1. General structure with special reference to
viroids and prions and RNA virus (TMV)
PRESENTED BY:-
NAME: ASHISH PANIGRAHI
Guided by:-
Namita panda ma’am
2. Introduction
• In the five kingdom classification ofWhittaker, there is no mention of lichens and
some acellular organisms like viruses, viroids, and prions.These are briefly
introduced here(viroids, and prions).
• Viruses are infectious agents, with simple, acellular organization and are obligatory
intracellular parasites.The study of virus is called virology.
Discovery of virology
• D.J Ivanowsky(1892) recognized certain microbes as causal organism of the
mosaic disease of tobacco.These were found to be smaller than bacteria because
they passed through bacterial proof filters.
• M.W. Beijerinck(1898) demonstrated that the extract of the infected plants of
tobacco could cause infection in healthy plants and called this fluid as contagium
vivum fluidum (infectious living fluid).
• W.M. Stanley(1935) crystallisedTMV (tobacco MosaicVirus) for the first time. He
showed that viruses could be crystallised and crystals consists largely of proteins.
3. SubViral Particles/Agents
• Viroids-They are the smallest self replicating particles which were
discovered by Diener (1971).Viroids are infectious RNA particles which are
devoid of protein coat.They are obligate parasites. Molecular weight is low.The RNA is
tightly folded to form circular or linear structures. Viroids are known to cause disease in plants
only, e.g., Potato SpindleTuber Disease (PSTD),Chrysanthemum stunt and Citrus exocortis.
• Virusoids- Discovered by Randle.These are RNA viruses but inside the capsid of other
larger viruses.They replicate within the hist and do not cause any infection.
• Prions- Discovered by Alper.These are proteinaceous infectious and self-reproducing
agent/pathogens mainly composed of proteins(abnormally folded). They cause diseases in
animals and humans. E.g., kuru Disease, BSE, Scrapie Disease, Creutz Feldt Jakob Disease.
4. Viroids
• Viroids are small, circular RNA molecules with a rod-like secondary structure
which possess no capsid or envelope and are associated with certain plant disease.
• The first viroid discovered was found to cause potato tuber spindle disease,
which causes slower sprouting and various deformities in potato plants.
• viroids do not have a protein coat to protect their genetic information.
• Viroids can result in devastating losses of commercially important agricultural food crops grown in fields
and orchards.
• They replicate by RNA-RNA transcription and lack protein coding.
Some other examples other than PSTV
1. Tomato planta macho viroid (TPMVd) infects tomato plants, which causes loss of chlorophyll,
disfigured and brittle leaves, and very small tomatoes, resulting in loss of productivity in this field crop.
2. Avocado sunblotch viroid (ASBVd) results in lower yields and poorer-quality fruit.ASBVd is the
smallest viroid discovered thus far that infects plants.
3. Peach latent mosaic viroid (PLMVd) can cause necrosis of flower buds and branches, and wounding of
ripened fruit, which leads to fungal and bacterial growth in the fruit.
PLMVd can also cause similar pathological changes in plums, nectarines, apricots, and cherries, resulting
in decreased productivity in these orchards, as well.
5. Classification of viroids
Viroids are classified into two families as given below
1. Family Pospiviroidae
• Genus Pospiviroid: type species: Potato spindle tuber viroid.
• Genus Hostuviroid: type species: Hop stunt viroid.
• Genus Cocadviroid: type species: Coconut cadangcadang viroid.
• Genus Apscaviroid: type specie: Apple scar skin viroid.
• Genus Coleviroid : type species: Coleus blumei viroid.
2. Family Avsunviroidae
• Genus Avsunviroid: type species: Avocado sunblotch viroid.
• Genus Pelamoviroid: type species: Peach latent mosaic viroid.
6. Viroid properties differ basically from those of
conventional viruses in atleast following five
important features
• The pathogen exists in vivo as an encapsulated in the RNA.
• Virion like particles are not detected in the infected tissues.
• The infectious RNA is of low molecular weight.
• Despite its small size, thee infectious RNA replicates autonomously in
susceptible cells i.e., no helper virus is required.
• The infectious RNA consists of one molecular species only.
7.
8. Infectious Stage
• When virus infects a cell, nucleic acid must be uncoated and gain access to
metabolic machinery of cell.
