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International Invention Journal of Agricultural and Soil Science (ISSN: 2408-7254) Vol. 3(3) pp. 36-42, September, 2015 Available online http://internationalinventjournals.org/journals/IIJAS Copyright ©2015 International Invention Journals Review Important of Plant viruses Hamid kheyrodin 1 and sadaf kheyrodin 1- 2 Assistant prof essor in semnan university -Semnan- Iran 2Student in Mashad Islamic azad university-Iran Abstract 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 are pathogenic to higher plants. While this article does not intend to list all plant viruses, it discusses some important viruses as well as their uses in plant molecular biology. Viruses are similar to, but not considered, living organisms. To be considered "living", an organism must be able to reproduce, either asexually or sexually. Viruses, however, are only able to replicate themselves by commandeering the reproductive apparatus of the cells of living beings and making them reproduce the virus's genetic structure instead. Thus, a virus cannot function or reproduce outside of another being's cell, thereby being totally dependent on a host cell in order to survive. Most viruses are species specific, and related viruses typically only infect a narrow range of plants, animals, bacteria, or fungi. We also provide information about viroids, which are infectious RNA molecules that cause diseases in various plants. Their genomes are much smaller than those of viruses (up to 400 nucleotides of circular singlestranded RNA) and do not code for any proteins. Key words: Plant viruses, Phatogenetic, Parasite. INTRODUCTION Although plant viruses are not nearly as well understood as the animal counterparts, one plant virus has become iconic. The first virus to be discovered (see below) was Tobacco mosaic virus (TMV). 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 that represent only a tiny fraction of the total number of plant species. Viruses in wild plants have been poorly studied, but those studies that exist almost overwhelming show that such interactions between wild plants and their viruses do not appear to cause disease in the host plants. (Roossinck, 2011). 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. Plants do not move, and so plant-to-plant transmission usually involves vectors (such as insects). Plant cells are *Corresponding Author Email: [email protected] surrounded by solid cell walls, therefore transport through plasmodesmata is the preferred path for virions to move between plant cells. Plants probably 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 (Colson et al., 2010). Plant defenses against viral infection include, among other measures, the use of siRNA in response to dsRNA (Colson et al., 2010). Most plant viruses encode a protein to suppress this response.Pl ants also reduce transport through plasmodesmata in response to injury (Colson et al., 2010). figure1 History of plant viruses The discovery of plant viruses causing disease 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. kheyrodin and kheyrodin 37 He thought that the causal agent was the 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 a Chamberland filter-candle. This was in contrast to bacteria microorganisms, 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", figure 2. After the initial discovery of the ‘viral concept’ there was need to classify any other known viral diseases based on the mode of transmission even though microscopic 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 by Wendell Stanley, who published his findings in 1935, although he did not determine that the RNA was the infectious material. However, he received the Nobel Prize in Chemistry in 1946. In the 1950s a discovery by two labs simultaneously proved that the purified RNA of the TMV was infectious which reinforced the argument. The RNA carries genetic 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 virus genomes, 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 by biotechnology companies. In particular, viral-derived sequences have been used to provide an understanding of novel forms of resistance. The recent boom in technology allowing humans to manipulate plant viruses may provide new strategies for production of value-added proteins in plants. Viruses Structure Viruses are extremely small and can only be observed under an electron microscope. The structure of a virus is given by its coat of proteins, which surround the viral genome. Assembly of viral particles takes place spontaneously. Over 50% of known plant viruses are rod-shaped (flexuous or rigid). The length of the particle is normally dependent on the genome but it is usually between 300–500 nm with a diameter of 15–20 nm. Protein subunits can be placed around the circumference 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 the nucleic acid genome in the middle (Gray and Banerjee, 1999). Figure 1, 2 and 3. The second most common structure amongst plant viruses are isometric 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 an integer. Some viruses may have 2 coat proteins that associate to form an icosahedral shaped