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Plant viruses areviruses that have the potential to affectplants. Like all other viruses, plant viruses are obligateintracellular parasites that do not have the molecular machinery toreplicate without ahost. Plant viruses can bepathogenic tovascular plants ("higher plants").
Many plant viruses arerod-shaped, with protein discs forming a tube surrounding the viralgenome; isometric particles are another common structure. They rarely have anenvelope. 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 73genera and 49families. 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 fromanimal viruses. Most plants do not move, and so plant-to-plant transmission usually involves vectors (such as insects). Plant cells are surrounded by solidcell walls, therefore transport throughplasmodesmata is the preferred path for virions to move between plant cells. Plants have specialized mechanisms for transportingmRNAs through plasmodesmata, and these mechanisms are thought to be used byRNA viruses to spread from one cell to another.[2]Plant defenses against viral infection include, among other measures, the use ofsiRNA in response todsRNA.[3] Most plant viruses encode a protein to suppress this response.[4] Plants also reduce transport throughplasmodesmata in response to injury.[2]
The discovery of plant viruses causingdisease is often accredited to A. Mayer (1886) working in the Netherlands demonstrated that the sap of mosaic obtained from tobacco leaves developed mosaic symptom when injected in healthy plants. However the infection of the sap was destroyed when it was boiled. He thought that the causal agent was bacteria. However, after larger inoculation with a large number of bacteria, he failed to develop a mosaic symptom.
In 1898, Martinus Beijerinck, who was a professor of microbiology at the Technical University the Netherlands, put forth his concepts that viruses were small and determined that the "mosaic disease" remained infectious when passed through aChamberland filter-candle. This was in contrast to bacteriamicroorganisms, which were retained by the filter.Beijerinck referred to the infectious filtrate as a "contagium vivum fluidum", thus the coinage of the modern term "virus".
After the initial discovery of the 'viral concept' there was need to classify any other knownviral diseases based on the mode of transmission even thoughmicroscopic observation proved fruitless. In 1939 Holmes published a classification list of 129 plant viruses. This was expanded and in 1999 there were 977 officially recognized, and some provisional, plant virus species.
The purification (crystallization) of TMV was first performed byWendell Stanley, who published his findings in 1935, although he did not determine that the RNA was the infectious material. However, he received theNobel Prize in Chemistry in 1946. In the 1950s a discovery by two labs simultaneously proved that the purifiedRNA of the TMV was infectious which reinforced the argument. The RNA carriesgenetic information to code for the production of new infectious particles.
More recently virus research has been focused on understanding the genetics and molecular biology of plant virusgenomes, with a particular interest in determining how the virus can replicate, move and infect plants. Understanding the virus genetics and protein functions has been used to explore the potential for commercial use bybiotechnology companies. In particular, viral-derived sequences have been used to provide an understanding of novel forms ofresistance. The recent boom in technology allowing humans to manipulate plant viruses may provide new strategies for production of value-added proteins in plants.
Viruses are so small that they can only be observed under anelectron microscope. The structure of a virus is given by its coat ofproteins, which surround the viralgenome. Assembly of viral particles takes placespontaneously.
Over 50% of known plant viruses arerod-shaped (flexuous or rigid). The length of the particle is normally dependent on the genome but it is usually between 300 and 500nm with adiameter of 15–20 nm. Protein subunits can be placed around thecircumference of a circle to form a disc. In the presence of the viral genome, the discs are stacked, then a tube is created with room for thenucleic acid genome in the middle.[5]
The second most common structure amongst plant viruses areisometric particles. They are 25–50 nm in diameter. In cases when there is only a single coat protein, the basic structure consists of 60 T subunits, where T is aninteger. Some viruses may have 2 coat proteins that associate to form anicosahedral shaped particle.
There are three genera ofGeminiviridae that consist of particles that are like two isometric particles stuck together.
A few number of plant viruses have, in addition to their coat proteins, alipid envelope. This is derived from the plant cell membrane as the virus particlebuds off from thecell.
Viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy one. Such contact may occur during agricultural practices, as by damage caused by tools or hands, or naturally, as by an animal feeding on the plant. Generally TMV, potato viruses and cucumber mosaic viruses are transmitted via sap.
