B.sc agriculture i principles of plant pathology u 4 viruses, viroids and bacteriphages

Viruses, Viroids and Bacteriphages

Course: B.Sc. (Agri.)Subject:- Principles of Plant PathologyUnit-4

• Definition of Virus:

• Mathwas (1981) considers a virus as a set of one or more template

molecules normally encased in a protective coat or coats of protein or

lipoprotein, which is able to organize its own replication only within

suitable host cells where its production is:

• i) Dependent on hosts protein synthesizing machinery (ribosomes).

ii) Organized from pools of required material rather than binary fission

and

iii) Located at sites which are not separated from the host cell contents

by a lipoprotein bilayer membrane.

• Bos (1983) defines virus as an infectious agent often causing disease,

invisible with the light microscope ( Sub-microscopic) , small enough

to pass through a bacterial filter, lacking a metabolism of its own and

depending on a living host cell for multiplication. Viruses are small

packages of host alien genetic information of one type. (RNA or

DNA), either in one strand or in a few segment. Encapsulated together

or separately and enclosed in a coat of one or more types of protein,

some time with an extra coat (envelope) and some other constituents.

Morphology of Viruses

Plant viruses are usually described as

1. Elongated (Rigid Rod or Flexious thread):

a. Rigid rod – (Ex) Tobacco mosaic virus (15 X300 nm)

Barley stripe mosaic virus (20x 10 nm).

b. Flexuous thread ( Ex) Potato virus X 10-13 wide upto 480 nm length , citrus tristeza

virus ( 10-14 X 2000nm).

2. Rhabdo Viruses:

Bacillus like- These are short bacillus like rods approximately 3 to 5 times as long as

they are wide.

(Ex): Potato yellow dwarf virus 75X 380 nm.

Wheat striate mosaic virus – 65X 270 nm.

Lettuce necrotic yellow virus – 52 X 300 nm.

3. Spherical (Isometric or Polyhedral) :

All spherical viruses are actually polyhedral ranging in diameter about 17 nm to 60 nm.

Ex: Tobacco Necrosis satellite virus 17 nm in diameter.

Wound tumour virus 60nm in diameter.

Tomato spotted wilt virus 70-80 nm in diameter.

PROPERTIES OF PLANT VIRUES

1. Biological Properties:

• Each virus produces its own protein, its function is to protect RNA from hostenzymes (Ribonuclease), heat, ultra violet light and chemical protein has noinfectivity.

Infectiousness of Virus:

• 1. Viruses are infectious and highly contagious, infectivity depends on virussynthesis.

2. After the entry of pathogen in host through natural opening or wounds orpollen grains, virus comes in contact with with inoculation.3. This is done by Host, since viruses do not produce enzymes. They lack theLipman Enzymatic system for the conversion of high energy into potentialenergy required for biological activity.4. So they have to depend on hosts. This is a major difference between thehost parasite relationship in viral diseases and those of other pathogen.5. The naked RNA induces host cell to form enzyme RNA polymerase. Theseenzyme in presence of viral RNA and nucleotides produce additional RNA.The new viral RNA induces host cell to produce specific protein moleculerequired for its coat.

2. Physiological Properties of Virus:

i) Dilution End Point (DEP):

Dilution of Juices containing the virus may or may not be affected the infectivity.

Tobacco mosaic virus is reported to remain infectious even in a dilution of

1:1000000 (6) Cucumber mosaic virus retains, Virulence at 1: 1000 Potatorugose

mosaic virus causes poor infection when diluted to 1:10 or 1: 100.

ii) Thermal End Point (TEP):

Effect temperature, no of viruses can be inactivated by Thermo therpy. Tomato

mosaic virus is destroyed by treating virus containing Juice at 85 to 90 degree

centigrade for 10 minutes, lower temp. Seems to have no effect on the viability of

plant viruses.

iii) Longevity in Vitro (LIV):

Retention of infectivity in storage Storability of Juice), if the Juice from mosaic

infected tobacco plant is kept; the virus can remain viable for a few hours to

several months. TMV in dried leaves or in Juice dried on filter paper remains

infective for many years

3. Chemical Properties:

i) Host Range:

Host range is useful in distinguishing the viruses from oneanother. The viruses are inoculated into indicator an plant whichdevelops typical symptoms, local lesions, ring spot systemic symptomsetc.

