1. Viroids,
Prions &
Phytoplasmas
Created by
Dr. M. Jayakara Bhandary
Associate Professor & Head
Department of Botany
Government First Grade College
Karwar, Uttara Kannada District
Karnataka, India
(Only For Educational Purpose,
Based on various Internet Resources)

2. Viroids
• Circular, ss RNA molecules of low mol. Wt. that
mainly cause diseases in plants.
• Smallest known infectious agents.
• Don’t code for any proteins– disrupt protein
synthesis mechanisms in plants.
• Discovered by T. O. Diener (1971), associated
with Potato Spindle Disease
• 1,00,000 – 1,40,000 d in size
• No protein coat like viruses
• 2 groups - Pospiviroidae (Ex. Potato Spindle Tuber Viroid,
PSTVd) & Avsunviroidae (Ex. Avocado SunBlotchViroiD
ASBVd).

3. Structure
• Circular ss RNA molecule
with double stranded regions
due to complementary base
pairing.
• 250-400 nucleotides
Electron microscopic picture of potato
• Appears like a Rod spindle tuber viroid (PSTVd)
(Courtesy Dr. G. Klotz)

5. Multiplication
• Viroids (RNA) multiply inside the nucleus
of infected plant cell by ‘Rolling Circle’
mechanism, using the host enzymes.
• Produce long, multimeric strands of RNA
which are cut into monomers, each
monomer then joined to form an individual
circular viroid.
• After replication, the progeny RNAs move
to neighboring healthy cells through the
connecting plasmodesmata or to distant
parts of the plant via the phloem.

7. Pathogenesis
• Viroids multiply inside host cells and cause
diseases by ’RNA interference’ (RNAi).
• RNAi is a mechanism that degrades
double-stranded RNAs (dsRNA).
• Short lengths of Viroid RNA base pair with
complementary host mRNA to form
dsRNAs that are then degraded.
• Destroying the host mRNAs affects
synthesis of proteins by host cell.

8. Viroid Diseases
• Potato Spindle Tuber :
Spindle or elongated
tubers, stunting of
growth.
Chrysanthemum stunt Disease
(CSVd),
Citrus exocortis (CEVd),
Coconut cadang-cadang
disease (CCCVd), and
Avocado sunblotch
disease (ASBVd).

9. Coconut Cadang Cadang Disease
• Common in Coconut and Oil
Palm
• leaf spots, reduced leaflets,
reduced crown, stunted growth,
• Reduction in number and size
of nuts
• Finally death of infected
palms.
• Common in Philippines

10. Prions
• Infectious proteins – proteins which cause
disesae
• Discovered by Prusiner in 1982 in Scrapie
(neurological disease of sheep)
• Prusiner won the Nobel Prize in Physiology or
Medicine in 1997
• Misfolded / abnormally folded proteins in
animals which are pathogenic. Normally folded
versions are non-pathogenic.
• Causes the similar proteins to also misfold

11. Prion Diseases
• Prions are responsible for the Transimissible
Spongiform Encephalopathy in Mammals,
including Bovine Spongiform
Encephalopathy (BSE, also known as "mad cow
disease") in Cattle and Creutzfeldt-Jakob
Disease (CJD) in humans.
• All known prion diseases affect the structure of
the Brain or other Neural tissue and all are
currently untreatable and universally fatal.

12. Creutzfeldt-Jakob Disease

13. Prion Reproduction
• When a prion enters a healthy organism, it induces
existing, properly folded normal proteins to convert
into the disease-associated, prion form.
• The normal form of the protein is called PrPC, while
the infectious (prion) form is called PrPSc
• These newly formed prions can then go on to convert
more proteins themselves; this triggers a chain
reaction that produces large amounts of the prion
form.
• Prions cause other similar proteins to also misfold
lose function, cause disease

14. A) Normal Protein b) infectious protein

15. Prion multiplication

16. Prion multiplication cycle

17. PHYTOPLASMAS
(PLANT MYCOPLASMA)
• Phytoplasmas are prokaryotes lacking cell walls and are obligate
parasites of plant phloem tissue and transmitting insects (vectors).
• They are currently classified in the class Mollicutes.
• Phytoplasmas are associated with plant diseases, and are known to
cause more than 600 diseases in several hundred plant species.
• Phytoplasmas are transmitted from plant to plant by insect vectors,
mainly leafhoppers and psyllids. .
• They were first discovered by scientists in 1967 and were
named mycoplasma-like organisms or MLOs, because they
physically resembled mycoplasmas.
• The organisms were renamed phytoplasmas in 1994. They cannot
be cultured in vitro in cell-free media.

18. Mycoplasmas
• A genus of parasitic or saprophytic bacteria that lack
a cell wall, pathogenic in animals.
• Several species are pathogenic in humans, including M.
pneumoniae, which is an important cause
of pneumonia and other respiratory disorders, and M.
genitalium, which is believed to be involved in pelvic
inflammatory diseases.
• The name Mycoplasma, from the Greek mykes (fungus)
and plasma (formed), was first used by A. B. Frank in
1889.
• He thought it was a fungus, due to fungus-like
characteristics.
• They are also called Pleuropneumonia-Like
Organisms (PPLO).

19. Structure of Phytoplasma
• Phytoplasmas lack cell walls and instead are
bound by a triple layered Membrane.
• The typical phytoplasma exhibits a
pleiomorphic or filamentous shape.
• less than 1 micrometer in diameter. Like
other prokaryotes, DNA is free in
the cytoplasm.

21. Electron micrographs of
Phytoplasma

22. Infection & Multiplication
• Phytoplasmas are transmitted from plant to
plant by insect vectors, mainly leafhoppers
and psyllids.
• They multiply in the hemolymph and the
salivary glands of insects.
• The insect vectors introduce phytoplasmas
along with salivary fluids into the phloem of
a new host plant.

24. General Symptoms
• yellowing or reddening of the leaves,
• shortening of the internodes with stunted growth,
smaller leaves,
• excessive proliferation of shoots resulting in a witches'
broom
• Phyllody (production of leaf-like structures in place
of flowers)
• Virescence (development of green flowers), sterile
flowers,
• necrosis of the phloem tissues,
• dieback of the branches of woody plants,
• general decline and death of the plant.

25. Control Measures
• Breeding and planting of disease resistance
varieties of crops.
• by the control of the insect vector.
• Cryotherapy and Tissue culture propagation
of infected plants.
• Treatment with the antibiotic ‘Tetracycline’