behavior of plants in response to hormones
DESCRIPTION
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 Produced in one structure/area Transported to a target area/structure Binds to a protein receptor at target site - PowerPoint PPT PresentationTRANSCRIPT
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Behavior of Plants in Response to Hormones
Chapter 39
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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
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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
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Types of Plant Hormones
I) Auxin or (Indoleacetic Acid - IAA)II) GibberellinsIII) CytokininsIV) EthyleneV) Abscisic Acid Growth InhibitorVI) PhytochromesVII) Florigen
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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
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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
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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
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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
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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
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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
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Gravitropism in Stem
Auxin accumulates on the bottom side of stem, causing elongation that turns the plant upwards
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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
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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
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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
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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)
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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
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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.
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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
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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
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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
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What triggers flowering?
Critical Night Length (not day length) triggers flowering
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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.)
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Florigen
Depending on what classification of plant they belong in, florigen hormone is produced at different periods of the season to trigger flowering