Virginia Tech and Penn State researchers: Parasitic plants may form weapons out of genes stolen from hosts
November 2, 2016
Sneaky parasitic weeds may be able to steal genes from the plants they are attacking and then use those genes against the host plant, according to a team of scientists.
In a recent study, researchers that included a team from Virginia Tech detected 52 incidences of the nonsexual transfer of DNA — known as horizontal gene transfer, or HGT — from a host plant that later became functional in members of a parasitic plant family known as the broomrapes, said Claude dePamphilis, a professor of biology from Penn State.
Although considered rare in more complex species, like plants, HGT may occur in some parasitic plants, an insight that could lead to better methods of controlling parasitic plants that threaten agriculture, he added.
The researchers, who released their findings in the current issue of Proceedings of the National Academy of Sciences, suggest that the transfer could boost the parasitic plant's ability to invade their hosts and overcome defenses the host creates to try to ward off attacks. HGT may also help reduce the risk of infection for the parasites.
“We looked at a range of different parasitic plant species and found that those with the greatest level of dependency on their hosts were the ones that stole the most genes,” he said.
Westwood works extensively with parasitic plant interactions and previously discovered a potential new form of plant communication that allows them to share an extraordinary amount of genetic information through exchange of RNA with one another.
This study was part of a larger project coordinated by Westwood, called The Parasitic Plant Genome Project. It compares several related parasitic plant species in order to understand the evolution of parasitism in plants.
"These parasitic plants that we study from the broomrape family include 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 we can use this information to find the best strategies to generate, or breed, resistant host plants."
While horizontal gene transfers in less complex species, such as bacteria, is common, most evolution in more complex organisms is driven by the sexual exchange of DNA, along with mutation and natural selection.
However, the researchers suggest that the close feeding connections of parasitic plants with their hosts may increase the chances of intact genes traveling from the host to the parasite's genome where it can quickly become functional.
"Parasitic plants seem to have a far greater rate of horizontal gene transfer than nonparasitic plants, and we think this is because of their very intimate connection they have with their host," said dePamphilis.
The roots of the parasite contact and enter the host and then begin extracting water, sugars, mineral nutrients, and even nucleic acids, including DNA and RNA, he 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," said dePamphilis.
Farmers throughout the world struggle with these types of parasitic plants, which are so numerous in some areas of the world that they become a major source of crop loss. In Sub-Saharan Africa, for example, Striga — or witchweed — is one of the most damaging sources of agricultural loss, according to dePamphilis.
To detect HGT in the plants, dePamphilis, Westwood, and colleagues used data generated by their collaborative research effort funded by the U.S. National Science Foundation to generate evolutionary histories for thousands of genes in the parasitic plants.
The researchers focused on transcriptomes — expressed gene sequences — of three parasitic plants, Triphysaria versicolor, also called yellowbeak owl's-clover; Striga hermonthica, or giant witchweed; and Phelipanche aegyptiaca, called Egyptian broomrape; as well the nonparasitic plant Lindenbergia philippensis and genome sequences from 22 other nonparasitic plants. Because the researchers considered mRNA, which can move between hosts and their parasites, as a possible source of the transfers, they tested and retested the data to rule out the experimental host as the source of the genetic material. Instead, they found that the foreign sequences had been derived from entire genes of past host plants and incorporated into the parasitic plants genomes.
Future research may investigate the mechanism of horizontal gene transfer to help engineer improved plant defenses against parasitic attacks.
Primary authors along with dePamphilis are Zhenzhen Yang, postdoctoral scholar in biology at Penn State, and Yeting Zhang, formerly doctoral student in genetics at Penn State and now a postdoctoral fellow at Rutgers University.
The researchers also worked with Christopher R. Clarke, postdoctoral research fellow at Virginia Tech; John I. Yoder, professor of plant sciences, and Siming Liu, postdoctoral scholar, both of UC Davis; Michael P. Timko, professor of biology, and Chun Su, graduate student in biology, both of University of Virginia.
Also Eric Wafula, bioinformatics programmer; Loren A. Honaas, postdoctoral scholar in biology, now at the U.S. Department of Agriculture; Huiting Zhang, doctoral student in plant biology; Sam Jones, doctoral student, plant biology; Paula E. Ralph, technician, biology; Naomi S. Altman, professor of statistics; and Stephan C. Schuster, professor of biochemistry and molecular biology, all of Penn State.
The U.S. National Science Foundation, the National Institute of Food and Agriculture Project of the U.S. Department of Agriculture, and the Huck Institutes of the Life Sciences supported this work.