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Viruses
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Some of this material was created by
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Viruses
The term virus was used in the Middle Ages and comes from
the Greek word for poison or Latin meaning slimy,
poisonous or malodorous liquid.
Its application in those early days did not have the modern
understanding of the word.
By the late 1930s scientists were regularly using the term
'filterable virus' to describe those agents capable of
passing filters fine enough to retain bacteria.
Viruses
Virus - a submicroscopic, obligate parasite consisting
primarily of protein and nucleic acid.
"Submicroscopic" - implies that the object can not be
seen using a light microscope; must use electron
microscope.
Viruses share traits with both living and non-living things.
Living - can replicate themselves; can infect hosts
Non-living - can be crystallized and stored.
Fathers of Plant Virology
In 1892 D. lvanovski - the passage of the tobacco mosaic agent, now
known as tobacco mosaic virus (TMV), through the bacteria-proof
Chamberland filter. – Living cells?
In 1898, M. Beijerinck- concept that viruses were small and
infectious. Infectious agent as a “contagium vivum fluidum”
• Discovered that the virus readily passed through a porcelain filter,
= smaller than bacteria.
• Observed that the 'agent' could diffuse through agar that retained
bacteria, and furthermore, that the virus could not be cultured
except in living, growing plants.
First report, suggesting that 'microbes' need not be cellular.
Viruses
First published record of a virus was in 1576.
Descriptions, paintings of tulips with a broken color pattern, now
known to be caused by Tulip Breaking Virus (TBV). This virus
induces in foliage of infected plants beautiful, variegated color
patterns.
Such bulbs were extremely expensive, giving rise to "tulipmania" in
the 17th century.
Viruses have been identified only recently.
1935 - W.M. Stanley purified and crystallized Tobacco Mosaic
Virus (TMV). late 1930's - electron microscope developed,
viruses visualized.
Characteristics of Viruses
1. Very small requires electron microscope to see them.
2. Virus particles (aka. virions) are composed of:
a. Nucleic acid - 5 to 40%, depending on virus.
Nucleic acid typically is RNA, but some have
DNA. Nucleic acid can be single- or doublestranded
b. Protein sheath (aka. capsid) - 60 to 95%
depending on virus.
protein,
Capsid protects virus from heat, enzymes, UV and other
denaturing agents.
Characteristics of Viruses
3. Shapes
a. Rigid rods - long, straight rods with nucleic acid core and protein coat.
Raw spaghetti.
Ex: Tobacco Mosaic Virus (TMV) is 15 x 300 nm.
b. Flexuous rods - long curved rods with nucleic acid core and protein
coat. Cooked spaghetti.
Ex: Citrus Tristeza Virus is ca 2,000 nm long.
c. Short rods (aka. Rhabdoviruses) - truncated, bullet shaped.
Ex: Potato Yellow Dwarf Virus (PYDV) is 75 x 380 nm.
d. Eicosahedrons (20-sided isometric particles). Formerly referred to as
spherical viruses, because they appeared spherical in the earliest
electron micrographs with poor resolution.
Ex: Wound Tumor Virus (WTV) can vary in diameter from 17 to 60
nm
Characteristics of Viruses
4. Virus replication depends on "directions" contained in
viral nucleic acid. Viruses code for their own enzymes
(replicases) to replicate themselves.
5. Multi-component viruses - complete genetic information
is carried in > particles.
Ex: Tobacco Rattle Virus (TRV) - consists of two particles
of different lengths.
Can infect but unstable because it lacks the gene for protein coat.
Carries info for protein coat.
Virus is stable only when both are present.
Satellite Viruses & Satellite RNAs
Satellite Viruses
Viruses that must always be associated with certain typical
viruses (helpers) because they depend on the helper for
multiplication and plant infections.
They often reduce the ability of the helper virus to
multiply and cause disease, so the satellite viruses act as
parasites.
