HISTORY OF
VIRUSES
The
first virus discovered was “Tobacco
Mosaic Virus” at the end of 19th century through the
complementary efforts of three scientists. Adolf Meyer proposed a name “Tobacco
Mosaic” to highly contagious disease of tobacco in Netherlands (Figure 1). He
suggested that the causal agent could not be abacteria since it was not seen under
microscope or to cultivate in vitro.
Dimitri Ivanovsky carried out a decisive experiment in 1886 in Russia showing
that “the mosaic disease retains its qualities even after filteration through
Chamberland filteration candles”. As bacteria are retained by such filters, a
new concept of filterable pathogens was born.
Figure
1.Mosaic Disease caused by virus (http://www.google.co.in/imgres?q=plants+virus+disease)
Martinus
Beijerinck in the Netherlands carried on the research work started by Meyer. He
published a paper in 1898, he named the agent of the tobacco disease “virus”,
and confirmed it to be filterable (“a soluble living germ”) and definitively
different from bacteria. He also showed that the “virus” multiplies in young
tobacco leaves and suggested a possible migration through the phloem.
Human
infections today known as viral diseases were identified a very long time ago, as
shown by records of poliomyelitis and chicken pox in ancient Egypt. In China,
by the 17th century, variolation was developed to limit smallpox outbreaks.
In the 19th century, vaccination against smallpox was extensively
used. In his studies on rabies, Louis Pasteur named the agent “virus”, a term
describing a poison or infectious matter. But the nature of the agent not
understood completely different from bacteria. Following Beijerinck’s results
on tobacco mosaic, filterable agents were recognized as responsible for foot
and mouth disease of cattle in 1898 by E. Loeffler and P. Frosch, foe yellow
fever by W. Reed and co-workers in 1901, and for poliomyelitis by K.
Landsteiner and E. Popper in 1909.
Numerous
viral diseases have been described since 1900, the viral nature of an
infectious agent being assessed by its filterability. In plants, attention was
first focussed upon the description of symptoms, host ranges and indicator
plants, and transmission by mechanical means or by insects. Studies on viral
structure and composition started in 1935 when W. Stanley published a paper in Nature showing that the viral particles
of TMV could be obtained as a proteinaceous crystal that was still infectious.
Later, Bawden and coworkers in 1936, showed that the TMV particle was a
nucleoprotein containing a pentose nucleic acid and Kausche et al. in 1939, using electron
microscope, could visualize for the first time the morphology of a virus
particle, the rod shaped TMV (Figure 2).
Figure
2.TMV under electron microscope
Viruses
multiply in a widest possible range of organisms: in archaebacteria, bacteria,
algae, fungi, plants, invertebrates and vertebrates. Viruses are responsible
for more than half of the infectious diseases that infect humans (including
influenza, smallpox, poliomyelitis, AIDS and some cancers). They cause some
diseases which reduce yields of plants.
A VIRAL
PARTICLE: A SIMPLE GEOMATRIC ARCHITECTURE
Viral
particles are smaller by several orders of magnitude than the cells they
infect. They are measured in nanometers and are invisible under microscope and
observation under electron microscope shows particles of a wide variety of
shapes sizes. They are surrounded by a protein capsid, which is an assemblage
of elements organized in regular geometric forms (icosahedral, helical) and
sometimes also a lipid membrane.
The viral
genome: relatively a small amount of information
The
analysis of elements constituting virus shows a very simple basic structures: a
nucleic acid (DNA and RNA) surrounded by a protein shell (capsid), sometimes
covered by an envelope. The quantity of the genetic information carried by the
virus, estimated by the number of genes, is much lesser than that of the
smallest of cells. Plant viruses generally encode for 4-12 proteins and the
more complex animal viruses up to 250 proteins. To express the proteins viruses
totally depend upon the host system of protein synthesis.
The support of
information: DNA or RNA
While
cellular organisms have double stranded genomes, viruses have both DNA and RNA
genomes. Viruses are the only taxonomic group in which the genomes are present
in such a great diversity of nature (DNA or RNA) and structure (single or
double stranded, monopartite or multipartite). The viral genera are divided
into various groups depending upon the nature of genome present.
Plant
viruses are present in five of the following six groups:
1.
Single-stranded
positive RNA: most plant viruses are found in this group. The genomic RNA is
“positive” or messanger.
2.
Single-stranded
negative RNA: the strand complementary to negative is the messanger.
3.
Double
stranded RNA: the genome is made up of several molecules of ssRNA.
4.
Single
stranded DNA: the genome is made up of several molecules of circular single-
stranded molecules.
5.
Double-stranded
retroid DNA: the genome consists of double-stranded, circular DNA molecules and
replication involves a reverse transcription step (Figure 3).
Figure 3.Classification of Viruses
Viruses are
intracellular parasites
The
viral particle has no metabolism of its own and is inert. To become active and
multiply, the virus needs to penetrate a cell and metabolize the metabolic
resources for its own replication. The energy it needs is provided by the
cellular metabolites. The physicochemical properties of the viral particles and
the characterstics of their relationship with a living cell together define a
virus as an “integrated biological system” (Van Regenmortel and Fauquet, 2000).
When virus has successfully entered the cell, it releases its genetic
information. If the cell provides the requisite machinary, a cycle begins
during which viral proteins and copies of genomes are produced. At the end of
the cycle, many daughter particles are assembled from the pool of components,
nucleic acids and capsid, as well as, viral proteins in some cases.
There
are various stages in viral cycle and they often overlap:
1.
Penetration and
decapsidation: viruses penetrate through the infections,
wounds present on the surface of plants and release their genome in cells.
2.
Translation: The viral
message is translated by the cellular ribosomes, which synthesize structural
and non-structural proteins. The proteins are involved in one or more functions
like replication, movement and transmission.
3.
Replication: this is the
process that ends in the multiplication of the incoming nucleic acid into many
copies, it involves the formation of complementary chains. One or several viral
proteins are necessary for the replication. They participate directly or
indirectly in the replication. Because of the simplicity of their structure and
their specific enzymes, which can escape certain regulations by host, viruses
can multiply much faster than the cells that harbor them. They use the
metabolism and the resources of the cell and rarely cause cell death. Their
presence is often manifested in external symptoms like mosaics, yellowing,
discoloring, rolling or stunting and as internal symptoms such as inclusions.
4.
Encapsidation: the components,
nucleic acids and proteins are assembled spontaneously to form new particles. A
viral particle cannot grow or divide.
From
the first infected cell, the virus invades rest of the cells of the plant by
using the intracellular communications that carry out physiological exchanges
between the cells. The invasion does not occur without the reaction from the
plant, which in each cell produces mechanisms to degrade the viral messanger.
Sometimes, the plant detects the infection at a very early stage and manages to
circumvent it. The plants sensitive to the virus make up for the host range of
that virus.
Transmission of
the viruses
In
nature, most plant viruses are selected continuously for transmission by
vectors. Viruses are strictly obligate parasites that, in order to multiply,
must enter living cells. To perpetuate themselves, viruses contaminate other
individuals derived from the infected plant or infect new individuals of the
same species or other plant species, most important through intermediary of a
vector.
Diagnosis of
viral diseases
Viruses
are difficult to diagnose and identify because of their diversity and number. The
symptomology alone is insufficient: current methods are based on infectious,
immunological, and genomic properties.
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