Download Plant Diseases Caused by Viruses and Viroids

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Plant Diseases Caused by
Viruses and Viroids
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History
1886 Mayer: Artificial symptoms produced in healthy
tobacco plants.
1892 Ivanowski: The causal agent of tobacco mosaic
was found to be smaller than bacteria.
1898 Beijerinck: Tobacco mosaic disease was caused
by a “contagious living fluid” called a virus.
1939 Kausche et al.: The first virus (TMV) particles
were seen with the electron microscope.
1956 Gierrer & Schramm: Viral RNA carried all the
genetic information needed for infection.
1971 Diener: Discovery of a viroid in potato infected
with spindle tuber disease.
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Plant Pathogenic Viruses
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Plant Pathogenic Viruses

A virus



is a nucleoprotein.
is a submicroscopic entity that multiplies
only in living cells.
has the ability to cause disease.
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Plant Pathogenic Viruses (cont’)
Important characteristics:




Obligate parasites
A virus particle is called a virion which is
divided into 2 main components:
1.
Nucleic acid (RNA or DNA)
2.
Protein coat (capsid)
Reproduction via replication
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Morphology of Viruses
Shapes and sizes
1.
Elongate (rigid rods or flexuous threads)
•
2.
Cylindrical rods (Bacillus-like)
•
3.
≈10-13 nm (width) & 480-2,000 nm (length)
≈52-75 nm (width) & 300-380 nm (length)
Spherical (isometric or polyhedral)
•
Most are polyhedral - 17-60 nm (diameter)

1 µm (micron) = 1/1000 mm

1 nm (nanometer) = 1/1,000,000 mm
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Morphology of Viruses (cont’)
Rigid rods
Flexuous threads
Spherical
Isometric
Bacillus-like rods
Twin isometric
Source: JPT
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Morphology of Viruses (cont’)
Shape, size & structure
A Flexuous thread
B Rigid rod
C Bacillus-like
D Isometric polyhedral
E Twin particles
Source: Agrios (2005)
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cucumber necrosis virus particle
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Viral Composition and Structure
1.
2.
Nucleic acid
Protein coat (capsid)
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Viral Nucleic Acids



Causes disease infectivity in plants
Two types:
Single strand or double strand

ssRNA/dsRNA or ssDNA/dsDNA


Majority: ssRNA
Content: 5-40%
Elongated viruses: lower % of nucleic acids

Spherical viruses: higher % of nucleic acids

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Viral Nucleic Acids
Source: Agrios (2005)
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Viral Proteins
Functions:





To protect viral nucleic acids
Increase infectivity of nucleic acids
Content: 60-95%
Composed of repeating subunits
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Viral Replication


New virions are formed 10 h after
inoculation.
Viral replication make use of its genetic
machinery as well as of the host cell.
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Viral Replication (cont’)
1.
2.
3.
4.
Schematic representation of viral RNA replication.
Source: Agrios (2005)
Nucleic acid freed
from protein coat.
Viral nucleic acid
replication.
Viral protein
formation.
Assembly of nucleic
acid & protein to
form a virion.
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Replication cycle of a virus that causes Barley Yellow Dwarf. Courtesy L.L. Domier.
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Classification of Plant Viruses
Kingdom: Viruses
Family (12)


Examples:





Potyviridae (Papaya ring spot virus)
Sequiviridae (Rice tungro spherical virus)
Bromoviridae (Cucumber mosaic virus)
Caulimoviridae (Rice tungro bacilliform virus)
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Classification of Plant Viruses (cont’)
Classification according to:

Shape of particle
1.

Rod, filamentous, isometric, bacilliform
Type of nucleic acid
2.




ssRNA: positive or negative, 1-3 molecules
dsRNA
ssDNA: positive
dsDNA
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Classification of Plant Viruses (cont’)
Source: Agrios (2005)
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Translocation and Distribution of
Viruses in Plants

Move from one cell to another through
plasmodesmata



Rate: 1 mm/day, 8-10 cells/day
Multiply in parenchyma cells they infect
Transportation through phloem


More rapid rate: 15 cm/6 min
Spread systematically throughout the
plant
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Translocation and Distribution of
Viruses in Plants
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Translocation and Distribution of
Viruses in Plants (cont’)
Source: Agrios (2005)
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Transmission of Plant Viruses

Mode of transmission
1.
2.
3.
4.
5.
6.
7.
8.
9.
Insect
Vegetative propagation
Parasitic plant, Dodder
Mechanical, via sap
Seed
Pollen
Nematode
Mite
Fungus
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Mode of Transmission
1.
Insect vectors
(most common
& important)
Aphids
Thrip
Leafhopper
Whitefly
Source: Agrios (2005)
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Mode of Transmission (cont’)
2.
Vegetative propagation (budding, grafting,
organs like bulb, etc.)
3.
Parasitic plant, Dodder (e.g., Cuscuta sp.)
Source: Agrios (2005)
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Mode of Transmission (cont’)
4.
5.
6.
Mechanical through
sap
Seed
Pollen (reduced
fruit set, infect
seeds & seedlings)
Source: Agrios (2005)
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Mode of Transmission (cont’)
8.
Nematode (e.g., Longidorus,
Paralongidorus, Xiphinema)
Mite
9.
Fungus
7.
Source: Agrios (2005)
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Types of Viruses
Non-persistent viruses (stylet-borne)
1.

