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PLANT VIRUS

CONTENTINTRODUCTION.FEATURES.HISTORY.VIROIDS.PRIONS.CHEMICAL COMPOSITION OF PLANT VIRUS. ARCHITECHTURE OF PLANT VIRUS. TOBACCO MOSAIC VIRUS. CAULIFLOWER MOSAIC VIRUS. TOMATO YELLOW LEAF CURL VIRUS. TRANSMISSION. SYMPTOMS. PROTECTION. CONCLUSIONS.

INTRODUCTION Plant viruses are viruses that affect plants. Plant viruses are pathogenic to higher plants. These are normally insects , but some fungi. Plant viruses are harmless to human and other animals.

FEATURES Outside protein coat. Consists of a single nucleic acid. Non-living and Non-cellular structures. Non-cellular sub microscopic entity. Most viruses infect only SPECIFIC host cells. May be transmitted in pollen.

HISTORY Beijerinck (1897) coined the Latin name VIRUS meaning Poison.He studied filtered plant juices and found they caused healthy plants to become sick. Wendell Stanley (1935) crystallized sap from sick Tobacco plants. He discovered viruses were made of nucleic acid and protein.

VIROIDS Small , circular RNA molecules. Infect plants. Have no protein coat. Potato famine in Ireland. Resemble introns cut out of eukaryotic. Consist solely of small single-stranded RNA molecule. Define group of pathogens much smaller and distinctly different from viruses.

PRIONS Prions are Infectious Proteins. They have no DNA or RNA. Causes neurons to rapidly degeneration. Mad Cow Disease (Bovine Spongiform Encephalitis : BSE) is an example. Inherited and transmissible diseases.

CHEMICAL COMPOSITION OF PLANT VIRUSES Protein (capsid) -Capsomere.

Nucleic acids DNA : - ssDNA , dsDNA.RNA : - +ve strand RNA , -ve strand RNA , ssRNA , dsRNA.

PROTEINS : - 60% - 95% of the virion. Protein protects virus genome from Degradation.Facilitates movement through the host.Transmission from one host to another. Repeating subunits , identical for each virus type but varies from virus to virus and even from strain to strain.

NUCLEIC ACID : - Nucleic acid represents the genetic material , indispensable for replication. Nucleic acid is 5 40% of the virion.ssRNA : - About 540 plant viruses.eg. TMV.dsRNA : - About 40 pv. eg. Wound tumour virus.ssDNA : - About 50.eg. Beet curly top virus.dsDNA : - About 30. eg. Cauliflower mosaic virus.

ARCHITECHTURE OF PLANT VIRUSISOMETRIC : - Apparently spherical and (depending on the species) from about 18 nm in diameter upwards.Eg. Tobacco necrosis virus.Genus Necrovirus , with particles 26 nm in diameter.Geminate Twinned isometric particles. ROD SHAPED : -Rigid rod. Eg. TMV.Flexuous rod. Eg. Potato virus Y.

TOBACCO MOSAIC VIRUSCLASSIFICATION : -

Family Virgaviridae. Genus Tobamovirus. Species Tobacco mosaic virus.

TOBACCO MOSAIC VIRUS

STRUCTURE : -

Rigid helical rods. ssRNA. Extremely stable structure. 95% protein and ~5% nucleic acid(RNA). 2130 copies of single 17kDa coat protein. Simple regulatory elements. 300 nm long and 18 nm dia. Rod-like appearance. There are three RNA nucleotides monomer.

CAULIFLOWER MOSAIC VIRUSCLASSIFICATION : -

Family Caulimoviridae. Genus Caulimovirus. Species Cauliflower mosaic virus.

CAULIFLOWER MOSAIC VIRUS

STRUCTURE : -

CaMV is an icosahedral virus. Diameter 52 nm. Genome is circular dsDNA. Capsid multilayered. Arrangement of capsomere not clear. The outermost layer is made up of a total of 420 coat protein subunits.

TOMATO YELLOW LEAF CURL VIRUSCLASSIFICATION : -

Family Geminiviridae. Genus Begomovirus. Species Tomato yellow leaf curl virus.

TOMATO YELLOW LEAF CURL VIRUS

STRUCTURE : -

TYLCV has a characteristic twinned morphology. The TYLCV capsid , like that of other geminiviruses , consists of two joined , incomplete icosahedra. TYLCV has a single 2787 nucleotides covalently closed genomic circular ssDNA.

TRANSMISSION There are two types of plant virus transmission Horizontal transmission. Vertical transmission.Horizontal transmission :- It is by vectors , human pruning shears and tools and other direct , external contamination.Vertical transmission :- It occurs when a plant gets it from its parent plant. Either through asexual propagation (cuttings) or in sexual reproduction via infected seeds.-

Mechanical Transmission :-

Occure when plant come in contact with other plant and leaves rub together.

By the action of humans.

Nematode Transmission :-

Two ssRNA virus genera , Nepovirus and Tobravirus , have nematode vectors.

NEPOVIRUSES Comoviridae family.TOBRAVIRUSES Family not yet assigned.

NEMATODE TRANSMISSION

Seed Transmission :-

It occurs in two ways Externally seed borne Due to external contamination of the seed with virus particles (TMV , PVX).

Internally seed borne Due to infection of the living tissues of the embryo.

SEED TRANSMISSION

PLANT VIRUS SYMPTOMS Mosaic. Mottle. Yellowing. Chlorosis. Vein clearing. Vein banding. Necrosis. Wilt. Ringspot. Stunting.

PROTECTION Discard virus infected plants. Purchase virus-free plants. Remove all crop debris from benches and the greenhouse structure. Remove all weeds since these may harbor both viruses and insects. Maintain strict insect and mite control.

CONCLUSIONS Plant viruses and viroids are unusual groups of plant pathogens that infect and cause disease in many crop plants. Because 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.

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