Transcript Plant_Responses_

Plant Responses Hormones
Chapter 39
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Plants, like animals, can sense changes in their environments.
However, they lack a nervous system and cannot respond
immediately. Their responses are limited to events that occur as
a result of the activity of a plant's hormonal system.
 Hormones are “chemical messengers” - they carry information
from one part of an organism's body to another. This information
is carried in chemical form.
 Typically, these bits of chemical information result in changes in
growth patterns in plants (because plants lack the two principle
animal effectors - muscles and glands.)
 Hormones also influence flowering and fruit production.
Phototropism
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A plant growing on a windowsill will grow toward light.
This recognition of light and bending is termed
phototropism. It is adaptive.
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If the cells of a plant growing toward light are inspected, it can
be seen that the cells on the dark side of the plant are larger.
 The different cellular growth patterns make plants bend
toward the light.
 In order for plants to grow toward light, their shoot tips are
necessary (a discovery made by Charles Darwin and his son
Francis).
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They also learned that the shoot grew straight up if an
opaque cover was placed on the end of the shoot.
Darwin and his son speculated that some "signal" was
transmitted downward from the tip to the meristems in
the shoot.
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In 1913, Danish botanist Peter Boysen-Jensen continued
Darwin's experiments.
In one group of seedlings, he inserted a block of gelatin
between the tip and the rest of the shoot. This gelatin
block allowed chemicals to pass through.
(Gelatin is like microscopic “Swiss Cheese”!
In the other group he placed a thin piece of impermeable
mica between the tip and the rest of the shoot. This is
what he discovered:
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In 1913, Dutch botanist Fritz Went discovered the
transmitted factor and he named it auxin.
Auxin diffuses to the dark side of a shoot and promotes
cell growth there. The lighted side, which experiences
reduced auxin, shows a slower growth rate.
The most common auxin is IAA (Indoleacetic Acid) - a
chemical that plants make from the amino acid
tryptophan (Name the other 19, out loud, now, please).
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Botanists have since identified five different types of plant
hormones. Three of these - the auxins, cytokinins, and
gibberellins - are actually hormone classes. Abscisic acid and
ethylene are single chemicals.
 General statements concerning plant hormones:
 They often affect growth.
 They are produced in very small quantities.
 Their effect is profound.
 Their effects are limited to target cells.
 Plant hormones exert their
effects by altering the
expression of genes, by
activating or inhibiting enzymes,
or by changing properties of
membranes or cell walls.
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Plant hormones affect plant growth by causing
target cells to elongate or divide (mitosis).
 http://www.youtube.com/watch?v=u6rAHdvZ
The Auxins
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The chief function of the auxins
is to promote the elongation of
developing shoots.
The most common auxin is IAA
(Indoleacetic acid).
The major site of auxin
synthesis is the apical
meristem at the tip of the
shoot.
When IAA is produced, it
migrates downward and
causes elongation of the cells
in the stem
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Lower than normal concentrations of IAA results in slower stem
growth and increased root growth.
Higher concentrations of IAA promote stem growth and diminish root
growth.
Auxins can also trigger the development of vascular tissue and
induce cell division in the vascular cambium.
Auxins are also found in seeds and promote the development of the
fruit. Some plants (tomatoes, cucumbers, eggplants) can be made to
produce fruits without fertilization if they are sprayed with auxin.
(The fruits are seedless!)
Auxins are also known to affect gravitotropism. An auxin shield
results in the growth of roots that do not grow toward gravity:
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Auxins seem to exert their influence by weakening cell walls. Auxins
may stimulate certain proteins in a plant cell's plasma membrane to
pump hydrogen ions into the cell wall. These hydrogen ions combine
with (and activate) enzymes that break the bonds that hold cellulose
molecules together. When the cell swells with water (turgor), the cell
elongates.
