Introduction, diseases and types

2.  Infectious pathogens
 Too small to be seen with a light microscope
 The simplest viruses are composed of
1. A small piece of nucleic acid
2. Surrounded by a protein coat
 Obligate parasites that depend on the cellular machinery
of their hosts
 Not active outside of their hosts
 Organisms including animals, plants, fungi, and bacteria
are hosts for viruses, but most viruses infect only one type
of host

3.  Plants have specific viruses
 Viruses cause many plant diseases
 They are responsible for losses in crop yield and quality
 Viroids are infectious RNAs that cause important plant
diseases
 These pathogens are similar to some plant viruses in that
they contain an RNA genome
 They differ from RNA plant viruses that they are composed
of naked RNAs and lack a protein coat
 Viroids do not produce any proteins when they infect a
plant cell despite the fact that they are made of RNA

4.  Obligate intracellular parasites
 Do not have the molecular machinery to replicate
without a host
 Pathogenic to higher plants
 Plant viruses infect plants
 A virus particle, also known as a virion is an extremely
small infectious agent
 Essentially a nucleic acid (DNA or RNA) enclosed in a
protein coat called a capsid

5.  Viral genetic material can be
1. Double-stranded DNA
2. Double-stranded RNA
3. Single-stranded DNA or
4. Single-stranded RNA
 Most plant viruses are classified as single-stranded RNA or
double-stranded RNA virus particles
 Cause various types of plant diseases
 These diseases do not typically result in plant death

6.  Plant diseases produce symptoms such as
 Ring spots
 Mosaic pattern development
 Leaf yellowing and distortion
 Deformed growth
 Some plant viruses are not limited to one particular plant
host
 May infect different varieties of plants
 Plants including tomatoes, peppers, cucumbers, and
tobacco may all be infected by the tobacco mosaic virus

7.  The first plant virus discovered
 Attacks members of the nightshade, or Solanaceae, family
including tobacco, pepper, potato, tomato, eggplant,
cucumber and petunia
 Spreads through entry into breaks of cell walls caused by
insects or other physical damage

8.  Infects several grains and staple crops, including wheat
 Aphids primarily spread the virus
 Causes discoloration of leaves and the tips of the plants
 Reduces photosynthesis, stunts growth and decreases
production of seed grains

9.  Infects soybeans, a staple crop
 Causes the stem to bend at the top and the buds to turn
brown and drop off the plant
 Nematodes spread this virus

10.  Discolors leaves of the sugarcane plant
 It thus restricts its ability to feed itself through
photosynthesis and grow
 Stunts the growth of young plants.
 Aphids and infected seed spread the virus

11.  Causes discoloration and distortion of the leaves
of peanuts and some other rhizomes, stunting
their growth
 Aphids and sap spread the virus

12.  Causes yellow spots and stripes on the leaves
of corn, stunting its growth
 Leafhoppers spread the virus

13.  The cauliflower mosaic virus infects members of the
Brassica family
 Members includes cabbage, Brussels sprouts, Cauliflower,
Broccoli and Rape seed
 It causes a mosaic or mottle on the leaves which stunts
growth
 Aphids and mechanical exposure spread the virus

14.  Mottles the leaves of almost all types of lettuce,
stunting its growth and eliminating its market
appeal
 Aphids and infected seeds spread the virus

15.  Infects cucumber, tomato, peppers, melons, squash,
spinach, celery, beet and other plants
 Aphids spread it, and they cause physical damage to the
plant
 Allows entry of the virus via wind, splashing or dripping sap
 Causes twisting in young leaves that stunts growth of the
entire plant and causes poor fruit or leaf production

16.  Tobacco mosaic virus
 Tomato spotted wilt virus
 Tomato yellow leaf curl virus
 Cucumber mosaic virus
 Potato virus Y
 Cauliflower mosaic virus
 African cassava mosaic virus
 Plum pox virus
 Brome mosaic virus
 Potato virus X
 Including Citrus tristeza virus, Barley yellow dwarf virus, Potato leafroll
virus and Tomato bushy stunt virus

17.  Some of the same types of viruses that infect
humans can also infect plants
 Plants and humans do not transmit viruses to
each other
 Humans can spread plant viruses through
physical contact
 Viruses also spread through infected seeds,
grafting, wind, splashing, pollination and
dripping sap
 Unlike humans, plants never recover from a virus

18.  Plant cells are eukaryotic cells that are
similar to animal cells
 Plant have a cell wall that is nearly
impossible for viruses to breach in order to
cause infection
 Plant viruses are typically spread by two
common mechanisms
 Horizontal transmission
 Vertical transmission

19.  The plant virus is transmitted as a result of an
external source
 In order to invade the plant, the virus must
penetrate the plant's outer protective layer
 Horizontal transmission also occurs by certain
artificial methods of vegetative reproduction
typically employed by horticulturists and farmers
 Plant cutting and grafting are common modes by
which plant viruses may be transmitted

