Guest post: Not alive, but deadly

By David Parrish


Illustration of the structure of a coronavirus particle. Centers for Disease Control and Prevention (CDC); Public domain.

The “novel coronavirus” has many of us cringing at every sniffle, cough, and sneeze we hear (or utter). Anxiety levels are high and justifiably so. Perhaps shedding some light on viruses can help us deal less anxiously with this new one. The old Irish proverb comes to mind: “better the devil you know.”

Viruses inhabit an interesting space in the field of biology. Only a few thousand have been named (versus more than a million species of plants, animals, and fungi), but viruses are likely the most abundant form that biologists study. Here’s the logic. Humans are subject to infection by many different viruses, most of which infect only us, and the same appears to be true for every species of animal, plant, fungus, and bacterium. Accordingly, we can reason that viruses are the most abundant things biologists study.

However, most biologists do not consider viruses to be truly alive. Since the 19th century, biologists have agreed that living things are made of cells – the complex building blocks of life. Viruses are not cellular. They are quite simple, made of DNA or RNA, a few proteins, and (sometimes) oily substances. They do not carry out the biochemistry associated with life. Antibiotics (the name means “against life” or “not fit for life”) are not effective against viruses exactly because viruses do nothing that an antibiotic can attack.

But viruses do some things that make them seem to be alive. They act a lot like bacteria and fungi in their ability to invade organisms, cause diseases, reproduce, disperse, and then repeat the cycle. It is an act, though, not really life. Living microbes that cause diseases (bacteria and fungi) generally grow and make more of themselves in cavities or on cell surfaces of their “hosts” (victims). They live outside of the hosts’ cells and cause diseases by multiplying, producing toxins, or other effects. By contrast, viruses enter a host cell, unpack their DNA or RNA, commandeer the cell’s metabolic machinery, and cause new virus parts to be made. The parts self-assemble and the resulting viruses can disperse and repeat the process. In essence, a virus is a non-living, stripped-down, nano-robot-like, biochemical assembly that invades cells and hijacks them to make more viruses.

In animals, if a virus invades cells lining air passages, the symptoms will be respiratory – from colds to bronchitis to pneumonia. Several different human viruses attack liver cells and can cause hepatitis. Some viruses can turn genetic switches in infected organs and cause cells to divide malignantly. The human papillomavirus (HPV) is a well-studied virus known to cause cancer, and HPV vaccines have been shown to be quite effective in preventing both HPV infections and associated cancers.

Where does a novel coronavirus – or any new virus – come from? How does a fatal disease like Covid-19 suddenly appear? Unfortunately, a never-before-seen virus with lethal potential can be produced in just one viral generation. The genes (DNA or RNA) that viruses carry are subject to mutation and new combinations. Pieces of DNA from host cells can be picked up by the virus, potentially making it more easily spread, more lethal, or both. In this way, viruses evolve to produce new forms just as living things do.

In another cruel twist, a host cell may be invaded simultaneously by two different viruses and forced to make parts for both. As those parts are self-assembling, mixtures of the two viruses may form. And those hybrids may have unique disease properties. Many virologists suspect that the virus causing Covid-19 resulted from this kind of mashup between something as innocuous as a common cold coronavirus and a coronavirus from a bat. Such cross-species viral hybrids seem inevitable in a world with ever increasing numbers of people and our continual encroachment on other animals’ territory. Both science and common sense are needed in this area.

Virologists, epidemiologists, and physicians still have much to learn about Covid-19 and the virus that causes it, but I am cautiously optimistic that the science being brought to bear on this new scourge will get us past it. And the hope is the knowledge gained will make us better prepared for scourges yet to come.

Final_GYROCover (1)About the author: After earning his PhD in plant science from Cornell, Parrish joined the faculty of Virginia Tech’s College of Agriculture and Life Sciences, where he taught crop ecology and environmental science. His research interests spanned seed physiology, sustainable cropping systems, and biological sources for renewable energy. In his book, “The Gyroscope of Life” (Pocahontas Press, June 2020), Parrish brings biological studies to the curious non-scientist in an accessible and relevant way, inspiring readers to consider the world around us in a new light.


How a virus gives back to its host



Image by F_A (CC BY 2.0)

A study by UK scientists has shown that tomato plants infected with a virus are more attractive to bumblebees than healthy plants. Why would a plant virus want to change the behaviour of bumblebees?

The virus in question – cucumber mosaic virus (CMV) – can infect many different species of plant including tomatoes and a model plant called Arabidopsis thaliana. In tomatoes it causes many symptoms including yellowing, mottling, leaf distortion and can reduce the yield of seeds. As a result there is pressure for populations of plants to evolve better defences against the virus. Since CMV can only multiply within plant cells you might expect that, over time, CMV might become less common, but this doesn’t appear to be the case. One way the virus might be able to combat this problem is to compensate for the decrease in seed production in infected plants by encouraging pollinators, such as bumblebees, to visit the flowers.

Bumblebees fertilise tomato flowers by a process called buzz pollination, in which sounds produced by the bees shake the flowers to release pollen. Although tomato flowers can fertilise themselves without help from the bumblebees, buzz pollination makes the process more efficient and also leads to the transfer of pollen between flowers. Volatile compounds (molecules that easily become gases) released from the plants may help to guide the bees to the flowers. CMV infection can change the mix of volatile compounds that plants produce, but it was not clear whether this changes the behaviour of the bees.

Simon Groen, Sanjie Jiang, Alex Murphy, Nik Cunniffe et al. found that the bees are more attracted to the volatiles produced by CMV-infected tomato plants than those produced by healthy, uninfected plants. In the absence of buzz pollination, CMV-infected plants produce fewer seeds than healthy plants. However, mathematical modeling indicates that, in the “wild”, the bee’s preference for virus-infected flowers may help to compensate for this so that CMV-infected plants may produce more seeds than uninfected plants. Further experiments in A. thaliana suggest that molecules of micro ribonucleic acid (or miRNA for short) produced by the plants might regulate the mix of volatiles that plants produce.

These findings suggest that in some environments it may be in a virus’ interest to help its host plant by making the plant more attractive to bumblebees or other pollinators. Bumblebees are important pollinators for many crop plants so these findings may help us to develop new ways to increase crop yields in the future.

Reference: Groen SC, Jiang S, M, Murphy AM, Cunniffe N, Westwood JH, Davey MP, Bruce TJA,  Caulfield JA, Furzer O, Reed A, Robinson SI, Miller E, Davis CN, Pickett JA, Whitney HM,  Glover BJ, Carr JP. 2016. Virus Infection of Plants Alters Pollinator Preference: A Payback for Susceptible Hosts? PLOS Pathogens