When dealing with pathogens of plants, it is those of the viral kind in conjunction with their vector pests that can prove to be the biggest threats. Aphids, especially, are known for transmitting dozens of dangerous viral diseases while also themselves managing to kill a host plant outright through sucking up too much sap. Therefore, wiping out aphids is often a top priority once they’ve been discovered in order to prevent a full blown infestation. At the same time, however, plant viruses have a unique relationship with vector insects and not in an entirely positive manner.
Plant Virus Dissemination
A team of researchers working at Cornell University and the USDA decided to look into the effect plant viruses have on these insects and, in turn, how this improved their ability to be transmitted to more plants. It is well known that viruses are also to mount offensive molecular assaults on their host plants, but the aphid-borne variety are also able to do the same to their insect assistants, altering their biochemistry and even their physiology in ways that benefit transmission. The primary line between those that do and those that don’t is whether the plant virus is a persistent, circulative type or a non-persistent, transient type.
The latter only interacts briefly with the aphids, allowing their virions to be picked up by the mouthparts of the insect and then released when the aphid goes to obtain sap from a new, uninfected plant. Since they have such minimal interaction with aphids, they do essentially nothing to them in the process. They are just a short vehicle to an end point. Persistent species, on the other hand, are as their name suggests a more permanent feature.
These types of plant viruses enter into the circulatory system of the aphids and remain a part of them for the rest of their lives. Every time the aphids then consume sap from a plant, they transfer an amount of the virus to the plant, spreading the infection further. It is the family of Luteoviruses that exclusively use aphids as a vector like this, with potato leafroll virus (PLRV) being the species the researchers were investigating.
Symbionts, Viruses, and RNA Expression
Known as the vector manipulation hypothesis for plant viruses, the hypothesis has existed for over half a decade, based on prior physical evidence of changes in insect vectors that transmit those viruses. On the flip side, aphids also have positive endosymbionts, such as Buchnera aphidicola, that help supply essential amino acids that the bugs can’t otherwise obtain from their sap-based diet. While these symbionts would have no reason to assist plant viruses in infecting aphids, it is possible that the plant viruses affect the Buchnera symbiosis with aphids in some way.
The team additionally wanted to see what relationship, if any, there was between the plant viruses and the aphid virus Myzus persicae densovirus (MpDNV). Since both viruses need aphids to transmit themselves, but in very different ways, their interaction could be positive or negative, but no research into such a thing has been done before. Until this team began to look into it.
The primary method of evidence collecting was based around small RNA (sRNA) production and how RNA interference (RNAi) pathways were biochemically altered by the creation of these sRNAs over time. Any viruses that begin to replicate in the aphid tissues would themselves begin producing virus-derived small interfering RNA (viRNA) to prevent the aphid immune system from stopping them. Sequencing of all small RNAs would thereby show how these changes occurred over time and how they affect gene expression of aphids.
Difference in sRNA Production
When testing general expression differences, the researchers found that piwi-interacting RNAs (piRNAs), a class of small RNAs that change gene expression and epigenetic methylation of DNA, were altered or “primed” in the aphid cells by the plant viruses and were able to cause germline changes even without having any active plant interaction happening. Similarly, when looking into the relationship with the symbiont, it was shown that the relative amount of tRNAs produced to utilize the essential amino acids provided by the bacteria varied when the aphid was also infected with a plant virus. The changes, however, didn’t provide any direct evidence toward how this alteration affected the aphid in general and whether it was a good or bad change.
Lastly was the impact the plant virus has on insect virus uptake and infection. It was already shown that the plant viruses themselves do have a suppressive effect on the aphid antiviral immune system, but all the plant viruses do this and not all of them show the same levels of insect virus accumulation. So the scientists zeroed in on P0, a suppressor gene that focuses on marking Ago1 for degradation. Ago1 is a member of the Argonaute family of proteins and is involved in RNA interference and silencing, something which could harm the plant viruses and so it is important for them to minimize its activity.
After 3 days of exposure, the MpDNV levels were highest in the experimental group made to eat leaves expressing the P0 protein. This effect was higher than even the group exposed to the entire virus. The protein alone had a stronger outcome. The team hypothesized that the protein may be altering the antiviral immune system directly in the aphid cells and so expression in leaf tissues gave it a more direct route to do so than having it have to be applied by plant viruses.
Another facet of MpDNV infection is the forced formation of wings in times outside of the migration season when the aphids normally grow them. The virus often causes them to have wings permanently throughout the year, increasing their mobility and thus greater contact with other aphids that the MpDNV can infect. When looking into RNA levels, it was observed by the researchers that the plant viruses like PLRV were helping to modulate this wing expression and improving the ability for it to be enacted. This results in greater winged morph variants being created than would otherwise happen by probability within an infected population.
A Kaleidoscope of Interactions
There remains to be seen whether any further or deeper effects results from a combined aphid infection with PLRV and MpDNV, but what is known thus far is that their mutually beneficial alterations to aphid morphology and biochemistry through genetic manipulation gives them both a greater chance at transmitting themselves to future hosts.
These kinds of interactions on a microorganismal level are likely more common than we currently know, but are especially difficult to tease out on what individual alterations are done by each species. But we now have a first look at such a thing in plant and insect viruses and that gives us the clues and tools we need for our ongoing foray into the biological world around us
Photo CCs: Blattlaus-Teens (14612009645) from Wikimedia Commons