Taxonomy of plant viruses

MAHAM NAVEED
M.Sc Botany
Semester 1
Roll no 5

What is Taxonmoy?
 Taxonomy is the science of naming,
describing and classifying organisms and
includes all plants, animals and
microorganisms of the world.
 Taxonomists provide unique names for
species.

 In the past 250 years taxonomist gives 1.78
million species of animals, plants and micro-organisms.
 Total number of species probably between 5
and 30 million.
 Names of orders, families and genera are in
italic script.

PLANT VIRUS
Plant viruses are viruses that affect
plants.
Plant viruses are obligate
intracellular parasites.
Plant viruses are pathogenic to
higher plants.

HISTORY
 In 1898 Martinus Beijerinck discover plant
viruses from tobacco leaves with the "mosaic
disease“.
 In 1939 Holmes published a classification list
of 129 plant viruses.
 In 1999 there were 977 officially recognized,
and some provisional, plant virus species.

Virus classification
 Virus classification is based mainly on
phenotypic characteristics, including
morphology (enveloped, shape of capsid),
nucleic acid type, and mode of replication,
host organisms, and the type of disease they
cause.
 David Baltimore, a Nobel Prize-winning
biologist, devised the Baltimore classification
system.

Classification and Taxonomy
Classical: morphology
1.Physical and chemical composition
2.Genetic releatedness
Modern: Phylogenetic, based on nucleic acid
sequence analysis.

How are plant viruses classified?
 Six major groups based on the nature of the
genome:
1. Single-stranded DNA (ssDNA): there are
two families of plant viruses in this group.
 small circular genome components, often
with two or more segments.

2.Reverse-transcribing viruses
• 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.
 A single component of circular dsDNA, the
replication of which is via an RNA
intermediate.

3.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.
 Well-known viruses in this group include the
herpes and pox viruses.

4. Double-stranded RNA (dsRNA):
Some plant viruses and many of the
mycoviruses are included in this group. e.g
Chrysoviridae, Endornaviridae
Partitiviridae.
5. Negative sense single-stranded RNA
(ssRNA): some or all of the genes are
translated into protein from an RNA strand
complementary to that of the genome.

6.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").
 The causal agents of polio and foot-and-mouth
disease.

Combination of characters
 Within each of these groups, many different
characteristics are used to classify the
viruses into families, genera and species.
1.Particle morphology: the shape and size
of particles as seen under the electron
microscope.

2. 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.

3. Biological properties
This may include the type of host and also
the mode of transmission.
4. Serological properties: the relatedness
(or otherwise) of the virion protein(s).

Taxonomy of Plant Viruses (ICTV)
 (i) To develop an internationally agreed
taxonomy for viruses.
 (ii) To develop internationally agreed names
for virus taxa.
 (iii) To communicate taxonomic decisions to
the international community of virologists.
 (iv) To maintain an Index of agreed names of
virus taxa.

Rules of Classification and Nomenclature
 The primary classification is of viruses into
species.
 Most species are classified into genera and
most genera are classified into families.
 Some families are classified into Orders, but
often the family is the highest level taxon in
use.

Scope of the classification
 The classification and naming of serotypes,
genotypes, strains, variants and isolates of
virus species is the responsibility of
acknowledged international specialist groups.
 Artificially created viruses and laboratory
hybrid viruses will not be given taxonomic
consideration.

Taxonomy scheme
 Present totals:
 Family: 73
 Subfamily: 9
 Genus: 287
 Species: 1950

Rules about naming Taxa
 Names proposed for taxa are "valid names" if
they conform to the Rules set out in the Code
and they pertain to established taxa.
 The rule of priority in naming taxa shall not
be observed. Subscripts, superscripts,
oblique bars and non-Latin letters may not be
used in taxon names.
 Short names are desirable and the number
of syllables should be kept to a minimum.

Rules about Species
 A species is the lowest taxonomic level in the
hierarchy approved by the ICTV .
 A species is a monophyletic group of viruses whose
properties can be distinguished from those of other
species by multiple criteria.
 Not capitalized, unless a geographical location.
 Not italicized.
 Species names shall not consist only of a host name
and the word "virus".