Ex: EMV and TMV incite symptoms on tobacco but EMV affectscucumber systematically where as TMV does not.

ii) Mutability and Strains:

The presence of genetic material in the form of RNA in plantviruses ensure that new strains of the viruses may develop probably bymutation of RNA. In tobacco mosaic virus alone, there are more than50 strains.

iii) Serological Reactions:

If a virus containing juice is injected into body of a rabbit, therabbits form antibodies, which will react with viral proteins to giveprecipitation. The reactions are specific. i. e the antibodies obtained byinoculation of strain A, of a virus will precipitate , the juice containingthe same specific type of virus

Transmission of Plant Viruses

Plant viruses are not disseminated as such by wind or water. Viruses

are transmitted from plant to plant in a number of ways such as vegetative,

propagation, mechanically through sap and by seed, pollen, insect, mites,

nematodes, dodder and fungi.

1. Transmission by Vegetative Propagation:

Plants are propagated vegetatively by budding or grafting or by

cutting or by the use of tubers, corms, bulbs or rhizome. Any virus present in

the mother plant from which these organs are taken will almost always be

transmitted to the progeny. Transmission of viruses may also occur through

natural root grafts of adjacent plant.

2. Mechanical Transmissions:

Such transmissions may takes place between closely spaced plants

after a strong wind by contact , when plants are wounded during cultural

operations, virus infected sap adhering to the tools, worker hands or cloth

accidently transmitted to the subsequently wounded plants.

E .g. Potato virus: (PV-X,TMV on Tobacco and Tomato)

3. Natural Modes of Transmission:

This includes air borne transmission through insects and

mites and soil borne transmission through nematodes and fungus.

4. Seed Transmission:

About 100 viruses have been reported to be transmitted by

seed only a small portion (1-30) of the seed derived from virus

infected. Plant transmit the virus. Frequencies varies with the host

virus combination. Tobacco ring spot virus is soybean may be

transmitted by almost all 100% of the seed of infected plant.

28-94% in squash mosaic virus in musk melon.

50-100% in barley stripe mosaic virus in barley.

5. Pollen Transmission:

Virus transmitted by pollen may infect not only the seed

but also seedling that will grow form it. Ex. Tomato and Tobacco

black ring virus.

6. Dodder Transmission:

Several plant virus can be transmit ted from one plant to

another plant through the bridge formed between the two plants

by twining the stem of parasitic plant, dodder ( Cuscuta species),

green strain of cucumber mosaic virus on N. glutinosa, lucern (

alfa-alfa) , mosaic virus to tobacco potato stem mottle virus to

tobacco. Bennett (1940) showed dodder transmission, sugar beet

curly top virus (BCTV), cucumber mosaic virus.

Natural Modes of Transmission of Viruses

This includes air borne transmission through insects and mites and soil bornetransmission through nematodes and fungus.

A)Air Borne through Insects:

The most common and economically most important means of virus transmission in the field.

Members of the order Homoptera- Aphids, Jassids, Leaf hopper, whiteflies, mealy bug , scale insects.

Thysanoptera – Thrips, coleopteran- Beetles.

Insects with sucking mouth parts carry plant viruses on their stylet-stylet borne or non persistances.

B) Circulative:

1. Circulative Viruses (Persistent):

Viruses that are acquired by their vectors through their Mouth Parts,accumulate internally, then they are passed through their tissues andintroduced into the plants again via mouth parts of the vectors.

2. Propagative Viruses:

Some Circulative viruses may multiply in their respective vector

are then called Propagative viruses.

The viruses are transmitted by diffeent types of insects and most

efficient vectorsare of sucking and bitting type of insects.

Group of viruses based upon the action of vector:

1. Non persistent viruses

2. Persistant Viruses

3. Latent viruses

4. Bacteriophage

5. Circulative viruses

C) Insect Vectors:

Virus transmitting insect is called vector Walker (1969) showed

that viruses are mostly really on insect for transmission ( 400

species of insect vector transmitting more than 200 viruses).

Initially, most plant viruses multiply at the site of infection, giving

rise to localized symptoms such as necrotic spots on the leaves. Subsequently,

the virus may be distributed to all parts of the plant either by direct cell-to-cell

spread or by the vascular system, resulting in a systemic infection involving

the whole plant. However, the problem these viruses face in re -infection and

recruitment of new cells is the same as they face initially - how to cross the

barrier of the plant cell wall. Plant cell walls necessarily contain channels

called plasmodesmata which allow plant cells to communicate with each other

and to pass metabolites between them. However, these channels are too small

to allow the passage of virus particles or genomic nucleic acids. Many (if not

most) plant viruses have evolved specialized movement proteins which modify

the plasmodesmata. One of the best known examples of this is the 30k protein

of tobacco mosaic virus (TMV).

