| Growth
and development of plants, like all organisms, is regulated
by a combination of genetic factors and environment influences.
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multitude of growth activities are under specific chemical
controls. Such chemicals are known as plant growth regulators,
or plant hormones. Other growth activities are subject
to environmental cues, including photoperiod, temperature,
and pressure and moisture changes. One of the differences
between plant responses and animal responses is that plant
responses to the environment often involve differential
growth patterns rather than behavioral activities in response
to positive or negative stimuli. |
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| Signal
Transduction Pathways in Plants |
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| Hormones
may function as signal molecules that trigger the signal
transduction pathways in cells. Such pathways often result
in the synthesis of transcription factors that in turn
promote synthesis of enzymes that facilitate chemical
reactions within the cell (the response). Signal transduction
pathways are equally important for chemical messaging
in plants as in animals and both environmental cues and
hormones serve as signal stimuli. |
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| Plant
hormones, or plant growth regulators, are chemicals produced
by plants that alter growth patterns and/or maintenance
of the plant. They can be found in many cells and tissues,
although plant hormones seem to be concentrated in meristems
and buds (which are dormant shoot meristems).One should
not confuse hormones, or growth regulators with enzymes.
Any chemical reaction that occurs in a cell requires a
specific catalyst, a molecule that brings the reactants
together in an arrangement in which the chemical reaction
can take place. Enzymes, which are proteins, are the catalysts
found in the cells of all living organisms. Growth regulators
function more in controlling events by sending chemical
signals or messages to cells to do something or to not
do something. Plant hormones inhibit as well as promote
cellular activities. |
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| In
contrast to animal hormones, which generally have very
specific effects, the hormones identified in plants most
often regulate division, elongation and differentiation
of cells. Most hormones have multiple effects in plants.
Plant hormones work in very small concentrations, affecting
membrane properties, controlling gene expression and affecting
enzyme activity. One example previously mentioned was
the action of plant hormones on increasing the extensibility
of cellulose for cell wall expansion. In most cases, the
effect plant hormones have on the plant depends on the
location of and concentration of the hormone relative
to other hormones in the specific tissues. Plant hormones
often work in conjunction with each other, and have overlapping
effects. They also work with environmental stimuli, as
we shall discuss. |
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| There
are several classes of plant hormones, including
a number of recently "discovered"
ones. Auxins |
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| The
concept of chemical messengers in plants was proposed
by Charles Darwin and his son, Francis, in 1881, who spent
time looking at the phenomenon of phototropism in wheat
seedlings. Plant shoots are positively phototropic. When
a seedling is illuminated from the side, the shoot will
bend towards the light. This directional growth makes
sense, since plants need light for photosynthesis. However,
the Darwins found that if the coleoptile of the wheat
seedling was removed, the plant no longer curved towards
light. They did a number of experiments and determined
that a chemical located in the coleoptile traveled to
the region of elongation and effected a differential elongation
of cells furthest from the light sources. The chemical
was subsequently studied and named auxin by Frits Went
in 1926. |
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| Chemically
auxin is indoleacetic acid (IAA) which is synthesized
from tryptophan. There are a number of synthetic "auxins"
too. Auxins promote growth in molar concentrations of
10-3 to 10-8. A primary site of auxin production is the
apical shoot meristem. Auxin moves down the stem parenchyma
cells by polar transport (auxin becomes negatively charged)
using proton pumps, an ATP requiring process. |
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The
most studied function of auxin is promotion of elongation
and cell enlargement For cell elongation, proton
pumps increase H+ concentration in cell walls which
stimulates expansions, proteins that disrupt hydrogen
bonds and break cross linkages in cellulose. This
facilitates wall expansion when cells take in more
water. |
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Auxins
are involved in tropic responses. Auxins migrate
away from a light source, which accounts for the
uneven elongation of cells on the shaded side of
a plant unevenly exposed to light. |
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Auxin
stimulates the production of secondary growth by
simulating cambium cells to divide and secondary
xylem to differentiate. |
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Auxin
produced in apical buds tends to inhibit the activation
of buds lower on the stems. This is known as apical
dominance. This effect lessens with distance
from the shoot tip. Cytokinins (another group of
plant hormones) counter the apical dominance effect
of auxins. |
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Auxins
also: |
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Promotes
lateral and adventitious root development |
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Promotes
other hormone production, especially ethylene
when auxin concentration increases |
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Promotes
flower initiation |
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Loss
of auxin initiates leaf abscission |
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Fruit
development requires auxin produced by the developing
seed. Can get seedless fruits using synthetic auxins |
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| Auxins
are toxic in large concentrations. They affect mostly
dicots but not monocots, suggesting the effect is on secondary
growth activities. Synthetic auxin herbicides include
2-4-D and 2-4-5-T (Agent orange of the Vietnam era contains
synthetic auxin) |
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Used
as defoliants |
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Used
as weed killers |
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| Cytokinins
are a group of phenyl urea derivatives of adenine, one
of the molecules in DNA. The first cytokinin was chemically
isolated in 1913 and cytokinins were studied using coconut
endosperm for a number of years starting in the 1940's.
However cytokinins were not chemically identified until
1963 in corn. That cytokinin was called zeatin. |
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| Biologists
have yet to identify the genes that code for the variety
of cytokinins found in plants. One hypothesis today is
that cytokinins are actually synthesized by symbiotic
methylobacteria that live within plant tissues. |
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Cytokinins
promote cytokinesis. They are found primarily in
root meristems, embryos and fruits, and migrate
from roots to the shoot systems of plants in xylem
tissue. |
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The
effects of cytokinins are often studied in tissue
culture. Parenchyma cells grown in tissue culture
will not divide and differentiate unless cytokinins
and auxin are present. The ratio of cytokinins to
auxin controls what tissues differentiate in the
tissue culture. |
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Cytokinins
can promote axillary bud growth by over-riding the
apparent inhibiting effect of auxin. This is one
of the ways in which plants balance root and shoot
growth. Shoot tip auxins inhibit lateral bud activation.
Cytokinins produced in root meristems travel upward
in xylem and if in higher concentration counter
the inhibition of auxins to activate lateral buds.
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Removing
shoot tip activates lateral shoot development. |
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Cytokinins
also retard leaf senescence, probably by stimulating
RNA and protein synthesis and delaying degradation
of chlorophyll. |
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Some
pathogens that promote tumor formation, called galls
in plants take advantage of cytokinin function.
The bacteria that form these tumors contain genes
for the synthesis of cytokinins so promote rapid
undifferentiated cell growth in the infected part
of the plant. Agrobacterium tumefaciens, a common
vector in plant biotechnology contains such genes.
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