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, Gramineae.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.

Any Question class????