Alfalfa mosaic virus

Taxobox
virus_group = IV
familia = "Bromoviridae"
genus = "Alfamovirus"
species = "Alfalfa mosaic virus"

Alfalfa mosaic virus (AMV), 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. [cite web
title=Alfalfa mosaic virus
author=ICTVdB Management
publisher=In: ICTVdB - The Universal Virus Database, version 4. Büchen-Osmond, C. (Ed), Columbia University, New York, USA
year=2006
url=http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.010.0.01.001.htm ] [cite web
title=Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist
author=Bisby FA, Roskov YR, Orrell TM, Nicolson D, Paglinawan LE, Bailly N, Kirk PM, Bourgoin T, van Hertum J, eds
publisher=Reading, U.K.
year=2008
url=http://www.catalogueoflife.org/annual-checklist/2008/]

tructure and genome

The virion has a capsid (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. [cite book
title=Plant pathology
author=Agrios G. N.
publisher=San Diego, Academic Press, 635 p
year=1997] [cite journal
title=The structure of alfalfa mosaic virus capsid protein assembled as T=1 icosahedral particle at 4.0-Å resolution
author=Kumar A., Reddy V. S., Yusibov V., Chipman P. R., Hata Y., Fita I., Fukuyama K., Rossmann M. G., Loesch-Fries L. S., Baker T. S., Johnson J. E.
publisher=Journal of Virology 71:7911- 7916
year=1997
url=http://jvi.asm.org/cgi/reprint/71/10/7911.pdf] 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. [cite journal
title=Spatial determinants of the alfalfa mosaic virus coat protein binding site
author=Laforest S.M., Gehrke L.
publisher=RNA 10:48- 58
year=2004
url=http://www.rnajournal.org/cgi/reprint/10/1/48
doi=10.1261/rna.5154104
journal=RNA
volume=10
pages=48] [cite journal
title=Genetic dissection of the multiple functions of alfalfa mosaic virus coat protein in viral RNA replication, encapsidation, and movement
author=Tenllado F., Bol J.
publisher=Virology 268:29-40
year=2000] [cite journal
title=Purification, characterization, assembly and crystallization of assembled alfalfa mosaic virus coat protein expressed in "Escherichia coli"
author=Yusibov V., Kumar A. , North A., Johnson J.E., Loesch-Fries L.S.
publisher=Journal of General Virology 77: 567- 573
year=1996
url=http://vir.sgmjournals.org/cgi/reprint/77/4/567.pdf]

Replication cycle

The AMV cycle can be split up in 5 steps:
* 1^st 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).
* 2^nd 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.
* 3^rd 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).
* 4^th step: Minus- strand RNAs are synthesized.
* 5^th step: Plus- strand RNAs and viral proteins are synthesized. Virions assemble.(Most details of the replication cycle are still unknown). [cite journal
title=Alfalfa mosaic virus: coat protein-dependent initiation of infection
author=Bol J.F.
publisher=Molecular Plant Pathology 4(1): 1–8
year=2003]

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 and chloroplast of the infected plant (as inclusion bodies).

"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. [cite journal
title=Some phenoma observed in systemic infection of alfalfa mosaic virus: the influences of air temperature and shading on symptom appearance
author=Kudo A., Misawa T.
publisher=Tohoku Journal of Agricultural Research 22: 199- 206
year=1971] [cite journal
title=Factors influencing the infection of cowpea mesophyll protoplasts by alfalfa mosaic virus
author=Alblas F., Bol J.F.
publisher=J.gen. Virol. 36: 175-185
year=1977
url=http://vir.sgmjournals.org/cgi/reprint/36/1/175.pdf] [cite journal
title=Effect of alfalfa mosaic virus (AMV) on the content of some macro- and micronutrients in alfalfa
author=Yardımcı N, Eryiğit H., Erdal I
publisher=Journal of culture collections 5: 90- 93
year=2006-2007
url=http://www.nbimcc.org/JCC/2007/JCC07513/JCC07513.pdf] [cite web
title=Alfalfa mosaic virus
author=Jaspars E.M.J., Bos L.
publisher=AAB Descriptions of Plant Viruses
year=1980
url=http://www.dpvweb.net/dpv/showdpv.php?dpvno=229]

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. [cite journal
title=Characterization and partial nucleotide sequence analysis of alfalfa mosaic alfamoviruses isolated from potato and azuki bean in Korea
author=Hyo Won Jung, Hye Jin Jung, Wan Soo Yun, Hye Ja Kim, Young ll Hahm, Kook-Hyung Kim, Jang Kyung Choi
publisher=The Plant Pathology Journal 16(5):269- 279
year=2000
url=http://www.riceblast.snu.ac.kr/file/44/43113_16(5)-5.pdf]

Transmission

The vectors are insects of the order "Hemiptera", 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.

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. [cite journal
title=Occurrence of alfalfa mosaic virus in Prince Edward Island
author=McDonald J. G., Suzuki M.
publisher=Canadian Plant Disease Survey 63:47- 50
year=1983
url=http://www.cps-scp.ca/download/cpds-archive/vol63/CPDS_Vol_63_No_2_(47-50)1983.pdf]

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. [cite journal
title=Temperature pulse viruses: alfalfa mosaic virus (AMV)
author=Freeman A., Aftab M.
publisher=Agriculture notes p. 1- 3
year=2006] 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. [cite book
title=Molecular breeding of forage crops
author=Spangenberg G.
publisher=Victoria, Springer, 356 p
year=2001]

References