Although often studied by plant biologists, viruses are not living organisms because they:
- Are not cellular and have no cytoplasm, membranes nor organelles.
- Can't metabolize; they lack the enzymes necessary for protein synthesis and energy transfer.
- Don't increase in size (they don't grow).
- Don't respond to external stimuli.
But, they are able to reproduce within a host cell by using the metabolic equipment of the host. Outside of a cell they are simply a collection of inert molecules of nucleic acids and proteins (which can be denatured easily). They are extremely small, between 20 nm–400 nm, the size of large molecules.
Viruses are obligate intracellular parasites that live within the cells of all kinds of organisms, frequently to the detriment and ultimate death of the host cells. Their presence may trigger a response within the host that produces disease symptoms. Each virus can enter only the cells of hosts with receptors specific for it; humans don't get canine distemper, and dogs don't get polio—both virus‐caused diseases—because the proteins don't match. Viruses attack hosts whose genomes are most like their own.
Bacteriophages (or simply phages) are viruses that invade bacteria. They have very small genomes consisting of only a few genes. A still smaller replicating particle, the viroid, is a small bit of RNA without a protein coat. Viroids are about a tenth the size of small viruses and replicate like viruses using the mechanisms of the host cell. Viroids have been identified as causing some infectious plant diseases and probably are responsible for many animal diseases as well.
The viruses that invade plants do so by entering an open wound or other breaks in the surface or from the actions of an animal invader. The first virus to be isolated and described was the tobacco mosaic virus, which earned its investigator a Nobel prize in 1946. Over 1,000 plant diseases are attributed to viruses and viroids and more than 400 kinds of viruses are involved. Some plants harbor several kinds of viruses and show no effects while others, such as Rembrandt tulips, owe their unique flower colors to the presence of a virus in their cells.
The symptoms of viral diseases usually are systemic rather than localized because the virus spreads throughout the plant in the phloem and from cell to cell through the plasmodesmata. There is no evidence that viruses can penetrate cell walls. In plants infected by some kinds of viruses, only the actively dividing cells of the growing points seem to be virus-free, presumably because the meristem cells divide faster than the virus can move through the plasmodesmata into tip cells.
The universal plant response to a viral infection is reduction in size—stunting of the whole plant, small leaves, and decreased flower and seed production—due to upset of normal metabolism and interference with translocation of materials in the phloem. Symptoms include yellowing of leaves and mottling, leaf spots, wilting, and tumors resulting in abnormal flowers and leaves.
Virus-infected plants are weakened and more susceptible to other diseases. Except for tobacco mosaic disease, there is no evidence, however, that viruses are spread from plant to plant by direct contact of plant parts. Most are spread by insects, nematodes, or other animals (such as slugs and snails) or by soil-inhabiting fungi. Humans contribute to the distribution by making and growing cuttings from diseased plants.
There are no vaccines with which to inoculate plants against viral diseases. Control, at present, consists of stopping the spread (such as burning diseased crop plants and potential weed hosts, sterilizing tools used to make cuttings, destroying the seeds of infected plants, and controlling insect and nematode vectors by insecticides and soil fumigation).