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Begomoviruses are monopartite or bipartite, whitefly-transmitted geminiviruses that are found in the Old (both genome types) and New Worlds (mostly bipartite genomes, with one recently described monopartite genome virus) (Brown et al., 2015). The whitefly vector Bemisia tabaci (Genn.) is currently considered to be a complex of cryptic species (De Barro et al., 2011). Whitefly vector specificity is associated with specific amino acid sequences in the viral coat protein (Briddon et al., 1990). Most monopartite begomoviruses are associated with two types of satellite molecules known as betasatellites and alphasatellites (Zhou 2013). Alphasatellites are related to the Rep-encoding components of nanoviruses. Betasatellites contain sequences non-homologous to viruses except for a TAATATT/AC sequence located in the loop of a potential stem-loop structure within the IR. The contribution of alphasatellites to the begomoviral infection cycle is not clear. Betasatellites in many cases modulate virulence by suppression of host gene silencing. Begomoviruses infect only dicot hosts. Bipartite begomoviruses encode between seven and eight proteins, and monopartite begomoviruses encode five or six proteins.
See discussion under family description.
The genomes of bipartite begomoviruses consist of two components, referred to as DNA-A and DNA-B, each of 2.5-2.6 kb. The DNA-A component of the bipartite begomoviruses can replicate autonomously and produce virions but requires the DNA-B component for systemic infection. Cognate DNA-A and DNA- B components share approximately 200 bp of of sequence within the intergenic region (termed the "common region", CR), encompassing the conserved stem-loop with the TAATATTAC sequence at the v-ori (Figure 1.Begomovirus). The organization of ORFs in the genomes of monopartite begomoviruses resembles that seen in the bipartite viral DNA-A component (Hanley-Bowdoin et al., 2000).
Figure 1.Begomovirus. Genomic organization of begomoviruses. The ORFs are denoted as being encoded on the virion-sense (V) or complementary-sense (C) strand, preceded by component designation (A or B) if bipartite (bottom). Corresponding protein products are indicated. ORF AV2/V2 is not present in begomoviruses from the New World. The "common region" (CRA and CRB; bottom) that are shared between the two genomic components of bipartite viruses are shown as grey boxes within the intergenic region (IR). The position of the stem-loop containing the conserved TAATATTAC sequence located in the IR is shown. CP, coat protein; Rep, replication-associated protein; TrAP, transcriptional activator protein; REn, replication enhancer protein; MP, movement protein; NSP, nuclear shuttle protein.
The DNA-A (and the monopartite genome) virion-sense strand encodes the coat protein (CP, cp ORF) that encapsidates the virion-sense ssDNA and may be involved in virus movement, and the AV2/V2 protein (ORF AV2/V2), which has also been implicated in virus movement. New World begomoviruses lack the AV2/V2 ORF. The DNA-A complementary-sense strand encodes the replication-associated protein (Rep, rep ORF), a transcriptional activator protein (TrAP, trap ORF), a replication enhancer protein (REn, ren ORF) and the AC4/C4 protein (AC4/C4 ORF) (Hanley-Bowdoin et al., 2000). Rep initiates viral DNA replication by binding to iterated motifs (iterons) within the intergenic region and introducing a nick into the conserved TAATATT/AC sequence (Laufs et al., 1995, Fontes et al., 1994). Rep also binds to the plant homologue of retinoblastoma protein (Rb) to regulate cell-cycle progression, altering the environment of terminally differentiated cells to provide host factors that support viral DNA replication (Arguello-Astorga et al., 2004). TrAP transactivates expression of virion-sense gene expression from both DNA-A and DNA-B, and also functions in the suppression of transcriptional and post-transcriptional gene silencing (Sunter and Bisaro 1992, Bisaro 2006). REn is required for efficient viral DNA replication (Sunter et al., 1990). AC4 is an important symptom determinant implicated in cell-cycle control, and C4 may counter a host response to Rep expression (Hanley-Bowdoin et al., 2013). The DNA-B encodes a nuclear shuttle protein (NSP, nsp ORF) on the virion-sense strand and a movement protein (MP, mp ORF) on the complementary-sense strand (Noueiry et al., 1994, Sanderfoot and Lazarowitz 1996).
Circular ssDNA satellites of approximately 1.3 kb are associated with many Old World monopartite begomoviruses and with some New World bipartite begomoviruses. Betasatellites (previously known as DNA-β), contain a single ORF termed βC1, the product of which has been shown to function as a suppressor of host gene silencing (Cui et al., 2005). This has the effect of dramatically increasing the virulence of the helper begomovirus. The only recognizable homologue of begomoviral sequence found in betasatellites is the stem-loop and TAATATTAC sequences. The remainder of the satellite sequence is otherwise not recognizably related to the helper begomovirus (Briddon and Stanley 2006). Betasatellites depend on the helper begomoviruses for their replication and encapsidation, and are, in some cases, essential for the maintenance of disease in the field (Mansoor et al., 2003, Saunders et al., 2000). They are thought to be promiscuous in that they may associate and function with more than a single helper begomovirus. Autonomously replicating nanovirus-like satellites are frequently associated with the begomovirus/betasatellite complexes, and are referred to as alphasatellites (previously known as DNA-1) (Saunders et al., 2000). At least three types of alphasatellites are known (Rosario et al., 2013). One type (type 1) has recently been shown to ameliorate symptoms when co-infecting plants with the helper virus and its associated betasatellite, suggesting that it down-regulates virulence to some degree, possibly by reducing the accumulation of betasatellite molecules (Idris et al., 2011). Alphasatellites of types 2 and 3 have been found in association with bipartite begomoviruses in the New World (Brazil, Cuba and Venezuela) (Paprotka et al., 2010, Romay et al., 2010).
Serological tests show all begomoviruses to be antigenically closely related. The use of monoclonal antibodies has shown that begomoviruses may be grouped geographically based on shared epitopes (Harrison and Robinson 1999).
Collectively, begomoviruses infect a wide range of dicotyledonous plants although, individually, most have limited host ranges (Rojas et al., 2005).
Begomoviruses are transmitted in nature by the whitefly B. tabaci, a cryptic species complex. Some begomoviruses are known to be differentially adapted for efficient transmission by their local B. tabaci species, but for the most part this proposed co-evolution has been poorly studied (Gilbertson et al., 2015). Some begomoviruses are transmissible by mechanical inoculation, although most require either Agrobacterium-mediated transfer (agroinoculation) from partially or tandemly repeated cloned genomic DNA or biolistic delivery of cloned genomic DNA for their experimental transmission (Rojas et al., 2005).
The following criteria should be used as a guideline to establish taxonomic status:
Due to the large number of species in the Begomovirus genus, the member species table is presented on a separate page here.
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