Plant Viruses

Plant viruses are viruses that infect plants. A 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 cause various types of plant 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 the tobacco mosaic virus.

Plant Viruses: 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 the weather, pruning, or vectors such as bacteria, fungi 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 Viruses: Viroids

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.

Plant Viruses: Satellite Viruses

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.