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Two of the toxins used in Bt crops with their middle domains, found to be important for the development of pest resistance, highlighted.

Over-estimating the pest-control abilities of genetically modified (GM) crops is leading to poor crop management, researchers at the universities of Sussex and Arizona have cautioned.

The scientists argue that computer-generated models used by biotech companies fail to give realistic predictions about how quickly insects adapt to crops that have been genetically engineered to produce multiple toxins that target pests.

Published today (Monday 19 January) in the journal Nature Biotechnology, the findings could improve crop-management practices and promote development of new crop varieties that are more effective and more durable.

Crops genetically engineered to produce proteins from the bacterium Bacillus thuringiensis (Bt) to control insect pests have been planted on a cumulative total of more than a billion acres worldwide since 1996. With some pests rapidly evolving resistance to Bt crops that make only one toxin, biotech companies introduced Bt crops called ‘pyramids’ that produce two or more Bt toxins active against the same pest. Such pyramids have been adopted in many countries since 2003, including the United States, India and Australia.

Dr Neil Crickmore, Senior Lecturer in Molecular Genetics at the University of Sussex - and his Arizona colleagues Professor Yves Carrière and Professor Bruce Tabashnik - analyzed data from 38 studies that looked at how well Bt toxins used in genetically modified crops performed against insect pests, and used these data to generate new quantitative measures of toxin effectiveness.

They found that, in many cases, the crops' actual effectiveness against pests did not live up to predictions.  The computer simulations underestimated how quickly pests adapt to Bt crops and led to inadequate management guidelines.

Dr Crickmore, an expert on Bt toxin structure and function, looked for structural features within the toxins that could be influencing how they interact with one another. A strong association was found between toxins that had independently lost effectiveness against a resistant pest and amino acid sequence similarity in the middle domain of the toxin.

“This was no great surprise” said Dr Crickmore. “There are a lot of data on the role of this domain in recognizing the receptor that the toxins target in the insects. Since the main cause of resistance is due to modification of this receptor, toxins which target the same receptor, through interactions with this middle domain, are all likely to be affected by such modifications.”

Unexpectedly, results from this new study also indicated that amino acid sequence similarity from another domain near the end of the toxin contributes to mortality of Bt-susceptible insects on pyramids. “While we have some ideas as to how this region of the toxin might influence toxicity within a pyramid,” Dr Crickmore explains, “further research is required to test these ideas”.

The data from this study could help modelers make more accurate predictions of how a certain pyramided Bt crop will perform and help policy makers determine pest-management strategies more realistically.

But understanding how the Bt toxins work and interact could have implications beyond agriculture, Dr Crickmore says: “The use of mathematical models to find correlations between structure and function from ever-increasing data sets has great potential. It’s certainly an approach that I hope to use for the main thrust of my lab at the moment – understanding how some Bt toxins have evolved to target human cancer cells.”

Notes for editors

University of Sussex press office contacts: James Hakner and Jacqui Bealing – press@sussex.ac.uk, 01273 678888.

The paper ‘Optimizing pyramided transgenic Bt crops for sustainable pest management’ is published in the journal Nature Biotechnology.

This study was supported by US Department of Agriculture (USDA) Biotechnology Risk Assessment Grant Award 2011-33522-30729.

Dr Neil Crickmore is a researcher in the Bt Lab within the School of Life Sciences at the University of Sussex. Research in the lab is based upon the bacterium Bacillus thuringiensis and its insecticidal toxins. The team is interested in discovering and developing novel biological insecticides and in studying the interaction between these and their insect targets. They also use this bacterium and its host as a model system for studying a variety of ecological, physiological, biochemical and genetic processes.

First photo, of corn field, by Seanpanderson (Own work) [CC BY-SA 3.0  http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Second photo by Neil Crickmore. Caption: ‘Two of the toxins used in Bt crops with their middle domains, found to be important for the development of pest resistance, highlighted.’

Further information: http://www.nature.com/nbt/journal/vaop/ncurrent/full/nbt.3099.html