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International Invention Journal of Agricultural and Soil Science (ISSN: 2408-7254) Vol. 3(3) pp. 36-42, September, 2015
Available online http://internationalinventjournals.org/journals/IIJAS
Copyright ©2015 International Invention Journals
Review
Important of Plant viruses
Hamid kheyrodin 1 and sadaf kheyrodin
1-
2
Assistant prof essor in semnan university -Semnan- Iran
2Student in Mashad Islamic azad university-Iran
Abstract
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. While this article does not intend to list all plant viruses, it discusses some
important viruses as well as their uses in plant molecular biology. Viruses are similar to, but not
considered, living organisms. To be considered "living", an organism must be able to reproduce, either
asexually or sexually. Viruses, however, are only able to replicate themselves by commandeering the
reproductive apparatus of the cells of living beings and making them reproduce the virus's genetic
structure instead. Thus, a virus cannot function or reproduce outside of another being's cell, thereby
being totally dependent on a host cell in order to survive. Most viruses are species specific, and related
viruses typically only infect a narrow range of plants, animals, bacteria, or fungi. We also provide
information about viroids, which are infectious RNA molecules that cause diseases in various plants.
Their genomes are much smaller than those of viruses (up to 400 nucleotides of circular singlestranded RNA) and do not code for any proteins.
Key words: Plant viruses, Phatogenetic, Parasite.
INTRODUCTION
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). This and other viruses
cause an estimated US$60 billion loss in crop yields
worldwide each year. 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 overwhelming show that such interactions
between wild plants and their viruses do not appear to
cause disease in the host plants. (Roossinck, 2011).
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
*Corresponding Author Email: [email protected]
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 (Colson et al., 2010). Plant defenses
against viral infection include, among other measures,
the use of siRNA in response to dsRNA (Colson et al.,
2010). Most plant viruses encode a protein to suppress
this response.Pl ants also reduce transport through
plasmodesmata in response to injury (Colson et al.,
2010). figure1
History of plant viruses
The discovery of plant viruses causing disease is often
accredited to A. Mayer (1886) working in the
Netherlands demonstrated that the sap of mosaic
obtained from tobacco leaves developed mosaic
symptom when injected in healthy plants. However the
infection of the sap was destroyed when it was boiled.
kheyrodin and kheyrodin 37
He thought that the causal agent was the bacteria.
However, after larger inoculation with a large number of
bacteria, he failed to develop a mosaic symptom. In
1898, Martinus Beijerinck, who was a Professor of
Microbiology at the Technical University the
Netherlands, put forth his concepts that viruses were
small and determined that the "mosaic disease"
remained infectious when passed through a
Chamberland filter-candle. This was in contrast to
bacteria microorganisms, which were retained by the
filter. Beijerinck referred to the infectious filtrate as a
"contagium vivum fluidum", thus the coinage of the
modern term "virus", figure 2.
After the initial discovery of the ‘viral concept’ there
was need to classify any other known viral diseases
based on the mode of transmission even though
microscopic observation proved fruitless. In 1939
Holmes published a classification list of 129 plant
viruses. This was expanded and in 1999 there were 977
officially recognized, and some provisional, plant virus
species. The purification (crystallization) of TMV was first
performed by Wendell Stanley, who published his
findings in 1935, although he did not determine that the
RNA was the infectious material. However, he received
the Nobel Prize in Chemistry in 1946. In the 1950s a
discovery by two labs simultaneously proved that the
purified RNA of the TMV was infectious which reinforced
the argument. The RNA carries genetic information to
code for the production of new infectious particles.
More recently virus research has been focused on
understanding the genetics and molecular biology of
plant virus genomes, with a particular interest in
determining how the virus can replicate, move and infect
plants. Understanding the virus genetics and protein
functions has been used to explore the potential for
commercial use by biotechnology companies. In
particular, viral-derived sequences have been used to
provide an understanding of novel forms of resistance.
The recent boom in technology allowing humans to
manipulate plant viruses may provide new strategies for
production of value-added proteins in plants.
