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VIRUSES

Plant virus diseases


Viral ring-spot on Geranium


Squash leaf chlorosis virus

Plants like animals (and computers) suffer from virus diseases. Typically they cause mottling, spots or streaks on leaves, flowers and fruits

This is sometimes desirable as with streaked tulips or African violets:

Sometimes viruses just cause growth reduction or no symptoms at all.

Clonally propagated plants tend to accumulate viruses whereas they are usually lost in the process of seed formation

Viruses are invisible under the light microscope; they can only be "seen" with the electron microscope

They are the smallest and simplest form of life (are they alive?)

Some method of reproduction is a (or the) basic necessity of life.

A virus is little more than a message to say "Make more of me"

As for all other life forms on this planet this genetic message is encoded in nucleic acid and viruses make use of the central molecular processes common to all life.

The catch for most plant viruses is that they use RNA to transmit the information to the next generation instead of DNA.

The mature virus is just a piece of RNA wrapped in a protein coat. Inside the plant cell this coat comes off and the virus has to use the host cell machinery to reproduce itself

Many plant viruses contain RNA rather than DNA as their genetic material. Normally, RNA is copied from DNA in the process of transcription. The RNA is then used to direct protein synthesis by ribosomes in the process of translation. RNA viruses carry replicase enzymes which copy their genome into a complementary RNA strand which is then copied back into more copies of the genome. If the genome is "sense RNA" it can be used to direct synthesis of the virus coat proteins and replicase. If it is "antisense RNA" the first-copied, complementary strand is translated into protein. The diagram shows DNA strands separating as RNA is transcribed; an amino acid (glycine) linked to messenger RNA (mRNA) through its transfer RNA (tRNA) and the beginnings of protein synthesis on a ribosome.

Viruses are totally dependent on their hosts for replication of their genomes and protein synthesis - they are the most extreme examples of heterotrophic organisms.

Virus dispersal

So how can viruses get from plant to plant to ensure their reproduction and survival? Direct contact is one way (this could be through stems and leaves, and less obviously through roots) Many viruses are transmitted by vectors that feed off plants: aphids, leafhoppers, nematodes. We have become quite effective virus vectors with pruning shears, knives and saws. If you are feeling aggressive you can accuse a smoker of infecting your tomatoes with tobacco mosaic virus.

Virus disease control

There are no sprays for virus diseases. Most viruses are excluded from seeds and for some reason the growing tips of infected plants tend to be free of virus. So seed and micropropagation will often eliminate virus disease. Conversely viruses tend to persist or accumulate in more conventional vegetative propagation. Many viruses are more heat sensitive than plants, so they can be cleaned up by a spell in a hot (40C) box. Control of vectors obviously helps minimize the chances of infection.

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Copyright © Michael Knee (images provided by Steve Nameth)
The Ohio State University
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