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VIROIDS: The Plant Invaders…
A VIROID is a…
Viroids are“sub-viruses” composed
exclusively of a single circular strand
of nucleic acid (RNA) that codes for a
Small, circular RNA molecules without
a protein coat
VIROIDS: DID YOU KNOW VIROIDS…
Very small, covalently closed, circular RNA molecules
capable of autonomous replication and induction of disease.
Range in size from approximately 20 nm
(8 x 10-7 inches) ...
Are infectious particles.
Cause certain plant and human diseases.
Consists only of Nucleic Acid (RNA) .
No coding capacity - do not program their own polymerase.
Use host-encoded polymerase for replication.
Mechanically transmitted; often seed transmitted.
More than 40 viroid species and many variants have been characterized.
“Classical” viroids have been found only in plants.
Viroids differ from viruses in that viruses, at their most basic level, consist
of genetic material (DNA or RNA) contained within a protective protein shell.
Viroids differ from prions, another type of subviral infectious agent, in that
prions are made only of protein, lacking nucleic acid.
viroids are non-
O’Brien develop a
bioassay for the
teams up with
Diener; they show
that agent is not a
the agent is a free
coins the term
stunt and citrus
exocortis as non-
1978: PSTVd is
Theodore O. Diener
PSTVd (potato spindle tuber)
HSVd (hop stunt)
CCCVd (coconut cadang-cadang)
ASSVd (apple scar skin)
CbVd 1 (coleus blumei 1)
ASBVd (avocado sunblotch)
PLMVd (peach latent mosaic)
• Potato spindle tuber viroid (PSTVd)
• May be limiting to potato growers
• First viroid characterized
• Many variants described
• Control with detection in mother
stock, clean seed
PSTVd in tomato
PSTVd in potato
• Citrus exocortis viroid (CEVd)
• Causes stunting of plants, shelling of bark
• May result in little yield loss
• May be useful to promote dwarfing for
• Transmitted through stock, graft
• Control by removal of infected plants,
detection, clean stock
Citrus exocortis viroid
1. Disinfection of cutting tools
2. Cold treatment
E.g storage at 4 °C for 6months or more, followed
by apical shoot-tip-culture, grafting, can be used
to eliminate CSVd and HSVd.
3. Pre-inoculation with protective mild strains of viroid
has proved effective to control PSTVd
Viroid disease control
Most mechanically transmitted
Some seed or pollen transmitted
Tomato planta macho viroid-aphid
(Ricardo Flores et al., 2005)
Human pathologies induced by viroids
The only human disease known to be caused
by a viroid is hepatitis D. This disease was
previously discribed to a defective virus
called the delta agent.
However, it now is known that the delta
agent is a viroid enclosed in a hepatitis B
For hepatitis D to occur there must be
simultaneous infection of a cell with both
the hepatitis B virus and the hepatitis D
Viroids and RNA silencing
The evidence indicates that when viroids replicate
via a double stranded intermediate RNA, they are
targeted by a dicer enzyme and cleaved into siRNAs
that are then loaded onto the RNA-induced
The viroid siRNAs actually contain sequences
capable of complementary base pairing with the
plant's own messenger RNAs and induction of
degradation or inhibition of translation is what
causes the classic viroid symptoms.
• Some viroids are ribozymes, having RNA enzyme
properties that allow self-cleavage and ligation of unit-
size genomes from larger replication intermediates. It
has been proposed that viroids are "escaped introns."
Not all viroids are known to be pathogenic, but some
are serious pathogens of plants. Viroids are usually
transmitted by seed or pollen, but may be transported by
farm implements as well.
Infected plants can show distorted growth and
sometimes are killed by the viroid.
Two main groups of viroids: self-cleaving and non-self-
Non-self cleaving viroids replicate in nucleus and fold into
“dog bone” or rod-like structure
All are covalently closed circular RNAs fold to tightly
Five domains identifiable in non-self-cleaving
Terminal left (TL)
Terminal right (TR)
The closed single-stranded RNA circle has
extensive intrastrand base pairing and
interspersed unpaired loops.
Viroids have five domains. Most changes in
viroid pathogenicity seem to arise from
variations in the P and TL domains.
Schematic models of viroid structures
i. (A) Rod-like secondary structure proposed for PSTVd, the
type member of family Pospiviroidae.
ii. (B) Quasi rod-like secondary structure proposed for ASBVd,
the type member of family Avsunviroidae.
iii. (C) Complex branched conformation proposed for PLMVd.
Intra cellular movement
cell-to-cell through plasmodesmata
long distance through phloem
Host factors responsible for viroid movement
Phloem proteins- Phloem lectin PP2(CsPP2)
Viroid binding protiens-Virp1
specific sequence or Structural motifs.
Viroid RNA does not code for any known protein;
some even lack the AUG initiation codon.
Nonetheless, they replicate autonomously in host
The replication mechanism involves interaction
with RNA polymerase II, an enzyme normally
associated with synthesis of messenger RNA, and
"rolling circle" synthesis of new RNA.
Asymmetric rolling circle replication
(Biao Ding et al., 2005)
Member of Pospiviroidae replicate via an asymmetric
Incoming (+)-circular RNA initially is transcribed into
concatemeric linear (-)-strand RNA
Which then serves as the replication intermediate for
the synthesis of concatemeric, linear (+)- strand RNA
This (+)- strand RNA subsequently is cleaved into unit
length monomers that are ligated into circles
viroids replicate autonomously by using host-encoded RNA
Concatemer: Multiple copies of a DNA sequence arranged
end to end in tandem.
Symmetric rolling circle replication
Member of Avsunviroidae replicate via an symmetric
The circular (+)-RNA is transcribed into linear, concatemeric (-)-
Instead of serving as the direct template for the synthesis of
linear concatemeric (+)- strand RNA
The concatemeric (-)- strand RNA is cleaved into unit length
molecules followed by circularization
The circular (-)- RNA then serves as the template for the
synthesis of linear, concatemeric (+)- strand RNA
When subsequently is cleaved into unit-length monomers and
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this presentation will give useful information about viroids..