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) Auxin or (Indoleacetic Acid - IAA)

II) Gibberellins

III) Cytokinins

IV) Ethylene

V) Abscisic Acid Growth Inhibitor

VI) Phytochromes

VII) 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

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

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.

Phototropism

Phototropism = 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 night

2. [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