Is our world remodeled by viruses?

The virus community is incredibly vast. Occupying all conceivable biological niches, scorching underwater winds on the icy tundra, these enigmatic invaders, floating between inert matter and life, travel around the globe in hundreds of trillions of billions. These are the most abundant life forms on Earth.

Viruses are rightly feared as ingenious pathogens, causing disease in everything they invade, including virtually all bacteria, fungi, plants and animals. Recent advances in the field of virology, however, suggest that viruses play a more important and complex role than previously thought, and could be essential to the functioning of various ecosystems.

We now know that humans contain about 100,000 viral DNA elements, which account for about 8% of our genome. Speculations about the role of these ancient virus fragments range from protection against disease to increased risk of cancer or other serious diseases, although researchers recognize that they have barely touched the surface of this puzzle.

A new journal article in the journal Nature Reviews Microbiology highlights the evolution and ecology of plant viruses. Arvind Varsani, a researcher at ASU's Biodesign Institute, joins an international team to explore many of the details of viral dynamics. They describe the subtle interaction between three components of the viral infection process, the virus itself, hosts of plant cells infected with the virus and the vectors that act as intermediates, a complex system evolving over some 450 millions of years. The three elements are embedded in broader relationships in the surrounding ecosystem.

Recent studies in the field of virology have shown that viruses are sometimes beneficial for the organisms they infect. "Before that, people have always seen viruses as pathogenic entities," says Varsani. "It breaks all the dogmas of our study of viruses. We have a section where we examine mutualism and symbiosis and also how some symbiotic relationships are dissociated. "

Elusive vagabond

In 1892, Dmitry Ivanovsky, a Russian botanist, conducted a simple experiment that would have major implications for science and medicine. He collected the sap from a sick tobacco plant, fed it through very fine pores and showed that this filtered fluid could infect a healthy tobacco plant. Filtering ensures that, whatever the pathogenic entity responsible, it is smaller than a bacterium. Dutch plant scientist and microbiologist Martinus Beijerinck described the mysterious pathogen as a virus, although its true form – invisible to light microscopy – only appeared in 1931, with the invention of the electron microscope. A stem-shaped plant invader, known as the tobacco mosaic virus, was revealed – the first virus ever recorded. Since that time, thousands of distinct species have been identified, but they represent only a tiny part of the viral universe, most of which remains unexplored.

Indeed, even the question of what constitutes a virus has no single answer. Their sizes vary enormously, from a virus like Ebola, carrying a tiny handful of genes, to giant viruses recently discovered. Rivaling the size of certain bacteria, giant viruses can contain elements of the machinery needed for translation, calling into question their non-living entity status.

"At the moment, I consider viruses from a philosophical angle," says Varsani. "They form a dynamic entity and have several lifestyles, ranging from basic mode, where the virus is completely dependent on the host for replication, in some cases where it only partially depends on the host. Since some viruses can evolve so rapidly and acquire new genetic elements, their genomes can become chimerical or even fragmented, making their appropriate clbadification a major challenge for the field of virology.

From the point of view of ecology, plant viruses are particularly important for several reasons. Plants make up more than 80% of the world's biombad and have a greater impact on the planet's various ecosystems than viruses that infect other kingdoms of life. Plant viruses are of obvious importance for food crops and ornamentals, and various viruses cause about $ 60 billion in crop losses worldwide each year.

To seize the astonishing richness of the viral universe of the planet, the researchers went beyond the old methods of identification and badysis of individual viral particles. The techniques of metaviromics are used to probe the environments of the complete panoply of viruses they contain. This method, which consists of reconstructing several DNA or RNA genomes from environmental samples, has recently been used to identify a large number of previously undocumented viruses. In the case of plant viruses, these viral fragments are often extracted from insect vectors that carry viruses from one plant to the other.

New methods discover a multitude of new viruses

Metaviromic sequencing is a particularly powerful technique for investigating viral communities. Unlike cell life, which has one and the same common origin, viruses are polyphyletic, which means that they are the result of multiple origins. No single gene shared by all viruses has been identified. Common protein motifs have been observed in viral capsids, but are probably due to convergent evolution or horizontal gene transfer, rather than inherited elements.

The strategy of metaviromics is particularly useful for breaking the mutual relationships between plants, vectors and viruses and their changing relationships over time. Since the beginning of virology, a great deal of research has been devoted to viruses as pathogenic agents in humans and plants, so that the nature and degree of mutualistic interactions between viruses, vectors and hosts are most likely under -représentés.

