Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses can be pathogenic to higher plants.
Most plant viruses are rod-shaped, with protein discs forming a tube surrounding the viral genome; isometric particles are another common structure. They rarely have an envelope. The great majority have an RNA genome, which is usually small and single stranded (ss), but some viruses have double-stranded (ds) RNA, ssDNA or dsDNA genomes. Although plant viruses are not as well understood as their animal counterparts, one plant virus has become very recognizable: tobacco mosaic virus (TMV), the first virus to be discovered. This and other viruses cause an estimated US $60 billion loss in crop yields worldwide each year. Plant viruses are grouped into 73 genera and 49 families. However, these figures relate only to cultivated plants, which represent only a tiny fraction of the total number of plant species. Viruses in wild plants have not been well-studied, but the interactions between wild plants and their viruses often do not appear to cause disease in the host plants.[1]
To transmit from one plant to another and from one plant cell to another, plant viruses must use strategies that are usually different from animal viruses. Most plants do not move, and so plant-to-plant transmission usually involves vectors (such as insects). Plant cells are surrounded by solid cell walls, therefore transport through plasmodesmata is the preferred path for virions to move between plant cells. Plants have specialized mechanisms for transporting mRNAs through plasmodesmata, and these mechanisms are thought to be used by RNA viruses to spread from one cell to another.[2] Plant defenses against viral infectioninclude, among other measures, the use of siRNA in response to dsRNA.[3] Most plant viruses encode a protein to suppress this response.[4] Plants also reduce transport through plasmodesmata in response to injury.[2]
Direct plant-to-human transmission[edit]
Researchers from the University of the Mediterranean in Marseille, France have found tenuous evidence that suggest a virus common to peppers, the Pepper Mild Mottle Virus (PMMoV) may have moved on to infect humans.[9] This is a very rare and highly unlikely event as, to enter a cell and replicate, a virus must "bind to a receptor on its surface, and a plant virus would be highly unlikely to recognize a receptor on a human cell. One possibility is that the virus does not infect human cells directly. Instead, the naked viral RNA may alter the function of the cells through a mechanism similar to RNA interference, in which the presence of certain RNA sequences can turn genes on and off," according to Virologist Robert Garry from the Tulane University in New Orleans, Louisiana.[10]
Translation of plant viral proteins[edit]
75% of plant viruses have genomes that consist of single stranded RNA (ssRNA). 65% of plant viruses have +ssRNA, meaning that they are in the same sense orientation as messenger RNA but 10% have -ssRNA, meaning they must be converted to +ssRNA before they can be translated. 5% are double stranded RNA and so can be immediately translated as +ssRNA viruses. 3% require a reverse transcriptase enzyme to convert between RNA and DNA. 17% of plant viruses are ssDNA and very few are dsDNA, in contrast a quarter of animal viruses are dsDNA and three-quarters of bacteriophage are dsDNA.[12] Viruses use the plant ribosomes to produce the 4-10 proteins encoded by their genome. However, since many of the proteins are encoded on a single strand (that is, they are polycistronic) this will mean that the ribosome will either only produce one protein, as it will terminate translation at the first stop codon, or that a polyprotein will be produced. Plant viruses have had to evolve special techniques to allow the production of viral proteins by plant cells.
5' Cap[edit]
For translation to occur, eukaryotic mRNAs require a 5' Cap structure. This means that viruses must also have one. This normally consists of 7MeGpppN where N is normally adenine or guanine. The viruses encode a protein, normally a replicase, with a methyltransferase activity to allow this.
Some viruses are cap-snatchers. During this process, a 7mG-capped host mRNA is recruited by the viral transcriptase complex and subsequently cleaved by a virally encoded endonuclease. The resulting capped leader RNA is used to prime transcription on the viral genome.[13]
However some plant viruses do not use cap, yet translate efficiently due to cap-independent translation enhancers present in 5' and 3' untranslated regions of viral mRNA.[14]
Readthrough[edit]
Some viruses (e.g. tobacco mosaic virus (TMV)) have RNA sequences that contain a "leaky" stop codon. In TMV 95% of the time the host ribosome will terminate the synthesis of the polypeptide at this codon but the rest of the time it continues past it. This means that 5% of the proteins produced are larger than and different from the others normally produced, which is a form of translational regulation. In TMV, this extra sequence of polypeptide is an RNA polymerase that replicates its genome.
