Seeds genetically identical to parent plant could revolutionize agriculture.
The production of exact genetic replicas of important food crops has come a step closer. By combining mutations that abolish the shuffling of genes during sexual reproduction, researchers have found a way to force sexually reproducing plants to clone themselves through seeds.
The method, published today in Science1, has so far only been tested in the model plant Arabidopsis thaliana (thale cress), but lead author Raphaël Mercier of the French National Institute for Agricultural Research in Versailles says his lab is working to extend the findings to crops. Such an advance could allow farmers to propagate their own crops, rather than buying seed each year. It would also speed up the time it takes for companies to generate new plant breeds.
Some in the field are already confident that the technique will find its way into agricultural use. "This is a new breeding tool," says Peter van Dijk, a plant geneticist at the plant breeding company KeyGene, in Wageningen, the Netherlands. "It opens up a whole new field."
Loss of vigour
Many of the hardiest, most productive crops are hybrids of two genetically disparate cultivars. But the beneficial combination of genes that makes the hybrids so robust disappears in the next generation because the genes are shuffled into new combinations during sexual reproduction.
Agricultural researchers have sought a way to clone prized hybrids in bulk, and have looked particularly at apomixis — asexual reproduction through the production of seeds. Some plants, including blackberries and dandelions, do this naturally, but most crops do not.
In 2009, Mercier and his colleagues took a step towards this goal when they reported2 that combining three specific genetic mutations in Arabidopsis abolished the normal genome shuffling that occurs during sexual reproduction (see 'Sexual gene shuffling suppressed in plants'). But the mutations came with a trade-off — offspring contained extra copies of the genome and the fertility of the plants plummeted over several successive generations.
Now Mercier's team reports that it has overcome this obstacle using a strain whose chromosomes are engineered to be eliminated after fertilization. This strain, first reported last year in Nature3, carries a modified version of a gene called CENH3, which encodes a protein located in the centromere — a region of the chromosome that is involved in cell division.
The team crossed a plant carrying this mutation with plants carrying one of two other genes, MiMe or dyad. Mutations in these two genes cause Arabidopsis to produce reproductive cells that are genetically identical to the parent. In up to a third of the progeny from these crosses, the chromosomes from the parent carrying the engineered CENH3 gene were eliminated, yielding a clone of the other parent.
The technique does not quite recapitulate apomixis because it still relies on crossing together two plants, says Mercier. One way to get around this requirement, he adds, is to genetically engineer plants that produce modified CENH3 protein in the male reproductive tissues and mutant dyad or MiMe proteins in female reproductive tissues, or vice versa.
Van Dijk predicts that the technique will find its way into commercial use. "In the end these genes will be cloned and we will be able to use them for plant breeding," he says. But seed companies will want a system that yields more than the 34% of clones generated in Mercier's study, he says. It is also unclear how well the technique will work in other species.
Nevertheless, there is a major incentive to make these improvements, because the technique could dramatically speed up plant breeding. At present, it often takes over a decade to generate a new agricultural cultivar, says van Dijk.
"For a long time people were excited about the potential of apomixis, but it still seemed to be so far away," says van Dijk. "Now you get findings like these and we're starting to realize that it's coming within reach."
Marimuthu, M. P. A. et al. Science 331, 876 (2011).
D’Erfurth, I. et al. PLoS Biol. 7, e1000124 (2009).
Ravi, M. & Chan, S. W. L. Nature 464, 615-618 (2010).
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Ledford, H. Genetic engineering brings cloned crops closer. Nature (2011). https://doi.org/10.1038/news.2011.102