Tomato

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Taxonomy
Geminiviruses are plant viruses that belong to the family Geminiviridae, first described by Goodman in 1977 (Goodman, 1977a, 1977b). Geminiviruses are characterized by the unique Gemini shape of a fused icosahedral viral particle.
The geminate virions consists a circular single-stranded DNA (ssDNA) genome. The family Geminiviridae. is comprised of three genera, all of which share similarities in
genome organization, insect transmission, and host range.
The genus Mastrevirus
Consists of geminiviruses with a monopartite genome, and the Mastreviruses are transmitted by leafhoppers, in most cases by a single species in a persistent, circulative, non-propagative manner.
The genus Curtovirus
Includes viruses with monopartite genomes, transmitted by leafhoppers or treehoppers in a persistent, circulative, non-propagative manner. Curtoviruses have very wide host ranges
The genus Begomovirus
Consists viruses with monopartite and bipartite genomes. Begomoviruses are transmitted by whiteflies in a persistent, circulative, non-propagative manner, and infect dicotyledonous plants. Bean golden mosaic virus (BGMV) is the type species.

Geminivirus Genome Organization
The geminivirus genome is organized in one (monopartite) or two (bipartite) covalently closed, circular, ssDNA molecules of about 2.5 - 2.9 Kb (Lazarowitz, 1992).
The genes in monopartite and bipartite geminiviruses are arranged in two divergent clusters 280 to 350 nucleotides each separated by the intergenic region (IR)each. The single genomic component of monopartite geminiviruses (mastreviruses and curtoviruses) contains all the information necessary for virus replication and infectivity
(Lazarowitz, 1992; Hanley-Bowdoin et al., 1996). Bipartite begomoviruses have seven genes distributed in the two genomic components designated A and B. The A component contains genes involved in virus replication and encapsidation, and the B component contains the genes involved in virus movement (Lazarowitz, 1992). The A and B components each have a common region, which consists of a block of approximately 200bp within the IR (Sunter and Bisaro, 1991; Lazarowitz, 1992). The common regions are virtually identical in sequence in a given bipartite begomovirus, but are completely different in sequence among the other geminiviruses, with the exception of a 30 nucleotide conserved region (stem loop) that has been identified as the origin of replication (Sunter and Bisaro, 1991). The common region also contains two divergent promoters which differentially regulate the temporal expression of the viral genes (Lazarowitz, 1992).

Begomoviruses
With the exception of tomato yellow leaf curl virus (TYLCV), which consists of only one component (Lazarowitz, 1992), the genomes of begomoviruses are made up of two components, each 2.5 - 2.8 Kb in size. The A component of begomoviruses typically
has one gene in the virion sense and four genes in the complementary sense (Lazarowitz,
1992).
AV1 (virion sense) encodes the coat protein, AC1 (complementary sense) encodes the replication-associated protein (Rep) (Elmer et al., 1988), and AC2encodes the transcriptional activator protein (TrAP) that transactivates the expression of the coat protein gene and the BV1 movement gene of the B component (Sunter and Bisaro, 1991). AC3 encodes the replication enhancer protein (Ren) that regulates the virus replication rate, possibly via the activation of an early gene (AV1) required for DNA synthesis (Azzam et al., 1994). AC4 encodes a protein which is a determinant of symptom expression in monopartite begomoviruses (Rigden et al., 1994). The B component has two genes, designated BC1 and BV1. The product of BV1 is localized in the cell nucleus and binds ssDNA, allowing the newly formed virus genome to be transported to the cytoplasm (Pascal et. al 1993; Pascal et. al 1994). The BC1 product has been extracted from cell wall and cellular membrane fractions, and its function is to increase the exclusion limit of plasmodesmata to facilitate cell to cell movement of the virus (Pascal et al., 1993). Both movement proteins define the viral host range but only BC1etermines symptom severity and pathogenicity in bipartite begomovirus (Ingham et al., 1995; Duan et al., 1997b).
Serological tests showed that all begomoviruses are related. In addition, there is a
group of epitopes unique to the begomoviruses which infect crops in the Old World
(Europe, Africa, and Australasia), and a distinct set of epitopes is shared by
begomoviruses that infect crops in the New World (North, Central and South America)
(Thomas et al., 1986).

