Alfalfa mosaic virus
Encyclopedia
Alfalfa mosaic virus also known as Lucerne mosaic virus or Potato calico virus, is a worldwide distributed phytopathogen that can lead to necrosis
and yellow mosaics on a large variety of plant species, including commercially important crops. It is the only Alfamovirus of the Bromoviridae family.
In 1931 Weimer J.L. was the first to report AMV in alfalfa
(Medicago sativa). Transmission of the virus occurs mainly by some aphids (plant lice), by seeds or by pollen to the seed.
(coat protein) but no envelope. The icosahedral symmetry of the capsid is round to elongated. The range for the length of the virion particle is about 30-57 nm. AMV is a multipartite virus
and is composed of 4 particles (3 bacilliform and 1 spheroidal) with a diameter of 18 nm.
The genetic material of AMV consists of 3 linear single strands RNAs (RNA 1, RNA 2 and RNA 3) and a subgenomic RNA (RNA 4) which is obtained by transcription of the negative- sense strand of RNA 3. RNA 1 and 2 encode proteins needed for replication (RNA 1 and 2 alone can only infect the protoplast
). RNA 3 is required for the synthesis of the protein responsible for cell-to-cell movement. RNA 4 encodes the capsid. Beside encapsidation and his role in movement the viral coat protein also plays a role in the initiation of RNA replication. This property is called genome activation and means that the genomic nucleic acid cannot be successfully infectious without the capsid. Specific association of the coat protein with the RNA 3’- terminal sequences or with the subgenomic mRNA is required for the infection. Bacilliform particles contain separately encapsidated RNAs 1, 2 and 3. Spheroidal particles each have two copies of RNA 4.
The nucleotide
sequence of the complete genome has been determined and the length of the genome is 8274 nucleotides ( or 9155 including the subgenomic RNA). RNA 1, 2, 3 and 4 are respectively 3644 (3.65kb), 2593 (2.6kb), 2037 (2.2kb) and 881 (0.88kb) nucleotides long.
(Most details of the replication cycle are still unknown).
Symptoms vary from wilting, white flecks, malformation like dwarfing, ringspots, mottles, mosaics to necrosis depending on the virus strain, host variety, stage of growth at infection and environmental conditions. Signs of infection can persist or disappear quickly. The virus can be detected in each part of the host plant. The virions are mainly found in the cytoplasm
and chloroplast
of the infected plant (as inclusion bodies
).
Inclusions of Alfalfa mosaic virus
In vitro
AMV has a longevity of 1–4 days (sometimes much longer). Air temperature and light are the environmental factors that have the greatest influence on the multiplication and movement of AMV in the plant and thus indirectly on the symptoms. Under low temperature the appearance of necrosis for example is less than that for high temperature. The virus usually reaches his inactivation temperature at 60-65 °C. Dark conditions slow down the virus multiplication, while light speeds it up. A hypothesis for this phenomenon is that shading causes a decrease in ATP production by photosynthesis. The optimum pH was found to be about pH 7-7.5 for AMV in sap (depending on the host species). It has been proved that in the important forage grass alfalfa, the infection by AMV leads to a decrease of Cu, Fe, Mn, P and Zn quantities. On the other hand an increase in N (viral protein) was observed. Infected alfalfa was also not seen to be harmful for domestic animals.
AMV is a very variable plant virus and several variants with minor differences exist (strain Q, strain S, strain 425, strain AlMV-B, strain AlMV- S,...). Distinction is based on different symptoms in one or two chosen hosts and also on, for example, differential physico-chemical properties.
, family Aphididae
; green peach aphids (Myzus persicae
) and at least 14 other species are known to play that role. AMV can also be transmitted by seed, pollen to the seed (rarely by ovules), through mechanical inoculation
by plant sap and by the parasitic plant dodder (Cuscuta). The combination of seed- infected plants and spreading by aphids results mostly in high levels of infection.
Necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
and yellow mosaics on a large variety of plant species, including commercially important crops. It is the only Alfamovirus of the Bromoviridae family.
In 1931 Weimer J.L. was the first to report AMV in alfalfa
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
(Medicago sativa). Transmission of the virus occurs mainly by some aphids (plant lice), by seeds or by pollen to the seed.
Structure and genome
The virion has a capsidCapsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
(coat protein) but no envelope. The icosahedral symmetry of the capsid is round to elongated. The range for the length of the virion particle is about 30-57 nm. AMV is a multipartite virus
Multipartite virus
A multipartite virus is a computer virus that infects and spreads in multiple ways. The term was coined to describe the first viruses that included DOS executable files and PC BIOS boot sector virus code, where both parts are viral themselves. For a complete cleanup, all parts of the virus must be...
and is composed of 4 particles (3 bacilliform and 1 spheroidal) with a diameter of 18 nm.
The genetic material of AMV consists of 3 linear single strands RNAs (RNA 1, RNA 2 and RNA 3) and a subgenomic RNA (RNA 4) which is obtained by transcription of the negative- sense strand of RNA 3. RNA 1 and 2 encode proteins needed for replication (RNA 1 and 2 alone can only infect the protoplast
Protoplast
Protoplast, from the ancient Greek πρῶτον + verb πλάθω or πλάττω , initially referred to the first organized body of a species.Protoplast has several biological definitions:...
). RNA 3 is required for the synthesis of the protein responsible for cell-to-cell movement. RNA 4 encodes the capsid. Beside encapsidation and his role in movement the viral coat protein also plays a role in the initiation of RNA replication. This property is called genome activation and means that the genomic nucleic acid cannot be successfully infectious without the capsid. Specific association of the coat protein with the RNA 3’- terminal sequences or with the subgenomic mRNA is required for the infection. Bacilliform particles contain separately encapsidated RNAs 1, 2 and 3. Spheroidal particles each have two copies of RNA 4.
The nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
sequence of the complete genome has been determined and the length of the genome is 8274 nucleotides ( or 9155 including the subgenomic RNA). RNA 1, 2, 3 and 4 are respectively 3644 (3.65kb), 2593 (2.6kb), 2037 (2.2kb) and 881 (0.88kb) nucleotides long.
Replication cycle
The AMV cycle can be split up in 5 steps:- 1st step: AMV enters the cell and the particles disassemble. The capsid remains attached to the coat protein binding site (CPB) at the 3’- end of the RNAs. The initiation factors elF4A, elF4E and elF4G of the host bind to the cap (5’-end).
- 2nd step: The coat protein interacts with an initiation factor. This triggers translation of RNA1 and 2 into replicase proteins P1 and P2. The complex P1/P2 bind to the RNA.
- 3rd step: Targeting of RNA to the tonoplast by P1/P2. The capsid dissociates from CPB. CPB undergoes a conformational change into TLS (tRNA-like structure). P1/P2 bind to the minus- strand promoter which is made up of TLS and hairpin E (directs initiation of some transcriptions).
- 4th step: Minus- strand RNAs are synthesized.
- 5th step: Plus- strand RNAs and viral proteins are synthesized. Virions assemble.
(Most details of the replication cycle are still unknown).
Pathology
AMV infects over 600 plant species in 70 families (experimental and natural hosts). Some hosts: potato (Solanum tuberosum), pea (Pisum sativum), tobacco (Nicotiana tabacum), tomato (Lycopersicon esculentum), bluebeard (Caryopteris incana), ...Symptoms vary from wilting, white flecks, malformation like dwarfing, ringspots, mottles, mosaics to necrosis depending on the virus strain, host variety, stage of growth at infection and environmental conditions. Signs of infection can persist or disappear quickly. The virus can be detected in each part of the host plant. The virions are mainly found in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
of the infected plant (as inclusion bodies
Inclusion bodies
Inclusion bodies are nuclear or cytoplasmic aggregates of stainable substances, usually proteins. They typically represent sites of viral multiplication in a bacterium or a eukaryotic cell and usually consist of viral capsid proteins...
).
Inclusions of Alfalfa mosaic virus
In vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
AMV has a longevity of 1–4 days (sometimes much longer). Air temperature and light are the environmental factors that have the greatest influence on the multiplication and movement of AMV in the plant and thus indirectly on the symptoms. Under low temperature the appearance of necrosis for example is less than that for high temperature. The virus usually reaches his inactivation temperature at 60-65 °C. Dark conditions slow down the virus multiplication, while light speeds it up. A hypothesis for this phenomenon is that shading causes a decrease in ATP production by photosynthesis. The optimum pH was found to be about pH 7-7.5 for AMV in sap (depending on the host species). It has been proved that in the important forage grass alfalfa, the infection by AMV leads to a decrease of Cu, Fe, Mn, P and Zn quantities. On the other hand an increase in N (viral protein) was observed. Infected alfalfa was also not seen to be harmful for domestic animals.
AMV is a very variable plant virus and several variants with minor differences exist (strain Q, strain S, strain 425, strain AlMV-B, strain AlMV- S,...). Distinction is based on different symptoms in one or two chosen hosts and also on, for example, differential physico-chemical properties.
Transmission
The vectors are insects of the order HemipteraHemiptera
Hemiptera is an order of insects most often known as the true bugs , comprising around 50,000–80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others...
, family Aphididae
Aphididae
Aphididae is a very large insect family in the aphid superfamily , of the order Hemiptera. There are several thousand species in this family, many of which are well known for being serious plant pests...
; green peach aphids (Myzus persicae
Myzus persicae
Myzus persicae, known as the green peach aphid, is a small green aphid. It is the most significant aphid pest of peach trees, causing decreased growth, shriveling of the leaves and the death of various tissues...
) and at least 14 other species are known to play that role. AMV can also be transmitted by seed, pollen to the seed (rarely by ovules), through mechanical inoculation
Inoculation
Inoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
by plant sap and by the parasitic plant dodder (Cuscuta). The combination of seed- infected plants and spreading by aphids results mostly in high levels of infection.
Economic importance
The host range of the virus is wide and includes food crops and pasture (peas, lentils, potatoes, clovers,…). Infection by AMV causes important yield losses, reduces winter survival and facilitates infection of the affected plant by other pathogens.Management
Insecticides against aphids are not effective for controlling AMV. Recommendations are sowing healthy seed (some seed companies sell seed tested for AMV), managing weeds, avoiding to grow crops adjacent to infected pasture and other cultural practices to minimize AMV. Work is still done on creating transgenic AMV resistant plants. For example DNA derived from AMV containing the gene encoding for the capsid is inserted in plants. This reduces the susceptibility of the plant to infection by AMV and the plant is less a source of virus for spread to other plants.See also
- Alfalfa mosaic virus coat protein binding (CPB) RNAAlfalfa mosaic virus coat protein binding (CPB) RNAThe Alfalfa mosaic virus coat protein binding RNA is an RNA element which is found in the 3' UTR of the genome. AMV CPB can stimulate the translation of AMV RNA by between 50 and 100-fold. This family contains at least two coat protein binding sites which are thought to be essential for efficient...
- Alfalfa mosaic virus RNA 1 5' UTR stem-loopAlfalfa mosaic virus RNA 1 5' UTR stem-loopThe Alfalfa mosaic virus RNA 1 5' UTR stem-loop represents a putative stem-loop structure found in the 5' UTR in RNA 1 of alfalfa mosaic virus. RNA 1 is responsible for encoding the viral replicase protein P1. This family is required for negative strand RNA synthesis in in the alfalfa mosaic virus...