Properties of Viruses

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Tom German 637 Russell Lab 262-2956 tlg_at_entomology
.wisc.edu Virology/Virus Vector Relationships
Plant Pathology- G. Agrios Mon Oct 20 p.
724-737, 747-750 General concepts Wed Oct 22
p. 737-743 Transmission Fri Oct 24 p.
751-757 Detection and control p. 764-774
Diseases caused by Potyviruses
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What are viruses and viroids ? Why do we care
about them? How do they multiply, spread and
cause disease? Can we control them ?
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Billions
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Tulip Flower Break Virus
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Tobacco Mosaic Virus (TMV) has played a central
role in the history of virology
The germ theory gained acceptance in the 19th
century with a recognition of bacteria as
infectious agents of disease.
- -Louis Pasteur -Robert Lister -Robert Koch
(1876, 1882)
Infectious filterable agents -Tobacco mosaic
disease (1886-1898) - Mayer - Ivanowski -
Beijerinck Stanley 1935
Bawdin Pirie 50s
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Tobacco mosaic virus symptoms on tobacco
Tobacco Mosaic Virus
From Mathews Plant Virology, Fourth Edition, 2001
http//www.apsnet.org/education/k-12plantpathways/
TeachersGuide/Activities/TMV/purpose.htm
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Tospovirus Symptoms
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Potato Leaf Roll Virus (PLRV) and Net Necrosis
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Tobacco Rattle Virus
Corky Ringspot Disease
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Maize fine streak virus causes fine chlorotic
streaks and dwarfing on maize.
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Cucumber mosaic virus (CMV)
Rugosity
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Fig. 10. TMV infection of plants with or without
the dominant N gene. (Left) Systemic mosaic
symptoms of TMV infection on nn tobacco plants.
(Right) Localized necrotic lesions typical of the
hypersensitive response on a leaf from a
TMV-infected tobacco plant carrying the N gene.
Photographs courtesy of Dr. Milt Zaitlin, Cornell
University.
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CMV-Fny CMV-M
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PVX PVY PVY ONLY
PVX ONLY
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Symptoms of Viroid Diseases
CCCVd
PSTVd
CSVd
CEVd
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Viroids are common plant pathogens which are a
serious economic problem. 25 different viroid
sequences have been determined and numerous
variants identified
Pospiviroidae are a large group, only 2 members
of the Avsunviroidae are currently known.
 
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The RNA genomes of viroids are 246-375
nucleotides in length and share many
similarities They are all single stranded
covalent circles There is extensive
intramolecular base pairing A DNA-directed RNA
polymerase makes both plus and minus strands
Replication does not depend on the presence of
a helper virus No proteins are encoded
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Circular and linear forms of the Potato Spindle
Tuber Viroid observed using a nucleic acid
spreading technique followed by high angle
shadowing with platinum metal.Magnified
350,000X.
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Host Range
Host range the Viral Perspective

• Wide host range One virus infects more than one
  plant species  
• Narrow host range Some viruses are host specific 

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Host TRange
The Plant Perspective

• Each plant species is usually susceptible to
  several different kinds of viruses
• A plant can be infected with more than one kind
  of virus at the same time
• mixed infections
• synergistic effect

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VIRUSEGeneS!! What they are ??
General Characteristics of Viruses

• Nucleoprotein
• nucleic acid surrounded by a protein coat
• acellular, lacks a metabolism of its own
• Obligate parasites
• can only multiply in living cell
• has the ability to cause disease
• Submicroscopic
• too small to be seen individually with a light
  microscope
• electron microscope (EM)

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Viruses The Ultimate Parasites

• No physiologic activity
• No energy production
• Do not divide
• Do not produce any kind of specialized
  reproductive structures
• Multiply by inducing host cells to make more virus

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• Cause disease by utilizing cellular
  
• substances during multiplication
• Interfering with host metabolism and
  developmental regulation
• Do not consume cells or kill them with toxins

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Viruses The Ultimate Parasites

• Smaller than the smallest prokaryotic cell
• Variable size range
• small--25 nm
• larger--300 nm

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Fig. 3. Classification of plant viruses. From
Van Regenmortel, M.H.V., Fauquet, C.M., Bishop,
D. et al. (1999). Virus Taxonomy Seventh Report
of the International Committee on Taxonomy of
Viruses.