Viruses Structure
Viruses are extremely small and can only be observed
under an electron microscope. The structure of a virus is
given by its 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 (Gray and Banerjee, 1999). Figure 1, 2 and 3.
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.
Transmission of plant viruses
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. Plant viruses need to be transmitted by a vector,
most often insects such as leafhoppers. One class of
viruses, the Rhabdoviridae, has been proposed to
actually be insect viruses that have evolved to replicate
in plants. The chosen insect vector of a plant virus will
often be the determining factor in that virus's host range:
it can only infect plants that the insect vector feeds upon.
This was shown in part when the old world white fly
made it to the USA, where it transferred many plant
viruses into new hosts. Depending on the way they are
transmitted, plant viruses are classified as nonpersistent, semi-persistent and persistent. In nonpersistent 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. Those viruses that manage to pass through
the gut into the haemolymph and then to the salivary
glands are known as persistent There are two subclasses of persistent viruses: propagative and
circulative. Propagative viruses are able to replicate in
both the plant and the insect (and may have originally
been insect viruses), whereas circulative can not.
Circulative viruses are protected inside aphids by the
chaperone protein symbionin, produced by bacterial
symbionts. Many plant viruses encode within their
genome polypeptides with domains essential for
transmission by insects. In non-persistent and semipersistent viruses, these domains are in the coat protein
and another protein known as the helper component. A
bridging hypothesis has been proposed to explain how
these proteins aid in insect-mediated viral transmission.
The helper component will bind to the specific domain of
the coat protein, and then the insect mouthparts —
creating a bridge. In persistent propagative viruses, such
as tomato spotted wilt virus (TSWV), there is often a lipid
38 Int. Inv. J. Agric. Soil Sci.
Figure 1. Viral life cycle
Figure 2. Yellow mosaic symptoms on
lettuce caused by Lettuce mosaic virus.
Figure from Description 399 (photo: INRA
Avignon, France).
Figure
3.
Yellow
vein-banding
symptoms on grapevine caused by
Grapevine fanleaf virus. Figure from
Description 385
coat surrounding the proteins that is not seen in other
classes of plant viruses. In the case of TSWV, 2 viral
proteins are expressed in this lipid envelope. It has been
proposed that the viruses bind via these proteins and are
then taken into the insect cell by receptor-mediated
endocytosis.
Nematodes and Plasmodiophorids
Soil-borne nematodes also have been shown to transmit
viruses (Kanyuka et al., 2003). They acquire and
transmit them by feeding on infected roots. Viruses can
be transmitted both non-persistently and persistently, but
kheyrodin and kheyrodin 39
there is no evidence of viruses being able to replicate in
nematodes. The virions attach to the stylet (feeding
organ) or to the gut when they feed on an infected plant
and can then detach during later feeding to infect other
plants. Examples of viruses that can be transmitted by
nematodes include tobacco ringspot virus and tobacco
rattle virus. 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. Examples include Polymyxa
graminis, which has been shown to transmit plant viral
diseases in cereal crops (Kneller et al., 2006), and
Polymyxa betae which transmits Beet necrotic yellow
vein virus. Plasmodiophorids also create wounds in the
plant's root through which other viruses can enter.
Seed and pollen borne viruses
Plant virus transmission from generation to generation
occurs in about 20% of plant viruses. When viruses are
transmitted by seeds, the seed is infected in the
generative cells and the virus is maintained in the germ
cells and sometimes, but less often, in the seed coat.
When the growth and development of plants is delayed
because of situations like unfavourable weather, there is
an increase in the amount of virus infections in seeds.
