Viruses are non-cellular agents that replicate inside host cells, often leading to disease in the host. Plant virus infections are associated with devastating losses in agriculture and threaten global food security. Viruses are responsible for nearly 50% of plant diseases accounting for an estimated annual economic loss of US$30 billion (Hilaire et al., 2022). Some sources attribute major crop yield losses of up to US$ 60 billion annually worldwide due to plant virus infections (Fingu-Mabola and Francis, 2021). However, not all viruses should be viewed in a negative light. As viruses are host-specific, there is potential to utilize them as bio-control agents against agricultural pests and pathogens. Furthermore, virus infection of plant endophytes has been linked to increased thermal and drought tolerance in plants. This article, therefore, aims to explore the nature of viruses’ pathogenicity in plants and their importance and applications in agriculture.

So what is a virus? A virus is an acellular infectious pathogen composed fundamentally of a ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) genome encased in a protein coat known as the capsid. In addition, some viruses may have an outer envelope made of a membrane containing lipids and proteins derived from the host cell. Many viruses have spike proteins on the capsid or on the envelope which help the virus bind to receptors on the host cell. The infectious form of a virus is called a virion. Plant viruses, also known as phytoviruses, generally come in the form of helical (roughly elongated) or icosahedral (roughly spherical) shaped capsids (Gergerich and Dolja, 2006). While spherical viruses are relatively small (diameter ~30nm), elongated viruses can be significantly bigger: tobacco mosaic virus (TMV) is 300 x 18 nm, and some filamentous viruses can reach the length of ~2000nm (2µm) (Gergerich and Dolja, 2006). A typical plant cell in comparison is ~50µm. Most phytoviruses contain an RNA genome, and DNA phytoviruses are rare (Gergerich and Dolja, 2006).

Why are viruses harmful to the host? Plant viruses are obligate, biotrophic parasites. The virion can penetrate the host cell mainly via wounds caused by damage to the cell wall or through insect vectors. Inside the cell, the viral genome leaves the capsid and takes over the host cell machinery to conduct transcription and translation producing new viral nucleic acids and proteins. New virions are assembled in the plant cell followed by the release of mature virions out of the cell to infect new host cells (Figure 1). In plants, virions can enter neighboring cells via the plasmodesmata (cell-to-cell movement) and can move to other parts of the plant in the phloem tissue (long-distance movement) (Gergerich and Dolja, 2006). The virus infection leads to adverse effects on plants such as stunted growth, abnormal flower and leaf formation, ring patterns and bumps on the foliage, necrotic spots, mosaic, and mottling of the leaves (Figure 2). This can have a severe impact on crop yields.

As negative an impact as most virus infections may have on plants, the relationship between virus and host can be blurred. Some viruses such as pararetroviruses in tomato plants are integrated in an inactive state within the genome of the host cell for long periods and will only become active under stress; the presence of an inactive virus in the host for an extended time may act as a method of immunization and give the host a selective advantage in fighting against other pathogens (Roossinck, 2015). Meanwhile, some viruses have mutualistic impacts on plants such as counteracting the effects of abiotic stress. In Yellow Stone National Park where geothermal soils can reach temperatures greater than 50˚C, one species of grass that is colonized by a fungal endophyte infected by the virus allows the plant to survive hot temperatures (Roossinck, 2015)! Greenhouse studies have also shown that some virus infected plants have conferred drought-tolerance to plants (Roossinck, 2015). Further study of this phenomena may have useful applications in the development of heat- and drought-tolerant plants that can survive global climate change.
Some plant viruses may also help fight against biotic stress. For instance, studies have shown that white clover mosaic virus infection helps the plant deter fungus gnats and zucchini yellow mosaic virus downregulates the production of volatile compounds in plants that attract beetles; as beetles are vectors of bacterial wilt, the viral infection has protected the plant from wilt disease (Roossinck, 2015). Studies are also being conducted on using bacteriophages (viruses that infect bacteria) as an agent to control bacteria-borne diseases in plants.

In considering this, it is highly important to thoroughly study phytoviruses to understand the nature of plant-viral relationships and explore applications of this knowledge to increase crop yields.

References
Carbonell, A., García, J.A. et al. (2018). eLS Plant Virus RNA Replication, accessed https://www.semanticscholar.org/paper/eLS-Plant-Virus-RNA-Replication-Carbonell-Garc%C3%ADa/85f49c32434c48dce5dbe0eec5d7b80e0af810c9
Fingu-Mabola, J.C. and Francis, F. (2021). Aphid-plant-phytovirus pathosystems: influencing factors from vector behaviour to virus spread. Agriculture, 11(6), https://doi.org/10.3390/agriculture11060502.
Gergerich, R.C. and Dolja, V.V. (2006) Introduction to plant viruses, the invisible foe. The Plant Health Instructor, DOI: 10.1094/PHI-I-2006-0414-01
Hilaire, J., Tindale, S., Jones, G. et al. (2022). Risk perception associated with an emerging agri-food risk in Europe: plant viruses in agriculture. Agriculture and Food Security, 11(2), https://doi.org/10.1186/s40066-022-00366-5 .
Roossinck, M. J. (2015) Move over, bacteria! Viruses make their mark as mutualistic microbial symbionts. Journal of Virology, 89(13), pp. 6532-6535.

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Figure 1: Carbonell, A., García, J.A. et al. (2018). eLS Plant Virus RNA Replication, accessed https://www.semanticscholar.org/paper/eLS-Plant-Virus-RNA-Replication-Carbonell-Garc%C3%ADa/85f49c32434c48dce5dbe0eec5d7b80e0af810c9

Figure 2 : University of Maryland Extension (2018-2020) https://extension.umd.edu/sites/extension.umd.edu/files/styles/optimized/public/2021-06/HGIC_flowers_peony_virus-symptoms_CC_600.jpg?itok=EEPJsGqA