2. (negative sense ssRNA)
Higher Order Categories
Family : Bunyaviridae
Genus :Tospovirus
Species
Tomato spotted wilt virus (TSWV);
Impatiens necrotic spot virus (INSV);
Watermelon silver mottle virus (WSMoV);
many others…
3. INTRODUCTION
Viruses in the genus Tospovirus cause significant worldwide
crop losses.
The genus name is derived from the name of its first member,
Tomato spotted wilt virus (TSWV).
However, discovery of a second tospovirus, Impatiens necrotic
spot virus (INSV), was followed by description of more
tospoviruses from several parts of the world.
In nature, TSWV is transmitted by thrips in a circulative and
propagative manner.
The virus is mechanically transmissible and is not seed-
transmitted.
Management of TSWV has proven to be challenging due to the
wide host range of both the virus and its vector.
4. During the 1980’s TSWV caused significant losses and
epidemic on peanuts, tobacco and tomatoes in the Southeastern
United States ,Eastern Europe and South America and forced
shifts in production practices.
United States caused major economic losses and Losses due to
TSWV outbreaks in peanut were estimated at more than US
$100 million in Georgia alone in the USA.
TSWV has caused serious losses in tomatoes and peppers in
Australia for many years and is still a serious problem.
In the late 1980’s a new virus was recognized, Impatiens
necrotic spot virus (INSV), which caused severe losses in the
floral crop industry in the United States and Europe.
5. Contd..
INSV may be found wherever ornamentals are produced in North
America and Europe, and more recently in Asia.
IYSV is currently found in the US onion and onion seed production
areas, the Middle East Europe and Australia.
Another tospovirus, Capsicum chlorosis virus (CaCV), was
identified in Australia where it has displaced TSWV in pepper and
tomato in some areas.
Distribution of these tospovirus species is currently restricted to
other regions of the world but could expand via the global plant
trade as tospoviruses and their vectors can be spread by the
transport of infected plant material.
6. MOLECULAR BIOLOGY
Particles are quasi-spherical,
80-120 nm in diameter.
The tospovirus genome consists of
three ssRNA molecules, known as a
tripartite RNA genome.
(Cortez et al., 2001).
The genome of tospoviruses includes three RNAs referred to as
Large (L), Medium (M) and Small (S) RNA.
L RNA is in negative sense while M and S RNAs are ambisense in
their genome organization - meaning that one gene is encoded in the
negative sense sequence and another gene is encoded in the positive
sense sequence (Cortez et.al., 2001).
8. Due to the negative strandedness of the genome, virions
contain several molecules of the RNA-dependent RNA
polymerase to initiate initial rounds of replication of the
virion RNAs.
The three genomic RNAs are tightly linked with the N
protein forming ribonucleoproteins (RNPs).
The ambisense genome organization consists of one open
reading frame at the 5’ end and a second open reading
frame near the 3’ end.
9. THE ENTIRE GENOME CODES FOR SIX PROTEINS –
L RNA (8.9kb) : It codes for RNA-dependent RNA polymerase
(RdRp) ( 331-kDa) for viral replication and genome transcription.
M RNA (5.4kb) : It encodes two structural glycoproteins that is
polypeptides each of which is glycosylated (GN and GC) and a non
structural protein (NSm).
(GN and GC)(127-kDa) -The C-terminal G1 is estimated to be 75
kDa and N-terminal G2 to be 46 kDa.
G1/G2 are found in the envelope and may be involved in recognition
of receptors in the vector.
( NSm -34 kDa ) for cell-to-cell
movement of the virus.
• The absence of a gene similar to NSm
in other genera of the family Bunyaviridae
suggests that NSm facilitates TSWV
infection of plants.
10. S RNA (2.9kb) : codes for nucleocapsid protein (N) used to
construct the virion capsid and a non-structural protein (NSs)
which shown to be a suppressor of RNA silencing.
• Both plays important role in the TSWV infection cycle.
• nonstructural proteins on the small (NSs) and middle( NSm)
genome segments are located near the 5’ end and 3’ end code for
structural proteins (N and G1/G2).
11.
12. Reproductive Cycle of a Tospovirus in a Host Cell (In
Plant)
Tospovirus enters the cell via an insect vector.
The virus sheds its capsid, and the viral RNA polymerase is produced
using host cell machinery.
At low concentrations of the virus nucleocapsid N protein (at initial
entry), the RNA polymerase makes mRNAs that encode the other viral
proteins necessary for packaging and transport.
The host cell machinery is then used to translate the viral proteins.
At high concentrations of N protein ( after translation of the viral
proteins has occurred), the RNA polymerase can begin replicating the
viral genome.
After virion packaging occurs, viral transmission can proceed in two
ways :1. The virus can be transmitted directly to another host cell
(without an envelope) via the viral movement protein (NSm) or 2.
indirectly (as a mature, enveloped particle) via uptake by an insect
vector.
13. replication
tospovirus replication occurs in the cytoplasm in association with
the Golgi Bodies.
It is thought that tospovirus replication is similar to that of other
bunyaviruses with the concentration of N protein regulating the
switch from the production of viral mRNAs to replication of the
genome (Storms, 1998).
The mature particle is usually enclosed in a lipid envelope, which is
formed by budding from the host's Golgi and incorporating the viral
envelope glycoproteins. (Prins & Goldbach, 1998)
14. In insect:
Virus attaches to host receptors though Gn-Gc
glycoprotein dimer, and is endocytosed into vesicles in
the host cell.
Fusion of virus membrane with the vesicle membrane;
ribonucleocapsid segments are released in the
cytoplasm.
Transcription, viral mRNAs are capped in the
cytoplasm.
Replication presumably starts when enough
nucleoprotein is present to encapsidate neo-
synthetized antigenomes and genomes.
The ribonucleocapsids buds at Golgi apparatus,
releasing the virion by exocytosis.
