Geminiviridae

Mastrevirus Curtovirus Begomovirus Topocuvirus

Geminiviruses are plant viruses which have ambisense single-stranded circular DNA genomes and are members of class II of the Baltimore classification of viruses. The genome can either be a single component of 2500-3000 nucleotides, or two similar-sized components. They have an elongated, geminate capsid with two incomplete T=1 icosahedra joined at the missing vertex. The capsids range from 18-20 nm in diameter with a length of about 30nm. Viruses with bipartite genomes (begomoviruses only) have these components separated into two different particles, therefore more than one virus particle is required to infect a cell.

Transmission of these viruses can be via leafhoppers (mastreviruses, curtoviruses) or via one species of whitefly (begomoviruses) or via treehoppers (topocuviruses).

Purified Maize streak virus (MSV) particles stained with uranyl acetate. Size bar indicates 50 nm. Picture courtesy of Kassie Kasdorf.

The Geminiviridae include the following genera:
 * Genus Mastrevirus; type species: Maize streak virus
 * Genus Curtovirus; type species: Beet curly top virus
 * Genus Begomovirus; type species: Bean golden mosaic virus
 * Genus Topocuvirus; type species: Tomato pseudo-curly top virus

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.

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 - and probably repair polymerases - in order to amplify their genomes, as well as transcription factors. Geminiviruses replicate via a rolling circle mechanism like bacteriophages such as M13, and many plasmids. Replication occurs within the nucleus 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 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.

These viruses tend to be introduced into and initially infect differentiated plant cells, via the piercing mouthparts f 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 from a quiescent state so that viral replication can occur.