A lot of the time when speaking of pathogens, the conversation stays on just the pathogenic organism itself and the host it exploits. But there are many other interactions that are possible and that complicate the scenario, especially those that involve an intermediate vector needed to spread the pathogen to the eventual host. It has already been well known that microorganisms are capable of altering their vector and host in order to make it easier for their eventual transmission, including by changing how either can retrieve the nutrients they require for survival.
War of the Aphids
But being able to directly influence the vector into changing how it responds to the host is a bit more special and there is an extensive lack of proper research into these community level interactions and their genetic complexities. It is, of course, something that is difficult to study, as it involves multiple completely different organisms that have to be cared for and their interactions with each other controlled as tightly as possible so as not to introduce any confounding variables that may interfere with accurate data collection. So ecological research on this level is often highly informative and useful for biology as a whole.
At Penn State University, researchers have been looking into the specifics of the barley yellow dwarf virus (BYDV) and its unique life cycle in infecting spring wheat crops. The complication here is the two aphid species that feed upon the wheat, Rhopalosiphum padi and Rhopalosiphum maidis, the former being a fair bit smaller than the latter by almost a full half. Which is an important distinction. This allows the larger aphid to essentially bully their smaller counterparts for space on any single plant and this competition is not to the benefit of R. padi.
Previous research has shown that the two aphids have overlapping, but still separate temperatures they grow, develop, and reproduce best at. For R. padi that is between 21 and 28 degrees Celsius and for R. maidis that is between 15 and 25 degrees Celsius. Depending on the environmental temperature, the amount of competition can be either fierce or practically nonexistent, particularly since they both prefer the lower, shadier stems to retrieve sap from.
Temperature Modulation
Studies involving other organisms have found that the vector species can have its temperature range modulated by the infecting virus and the scientists here were interested in seeing if BYDV was conducting the same strategic maneuver. To test this, they set up separate experiments both in the lab and out in a test field using different strains of BYDV that are particular to either R. padi or R. maidis. Then they measured three different factors: the surface temperature distribution of the plant tissues themselves, the spatial distribution of the two aphid species across the plant, and the overall heat tolerance of the two aphid species during this process, both for those infected with BYDV and those not.
What they found was very one-sided and specific to the virus strain that infected R. padi and not its counterpart. The viruliferous R. padi (those that were infected) exhibited a particular change on the genetic level, an upregulation of three heat shock proteins that resulted in a phenotype of being able to endure higher levels of heat for a longer period of time. It appears that the virus is able to actively alter the gene expression of its vector organism to remove it from competition with its sibling species and have it move to an upper, higher temperature portion of the plant in order to increase the likelihood of the virus being able to infect and infiltrate the host wheat plant. Since that is its true goal after all.
But that’s not all they found. Not only did the R. padi aphids move to the higher portions of the stem that had greater temperature, but those temperatures were not due to just greater amounts of sunlight. No, instead, it appears that infected plants are additionally modified by the virus as well to increase the temperature in its stem by a few degrees so that the environment is better suited for R. padi without as much competition from its larger, less heat tolerant friend. And it is completely unknown what the virus is doing in that case to make that happen.
The Versatility of Viruses
One hypothesis presented by the researchers in relation to prior published work is that the virus is able to disrupt the stomata on the leaf tissue so that respiration is less effective, causing the plants to be unable to cool themselves as efficiently. This is something that has been proven for tobacco plants infected with tobacco mosaic virus, so the mechanism is at least possible. Perhaps being infected with the virus causes an expanded hormonal response involving salicylic acid from the plant, which in turn causes depolarization of the stomata cellular membranes. The researchers suggest that this doesn’t happen naturally in the opposing infected R. maidis because the virus strain there has a lower replication rate, so the plant immune response is more subdued.
This sort of extensive genetic manipulation of vector and host by a virus to suit its own transmission needs is both incredible and dangerous. It showcases that we still have much to learn about broader interactions in the environmental community between viruses, their hosts, and any other involved organisms. If they can develop mechanisms capable of this sort of dual fine-tuning of genetic expression, then there are plenty of other harmful things they can do to plants, insects, animals, and all organisms in general. How food webs are affected by pathogens and the fine line between symbiotic mutualism and parasitism is something we clearly need to learn more about.
At the same time, this knowledge could prove to be highly beneficial in a direct manner, since providing heat tolerance is one of the most desired phenotypic traits right now due to the ongoing impacts from climate change. So figuring out how these gene changes are done on the genetic level may give us more options to purposefully enact those changes ourselves or, at minimum, perhaps enable us to use viruses like this to cause the changes we want by borrowing their capabilities. For now, what we do know is that we need to know more and that there’s much more work and research to be done.
Photo CCs: Wheat P1210892 on Wikimedia Commons
