We have been accidentally genetically engineering plants 鈥� and eating GMOs 鈥� for millennia.
That is the implication of a series of studies showing the ancient practice of grafting can allow even distantly related plants to swap all three kinds of genomes they possess.
鈥泪迟鈥檚 genetic engineering done by mother nature,鈥� says of the Max Planck Institute of Molecular Plant Physiology in Potsdam, Germany.
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Grafting involves transplanting part of one plant onto another so they fuse and continue to grow.
Farmers have been grafting plants for thousands of years to combine, say, a tree that bears delicious fruit with one that has disease-resistant roots. Grafting also occurs naturally, when branches press together.
Bock鈥檚 2009 study showed that cells on either side of a graft could exchange chloroplasts聽鈥� organelles that carry out photosynthesis and have their own small genome.
Then, in 2014, another study found that the entire nucleus of a cell, containing the main genome, . The transferred nucleus can be added to an existing cell nucleus 鈥� fusing the two genomes and potentially creating a new species.
Triple whammy
Now a team led by of Rutgers University in New Jersey has shown that cells also swap mitochondria 鈥� energy-generating organelles with a small genome of their own 鈥� across grafts.
And once entire mitochondria from one plant get into the cells of another, they mix their DNA with that of the existing mitochondria.
This means all three kinds of plant genome can be swapped via grafts.
There has been growing evidence from genome sequencing that plants sometimes exchange mitochondria, but this study is the first to show it actually happening.
To do so, Maliga鈥檚 team grafted one tobacco species onto another. One had a mitochondrial mutation that prevents the male parts of flowers developing normally.
Unintentional engineering
They then took slices from the sterile-male side of the grafts and grew whole plants from them. Some of these plants grew flowers with normal male parts, thanks to mitochondrial transfer between the two species.
Genome swapping only takes place close to the site of a graft, but new shoots often spring up in this region. These shoots can give rise to new plants with mixed genomes.
Because grafting has been widely used for millennia, it is highly likely that some of the plants we eat were created by this kind of unintentional genetic engineering by farmers, Maliga and Bock think.
Nobody has looked for evidence yet, says Maliga. 鈥淏ut I would be very surprised if people didn鈥檛 find any sign of this.鈥�
Bock points out that many crop plants have more than two sets of chromosomes. Such polyploidy, as it is called, is usually attributed to genome duplication, but some cases could be evidence of genome exchange in grafted plants.
Nature blurring the lines
The idea that we have been unintentionally modifying plants by grafting will not be welcome to those who like to claim that grafting is very different to genetic modification.
鈥淚t is quite shocking to people,鈥� Bock says. 鈥淚t blurs the boundaries between man-made and natural genetic engineering.
These聽findings taken together could聽provide plant breeders with聽new tools聽to create novel traits and crops.聽Bock is already trying to聽use grafting to聽create new species, such as a tomato-chilli mix.
While it is possible to genetically modify chloroplasts and the nucleus, there has been no way of altering mitochondria in plants so far.聽Now the聽latest result offers a way to transfer traits encoded by mitochondrial genes, such as male sterility, to plants that lack them.
Sterile-male plants make it much easier and cheaper to cross strains of the same species to produce the vigorous hybrids prized by farmers and gardeners. If plants aren鈥檛 sterile-male, they can fertilise themselves and fewer of the seeds they produce will be hybrids. 聽At present, the only way to prevent some plants self-fertilising is to remove the male parts of flowers by hand, which is very labour intensive.
Grafting is increasingly used for vegetable production, for instance to boost yields by using plant varieties with more vigorous roots. Some more unusual combinations are also becoming available, such as tomatoes grafted onto a potato root to create a crop that produces both.
Journal reference: PNAS, DOI:
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