Transcript Behavior of Plants in Response to Hormones

Behavior of Plants in
Response to Hormones
Chapter 39
Plants Respond to Hormones
Hormone = chemical signals that coordinates
the structure and function of an organism
1) Produced in one structure/area
2) Transported to a target area/structure
3) Binds to a protein receptor at target site
4) Triggers a signal transduction response at
target cells/tissues
Tropism
Tropism = Growth pattern in response to an
environmental stimulus
1) Phototropism (response to light)
(+) = towards
(-) = away
2) Gravitropism (response to gravity)
(+) = towards earth
(-) = away from earth
3) Thigmotropism (response to touch)
- ex. Climbing vines
(+) = towards contact (-) = away from contact
Types of Plant Hormones
I)
II)
III)
IV)
V)
VI)
VII)
Auxin or (Indoleacetic Acid - IAA)
Gibberellins
Cytokinins
Ethylene
Abscisic Acid Growth Inhibitor
Phytochromes
Florigen
Auxin (Indoleacetic Acid or IAA)
Auxin = Hormone that promotes elongation
in parts of cells
Produced in apical meristem of shoots and
transported to areas in the plant where cell
elongation is needed
Auxin Transport
Anionic form of auxin is
transported across
membrane through a
protein into the cell
wall, where a a
hydrogen ion (proton)
is picked up
Auxin Transport
• In the cytoplasm, the
pH of the cell causes
the auxin to ionize
again.
• The H+ ion is
transported by
ATPase back into the
cell wall, maintaining
a voltage difference
(or membrane
potential) between
the cytoplasm and
wall
Auxin Transport
• Voltage difference
contributes to the
favoring of anion
transport out of the
cytoplasm, so anionic
auxin leaves the
cytoplasm of the cell
• … as this cycle
continues, auxin can
be transported
throughout the plant
Phototropism in Plant Stem
Normalsized cells
on the
other side
Elongation of cells
on one side of the
stem (due to
auxin) causes
bending of the
stem
If apical meristem is
removed, no
phototropism can occur
because that is where
auxin is produced
The Acid-Growth Hypothesis
ADP
ATP
H+
H+
H+
H+
Expansin protein
H+
Protons
activate occurs as
Cell elongation
Expansin
which
cell wallProtein,
stretches
in
(breaks
downtoHydrogen
response
turgor pressure
bonds
cellvacuole
wall)
frominthe
Gravitropism in Stem
Auxin accumulates on the bottom side of stem,
causing elongation that turns the plant upwards
Auxin has opposite effect in roots!
In roots, instead of expanding and
elongating the cell, high auxin
concentration tends to inhibit growth in
roots.
http://www.bio.psu.edu/People/Faculty/gilroy
/ali/graviweb/toc.htm
Auxin produced by apical meristem of roots
accumulate at the bottom and inhibits growth
on this side, causing a bend in the roots
towards gravity
Gibberellins (Gibberellic Acid – GA)
Gibberellins = a group of plant hormones (>100
types) that promotes cell growth
1. Causes “bolting” = rapid elongation
(evident when dwarf plants are treated with GA,
they grow to normal size)
2. Often works with auxin in the following:
a) fruiting – auxin + gibberellins are necessary
for fruit to set
b) germination – auxin + gibberellins are
necessary to cause seeds to break dormancy
Cytokinins (CK)
Cytokinins = hormones that stimulate
cytokinesis
1. Effect of Cytokinins depends on relative
concentration of auxin (IAA)
[IAA] = [CK]  cell dividision w/o
differenctiation
[IAA] < [CK]  shoots form
[IAA] > [CK]  roots form
Cytokinins (CK)
2. CK weakens apical dominance and
promotes the growth of auxillary bud
3. Anti-aging properties of plant organs by
inhibiting breakdown of plant proteins
(florists often use CKs to keep flowers
fresh)
Ethylene (CH2)
Ethylene = a gas that acts like a hormone
and is used by plants to cope with stress
1. (CH2) produced during times of stress
like drought, flooding, etc.)
- Stimulates flowering and fruit ripening
2. w/ auxin (IAA), promotes dropping of
leaves (abscission) during the fall and
prevents elongation of roots and stems
Abscisic Acid Growth Inhibitor
(ABA)
Abscisic Acid = hormone responsible for
preventing growth
1. Acts as anti-auxin, cytokinins, and
gibberrelins
2. Keeps seeds dormant during drought
- once rains come, the rains wash out the
ABA, allowing seeds to break dormancy
with the help of gibberrellins and auxins.
Phototropism
Phototropism = the response of plants to changes in
season
1. Photoperiod = relative length of night and day
2. Circadian rhythm – internal clock that measures
the length of night and day
3. Circadian rhythm is controlled by:
- endogenous (internal) factors and/or
- exogenous (external) factors
4. Phytochrome protein (has a light absorbing
chromophore) helps maintain the circadian
rhythm
Phytochromes have 2 isomeric
forms
Pr = the “inactive” form that absorbs
wavelengths of red light (660 nm)
Pfr = the “active” form that absorbs
wavelengths of far-red light (730 nm)
Red
Far Red
600nm
730 nm
Absorbs red
Pfr
Pr
Absorbs far red
How are phytochromes used by
plants to measure day and night?
1. Pr (inactive) is made by plants at night
2. [Pr] is high
3. As daybreak approaches and more red
light is available, [Pr]  [Pfr]
4. Since sunlight has both red and far-red
spectrums, [Pr] = [Pfr] at mid-day
5. Evening decreases the [Pfr] while
increases in the [Pr] helps reset the
circadian rhythm
What triggers flowering?
Critical Night Length (not day length)
triggers flowering
Flowering Responses to Changes
in Photoperiod
Three classifications:
1. Short-day plants (flower when daylight
decreases in early fall/late summer)
Critical night length > daylight
2. Long-day plants (flower when daylight
increases in spring/early summer)
Critical night length < daylight
3. Day-neutral plants (other factors trigger
flowering, like availability of water, etc.)
Florigen
Depending on what classification of plant
they belong in, florigen hormone is
produced at different periods of the
season to trigger flowering