Production of sub-genomic RNAs[edit]
Some viruses use the production of subgenomic RNAs to ensure the translation of all proteins within their genomes. In this process the first protein encoded on the genome, and is the first to be translated, is a replicase. This protein will act on the rest of the genome producing negative strand sub-genomic RNAs then act upon these to form positive strand sub-genomic RNAs that are essentially mRNAs ready for translation.
Segmented genomes[edit]
Some viral families, such as the Bromoviridae instead opt to have multipartite genomes, genomes split between multiple viral particles. For infection to occur, the plant must be infected with all particles across the genome. For instance Brome mosaic virus has a genome split between 3 viral particles, and all 3 particles with the different RNAs are required for infection to take place.
Polyprotein processing[edit]
Polyprotein processing is adopted by 45% of plant viruses, such as the Potyviridae and Tymoviridae.[11] The ribosome translates a single protein from the viral genome. Within the polyprotein is an enzyme (or enzymes) with proteinase function that is able to cleave the polyprotein into the various single proteins or just cleave away the protease, which can then cleave other polypeptides producing the mature proteins.
Genome packaging[edit]
Besides involvement in the infection process, viral replicase is a directly necessary part of the packaging of RNA viruses' genetic material. This was expected due to replicase involvement already being confirmed in various other viruses.[15]
Applications of plant viruses[edit]
Plant viruses can be used to engineer viral vectors, tools commonly used by molecular biologists to deliver genetic material into plant cells; they are also sources of biomaterials and nanotechnology devices.[16][17] Knowledge of plant viruses and their components has been instrumental for the development of modern plant biotechnology. The use of plant viruses to enhance the beauty of ornamental plants can be considered the first recorded application of plant viruses. Tulip breaking virusis famous for its dramatic effects on the color of the tulip perianth, an effect highly sought after during the 17th-century Dutch "tulip mania." Tobacco mosaic virus (TMV) and cauliflower mosaic virus (CaMV) are frequently used in plant molecular biology. Of special interest is the CaMV 35S promoter, which is a very strong promoter most frequently used in plant transformations. Viral vectors based on tobacco mosaic virus include those of the magnICON® and TRBO plant expression technologies.[17]
Representative applications of plant viruses are listed below.
| Use | Description | References |
|---|---|---|
| Enhanced plant aesthetics | Increase beauty and commercial value of ornamental plants | [18] |
| Cross‐protection | Delivery of mild virus strains to prevent infections by their severe relatives | [19] |
| Weed biocontrol | Viruses triggering lethal systemic necrosis as bioherbicides | [20] |
| Pest biocontrol | Enhanced toxin and pesticide delivery for insect and nematode control | [21] |
| Nanoparticle scaffolds | Virion surfaces are functionalized and used to assemble nanoparticles | [22] |
| Nanocarriers | Virions are used to transport cargo compounds | [23] |
| Nanoreactors | Enzymes are encapsulated into virions to engineer cascade reactions | [24] |
| Recombinant protein/peptide expression | Fast, transient overproduction of recombinant peptide, polypeptide libraries and protein complexes | [25] |
| Functional genomic studies | Targeted gene silencing using VIGS and miRNA viral vectors | [26] |
| Genome editing | Targeted genome editing via transient delivery of sequence‐specific nucleases | [27][28] |
| Metabolic pathway engineering | Biosynthetic pathway rewiring to improve production of native and foreign metabolites | [29][30] |
| Flowering induction | Viral expression of FLOWERING LOCUS T to accelerate flowering induction and crop breeding | [31] |
| Crop gene therapy | Open‐field use of viral vectors for transient reprogramming of crop traits within a single growing season | [16] |
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