A discussion on plant viruses

1. Pawan Kumar
M.Sc. Bioinformatics
1st semester
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2. Learning Objectives
 Introduction to Plant viruses
 Infections
 Virus life cycle
 Transmission
 Structure
 Classification
 Replication
 Symptom
 Identification
 Control.
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3. Plant viruses
 Non-cellular form of life.
 Nucleic acid in protein capsid (no membrane envelop).
 Virus is a nucleo-protein having RNA or DNA as a genetic
material.
 RNA or DNA may be ss or ds, ssRNA may be +ve or –ve sense.
 The fundamental characteristic is their absolute dependence
on a living host organism for their reproduction.
 Other necessary processes (enzymatic activities) by host cell.
 Encode just few genes (x bacteriophages up to 70)
 Most of plant virus infect a limited number of different plant
species and a few have a wide host range.
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4. chronic degenerative desease decreasing fitness
growth reductionchlorotic lesions mozaic
intervein chloroses necroses leaf curling
Viral infection
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6. Plant response to inoculation
with a virus
 Plant is immune: virus does not replicate in protoplasts nor
in cells of the intact plant
 Infection is limited to initially infected cells: replication, but
no cell-to-cell movement (could be due to an ineffectual
virus movement protein)
 Infection is limited to initial leaf: replication, cell-to-cell
movement, but no long distance movement (inability to
enter phloem or due to a host resistance gene-mediated
response, i.e. hypersensitive response when the plant
resistance gene product recognizes some viral product,
local lesions formation)
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7. MODE OF ENTRY
 Mechanically TMV, BCMV, PVX, PYDV.
 Vectors PLRV,TLCV, WTV.
 Pollen Grain necrotic ring spot, ring spot of soybean.
 Seeds or vegetative:
Propagation BCMV, CMV, Pea stripe virus.
 Pinocyte CPMV, Pea enation mosaic
 Fungal parasite TNV, Barley yellow mosaic virus,
Tobacco stunt virus.
 Ectodesmata TMV.
 Epidermal hair TMV.
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8. Virus life cycle
Invasion
Genome
uncoating,
expression
and
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9. Systemic transport through phloem
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10. Spreading of viral infection
Within a plant
- plasmodesmata (movement proteiny)
- vascular tissue (phloem)
Movement proteins: - interaction with virion
- interaction with plasmodesmata
(increase of size exclussion limit)
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11. Spreading of viral infection
Between plants –
natural barriers of entrance: cuticle, cell wall
- mechanical injury, direct contact (wind)
- vectors – sucking insects,
other insects, nematods, fungi
- grafting, root coalescence,
- parasitic plants (Cuscuta)
- vegetative propagation
- some viruses also via seeds and polen!
Protection – elimination of infected plants and vector insects!
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12. Non-persistant
• adsorbtion on styletes
(specific binding sites on acrostyle)
• infectiousness:
immediate, persists only minutes to hours
Circulative
• circulation of virus in insect body – salivary glands
• infectiousness:
latent period (hours to days), gradually decreasing many days
Propagative
• virus replication in transmittor
• infectiousness:
latent period (hours to days), life-long (also transmission to progeny)
Transmission via sucking insects
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13. Proteins encoded by plant viruses
 Polymerases of NA (helicases)
 Movement proteins
- transport through plasmodesmata
 Capsid proteins
 Proteases
- cleavage of polyproteins.silencing
Different representation of these proteins in
different viruses
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14. TMV
Viral capsids
Capsomers – structural subunits (one or more capsid
proteins)
Basic shapes:
A. Helical – capsomers in helical arrangement
(e.g. Tobacco mosaic virus)
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15. capsomers form usually triangles arranged to polyhedron
(usually icosahedra – twenty sides)
- various number of proteins in a capsomer.
12 pentagons
20 hexagons
Viral capsids
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16. Classification of plant viruses
- genom/replication
 ssRNA, also dsRNA, ssDNA, dsDNA
 ssRNA - coding ssRNA(+)
- non-coding ssRNA(-)
- replication via RT (also dsDNA viruses)
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17. DNA Viruses
- transcription by RNA polymerase II from dsDNA
dsDNA viruses – replication through RNA intermediate
(reverse transcription)
ssDNA viruses – replication through dsDNA intermediate
(by host DNA polymerase)
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18. Replication cycle of ssDNA viruses
(Geminiviridae)
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19. RNA viruses
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20. dsRNA viruses
e.g. Phytoreoviridae - 12 dsRNA segments,
- viral polymerase
- transcription in cytoplasma (viroplasma)
- minus strands synthetized after encapsidation
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21. ssRNA viruses
 Ss RNA(RT):Pseudoviridae
again derived from retrotransposons.
 Classical RNA viruses:
enkapsidation of + or –RNA
 RNA+ : most frequent (Tombusviridae,
Bromoviridae, Potyviridae)
 RNA- : Rhabdoviridae
all propagate also in insect vectors
RNA dep. RNA-polymerase in capsid.
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22. Ss RNA (RT) viruses:
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Reverse transcription is applied to first make ds DNA
and then transcription and translation.

23. Ss RNA+ viruses:
 e.g. tobacco mosaic virus (TMV)
 Release of RNA
 Translation of polymerase
 RNA replication
 Translation of viral proteins (polymerase, capsid, ….)
 new virions spontaneously pack with coating protein.
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24. Infection cycle of TMV.
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25. Ss RNA- viruses:
 -ve ss RNA require RDRP(RNA dependent RNA polymerase).
 Which make +ve ss RNA from its original stand.
 These newly formed +ve ss RNA will make –ve ss RNA and
also translate to form protein capsid.
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26. Some example
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28. Symptoms:
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Systemic Symptoms:
 General:
Abnormal growth and developmental
malformations (reduced growth, dwarfing,
stunting of plants, etc.)
Reduced life span of plants, death.
 Foliage symptoms:
Mosaics
Yellows Ring spots.
 Other:
Symptoms on stems, fruit, roots.
 Local symptoms:
Necrotic lesions.

29. Detection and identification
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 Symptoms (possible causes by nutrient deficiencies,
toxicities, insects, etc.
 dsRNA isolation.
 PCR, RT-PCR (for known viruses).
 Virus genome sequencing.
 Microarray.

30. Control of plant viruses
 Keep out of an area through quarantine, inspection, and
certification programs that would prevent or limit local or
international virus movement and warranty the use of virus-free
seed, tubers, budwood .
 Eradication of diseased plants
 Controlling the insect vectors (oil sprays, repellents, insecticides,
predators…)
 Breeding plants for resistance
 Transgenic resistance (natural resistance genes, viral sequences,
genes from other sources).
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31. Reference:
 Prescott Harley Klein's Microbiology - Willey, Sherwood & Woolvertion
(7th ed.).
 Molecular Biology of the Gene - James D. Watson, Tania A. Baker,
Stephen P. Bell (5th ed.).
 Wikipedia source.
 YouTube source.
 Characteristic of Plant viral pathogen by Svetlana Folimonova (Ass.
Prof. -Dept. of Plant Pathology, University of Florida).
 Introduction to Plant Viruses, the Invisible Foe (www.apsnet.org).
 Viron | Biology Field Guide | Biowars (www.biowars.com).
 Plant Virus replication by P.N. Sharma Department of Plant Pathology,
CSK HPKV, Palampur (H.P.).
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