WORLD OF MICROBIOLOGY AND IMMUNOLOGY Plant viruses441•
Phytoplankton are also being recognized as an indicator
for the physical status of the oceans. They require a fairly lim-
ited range of water temperature for healthy growth. So, a
downturn in phytoplankton survival can be an early indicator
of changing conditions, both at a local level (such as the pres-
ence of pollutants) and at a global level (global warming).
Planktonic bacteria are free-living bacteria. They are the
populations that grow in the familiar test tube and flask cul-
tures in the microbiology laboratory. The opposite mode of
growth is the adherent, or sessile, type of growth.
Planktonic bacteria have been recognized for cen-
turies. They are some of the “animalcules” described by
Antoni van Leeuwenhoekin 1673 using a microscopeof his
own design. Indeed, much of the knowledge of microbiology
is based on work using these free-floating organisms.
Research over the past two decades has shown that the
planktonic mode of growth is secondary to the adherent type
of growth. Additionally, the character of planktonic bacteria
is very different from their adherent counterparts. Planktonic
bacteria tend to have surfaces that are relatively hydrophilic
(water loving), and the pattern of gene expression is
markedly different from bacteria growing on a surface. Also,
planktonic bacteria tend not to have a surrounding coat made
of various sugars (this coat is also called a glycocalyx), and
so the bacteria tend to be more susceptible to antibacterial
agent such as antibiotics. Paradoxically, most of the knowl-
edge of antibiotic activity has been based on experiments
with planktonic bacteria.
When grown in a culturewhere no new nutrients are
added, planktonic bacteria typically exhibit the four stages of
population development that are known as lag phase, logarith-
mic (or exponential) phase, stationary phase, and death (or
decline) phase. It is also possible to grow planktonic bacteria
under conditions where fresh food is added at the same rate as
culture is removed. Then, the bacteria will grow as fast as the
rate of addition of the new food source and can remain in this
state for as long as the conditions are maintained. Thus, plank-
tonic bacteria display a great range in the speeds at which they
can grow. These abilities, as well as other changes the bacte-
ria are capable of, is possible because the bacteria are pheno-
typically “plastic;” that is, they are very adaptable. Their
adherent counterparts tend to be less responsive to environ-
mental change.
Planktonic bacteria are susceptible to eradication by the
immune systemof man and other animals. Examination of
many infectious bacteria has demonstrated that once in a host,
planktonic bacteria tend to adopt several strategies to evade
the host reaction. These strategies include formation of the
adherent, glycocalyx enclosed populations, the elaboration of
the glycocalyx around individual bacteria, and entry into the
cells of the host.
It is becoming increasingly evident that the planktonic
bacteria first observed by Leeuwenhoek and which is the sta-
ple of lab studies even today is rather atypical of the state of
the bacteria in nature and in infections. Thus, in a sense, the
planktonic bacteria in the test tube culture is an artifact.See alsoCarbon cycle in microorganismsPPlant virusesLANT VIRUSES
Plant virusesare viruses that multiply by infecting plant cells
and utilizing the plant cell’s genetic replication machinery to
manufacture new virus particles.
Plant viruses do not infect just a single species of plant.
Rather, they will infect a group of closely related plant
species. For example, the tobacco mosaic viruscan infect
plants of the genus Nicotiana. As the tobacco plant is one of
the plants that can be infected, the virus has taken its name
from that host. This name likely reflects the economic impor-
tance of the virus to the tobacco industry. Two other related
viruses that were named for similar economic reasons are the
potato-X and potato-Y viruses. The economic losses caused by
these latter two viruses can be considerable. Some estimates
have put the total worldwide damage as high as $60 billion a
year.
The tobacco mosaic virus is also noteworthy as it was
the first virus that was obtained in a pure form and in large
quantity. This work was done by Wendall Meredith Stanley in- For this and other work he received the 1946 Nobel
Prize in Chemistry.
Plants infected with a virus can display lighter areas on
leaves, which is called chlorosis. Chlorosis is caused by the
degradation of the chlorophyllin the leaf. This reduces the
degree of photosynthesisthe plant can accomplish, which can
have an adverse effect on the health of the entire plant.
Infected plants may also display withered leaves, which is
known as necrosis.
Sometimes plant viruses do not produce symptoms of
infection. This occurs when the virus become latent. The viral
nucleic acid becomes incorporated into the host material, just
as happens with latent viruses that infect humans such as her-
pesviruses and retroviruses.
Most of the known plant viruses contain ribonucleic
acid(RNA). In a virus known as the wound tumor virus, the
RNA is present as a double strand. The majority of the RNA
plant viruses, however, possess a single strand of the nucleic
acid. A group of viruses known as gemini viruses contain sin-
gle stranded deoxyribonucleic acid(DNA) as their genetic
material, and the cauliflower mosaic virus contains double
stranded DNA.
As with viruses of other hosts, plant viruses display dif-
ferent shapes. Also as with other viruses, the shape of any par-
ticular virus is characteristic of that species. For example, a
tobacco mosaic virus is rod-shaped and does not display vari-
ation in this shape. Other plant viruses are icosahedral in shape
(an icosahedron is a 20-sided figure constructed of 20 faces,
each of which is an equilateral triangle).
There are no plant viruses known that recognize specific
receptors on the plant. Rather, plant viruses tend to enter plant
cells either through a surface injury to a leaf or the woody
stem or branch structures, or during the feeding of an insect or
the microscopic worms known as nematodes. These methods
of transmission allow the virus to overcome the barrier
imposed by the plant cell wall and cuticle layer. Those viruses
that are transmitted by insects or animals must be capable of
multiplication in the hosts as well as in the plant.
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