The authors badume that viruses could play an important role in maintaining biodiversity and helping plants adapt to their environment by limiting the growth of genetically homogeneous plants, including crops. New studies on viral ecology seek to understand the extent and importance of pathogenic and mutual interactions. The behavior of certain vector insects and their modes of viral transmission are an essential link in the chain of infection, although many other factors come into play, including nutrients, water resources, stress due to heat and cold and unfavorable soil conditions.

Viral intermediates

Vectors play a disproportionate role in the world of plant viruses. Unlike animal viruses, plant viruses are not usually transmitted by direct contact between infected and uninfected individuals. Instead, plant viruses are spread by vectors (including insects) as well as by pollen and seeds.

The mode of viral transmission is thought to play a role in the effect of the virus on its host. If the virus is transmitted through seeds or pollen, it should limit its adverse effects on the reproductive success of the host plant, or even confer an adaptive advantage over uninfected plants.

The viral pbadage from the mother plant to the daughter plant is called vertical transmission. On the other hand, horizontal virus transmission occurs when insect vectors pbad the virus from one plant to another. Such vector aggression can be more ruthless to the infected plant and it is sufficient to ensure their continued spread to an appropriate number of healthy plants for the virus to succeed.

Many types of vectors can transmit plant viruses, including arachnids, fungi, nematodes and some protists, although more than 70% of known plant viruses are transmitted by insects, most of them Hemiptera, which includes cicadas, aphids, leafhoppers and leafhoppers. and shield the insects.

Insects of this type may use mouthparts designed to pierce and extract sap or plant cell material. Insect transmission by plant viruses can occur through the excretion of virus particles in saliva after eating an infected plant. Alternatively, the plant virus can integrate permanently into the salivary glands of the insect, allowing the vector to transmit it to new plants throughout its life.

Curiously, a number of insect-transmitted plant viruses may have developed mechanisms to influence vector behavior, making infected plants more attractive to insects feeding on sap or ensuring that infected plants produce chemicals that promote the behavior of insects facilitating transmission.

In addition to their complex and varied infection chains, some plant viruses have another unique property. These viruses transmit their genomes in several packets, each containing only a part of the complete genetic code of the virus, encapsulated in a separate virus particle. This particular strategy, which requires the co-transmission of several viral particles to a new host to ensure the integrity of the viral genome, is considered a unique feature of plant viruses. The nature and evolution of these so-called multiparty viruses remain a biological puzzle.

Plant viruses show considerable ingenuity in their strategies, which are highly dependent on their environment. Some are generalists, invading many species, while others are specialists who favor a restricted range of plant hosts. This selectivity can evolve over time, according to a process called adaptive radiation. This usually happens when a virus faces a heterogeneous habitat and adaptively specializes to exploit particular ecological resources while becoming unsuited to exploiting others. This specialization limits competition between different lines or species of viruses. Alternatively, generalist viruses infect several plant hosts, but they must compete with other viruses for these resources. This situation tends to lead to a low diversity viral population dominated by the most adapted viral genotypes.

The arrival of viruses

Although researchers agree that viruses do not have a single common ancestor, a detailed picture of how they emerged (and when) in the network of life remains deeply contested. Three common hypotheses vie for domination as an explanatory framework, although they do not exclude each other. Perhaps viruses have evolved from free cells, as stipulated by the decentralization or regression hypothesis. They could also come from RNA and DNA molecules escaped from living cells. Alternatively, viruses may have existed sometime as self-replicating entities that evolved parallel to cells, eventually losing their independent status.

Current metaviromic research on viral diversity is helping to uncover the fundamental relationships between viruses and to identify the common origins of many plant, fungus and arthropod viruses. The future is particularly concerned about the way in which human-caused disturbances to the planet's ecosystems, occurring at a rate unprecedented in the history of the planet, consist in reforming the relations between virus, vector and host. .

The effects of these disturbances may be to promote the emergence of viruses with an increased ability to cause disease in their hosts. While ecological communities are increasingly intertwined with the changes in human land use, the existing interaction networks that have operated during the course of evolution for Stabilize host relationships with native vectors and viruses can change suddenly. Any mortal entity entering this type of disrupted ecosystem is more likely to spread quickly across the population and aggressively sweep different organisms. The future health and sustainability of human and plant populations will benefit from a better understanding of the many subtle interrelationships that govern the most prevalent viruses – colonizing plants.

Reference
Evolution and ecology of plant viruses. Pierre Lefeuvre, Darren P. Martin, Santiago F. Elena, Dionne N. Shepherd, Philippe Roumagnac and Arvind Varsani. Nature Reviews Microbiology (2019), https://doi.org/10.1038/s41579-019-0232-3.

This article has been republished from the following materials. Note: Content may have changed for length and content. For more information, please contact the cited source.