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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
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.
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)
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.
Epidermal hair TMV.
Virus life cycle
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)
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!
• adsorbtion on styletes
(specific binding sites on acrostyle)
immediate, persists only minutes to hours
• circulation of virus in insect body – salivary glands
latent period (hours to days), gradually decreasing many days
• virus replication in transmittor
latent period (hours to days), life-long (also transmission to progeny)
Transmission via sucking insects
Proteins encoded by plant viruses
Polymerases of NA (helicases)
- transport through plasmodesmata
- cleavage of polyproteins.silencing
Different representation of these proteins in
Capsomers – structural subunits (one or more capsid
A. Helical – capsomers in helical arrangement
(e.g. Tobacco mosaic virus)
capsomers form usually triangles arranged to polyhedron
(usually icosahedra – twenty sides)
- various number of proteins in a capsomer.
Classification of plant viruses
ssRNA, also dsRNA, ssDNA, dsDNA
ssRNA - coding ssRNA(+)
- non-coding ssRNA(-)
- replication via RT (also dsDNA viruses)
- transcription by RNA polymerase II from dsDNA
dsDNA viruses – replication through RNA intermediate
ssDNA viruses – replication through dsDNA intermediate
(by host DNA polymerase)
Replication cycle of ssDNA viruses
e.g. Phytoreoviridae - 12 dsRNA segments,
- viral polymerase
- transcription in cytoplasma (viroplasma)
- minus strands synthetized after encapsidation
again derived from retrotransposons.
Classical RNA viruses:
enkapsidation of + or –RNA
RNA+ : most frequent (Tombusviridae,
RNA- : Rhabdoviridae
all propagate also in insect vectors
RNA dep. RNA-polymerase in capsid.
Ss RNA (RT) viruses:
Reverse transcription is applied to first make ds DNA
and then transcription and translation.
Ss RNA+ viruses:
e.g. tobacco mosaic virus (TMV)
Release of RNA
Translation of polymerase
Translation of viral proteins (polymerase, capsid, ….)
new virions spontaneously pack with coating protein.
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.
Abnormal growth and developmental
malformations (reduced growth, dwarfing,
stunting of plants, etc.)
Reduced life span of plants, death.
Yellows Ring spots.
Symptoms on stems, fruit, roots.
Detection and identification
Symptoms (possible causes by nutrient deficiencies,
toxicities, insects, etc.
PCR, RT-PCR (for known viruses).
Virus genome sequencing.
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,
Breeding plants for resistance
Transgenic resistance (natural resistance genes, viral sequences,
genes from other sources).
Prescott Harley Klein's Microbiology - Willey, Sherwood & Woolvertion
Molecular Biology of the Gene - James D. Watson, Tania A. Baker,
Stephen P. Bell (5th ed.).
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.).