20.  The virus is inherited from a parent
 This type of transmission occurs in both asexual and sexual
reproduction
 In asexual reproductive methods such as vegetative
propagation, the offspring develop from and are
genetically identical to a single plant
 In sexual reproduction, viral transmission occurs as a result
of seed infection
 In most cases it is unable to find cures for plant viruses, so
the main focus is on reducing the occurrence and
transmission of the viruses

21.  There are a number of routes by which plant
viruses may be transmitted:
 Seeds: These may transmit virus infection either due to
external contamination of the seed with virus particles,
or due to infection of the living tissues of the embryo.
 Vegetative propagation/grafting: These techniques are
cheap and easy methods of plant propagation, but
provide the ideal opportunity for viruses to spread to
new plants.
 Vectors: Many different groups of living organisms can
act as vectors and spread viruses from one plant to
another

22.  Bacteria (e.g. Agrobacterium tumefaciens - the Ti plasmid of this
organism has been used experimentally to transmit virus genomes
between plants)
 Fungi
 Nematodes
 Arthropods: Insects - aphids, leafhoppers, plant hoppers, beetles, thrips
 Arachnids – mites
 Mechanical:
 Mechanical transmission of viruses is usually achieved by
rubbing virus-containing preparations into the leaves
 In plant species are particularly susceptible to infection
 This is also an important natural method of transmission
 Virus particles may contaminate soil for long periods and
may be transmitted to the leaves of new host plants as
wind-blown dust or as rain-splashed mud

23.  This method is of particular agricultural importance
 Extensive areas of monoculture and the inappropriate use
of pesticides which kill natural predators can result in
massive population booms of insects such as aphids
 Plant viruses rely on a mechanical breach of the integrity
of a cell wall to directly introduce a virus particle into a
cell
 This is achieved either by the vector associated with
transmission of the virus or simply by mechanical damage
to cells

24.  Transfer by insect vectors is a particularly efficient means
of virus transmission
 Non propagative transmission:
Insects which bite or suck plant tissues are the ideal means
of transmitting viruses to new hosts. This is known as non-
propagative transmission
 Propagative transmission:
The virus may also infect and multiply in the tissues of the
insect as well as those of host plants called propagative
transmission for example many plant rhabdoviruses

25.  Plant viruses are obligate, biotrophic parasites
 Their life cycles start by penetration of the virion into the
cell
 Plant viruses are unable to penetrate the plant cuticle and
cell wall
 The virion enters the cytoplasm of the cell passively
through wounds caused by mechanical damage to the
cuticle and cell wall
 The next phase of virus infection is the partial or complete
removal of the coat protein shell of the virion in the
cytoplasm

26.  Next the cell mediates expression of the viral genome
 By providing a transcription apparatus (for DNA viruses)
and a translation apparatus (for all viruses)
 The DNA viruses must be transported to the nucleus for
transcription
 To gain access to the cell proteins required for the
production of messenger RNA from viral DNA
 Translation of viral RNA in the cytoplasm produces viral
proteins that are required for completion of the virus life
cycle

27.  All viruses must direct the formation of at least
three types of proteins:
 Replication proteins that are essential for
nucleic acid production
 Structural proteins that form the protein shell
and other components contained in the virions
 Movement proteins that mediate virus
transport between plant cells

28.  The viral replication proteins combine with
cellular proteins
 Produce a complex of proteins that
manufactures multiple copies of the virus
genome
 These newly made genomes interact with the
structural proteins to form new virions

29.  The outer surfaces of plants are composed of
protective layers of waxes and pectin
 More significantly each cell is surrounded by
a thick wall of cellulose overlying the
cytoplasmic membrane

30.  To date no plant virus is known to use a specific cellular
receptor of the type that animal and bacterial viruses use
to attach to cells
 Plant viruses rely on a mechanical breach of the integrity
of a cell wall to directly introduce a virus particle into a
cell
 This is achieved either by the vector associated with
transmission of the virus or simply by mechanical damage
to cells
 After replication in an initial cell, the lack of receptors
poses special problems for plant viruses in recruiting new
cells to the infection

31.  Segmented virus genomes are those which are divided into
two or more physically separate molecules of nucleic acid,
all of which are then packaged into a single virus particle
 Although multipartite genomes also segmented, each
genome segment is packaged into a separate virus particle
 These discrete particles are structurally similar and may
contain the same component proteins but often differ in
size
 Genome segmentation reduces the probability of
breakages due to shearing, thus increasing the total
potential coding capacity of the genome

32.  The disadvantage of this is that all the individual genome
segments must be packaged into each virus particle, or the virus
will be defective as a result of loss of genetic information
 All the discrete virus particles must be taken up by a single host
cell to establish a productive infection.
 This is perhaps the reason multipartite viruses are only found in
plants
 Many of the sources of infection by plant viruses, such as
inoculation by sap-sucking insects or after physical damage to
tissues, result in a large inoculum of infectious virus particles
 Providing opportunities for infection of an initial cell by more
than one particle

