Viruses, viroids and lichens were not considered as true living beings as they lacked cell structures and were therefore, not placed anywhere in the classification system. The word ‘virus’ means venom, and was coined by Louis Pasteur. It is so named because when a virus infects a cell, it takes over the host cell’s machinery to replicate and kills it. A virus is non-living outside its host cell, thus, viruses are known as ‘obligate parasites’. It was D J Ivanowsky, who in 1882, discovered the Tobacco Mosaic Virus (TMV), the microbe that causes mosaic disease in tobacco and is smaller than bacteria.
Later, in 1898, M W Beijerinek demonstrated that extracts of infected tobacco plants could also cause infection in other healthy plants. He referred to the virus as Contagium vivum fluidum, meaning “infectious living fluid”. W M Stanley, in 1935, further proved that viruses could be crystallised, which means that viral crystals are mainly protein crystals. In addition to proteins, viruses have genetic material- either RNA or DNA, which acts as the infectious agent. Usually, viruses that infect plants have single-stranded RNA while those that infect animals have double-stranded RNA or DNA.
Bacteriophage, the virus that infects bacteria, usually has a protein coat called a ‘capsid’ that envelops the inner double-stranded DNA. A capsid is made of small sub-units called ‘capsomeres’ that are arranged either helically or in a polyhedral manner. Viruses infect plants, causing symptoms such as mosaic formation, leaf-rolling and yellowing, dwarfing and stunted growth. Viruses infect humans causing diseases such as the common cold, mumps, small pox and AIDS.
Viroids contain RNA, but lack the protein coat and are also infectious. They were first discovered by T O Diener in 1971. They are known to cause potato spindle tuber disease.
Lichens have both algae and fungi living together symbiotically. The algae component known as ‘phycobiont’ is autotrophic while the fungal component, called ‘mycobiont’ is heterotrophic. So, algae prepare food for fungi and fungi provide shelter and absorb water and nutrients for algae.