Welcome to my website where I record information, gathered from many sources on the web and through print media, about the benefits of attracting useful creatures to our gardens. I include creatures found in my own garden and explain how they help grow healthy and productive plants without using synthetic chemical pesticides and fertilisers.................................................John Ashworth 27th July 2015.
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.
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
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.
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.
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.
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.