33. Family: Segments:
Geminivirus (group III) (single-stranded DNA) Bipartite
Comovirus (single-stranded RNA) Bipartite
Furovirus (single-stranded RNA) Bipartite
Tobravirus (single-stranded RNA) Bipartite
Partitiviridae (double-stranded RNA) Bipartite
Bromoviridae (single-stranded RNA) Tripartite
Hordeivirus (single-stranded RNA) Tripartite

34.  Initially, most plant viruses multiply at the site of infection,
giving rise to localized symptoms such as necrotic spots on
the leaves
 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
 Plant cell walls necessarily contain channels called
plasmodesmata which allow plant cells to communicate
with each other and to pass metabolites between them
 These channels are too small to allow the passage of virus
particles or genomic nucleic acids
 Many plant viruses have evolved specialized movement
proteins which modify the plasmodesmata

35.  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:
 Necrosis of cells is caused by direct damage due to virus
replication
 Hypoplasia is localized retarded growth frequently leading to
mosaicism (the appearance of thinner, yellow areas on the leaves)
 Hyperplasia is excessive cell division or the growth of abnormally
large cells, resulting in the production of swollen or distorted
areas of the plant

36.  Viruses are capable of infecting virtually all species of cultivated
and wild plants. However, host ranges of individual viruses vary
from very narrow to very broad
 For example Citrus tristeza virus infects only a few species in the
Citrus genus, whereas Cucumber mosaic virus infects over 1000
species in 85 plant families
 Susceptibility or resistance of plant species and cultivars to
viruses is determined primarily by the plant genotype
 Plants possess active and passive means of preventing virus
infection. Passive defenses are due to the failure of the plant to
produce one or more host factors required for virus reproduction
and spread within the host

37.  Active defenses include detection and destruction of the
virus-infected cells due to the function of specific
resistance genes in the plant
 In addition, plants possess a general defense system that is
analogous to the animal immune system.
 The major difference between the two is that the immune
system in animals targets a pathogen’s proteins, whereas
the plant defense system, which is called RNA silencing,
detects and degrades viral RNAs
 Depending on the particular combination of virus and host,
and on environmental conditions, a plant’s response to
infection may range from a symptomless condition to
severe disease and plant death

38.  Different viruses may elicit similar symptoms
 The disease phenotype can provide only limited, although
important, information for disease diagnosis
 More specific and reliable methods of virus identification are
based on various properties of the virus
 These properties and corresponding approaches include the
following:
1.Pathogenicity
Two major types of responses are :
 local lesions which are confined to inoculated leaves (local lesion
hosts)
 systemic infections which produce symptoms on leaves distant from
the inoculation site (systemic hosts)

39. 2.Transmissibility
Due to vector specificity, identification of the organism that transmits the
virus may provide important information for virus identification
3.Architecture of virus particles
By using electron microscopy the shape and size of virions can be
distinguished that is they are rod-shaped, filamentous, icosahedral, or
large enveloped particles
4.Presence of virus-specific structures in infected cells
Due to their intimate association with components of the cell, viruses
often form unusual structures within plant cells as a result of virus
infection
For example virus-specific inclusions have been characterized for a
number of plant virus families and genera, and the detection of these
inclusions indicates the presence of a virus within that group

40. 5.Properties of the protein coat
These tests rely on identification of a virus (the antigen)
through its reaction with specific antibodies
One of the most widely used diagnostic tests for plant
viruses is an antibody-based procedure called the
Enzyme-Linked Immunosorbent Assay (ELISA)

41.  Although there are virtually no antiviral compounds available to cure
plants with viral diseases, efficient control measures can greatly mitigate
or prevent disease from occurring
 Virus identification is a mandatory first step in the management of a
disease caused by a virus
 The strategy for management will depend on the means by which a
particular virus enters a crop, how the virus is transmitted between
plants within a crop, and how the virus survives when the crop is not
being grown
 Preventative measures may include use of certified virus-free seed or
vegetative stocks, elimination of the virus reservoirs in the surrounding
wild vegetation, and modification of planting and harvesting practices
 If the virus is known to be transmitted by a particular vector, control or
avoidance of this vector is important. For instance, insect, nematode or
fungal vectors can be controlled by insecticides, nematicides, or
fungicides, respectively

42.  Plant viruses and viroids are diverse and unusual groups of
plant pathogens that infect and cause disease in many crop
plants
 These pathogens depend on the normal cellular machinery of
their plant host for reproduction, it is difficult to eliminate
them without damaging the host plant
 Therefore, most management strategies for diseases caused
by plant viruses and viroids are directed at preventing
infection of the plant