There does not seem to be a correlation between the
location of the seed on the plant and its chances of
being infected (Gray and Banerjee, 1999). Little is
known about the mechanisms involved in the
transmission of plant viruses via seeds, although it is
known that it is environmentally influenced and that seed
transmission occurs because of a direct invasion of the
embryo via the ovule or by an indirect route with an
attack on the embryo mediated by infected gametes
(Gray and Banerjee, 1999; Hull, 2001). These processes
can occur concurrently or separately depending on the
host plant. It is unknown how the virus is able to directly
invade and cross the embryo and boundary between the
parental and progeny generations in the ovule (Hull,
2001). Many plants species can be infected through
seeds including but not limited to the families
Leguminosae, Solanaceae, Compositae, Rosaceae,
Cucurbitaceae, Gramineae (Gray and Banerjee, 1999).
Bean common mosaic virus is transmitted through
seeds.
Direct plant-to-human transmission
Researchers from the University of the Mediterranean in
Marseille, France have found tenuous evidence that
suggest a virus common to peppers, the Pepper Mild
Mottle Virus (PMMoV) may have moved on to infect
humans (Oparka and Alison 2001). This is a very rare
and highly unlikely event as, to enter a cell and replicate,
a virus must "bind to a receptor on its surface, and a
plant virus would be highly unlikely to recognize a
receptor on a human cell. One possibility is that the virus
does not infect human cells directly. Instead, the naked
viral RNA may alter the function of the cells through a
mechanism similar to RNA interference, in which the
presence of certain RNA sequences can turn genes on
and off," according to Virologist Robert Garry from the
Tulane University in New Orleans, Louisiana (Roossinck,
2011).
Translation of plant viral proteins
75% of plant viruses have genomes that consist of single
stranded RNA (ssRNA). 65% of plant viruses have
+ssRNA, meaning that they are in the same sense
orientation as messenger RNA but 10% have -ssRNA,
meaning they must be converted to +ssRNA before they
can be translated. 5% are double stranded RNA and so
can be immediately translated as +ssRNA viruses. 3%
require a reverse transcriptase enzyme to convert
between RNA and DNA. 17% of plant viruses are ssDNA
and very few are dsDNA, in contrast a quarter of animal
viruses are dsDNA and three quarters of bacteriophage
are dsDNA (TechVert. 2010).
Viruses use the plant
ribosomes to produce the 4-10 proteins encoded by their
genome. However, since many of the proteins are
encoded on a single strand (that is, they are
polycistronic) this will mean that the ribosome will either
only produce one protein, as it will terminate translation
at the first stop codon, or that a polyprotein will be
produced. Plant viruses have had to evolve special
techniques to allow the production of viral proteins by
plant cells. At one extreme, the association occurs within
the feeding apparatus of the insect, where the virus can
be rapidly adsorbed and then released into a different
plant cell. The feeding insect looses the virus rapidly
when feeding on a non-infected plant. Such a
relationship is termed "non-persistent". The best studied
examples are of potyvirus transmission by aphids. At
the other extreme, the virus is taken up into the vector,
circulates within the vector body and is released through
the salivary glands. The vector needs to feed on an
infected plant for much longer and there is an interval
(perhaps several hours) before it can transmit. Once it
becomes viruliferous, the vector will remain so for many
days and such a relationship is therefore termed
"persistent" or "circulative". The best studied examples
are of luteovirus transmission by aphids. In some
examples of this type (e.g. some hoppers and thrips),
the virus multiplies within the vector and this is termed
"propagative".
Genome properties: Important features include:
 Nature of the genome: circular (as in all known plant
DNA viruses) or linear.
 Number of genome components: This varies from a
40 Int. Inv. J. Agric. Soil Sci.
single component (e.g. in the genera Potyvirus and
Tobamovirus) to 11 (in some members of the genus
Nanovirus). Individual components vary in size from
about 1kb (Nanovirus components) to about 20 kb (in
the genus Closterovirus).
 Number of genes: These vary considerably. Most
plant viruses have at least 3 genes: 1 (or more)
concerned with replication of the nucleic acid, 1 (or
more) concerned with cell-to-cell movement of the virus
and 1 (or more) encoding a structural protein that is
assembled into the virus particle (usually called the
"coat" or "capsid" protein). There may also be additional
genes that have a regulatory function or which are
required for transmission between plants (association
with a vector).
