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Behavior of Plants in Response to Hormones

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1 Behavior of Plants in Response to Hormones
Chapter 39

2 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 Triggers a signal transduction response at target cells/tissues

3 Tropism Tropism = Growth pattern in response to an environmental stimulus Phototropism (response to light) (+) = towards (-) = away Gravitropism (response to gravity) (+) = towards earth (-) = away from earth 3) Thigmotropism (response to touch) - ex. Climbing vines (+) = towards contact (-) = away from contact

4 Types of Plant Hormones
Auxin or (Indoleacetic Acid - IAA) Gibberellins Cytokinins Ethylene Abscisic Acid Growth Inhibitor Phytochromes Florigen

5 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

6 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

7 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

8 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

9 Phototropism in Plant Stem
Elongation 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

10 The Acid-Growth Hypothesis
ATP ADP H+ H+ H+ Expansin protein 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


12 Gravitropism in Stem Auxin accumulates on the bottom side of stem, causing elongation that turns the plant upwards

13 Auxin has opposite effect in roots!
In roots, instead of expanding and elongating the cell, high auxin concentration tends to inhibit growth in roots. 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

14 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

15 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

16 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)

17 Ethylene (CH2) Ethylene = a gas that acts like a hormone and is used by plants to cope with stress (CH2) produced during times of stress like drought, flooding, etc.) - Stimulates flowering and fruit ripening w/ auxin (IAA), promotes dropping of leaves (abscission) during the fall and prevents elongation of roots and stems

18 Abscisic Acid Growth Inhibitor (ABA)
Abscisic Acid = hormone responsible for preventing growth Acts as anti-auxin, cytokinins, and gibberrelins 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.

19 Phototropism Phototropism = the response of plants to changes in season Photoperiod = relative length of night and day Circadian rhythm – internal clock that measures the length of night and day 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

20 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) Far Red 730 nm Pfr Red 600nm Pr Absorbs red Absorbs far red

21 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

22 What triggers flowering?
Critical Night Length (not day length) triggers flowering

23 Flowering Responses to Changes in Photoperiod
Three classifications: 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.)

24 Florigen Depending on what classification of plant they belong in, florigen hormone is produced at different periods of the season to trigger flowering

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