterpillars with disabled receptors. For example, while the unmodified cells were not killed by Cry1Ab or Cry1Fa, cells modified to produce ABCC2 were killed by both Bt proteins, confirming the conclusion that ABCC2 facilitates a toxic pathway for both. Also, cells modified to produce cadherin and ABCC3 were susceptible to Cry1Ab but not Cry1Fa. As expected, this modification provided the second pathway for Cry1Ab, which does not exist for Cry1Fa.
Potential solution of a mystery about a major pest in North America and Europe
The new results with the Asian corn borer could elucidate a previously unexplained pattern observed in its close relative that is a major pest in North America and Europe, the European corn borer (Ostrinia nubilalis). In the lab and field, the European corn borer has evolved resistance slower to Cry1Ab than Cry1Fa. For example, in Canada, practical resistance reducing the efficacy of Bt corn against this pest in the field was not evident after 21 years of exposure to Bt corn producing Cry1Ab whereas practical resistance was first documented after just 12 years of exposure to Bt corn producing Cry1Fa. One plausible explanation is that, like the Asian corn borer, the European corn borer has two toxic pathways for Cry1Ab but only one for Cry1Fa. This idea could be tested directly by conducting the same type of experiments with the European corn borer that were used to analyze the Asian corn borer.
Implications for enhancing sustainability
Tabashnik noted, "Functional redundancy, the use of more than one toxic pathway by a single Bt protein, is not limited to Cry1Ab and the Asian corn borer – it also occurs with other Bt proteins and other major lepidopteran pests. This natural strategy for delaying pest resistance could be harnessed to enhance sustainability by seeking native Bt proteins or designing novel Bt proteins that attack pests via multiple pathways."
