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/area2) Transported to a target area/structure3) Binds to a protein receptor at target site4) 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 earth3) Thigmotropism (response to touch) - ex. Climbing vines (+) = towards contact (-) = away from contact
Types of Plant Hormones
I) Auxin or (Indoleacetic Acid - IAA)II) GibberellinsIII) CytokininsIV) EthyleneV) Abscisic Acid Growth InhibitorVI) PhytochromesVII) 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 StemElongation of cells on one side of the stem (due to auxin) causes bending of the stem
Normal-sized cells on the other side
If apical meristem is removed, no phototropism can occur because that is where auxin is produced
The Acid-Growth Hypothesis
H+
H+
H+
Protons activate Expansin Protein, which (breaks down Hydrogen bonds in cell wall)
Cell elongation occurs as cell wall stretches in response to turgor pressure from the vacuole
H+
H+
ATPADP
Expansin protein
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 setb) 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.
PhototropismPhototropism = the response of plants to changes in
season
1. Photoperiod = relative length of night and day2. Circadian rhythm – internal clock that measures
the length of night and day3. Circadian rhythm is controlled by: - endogenous (internal) factors and/or - exogenous (external) factors4. 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
600nm
Pr
Far Red
730 nm
Pfr
Absorbs red
Absorbs far red
How are phytochromes used by plants to measure day and night?
1. Pr (inactive) is made by plants at night2. [Pr] is high3. 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