US regulators said the wheat carries no risks but the outcome of the probe -- namely finding out whether such wheat is growing elsewhere -- could have a wider impact on world markets, experts said.
"No one wants genetically engineered wheat," said Bill Freese, science policy analyst at the Center for Food Safety, recalling that massive opposition in 2004 led seed giant Monsanto to pull back from its bid to commercialize it.
Market jitters ensued as Japan cancelled a bid for 25,000 tons of US wheat and the European Union told its member states to test imports from the area, saying any genetically modified wheat would not be sold to consumers.
Monsanto's share price was down 3.3 percent at USD 101.53 in midday trade after climbing for the past 10 days.
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The altered wheat is glyphosate resistant, which means it contains a transgene that allows it to survive when a popular weedkiller made by Monsanto, called Roundup, is sprayed on fields.
Monsanto said it was cooperating with the probe and vowed to "conduct a rigorous investigation to validate the scope of and to address any presence of a Monsanto Roundup Ready event in commercial wheat seed."
The same GE technology is already widely used in soybeans and maize in the United States and allows farmers to spray their fields with weedkillers without harming the main crop.