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PPATH 221: Plant Virology. K. M. Golam Dastogeer Lecturer Department of PLANT PATHOLOGY BANGLADESH AGRICULTURAL university. WELCOME. Definition of a Virus.

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PPATH 221: Plant Virology

K. M. Golam Dastogeer





Definition of a Virus
  • Highly infectuous, submicroscopic and obligate intercellular parasite consisting of one or more molecules of nucleic acid surrounded by a protein coat and capable of replication only within the living cells.
  • Do not have cells or cellular organells like mitochondria, nucleus, ribosomes etc.
  • Exists as particle
  • Not visible under light microscope but can be viewed by electron microscope
  • Composed of one type of nucleic acid either RNA or DNA and protein
  • The necleic acid is the infectuou part and protein the protective part
  • Unable to grow by bunary fission but can multiplicate by means of replication using host ribosome
  • Neither living nor dead, rather they form bridge between them
  • Not self motile, therefore depend on vectos for transmission
  • Can not penetrate host cell by themselves but can enter into host cells through wound or vectors
is virus living or non living
Is virus living or non living?

(I) Living characters of viruses:

  • (a) They multiply within host cells.
  • (b) They possess genetic material, either DNA or RNA.
  • (c) There are definite races or strains.
  • (d) They exhibit mutations.
  • (e) they can be transmitted from diseased host to healthy one
(II) Non-living Characters of Viruses:

Following characters of viruses assign them as non-living:

(a) They can be crystallized.

(b) Outside the cell, they behave like inert chemicals.

(c) They do not show growth, development, nutrition, reproduction, etc.

(d) No physiological activity

(e) Do not reepire or excrete

(f) They can be precipitated.

Because of the above reasons, viruses form unique bridge between living and non-living things.

Single-stranded RNA (ssRNA): about 540 plant viruses e.g. TMV
  • Double-stranded RNA (dsRNA): ab. 40 pv e.g. Wound tumour virus
  • Single-stranded DNA (ssDNA): ab. 50 e.g. Beet curly top virus
  • Double-stranded DNA (dsDNA): ab. 30 e.g. Cauliflower mosaic virus
architechture of plant virus
Architechture of plant virus
  • 1. Isometric: apparently spherical and (depending on the species) from about 18nm in diameter upwards. The example here shows Tobacco necrosis virus, genus Necrovirus with particles 26 nm in diameter.
2 rod shaped
2. Rod-shaped:
  • 2.1. Rigid rod: eg. TMV
  • 2.2. Flexuous rod (filamentous): e.g. Potato virus Y
  • There are two types of plant virus transmission :

horizontal transmission

  • Horizontal transmission is by vectors, human pruning shears and tools, and other direct, external contamination.

Vertical transmission

  • Vertical transmission occurs when a plant gets it from its parent plant. Either through asexual propagation (cuttings) or in sexual reproduction via infected seeds.
persistent transmission: (syn. circulative transmission) a type of virus transmission in which the virus is acquired and transmitted by the vector after relatively long feeding times and remains transmissible for a prolonged period.
nematode transmission
Nematode transmission
  • Two single-stranded RNA virus genera, Nepovirus (NEPO) and Tobravirus (TOBRA), have nematode vectors
  • Nepoviruses: Comoviridae family
  • Tobraviruses: family not yet assigned
seed transmission
Seed Transmission:
  • Seed transmission occurs in two ways
  • Externally seed borne
  • due to external contamination of the seed with virus particles (TMV, PVX)
  • Internally seed borne (BCMV, CMV, BYMV, ULCV)
  • due to infection of the living tissues of the embryo.
Prion = a small infectious particle consistingof protein and lack nucleic acid.
  • Viroid = an infectious RNA particle, smaller than a virus, lacking a capsid, that causes various plant diseases
  • Virusoid (satellite nucleic acids) = same as viroid; small, ssRNA molecule, usually 500 to 2000 nucleotides in length, lacking a capsid, lack genes required for the replication virusoid require a helper (satellite) virus to replicate, causes various plant diseases.
methods of detection
Methods of detection

Based on biological properties

1. Symptomatology

2. Transmission tests

mechanical, graft,

and vector transmission

3. Physical properties

thermal inactivation point, dilution end point, and

longevity in vitro)

4. Microscopy

Electron microscopy

based on viral coat protein
based on viral coat protein
  • Precipitation and agglutination tests
  • Immunosorbent electron microscopy
  • Enzyme-linked immunosorbent assay (ELISA)
    • double antibody sandwich (DAS)ELISA
    • triple antibody sandwich (TAS) ELISA
based on virus nucleic acid
based on virus nucleic acid
  • Nucleic acid hybridization assays
    • The dot- or spot-blot hybridization assay
  • Polymerase chain reaction
    • real-time quantitative PCR assay
    • Microarrays for rapid identification of plant viruses.
the steps of a sandwich elisa
The steps of a "sandwich" ELISA
  • A surface is prepared to which a known quantity of capture antibody is bound.
  • Any nonspecific binding sites on the surface are blocked.
  • The antigen-containing sample is applied to the plate.
  • The plate is washed to remove unbound antigen.
  • A specific antibody is added, and binds to antigen (hence the 'sandwich': the Ag is stuck between two antibodies)
  • Enzyme-linked secondary antibodies are applied as detection antibodies that also bind specifically to the antibody's Fc region (nonspecific).
  • The plate is washed to remove the unbound antibody-enzyme conjugates.
  • A chemical is added to be converted by the enzyme into a color or fluorescent or electrochemical signal.
  • The absorbency or fluorescence or electrochemical signal (e.g., current) of the plate wells is measured to determine the presence and quantity of antigen.
symptoms of plant virus diseases
Symptoms of plant virus diseases
  • Symptoms around the site of virus inoculation are denoted local symptoms.
  • When virus spreads from the site of inoculation and causes symptoms in other parts of the plant, this is referred to as systemic symptoms. 
Mottling: abnormal coloration
  • MosaicIf a mottle is light and creates a mosaic
Chlorosis: reduced amount of chorophyll resulting in light color.

Vein clearing:

eins become light and more distinct

leaf morphological change
Leaf morphological change

Leaf rolling and curling: Rolling is folding of leaves along their mid axes resulting in a more or less tube-like structure. When the folding is more irregular or does not result in a tube-like structure it is usually referred to as leaf curling.

Leaf distortion: deviations from normal leaf shape 

RugoseRugose means : "rough leaves". This term covers both crinkling (leaves looking edged or wrinkled) and leaf puckering (blister-like irregularities formed on the leaf) 

EnationAbnormal outgrowth of vascular tissue in leaves or on the stem

Pea enation mosaic virus (PEMV)

Shoe string

Crown gall/ tumor

Rosette: cluster leaves