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''Geminiviridae'' is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: bright yellow mosaic, yellow mosaic, yellow mottle, leaf curling, stunting, streaks, reduced yields. They have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication (i.e. geminivirus genomes are ambisense). According to the Baltimore classification they are considered class II viruses. It is the largest known family of single stranded DNA viruses. Mastrevirus and curtovirus
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...
is via various
leafhopper A leafhopper is the common name for any species from the family Cicadellidae. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or trees. Their hind legs are modified for jumping, and ...
species (e.g.
maize streak virus ''Maize streak virus'' (MSV) is a virus primarily known for causing maize streak disease (MSD) in its major host, and which also infects over 80 wild and domesticated grasses. It is an insect-transmitted maize pathogen in the genus ''Mastrevirus ...
and other African streak viruses are transmitted by ''
Cicadulina mbila ''Cicadulina mbila'', the maize leafhopper, is a leafhopper species in the genus '' Cicadulina''. It is found in sub-Saharan Africa (Kenya, Tanzania and Uganda), the Middle East, West Asia and India. It is one of the vectors of the maize strea ...
''), the only known topocuvirus species, '' Tomato pseudo-curly top virus'', is transmitted by the treehopper '' Micrutalis malleifera'', and begomoviruses are transmitted by the
whitefly Whiteflies are Hemipterans that typically feed on the undersides of plant leaves. They comprise the family Aleyrodidae, the only family in the superfamily Aleyrodoidea. More than 1550 species have been described. Description and taxonomy The ...
species, ''
Bemisia tabaci The silverleaf whitefly (''Bemisia tabaci'', also informally referred to as the sweet potato whitefly) is one of several species of whitefly that are currently important agriculture, agricultural pest (animal), pests. A review in 2011 concluded t ...
''. These viruses are responsible for a significant amount of crop damage worldwide. Epidemics of geminivirus diseases have arisen due to a number of factors, including the recombination of different geminiviruses coinfecting a plant, which enables novel, possibly virulent viruses to be developed. Other contributing factors include the transport of infected plant material to new locations, expansion of agriculture into new growing areas, and the expansion and migration of vectors that can spread the virus from one plant to another.


Virology

The genome of ssDNA can either be a single component between 2500–3100
nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecu ...
s, or, in the case of some begomoviruses, two similar-sized components each between 2600 and 2800 nucleotides. They have elongated, geminate
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or ma ...
s with two incomplete T=1
icosahedra In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
joined at the missing vertex. The capsids range in size from 18–20 nm in
diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid f ...
with a length of about 30 nm. Begomoviruses with two component (i.e. bipartite) genomes have these components separated into two different particles both of which must usually be transmitted together to initiate a new infection within a suitable host
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
.


Taxonomy

The following genera are recognized: *'' Becurtovirus'' *'' Begomovirus'' *'' Capulavirus'' *'' Citlodavirus'' *'' Curtovirus'' *'' Eragrovirus'' *'' Grablovirus'' *'' Maldovirus'' *'' Mastrevirus'' *'' Mulcrilevirus'' *'' Opunvirus'' *'' Topilevirus'' *''
Topocuvirus ''Topocuvirus'' is a genus of viruses, in the family ''Geminiviridae''. Dicotyledonous plants serve as natural hosts. There is only one species in this genus: ''Tomato pseudo-curly top virus''. Diseases associated with this genus include: vein s ...
'' *''
Turncurtovirus ''Turncurtovirus'' is a genus of viruses, in the family ''Geminiviridae''. Dicotyledonous plants serve as natural hosts. There are three species in this genus. Structure Viruses in ''Turncurtovirus'' are non-enveloped, with icosahedral geometri ...
'' Several additional genera have been proposed: Baminivirus, Nimivirus and Niminivirus.Ng TF, Marine R, Wang C, Simmonds P, Kapusinszky B, Bodhidatta L, Oderinde BS, Wommack KE, Delwart E (2012) High variety of known and new RNA and DNA viruses of diverse origins in untreated sewage. J Virol


Replication

Geminivirus genomes encode only a few proteins; thus, they are dependent on host cell factors for replication: these include factors such as
DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to crea ...
—and probably repair polymerases—in order to amplify their genomes, as well as transcription factors. Geminiviruses replicate via a rolling circle mechanism like
bacteriophages A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacter ...
such as M13, and many plasmids. Replication occurs within the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
of an infected plant cell. First the single-stranded circular DNA is converted to a double-stranded circular intermediate. This step involves the use of cellular DNA repair enzymes to produce a complementary negative-sense strand, using the viral genomic or plus-sense DNA strand as a template. The next step is the rolling circle phase, where the viral strand is cleaved at a specific site situated within the
origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by se ...
by the viral Rep protein in order to initiate replication. This process in a eukaryotic nucleus can give rise to concatemeric double-stranded forms of replicative intermediate genomes, although double-stranded unit circles can be isolated from infected plants and cells. New single-stranded DNA forms of the virus genome (plus-sense) are probably formed by interaction of the coat protein with replicating DNA intermediates, as genomes lacking a CP gene do not form ssDNA. The ssDNA is packaged into germinate particles in the nucleus. It is not clear if these particles can then leave the nucleus and be transmitted to surrounding cells as virions, or whether ssDNA associated with coat protein and a movement protein is the form of the genome that gets trafficked from cell to cell via the
plasmodesmata Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Plasmodesmata evolved independently in several lineages, and spec ...
. These viruses tend to be introduced into and initially infect differentiated plant cells, via the piercing mouthparts of the vector insect: however, these cells generally lack the host enzymes necessary for DNA replication, making it difficult for the virus to replicate. To overcome this block geminiviruses can induce plant cells to reenter the
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and sub ...
from a quiescent state so that viral replication can occur.


Evolution

These viruses may have evolved from a
phytoplasma Phytoplasmas are obligate intracellular parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-lik ...
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...
. Geminiviruses are capable of
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
of genetic information to the plant host.


References


External links


ICTV Online (10th) Report; Geminiviridae

Description of Plant Viruses




{{Taxonbar, from=Q2554769 Viral plant pathogens and diseases Virus families