Biology Reference
In-Depth Information
Viruses, Viroids, Phytoplasmas
Viruses
viruses contain deoxyribonucleic acid (DNA)
instead of RNA. X-ray diffraction and electron
microscopy have shown something of the mor-
phology of virus particles. Some are rods, some
filiform, and some are isometric, but polyhedral
rather than spherical. They apparently act not as
organisms but as disturbances in the host metab-
olism of nucleic acid.
There are over 850 described plant virus spe-
cies. Many of the described viruses are definitive
members of genera, whose names have been
approved by the ICTV (International Committee
on Taxonomy of Viruses) to be viruses and could
be agents of other kinds. For example, aster yel-
lows and elm phloem necrosis were thought for
some time to be caused by viruses, but have now
besen determined to be caused by phytoplasmas.
Moreover, in
Part 4
some phytoplasma may still
be positioned under bacterial or viral caused dis-
ease since their true identity is not yet known or
that the confirmation of identity has been made
but missed for inclusion in the 7th edition. In
addition, potato spindle tuber and chrysanthe-
mum stunt disease were long thought to be caused
by viruses, but have now been determined to
be caused by viroids. Viroids consist solely of
small RNAs with no protein coat. There are now
about 30 plant diseases that have been identified
as having viroid causal agents including potato
spindle tuber, chrysanthemum stunt, citrus
exocortis, chrysanthemum chlorotic mottle, and
cadangcadang of palm. More diseases caused by
viroids will probably be identified in future years.
There are now about 68 identified phytoplasmas
The word virus means poison or venom. When it
is used in connection with a plant disease, it
means a filterable virus, an infective principle or
etiological agent so small it passes through filters
that will retain bacteria. Virus diseases in man
range from infantile paralysis to the common
cold and in plants from “breaking” of tulip
flowers to the deadly raspberry ringspot disease
on the Malling Jewel variety of raspberry.
Viruses are obligate parasites in that they are
capable of increasing only in living cells. They
are not organisms in the usual sense because they
do not multiply by growth and fission, and they
are too complex to be chemical molecules. F. C.
Bawden, in the 1964 edition of his
Plant Viruses
and Virus Diseases
, defines viruses as “submi-
croscopic infective entities that multiply only
intracellularly and are potentially pathogenic.”
Virus diseases are old; our knowledge of them
is relatively recent. Tulip mosaic, shown as
breaking of flower color, was described in
a topic published in 1576. In 1892 it was shown
that the cause of tobacco mosaic could pass
through a bacteria-proof filter, and in 1935
a crystalline protein was prepared from tobacco
mosaic virus juice. At present we believe that
virus particles contain only two major compo-
nents, nucleic acid embedded in a protein struc-
ture, and that they are built of uniform-sized
subunits arranged in a fixed and regular manner.
Many plant viruses contain ribonucleic acid
(RNA). Some plant viruses and many animal