• Virus life cycle is characterized by:
1. Attachment penetration.
2.With entry of nucleic acid into cell early expression of virus genes (either
directly by translation, if virus contains "+" RNA, or indirectly after
transcription and then translation).
3. Replication of virus nucleic acid synthesis of new virion components
packaging and assembly of new virions exit from cell.
9. Replication of viroid
There are two ways to explain the viroidal replication
• DNA dependent replication : This is based on the premise that viroids, like Retro
viruses, produce a novel DNA by reverse transcription.This novel DNA later
produce viroid RNA by normal transcription process.
• RNA dependent replication : DNA directed RNA polymerases are present in the
normal cells.They direct the RNA synthesis. Although this is found in some plants
and E. coli and it is against the conventional concept of cell biosynthetic machinery.
It is suggested that the DNA dependent RNA polymerase II help in the synthesis of
RNA molecules. It is based on the fact that viroid replication is inhibited by RNA
polymerase II inhibitors like actinomycin-D.
Due to circular nature of the viroid, their replications occurs through a rolling circle
mechanism.
11. Diseases Host
Potato spindle tuber viroid Potato
Hop stunt viroid Hop plant
Coconut cadangcadang viroid Coconut
Apple scar skin viroid Apple
Coleus blumei viroid Coleus plant
Avocado sunblotch viroid Avocado
Peach latent mosaic viroid Peach
12. Prions
• In 1982, Stanley Prusiner, a medical doctor studying scrapie (a fatal, degenerative
disease in sheep) discovered that the disease was caused by proteinaceous
infectious particles, or prions and he received the Nobel Prize in Physiology or
Medicine in 1997.
• A prion is a misfolded rogue form of a normal protein (PrPc) found in the cell.This
rogue prion protein (PrPsc), which may be caused by a genetic mutation or occur
spontaneously, can be infectious, stimulating other endogenous normal proteins to
become misfolded, forming plaques.
• prions are known to cause various forms of transmissible spongiform
encephalopathy (TSE) in human and animals.
• Prions are extremely difficult to destroy because they are resistant to heat,
chemicals, and radiation. Even standard sterilization procedures do not ensure the
destruction of these particles.
13.
14. • TSE is a rare degenerative disorder that affects the brain and nervous system.The
accumulation of rogue proteins causes the brain tissue to become sponge-like, killing brain
cells and forming holes in the tissue, leading to brain damage, loss of motor coordination,
and dementia.
• TSEs in humans include kuru, fatal familial insomnia, Gerstmann-StrausslerScheinker
disease, and Creutzfeldt-Jakob disease.
• TSEs in animals include mad cow disease, scrapie (in sheep and goats), and chronic wasting
disease (in elk and deer).
Transmission ofTSE
• TSEs can be transmitted between animals and from animals to humans by eating
contaminated meat or animal feed.
• Transmission between humans can occur through heredity (as is often the case with GSS
and CJD) or by contact with contaminated tissue, as might occur during a blood transfusion
or organ transplant.
• There is no evidence for transmission via casual contact with an infected person.
Treatment ofTSE
• There is no treatment or cure forTSE disease, and contaminated meats or infected animals
must be handled according to federal guidelines to prevent transmission.
15. Characteristics of prions
Prions have the following characteristics :
• Prions are resistant to inactivation by heating to 900C, which will inactivate viruses.
• Prion infection is not sensitive to radiation treatment that damages virus genomes.
• Prions are not destroyed by enzymes that digest DNA or RNA.
• Prions are sensitive to protein denaturing agents, such as phenol and urea.
• Prions have direct paring of amino acids.
16. Prion Propagation
• Endogenous normal prion protein (PrPc) is converted into the disease-
causing form (PrPsc) when it encounters this variant form of the protein.
• PrPsc may arise spontaneously in brain tissue, especially if a mutant form of
the protein is present, or it may originate from misfolded prions consumed
in food that eventually find their way into brain tissue.
17. Sr.No. Diseases Natural host
1. Scrapie Sheep and goats
2. Mad Cow Disease or Bovine Spongiform
Encephalopathy (BSE)
Cattle
3. Transmissible in mink encephalopathy Mink
4. Chronic wasting disease Male deer and elk
5. Kuru Humans (Fore tribe of papua New
Guinea)
6. Creutzfeldt-Jakob disease (CJD) Humans
7 Gerstmann-Straussler Syndrome (GSS) Humans
8. Parkinson’s disease Humans
9. Multiple sclerosis Humans
18.