Plant viruses need to be transmitted by avector, most ofteninsects such asleafhoppers. One class of viruses, theRhabdoviridae, has been proposed to actually be insect viruses that have evolved to replicate in plants. The chosen insect vector of a plant virus will often be the determining factor in that virus's host range: it can only infect plants that the insect vector feeds upon. This was shown in part when theold worldwhite fly made it to the United States, where it transferred many plant viruses into new hosts. Depending on the way they are transmitted, plant viruses are classified as non-persistent, semi-persistent and persistent. In non-persistent transmission, viruses become attached to the distal tip of thestylet of the insect and on the next plant it feeds on, it inoculates it with the virus.[6] Semi-persistent viral transmission involves the virus entering theforegut of the insect. Those viruses that manage to pass through the gut into thehaemolymph and then to thesalivary glands are known as persistent. There are two sub-classes of persistent viruses: propagative and circulative. Propagative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses), whereas circulative can not. Circulative viruses are protected inside aphids by the chaperone proteinsymbionin, produced by bacterialsymbionts. Many plant viruses encode within their genomepolypeptides with domains essential for transmission by insects. In non-persistent and semi-persistent viruses, these domains are in the coat protein and another protein known as the helper component. A bridginghypothesis has been proposed to explain how these proteins aid in insect-mediated viral transmission. The helper component will bind to the specific domain of the coat protein, and then the insect mouthparts – creating a bridge. In persistent propagative viruses, such astomato spotted wilt virus (TSWV), there is often a lipid coat surrounding the proteins that is not seen in other classes of plant viruses. In the case of TSWV, 2 viral proteins are expressed in this lipid envelope. It has been proposed that the viruses bind via these proteins and are then taken into the insectcell by receptor-mediatedendocytosis.
Soil-bornenematodes have been shown to transmit viruses. They acquire and transmit them by feeding on infectedroots. Viruses can be transmitted both non-persistently and persistently, but there is no evidence of viruses being able to replicate in nematodes. Thevirions attach to the stylet (feeding organ) or to the gut when they feed on an infected plant and can then detach during later feeding to infect other plants. Nematodes transmit viruses such astobacco ringspot virus andtobacco rattle virus.[7]
A number of virus genera are transmitted, both persistently and non-persistently, by soil bornezoosporicprotozoa. These protozoa are not phytopathogenic themselves, butparasitic. Transmission of the virus takes place when they become associated with the plant roots. Examples includePolymyxa graminis, which has been shown to transmit plant viral diseases in cereal crops[8] andPolymyxa betae which transmitsBeet necrotic yellow vein virus. Plasmodiophorids also create wounds in the plant's root through which other viruses can enter.
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Plant virus transmission from generation to generation occurs in about 20% of plant viruses. When viruses are transmitted by seeds, the seed is infected in the generative cells and the virus is maintained in the germ cells and sometimes, but less often, in the seed coat. When the growth and development of plants is delayed because of situations like unfavorable weather, there is an increase in the amount of virus infections in seeds. There does not seem to be a correlation between the location of the seed on the plant and its chances of being infected. Little is known about the mechanisms involved in the transmission of plant viruses via seeds, although it is known that it is environmentally influenced and that seed transmission occurs because of a direct invasion of the embryo via the ovule or by an indirect route with an attack on the embryo mediated by infected gametes. These processes can occur concurrently or separately depending on the host plant. It is unknown how the virus is able to directly invade and cross the embryo and boundary between the parental and progeny generations in the ovule. Many plants species can be infected through seeds including but not limited to the familiesLeguminosae,Solanaceae,Compositae,Rosaceae,Cucurbitaceae,Gramineae. Bean common mosaic virus is transmitted through seeds.
There is tenuous evidence that a virus common to peppers, thePepper Mild Mottle Virus (PMMoV) may have moved on to infect humans.[9] This is a rare and 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 toRNA interference, in which the presence of certain RNA sequences can turn genes on and off," according to Virologist Robert Garry.[10]
The intracellular life of plant viruses in hosts is still understudied especially the earlieststages of infection.[11] Many membranous structures which viruses induce plant cells to produce are motile, often being used to traffic new virions within the producing cell and into their neighbors.[11] Viruses also induce various changes to plants' ownintracellular membranes.[11] The work of Perera et al. 2012 inmosquito virus infection and various others studyingyeast models of plant viruses find this to be due to changes inhomeostasis of the lipids that composetheir intracellular membranes, including increasingsynthesis.[11] These comparable lipid alterations inform our expectations and research directions for the lesser understood area of plant viruses.[11]
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 asmessenger 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 areverse 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 ofbacteriophage are dsDNA.[13] Viruses use the plantribosomes 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 arepolycistronic) this will mean that the ribosome will either only produce one protein, as it will terminate translation at the firststop codon, or that apolyprotein will be produced. Plant viruses have had to evolve special techniques to allow the production of viral proteins byplant cells.