 Translation strategy: A variety of strategies are
employed to translate the genes from the genome
components either directly or via mRNA intermediates
and (in some cases) to permit different amounts of
protein to be produced from the different genes. These
are summarised for each genus in the genus description
pages but 3 examples here serve to illustrate some of
the variety:
Genus Potyvirus: in this very large genus, there is
one ssRNA component that encodes one large (c. 350
kDa) polyprotein. This is cleaved by 3 different proteases
(all encoded by the virus itself) into 10 different mature
proteins. The two proteins at the C-terminus of the
polyprotein are respectively an RNA-dependent RNA
polymerase (NIb, involved in replication of the virus) and
the (single) coat protein (CP). Many of the proteins have
multiple functions. The genome organisation of a typical
member is shown here, indicating the 10 mature proteins
and the nine cleavage sites (arrowed) figure 4.
Genus Furovirus: in this genus there are two ssRNA
components. The 5'-proximal gene on each RNA is
translated directly from the genomic RNA: on RNA1 (the
larger RNA component) this gene encodes a replication
protein and on RNA2 it is the coat protein. The stop
codons of both of these genes are "leaky" and in a small
percentage of cases, translation continues to produce a
larger ("readthrough") protein. On RNA1, the replication
protein is extended to include an RNA-dependent RNA
polymerase (RdRp) while the readthrough region of the
coat protein is probably required for particle assembly
and for transmission by the plasmodiophorid vector.
There is a further (3'-proximal) gene on each of the
RNAs and these are translated from shorter RNA
molecules transcribed from the 3'-end of the genomic
RNA ("subgenomic" mRNAs). That from RNA1 is a cellto-cell movement protein (MP) that enables the virus to
move between adjacent plant cells via the
plasmodesmata while the function of the product from
RNA2 is uncertain but may involve supression of the
host plant defence reaction. The genome organisation of
a typical member is shown here figure 5.
o
Genus Fijivirus: in this genus there are 10
components of dsRNA. Most of the components encode
a single protein and at least 3 of these are structural
proteins assembled into the complex virion.
 Genome relatedness: the degree of nucleotide
identity (or amino acid identity in the protein sequence)
between sequences is often used to examine the
relationship between different viruses or isolates. For
example, recent studies in the genus Carlavirus show
that when different species are compared, they have
less than 73% nucleotide identity (or 80% amino acid
identity) in their coat proteins.
Biological properties:
 In some families, the type of host is a useful feature
for classification. For example, in the family Reoviridae,
there are currently 3 genera with plant-infecting
members (Fijivirus, Oryzavirus, Phytoreovirus), 1 genus
of mycoviruses (Mycoreovirus), 1 genus containing
viruses of fish and cephalopods (Aquareovirus), two
genera that are restricted to insects (Cypovirus and
Entomoreovirus) and 5 genera of vertebrate viruses that
sometimes also infect insects.
 The mode of transmission is also a useful
characteristic of some groups of plant viruses. For
example in the family Potyviridae, members of the
largest genus (Potyvirus) are transmitted by aphids,
while viruses in the genera Rymovirus and Tritimovirus
are transmitted by mites of the genus Abacarus or
Aceria respectively, those in the genus Ipomovirus are
transmitted by whiteflies and those in the genus
Bymovirus by plasmodiphorids (root-infecting parasites
once considered to be fungi but probably more closely
related to protists).
Serological properties: Many viruses are good
antigens (elicit strong antibody production when purified
preparations are injected into a mammal) and this
property has been widely exploited to produce specific
antibodies that can be used for virus detection and for
examining relationships between viruses. Earlier studies
used agar diffusion plates but in the last 20 years these
have been largely superseded by ELISA (enzyme-linked
immunosorbent assay) procedures. Although serological
properties are still important, their significance in
taxonomy has declined to some extent now that
nucleotide sequence data are available.