Typically, virus infections of plants might result in effects such

as growth retardation, distortion, mosaic patterning on the leaves,

yellowing, wilting, etc. These macroscopic symptoms result from:

This protein is expressed from a sub-genomic mRNA and its

function is to modify plasmodesmata causing genomic RNA coated

with 30k protein to be transported from the infected cell to

neighbouring cells. Other viruses, such as cowpea mosaic virus (CPMV

- Comovirus family) have a similar strategy but employ a different

molecular mechanism. In CPMV, the 58/48k proteins form tubular

structures allowing the passage of intact virus particles to pass from one

cell to another.

Necrosis of cells, caused by direct damage due to virus replication

Hypoplasia, i.e. localized retarded growth frequently leading to

mosaicism (the appearance of thinner, yellow areas on the leaves)

Hyperplasia, which is excessive cell division or the growth of

abnormally large cells, resulting in the production of swollen or

distorted areas of the plant

Aphids: Transmit viruses from many different genera, including Potyvirus,

Cucumovirus and Luteovirus.The picture shows the green peach aphid Myzus

persicae, the vector of many plant viruses, including Potato virus Y

These are the most important insect vector of plant viruses and transmitgreat majority ( about 170) of the all stylet borne viruses Ex. Soybean mosaic ,cowpea aphids, borne mosaic, potato leaf roll, pea enation mosaic, lettucenecrotic yellows, red clover mosaic BYMV, MCMV, AMV

Fig-1

Whiteflies: transmit viruses from several genera but particularly those in the

genus Begomovirus. The picture shows Bemisia tabaci, the vector of many

viruses including Tomato yellow leaf curl virus and Lettuce infectious yellows

virusTobacco leaf curl, yellow vein of okra, pumpin yellow mosaic , mungyellow mosaic, cotton leaf crumple, cassava mosaic, sweet potato mosaic.

Fig-2

Hoppers: transmit viruses from several genera, including those in the families

Rhabdoviridae and Reoviridae. The picture shows Micrutalis malleifera, the treehopper

vector of Tomato pseudo-curly top virus Atleast 40 plant viruses are transmitted by leafhoppers, all leaf hoppers transmitted viruses are circulatory, several are known tomultiply in the Vector ( Propagative), cause disturbance an phloem region ex. Rice,dwarf viruses, Maize mosaic, Rice tungro viruses, Beet curly top, Potato yellow dwarf,Maize Rough dwarf.

Fig-3

• Thrips: transmit viruses in the genus Tospovirus. The picture

shows Frankinella occidentalis, the western flower thrips that

is a major vector of Tomato spotted wilt virus..

Fig-4

Nematodes: these are root-feeding parasites, some of whichtransmit viruses in the genera Nepovirus and Tobravirus. Thepicture shows an adult female of Paratrichodoruspachydermus, the vector of Tobacco rattle virus

Fig-5

• Plasmodiophorids: these are root-infecting obligate parasites traditionally

regarded as fungi but now known to be more closely related to protists.

They transmit viruses in the genera Benyvirus, Bymovirus, Furovirus,

Pecluvirus and Pomovirus. The picture shows Polymyxa graminis, the

vector of several cereal viruses including Barley yellow mosaic virus,

growing within a barley root cell

Fig-6

Root infecting fungus-Olpidium transmits 4 plants viruses.

Sr. No Fungus Virus Vectors Example

1 Olpidium brassicae

Tobacco necrosis,

cucumeber, necrosis (O.

Cucubitacearum) lettuce

big vein tobacco stunt

virus

2 Polymyxe graminis

Wheat mosaic virus, beat

necrotic yellow vein,

virus.

3 Spon Gospora subteranea Potato mop top virus

4 Synchytrium endobioticum PU-X virus

Mites: these transmit viruses in the genera Rymovirus and Tritimovirus. Thepicture shows Aceria tosichella, the vector of Wheat streak mosaic virus

Fig-7

Reference

Books

Principles of Plant Pathology by R. S. Singh

Plant Pathology by G N Agrios

Web resources

http://agriinfo.com

http://agridr.in/tnauEAgri/eagri50/PATH171/index.html

http://www.dpvweb.net/intro/

Image Reference

Fig 1-7 http://www.dpvweb.net/intro