It is about Virology (study of viruses).

1. Madeby:MadihaSiddiqui
B.SMicrobiology
Natureofanimalandplantvirus

2. NATUREOFPLANTANDANIMALVIRUSES!
Virus:
A piece of a bad news wrapped in a protein coat
(PETER MEDAWAR)
A virus is the set of genes composed of either DNA or RNA
packaged in a protein containing coat. Some viruses also have
an outer lipid bilaye membrane external to the coat called an
envelope. The resulting complete virus particle is called virion.
Viruses have an obligate requirement for intra cellular
growthand a heavy dependence on host cell structural and
metabolic components. Viruses don’t have a nucleus, cytoplasm,
mitochondria or other cell organelles.
 Viruses that infect humans and animals are called animal
viruses
 Viruses that infect plant are called plant viruses
 Viruses that infect bacteria are called bacteriophages

3. Plant viruses:
Plant viruses are viruses that affect plants. Like all other
viruses, plant viruses are obligate intracellular parasites that
do not have the molecular machinery to replicate without a
host. Plant viruses are pathogenic to higher plants
Although plant viruses are not nearly as well understood as
the animal counterparts, one plant virus has become iconic.
The first virus to be discovered (see below) was Tobacco
mosaic virus (TMV). Plant viruses are grouped into 73 genera
and 49 families. However, these figures relate only to
cultivated plants that represent only a tiny fraction of the
total number of plant species. Viruses in wild plants have been
poorly studied, but those studies that exist almost

4. overwhelming show that such interactions between wild plants
and their viruses do not appear to cause disease in the host
plants.
To transmit from one plant to another and from one plant cell
to another, plant viruses must use strategies that are usually
different from animal viruses. Plants do not move, and so
plant-to-plant transmission usually involves vectors (such as
insects). Plant cells are surrounded by solid cell walls,
therefore transport through plasmodesmata is the preferred
path for virions to move between plant cells. Plants probably
have specialized mechanisms for transporting mRNAs through
plasmodesmata, and these mechanisms are thought to be used
by RNA viruses to spread from one cell to another.
Plant defenses against viral infection include, among other
measures, the use of siRNA in response to dsRNA. Most plant
viruses encode a protein to suppress this response.Plants also
reduce transport through plasmodesmata in response to
injury.
Structure of Plant virus:
Viruses are extremely small and can only be observed under an
electron microscope. The structure of a virus is given by its

5. coat of proteins, which surround the viral genome. Assembly
of viral particles takes place spontaneously.
Over 50% of known plant viruses are rod-shaped (flexuous or
rigid). The length of the particle is normally dependent on the
genome but it is usually between 300–500 nm with a diameter
of 15–20 nm. Protein subunits can be placed around the
circumference of a circle to form a disc. In the presence of
the viral genome, the discs are stacked, then a tube is
created with room for the nucleic acid genome in the middle.
The second most common structure amongst plant viruses are
isometric particles. They are 25–50 nm in diameter. In cases
when there is only a single coat protein, the basic structure
consists of 60 T subunits, where T is an integer. Some viruses
may have 2 coat proteins that associate to form an
icosahedral shaped particle.
There are three genera of Geminiviridae that possess
geminate particles which are like two isometric particles stuck
together.
A very small number of plant viruses have, in addition to their
coat proteins, a lipid envelope. This is derived from the plant
cell membrane as the virus particle buds off from the cell.

6. Transmission of plant viruses
Through sap
Viruses can be spread by direct transfer of sap by contact of a wounded
plant with a healthy one. Such contact may occur during agricultural
practices, as by damage caused by tools or hands, or naturally, as by an
animal feeding on the plant. Generally TMV, potato viruses and
cucumber mosaic viruses are transmitted via sap.
Insects
Plant viruses need to be transmitted by a vector, most often insects
such as leafhoppers.
Depending on the way they are transmitted, plant viruses are classified
as
1. non-persistent,
2. semi-persistent
3. persistent.
In non-persistent transmission, viruses become attached to the distal
tip of the stylet of the insect and on the next plant it feeds on, it
inoculates it with the virus.
Semi-persistent viral transmission involves the virus entering the
foregut of the insect.
Viruses that manage to pass through the gut into the haemolymph and
then to the salivary glands are known as persistent.
Nematodes

7. Soil-borne nematodes also have been shown to transmit viruses. They
acquire and transmit them by feeding on infected roots. Viruses can be
transmitted both non-persistently and persistently
Plasmodiophorids
A number of virus genera are transmitted, both persistently and non-
persistently, by soil borne zoosporic protozoa. These protozoa are not
phytopathogenic themselves, but parasitic. Transmission of the virus
takes place when they become associated with the plant roots.
Seed and pollen borne viruses
Direct plant-to-human transmission
Well understood plant viruses
Tobacco mosaic virus (TMV) and Cauliflower mosaic virus (CaMV) are
frequently usedin molecular microbiology
Animal viruses:
Animal viruses are viruses that infect animals. Viruses infect
all cellular life and although viruses infect every animal, plant
and protist species, each has their own specific range of
viruses that often infect only that species.
Animal viruses, unlike the viruses of plants and bacteria, do
not have to penetrate a cell wall to gain access to the host
cell. Non-enveloped or "naked" animal viruses may enter cells

8. in two different ways. When a protein in the viral capsid binds
to its receptor on the host cell, the virus may be taken inside
the cell via a vesicle during the normal cell process of
receptor-mediated endocytosis.
An alternative method of cell penetration used by non-
enveloped viruses is for capsid proteins to undergo shape
changes after binding to the receptor, creating channels in
the host cell membrane. The viral genome is then "injected"
into the host cell through these channels in a manner
analogous to that used by many bacteriophages.
Enveloped viruses also have two ways of entering cells after
binding to their receptors:
 receptor-mediated endocytosis
 fusion.
Many enveloped viruses enter the cell by receptor-mediated
endocytosis in a fashion similar to some non-enveloped
viruses. On the other hand, fusion only occurs with enveloped
virions. These viruses, which include HIV among others, use
special fusion proteins in their envelopes to cause the
envelope to fuse with the plasma membrane of the cell, thus
releasing the genome and capsid of the virus into the cell
cytoplasm.

9. After making their proteins and copying their genomes, animal
viruses complete the assembly of new virions and exit the cell.
Using the example of HIV, enveloped animal viruses may bud
from the cell membrane as they assemble themselves, taking a
piece of the cell's plasma membrane in the process. On the
other hand, non-enveloped viral progeny, such as rhinoviruses,
accumulate in infected cells until there is a signal for lysis or
apoptosis, and all virions are released together.
Different viruses can infect all the organs and tissues of the
body and the outcomes range from mild or no symptoms, to
life-threatening diseases. Humans cannot be infected by plant
or insect viruses, but they are susceptible to infections with
viruses from other vertebrates. These are called viral
zoonoses or zoonotic infections. Examples include, rabies,
yellow fever and pappataci fever.
receptor-mediated endocytosis is a process by which
cells internalize molecules (endocytosis) by the inward budding
of plasma membrane vesicles containing proteins with
receptor sites specific to the molecules being internalized
Some important plant viruses are :
Peanut stunt virus
Maize mosaic virus
Lettuce mosaic virus

10. Cauliflower mosaic virus
Sugarcane mosaic virus
Bud blight
Cucumber mosaic virus
Tobacco mosaic virus
Some important animal viruses are:
African swine fever virus
Herpesviruses
Porcine circoviruses
Bluetongue virus
Avian influenza
Hendra and Nipha virus (paramyxoviruses)
Coronavirus
Arterivirus
Pestiviruses
Alphaviruses
Ebola virus