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Plant viruses, like all other viruses, are obligate intracellular parasites that do not have the molecular machinery to replicate without the host. The plant viruses are defined as viruses 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.
Plant viruses are not nearly as well understood as the animal counterparts, this is ironic as the first virus to be discovered (see below) was the Tobacco Mosaic Virus (TMV) as well as the fact that they have an estimated US$ 60,000,000,000 per year economic influence on crops worldwide. Plant viruses are grouped into 73 genera and 49 families.
The discovery of plant viruses causing disease is often accredited to Martinus Beijerinck who discoved, in 1898, that even after passing infective tree sap through a porcelain filter remained infectious but was sterile of microorganisms.
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 recognised, and some provisional, plant virus species.
The purification of the TMV (the first purification) was first performed by Wendell Stanley, who published his findings in 1936. He later was accredited with 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, that had a lot of opposition at the time, that RNA was carrying genetic information to code for the production of new infectious particles.
More recently the research has been focused on the manipulation and modification of plant virus genomes do discover function and for commercial gain in the agriculture business by using viral-derived sequences to provide understanding of novel forms of resistance. The recent boom in technology allowing humans to manipulate plant viruses has really helped bring the subject out of an Aristolean science age (observation and description of the subject matter) and into the 21st centaury.
Viruses are very small and can only be seen under an electron microscope. The structure of a virus is given by its coat proteins, which surround the viral genome. Assembly of viral particles takes place spontaneously.
Over 50% of known plant viruses are Rod shaped (flexious or rigid). Exact length is normally dependent on the genome but it is usually between 300 - 500 nm with a diameter of 15-20nm. 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.
The second most common structure amongst plant viruses are isometric particles. They are 40-50nm 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 are the formation of the particle is analogous to a football.
There are three genera of Geminiviridae that posses 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
It implies direct transfer of sap by contact of and wounded plant with a healthy one. Such process occurs during agricultural practices by tools, hands, or by 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, have 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' 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 onto 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 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 sub-classes of persistent viruses: propergative and circulative. Propergative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses), whereas circulative can not.
Many plant viruses encode within their genome polypeptides 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 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 propergative viruses, such as tomato spotted wilt virus (TSWV), there is often a lipid coat surrounding the proteins that is not seen in the 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.
Soil-bourne nematodes also have been shown to transmit viruses. They acquire and transmit them by feeding on infected roots. Viruses can be transmitted by non-persistently and persistently, but there is no evidence of viruses being able to replicate in nematodes.
A number of viral genra are transmitted, both persistently and non-persistently, by soil bourne zoosporic protozoa. These protozoa are not phytopathogenic themselves, but parasitic. Transmission of the virus takes place when they become associated with the plant roots.
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.  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 families Leguminoseae, Solanacease, Compositae, Rosaceae, Curcurbitaceae, Gramineae. 
As mentioned above, 90% of plant viruses have genomes that consist of single stranded RNA, meaning that they are in the same sense orientation as messenger RNA. Viruses use the plant ribosomes to produce the 4-10 proteins encoded by their genome. However, since all of the proteins are encoded on a single strand (that is, they are ploycistronic) 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.
In order 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.
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 
Production of sub-genomic RNAs
Some viruses use the production of sub-genomic RNAs to ensure the translation of all proteins within their genomes. In this process the first protein encoded on the genome, and this the first to be translated, is a replicase. This protein will act of 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 the Bromoviridae instead opt to have multi-partite 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.
This stratergy is adopted by viral genra such as the Potyviridae and Tymovirus. The ribosome translates a single protein from the viral genome. Within the polyprotein is an ezyme with proteinase function that is able cleave the polyprotein into the various single proteins or just cleave away the replicase, which can then produce sub-genomic RNAs.
Well understood plant viruses
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.
- plant virus-route of infection
- Plant Viruses Online, a full list of plant viruses
- MicrobiologyBytes: Plant Viruses
- DPVweb, on-line plant virus database
- ↑ http://www.tulane.edu/~dmsander/WWW/224/Structure224.html
- ↑ Stewart M. Gray1 and Nanditta Banerjee - Mechanisms of Arthropod Transmission of Plant and Animal Viruses. Microbiology and Molecular Biology Reviews, Volume 63, pages 128-148.
- ↑ Konstantin Kanyuka, Elaine Ward and Michael J. Adams. Polymyxa graminis and the cereal viruses it transmits: a research challenge. Molecular Plant Pathology, Vol 4 pages 393-406.
- ↑ Duijsings et al., In vivo analysis of the TSWV cap-snatching mechanism: single base complementarity and primer length requirements. The EMBO Journal, Vol. 20 pages 2545-2552.
- ↑ [Kneller et all. Cap-independent translation of plant viral RNAs. Virus Research, Volume 119, Issue 1, July 2006, Pages 63-75] .
- Milton Zaitlin and Peter Palukaitis (2000), Advances in Understanding Plant Viruses and Virus Diseases. Vol. 38: 117-143 (doi:10.1146/annurev.phyto.38.1.117)
- Milton Zaitlin (1998), Discoveries in Plant Biology, New York 14853, USA. Pp.: 105-110. S.D Kung and S. F. Yang (eds).
- Dickinson, M - Molecular Plant Pathology (2003). BIOS Scientific Publishers.
- Wang, Daowen and Andrew J. Maule (June 1994). "A Model for Seed Transmission of a Plant Virus: Genetic and Structural Analyses of Pea Embryo Invasion by Pea Seed-Borne Mosaic Virus" The Plant Cell, Vol 6, 777-787.
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