CONCLUSION
Viruses are very small (submicroscopic) infectious
particles (virions) composed of a protein coat and a
nucleic acid core. They carry genetic information
encoded in their nucleic acid, which typically specifies
two or more proteins. Translation of the genome (to
produce proteins) or transcription and replication (to
produce more nucleic acid) takes place within the host
cell and uses some of the host's biochemical
"machinery". Viruses do not capture or store free energy
and are not functionally active outside their host. They
are therefore parasites (and usually pathogens) but are
kheyrodin and kheyrodin 41
Figure 4. Genus Potyvirus
Figure 5. Genus Furovirus
not usually regarded as genuine microorganisms. The
highest level of virus classification recognises six major
groups, based on the nature of the genome:
 Double-stranded DNA (dsDNA): there are no plant
viruses in this group, which is defined to include only
those viruses that replicate without an RNA intermediate
(see Reverse-transcribing viruses, below). It includes
those viruses with the largest known genomes (up to
about 400,000 base pairs) and there is only one genome
component, which may be linear or circular. Well-known
viruses in this group include the herpes and pox viruses.
 Single-stranded DNA (ssDNA): there are two families
of plant viruses in this group and both of these have
small circular genome components, often with two or
more segments.
 Reverse-transcribing viruses: these have dsDNA or
ssRNA genomes and their replication includes the
synthesis of DNA from RNA by the enzyme reverse
transcriptase; many integrate into their host genomes.
The group includes the retroviruses, of which Human
immunodeficiency virus (HIV), the cause of AIDS, is a
member. There is a single family of plant viruses in this
group and this is characterised by a single component of
circular dsDNA, the replication of which is via an RNA
intermediate.
 Double-stranded RNA (dsRNA): some plant viruses
and many of the mycoviruses are included in this group.
 Negative sense single-stranded RNA (ssRNA-): in
this group, some or all of the genes are translated into
protein from an RNA strand complementary to that of the
genome (as packaged in the virus particle). There are
some plant viruses in this group and it also includes the
viruses that cause measles, influenza and rabies.
 Positive sense single-stranded RNA (ssRNA+): the
majority of plant viruses are included in this group. It also
includes the SARS coronavirus and many other viruses
that cause respiratory diseases (including the "common
cold"), and the causal agents of polio and foot-andmouth disease.
Within each of these groups, many different
characteristics are used to classify the viruses into
families, genera and species. Typically, combinations of
characters are used and some of the most important are:
 Particle morphology: the shape and size of particles
as seen under the electron microscope.
 Genome properties: this includes the number of
genome components and the translation strategy. Where
genome sequences have been determined, the
relatedness of different sequences is often an important
factor in discriminating between species.
 Biological properties: this may include the type of host
and also the mode of transmission.
 Serological properties: the relatedness (or otherwise)
of the virion protein(s).
The survey generated more than 250 votes from the
international community, and allowed the generation of a
Top 10 plant virus list for Molecular Plant Pathology. The
Top 10 list includes, in rank order, (1) Tobacco mosaic
42 Int. Inv. J. Agric. Soil Sci.
virus, (2) Tomato spotted wilt virus, (3) Tomato yellow
leaf curl virus, (4) Cucumber mosaic virus, (5) Potato
virus Y, (6) Cauliflower mosaic virus, (7) African cassava
mosaic virus, (8) Plum pox virus, (9) Brome mosaic virus
and (10) Potato virus X, with honourable mentions for
viruses just missing out on the Top 10, including Citrus
tristeza virus, Barley yellow dwarf virus, Potato leafroll
virus and Tomato bushy stunt virus. This review article
presents a short review on each virus of the Top 10 list
and its importance, with the intent of initiating discussion
and debate amongst the plant virology community, as
well as laying down a benchmark, as it will be interesting
to see in future years how perceptions change and
which viruses enter and leave the Top 10.
ACKNOWLEDGEMENTS
Research presented in this manuscript is supported by
Semnan University, Iran and is gratefully acknowledged.
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How to cite this article: kheyrodin H, kheyrodin S (2015). Important of
Plant viruses. Int. Inv. J. Agric. Soil Sci. Vol. 3(3): 36-42