Introductory Microbiology

Plant viruses

* most of the plant viruses are naked viruses
* they don’t have specific mechanism for entering the host plants. The injuries, insect vectors are the means of entry.
* the cell wall and cuticle layers of plant are the basis for the entry of virus.
* the virus diseases of plants are very rare because infection is not strong enough to kill the plant.
* mostly the virus infection can be recognized by mosaic like leaf pattern, in which loss of chlorophyll leads to yellow in color.
* some viruses multiply in the host plant, without showing the symptom referred as latent infection.

Diseases of plant viruses:
Tobacco mosaic (TMV), Turnip crinkle virus, cucumber mosaic virus, wound tumour virus, maize streak virus, tomato golden mosaic virus, cauliflower mosaic virus.
Viroids: Viroids are the smallest infectious agents which are having a circular RNA alone as genome. They use DNA dependent RNA polymerase of host plant for their multiplication.
Prions: Prions are infections protein particles that cause several diseases in animals and humans. Prions are the only infectious agents that donot contain any nucleic acids and only proteins are present. Prion diseases are often called spongiform encephalopathies because of the post mortem appearance of the brain with large vacuoles. Then cause brain diseases in sheep (scrapie disease), cows (mad cow disease) humans (kuru and creutzfeld jakob-disease) and other animals.