Geminivirus Replication

Geminivirus replication strategy
The ssDNA genomes of geminiviruses replicate in the nucleus of infected cells via a rolling circle mechanism using a dsDNA intermediate (Saunders et al., 1991; Stenger et al., 1991). The replication process is analogous to that used by ssDNA phages, such as ØX174 (Kornberg and Baker, 1992) and ssDNA plasmids such as pT181 and pC194 (Gros et al., 1987). Moreover, sequence comparisons have shown that the geminiviral replication-associated proteins are DNA binding proteins and are related to proteins involved in the initiation of replication of some ssDNA plasmids (pMV158 family) (Koonin and Ilyina, 1992).
The origin of replication includes a conserved 30 nucleotide putative stem-loop element that is present in all geminiviruses (Revington et al., 1989). A 5'- TAATATTAC- 3' motif, present in the stem-loop element, is analogous to the A protein replication cleavage sequence in phage 1X140 (Saunders et al., 1991; Stenger et al., 1991; Arguello-Astorga et al., 1994). The specific binding site for the replication associated-protein in TGMV and SqLCV has been mapped to a region of about 60 nucleotides upstream of the stem-loop element (Fontes et al., 1994; Lazarowitz et al., 1992).
The Rep protein is a multifunctional protein that binds double-stranded DNA, catalyzes cleavage and ligation of single-stranded DNA and forms oligomers (Orozco and Hanley-Bowdoin, 1998). Rep protein initiates the replication cycle by making a single stranded-cleavage of the virion sense strand at the TAATATTAC sequence in the origin of replication. After the DNA cleavage and strand transfer reaction at the origin of replication, the Rep protein become covalently linked to the 5' end of the cleaved DNA
(Laufs et al 1995). Luafs et al. (1995) demostrated that in TYLCV tyrosine-103, located in the stem loop motif initiates DNA cleavage and is the physical link between Rep protein and its origin DNA. Orozco and Hanley-Bowdoin (1998) showed that the DNA binding motif in the Rep protein of TGMV is located between amino acids 1 and 130.
The recognition of the origin of replication by the TGMV Rep protein depends on a domain located between amino acids 121 and 200. The transcriptional specificity is conferred primarily by amino acids 1 to 193 (Gladfelter et al., 1997). The synthesis of ssDNA is regulated by the synergistic activity of TrAp and Ren proteins that act as activator of transcription and enhancer of replication, respectively (Sunter and Bisaro, 1991).
It has been proposed that geminiviruses depend on cellular factors to complete their replicative cycles (Xie et al., 1999). A family of proteins termed GRAB (for geminivius Rep-A binding) has been shown to bind to the Rep A protein of wheat dwarf geminivirus. Two members of the family, GRAB 1 and GRAB 2, have been characterized (Xie et al., 1999). The N-terminal domain of GRAB proteins exhibit a significant amino acid homology to the NAC domain present in proteins involved in plant development and senescence.

Cytological effects of geminivirus replication
The replication of geminiviruses induces micro-structural changes in the nucleus of the host cells. Ultrastructural studies of Jatropha gossypifolia, infected with the whitefly- transmitted begomovirus, jatropha mosaic virus, showed fibrillar bodies and virus-like particles in the nuclei of phloem-associated parenchyma cells and sieve elements (Kim et al., 1986). The fibrillar bodies consists of two structural components with different electron densities: the highly electron-dense beads and the less electrondense matrix (Kim et al., 1986). Light microcopy studies of leaf tissue of plants infected with BGMV and of lima bean golden mosaic, euphorbia mosaic, malvaceous chlorosis, and rhyncosia mosaic begomoviruses revealed nuclear inclusions that appear as large blue- violet bodies when the tissue is stained with azure A (Christie et al., 1986). These inclusions consist of aggregated virus particles. Small, ring-shaped blue-violet, inclusions were also observed in the nuclei of the phloem parenchyma cells. The nuclear inclusions were not observed in stained tissues of non-infected host plants (Christie et al., 1986).
Pinner et al. (1990) observed four types of cytoplasmic inclusions induced by maize streak virus and serologically related isolates: crystalline, non-crystalline, sheet-like and open lattice. These distinctions enable certain geminiviruses to be identified at the strain level.

