Control Systems In Plants
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I.
Research on how plants grow toward light led to the discovery of plant
hormones.
A.
Hormones
are chemical signals that coordinate the parts of the organism.
B.
Only
minute concentrations are required to induce substantial changes in the
organism.
C.
Positive
phototropism is the growth of a shoot toward light.
D.
Charles
and Francis Darwin observed that a grass seedling could bend toward light only
if the tip of the coleoptile was present.
1.
If the tip
were removed, the coleoptile would not curve.
2.
The actual
growth response occurred some distance below the tip.
3.
They
proposed the hypothesis that some signal was transmitted downward from the tip
to the elongating region of the coleoptile.
E.
Peter
Boysen-Jenson demonstrated that the signal was a mobile substance by separating
the tip from the remainder of the coleoptile by a block of gelatin.
1.
Cellular
contact was block but chemicals could pass.
2.
The
seedlings behaved normally.
F.
F.W. Went
extracted the chemical messenger by removing the coleoptile tip and placing it
on a block of agar.
1.
He
concluded the block contained a chemical produced in the coleoptile tip and it
stimulated growth as it passed down the coleoptile.
2.
Named the
hormone auxin.
II.
Plant hormones help coordinate growth, development, and responses to
environmental stimuli.
A.
Five major
classes of plant hormones.
B.
Control
plant growth and development by affecting the division, elongation, and
differentiation of cells.
C.
Some also
mediate shorter-term physiological responses.
D.
Each has a
multiplicity of effects depending on the site of action, the developmental stage
of the plant, and the concentration of the hormone compared to others.
E.
Transport
from cell to cell involves the passage across cell walls.
F.
Found in
very small concentrations.
G.
May act by
altering the expression of genes, by affecting the activity of existing enzymes,
or by changing properties of membranes.
H.
Reaction
usually depends not on the absolute amount of the hormones but its relative
concentration compared to others.
I.
Hormonal
balance may control the growth and development of the plant.
J.
Auxin
1.
Describes
any chemical substance that promotes elongation of the coleoptiles.
2.
The
natural auxin is called indoleacetic acid of IAA.
3.
The apical
meristem is a major site of auxin synthesis.
4.
The auxin
moves down the shoot apex to the region of cell elongation and stimulates growth
of cell.
5.
At high
concentrations, it may inhibit cell elongation.
6.
This is
probably due to a high level of auxin inducing the synthesis of another hormone,
ethylene, which generally acts as an inhibitor of plant growth.
7.
Transported
directly through parenchyma tissue from one cell to the next.
8.
Moves only
from shoot tip to base and is called polar transport although it has nothing to
do with gravity.
9.
Polar
auxin transport requires energy using proton pumps driven by ATP.
10.
Auxin
exits each cell by a specific carrier protein that is restricted to the basal
end of the cell.
11.
Proton
pumps located at the plasma membrane also play a response by lowering the pH of
the wall which makes it more plastic and is free to take up additional water by
osmosis and this allows for elongation.
12.
Also
stimulates longer-term growth responses.
13.
Affects
secondary growth by inducing cell division in the vascular cambium and by
influencing the differentiation of secondary xylem.
14.
Promotes
the formation of adventitious roots at the cut base of the stem.
15.
Developing
seeds also synthesize auxin, which promotes the growth of fruit in many plants.
16.
Used as a
herbicide (2,4-D)
K.
Cytokinins
1.
Discovered
by accident by Johannes van Overbeek using coconut milk on developing embryos.
2.
Named
because they stimulate cytokinesis or cell division.
3.
Produced
in actively growth tissues in roots embryos and fruits.
4.
Reach
target tissues by moving up the plant by the xylem sap.
5.
Stimulate
cell division and influence the pathway of differentiation.
6.
The ration
of Cytokinin to auxin controls the differentiation of cells.
7.
If more
Cytokinin than auxin, shoot buds appear.
8.
If more
auxin that Cytokinin, roots form.
9.
Auxin
transported from the shoot restrains axillary buds from growing causing the
shoot to lengthen.
10.
Cytokinin
entering the shoot system from the roots counter the action by signaling the
axillary buds to grow.
11.
As roots
become more extensive, the increased level of cytokinins would signal the shoot
system to form more branches.
12.
Used to
retard aging of some plant organs by inhibiting protein breakdowns, by
stimulating RNA and protein synthesis and by mobilizing nutrients from
surrounding tissues.
L.
Gibberellins
1.
E.
Kurosawa discovered that the disease of rice was caused by a fungus and then
later determined that the hyperelongation of rice stems came from a chemical
called gibberellin.
2.
Roots and
young leaves are major sites of production.
3.
Stimulate
growth in both the leaves and stem but have little effect on root growth.
4.
Stimulate
cell elongation and cell division.
5.
Gibberellins
and auxin must be acting simultaneously.
6.
Also used
in fruit growth.
7.
In some
plants both gibberellins and auxins must be present.
8.
Many seed
have a high concentration of gibberellins from the embryo.
9.
