Viruses, Viroids, and Lichens
(a) Tobacco Mosaic Virus (T.M.V.)
The name virus that means venom or poisonous fluid was given by Pasteur. 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 bacteria-proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins. They are inert outside their specific host cell. Viruses are obligate parasites.
In addition to proteins viruses also contain genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. In general, viruses that infect plants have single-stranded RNA and viruses that infect animals have either single or double-stranded RNA or double-stranded DNA. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double-stranded DNA viruses. The protein coat called capsid made of small sub-units called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms. Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
Viroids: In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.
Lichens: Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which is autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner. So close is their association that if one saw a lichen in nature would never imagine that they had two different organisms within them. Lichens are very good pollution indicators – they do not grow in polluted areas.