Aphids reproduce better on faster-growing plants
The idea that breeding plants for faster growth and higher yield will lead to a generation of crops vulnerable to pests and disease has been supported by new research.
By using 'knockout' mutations, researchers have disabled one gene at a time and found that genes for high yield and fast growth are closely linked to defence against pests. Plants are able to put more resources into growth by shutting down some defence genes.
For decades farmers have selectively bred crops for their yield, but Tobias Züst, lead author and a researcher at the University of Zurich, in Switzerland, said the new research shows that "you cannot just select for fast growth and high yield if you don't want your plants to be completely defenceless".
Züst's team grew the model plant Arabidopsis thaliana with the genes for trichomes (leaf hairs that discourage herbivores) and glucosinolate (a chemical toxic to certain pests) disabled. They introduced aphids to show the effect of removing the plant's defences and measured the growth of the plants and the aphids' rate of reproduction.
Most of the mutants had significantly higher growth rates than normal plants in early life, but the aphids reproduced faster on these plants compared with those on slow-growing plants with intact defences.
Züst believes that breeding for higher yields has led to greater susceptibility to pests and pathogens, resulting in increased pesticide use worldwide.
Zeyaur Khan, an entomologist at the International Centre of Insect Physiology and Ecology in Nairobi said that since the Green Revolution, "breeders bred plants for high yield and did not pay much attention to insect or disease resistance".
Farmers had used pesticides to defeat the pests and diseases that attacked their crops because lower oil prices made them cheaper, and farmers were not aware of the environmental costs, said Khan. The problem is more acute in the developing world where the large-scale use of pesticides is too expensive, he added.
But Eric Danquah, director of the West Africa Centre for Crop Improvement in Ghana, said that researchers at the centre "have in the past successfully bred early maturing and disease-resistant varieties which are [also] high yielding".
"A generalisation that faster growth will always compromise plant defence needs to be treated with caution," he warned.
And Mark Laing, director of the African Centre for Crop Improvement in South Africa, does not think farmers should move away from high-yield crops completely.
"Farmers mix their risk by planting a mixture of high 'yield potential' varieties with high 'yield stability' varieties," he said.
In a good year, with fewer pathogens and pests, the plants with high yield potential produce a bumper crop and those with high yield stability provide a stable, moderate crop, he explains. But in a bad year, where plants with high yield potential produce very little, farmers can rely on the more resistant plants to provide a stable, moderate crop.
Proceedings of the Royal Society B doi:10.1098/rspb.2010.2475 (2011)
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