Persist in the vector for only a few to several
hours
Semi-persistent viruses
2.

Persist in the vector for a few (1-4) days
Circulative or persistent viruses
3.


Ability to multiply in their respective vectors
Ability to re-introduce the virus into plants
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Mode of Transmission
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Symptoms Caused by Plant Viruses
1.
2.
3.
Systemic symptoms
Local lesions
Other symptoms
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1. Systemic Symptoms
i.
Mosaic
Normal green + light

yellow, or + yellow, or
+ white

Infect leaves or fruits
Pattern & intensity of

discoloration differ –
mottling, streak, vein
clearing, vein banding
Source: Agrios (2005)
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1. Systemic Symptoms (cont’)
Ring Spot
ii.

Necrotic or chlorotic spots on leaves,
fruits & stems
Source: Agrios (2005) & JPT
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2. Local Lesions


Artificial inoculation
Small & chlorotic lesions at points of
entry
Source: JPT
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3. Other symptoms




Stunt
Dwarf
Leaf roll
Yellows
Source: JPT
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Potato leafroll caused
by the potato leafroll
virus
tomato yellow leaf curl virus (TYLCV)
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A diseased plant of
sunn hemp showing
leaf curl symptoms
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Diseases Caused by Plant Viruses
1.
2.
“Penyakit Merah” of Rice
Mosaic diseases
4.
Citrus Tristeza
Papaya Ring Spot
5.
Cacao Swollen Shoot
3.
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Diseases Caused by Plant Viruses
“Penyakit Merah” of
Rice
1.




Other names: ‘Tungro’
(Philippines), ‘Mentek’
(Indonesia)
Pathogen: Rice tungro
spherical virus (RTSV,
ssRNA) & Rice tungro
bacilliform virus (RTBV,
dsDNA)
Symptoms: Stunting,
mottling, yellow-orange
discoloration of the leaves
Disease impact: 20-100%
yield reduction
Source: Agrios (2005)
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Diseases Caused by Plant Viruses (cont’)
“Penyakit Merah” of Rice (cont’)
1.
Vector: Green planthopper - Nephotettix
virescens, Nephotettix nigropictus
Infection process:





Nephotettix suck sap from diseased plants & transmit
viruses to healthy plants.
Infective period in vector, < 5 days (semi-persistent
manner).
One Nephotettix can infect 40 rice plants/day.
Source: Agrios (2005)
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Diseases Caused by Plant Viruses (cont’)
Mosaic diseases
2.




Hosts: Wide range,
e.g., chili, tobacco,
cucurbits, legumes
Pathogen: Cucumber
mosaic virus (CMV) –
isometric, ssRNA
Symptoms:
Discoloration &
distortion, flowers &
fruits, stunting
Vectors: Insects &
humans
Agrios, 2005
Source: JPT
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Diseases Caused by Plant Viruses (cont’)
Citrus Tristeza
3.


Pathogen: Citrus
tristeza virus (CTV),
flexuous thread,
ssRNA
Vector: Brown
aphids - Toxoptera
citricida
Source: Agrios (2005)
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Diseases Caused by Plant Viruses (cont’)
Citrus Tristeza (cont’)
3.


Dissemination: Grafting & aphids in semipersistent manner
Symptoms: Discoloration on leaves & fruits,
quick-decline on the whole tree
Source: Agrios (2005) & JPT
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Diseases Caused by Plant Viruses (cont’)
Papaya Ring Spot
4.



Pathogen: Papaya ring spot virus (PRSV),
flexuous thread, ssRNA
Symptoms: Ring spots on leaves or fruits,
stunting
Vector: Aphids (nonpersistent manner)
Source: Agrios (2005) & JPT
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Diseases Caused by Plant Viruses (cont’)
Cacao Swollen Shoot
5.




Occurs in West Africa only,
not found in Asia
Pathogen: Cacao swollen
shoot virus (CSSV),
bacilliform
Vectors: Mealy bugs
Planoccoides njalensis, P.
citri) (semipersistent
manner)
Symptoms: Swollen stem
& main root, necrosis on
lateral roots, chlorosis on
leaves & fruits
Source: Agrios (2005)
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Plant Pathogenic Viroids
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Plant Pathogenic Viroids




≈ 40 plant diseases caused by virioids
Virioid diseases not discovered yet in animals
& humans.
Viroids are small, low molecular weight RNAs.
Important characteristics:



Able to infect plant cells
Able to replicate themselves
Able to cause disease
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Morphology of Viroids
Differ from viruses in 2 ways:

1.
2.
Size of RNA: 246-401 bases (much <
viruses, 4-20 kb)
Lack a protein coat (exist as free or
naked RNA)
Shape:



Circular single-stranded RNA
Hairpin structure (due to base pairing of
RNA strand)
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Classification of Viroids
Classification is based on the absence or
presence of a conserved central region.
Avsunviroids (ASBVd-typed species)

1.

