| MicrobiologyBytes: Virology: Plant viruses | Updated: November 2, 2007 | Search |
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This document is in no way intended to be a complete or even thorough discussion of plant viruses. More detailed information is available elsewhere!
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Plant viruses face special problems initiating an infection. The outer surfaces of plants are composed of protective layers of waxes and pectin, but more significantly, each cell is surrounded by a thick wall of cellulose overlying the cytoplasmic membrane. |
To date, no plant virus is known to use a specific cellular receptor of the type that animal and bacterial viruses use to attach to cells. Rather, plant viruses rely on a mechanical breach of the integrity of a cell wall to directly introduce a virus particle into a cell. This is achieved either by the vector associated with transmission of the virus or simply by mechanical damage to cells. After replication in an initial cell, the lack of receptors poses special problems for plant viruses in recruiting new cells to the infection.
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Transmission of plant viruses by insects is of particular agricultural importance. Extensive areas of monoculture and the inappropriate use of pesticides which kill natural predators can result in massive population booms of insects such as aphids. Plant viruses rely on a mechanical breach of the integrity of a cell wall to directly introduce a virus particle into a cell. This is achieved either by the vector associated with transmission of the virus or simply by mechanical damage to cells. |
Transfer by insect vectors is a particularly efficient means of virus transmission. In some instances, viruses are transmitted mechanically from one plant to the next by the vector and the insect is merely a means of distribution, flying or being carried on the wind for long distances (sometimes hundreds of miles). Insects which bite or suck plant tissues are, of course, the ideal means of transmitting viruses to new hosts. This is known as non-propagative transmission. However, in other cases (e.g. many plant rhabdoviruses) the virus may also infect and multiply in the tissues of the insect (propagative transmission) as well as those of host plants. In these cases, the vector serves as a means not only of distributing the virus, but also of amplifying the infection:
Begomoviruses (Geminiviridae) are transmitted by whiteflies. Most Begomovirus genomes consist of two circular, single-stranded DNA molecules. These viruses cause a great deal of crop damage in plants such as tomatoes, beans, squash, cassava and cotton and their spread may be directly linked to the inadvertent world-wide dissemination of the "B" or silverleaf biotype of the whitefly Bemisia tabaci. This vector is an indiscriminate feeder, encouraging rapid and efficient spread of viruses from indigenous plant species to neighbouring crops.
| Family: | Segments: |
|---|---|
| Begomovirus (Geminiviridae) (single-stranded DNA) | Bipartite |
| Comovirus (single-stranded RNA) | Bipartite |
| Furovirus (single-stranded RNA) | Bipartite |
| Tobravirus (single-stranded RNA) | Bipartite |
| Partitiviridae (double-stranded RNA) | Bipartite |
| Bromoviridae (single-stranded RNA) | Tripartite |
| Hordeivirus (single-stranded RNA) | Tripartite |
These discrete particles are structurally similar and may contain the same component proteins, but often differ in size depending on the length of the genome segment packaged. In one sense, multipartite genomes are, of course, segmented, but this is not the strict meaning of these terms as they are used here.
Genome segmentation reduces the probability of breakages due to shearing, thus increasing the total potential coding capacity of the genome. However, the disadvantage of this strategy is that all the individual genome segments must be packaged into each virus particle, or the virus will be defective as a result of loss of genetic information. Separating the genome segments into different particles (the multipartite strategy) removes the requirement for accurate sorting, but introduces a new problem in that all the discrete virus particles must be taken up by a single host cell to establish a productive infection. This is perhaps the reason multipartite viruses are only found in plants. Many of the sources of infection by plant viruses, such as inoculation by sap-sucking insects or after physical damage to tissues, result in a large inoculum of infectious virus particles, providing opportunities for infection of an initial cell by more than one particle.
| Plant cell walls necessarily contain channels called plasmodesmata
which allow plant cells to communicate with each other and to pass metabolites
between them.
However, these channels are too small to allow the passage of virus particles or genomic nucleic acids. |
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Many (if not most) plant viruses have evolved specialized movement proteins which modify the plasmodesmata. One of the best known examples of this is the 30k protein of tobacco mosaic virus (TMV). This protein is expressed from a sub-genomic mRNA and its function is to modify plasmodesmata causing genomic RNA coated with 30k protein to be transported from the infected cell to neighbouring cells. Other viruses, such as cowpea mosaic virus (CPMV - Comovirus family) have a similar strategy but employ a different molecular mechanism. In CPMV, the 58/48k proteins form tubular structures allowing the passage of intact virus particles to pass from one cell to another.
Typically, virus infections of plants might result in effects such as growth retardation, distortion, mosaic patterning on the leaves, yellowing, wilting, etc. These macroscopic symptoms result from:
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Plants might be seen as sitting targets for virus infection - unlike animals, they cannot run away. However, plants exhibit a range of responses to virus infections designed to minimize their effects. Initially, infection results in a 'hypersensitive response', manifested as:
Plants might be seen as sitting targets for virus infection - unlike animals, they cannot run away. However, plants exhibit a range of responses to virus infections designed to minimize their effects. Initially, infection results in a 'hypersensitive response', manifested as:
Although this system is poorly understood, at least some of these proteins have been characterized and have been shown to be proteases, which presumably destroy virus proteins, limiting the spread of the infection. There is some similarity here between this response and the production of interferons by animals. In addition, systemic resistance to virus infection is a naturally occurring phenomenon in some strains of plant, e.g. the tobacco N gene encodes a cytoplasmic protein with a nucleotide binding site which interferes with the TMV replicase. This is clearly a highly desirable characteristic and is highly prized by plant breeders, who try to spread this attribute to economically valuable crop strains. There are probably many different mechanisms involved in systemic resistance, but in general terms there is a tendency towards increased local necrosis as substances such as proteases and peroxidases are produced by the plant to destroy the virus and to prevent its spread and subsequent systemic disease. An example of this is the N gene, which when present in plants causes TMV to produce a localized, necrotic infection rather than the systemic mosaic symptoms normally seen (Marathe R. et al. The tobacco mosaic virus resistance gene, N. Molecular Plant Pathology 3:167-172, 2002). Other examples include:
Virus-resistant plants have been created by the production of transgenic plants expressing recombinant virus proteins or nucleic acids which interfere with virus replication without producing the pathogenic consequences of infection, e.g:
Another developing aspect of plant biotechnology is the construction of CVPs - Chimaeric Virus Particles - recompinant viruses such as:
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