GenusBecurtovirusBecurtovirusesare transmitted by leafhoppers and infect dicots causing curly top disease.
Thebiological characteristics of the becurtoviruses are similar to members of the genusCurtovirus. They have a monopartitegenome which encodes five proteins and genome organization resembles that of thegenus Mastrevirus (Sahu et al., 2013). GenusCapulovirusCapuloviruses have four proteins in thevirion sense strand and their arrangements are complex with respect to othergenus. The ORF MP (movement protein) overlaps with the CP (coat protein) ORFand two or more MP ORFs constitute an intron containing MP.
Capuloviruses have three proteins in theircomplementary sense strand in which, ORFs C1 and C2 overlaps with each otherand predicted to encode replication-associated protein (Rep) from a splicedtranscript. As similar to mastreviruses, it is possible that capuloviruses mayexpress a RepA protein from an unspliced transcript. The other ORF C3 completelylies within the ORF C1 (Fig.
1.1). Capuloviruses have nonanucleotide motifTAATATTAC at their origin of replication (Bernardo et al. 2013). Four species: Alfalfa leaf curl virus, Euphorbiacapt-medusae latent virus, French bean severe leaf curl virus, Plantago lanceolatalatent virus are identifiedin this genus. Of these, Alfalfaleaf curl virus is shown to transmit by aphid (Roumagnac et al.
2015). Novector has been identified for the other three species in this genus.GenusCurtovirusCurtoviruses are transmitted byleafhoppers, infect dicotyledonous plants and have a monopartite genome(Stanley et al. 2005). Curtoviruseshave a genome size of nearly 3 kb with an intergenic region that contains the originof replication and encodes seven proteins bidirectionally (Fig. 1.1).
The three proteins V1 (capsid protein, CP), V2 (a single stranded (ss)/double stranded (ds) DNA regulator) and V3 (a putative movement protein, MP) inthe virion–sense and four proteins C1 (the replication associated protein,Rep), C2 (a pathogenicity- associated protein involved in a recovery phenotype),C3 (a replication enhancer protein, REn), and C4 (a protein affecting celldivision and symptom development) in the complementary-sense (Briddon et al.,1990). Beet culy topvirus (BCTV) belongs to this genus.GenusEragrovirusEragroviruses are monopartite geminiviruses with a uniqueTAAGATTCC virion strand origin of replication and infect dicots. The genome of eragroviruses encodes two proteins in the virion sense V1 (Coat protein) and V2 (Movement protein) and two in the complementary sense, C1 (Replication initiation protein) and C2 (possible transcription activator protein). The insect vector is yet to be identified (Sahu et al., 2013). Genus GrablovirusGrabuloviruses are transmitted by treehoppers, have a monopartite genomewith 3.
2 kb which is significantly larger than those of other monopartitegeminiviruses (2.7-3.0 kb) (Bahder et al. 2016). The genome of grabulovirusesencodes three proteins in the virion sense V1 (Coat protein), V2 and V3(Movement protein) and three in the complementary sense, C1 and C2 (Replicationassociated protein), C3 protein function is unknown and is completely nested inthe ORF of C1 (Sudarshana et al. 2015; Krenz et al. 2014) (Fig.
1.1). Grapevine red blotch virus is a memberof this genus. GenusMastrevirusMastreviruses are transmitted byleafhoppers, have a monopartite genome and mostly infect monocotyledonousspecies.
The genus includes Maize streak virus (MSV) and Wheat dwarf virus (WDV). Some members Tobacco yellow dwarf virus (TYDV), and Bean yellow dwarf virus (BeYDV) infect dicotyledonous plants(Morris et al. 1992; Liu et al. 1998). There are two intergenic regions: the long intergenic region (LIR) andthe short intergenic region (SIR), required for completing the DNA replicationcycle. Mastrevirus genome encodes four proteins:RepA protein (exclusive to this genus) and Rep protein on the complementarysense strand and the movement protein (MP) and the coat protein (CP) on theviral sense strand (Palmer and Rybicki, 1998).GenusTopocuvirusThe topocuvirusesinfect dicots and aretransmitted by tree hoppers.
