Pl Path 502-Viroids - CSK Himachal Pradesh Agricultural University
Pl Path 502-Viroids - CSK Himachal Pradesh Agricultural University
Pl Path 502-Viroids - CSK Himachal Pradesh Agricultural University
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<strong>Viroids</strong><br />
<strong>Pl</strong> <strong>Path</strong> <strong>502</strong><br />
Dr. PN SHARMA<br />
Department of <strong>Pl</strong>ant <strong>Path</strong>ology<br />
<strong>CSK</strong> HP <strong>Agricultural</strong> <strong>University</strong><br />
Palampur-176 062 (HP State) INDIA
Developments in molecular biology of the 20 th century<br />
<br />
<br />
Discovery of double helical DNA<br />
Cracking of genetic code<br />
Development of recombinant DNA and PCR<br />
techniques<br />
<br />
<br />
Elucidation of 3D protein structure<br />
<strong>Viroids</strong> and Prions – molecules at the threshold of<br />
origin of life
<strong>Viroids</strong><br />
(T.O. Diener, 1971): are small, low mol<br />
wt. RNA units (250-370 bp.), lack protein<br />
coat, replicate themselves and cause<br />
disease<br />
Example: Potato spindle tuber<br />
viroid, coconut codang-cadang.<br />
Autonomously replicating<br />
<strong>Path</strong>ogens, unencapsidated<br />
Single<br />
Yellow green rods denote the first<br />
viroid as seen in electron micrograph<br />
Therefore often denoted as subviral<br />
particles or agents<br />
THEODOR O. DIENER<br />
Discoverer of the viroid 1971
• Self replicating circular, low molecular weight RNA<br />
without protein coat<br />
• Infect only plant cells<br />
• Produce variable symptoms on different hosts like<br />
stunting, bark scaling, proliferation, veinal necrosis and<br />
also symptom less carrier (No symptoms)<br />
• Vegetatively propagated, highly seed and pollen<br />
transmitted
Losses caused byviroid diseases<br />
DISEASE<br />
FIRST<br />
REPORT<br />
COUNTRY<br />
LOSS<br />
VIROID<br />
ETIOLOGY<br />
VIROID<br />
NAME<br />
Potato Spindle 1917 USA 26 - 90% 1971 PSTVd<br />
Tuber USSR 54%<br />
China 60%<br />
Canada 64%<br />
Cadang Cadang 1927 Philippines 20 million 1975 CCCVd<br />
of coconut<br />
nuts<br />
Hop Stunt 1952 Japan 17 - 60% 1977 HSVd
• RNA, Low molecular weight 0.8-1.3x10 5 D<br />
• Single stranded - 246-375 nucleotides<br />
• Circular forms with secondary structure - Highly base paired<br />
• Rich in G+C Content<br />
• To date sequences of 25 viroids and 160 viroid variants are available in<br />
gene databases<br />
Model of viroid domain<br />
T1 and T2; Terminal Domains, P; <strong>Path</strong>ogenicity Domain, V; Variable Domain and<br />
C; Central Conserved Domain
• Self Replicating -<br />
• Auto cleaving -Due to Presence of Ribozymes<br />
• By rolling circle mechanism<br />
• No translation<br />
Ribozymes are catalytic RNAs with intrinsic ability to break and<br />
form covalent bonds. They cleave RNA in 2 fragments with 5’<br />
hydroxyl and 2’ – 3’ cyclic phosphate in a non hydrolytic reaction.<br />
The process is often referred to as catalytic cleavage
ROLLING CIRCLE MECHANISM<br />
Asymmetric model<br />
Symmetric model
• Common in plasmid or bacteriophage DNA and the circular RNA genome of e.g.<br />
<strong>Viroids</strong>, and DNA viruses e.g. geminiviruses<br />
• Rolling circle DNA replication is initiated by an initiator protein encoded by the plasmid or<br />
bacteriophage DNA, which nicks one strand of the double-stranded, circular DNA<br />
molecule at a site called the double-strand origin, or DSO.<br />
• The initiator protein remains bound to the 5' phosphate end of the nicked strand, and the free 3' hydroxyl end<br />
is released to serve as a primer for DNA synthesis by DNA polymerase II.<br />
• Using the unnicked strand as a template, replication proceeds around the circular DNA molecule, displacing<br />
the nicked strand as single-stranded DNA. Displacement of the nicked strand is carried out by a host-encoded<br />
helicase called PcrA (plasmid copy reduced) in the presence of the plasmid replication initiation protein.<br />
• Continued DNA synthesis can produce multiple single-stranded linear copies of the original<br />
DNA in a continuous head-to-tail series called a concatamer.<br />
• These linear copies can be converted to double-stranded circular molecules through the<br />
following process:<br />
• First, the initiator protein makes another nick to terminate synthesis of the first (leading) strand. RNA<br />
polymerase and DNA polymerase III then replicate the single-stranded origin (SSO) DNA to make another<br />
double-stranded circle. DNA polymerase I removes the primer, replacing it with DNA, and DNA ligase joins<br />
the ends to make another molecule of double-stranded circular DNA.