Satellite RNAs - small RNAs found inside virions of certain
multi-component viruses. The RNA is not related or
could be only partially related to that inside the virions. –
They may increase or decrease the severity of viral
infections.
Viroids
1. They are small, low molecular weight RNAs that
can infect plants, replicate themselves, and cause
disease.
2. Naked nucleic acid (no protein coat). However they
are stable because of nucleic acid side chains
"double over" and block sites of attachment for
enzymes and protect against other environmental
variables.
3. Much smaller than viruses. RNA contains 250-376
base compared to 4,000-20,000 bases in viruses.
Viroids
4. This is too small to encode for even the smallest
replicase! So how do they replicate?
Apparently they rely on host somehow, but the
method is not understood.
5. Recently discovered in 1971 as the cause for potato
spindle tuber disease. Now known to cause ~ 20
diseases.
How do viruses enter plants and cause
symptoms?
1. They require wounds - either through vector
(insects, nematodes) feeding or mechanical.
2. Once inside host cell, protein coat
disassociates and nucleic acid replicates.
Virus and nucleic acid spread from cell to
cell.
How do viruses enter plants and cause
symptoms?
1. They require wounds - wither through vector feeding or
mechanical.
2. Once inside host cell, protein coat disassociates and nucleic acid
replicates. Virus and nucleic acid spread from cell to cell.
3. Some viruses cause local lesions only, but the
majority are systemic.
How do viruses cause symptoms?
1. They require wounds - wither through vector feeding or mechanical.
2. Once inside host cell, protein coat disassociates and nucleic acid replicates. Virus
and nucleic acid spread from cell to cell.
3. Some viruses cause local lesions only, but the majority are systemic.
4. Viruses produce a variety of symptoms, depending on the
specific virus/host combination. Some viruses induce one type
of symptom in one host but a different symptom in another
host. They usually do not kill their hosts.
Ex: mosaic, deformed plant parts, vein necrosis, ringspots,
yellowing, dwarfing, stunting, and rarely death.
Diagnosis of Plant Viruses
1. Symptoms are NOT reliable.
2. Host reaction - using different hosts that are known to react
differently based on the virus.
3. Serology - using antiserum prepared against a particular
virus in tests such as ELISA and immunodiffusion.
ELISA
Agar
Immunodiffusion
Diagnosis of Plant Viruses
1. Symptoms are NOT reliable.
2. Host reaction - using different hosts that are known to react differently based
on the virus.
3. Serology - using antiserum prepared against a particular virus in tests such as
ELISA and immunodiffusion.
4. Microscopy - usually electron microscopes, although viral
inclusion bodies can be seen using a light microscope. Some
viruses aggregate into large masses in cells, termed inclusion
bodies. These can be very different in shape (i.e., pinwheels)
and appearance (crystalline or amorphous)
5. Detection of nucleic acids using electrophoresis or molecular
hybridization.
Aspects of Symptom Expression
1. Localized – symptoms
occur at the point of
inoculation only; do not
spread. Local lesions.
2. Systemic- symptoms are not
limited to point of
inoculation. Chlorosis,
stunting, etc.
Aspects of Symptom Expression
1. Localized – symptoms occur at the point of
inoculation only; do not spread. Local lesions.
2. Systemic- symptoms are not limited to point of
inoculation. Chlorosis, stunting, etc.
3. Specific virus/host interactions for symptoms
expression.
4. Symptomless carriers – virus is present in a host but
symptoms do not occur.
Aspects of Symptom Expression
5.
Masked symptoms – virus is present but symptoms are expressed
only in response to some conditions exclusive of virus.
Ex. Temperature A = no symptoms; Temp. B = symptoms
6.
Symptom synergy- plant is infected by more than one virus, and
resulting symptom combination is more severe than symptoms of
each virus alone.
7.
Cross Protection – inoculation with a mild strain of a virus
provided resistance to a more sever strain.