Botanists attribute auxin movements in plants to active transport
mechanisms - they seem to have specific target tissues and they are
faster than diffusion.
http://www.youtube.com/watch?v=HvZJqWSdMMk
The Cytokinins
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This is the plant hormone group that promotes cytokinesis (AKA cell
division) - making it a pretty easy-to-remember hormone.
Most often associated with mitosis in roots, embryos, and fruits.
The most commonly cited effect of the cytokinin hormones is to
promote lateral branch growth in the absence of auxin production in
stem tips. To explain, if a plant is left to grow with an intact terminal
bud, its growth habit is to increase in length, with a smaller amount
of lateral stem development. If, however, the stem tip is CUT OFF
(negating auxin production), then there is no inhibitory effect on the
axillary buds and the plant grows laterally. Cytokinins, transported
upward from the root, activate the axillary buds, making the plant
grow more "bushy".
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Most plant growth can be explained as a combination of the
effects of auxins and cytokinins. Cytokinins traveling upward
from the roots counter the effects of auxins being produced in
the apical shoot meristems. Growth in the lower parts of the
plant are more prominently lateral because of their proximity
to the cytokinins, whereas apical dominance remains the
growth pattern in the higher parts of the plant.
 Cytokinins also retard the aging of flowers and fruits. This
aging is termed senescence.
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The Gibberellins
The gibberellins are a category of
at least 70 different hormones.
They are produced in both shoot
and root tip meristems.
The function most likely to be
addressed on the A.P. exam is the
role they play in the lengthening of
the stem between nodes (and the
growth of leaves).
•Gibberellins also influence fruit
development. They (like auxins)
can be sprayed on certain flowers
to induce fruit development without
seeds – apples, currants,
eggplants.
Gibberellins are known to influence
seed germination. If they are
sprayed on seeds, they can cause
germination in the absence of
factors that are normally required
for seed germination to begin
(such as colder temperatures,
increased photoperiod).
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Abscisic Acid
Abscisic acid (ABA) is produced in buds and inhibits cell division in
apical meristems and cambium. It is useful in maintaining dormant
periods. It also signals the formation of bud scales, which protect
buds from harsh conditions.
ABA is also generated to help maintain seed dormancy. A downpour
or monsoon will leach out ABA, enabling germination. This helps to
insure that seeds do not germinate until sufficient water is present in
the environment.
It is often helpful to think of the effects of Abscisic Acid to counter (or
balance) the effects of the Gibberellins.
Abscisic acid also helps plants deal with adverse conditions. If a
plant is dehydrated, ABA causes the stomata to close and remain
closed until more water is available.
Ethylene
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Plants produce ethylene, a gas, to
function as a hormone that
triggers a variety of aging
responses - the most important is
the ripening of fruit.
Fruit ripens when:
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Cell walls are weakened or
broken down.
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The color of the skin changes
(sometimes) Drying occurs.
Ethylene is produced in the cells
of the fruit to encourage all fruits
nearby to ripen at the same time.
Stored fruits can be stored under
high CO2 conditions to counter
the effects of ethylene and retard
ripening.
The change of color that occurs in
trees in autumn is also a function
of ethylene.
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The separation of leaves from their stems in the autumn
(abscission) is regulated by ethylene.
 Separation occurs at a site known as the abscission layer at
the base of the leafstalk (petiole). This layer is dominated by
parenchyma cells. Under the influence of ethylene, these
parenchyma cells are sealed off from the vascular bundles
(their source of water and nutrients). When the parenchyma
cells die, the abscission layer weakens until the leaf falls from
the tree noiselessly and floats daintily to the ground. (Note:
During the summer, auxin prevents abscission).
How are plant hormones different
than animal hormones?
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Plant hormones are not produced in glands (but they are transported
around the plant to act on distant tissues).
Plant hormones are not specific in their action. Or more simply, they
can produce different responses in different tissues.
Plant hormones are small, simple molecules but animal hormones are
large, complex molecules
There are many types of animal hormones but there are only five
classic plant hormones.