Genus are complex with respect to other genus.

Genus
Becurtovirus

Becurtoviruses
are transmitted by leafhoppers and infect dicots causing curly top disease. The
biological characteristics of the becurtoviruses are similar to members of the genus
Curtovirus. They have a monopartite
genome which encodes five proteins and genome organization resembles that of the
genus Mastrevirus (Sahu et al., 2013).

Genus
Capulovirus

Capuloviruses have four proteins in the
virion sense strand and their arrangements are complex with respect to other
genus. The ORF MP (movement protein) overlaps with the CP (coat protein) ORF
and two or more MP ORFs constitute an intron containing MP.  Capuloviruses have three proteins in their
complementary sense strand in which, ORFs C1 and C2 overlaps with each other
and predicted to encode replication-associated protein (Rep) from a spliced
transcript. As similar to mastreviruses, it is possible that capuloviruses may
express a RepA protein from an unspliced transcript. The other ORF C3 completely
lies within the ORF C1 (Fig. 1.1). Capuloviruses have nonanucleotide motif
TAATATTAC at their origin of replication (Bernardo et al. 2013). Four species: Alfalfa leaf curl virus, Euphorbia
capt-medusae latent virus, French bean severe leaf curl virus, Plantago lanceolata
latent virus are identified
in this genus. Of these, Alfalfa
leaf curl virus is shown to transmit by aphid (Roumagnac et al. 2015). No
vector has been identified for the other three species in this genus.

Genus
Curtovirus

Curtoviruses are transmitted by
leafhoppers, infect dicotyledonous plants and have a monopartite genome
(Stanley et al. 2005). Curtoviruses
have a genome size of nearly 3 kb with an intergenic region that contains the origin
of 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) in
the 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 cell
division and symptom development) in the complementary-sense (Briddon et al.,
1990). Beet culy top
virus (BCTV) belongs to this genus.

Genus
Eragrovirus

Eragroviruses are monopartite geminiviruses with a unique
TAAGATTCC 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 Grablovirus

Grabuloviruses are transmitted by treehoppers, have a monopartite genome
with 3.2 kb which is significantly larger than those of other monopartite
geminiviruses (2.7-3.0 kb) (Bahder et al. 2016). The genome of grabuloviruses
encodes three proteins in the virion sense V1 (Coat protein), V2 and V3
(Movement protein) and three in the complementary sense, C1 and C2 (Replication
associated protein), C3 protein function is unknown and is completely nested in
the ORF of C1 (Sudarshana et al. 2015; Krenz et al. 2014) (Fig. 1.1). Grapevine red blotch virus is a member
of this genus.

Genus
Mastrevirus

Mastreviruses are transmitted by
leafhoppers, have a monopartite genome and mostly infect monocotyledonous
species.  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) and
the short intergenic region (SIR), required for completing the DNA replication
cycle.  Mastrevirus genome encodes four proteins:
RepA protein (exclusive to this genus) and Rep protein on the complementary
sense strand and the movement protein (MP) and the coat protein (CP) on the
viral sense strand (Palmer and Rybicki, 1998).

Genus
Topocuvirus

The topocuviruses
infect dicots and are
transmitted by tree hoppers. They have monopartite genome
which encodes six proteins.  The coat protein has features of the leaf
hopper-transmitted mastreviruses whereas the organization of the
complementary-sense genes is similar to that of the single-component
begomoviruses. Therefore it might have arisen due to
the recombination between a curtoviruses and
begomoviruses (Briddon et al., 1996; Rojas et al, 2005). Tomato pseudo curly top virus belongs to this genus.

Genus Turncurtovirus

Turncurtovirus constitutes a single member (Turnip
curly top virus) with a monopartite genome which closely resembles members
of Curtovirus. The genome comprises
of six ORFs rather than seven ORFs: V1 (Coat
protein) and V2 (Movement
protein) in the virion sense and C1 (Replication initiation protein), C2 (Transcription activator protein), C3 (Replication
enhancer protein) and C4 (Symptom determinant) in the complementary sense. The insect vector is yet to be identified (Sahu et al., 2013).    

 

 Genus Begomovirus

Begomovirus is the largest genus of family Geminiviridae
and is transmitted by the whitefly Bemicia tabaci and infects
dicotyledonous plants. The name of the genus Begomovirus is derived from the first
two letters of the each word of the type species, Bean golden mosaic virus (BGMV) causing golden mosaic disease in bean in Central
America. 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 been
officially recognised under the genus Begomovirus, which is the maximum
number
of members so far known in any genera of plant viruses.  Symptoms of begomoviruses infected plants are
yellow mosaic, leaf distortion, curling and stunting. Infection in early
seedling stage leads to poor fruit set and infertile seeds, resulting in severe
yield loss. Begomoviruses infect a large number of dicots such as bhendi,
cassava, cotton, legumes, tomato, chilli, and many more. Some of the diseases
like bhendi yellow vein mosaic, okra enation leaf curl, cassava mosaic and
cotton leaf curl are known for century and cause huge economic loss (Varma and
Malathi, 2003).

            Genus Begomovirus consists of both
bipartite and monopartite members. Based on the geographical distribution
begomoviruses are classified as “Old world” (Eurasia, Africa and Australasia)
or “New World” (America). Most of
the “Old World” and all the “New World” begomoviruses have bipartite genomes
whereas some of the “Old World” begomoviruses have monopartite genomes. The
genome of bipartite begomoviruses consists of two genomic components,
designated DNA A and DNA B  which are
covalently closed, circular, 2.5- to 2.9-kb-long single-stranded (ss) DNA
molecules (Lazarowitz, 1992).  DNA A depends on DNA B for intracellular and
intercellular movement. DNA B depends on DNA A for replication and
encapsidation (Rogers et al., 1986;
Sunter et al., 1987; Townsend et al., 1986). In
order to facilitate recognition of DNA B by the Rep encoded by DNA A. a segment
of 110-200 nt sequences present within the intergenic region are highly
conserved between DNA A and DNA B component. This region is denoted as common
region (CR) and is highly identical between DNA A and DNA B component. The
intergenic region consists of repetitive elements called iterons upstream of
the highly conserved stem-loop structure. The iterons, represent the binding
sites of Rep. The invariant nonanucleotide sequence TAA TAT TAC present in the
loop is converased in all geminiviruses and the nicking between seventh and
eight nucleotide by Rep protein is proved to initiate replication. The bipartite begomoviruses have five or six
ORFs on DNA A, one or two on the virion-sense strand and four or five in the
complementary sense strand. The DNA B component encodes two ORFs one each in
the complementary and virion sense respectively.  In
DNA A, AC1 encodes Rep which is involved in the
replication 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 et
al., 2005), AC3 encodes a
Replication enhancer (REn)
protein that increases the efficiency of viral replication (Settlage et al., 2005) and AC4 protein is believed to be involved in
controlling the cell cycle progression and is a  symptom determinant and suppressor of
post-transcriptional gene silencing (Bisaro, 2006; Vanitharani et al., 2004).  AV1
encodes the coat protein, and AV2 encodes the pre-coat protein. AV2 is not
present in “New World” begomoviruses.  BV1
encodes the Nuclear shuttle protein (NSP) and BC1
encodes the Movement protein (MP) involved in cell to cell movement (Rojas et
al., 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.

Comments are closed.