Movement
The establishment of a virus infection depends upon the spread of the virus through the plant host. The movement of the virus in the plant occurs at two different levels: a) short distance cell- to -cell movement and b) long-distance movement that involves delivery of the virus to distal parts of the plant by the vascular system (Lazarowitz, 1992). The BV1 and BC1 genes encode movement proteins in bipartite geminiviruses. Studies of SqLCV (Pascal et al., 1994) and bean dwarf mosaic virus (Noueiry et al., 1994) have shown that the BV1 and BR1 products act in a cooperative manner to move the viral genome from the nucleus to the cytoplasm and across the wall cell to a contiguous cell. It has been proposed that BV1 is a nuclear shuttle protein. BV1 binds newly replicated ssDNA viral genomes and transports them to the cytoplasm (Pascal et al., 1994: Sanderfoot et al., 1996). Then, the BV1-genome complexes are directed to the cell periphery through interactions with the BC1 product (Sanderfoot et al., 1996; Sanderfoot and Lazarowitz, 1995). It has been suggested that the BC1 protein allows the movement of BV1-genome complexes from one cell to the next by increasing the exclusion limit of plasmodesmata (Sanderfoot et al., 1996). The synthesis of BV1 is regulated at the transcriptional level by AC2 transactivation (Sunter and Bisaro, 1991).

Whitefly-Transmission of Geminiviruses
Bemisia tabaci B - biotype
Begomoviruses are transmitted by the sweetpotato whitefly, Bemisia tabaci (Gennadius). B. tabaci was first described in the genus Aleyrodes in 1889 (Gennadius, 1889), and was first reported as a pest in 1919 in India (Husain and Trehan, 1933). Since then, B. tabaci has been recognized as a pest of crops in tropical and subtropical countries. B. tabaci has a very wide host range, consisting of 500 species in 74 plant families (Greathead, 1986). The whitefly is a vector of viruses in the Geminiviridae, Potyviridae and Comoviridae families and the genera Carlavirus and Closterovirus.
Approximately 60 different geminiviruses have been reported to be transmitted by B.
tabaci (Markam et al., 1994). In the New World before 1986, B. tabaci was considered a pest of a limited number of crops (tobacco, cotton, potato, bean, soybean), but by 1986 a sudden increase of the whitefly population in ornamentals in Florida was observed (Osborne, 1988).
Shortly after that, whiteflies were reported in other crops in Florida (Schuster et al., 1991), in California (Perring et al., 1991), and in Texas, Arizona, Central America and South America (Brown, 1994). In Florida the whitefly infestation was associated with silverleaf of squash and irregular ripening of tomato (Maynard and Cantliffe, 1989). The whitefly population causing these disorders was physiologically, behaviorally, reproductively and genetically different from the population that was present before 1989 in California and Arizona, and was first called the “poinsettia strain”. Later, this whitefly became known as the B strain or B biotype. Perring et al. (1991) suggested that A and B strains were separate species, and thus, named the B strain (or B biotype) the “silverleaf whitefly, B. argentifolii (Bellows et al., 1994). The B biotype has a very wide host range, which has contributed to the spread of geminiviruses to new hosts (Bedford et al., 1993) and the outbreak of apparently new geminiviruses (Polston and Anderson, 1997).

