Structure and replication

Of plant virus

GUIDED BY: SMITHA GRACE

Seminarian: ADARSH N

What are viruses?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 not usually regarded as genuine microorganisms.

Most viruses are restricted to a particular type of host. Some infect bacteria, and are known as bacteriophages, whereas others are known that infect algae, protozoa, fungi (mycoviruses), invertebrates, vertebrates or vascular plants. However, some viruses that are transmitted between vertebrate or plant hosts by feeding insects (vectors) can replicate within both their host and their vector. This web site is mostly concerned with those viruses that infect plants but we also provide some taxonomic and genome information about viruses of fungi, protozoa, vertebrates and invertebrates where these are related to plant viruses. 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 single-stranded RNA) and do not code for any proteins. 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 single-stranded RNA) and do not code for any proteins.

Why are viruses important?Viruses also cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches), leaf distortion (e.g. curling) and/or other growth distortions (e.g. stunting of the whole plant, abnormalities in flower or fruit formation).

Yellow mosaic symptom Yellow vein-banding symptoms Fruit distortion Bark scaling

The highest level of virus classification recognises six major groups, based on the nature of the genome:

• Double-stranded DNA (dsDNA)• Single-stranded DNA (ssDNA)• Reverse-transcribing viruses• Double-stranded RNA (dsRNA)• Negative sense single-stranded RNA (ssRNA-)• Positive sense single-stranded RNA (ssRNA+)

Amongst plant viruses, the most frequently encountered shapes are:

• Isometric• Rod-shaped• Filamentous• Geminate• Bacilliform

Isometric: apparently spherical and (depending on the species) from about 18nm in diameter upwards. The example here shows Tobacco necrosis virus, genus Necrovirus with particles 26 nm in diameter. tered shapes are:

• Rod-shaped: about 20-25 nm in diameter and from about 100 to 300 nm long. These appear rigid and often have a clear central canal (depending on the staining method used). Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Tobacco mosaic virus, genus Tobamovirus with particles 300 nm long.

• Filamentous: usually about 12 nm in diameter and more flexuous than the rod-shaped particles. They can be up to 1000 nm long, or even longer in some instances. Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Potato virus Y, genus Potyvirus with particles 740 nm long.

• Geminate: twinned isometric particles about 30 x 18 nm. These particles are diagnostic for viruses in the family Geminiviridae which are widespread in many crops especially in tropical regions. The example here shows Maize streak virus, genus Mastrevirus.

• Bacilliform: Short round-ended rods. These come in various forms up to about 30 nm wide and 300 nm long. The example here shows Cocoa swollen shoot virus, genus

Badnavirus with particles 28 x 130 nm.

• Some important animal and human viruses can be spread through aerosols. The viruses have the "machinery" to enter the animal cells directly by fusing with the cell membrane (e.g. in the nasal lining or gut).

• plant cells have a robust cell wall and viruses cannot penetrate them unaided. Most plant viruses are therefore transmitted by a vector organism that feeds on the plant or (in some diseases) are introduced through wounds made, for example, during cultural operations (e.g. pruning). A small number of viruses can be transmitted through pollen to the seed (e.g. Barley stripe mosaic virus, genus Hordeivirus) while many that cause systemic infections accumulate in vegetatively-propagated crops. The major vectors of plant viruses are:

REPLICATION OF VIRUS

Since tobacco mosaic virus (TMV) has been studied the most extensively, its reproduction will be briefly described. The replication of virus RNA is an essential part of the reproduction process. Most plants contain RNA-dependent RNA polymerases, and it is possible that these normal constituents replicate the virus RNA. However, some plant virus genomes (e.g., turnip yellow virus and cowpea mosaic virus) appear to be copied by a virus-specific RNA replicase. Possibly TMV RNA is also replicated by a viral RNA polymerase, but the evidence is not clear on this matter. Four TMV-specific proteins, one of them the coat protein, are known to be made. After the coat protein and RNA genome have been synthesized, they spontaneously assemble into complete TMV virions in a highly organized process.

• As the rod lengthens, the RNA passes through a channel in its center and forms a loop at the growing end. In this way the RNA can easily fit as a spiral into the interior of the helical capsid. Reproduction within the host depends on the virus’s ability to spread throughout the plant. Viruses can move long distances through the plant vasculature; usually they travel in the phloem. The spread of plant viruses in nonvascular tissue is hindered by the presence of tough cell walls. Nevertheless, a virus such as

• TMV does spread slowly, about 1 mm/day or less. It moves from cell to cell through the plasmodesmata.

• The TMV movement protein accumulates in the plasmodesmata, but the way in which it promotes virus movement is not understood. Several cytological changes can take place in TMV-infected cells. Plant virus infections often produce microscopically visible

• intracellular inclusions, usually composed of virion aggregates, and hexagonal crystals of almost pure TMV virions sometimes do develop in TMV-infected cells. The host cell chloroplasts become abnormal and often degenerate, while new chloroplast synthesis is inhibited.

• Insects. This forms the largest and most significant vector group and particularly includes:

– Aphids: transmit viruses from many different genera, including Potyvirus, Cucumovirus and Luteovirus.The picture shows the green peach aphid Myzus persicae, the vector of many plant viruses, including Potato virus Y. (Figure from Nuessly & Webb, Insect Management for Leafy Vegetables, ENY-475, September 2003, University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)).

– Whiteflies: transmit viruses from several genera but particularly those in the genus Begomovirus. The picture shows Bemisia tabaci, the vector of many viruses including Tomato yellow leaf curl virus and Lettuce infectious yellows virus.

– Hoppers: transmit viruses from several genera, including those in the families Rhabdoviridae and Reoviridae. The picture shows Micrutalis malleifera, the treehopper vector of Tomato pseudo-curly top virus.

– Thrips: transmit viruses in the genus Tospovirus. The picture shows Frankinella occidentalis, the western flower thrips that is a major vector of Tomato spotted wilt virus.

– Beetles: transmit viruses from several genera, including Comovirus and Sobemovirus

• Nematodes: these are root-feeding parasites, some of which transmit viruses in the genera Nepovirus and Tobravirus. The picture shows an adult female of Paratrichodorus pachydermus, the vector of Tobacco rattle virus. (Figure from Description 398, courtesy of the Scottish Crop Research Station).

• Plasmodiophorids: these are root-infecting obligate parasites traditionally regarded as fungi but now known to be more closely related to protists. They transmit viruses in the genera Benyvirus, Bymovirus, Furovirus, Pecluvirus and Pomovirus. The picture shows Polymyxa graminis, the vector of several cereal viruses including Barley yellow mosaic virus, growing within a barley root cell.

• Mites: these transmit viruses in the genera Rymovirus and Tritimovirus. The picture shows Aceria tosichella, the vector of Wheat streak mosaic virus. (bar represents 10 µm).

Thank you

REFERENCES

• PRESCOTT’S MICROBIOLOGY (October 2002) Prescott, Harley and Klein's 5th edition 415-440

• PETER ,J. RUSSELL(2000) “CHROMOSOMAL BASIC INHERITANCE” .In., ‘GENETICS’.BENJAMIN/CUMMINGS IS AN IMPRINT OF ADDISON WESLEY LONGMAN(FIFTH Edn)PAGE NO.47-55

• http://ghr.nlm.nih.gov/handbook/basics/structure of virus replication

• http://en.wikipedia.org/wiki/virusreplication • http://microbiology.suite101.com/article.cfm/plantvirus