Findings of a new study on multiple Bt toxins in biotech crops could improve the way crops are managed and promote development of varieties that are more effective at pest control, University of Arizona researchers say.
The study, published as an advance online publication by the journal Nature Biotechnology, reviews the potential of crops with multiple Bt toxins to delay evolution of pest resistance.
Yves Carrière, and co-author Bruce Tabashnik, both in the UA College of Agriculture and Life Sciences, analyzed data from 38 studies that report effects of 10 Bt toxins used in transgenic crops against 15 insect pests.
They found that in many cases, the crops' actual efficacy against pests did not live up to the expectations used to inform computer simulation models that aim to predict the evolution of pest resistance.
Thus, the simulations could underestimate how quickly pests adapt to Bt crops and lead to inadequate management guidelines, the researchers said.
How Bt 'pyramids' work
Tabashnik likened multiple Bt toxins in a plant to a lock and key. "The lock on the door is the receptor protein in the insect's gut, and the key is the Bt toxin that binds to that receptor. To be able to kill the insect, the toxin must fit the lock to open the door and get inside," he said.
But if you have only one "key" or toxin, and a mutation has changed the "lock" or receptor, insects will live on because the toxin can't get inside.
It can be a similar situation if a second toxin is added, and another mutation is generated.
"If you can't kill the insect one way, you can kill it another way. That's how pyramids work. It's like having two different keys, so the insect needs two different mutations to become resistant," Tabashnik said.
However, this scenario is an ideal situation that is often not achieved in the real world, according to the new study. At the other extreme, some Bt crop pyramids could have two toxins that bind to the same receptor.
"In that scenario, the keys are so similar that each only opens the front door, and if that lock is changed, you're out of luck," Tabashnik said.
The reality, the authors found in this study, is often somewhere in between. If each toxin is effective on its own and two toxins act independently, the pyramid should kill at least 99.75% of the Bt-susceptible pests, Carrière said.
Reviewing the studies, helping biotech advancement
In the scientific literature, Carrière and Tabashnik discovered that this assumption was met only in about half of the cases. They also found that, contrary to the ideal scenario typically assumed, selection for resistance to one toxin in a pyramid often causes cross-resistance to another toxin in the pyramid.
One goal of this study, Carrière explained, is to help biotech companies decide which toxins to put in their pyramided crops based on data that already exist, rather than by a time-consuming process of trial and error.
"Will two toxins behave as one key or two keys, or somewhere in between?" he asked. "And can we use understanding of how these toxins work to answer that question?"
Determining refuge plantings
To help find answers, study co-author Neil Crickmore of the University of Sussex, analyzed the similarity of toxins in each of their three component parts, called domains.
The new study shows that cross-resistance between toxins is associated with their amino acid sequence. Carrière said the research showed that toxins with different amino acid similarity could be combined to produce better pyramids.
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"With the available technology, it is now possible to swap domains and engineer each Bt toxin with the desired domain configuration," he said, which could help the designs of future pyramids.
"Our results mean that the keys — toxins — used in Bt crops by farmers worldwide are often not as different from each other as we would like," Tabashnik said.
He suspects that will have "huge implications" for determining the appropriate amount of refuge to be planted.
The data from the study could help modelers make more accurate predictions of how a certain pyramided Bt crop will perform, UA said.
"We provide a realistic assessment of which Bt toxins do meet the two-key assumption and work well together so farmers can use small refuges," Tabashnik said, "and which ones are closer to the one-key scenario, so larger refuges are needed or we'll have problems."