Viruses

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Viruses
Images and lecture material were not entirely
created by J. Bond. Some of this material was
created by others.
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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.

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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.

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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.

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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.

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Characteristics of Viruses

• 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
  double- stranded
• b. Protein sheath (aka. capsid) - 60 to 95
  protein, depending on virus.
• Capsid protects virus from heat, enzymes, UV and
  other denaturing agents.

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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

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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 gt 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.
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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.

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Viroids

• 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.

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Viroids

• 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.
• Recently discovered in 1971 as the cause for
  potato spindle tuber disease. Now known to cause
  20 diseases.

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How do viruses enter plants and cause symptoms?

• 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.

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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.

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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.

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Diagnosis of Plant Viruses

• 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.

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ELISA
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Agar Immunodiffusion
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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.

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Aspects of Symptom Expression

• Localized symptoms occur at the point of
  inoculation only do not spread. Local lesions.
• Systemic- symptoms are not limited to point of
  inoculation. Chlorosis, stunting, etc.

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Aspects of Symptom Expression

• Localized symptoms occur at the point of
  inoculation only do not spread. Local lesions.
• Systemic- symptoms are not limited to point of
  inoculation. Chlorosis, stunting, etc.
• Specific virus/host interactions for symptoms
  expression.
• Symptomless carriers virus is present in a host
  but symptoms do not occur.

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Aspects of Symptom Expression

• 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
• Symptom synergy- plant is infected by more than
  one virus, and resulting symptom combination is
  more severe than symptoms of each virus alone.
• 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

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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.

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Virus Dispersal

• Insects- account for the largest part of virus
  dispersal.
• Aphids, leafhoppers, planthoppers, beetles,
  thrips, whiteflies, etc.
• Insect species vary in the number of viruses they
  can carry.
• Viruses vary in the number of insect species
  that can vector them.
• Dispersal depends on
• Amount of virus in plant (aka. Titer)
• Insect mobility
• Relationship between insect and virus. Insects-
  account for the largest part of virus dispersal.

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Insect Vectors
Aphids
Whiteflies
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Insect Vectors
Thrips
Leafhopper Plant Hoppers
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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.
• 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

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Tobacco Mosaic Virus

• Non persistent
• Tobacco, tomato, and other
• solanaceous plants

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Barley Yellow Dwarf

• Hosts - barley,wheat,
• maize, rice and other grasses
• Persistent, circulative virus

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Diagram of an aphid vector feeding on a plant
host showing the internal route of the viruses
that cause BYD.
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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.

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Virus Dispersal

• Insects
• Nematodes Microscopic, nonsegmented worms
• NEPO Viruses (Nematode transmitted Polyhedral
  viruses)
• Examples vectors Longidorus, Xiphenema
• Example viruses tomato black ring virus, cherry
  leaf roll virus
• NETU Viruses (Nematode transmitted Tubular
  viruses)
• Examples vectors Trichodorus, Paratrichodorus
• Example viruses tobacco rattle virus, pea early
  browning virus
• Nematodes acquire virus when feeding on roots of
  infected plants.
• Both adults and juveniles can transmit virus.
• Transmitted not persistently.

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Nematode Vectors
Xiphinema index Xiphinema spp. Longidorus spp.
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Virus Dispersal

• Insects
• Nematodes
• Soilborne fungi Primarily Plasmodiophoromycetes
  and Chytridiomycetes. Viruses are transmitted
  through zoospores.
• Olpidium brassicae transmits tobacco necrosis
  and lettuce big vein virus
• Polymyxa graminis transmits soilborne wheat
  mosaic virus and barley yellow mosaic virus
• Seed
• Fewer than 100 viruses are transmitted through
  seed.
• Generally, lt 10 of seed is infected by virus.
  Exception tobacco ringspot virus on soybean, in
  which seed transmission is 100.

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Virus Dispersal

• Insects
• Nematodes
• Soilborne fungi
• Seed
• Vegetative Plant Parts if parent is infected,
  cutting is infected
• Parasitic Seed Plants some viruses transmitted
  only through dodder.
• Mechanical
• Cutting and harvesting equipment
• Simple touch of hand or clothing.
• Carborundum a powdered abrasive used to wound
  leaf surface before virus inoculation.