How Plant Viruses, Viroids, and Satellite Viruses Cause Disease

Brome Mosaic Virus - Plant
Brome mosaic virus (BMV) is a small, positive-stranded, icosahedral RNA plant virus of the alphavirus-like superfamily. Laguna Design/Oxford Scientific/Getty Images

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

Ringspots Leaves
This photo shows orchid leaves with symptoms of ringspots resulting from a strain of tobacco mosaic virus.

Department of Plant Pathology, North Carolina State University/Bugwood.org/CC BY-NC 3.0

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

Green Peach Aphid
This green peach aphid (Myzus persicae) is an important transmission vector of plum pox virus.

Scott Bauer/USDA Agricultural Research Service/Bugwood.org/CC BY-NC 3.0

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.

Plant Viroids

Potato Spindle Tuber Viroid
The potato tubers on the left are infected with the potato spindle tuber viroid. The reduced size and yield of these tubers can be seen as compared to the healthy tubers on the right.

European and Mediterranean Plant Protection Organization/Bugwood.org/CC BY-NC 3.0

Viroids are extremely small plant pathogens that consist of tiny single-stranded molecules of RNA, usually only a few hundred nucleotides long. Unlike viruses, they lack a protein capsid to protect their genetic material from damage. Viroids don't code for proteins and are commonly circular in shape. Viroids are thought to interfere with a plant's metabolism leading to underdevelopment. They disrupt plant protein production by interrupting transcription in host cells.

Transcription is a process that involves the transcribing of genetic information from DNA to RNA. The transcribed DNA message is used to produce proteins. Viroids cause a number of plant diseases that severely impact crop production. Some common plant viroids include the potato spindle tuber viroid, peach latent mosaic viroid, avocado sunblotch viroid, and the pear blister canker viroid.

Satellite Viruses

Satellite Tobacco Necrosis Virus
This is a computer model of a satellite tobacco necrosis virus.

Mehau Kulyk/Science Photo Library/Getty Images

Satellite viruses are infectious particles that are capable of infecting bacteria, plants, fungi, and animals. They code for their own protein capsid, however they rely on a helper virus in order to replicate. Satellite viruses cause plant diseases by interfering with specific plant gene activity. In some instances, plant disease development is dependent upon the presence of both the helper virus and it's satellite. While satellite viruses alter the infectious symptoms caused by their helper virus, they do not influence or disrupt viral replication in the helper virus.

Plant Virus Disease Control

Tomato Spotted Wilt Virus
These tomato fruits are showing symptoms of Tomato Spotted Wilt Virus (TSWV).

William M. Brown Jr./Bugwood.org/CC BY-NC 3.0

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.

Plant Viruses Key Takeaways

  • Plant viruses are particles of RNA or DNA that infect plants and cause disease.
  • Most plant viruses are single-stranded RNA or double-stranded RNA viruses.
  • Common plant viruses include mosaic viruses, spotted wilt viruses, and leaf curl viruses.
  • Plant viruses are typically spread by either horizontal or verticle transmission.
  • Viroids are single-stranded molecules of RNA that cause plant diseases that lead to underdevelopment.
  • Satellite viruses are extremely small infectious particles that rely on a helper virus in order to replicate and cause plant diseases.
  • There is no cure for plant viral diseases, thus prevention remains the focus of control.