A small infectious agent that is only able to replicate by modifying the genetic machinery of living host cells. Outside the host cell a virus consists of DNA or RNA surrounded by a protein shell (see capsid). In this inert state a given virus has a characteristic size and shape (e.g. polyhedral, spherical, rod-shaped, etc.). Some of the simpler viruses, e.g. tobacco mosaic virus (TMV), can be crystallized.
Approximately 400 plant viruses are known, most of which are single-stranded RNA viruses. Some, e.g. TMV and cucumber mosaic virus, have a wide host range while others are limited to a few species. Viruses can produce a variety of symptoms, e.g. mosaics, leaf spots, and deformed growth of certain organs. The broken flower colours of certain ornamentals, e.g. Rembrandt tulips, are due to virus infection. Some viruses are symptomless though they may still markedly reduce yield. Viruses are transmitted by vectors and by infected seed and pollen. Control is by using virus-free seed or by breeding for hypersensitivity. Viruses are generally not found in meristematic shoot tips and virus-free stocks of certain species can be obtained by tissue culture of meristem explants. *Cross protection
is successful in some crops.
A virus that infects bacteria is termed a *bacteriophage