<strong>Pl</strong>ant Appeareance<br />
Stunting/ dwarfing; Proliferation leading to bunching<br />
Symptomless / latent<br />
Leaf<br />
Epinasty, venial necrosis, yellow/corky vein, puckering<br />
Stem<br />
Bark scaling/ splitting particularly at bud union region.<br />
Stem discolouration<br />
Flower<br />
No symptoms, no sterility<br />
Fruit/ Seed<br />
Rough skin, scar skin
• Sap<br />
Tomato bioassay<br />
• Graft<br />
Citron bioassay, Cucumber bioassay<br />
• Vegetative<br />
Pruning / Cutting Knives<br />
• Seed<br />
Very high rate<br />
• Pollen<br />
High rate<br />
• Insect<br />
Not yet confirmed universally
Symptoms on Inoculated Tomato<br />
Spindle Shaped Tubers
Artificially inoculated seedling (left), 6<br />
years after inoculation, showing stunting,<br />
sterility and disordered pinnae, compared<br />
with a healthy seedling.
Severe infection leading to tree decline
Stunting
CEVd<br />
CEVd-t
Intensity of disease known only<br />
after deformation of fruit is<br />
observed<br />
Symptoms on leaves appear as mild<br />
chlorosis
Viroid Diseases in India<br />
Disease First report Etiology<br />
Citrus exocortis 1968 1992<br />
Tomato Bunchy top 1982 1989,1992<br />
Potato spindle tuber 1989 1991<br />
Tobacco Proliferation 1991 1991<br />
Coleus Symptomless 1991 1992<br />
Citrus latent 1991 1992<br />
Apple Scar Skin (Dapple) 1995 1995<br />
Citrus yellow corky vein 1974 1996
VIROID INFECTIONS IN DIFFERENT<br />
PLANT FAMILIES<br />
ASTERACEAE : CCMVd, CSVd (2)<br />
CARYOPHYLLACEAE: CSVd (1)<br />
CUCURBITACEAE: CPFVd (1)<br />
GESNERIACEAE: CLVd (1)<br />
LABIATAE: CYVd, CbVd (2)<br />
LAURACEAE: ASBVd (1)<br />
PALMAE: CCCVd, CTVd, OPFYVd (3)<br />
ROSACEAE: ASSVd, PLMVd, PDVd, PBCVd (4)<br />
RUTACEAE: CEVd, HSVd (citron), CiVVd, CIT.<br />
CACHEXIA (4)<br />
VITACEAE : HSVd (gv), HSVd (ggv), AGVd,<br />
GYSVd, G 1bVd , HSVd (hop), HLVd,<br />
CEVd (gv) (8)<br />
SOLANACEAE: PSTVd, ITBTVd, TASVd,<br />
TAPMVd, NgPVd (5)<br />
THEACEAE: TDDVd (1)
• primary sources of inoculum: Seed and pollen<br />
• Vegetative propagation<br />
• Large scale monoculture<br />
• Escape from natural host to commercial crop and<br />
vice-versa<br />
• Evolution of natural recombinants<br />
• Lack of adequate quarantine check
Detection of <strong>Viroids</strong>
Detection of Citrus <strong>Viroids</strong> by PCR<br />
L-R: Marker (100bp), CEVd (2,3), CVd II(5),<br />
CVd gr III (6,7), CYCVVd (9,10)
Management of viroid diseases<br />
<br />
<br />
<br />
<br />
Eradication of Sources of inoculum<br />
Cultural Practices<br />
Quarantine Regulations<br />
Biotechnological Approach<br />
Biotechnological Approach