Fortranslation to occur,eukaryotic mRNAs require a5' Cap structure. This means that viruses must also have one. This normally consists of 7MeGpppN where N is normallyadenine orguanine. The viruses encode a protein, normally areplicase, with amethyltransferase activity to allow this.
Some viruses are cap-snatchers. During this process, a7mG-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.[14]
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.[15]
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 oftranslational regulation. In TMV, this extra sequence of polypeptide is anRNA polymerase that replicates its genome.
Some viruses use the production ofsubgenomic 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 areplicase. 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.
Some viral families, such as theBromoviridae instead opt to havemultipartite genomes, genomes split between multiple viral particles. For infection to occur, the plant must be infected with all particles across the genome. For instanceBrome mosaic virus has a genome split between 3 viral particles, and all 3 particles with the different RNAs are required forinfection to take place.
Polyprotein processing is adopted by 45% of plant viruses, such as thePotyviridae andTymoviridae.[12] The ribosome translates a single protein from the viral genome. Within the polyprotein is an enzyme (or enzymes) withproteinase 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.
Besides involvement in the infection process,viral replicase is a directly necessary part of thepackaging of RNA viruses' genetic material. This was expected due to replicase involvement already being confirmed in various other viruses.[16]
The genome of Beet necrotic yellow vein virus (BNYVV) consists of five RNAs, each encapsidated into rod-shaped virus particles. RNA 1, which is 6746 nucleotides long, encodes a single open reading frame (ORF) that produces the 237 kDa protein P237. This protein is cleaved into P150 and P66 by a papain-like proteinase. RNA 2, 4612 nucleotides long, encodes six proteins, including movement proteins (P42, P13, P15), a coat protein (P21), and a regulatory protein (P14). RNA 3, 1775 nucleotides long, encodes P25, which is involved in symptom expression. RNA 4, 1431 nucleotides long, encodes P31, crucial for vector transmission. RNA 5, found in certain isolates, encodes P26 and is associated with more severe symptoms.[17]
Plant viruses can be used to engineerviral vectors, tools commonly used by molecularbiologists to delivergenetic material into plantcells; they are also sources of biomaterials and nanotechnology devices.[18][19] 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 virus is famous for its dramatic effects on the color of the tulipperianth, an effect highly sought after during the 17th-century Dutch "tulip mania."Tobacco mosaic virus (TMV) andcauliflower mosaic virus (CaMV) are frequently used in plant molecular biology. Of special interest is the CaMV 35Spromoter, which is a very strong promoter most frequently used in planttransformations. Viral vectors based ontobacco mosaic virus include those of themagnICON® and TRBO plant expression technologies.[19]
Building on the market approvals and sales of recombinant virus-based biopharmaceuticals for veterinary and human medicine, the use of engineered plant viruses has been proposed to enhance crop performance and promote sustainable production.[20]
Representative applications of plant viruses are listed below.
| Use | Description | References |
|---|---|---|
| Enhanced plant aesthetics | Increase beauty and commercial value of ornamental plants | [21] |
| Cross‐protection | Delivery of mild virus strains to prevent infections by their severe relatives | [22] |
| Weed biocontrol | Viruses triggering lethal systemic necrosis as bioherbicides | [23] |
| Pest biocontrol | Enhanced toxin and pesticide delivery for insect and nematode control | [24] |
| Nanoparticle scaffolds | Virion surfaces are functionalized and used to assemble nanoparticles | [25] |
| Nanocarriers | Virions are used to transport cargo compounds | [26] |
| Nanoreactors | Enzymes are encapsulated into virions to engineer cascade reactions | [27] |
| Recombinant protein/peptide expression | Fast, transient overproduction of recombinant peptide, polypeptide libraries and protein complexes | [28] |
| Functional genomic studies | Targeted gene silencing usingVIGS and miRNA viral vectors | [29] |
| Genome editing | Targeted genome editingvia transient delivery of sequence‐specific nucleases | [30][31] |
| Metabolic pathway engineering | Biosynthetic pathway rewiring to improve production of native and foreign metabolites | [32][33] |
| Flowering induction | Viral expression ofFLOWERING LOCUS T to accelerate flowering induction and crop breeding | [34] |
| Cropgene therapy | Open‐field use of viral vectors for transient reprogramming of crop traits within a single growing season | [20][35] |
| Components |  | |
|---|---|---|
| Viral life cycle | ||
| Genetics | ||
| By host | ||
| Other | ||
