This document discusses plant viruses, including their definition, morphology, properties, transmission, and movement within plants. It provides definitions of viruses from Mathwas and Bos, and describes three main shapes of plant viruses: elongated, rhabdo, and spherical. It covers biological properties like infectivity, physiological properties like thermal endpoint, and chemical properties including host range and serological reactions. Methods of virus transmission include vegetative propagation, mechanical transmission, and natural transmission by insects, mites, nematodes, seed, pollen, and dodder. Viruses spread systemically within plants by modifying plasmodesmata to move between cells.

1. Viruses, Viroids and Bacteriphages
Course: B.Sc. (Agri.)
Subject:- Principles of Plant Pathology
Unit-4

2. • 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.

3. 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.

4. PROPERTIES OF PLANT VIRUES
1. Biological Properties:
• Each virus produces its own protein, its function is to protect RNA from host
enzymes (Ribonuclease), heat, ultra violet light and chemical protein has no
infectivity.
Infectiousness of Virus:
• 1. Viruses are infectious and highly contagious, infectivity depends on virus
synthesis.
2. After the entry of pathogen in host through natural opening or wounds or
pollen grains, virus comes in contact with with inoculation.
3. This is done by Host, since viruses do not produce enzymes. They lack the
Lipman Enzymatic system for the conversion of high energy into potential
energy required for biological activity.
4. So they have to depend on hosts. This is a major difference between the
host parasite relationship in viral diseases and those of other pathogen.
5. The naked RNA induces host cell to form enzyme RNA polymerase. These
enzyme in presence of viral RNA and nucleotides produce additional RNA.
The new viral RNA induces host cell to produce specific protein molecule
required for its coat.

5. 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

6. 3. Chemical Properties:
i) Host Range:
Host range is useful in distinguishing the viruses from one
another. The viruses are inoculated into indicator an plant which
develops typical symptoms, local lesions, ring spot systemic symptoms
etc.
Ex: EMV and TMV incite symptoms on tobacco but EMV affects
cucumber systematically where as TMV does not.
ii) Mutability and Strains:
The presence of genetic material in the form of RNA in plant
viruses ensure that new strains of the viruses may develop probably by
mutation of RNA. In tobacco mosaic virus alone, there are more than
50 strains.
iii) Serological Reactions:
If a virus containing juice is injected into body of a rabbit, the
rabbits form antibodies, which will react with viral proteins to give
precipitation. The reactions are specific. i. e the antibodies obtained by
inoculation of strain A, of a virus will precipitate , the juice containing
the same specific type of virus

7. 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)

8. 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.

9. 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.

10. Natural Modes of Transmission of
Viruses

11. This includes air borne transmission through insects and mites and soil borne
transmission 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, white
flies, 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 and
introduced into the plants again via mouth parts of the vectors.

12. 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).

14. 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).

15. 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

16. 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 transmit
great majority ( about 170) of the all stylet borne viruses Ex. Soybean mosaic ,
cowpea aphids, borne mosaic, potato leaf roll, pea enation mosaic, lettuce
necrotic yellows, red clover mosaic BYMV, MCMV, AMV
Fig-1

17. 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 , mung
yellow mosaic, cotton leaf crumple, cassava mosaic, sweet potato mosaic.
Fig-2

18. 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 leaf
hoppers, all leaf hoppers transmitted viruses are circulatory, several are known to
multiply 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

19. • 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

20. Nematodes: these are root-feeding parasites, some of which
transmit viruses in the genera Nepovirus and Tobravirus. The
picture shows an adult female of Paratrichodorus
pachydermus, the vector of Tobacco rattle virus
Fig-5

21. • 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

22. 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

23. Mites: these transmit viruses in the genera Rymovirus and Tritimovirus. The
picture shows Aceria tosichella, the vector of Wheat streak mosaic virus
Fig-7

24. 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/

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