19. TMV
• Tobacco mosaic virus (TMV) is the first virus discovered.
• It was discovered by D. Ivanovsky andW. Stanley was the first to crystallize the
virus and showed thatTMV remain active even after crystallization.
• Tobacco mosaic virus (TMV) is a single stranded RNA virus that infects plants,
especially tobacco and other members of the family Solanaceae.
• The infection causes characteristic patterns, such as "mosaic"-like mottling and
discoloration on the leaves.
• TMV is a thermostable virus.
20. Structure and Morphology
• A rod like virus and the center is hollow.
• Length-300nm, width- 18nm, molecular weight-
40million KD mass, nucleotides-6395.
• Its capsid is made from 2130 molecules of coat
protein and one molecule of genomic single strand
RNA 6400 bases long.
• The protein monomer consists of 158 amino acids
which are assembled into four main alpha-helices,
which are joined by a prominent loop proximal to
the axis of the virus.
• Each subunit protein consists of 4 large and 1 small
helices and 6 beta sheets with N-and C-terminals
towards the periphery.
• The [protein sub units are bounded at the inner
side end with the nucleic acid.
• The structure of it is stable at pH and at higher pH
the structure becomes unstable and falls apart.
• On a dried leaf, it can withstand up to 120 degrees
Fahrenheit (50 °C) for 30 minutes.TMV has an
index of refraction of about 1.57.
21. Mode of transmission
• TMV enters plants through wounds. Damage to the
plant cell membrane permits entry of infectious
particles into the cytoplasm.
• TMV attacks the hosts protein synthesis machinery to
make viral proteins andTMV makes many copies of its
RNA.
• TMV spreads into neighboring cells using its movement
protein which modifies tiny channels, called
plasmodesmata, which connect nearly all plant cells.
• The virus spreads from an initial infection site to all
parts of the plant via the phloem, the plant’s nutrient
transport network.
• The mostlyTMV spreads from plant to plant through
workers' hands, clothing or on tools.
• Vegetative propagation of infected plant carriesTMV
and other virus diseases.
• The virus particles are found in all parts of the plant
except the few cells at the tips of the growing points.
Mechanical transmission
Normal transmission
22. Multiplication and infection
• The replication of virus RNA is an essential part of theTMV reproduction or multiplication.
• Most of the plants contain RNA dependent RNA Polymerases which helps in replication of viral RNA.
• FourTMV – specific proteins are known to be made.
• TMV viral RNA directly serve as mRNA in the production of messenger in complex.
• After the coat protein and RNA genome have been synthesized they automatically assemble into complete
TMV virion.
• The protomere come together to form disks composed of two layers of protomere arranged in a helical
spiral. After its multiplication, it enters the neighboring cells through plasmodesmata.
• For its smooth entry,TMV produces a 30 kDa movement protein called P30 which enlarge the
plasmodesmata.TMV most likely moves from cell-to-cell as a complex of the RNA, P30, and replicase
proteins.
• It can also spread through phloem for longer distance movement within the plant.
• Moreover,TMV can be transmitted from one plant to another by direct contact. AlthoughTMV does not
have defined transmission vectors, the virus can be easily transmitted from the infected hosts to the
healthy plants, by human handling.
23.
24. Detection
• Most viral symptoms are easily
confused with environmental effects
or other plant pathogens, so correct
diagnosis is important and relies
upon detection assays that use
antibodies or PCR-based assays.
25.
26. Symptoms
Symptoms associated withTMV infections:
• Stunting.
• Mosaic pattern of light and dark green (or yellow and green) on the leaves.
• Malformation of leaves or growing points.
• Yellow streaking of leaves (especially monocots).
• yellow spotting on leaves.
• Distinct yellowing only of veins Some of the above symptoms can also be
caused by high temperature, insect feeding, growth regulators, herbicides,
mineral deficiencies, and mineral excesses.
TMV diseases cannot be diagnosed on the basis of symptoms alone.
27. Control
• Measures include strict glasshouse management – watering hoses/cans should not
be allowed to make contact with plants.
• Dead plants/old leaves are bagged and removed carefully.
• Contaminated soil discarded.
• Tools should be treated with soap or 10% bleach.
• Try to avoid ‘wounding’ healthy plants.
• Control is by crop rotation, effective sanitation and use of resistant cultivars of
tobacco.