They have monopartite genomewhich encodes six proteins. The coat protein has features of the leafhopper-transmitted mastreviruses whereas the organization of thecomplementary-sense genes is similar to that of the single-componentbegomoviruses. Therefore it might have arisen due tothe recombination between a curtoviruses andbegomoviruses (Briddon et al., 1996; Rojas et al, 2005). Tomato pseudo curly top virus belongs to this genus.Genus Turncurtovirus Turncurtovirus constitutes a single member (Turnipcurly top virus) with a monopartite genome which closely resembles membersof Curtovirus. The genome comprisesof six ORFs rather than seven ORFs: V1 (Coatprotein) and V2 (Movementprotein) in the virion sense and C1 (Replication initiation protein), C2 (Transcription activator protein), C3 (Replicationenhancer protein) and C4 (Symptom determinant) in the complementary sense. The insect vector is yet to be identified (Sahu et al.
, 2013). Genus BegomovirusBegomovirus is the largest genus of family Geminiviridaeand is transmitted by the whitefly Bemicia tabaci and infectsdicotyledonous plants. The name of the genus Begomovirus is derived from the firsttwo letters of the each word of the type species, Bean golden mosaic virus (BGMV) causing golden mosaic disease in bean in CentralAmerica.
Begomoviruses are transmitted by only one vector species, whitefly (Bemisia tabaci). About 68.1% of geminiviruses belong to the genus Begomovirus. Presently,322 virus species have beenofficially recognised under the genus Begomovirus, which is the maximumnumberof members so far known in any genera of plant viruses. Symptoms of begomoviruses infected plants areyellow mosaic, leaf distortion, curling and stunting. Infection in earlyseedling stage leads to poor fruit set and infertile seeds, resulting in severeyield loss.
Begomoviruses infect a large number of dicots such as bhendi,cassava, cotton, legumes, tomato, chilli, and many more. Some of the diseaseslike bhendi yellow vein mosaic, okra enation leaf curl, cassava mosaic andcotton leaf curl are known for century and cause huge economic loss (Varma andMalathi, 2003). Genus Begomovirus consists of bothbipartite and monopartite members. Based on the geographical distributionbegomoviruses are classified as “Old world” (Eurasia, Africa and Australasia)or “New World” (America).
Most ofthe “Old World” and all the “New World” begomoviruses have bipartite genomeswhereas some of the “Old World” begomoviruses have monopartite genomes. Thegenome of bipartite begomoviruses consists of two genomic components,designated DNA A and DNA B which arecovalently closed, circular, 2.5- to 2.9-kb-long single-stranded (ss) DNAmolecules (Lazarowitz, 1992).
DNA A depends on DNA B for intracellular andintercellular movement. DNA B depends on DNA A for replication andencapsidation (Rogers et al., 1986;Sunter et al., 1987; Townsend et al., 1986). Inorder to facilitate recognition of DNA B by the Rep encoded by DNA A. a segmentof 110-200 nt sequences present within the intergenic region are highlyconserved between DNA A and DNA B component. This region is denoted as commonregion (CR) and is highly identical between DNA A and DNA B component.
Theintergenic region consists of repetitive elements called iterons upstream ofthe highly conserved stem-loop structure. The iterons, represent the bindingsites of Rep. The invariant nonanucleotide sequence TAA TAT TAC present in theloop is converased in all geminiviruses and the nicking between seventh andeight nucleotide by Rep protein is proved to initiate replication. The bipartite begomoviruses have five or sixORFs on DNA A, one or two on the virion-sense strand and four or five in thecomplementary sense strand. The DNA B component encodes two ORFs one each inthe complementary and virion sense respectively. InDNA A, AC1 encodes Rep which is involved in thereplication of the virus genome, AC2 encodes a transcription activator protein(TrAP) which activates transcription from AV1, BC1 and BV1 promoters(Sunter and Bisaro, 1991; Shivaprasad etal., 2005), AC3 encodes aReplication enhancer (REn)protein that increases the efficiency of viral replication (Settlage et al.
, 2005) and AC4 protein is believed to be involved incontrolling the cell cycle progression and is a symptom determinant and suppressor ofpost-transcriptional gene silencing (Bisaro, 2006; Vanitharani et al., 2004). AV1encodes the coat protein, and AV2 encodes the pre-coat protein. AV2 is notpresent in “New World” begomoviruses.
BV1encodes the Nuclear shuttle protein (NSP) and BC1encodes the Movement protein (MP) involved in cell to cell movement (Rojas etal., 2005). Tomato golden mosaic virus (TGMV), SriLankan cassava mosaic virus (SLCMV), Mungbean yellow mosaic virus (MYMV), African cassava mosaic virus (ACMV)belong to this genus.