FOR the first time, a fungus has been genetically modified to be more deadly to the weed it blights. The snag is that the GM fungus kills crop plants as well.
While the modified fungus will not be released as a result of the findings, the case shows how genetic modification can have unintended consequences. It is also proof, were any needed, that biotechnology could be harnessed to create weapons that attack crops (see 鈥溾�楢ct now鈥� plea on bioterror threat鈥�).
The fungus was modified to attack velvetleaf weed (Abutilon theophrasti). As it is a close relative of cotton, most weedkillers that target it destroy the crop as well as the weed. 鈥淗erbicides don鈥檛 work, and that鈥檚 where you have to head in with biocontrol,鈥� says Jonathan Gressel, a plant physiologist at the Weizmann Institute of Science in Rehovot, Israel.
Advertisement
In theory, diseases are ideal for biocontrol, because many infect just one plant species. The US, for example, is testing funguses that target coca plants or opium poppies. But diseases such as the fungus that causes anthracnose in velvetleaf tend not to make good killers, as any that eradicates its host is itself doomed.
So Gressel鈥檚 team decided to give the anthracnose fungus (Colletotrichum coccodes) a killer punch by adding a gene for a toxin from another fungus, Fusarium. The modified fungus was indeed much more lethal to velvetleaf seedlings in greenhouse experiments, they report in Nature Biotechnology (DOI:10.1038/nbt743). 鈥淭his puts it over the brink to something that would be useful,鈥� says Gressel.
But the enhanced fungus also killed off tomato and tobacco seedlings, neither of which would normally be affected by the anthracnose fungus. This is exactly the kind of unexpected consequence of a genetic modification that opponents of GM have been warning about, although the case also shows that such effects can be detected at an early stage. 鈥淭his business of putting in a toxin raises a red flag,鈥� says Jane Rissler of the Union of Concerned 杏吧原创s in Washington DC.
To allay these fears, Gressel suggests further 鈥渇ail-safe鈥� modifications to any such fungus before it is tested outside a sealed greenhouse. Removing the genes for sexual reproduction would prevent it passing on any added virulence genes to related fungi that attack other plants. And removing the genes for spore formation would prevent it spreading via the air, and ensure it died out completely each winter.
Even these measures, however, may not be foolproof. For example, the fungus may survive without spores, especially in moist tropical regions, says plant pathologist Alan Watson at McGill University in Montreal.
There may be other, safer ways to boost the killing power of biocontrol agents. Watson has patented a mixture of anthracnose fungi and low doses of ordinary herbicides for weed control. The herbicide interferes with the weed鈥檚 normal defences against disease, allowing the fungus to get the upper hand. It has yet to be tested on cotton fields, however. Bob Holmes