particle. There are three genera of Geminiviridae that possess geminate particles which are like two isometric particles stuck together. A very small number of plant viruses have, in addition to their coat proteins, a lipid envelope. This is derived from the plant cell membrane as the virus particle buds off from the cell. Transmission of plant viruses 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 a vector, most often insects such as leafhoppers. One class of viruses, the Rhabdoviridae, 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 the old world white fly made it to the USA, where it transferred many plant viruses into new hosts. Depending on the way they are transmitted, plant viruses are classified as nonpersistent, semi-persistent and persistent. In nonpersistent transmission, viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on; it inoculates it with the virus. Semi-persistent viral transmission involves the virus entering the foregut of the insect. Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent There are two subclasses 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 protein symbionin, produced by bacterial symbionts. Many plant viruses encode within their genome polypeptides with domains essential for transmission by insects. In non-persistent and semipersistent viruses, these domains are in the coat protein and another protein known as the helper component. A bridging hypothesis 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 as tomato spotted wilt virus (TSWV), there is often a lipid 38 Int. Inv. J. Agric. Soil Sci. Figure 1. Viral life cycle Figure 2. Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus. Figure from Description 399 (photo: INRA Avignon, France). Figure 3. Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus. Figure from Description 385 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 insect cell by receptor-mediated endocytosis. Nematodes and Plasmodiophorids Soil-borne nematodes also have been shown to transmit viruses (Kanyuka et al., 2003). They acquire and transmit them by feeding on infected roots. Viruses can be transmitted both non-persistently and persistently, but kheyrodin and kheyrodin 39 there is no evidence of viruses being able to replicate in nematodes. The virions 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. Examples of viruses that can be transmitted by nematodes include tobacco ringspot virus and tobacco rattle virus. A number of virus genera are transmitted, both persistently and non-persistently, by soil borne zoosporic protozoa. These protozoa are not phytopathogenic themselves, but parasitic. Transmission of the virus takes place when they become associated with the plant roots. Examples include Polymyxa graminis, which has been shown to transmit plant viral diseases in cereal crops (Kneller et al., 2006), and Polymyxa betae which transmits Beet necrotic yellow vein virus. Plasmodiophorids also create wounds in the plant's root through which other viruses can enter. Seed and pollen borne viruses 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 unfavourable 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 (Gray and Banerjee, 1999). 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 (Gray and Banerjee, 1999; Hull, 2001). 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 (Hull, 2001). Many plants species can be infected through seeds including but not limited to the families Leguminosae, Solanaceae, Compositae, Rosaceae, Cucurbitaceae, Gramineae (Gray and Banerjee, 1999). Bean common mosaic virus is transmitted through seeds. Direct plant-to-human transmission 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 (Oparka and Alison 2001). 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 (Roossinck, 2011). Translation of plant viral proteins 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 (TechVert. 2010). 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. At one extreme, the association occurs within the feeding apparatus of the insect, where the virus can be rapidly adsorbed and then released into a different plant cell. The feeding insect looses the virus rapidly when feeding on a non-infected plant. Such a relationship is termed "non-persistent". The best studied examples are of potyvirus transmission by aphids. At the other extreme, the virus is taken up into the vector, circulates within the vector body and is released through the salivary glands. The vector needs to feed on an infected plant for much longer and there is an interval (perhaps several hours) before it can transmit. Once it becomes viruliferous, the vector will remain so for many days and such a relationship is therefore termed "persistent" or "circulative". The best studied examples are of luteovirus transmission by aphids. In some examples of this type (e.g. some hoppers and thrips), the virus multiplies within the vector and this is termed "propagative". Genome properties: Important features include: Nature of the genome: circular (as in all known plant DNA viruses) or linear. Number of genome components: This varies from a 40 Int. Inv. J. Agric. Soil Sci. single component (e.g. in the genera Potyvirus and Tobamovirus) to 