Beneath our feet, plants are locked in a slow-motion struggle for the elements necessary for survival: water, sunlight, and nutrition. But some plants have learned to game the system, stealing water and nutrients from their neighbors. Some of these parasites even steal genetic material.
A recent paper in the journal Science examined interactions between the well-studied parasitic plant, dodder (Cuscuta petagona) and two other common laboratory plants preferred by this parasite, the tomato, and Arabidopsis thaliana (which is essentially the white mouse of the plant world). Dodder uses specialized structures called haustoria to tap into the stems of fellow plants and extract food and water. In fact, dodder can chemically sense its preferred host plants, moving toward them to form connections. Once the plant makes those haustorial connections, linking its vascular system with that of its host, it lets its original roots die and lives full-time as a parasite.
The existence of parasitic plants is well known and fairly well understood, but increasingly, scientists have realized that these haustorial connections are like plant-to-plant highways, allowing transfer of not only nutrients and water, but pathogens and other molecules. Researchers hypothesized that these connections could allow plants to directly transfer genetic material in what is termed "horizontal gene transfer". Transfer of messanger RNA (mRNA) is of particular interest, because mRNA tells plants what to do, and can impact plant growth and defense.
Scientists allowed dodder to latch onto tomato and Arabidopsis, and measured the number of gene clusters that were being taken up by both the host plant, and the parasite. About 24% of dodder's mobile gene clusters were found in the host-plant's stem, meaning that dodder is dumping a lot of mRNA into the host plant. Dodder is also getting some mRNA from its host, but at a much lower level, and these mRNA degrade within a few hours inside the parasite.
It is currently unclear what exactly these transferred mRNA are doing. They may be acting sending information about the host plant back to dodder, filling it in on how healthy the plant is, how old it is, and how stressed it is. These mRNA may also in a sense "zombify" the plant, changing the host plant's metabolism or defense to make it easier for the parasite to extract food and water. Interestingly, dodder can attach to multiple species of plant at once, and may actually act as a gene-transfer highway, moving genes between itself and many other host-plants. These interactions may fundamentally alter the genetic code of host plants, spurring evolution.