Characteristics of whitefly transmission
Whitefly-transmitted geminiviruses affect a wide variety of vegetable crops worldwide.
In the 1930s, the first transmission of geminiviruses by whiteflies was demonstrated with tobacco leaf curl virus and African cassava mosaic virus in tobacco and cassava, respectively (Storey, 1934; Storey, 1936; Storey and Nichols, 1938).
Geminivirus transmission by B. tabaci is circulative and non-propagative (Duffus, 1987). Whiteflies can acquire and inoculate bipartite begomoviruses in short periods of time (10 min), but the efficiency of acquisition increases when the feeding period increases up to 24 h. Latent periods of four to 21 h between virus acquisition and the ability of the whitefly to transmit have been observed (Duffus, 1995) Studies of the transmission of TYLCV, a monopartite begomovirus, showed that whitefly feeding periods of 4 h or longer were necessary to achieve TYLCV transmission rates near to 90% (Zeidan and Czeszcnek, 1991).
Hunter et al. (1998) established the location of tomato mottle begomovirus (ToMoV) and cabbage leaf curl begomovirus (CaLCV) in various tissues of B. tabaci Bbiotype by immunofluorescent labeling of viral coat protein in freshly dissected whiteflies. Hunter et al. (1998) proposed the following model for the movement of begomoviruses in the whitefly vector: virus particles are ingested along with plant fluids into the whitefly esophagus and foregut, after which nutrients and begomoviruses are concentrated in the filter chamber. Begomovirus particles adsorb to specific sites on the alimentary membrane or to sites along the anterior region of the midgut. Begomovirus particles move out of these tissues into the hemolymph, eventually invading the salivary glands.

Economic Impact of Whitefly-Transmitted Geminiviruses
As early as the 1950s, there were reports of a correlation between the presence of B. tabaci and plant diseases characterized by foliar malformation, leaf curling, stunting and yellow mosaic in a variety of crops and weeds in the Americas and the Caribbean basin (Brown and Bird, 1992). Many of those diseases were later determined to be caused by geminiviruses (Brown and Bird, 1992). Until the early 1990s, whitefly-transmitted geminiviruses were primarily a problem in legume production in the Western Hemisphere. Since then, high incidences of geminivirus diseases in tomato-producing areas of Florida, the Caribbean, Mexico, Central America, Venezuela, and Brazil have been reported (Polston and Anderson, 1997). Currently, at least 17 geminiviruses have been reported infecting tomato in the Americas and Caribbean region i. e. chino del tomato virus, tomato leaf crumple virus, pepper huasteco virus, potato yellow mosaic virus, Sinaloa tomato leaf curl virus, Texas pepper virus, pepper jalapeno virus, TYLCV, ToMoV, serrano golden mosaic virus, tomato geminivirus BZ-Ub, tomato geminivirus BZ-Ig, TGMV, tomato yellow mosaic virus, tomato yellow streak virus, Tom GV1 virus, and Tom GV2 virus, with incidences ranging from 20 to 100% and causing crop losses up to 100% (Polston and Anderson, 1997).
Tomato geminiviruses have been reported to cause important losses in the tomato producing areas of the Caribbean basin and Florida (Polston and Anderson, 1997). The crop damage due to geminiviruses in the Dominican Republic between 1988 and 1995 ranged from 5 to 95%, and the economic losses from 1989 to 1995 were estimated at $50 million (Alvarez and Abud-Antún, 1995). Tomato geminiviruses caused losses estimated at $ 4.6 million in the Comayagua Valley of Honduras in 1992 (Caballero and Rueda, 1993). In Venezuela, the area of tomato production was reduced by 50% due to losses caused by tomato yellow mosaic virus (Salas and Mendoza, 1995). In central America geminiviruses are thought to be responsible for a significant portion of the crop losses estimated at $40 million from 1989 to 1995 (Bird et al., 1995). The yields of the tomato crop in Florida have been adversely affected by whitefly-transmitted geminiviruses. In 1990 to 1991, crop losses due to ToMoV were estimated at $140 million. (Schuster, 1992).In Florida and the Caribbean basin diseases caused by whitefly-transmitted geminivirus diseases are also serious concerns for many different crops such as beans, cassava, tobacco, potato, cotton, pepper, squash, and cabbage (Polston and Anderson, 1997).

Readings
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Prepared by Mohd. K. Abhary (mohammedjabri@hotmail.com)