A new Penn State university study found 52 cases of nonsexual transfer of DNA — or horizontal gene transfer (HGT) — from a host plants into a parasitic species known as broomrapes (genus Orobanche).
The transferred genes became functional in the parasites, said Claude dePamphilis, professor of biology at Penn State and co-author of the paper. Although HGT is rare in complex life, discovering that it can occur in parasitic plants could help us better defend our crops against them.
The team used genetic data to generate the evolutionary histories of thousands of genes in the parasite plants, dePamphilis said. They then looked at the transcriptomes — the expressed gene sequences — of three of these plants: Triphysaria versicolor (yellowbeak owl’s-clover), Striga hermonthica (giant witchweed) and Phelipanche aegyptiaca (Egyptian broomrape). They also examined the non-parasitic plant Lindenbergia philippensis, and genome sequences from 22 other non-parasitic plants. Because they also considered mithocondrial RNA (which can move between the host and the parasite) as a possible source for the transfers, they had to test all the data and rule out their experimental hosts as the source of genetic material. They found that the “foreign” sequences in the parasites had been derived from entire genes of past hosts, incorporated into the parasites’ genomes.
“[The broomrape family] includes some of the the world’s most devastating agricultural weeds,” said dePamphilis.
“The HGT discovery is really part of our effort to try to better understand how parasitic plants work and how we can better control them. Our hope is that we can use this information to find the best strategies to generate, or breed, resistant host plants.”
The researchers believe this transfer boosts the parasite’s ability to invade their host and overcome its natural defenses. The genes stolen this way could also provide the parasite with increased resistance to infection and pathogens the host plant has evolved to fight against.
HGT is actually pretty common in simple organisms, like bacteria. But complex life, such as you, or me, or a cucumber, transfers genes vertically — through the sexual exchange of DNA, with mutations and natural selection providing the means and incentive for evolution. But the researchers think the close feeding connections between the parasite plants and their hosts increases the chance of genes finding their way to the parasite, where they can become functional.
“Parasitic plants seem to have a far greater rate of horizontal gene transfer than non-parasitic plants and we think this is because of their very intimate connection they have with their host,” said dePamphilis.
Parasite plants push roots into their host, which they use to extract water, sugars, minerals, even nucleic acids such as DNA and RNA, dePamphilis added.
“So, they are stealing genes from their host plants, incorporating them into the genome and then turning those genes back around, very often, as a weapon against the host.”
This kind of plants plague farmers (figuratively speaking) around the world. In some areas, they are so numerous that they’re a major driver behind crop loss. In Sub-Saharan Africa the witchweed (Striga) is one of the biggest source of crop yield loss.
Future research may investigate the mechanism of horizontal gene transfer to help engineer improved plant defenses against parasitic attacks, dePamphilis concluded.
The full paper titled “Horizontal gene transfer is more frequent with increased heterotrophy and contributes to parasite adaptation” has been published in the journal Proceedings of the National Academy of Sciences.
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