ABOUT one in 20 flowering plants are naturally transgenic, carrying bacterial DNA within their genomes. The added genes can make them produce unusual chemicals, and the species they have been found in include tea, bananas and peanuts.
Other plants that carry bacterial genes include sweet potatoes, yams, American cranberries, Surinam cherries and the hops used to flavour beer. What effect the added genes have on the plants that contain them is still far聽from clear. 鈥淲e are only at the start of this,鈥� says L茅on Otten at the Institute of Molecular Biology of Plants in Strasbourg, France.
Advertisement
The culprit is a microbe called Agrobacterium that infects plants. When this bacterium gets inside a聽plant cell, it inserts a 鈥渃assette鈥� of聽DNA containing hundreds of genes into the genome of the cell. These genes include ones that encode hormones that make plants grow tumour-like lumps called crown galls and enzymes that make chemicals the bacteria feed on.
Agrobacterium is the main tool used to create the genetically engineered crops grown globally. Biologists swap out the microbe鈥檚 cassette of genes for whatever DNA they want the bacterium to splice in for them. 鈥淎grobacterium is nature鈥檚 own genetic engineer,鈥� Mary-Dell Chilton, once wrote. In 1980, she was the first to use it to modify plants.
In the wild, though, it was thought that the genes added by聽Agrobacterium hardly ever got passed on to the next generation. For this to happen, an infected cell聽has to grow into an entire new聽plant, says Otten. That plant then聽has to flower and produce offspring, and those offspring have to thrive despite harbouring alien genes meant to hijack them.
Until now, the only known examples of Agrobacterium DNA persisting in a plant genome were in tobacco and the sweet potato. Otten and Tatiana Matveeva of St聽Petersburg State University in聽Russia have now found dozens more by analysing the genomes of聽hundreds of plants.
Their results suggest that about 5 per cent of the hundreds of thousands of species of flowering plants carry Agrobacterium DNA. 鈥淭hey did a good job,鈥� says Jan Kreuze at the International Potato Center in Lima, Peru, who found in聽2015 that sweet potatoes are transgenic. 鈥淚 think it鈥檚 true.鈥�
This has only just been discovered because no one had looked before, says Otten. Of the Agrobacterium genes identified by聽Otten and Matveeva, most contained mutations that should聽disable them, but some are聽still likely to be active.
Plants that are transgenic in this聽way don鈥檛 count as genetically modified under European Union regulations, which specifically exclude organisms modified by聽鈥渘atural鈥� processes.
The discovery is good news聽for聽Henrik L眉tken at the University of聽Copenhagen in Denmark, who plans to test the聽limits of this definition.
He is creating new plant varieties using natural strains of Agrobacterium. For instance, he has created a compact variety of a聽house plant called Kalanchoe blossfeldiana, which is now ready for commercial sale. He聽thinks these plants shouldn鈥檛 count as聽GM and the latest findings will聽bolster his case.
Because the genes inserted into聽plants by Agrobacterium can produce big changes, Otten thinks this process could drive the evolution of new plant species. His聽research suggests that tobacco plants have been modified by Agrobacterium several times in聽the past few million years, and聽these events seem to have coincided with the emergence of聽new species.
Infection by Agrobacterium isn鈥檛聽the only way that transgenic organisms can be created naturally. Viruses often move genes between species. For instance, monarch butterflies have acquired genes from wasps in聽this way, and gonorrhoea bacteria have some human DNA inside them.
It has also been discovered that聽the horticultural process of聽grafting different plants together can lead to the exchange of genes, meaning humans have inadvertently been creating transgenic plants for millennia. From genome studies, we can see聽that gene swapping has been going on since the dawn of life.
Plant Molecular Biology