After
water is imbibed, the release of gibberellins from the embryo signals the seeds
to break dormancy and germinate.
10.
Also
function to break dormancy in the resumption of growth by apical buds in spring.
M.
Abscisic
Acid
1.
Produced
in the terminal bud.
2.
Slows
growth and directs leaf primordial to develop into the scales that will protect
the dormant buds during winter.
3.
Inhibits
cell division of the vascular cambium.
4.
Suspends
both primary and secondary growth.
5.
Also used
as a stress hormone to help the plant cope with adverse conditions.
N.
Ethylene
1.
Mistakenly
discovered by ripening fruit in the presence of kerosene heaters.
2.
Kerosene
produces ethylene which is a gaseous by-product.
3.
It is a
gas that moves through the plants in the air spaces between cells.
4.
Can also
move in the cytosol traveling from cell to cell through the symplast and in the
phloem.
5.
Inhibits
cell elongation.
6.
Associated
with a variety of aging processes in plants.
7.
Aging or
senescence is a progression of irreversible change that eventually leads to
death.
8.
May occur
at cell level, organ level or whole plant levels.
9.
Includes
the degradation of cell walls which softens the fruit and decreases the
chlorophyll content causing the fruiting to ripen.
10.
As it
triggers the aging process, the aging cells then release more ethylene.
11.
The signal
to ripen even spreads from fruit to fruit.
12.
Circulating
air prevents ethylene from accumulating and carbon dioxide inhibits the action
of whatever has not been flushed away.
13.
The loss
of leaves is controlled by the ethylene and auxin levels.
14.
Abscission
is the loss of leaves at the base of the petiole.
15.
Parenchyma
cells of this layer are very thin. And enzymes hydrolyze the polysaccharides
which causes the leaves to fall.
16.
An aging
leaf produces less and less auxin.
III.
Tropisms orient the growth of plant organs toward or away from stimuli
A.
Tropisms
are growth responses that result in curvatures of whole plant organs toward or
away from stimuli.
B.
The
mechanism is differential rate of elongation of cells opposite sides of the
organ.
C.
Three
stimuli are used to induce and are gravity, light, and touch.
D.
Phototropism
1.
Results
from auxin stimulating cell elongation on the darker side of the stem or some
other chemical messenger inhibiting elongation on the lighter side.
2.
Shot tip
is the site of photoreception that triggers the growth response and are made of
pigment molecules that are most sensitive to blue light.
These are probably yell and related to riboflavin.
E.
Gravitropism
1.
Roots
curve downward in response to gravity.
2.
Roots
display positive gravitropism and shoots display negative gravitropism.
3.
Uses
statoliths which are specialized plastids containing dense starch grains and
moving them to the low points of the cells.
4.
In roots
they are located in certain cells of the root cap.
F.
Thigmotropism
1.
Grasping
organs usually grow straight until they touch something and the contact
stimulates a coiling response caused by differential growth of cells on opposite
sides of the tendril.
2.
The
developmental response to mechanical perturbation is called thigmomorphogenesis
and usually results from an increased production of ethylene in response to
chronic mechanical stimulation like wind.
IV.
Turgor movements are relatively rapid, reversible plant responses
A.
Rapid leaf
movements
1.
Results
from a rapid loss of turgor by cells within pulvini which are specialized motor
organs located at the joints of the leaf.
2.
The motor
cells suddenly become flaccid after stimulation because the lose potassium which
causes water to leave the cells by osmosis.
3.
Helps the
plant conserve water.
4.
From the
point of stimulation, the message that produces this response travels wavelike
through the plant at a speed of about a centimeter per second.
5.
Chemical
messengers probably have a role in transmission but an electrical impulse can
also be detected and is called action potentials.
B.
Sleep
Movements
1.
Many
plants lower their leaves in the evening and raise them to a horizontal position
in the morning.
2.
Powered by
daily changes in the turgor pressure of motor cells in pulvini.
3.
When
leaves are horizontal the cells on side of the pulvinus are turgid and those on
the other side are flaccid.
4.
Paralleling
the opposing changes in volume is massive migration of potassium from one side
to another.
V.
Photoperiodism synchronizes many plant responses to changes of season.
A.
A physical
response to day length such as flowering is photoperiodism
B.
Photoperiodic
control of flowering
1.
Studied by
Garner and Allard
2.
Found that
if the plants were kept in light-tight boxes so that lamps could manipulate the
duration of light and dark, flowering would occur at other times.
3.
Short-day
plants require a light period shorter than a critical length to flower.
4.
These
include mums, poinsettias, and soybeans.
5.
Long-day
plants tend to flower in late spring or early summer when the light period is
longer.
6.
Day-neutral
plants flower when they reach the correct stage of maturity regardless of day
length at the time.
7.
Researchers
discovered in the 1940’s that is was the length of night that controlled these
plants and not the length of day.
8.
Some have
a pretreatment to cold required before they will flower.
This is called vernalization.
9.
Buds
produce flowers but leaves detect the photoperiod.