New Bt toxin developed to fight immunity in cotton pests5 min read . Updated: 01 Nov 2007, 11:30 PM IST
New Bt toxin developed to fight immunity in cotton pests
New Bt toxin developed to fight immunity in cotton pests
Bangalore: Even as concerns rise that the widespread use of Bt crops which are meant to be resistant to pests will lead to these very pests becoming immune to Bt, the active ingredient or toxin, a group of scientists claims to have created new versions of the toxin that prevent pests from becoming resistant to it.
Almost 40% of the genetically modified (GM) crops being developed in India use the Bt gene.
Most GM crops use this gene—a toxin-producing gene from a soil bacterium Bacillus thuringiensis—to become resistant to pests. A wide variety of crops have been infused with this gene including Bt cotton. However, the Bt gene is used extensively, and sometimes indiscriminately and researchers, particularly in India, fear pests will become resistant to the toxin.
The 2 November issue of popular science journal Science carries a report on the new versions.
A group of scientists, led by Mario Soberon at Instituto de Biotecnologia in Mexico, and Bruce E. Tabashnik from the University of Arizona in Tucson, Arizona, has shown that insects develop resistance to the toxin due to a mutation in a protein called cadherin found in the pest’s stomach.
To be effective, most varieties of Bt toxins used today need to bind to cadherin receptors. The mutation in cadherin deactivates the toxin; the scientists say a modification in the toxin, making it independent of cadherin, kills the pest.
Scientists call this an exciting breakthrough and a big step forward in pest management. “This is an absolute eye-opener," says Keshav Raj Kranthi, head of crop protection at the Central Institute for Cotton Research (CICR) in Nagpur. “This path-breaking finding shows us a very elegant way of tailor-making genes for fighting pest resistance," he adds, admitting that even though Indian researchers haven’t made any breakthrough in their fight against Indian pests, this finding would help “in reconstructing genes that can be more toxic" to some Indian bollworm species that are resistant to the Bt toxin.
The scientists who conducted the test were able to use the modified toxins to kill two species of pink bollworm. Experts say that if these results extend to other bollworms, then the modified toxin can be used to combat them.
Fear of immunity
There is a universal fear of pests becoming resistant to GM crops created to kill those very pests.
While there’ve been no reports of “field-evolved" resistance to Bt crops yet, scientists dread it’s a matter of time before this happens. “Diamondback moth (Plutella xylostella), has evolved resistance to Bt sprays in the field in many parts of the world. Cabbage looper (Trichoplusia ni) has evolved resistance to Bt sprays in greenhouses in western Canada," Tabashnik says.
The debate on the use of GM crops continues in India. The best-known GM crop in the country is Bt cotton and its use was prompted by pests becoming increasingly resistant to traditional insecticides—in some cases, a pest that could have once been killed with a single dose of insecticide needed 21,000 doses.
The use of Bt cotton in India has increased cotton yield from 308kg per ha in 2001 to 508kg per ha in 2006, according to the Cotton Corp. of India Ltd, a state-owned company that helps in the marketing of cotton.
The increased yield has transformed India into a net exporter of cotton from a net importer barely four years ago. However, India has allowed innumerable Bt varieties, both legal and illegal, to be grown.
Since Bt cotton was first approved for commercial cultivation in India in 2002, 162 varieties of Bt cotton have entered the market and 155 are in the pipeline. “There’s Bt gene everywhere and nothing in the universe can prevent resistance to this toxin," says Suman Sahai, a geneticist and convenor of Gene Campaign, a research and advocacy group. Gene Campaign and CICR claim they have already noticed anecdotal evidence of Bt resistance, though Sahai admits a systematic study has not been done.
Scientists are also worried that the right kind of Bt cotton is not being grown in India. Instead of true breeding varieties (where both the parents carry Bt gene) which are grown in other countries including the US, China, South Africa, and Australia, India grows hybrid Bt varieties where only one parent carries the Bt gene, rendering it more vulnerable to pest resistance.
Prevented from release
Newer Bt varieties, using diverse genetic sources, cannot be released because a Supreme Court ruling in response to a public interest litigation filed in 2003 has stayed release of new varieties.
“This is preventing new and diverse Cry (toxin) genes from being introduced...," says K.K. Narayanan, founder and managing director of MetaHelix Life Sciences Pvt. Ltd, an agri-biotech firm in Bangalore.
As for true breeding varieties, both Sahai and Kranthi say that since these will allow farmers to save their seeds for the next season, instead of buying them again, seed companies are not keen to introduce them in India.
India’s Genetic Engineering Approval Committee (GEAC), the government body that approves GM crops here, picked up several Bt varieties indiscriminately, but has not been able to enforce the mandatory pest-management regimen, an absolute must with this technology.
Narayanan says GEAC has even failed to evaluate the performance of Bt seeds. “There’s no assessment of what’s the impact of new varieties, what’s the level of resistance, allergenicity, toxicity, and so on," says Sahai.
The impact of this could be significant because more than 40% of genetically engineered crops under development in India use the Bt gene (cabbage, cauliflower, tomato, potato, brinjal, okra, rapeseed, mustard, etc., and even staple crops such as rice, maize and wheat) and if this toxin loses its potency, these crops could become vulnerable to pests.
Some Indian scientists, including Rakesh Tuli at the National Botanical Research Institute in Lucknow, are trying to develop modified toxin creating genes, but this is a time-consuming process.
The work of the scientist group profiled in Science will provide new insights, but this finding is limited to cadherin protein. Pest resistance due to cadherin-unrelated mechanisms would require a different approach.