11 (in some members of the genus Nanovirus). Individual components vary in size from about 1kb (Nanovirus components) to about 20 kb (in the genus Closterovirus). Number of genes: These vary considerably. Most plant viruses have at least 3 genes: 1 (or more) concerned with replication of the nucleic acid, 1 (or more) concerned with cell-to-cell movement of the virus and 1 (or more) encoding a structural protein that is assembled into the virus particle (usually called the "coat" or "capsid" protein). There may also be additional genes that have a regulatory function or which are required for transmission between plants (association with a vector). Translation strategy: A variety of strategies are employed to translate the genes from the genome components either directly or via mRNA intermediates and (in some cases) to permit different amounts of protein to be produced from the different genes. These are summarised for each genus in the genus description pages but 3 examples here serve to illustrate some of the variety: Genus Potyvirus: in this very large genus, there is one ssRNA component that encodes one large (c. 350 kDa) polyprotein. This is cleaved by 3 different proteases (all encoded by the virus itself) into 10 different mature proteins. The two proteins at the C-terminus of the polyprotein are respectively an RNA-dependent RNA polymerase (NIb, involved in replication of the virus) and the (single) coat protein (CP). Many of the proteins have multiple functions. The genome organisation of a typical member is shown here, indicating the 10 mature proteins and the nine cleavage sites (arrowed) figure 4. Genus Furovirus: in this genus there are two ssRNA components. The 5'-proximal gene on each RNA is translated directly from the genomic RNA: on RNA1 (the larger RNA component) this gene encodes a replication protein and on RNA2 it is the coat protein. The stop codons of both of these genes are "leaky" and in a small percentage of cases, translation continues to produce a larger ("readthrough") protein. On RNA1, the replication protein is extended to include an RNA-dependent RNA polymerase (RdRp) while the readthrough region of the coat protein is probably required for particle assembly and for transmission by the plasmodiophorid vector. There is a further (3'-proximal) gene on each of the RNAs and these are translated from shorter RNA molecules transcribed from the 3'-end of the genomic RNA ("subgenomic" mRNAs). That from RNA1 is a cellto-cell movement protein (MP) that enables the virus to move between adjacent plant cells via the plasmodesmata while the function of the product from RNA2 is uncertain but may involve supression of the host plant defence reaction. The genome organisation of a typical member is shown here figure 5. o Genus Fijivirus: in this genus there are 10 components of dsRNA. Most of the components encode a single protein and at least 3 of these are structural proteins assembled into the complex virion. Genome relatedness: the degree of nucleotide identity (or amino acid identity in the protein sequence) between sequences is often used to examine the relationship between different viruses or isolates. For example, recent studies in the genus Carlavirus show that when different species are compared, they have less than 73% nucleotide identity (or 80% amino acid identity) in their coat proteins. Biological properties: In some families, the type of host is a useful feature for classification. For example, in the family Reoviridae, there are currently 3 genera with plant-infecting members (Fijivirus, Oryzavirus, Phytoreovirus), 1 genus of mycoviruses (Mycoreovirus), 1 genus containing viruses of fish and cephalopods (Aquareovirus), two genera that are restricted to insects (Cypovirus and Entomoreovirus) and 5 genera of vertebrate viruses that sometimes also infect insects. The mode of transmission is also a useful characteristic of some groups of plant viruses. For example in the family Potyviridae, members of the largest genus (Potyvirus) are transmitted by aphids, while viruses in the genera Rymovirus and Tritimovirus are transmitted by mites of the genus Abacarus or Aceria respectively, those in the genus Ipomovirus are transmitted by whiteflies and those in the genus Bymovirus by plasmodiphorids (root-infecting parasites once considered to be fungi but probably more closely related to protists). Serological properties: Many viruses are good antigens (elicit strong antibody production when purified preparations are injected into a mammal) and this property has been widely exploited to produce specific antibodies that can be used for virus detection and for examining relationships between viruses. Earlier studies used agar diffusion plates but in the last 20 years these have been largely superseded by ELISA (enzyme-linked immunosorbent assay) procedures. Although serological properties are still important, their significance in taxonomy has declined to some extent now that nucleotide sequence data are available. CONCLUSION Viruses are very small (submicroscopic) infectious particles (virions) composed of a protein coat and a nucleic acid core. They carry genetic information encoded in their nucleic acid, which typically specifies two or more proteins. