Plant
Viruses
Plant
Viruses
- The
first
virus
to
be
discovered
was
a
plant
virus.
In
1892
Iwanoski
showed
that
the
tobacco
mosaic
disease
could
be
transmitted
through
the
sap
of
diseased
plants
even
after
the
sap
was
passed
through
filters
fine
enough
to
trap
all
bacteria.
The
sap
contained
the
particles
of
the
tobacco
mosaic
virus
(TMV)
which
were
ultramicroscopic
in
size.
In
1935
Stanley
crystallized
the
virus
and
showed
that
the
crystals
were
aggregations
of
submicroscopic
nucleoprotein
complexes.
Since
then
more
than
170
plant
viruses
have
been
visualized.
The
vast
majority
of
the
plant
viruses
have
an
RNA
genome.
Small
viruses
containing
ssRNA
can
be
considered
as
picorrnaviruses.
This
term
is,
however,
used
more
specifically
with
reference
animal
enteroviruses
and
rhinoviruses,
and
will
therefore,
not
to
be
used
,
with
reference
to
the
plant
ssRNA
viruses.
The
vast
majority
of
the
ssRNA
viruses
are
rod
like,
sometimes
flexous,
(helical
symmetry).
An
ssRNA
group
characterized
by
complex
bacilliform
or
bullet
shaped
lipid
membrane
containing
particles
has
been
included
in
the
rhabdoviruses.
This
group
also
contains
animal
viruses,
e.
g.
the
rabies
virus.
The
large,
complex,
membrane
covered
tomato
spotted
wilt
virus
is
a
pleomorphic
myxovirus.
The
plant
viruses
of
the
Reoviridae,
e.g.
the
wound
tumour
virus,
contain
a
dsRNA
genome,
and
are
similar
to
animal
reoviruses.
The
cauliflower
mosaic
group
of
viruses
are
the
only
viruses
to
have
a
DNA
genome
(dsDNA).
No
ssDNA
plant
viruses
are
known.
Some
plant
viruses
also
infect
arthropods,
and
thus
form
a
bridge
between
plant
and
animal
viruses.
The
vectors
of
plant
viruses
belong
mostly
to
the
order
Hemiptera
of
insects.
This
group
includes
bugs
that
feed
by
sucking
plant
juice.
The
majority
of
the
plant
viruses
are
transferred
by
aphids,
particularly
the
species
Myzus
persicae,
the
potato
and
peach
aphid.
Other
arthropod
vectors
of
plant
viruses
belong
to
the
order
Orthoptera
(grasshoppers
and
locusts)
and
Coleoptera
(beetles).
Viruses
may
be
rapidly
lost
by
the
insect
host
after
feeding
(non
persistent
viruses)
or
may
be
retained
for
long
periods,
often
for
the
rest
of
the
life
(persistent
viruses).
Transfer
of
non
persistent
viruses
may
be
simply
a
mechanical
process,
without
any
biological
relationship
between
the
virus
and
the
host.
In
the
case
of
persistent
viruses
there
may
be
multiplication
within
the
vector.
The
potato
leaf
roll
virus
transmitted
by
aphids
and
the
equine
encephalomyelitis
virus
transmitted
by
the
mosquito
are
examples
where
there
is
evidence
for
the
multiplication
of
the
virus
within
the
insect
host.
Evidence
for
the
biological
relationship
between
the
insect
host
and
the
virus
is
more
direct
in
the
case
of
leafhoppers
and
the
viruses
they
transmit.
The
virus
for
the
dwarf
disease
of
rice
is
transmitted
via
the
egg
of
its
leafhopper
vector
Nephotettix
apicalis.
The
clover
club
leaf
virus
is
also
transmitted
through
the
egg
of
its
leafhopper
vector
Agalliopsis
novela.
These
two
viruses
offer
clear
evidence
of
multiplication
of
plant
viruses
in
their
insect
vectors.
Several
other
instances
have
also
been
recorded.
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