CROPS can now be genetically engineered to resist herbicides without the
danger of creating superweeds. Farmers want crops that are engineered to resist
herbicides so they can use them liberally to kill weeds. But resistance to the
weedkiller could be spread by pollen.
Now researchers have engineered resistance to glyphosate, a common
weedkiller, in a tobacco plant so that the resistance cannot be spread by pollen
to other plants (Nature Biotechnology, vol 16, p 347). The trick was to
insert the resistance gene into the plants鈥� chloroplasts. The DNA from
chloroplasts is only spread through female plants鈥攑ollen comes from
males.
Glyphosate is a herbicide that kills plants by blocking the creation of an
enzyme called EPSP synthase, which the plant needs to synthesise amino acids.
But crops and weeds are killed by glyphosate. Over the past few years, Monsanto
has marketed soya beans and cotton which have been engineered to resist
glyphosate.
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But environmentalists are concerned that the resistance could spread
to wild relatives of some crops, creating superweeds that would not be easy to
control. The concern is greatest for cereal crops, which can interbreed with
some grassy weeds.
Henry Daniell and his colleagues at Auburn University in Auburn, Alabama,
tackled the problem by inserting the resistance gene, taken from a petunia, into
the plant鈥檚 choloroplasts. Chloroplasts are the structures which manufacture
chlorophyll. They have their own genome, and the chloroplasts are passed down
only through the female plant so that pollen cannot spread the resistance.
Daniell constructed a resistance gene which would bind only to a piece of DNA
in the chloroplasts, and used a gene gun to insert it. The altered chloroplasts
manufactured large amounts of EPSP synthase, overwhelming the glyphosate鈥檚
effect.
If the technique can be applied to crops, it removes worries about crops
interbreeding with weeds, and makes it easier for seed companies to win approval
for the products. Ingo Potrykus, a plant developmental biologist at the
Institute of Plant Sciences in Zurich, has his doubts. He says Daniell鈥檚
technique will be much harder to implement in important cereal crops than in
tobacco. 鈥淓very big company would like to have this for cereal. Nobody has so
far been able to achieve it,鈥� he says.
But environmentalists also have reservations. 鈥淭his is a technology that is
being used to perpetuate a dependence on herbicides. It is not being used to
help farmers get away from herbicides,鈥� says Jane Rissler, a scientist with
the Union of Concerned 杏吧原创s in Washington, DC.