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host's biochemical "machinery". Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are kheyrodin and kheyrodin 41 Figure 4. Genus Potyvirus Figure 5. Genus Furovirus not usually regarded as genuine microorganisms. The highest level of virus classification recognises six major groups, based on the nature of the genome: Double-stranded DNA (dsDNA): there are no plant viruses in this group, which is defined to include only those viruses that replicate without an RNA intermediate (see Reverse-transcribing viruses, below). It includes those viruses with the largest known genomes (up to about 400,000 base pairs) and there is only one genome component, which may be linear or circular. Well-known viruses in this group include the herpes and pox viruses. Single-stranded DNA (ssDNA): there are two families of plant viruses in this group and both of these have small circular genome components, often with two or more segments. Reverse-transcribing viruses: these have dsDNA or ssRNA genomes and their replication includes the synthesis of DNA from RNA by the enzyme reverse transcriptase; many integrate into their host genomes. The group includes the retroviruses, of which Human immunodeficiency virus (HIV), the cause of AIDS, is a member. There is a single family of plant viruses in this group and this is characterised by a single component of circular dsDNA, the replication of which is via an RNA intermediate. Double-stranded RNA (dsRNA): some plant viruses and many of the mycoviruses are included in this group. Negative sense single-stranded RNA (ssRNA-): in this group, some or all of the genes are translated into protein from an RNA strand complementary to that of the genome (as packaged in the virus particle). There are some plant viruses in this group and it also includes the viruses that cause measles, influenza and rabies. Positive sense single-stranded RNA (ssRNA+): the majority of plant viruses are included in this group. It also includes the SARS coronavirus and many other viruses that cause respiratory diseases (including the "common cold"), and the causal agents of polio and foot-andmouth disease. Within each of these groups, many different characteristics are used to classify the viruses into families, genera and species. Typically, combinations of characters are used and some of the most important are: Particle morphology: the shape and size of particles as seen under the electron microscope. Genome properties: this includes the number of genome components and the translation strategy. Where genome sequences have been determined, the relatedness of different sequences is often an important factor in discriminating between species. Biological properties: this may include the type of host and also the mode of transmission. Serological properties: the relatedness (or otherwise) of the virion protein(s). The survey generated more than 250 votes from the international community, and allowed the generation of a Top 10 plant virus list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Tobacco mosaic 42 Int. Inv. J. Agric. Soil Sci. virus, (2) Tomato spotted wilt virus, (3) Tomato yellow leaf curl virus, (4) Cucumber mosaic virus, (5) Potato virus Y, (6) Cauliflower mosaic virus, (7) African cassava mosaic virus, (8) Plum pox virus, (9) Brome mosaic virus and (10) Potato virus X, with honourable mentions for viruses just missing out on the Top 10, including Citrus tristeza virus, Barley yellow dwarf virus, Potato leafroll virus and Tomato bushy stunt virus. This review article presents a short review on each virus of the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant virology community, as well as laying down a benchmark, as it will be interesting to see in future years how perceptions change and which viruses enter and leave the Top 10. ACKNOWLEDGEMENTS Research presented in this manuscript is supported by Semnan University, Iran and is gratefully acknowledged. REFERENCES Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). Molecular Biology of the Cell. Garland Science Chapter 7: Control of Gene Expression, pp 451-452. Colson P, Richet H, Desnues C, Balique F, Moal V (2010). Mylonakis, Eleftherios, ed. "Pepper Mild Mottle Virus, a Plant Virus Associated with Specific Immune Responses, Fever, Abdominal Pains, and Pruritus in Humans". PLoS ONE (PLoS ONE) 5 (4): e10041. Ding SW, Voinnet O (2007). "Antiviral Immunity Directed by Small RNAs". 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A Not So Open-andShut Case. Plant Physiology, Jan., Vol. 125, pp 123-126 Roossinck MJ (2011). The good viruses: viral mutualistic symbioses . Nature Reviews Microbiology, 9, 99-108 DOI:10.1038/nrmicro2491 TechVert. (2010). Evidence of First Virus That Moves from Plants to Humans". TechVert. 2010-04-15. Verchot-Lubicz J (2003). "Soilborne viruses: advances in virus movement, virus induced gene silencing, and engineered resistance", Physiological and Molecular Plant Pathology 62, p. 56 How to cite this article: kheyrodin H, kheyrodin S (2015). Important of Plant viruses. Int. Inv. J. Agric. Soil Sci. Vol. 3(3): 36-42