Avsunviroideae


Pospiviroids (PSTVd-typed species)
2.

Pospiviroideae:

3.
Avsunviroid
Pelamoviroid
5 subgroups: Pospiviroid, Apscaviroid, Cocadviroid,
Coleviroid, Hostuviroid
Unassigned viroids
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Avsunviroideae
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Disease Caused by Plant Viroids
1.
Coconut Cadang-Cadang
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Disease Caused by Viroids
Coconut Cadang-Cadang


Occurs in the
Philippines
Pathogen: Coconut
cadang-cadang viroid

(CCCVd), 246
nucleotides (smallest
viroid known)
Symptoms: Develop
slowly (8-15 years),
chlorosis on leaves, bud
dies & fronds fall off,
leaving trunks standing
Source: Agrios (2005)
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Thank You
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DNA and RNA


The nucleic acids are informational molecules
because their primary structure contains a
code or set of directions by which they can
duplicate themselves and guide the synthesis
of proteins.
The synthesis of proteins - most of which are
enzymes - ultimately governs the metabolic
activities of the cell.
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DNA and RNA

There are two types of nucleic acids which
are polymers found in all living cells.



Deoxyribonucleic Acid (DNA) is found mainly
in the nucleus of the cell, while
Ribonucleic Acid (RNA) is found mainly in the
cytoplasm of the cell although it is usually
synthesized in the nucleus.
DNA contains the genetic codes to make RNA
and the RNA in turn then contains the codes
for the primary sequence of amino acids to
make proteins.
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Nucleic Acid Parts List:



pentose sugars,
phosphates,
and heterocyclic amines (or bases).
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Pentose Sugars:

There are two types of pentose sugars
found in nucleic acids. This difference is
reflected in their names:


deoxyribonucleic
acid
indicates
the
presence of deoxyribose
ribonucleic acid indicates the presence of
ribose.
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Phosphate:

A major requirement of all living things
is a suitable source of phosphorus. One
of the major uses for phosphorus is as
the phosphate ion which is incorporated
into DNA and RNA.
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Heterocyclic Amines:


Heterocyclic amines are sometimes called
nitrogen bases or simply bases. The
heterocyclic amines are derived from two root
structures: purines or pyrimidines. The
purine root has both a six and a five member
ring; the pyrimidine has a single six member
ring.
There are two major purines, adenine (A) and
guanine (G), and three major pyrimidines,
cytosine (C), uracil (U), and thymine (T).
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DNA
sugar phosphate sugar phosphate sugar phosphate sugar ...
T
A
C
G
¦
¦
¦
¦
A
T
G
C
sugar phosphate sugar phosphate sugar phosphate sugar ...
A
U
G
C
sugar phosphate sugar phosphate sugar phosphate sugar ...
RNA
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Messenger RNA (mRNA)

Messenger RNA is RNA that carries
information from DNA to the ribosome
sites of protein synthesis in the cell.
Once mRNA has been transcribed from
DNA, it is exported from the nucleus
into the cytoplasm (in eukaryotes mRNA
is "processed" before being exported),
where it is bound to ribosomes and
translated into protein.
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Transfer RNA (tRNA)

Transfer RNA is a small RNA chain of about
74-93 nucleotides that transfers a specific
amino acid to a growing polypeptide chain at
the ribosomal site of protein synthesis during
translation. It has sites for amino-acid
attachment and an anticodon region for
codon recognition that binds to a specific
sequence on the messenger RNA chain
through hydrogen bonding. It is a type of
non-coding RNA.
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Ribosomal RNA (rRNA)

Ribosomal RNA (rRNA) is a component of the
ribosomes, the protein synthetic factories in
the cell. Eukaryotic ribosomes contain four
different rRNA molecules: 18S, 5.8S, 28S, and
5S rRNA. Three of the rRNA molecules are
synthesized in the nucleolus, and one is
synthesized elsewhere. rRNA molecules are
extremely abundant. They make up at least
80% of the RNA molecules found in a typical
eukaryotic cell.
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Non-coding RNA or "RNA genes"

RNA genes (sometimes referred to as noncoding RNA or small RNA) are genes that
encode RNA that is not translated into a
protein. The most prominent examples of
RNA genes are transfer RNA (tRNA) and
ribosomal RNA (rRNA), both of which are
involved in the process of translation.
However, since the late 1990s, many new
RNA genes have been found, and thus RNA
genes may play a much more significant role
than previously thought.