Ex. Citrus – mild strain of citrus tristeza virus
Tomato - mild strain of tobacco mosaic virus
Problems with cross protection
Mild strains are not available for all viruses
May not be effective against all strains
Labor intensive
Danger of mutations so that mild strain becomes severe.
Virus Dispersal
1. Insects- account for the largest part of virus
dispersal.
–
Aphids, leafhoppers, planthoppers, beetles, thrips,
whiteflies, etc.
a. Insect species vary in the number of viruses they can
carry.
Viruses vary in the number of insect species that can
vector them.
b. Dispersal depends on:
i. Amount of virus in plant (aka. Titer)
ii. Insect mobility
iii. Relationship between insect and virus. Insects- account for the
largest part of virus dispersal.
Insect Vectors
Aphids
Whiteflies
Insect Vectors
Thrips
Leafhopper
Plant Hoppers
Relationships
1) Non-persistent – virus is simply a contaminant on the mouthparts of insect.
It is transmitted immediately and for a fairly short time. Insect vectors =
aphids, beetles, etc.
2) Persistent – insect can transmit the virus for a long time after acquisition.
a. Non-propagative-does not multiply in vector.
b. Propagative – virus multiplies in the insect vector
i. Circulative – virus acquired by the insect, enters gut in plant sap,
enters hemolymph of insect, travels to the salivary glands, multiplies and
reinoculated into plants at subsequent feedings.
** Most propagative are circulative.
ii. Transovarial – Similar to cirulative except that the virus also passes
into ovaries and enters insect eggs. Egg viability is decreased but vector
is infective from hatch. Leafhoppers, planthoppers
Tobacco Mosaic Virus
• Non – persistent
• Tobacco, tomato, and other
solanaceous plants
Barley Yellow Dwarf
• Hosts - barley,wheat,
maize, rice and other grasses
• Persistent, circulative virus
Diagram of an aphid vector feeding on a plant host showing the internal route
of the viruses that cause BYD.
Relationships
A necessary latent period between acquisition and transmission
ability is required usually several hours.
Often with propagative viruses a substantial period of feeding is
required before the viruses is acquired as well as transmitted.
Virus Dispersal
1.
2.
Insects
Nematodes – Microscopic, nonsegmented worms
a. NEPO Viruses (Nematode transmitted Polyhedral viruses)
i. Examples vectors = Longidorus, Xiphenema
ii. Example viruses = tomato black ring virus, cherry leaf roll
virus
b.
NETU Viruses (Nematode transmitted Tubular viruses)
i. Examples vectors = Trichodorus, Paratrichodorus
ii. Example viruses = tobacco rattle virus, pea early browning
virus
c.
d.
e.
Nematodes acquire virus when feeding on roots of infected
plants.
Both adults and juveniles can transmit virus.
Transmitted not persistently.
Nematode Vectors
Xiphinema index
Xiphinema spp.
Longidorus spp.
Virus Dispersal
1.
2.
Insects
Nematodes
3. Soilborne fungi – Primarily Plasmodiophoromycetes and
Chytridiomycetes. Viruses are transmitted through zoospores.
a. Olpidium brassicae transmits: tobacco necrosis and lettuce big
vein virus
b. Polymyxa graminis transmits: soilborne wheat mosaic virus
and barley yellow mosaic virus
4. Seed
a. Fewer than 100 viruses are transmitted through seed.
b. Generally, < 10% of seed is infected by virus. Exception –
tobacco ringspot virus on soybean, in which seed transmission
is 100%.
Virus Dispersal
1.
2.
3.
4.
Insects
Nematodes
Soilborne fungi
Seed
5. Vegetative Plant Parts – if parent is infected, cutting is
infected
6. Parasitic Seed Plants – some viruses transmitted only
through dodder.
7. Mechanical
1. Cutting and harvesting equipment
2. Simple touch of hand or clothing.
3. Carborundum – a powdered abrasive used to wound leaf
surface before virus inoculation.