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Revision as of 18:03, 19 January 2007 by M G Tuffen (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Plant viruses are viruses affecting plants.
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.
Over view
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 a predicted US$ 6 x 1010 per year economic influence on crops worldwide. Plant viruses have been 73 genera, of which 49 have been classed into families.
History
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.
Structure
Viruses are very small and can only be seen under a electron microscope. The structure of a virus is given by its coat proteins, which surrong 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 - 500nm with a diameter of 15-20nm. Protein subunits can be placed around the circumference of a circle to form a disc. In the prescence 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 60T subunits, where T is an interger. Some viruses may have 2 coat proteins are the formation of the particle is analogous to a football.
There are three genra of Geriniviridae that posses germinate 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
Through sap
It implies direct transfer of sap by contact of an 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.
Insects
Plant viruses need to be trasmitted by a vector, most ofteh insects such as leafhoppers. One class of viruses, the Rhabdoviridae, have been prosed to actually be insect viruses that have learnt to replicate in plants. The chosen insect vector of a plant virus will often be the determining factor in that viruses 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 transfered many plant viruses onto new hosts.
Depending on the way they are transmitted, plant viruses are classified as non-persistant, semi-persistant and persistant.In non-persistant transmission, viruses become attached to distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus.Semi-persistant 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 persistant. There are two sub-classes of persistant 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-persistant and semi-persistant viruses, these domains are in the coat protein and another protein known as the helper companant. A bridging hypothesis has been proposed to explain how these proteins aid in insect-mediated viral trasmission. The helper componant will bind to the specific domain of the coat protein, and then the the insect mouthparts - creating a bridge.
In persistant propergative viruses, such as tomato spotted wilt virus (TSWV), there is often a lipid coat surronding the proteins that is not seen in the other classes of plant viruses. In the cause 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
Soil-bourne nematodes also have been shown to transmit viruses. They acquire and transmit them by feeding on infected roots. Viruses can be transmited by non-persistantly and persistantly, but there is no evidence of viruses being able to replicate in nematodes.
Plasmodiophorids
A number of viral genra are transmitted, both persistantly and non-persistantly, 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.
An example is Polymyxa graminis, which has been shown to transmit a plant viral diseases in ceral crops.
Seed and Pollen Bourne Viruses
Around 20% of plant viruses can be transmited through the seed of more or more plant species, mostly contained within the embryo. Viruses can inavde the developing embryo in one of two ways: either indirectly by moving into the gametes prior to fertilization or by directly infecting the embryo.
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.
External links
- Plant Viruses Online, a full list of plant viruses
- Virology page based on Plant Viruses, University of Leicester
- DPVweb, on-line plant virus database
References
Milton Zaitlin and Peter Palukaitis (2000), Advances IN Understanding Plant Viruses AND Viruas 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.
- 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.