MicrobiologyBytes: Virology: Plant viruses Updated: April 8, 2009 Search

Plant Viruses

MicrobiologyBytes: Latest Updates

This document is in no way intended to be a complete or even thorough discussion of plant viruses. More detailed information is available elsewhere!

Virus Infection of Plants

In economic terms, viruses are only of importance if it is likely that they will spread to crops during their commercial lifetime, which of course varies greatly between very short extremes in horticultural production and very long extremes in forestry. Some estimates put total worldwide damage due to plant viruses as high as US$ 6 x 1010 per year.
Plant cell 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.

Transmission of Plant Viruses

There are a number of routes by which plant viruses may be transmitted:
Whitefly 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.

Multipartite Plant Viruses

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

Geminiviruses

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.

Pathogenesis of Plant Virus Infections

Initially, most plant viruses multiply at the site of infection, giving rise to localized symptoms such as necrotic spots on the leaves. Subsequently, the virus may be distributed to all parts of the plant either by direct cell-to-cell spread or by the vascular system, resulting in a systemic infection involving the whole plant. However, the problem these viruses face in reinfection and recruitment of new cells is the same as they face initially - how to cross the barrier of the plant cell wall.
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.

Computer reconstruction of a plasmodesmata

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:

Plant virus infection
  • necrosis of cells, caused by direct damage due to virus replication

  • hypoplasia, i.e. localized retarded growth frequently leading to mosaicism (the appearance of thinner, yellow areas on the leaves)

  • hyperplasia, which is excessive cell division or the growth of abnormally large cells, resulting in the production of swollen or distorted areas of the plant.

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:

This is a very promising technology which offers the possibility of substantial increases in agricultural production without the use of expensive, toxic and ecologically damaging chemicals (fertilizers, herbicides, or pesticides), but is at present still in its infancy.

Another developing aspect of plant biotechnology is the construction of CVPs - Chimaeric Virus Particles - recompinant viruses such as:

which express short antigenic peptides on their surface. Several grams of CVPs can be produced cheaply and easily per kilogram of plant leaves. This is a very promising vaccine technology.

 

Bestsellers - Music - DVDs - Videos - Electronics
Search for ... (keywords):
Search for ... (keywords):

Bestsellers - Music - DVDs - Videos - Electronics

 

Emergent Plant Viruses

Rarely, there seems to be an example of an emergent virus which has acquired extra genes and as a result of new genetic capacity, becomes capable of infecting new species. A possible example of this phenomenon is seen in tomato spotted wilt virus (TSWV). TSWV is a Bunyavirus with a very wide plant host range, infecting over 600 different species from 70 families. In recent decades, this virus has been a major agricultural pest in Asia, the Americas, Europe Africa. Its rapid spread has been due to dissemination of its insect vector (the thrip Frankinellia occidentalis ) and diseased plant material. TSWV is the type species of the Tospovirus genus and has a similar morphology and genomic organization to the other Bunyaviruses. However, TSWV undergoes propagative transmission and it has been suggested that it may have acquired an extra gene in the M segment via recombination, either from a plant or from another plant virus. This new gene encodes a movement protein, conferring the capacity to infect plants and cause extensive damage.



© MicrobiologyBytes 2009.