Transcript 25–1 Hormones and Plant Growth

Biology
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25–1 Hormones and Plant
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25–1 Hormones and Plant
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Patterns of Plant Growth
Patterns of Plant Growth
Biologists have discovered that plant cells send
signals to one another that indicate when to divide
and when not to divide, and when to develop into a
new kind of cell.
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25–1 Hormones and Plant
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Patterns of Plant Growth
One difference between growth in plants and animals
is that most animals stop growing once they reach
adulthood.
In contrast, plants continue to grow new needles, add
new wood, and produce cones or new flowers.
The secrets of plant growth are found in meristems,
regions of tissue that can produce cells that later
develop into specialized tissues.
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25–1 Hormones and Plant
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Patterns of Plant Growth
Plants grow in response to environmental factors
such as light, moisture, temperature, and gravity.
Specific chemicals direct, control, and regulate plant
growth.
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25–1 Hormones and Plant
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Plant Hormones
What are plant hormones?
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25–1 Hormones and Plant
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Plant Hormones
Plant Hormones
A hormone is a substance that is produced in one
part of an organism and affects another part of the
same individual.
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25–1 Hormones and Plant
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Plant Hormones
Plant hormones are chemical
substances that control a plant's
patterns of growth and development
and its responses to environmental
conditions.
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25–1 Hormones and Plant
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The portion of an
organism affected by
a particular hormone
is known as its
target cell or target
tissue.
Plant Hormones
Movement of
hormone
Hormone
producing cells
Target cells
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Plant Hormones
To respond to a hormone, the target cell must contain
a receptor to which the hormone binds.
If the receptor is present, the hormone can influence
the target cell by:
•
changing its metabolism
•
affecting its growth rate
•
activating the transcription of certain genes
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25–1 Hormones and Plant
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Plant Hormones
Cells that do not contain receptors are generally
unaffected by hormones.
Different kinds of cells may have different receptors
for the same hormone.
As a result, a single hormone may affect two different
tissues in different ways.
For example, a particular hormone may stimulate
growth in stem tissues but inhibit growth in root
tissues.
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25–1 Hormones and Plant
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Plant Hormones
How do auxins, cytokinins, gibberellins,
and ethylene affect plant growth?
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25–1 Hormones and Plant
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Auxins
Auxins
Charles Darwin and his son Francis carried out the
experiment that led to the discovery of the first
plant hormone.
They described an experiment in which oat
seedlings demonstrated a response known as
phototropism—the tendency of a plant to grow
toward a source of light.
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25–1 Hormones and Plant
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Auxins
In the experiment, they
placed an opaque cap
over the tip of one of the
oat seedlings.
This plant did not bend
toward the light, even
though the rest of the
plant was uncovered.
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25–1 Hormones and Plant
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Auxins
However, if an opaque
shield was placed a few
centimeters below the tip,
the plant would bend
toward the light as if the
shield were not there.
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25–1 Hormones and Plant
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Auxins
The Darwins suspected that the tip of each
seedling produced substances that regulated cell
growth.
Forty years later, these substances were identified
and named auxins.
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Auxins
Auxins are produced in the apical
meristem and are transported
downward into the rest of the plant.
They stimulate cell elongation.
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Auxins
When light hits one side
of the stem, the shaded
part develops a higher
concentration of auxins.
This change in
concentration stimulates
cells on the dark side to
elongate.
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25–1 Hormones and Plant
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Auxins
As a result, the stem
bends away from the
shaded side and
toward the light.
Recent experiments
have shown that auxins
migrate toward the
shaded side of the
stem.
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Auxins
Auxins and Gravitropism
Auxins are also responsible for gravitropism—the
response of a plant to the force of gravity.
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Auxins
Auxins build up on the
lower sides of roots and
stems. In stems, auxins
stimulate cell elongation,
helping turn the trunk
upright.
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Auxins
In roots, their effects are exactly the opposite.
There, auxins inhibit cell growth and elongation,
causing the roots to grow downward.
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Auxins
Auxins also influence how roots grow around objects
in the soil.
If a growing root is forced sideways by an obstacle,
auxins accumulate on the lower side of the root.
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25–1 Hormones and Plant
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Auxins
High concentrations of auxins inhibit the elongation of
root cells.
Uninhibited cells on the top elongate more than
auxin-inhibited cells on the bottom and the root grows
downward.
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25–1 Hormones and Plant
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Auxins
Auxins and Branching
Auxins also regulate
cell division in
meristems.
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25–1 Hormones and Plant
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Auxins
As a stem grows in
length, it produces lateral
buds.
A lateral bud is a
meristematic area on the
side of a stem that gives
rise to side branches.
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Auxins
Most lateral buds do not
start growing right away.
The reason for this
delay is that growth at
the lateral buds is
inhibited by auxins.
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25–1 Hormones and Plant
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Auxins
Because auxins move
out from the apical
meristem, the closer a
bud is to the stem's tip,
the more it is inhibited.
This phenomenon is
called apical
dominance.
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25–1 Hormones and Plant
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Auxins
Apical meristem removed
When the apical
meristem is removed,
the concentration of
auxin is reduced and
the side branches
begin to grow more
rapidly.
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25–1 Hormones and Plant
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Auxins
Auxinlike Weed Killers
Chemists have produced compounds that mimic
the effects of auxins.
Since high concentrations of auxins inhibit growth,
many of these are used as herbicides—
compounds toxic to plants.
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25–1 Hormones and Plant
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Cytokinins
Cytokinins
Cytokinins are plant hormones produced in
growing roots and developing fruits and seeds.
Cytokinins delay the aging of leaves and play
important roles in early stages of plant growth.
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Cytokinins
In plants, cytokinins stimulate cell division
and the growth of lateral buds, and cause
dormant seeds to sprout.
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25–1 Hormones and Plant
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Cytokinins
Cytokinins and auxins often produce opposite effects.
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Auxins stimulate cell elongation.
•
Cytokinins inhibit cell elongation and cause cells
to grow thicker.
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Auxins inhibit the growth of lateral buds.
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Cytokinins stimulate lateral bud growth.
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Cytokinins
Recent experiments show that the rate of cell growth
in most plants is determined by the ratio of the
concentration of auxins to cytokinins.
In growing plants, therefore, the relative
concentrations of auxins, cytokinins and other
hormones determine how the plant grows.
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Gibberellins
Gibberellins
A gibberellin is a growth-promoting
substance in plants.
Gibberellins produce dramatic increases
in size, particularly in stems and fruit.
Gibberellins are also produced by seed
tissue and are responsible for the rapid early
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growth of many plants.
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25–1 Hormones and Plant
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Ethylene
Ethylene
In response to auxins, fruit tissues release
small amounts of the hormone ethylene.
Ethylene is a plant hormone that causes fruits
to ripen.
Commercial producers of fruit sometimes use
this hormone to control the ripening process.
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25–1
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25–1
The tendency of a plant to grow toward a source
of light is
a. gravitropism.
b. phototropism.
c. meristematic growth.
d. apical dominance.
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25–1
A plant part in which hormones are produced is
a. the apical meristem.
b. a target cell.
c. a hormone receptor.
d. xylem.
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25–1
If you snip off the tip of a stem, the
a. plant grows tall and narrow.
b. plant dies.
c. side branches begin to grow more quickly.
d. stem stops growing.
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Ethylene is a plant hormone that causes
a. plant cells to grow longer.
b. flowers to develop.
c. fruit to ripen.
d. roots to grow downward.
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25–1
The herbicides produced by chemists have a
structure that is similar to
a. auxins.
b. gibberellins.
c. cytokinins.
d. ethylene.
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