- Plant viruses are viruses that infect plants. Control of plant viruses is of great economic importance world-wide, as these viruses cause diseases that destroy commercial crops.
- Like other viruses, a plant virus particle, also known as a virion, is an extremely small infectious agent. It is essentially a nucleic acid (DNA or RNA) enclosed in a protein coat called a capsid.
Viral genetic material can be double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. Most plant viruses are classified as single-stranded RNA or double-stranded RNA virus particles. Very few are single-stranded DNA and none are double-stranded DNA particles.
Plant Viruses and Disease
Plant viruses cause various types of diseases, but the diseases do not typically result in plant death. They do however, produce symptoms such as ringspots, mosaic pattern development, leaf yellowing and distortion, as well as deformed growth.
The name of the plant disease is often related to the symptoms the disease produces in the particular plant. For example, papaya leaf curl and potato leaf roll are diseases that cause specific types of leaf distortion. Some plant viruses are not limited to one particular plant host, but may infect different varieties of plants. For example, plants including tomatoes, peppers, cucumbers, and tobacco may all be infected by mosaic viruses. The brome mosaic virus commonly infects grasses, grains, and bamboos.
Plant Virus Transmission
Plant cells are eukaryotic cells that are similar to animal cells. Plant cells however, have a cell wall that is nearly impossible for viruses to breach in order to cause infection. As a result, plant viruses are typically spread by two common mechanisms: horizontal transmission and vertical transmission.
- Horizontal Transmission
In this type of transmission, the plant virus is transmitted as a result of an external source. In order to “invade” the plant, the virus must penetrate the plant’s outer protective layer. Plants that have been damaged by weather, pruning, or plant vectors (bacteria, fungi, nematodes, and insects) are typically more susceptible to a virus. Horizontal transmission also occurs by certain artificial methods of vegetative reproduction typically employed by horticulturists and farmers. Plant cutting and grafting are common modes by which plant viruses may be transmitted.
- Vertical Transmission
In vertical transmission, the virus is inherited from a parent. This type of transmission occurs in both asexual and sexual reproduction. In asexual reproductive methods such as vegetative propagation, the offspring develop from and are genetically identical to a single plant. When the new plants develop from the stems, roots, bulbs, etc. of the parent plant, the virus is passed along to the developing plant. In sexual reproduction, viral transmission occurs as a result of seed infection.
In most cases, scientists have been unable to find cures for plant viruses, so they have been focusing on reducing the occurrence and transmission of the viruses. Viruses are not the only plant pathogens. Infectious particles known as viroids and satellite viruses cause several plant diseases as well.
Currently, there is no cure for plant viral disease. This means that any infected plants must be destroyed for fear of spreading disease. The best methods being employed to combat plant viral diseases are aimed at prevention. These methods include ensuring seeds are virus-free, control of potential virus vectors through pest control products, and ensuring that planting or harvesting methods do not promote viral infection.