Rules about Genera
 A genus is a group of species sharing certain
common characters.
 Capitalized, Italicized, ends A genus name shall be
a single word ending in ...virus .
Flavivirus-yellow fever virus
Pestivirus- Bovine Diarrhea virus 1
 Approval of a new genus must be accompanied by
the approval of a type species.

Family
 A group of genera with common
characteristics.
 Capitalized, Italicized, and end in -viridae.
Examples:
– Picornaviridae (picornavirus family is also
acceptable).
– Herpesviridae (herpesvirus family).
– Flaviviridae (flavivirus family)

Rules about Subfamilies
 A subfamily is a group of genera sharing certain
common characters.
 Groups within some large families.
 A subfamily name shall be a single word ending
in ...virinae.
 Rules about Families A family name shall be a single
word ending in ...viridae.
 Rules about Orders An order is a group of families
sharing certain common characters.An order name
shall be a single word ending in ...virales.

Rules for Orthography
 In formal taxonomic usage, the accepted
names of virus Orders, Families,
Subfamilies, and Genera are printed in italics
and the first letters of the names are
capitalized.
 Species names are printed in italics and
have the first letter of the first word
capitalized.

Examples
 When taxon names are used informally, italics and
capital initial letters are not needed. This applies at
all taxonomic levels; examples are:
 (1) "the tobacco mosaic virus polymerase", when
describing the properties of the polymerase in
members of the species Tobacco mosaic virus.
 (2) "three pestiviruses", to describe viruses that are
members of the genus Pestivirus.

Taxonomy examples
Example 1: herpes simplex virus 1
 Family: Herpesviridae or herpesvirus family
 Subfamily: Alphaherpesvirinae;
 Genus: Simplexvirus;
 Species: herpes simplex virus 1.

Plant-Virus Interactions
 Viruses are capable of infecting virtually all
species of cultivated and wild plants.
 For instance Citrus tristeza virusinfects
only a few species in the Citrus genus,
whereas Cucumber mosaic virus infects
over 1000 species in 85 plant families.
 Plants possess active and passive means of
preventing virus infection.

 Tomato spotted wilt
virus infects a wide
variety of plants
including tomato,
peanuts, and tobacco.
 In some cases, small
necrotic or chlorotic
spots called local
lesions develop at the
site of infection.

Symptoms
 Typical leaf symptoms
of viral diseases include
mosaic patterns
chlorotic or necrotic
lesions yellowing,
stripes or streaks , vein
clearing, vein banding,
and leaf rolling and
curling

In most cases, viruses spread throughout
the whole plant and cause a systemic
infection.
Most of the symptoms induced by viruses
can also occur due to adverse
environmental conditions or diseases
caused by other plant pathogens.
The symptoms induced by plant viruses
lead to reduced crop quality and yield.

Transmission of plant viruses
1.Through sap TMV, potato viruses and
cucumber mosaic viruses are transmitted via
sap.
2.Insects One class of viruses, the
Rhabdoviridae, has been proposed to
actually be insect viruses

3. 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.e.g tobacco ringspot
virus andtobacco rattle virus
.

Plasmodiophorids
 Plasmodiophorids also create wounds in the
plant's root through which other viruses can
enter.
 Polymyxa graminis, transmit plant viral
diseases in cereal crops and Polymyxa
betae which transmits
Beet necrotic yellow vein virus .

Seed and pollen borne viruses
 Plant virus transmission from generation to
generation occurs in about 20% of plant
viruses.
 Families Leguminosae, Solanaceae
, Compositae, Rosaceae, Cucurbitaceae, Gr
amineae.Bean common mosaic virus is
transmitted through seeds.

Management of Plant Virus Diseases:
 Chemical or biological control of the vector
 Growing resistant crop varieties
 Use of virus-free planting material
 Control or avoidance of this vector is of
paramount importance.
 An alternative strategy for virus control is
utilization of natural or engineered resistance
to virus infection.

Taxonomy of plant viruses

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