Agricultural parasite takes control of host plant's genes
Dodder, a parasitic plant that causes significant crop damage in the United States and around the world each year, can silence gene expression in host plants from which it obtains water and nutrients. This interspecific genetic regulation, which includes genes that contribute to the host plant's defense against pests, has never before been seen from a parasitoid plant. Understanding this system could provide researchers with a method for designing plants that are resistant to the pest. An article describing the research of a team including scientists from Penn State and Virginia Tech was published on January 4, 2018 in the journal Nature.
"Dodder is a mandatory parasite, which means he can't live alone," said Michael J. Axtell, professor of biology at Penn State and an author of the article. "Unlike most plants that derive their energy from photosynthesis, dodder siphons water and nutrients from other plants by connecting to the host's vascular system through structures called haustoria. We were able to show that in addition to the nutrients that flow into the dodder from the host plant through the haustoria, the dodder transmits microRNAs to its host plant that regulate the expression of host genes in a very direct way."
MicroRNAs are very short fragments of nucleic acid - the material of DNA and RNA - that can bind to messenger RNAs that code for proteins. This binding of the microRNA to the messenger RNA prevents the production of the protein, either by directly blocking the process or by triggering other proteins that cut the messenger RNA into small pieces. It is important to note that small remnants of messenger RNA can then function as additional microRNAs, binding to other copies of messenger RNA, resulting in silencing of the gene.
"Dodder appears to activate the expression of these microRNAs when they come into contact with the host plant," said James H. Westwood, Virginia Tech's professor of plant pathology, physiology and weed science and other author of the article. "What is interesting is that microRNAs specifically target the host genes that are involved in the plant's defense against the parasite."
When a plant is attacked by a pest, it triggers a number of defence mechanisms. In one of these mechanisms, similar to blood coagulation after a cut, plants produce a protein that coagulates the flow of nutrients to the parasite site. The microscopic RNA of the dodder targets the messenger RNA that encodes this protein, which then helps to maintain a free flow of nutrients to the parasite. The gene that codes for this coagulation protein has a very similar sequence in many plant species, and researchers have shown that the dodder microRNA targets the regions of the genetic sequence that are best preserved in plants. For this reason, doddering can probably silence this coagulation protein in a wide variety of plant species and, therefore, parasitize it.
The researchers sequenced all microRNAs present in the tissues of the parasite alone, the host plant alone and a combination of both. By comparing the sequencing data from these three sources, they were able to identify the microRNAs of the dodder that had entered the plant tissue. They then measured the amount of messenger RNA from the genes targeted by dodder microRNAs and found that the host's messenger RNA level was reduced when dodder microRNAs were present.
"With previous examples of exchanges of small RNAs between fungi and plants, our results imply that this interspecies genetic regulation could be more widespread in other plant-parasite interactions," explains Axtell. "So, with this knowledge, the dream is that we could eventually use gene editing technology to modify microRNA target sites in host plants, preventing microRNAs from binding and silencing these genes. Such resistance could reduce the economic impact of the parasite on cultivated plants."
