Soil and Plant Viruses

photo: astrobio
Latest Update 19th January 2015.

Foreword 
  • This website is about useful garden creatures, and it may be hard to see viruses as useful in any way whatsoever.  Despite this I have included them because they exist in such fantastic numbers, are ubiquitous and are incomprehensibly small as individuals.  
  • Consequently we have very little knowledge of them except as plant and animal pathogens, but logic tells me that as we find out more, as we have already done with bacteria, we may discover that they play an important role in maintaining the worlds natural ecosystems.  
Soil and Plant Viruses.
  • A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea. 
  • Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. 
  • Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. 
  • The average virus is about one one-hundredth the size of the average bacterium, and most viruses are too small to be seen directly with an optical microscope. 
  • Viruses spread in many ways. Influenza viruses are spread by coughing and sneezing.  In plants they are often transmitted from plant to plant by insects that feed on sap, such as aphids.  In animals they can be carried by blood-sucking insects.  These disease-bearing organisms are known as vectors.  
Plant Viruses.
  • There are many types of plant virus.  Some of them effect crop yields and can't be controlled because of the cost of remedial measures. 
  • Plant viruses can be spread from plant to plant by organisms, known as vectors
  • These are normally insects, but some fungi, nematode worms, and single-celled organisms have also been shown to be vectors. 
  • In some cases plant virus infections can be controlled economically, by killing the vectors and removing alternate hosts such as weeds. 
  • Plant viruses can't infect humans and other animals because they only reproduce in living plant cells. 
  • Plants have elaborate and effective defence mechanisms against viruses. One of the most effective is the presence of so-called resistance (R) genes. 
  • Each R gene confers resistance to a particular virus by triggering localised areas of cell death around the infected cell, which can often be seen with the unaided eye as large spots. This stops the infection from spreading.  
  • RNA interference is also an effective defence in plants.  When they are infected, plants often produce natural disinfectants that kill viruses, such as salicylic acid, nitric oxide, and reactive oxygen molecules. 

Bacterial Viruses.

  • Bacteriophages are a common and diverse group of viruses and are the most abundant form of biological entity in aquatic environments – there are up to ten times more of these viruses in the oceans than there are bacteria, reaching levels of 250,000,000 bacteriophages per millilitre of seawater.  
  • These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases just minutes, bacterial polymerase starts to release hundreds of new phages. 
  • Most bacteria defend themselves from bacteriophages by producing enzymes that destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells.
  • Bacteria also contain a system that retains fragments of the genomes of viruses the bacteria have come into contact with.  It allows them to block the virus's replication through a form of RNA interference.
  • This genetic system provides bacteria with acquired immunity to infection.
Archaean Viruses.
  • Some viruses replicate within archaea which are double-stranded DNA viruses with unusual and sometimes unique shapes.
  • These viruses have been studied in detail in thermophilic archaea.  Defence against them may involve RNA interference.
In my Garden.
  • Although well advanced in the battle against human viral pathogens, the science does not seem to have advanced very far in the study of plant and bacterial viral pathogens.
  • However, it seems to me that if plants and soil microbes are kept healthy, they will have adequate defensive systems to deal with most viral infections.
  • I have seen nothing in my reading so far to suggest there are viruses which can impart a benefit in the soil foodweb, but I would be surprised if their role is only that of a pathogen.
Information from.