Chapter 39

Examples of some hormones Phototropism experiments Auxin’s mode of action Apical dominance Other tropisms Seed dormancy, germination Fruit ripening Photoperiodism

Hormone: a chemical substance produced in one part of the body and has a physiological response in another part of the body at very low concentrations.

coordinates metabolic activities active in small amounts

Phototropism plants grow towards light Gravitropism- roots grow down into soil

statoliths in root cap Thigomotropism- tendrils curl around branch,

etc. Responds to touch. wind blowing

Darwin and Son, 1880 light sensitivity in tip.

Boysen-Jensen, 1913 signal moving down from tip

Went, 1926 signal is a chemical substance

Thimman isolated the structure of Auxin

Differential cell elongation causes stem to bend. Cells on shaded side elongate.

How do cells elongate? Auxin in higher concentration on

shaded side of stem. Only cells directly underneath

stimulated cells elongate Auxin moves basipetally down stem How?

Cell wall has constitutive enzymes that loosen connections between cellulose when activated.

An increase in cell wall acidity (lower pH) activates the enzymes.

Auxin in cytoplasm causes cell wall to acidify.

Water moves in by osmosis, swelling cell.

Cell membrane lays down additional cell wall material keeping new size / shape

How cellulose microfibrils are laid down determine the direct of elongation.

Elongation takes place when a cell only has primary cell wall.

Once proper size and shape is attained, secondary cell wall material is added inside the primary cell wall.

Determines the cell shape

higher pH in cytoplasm activates Auxin

Can only leave cell via transport proteins at base of cell.

Diffuse across into next cell, in inactive form.

Entering next cell down, auxin is activated by pH, causing proton pumps to start

Cell wall acidifies, activating enzymes.

Auxin moves down to next cell.

Now more diluted by process

Signal fades out farther down

Fig. 39-7

100 µm

RESULTS

Cell 1

Cell 2

Epidermis

Cortex

Phloem

Xylem

PithBasal endof cell

25 µm

Statoliths fall to bottom of cell at root cap.

Inhibit auxin production in lower cells elongate bending tip downward.

Flowers or leaves bend towards sun’s path in the sky during the day.

Motor cells at base of flower or leaf uptake K+ and other ions causing them to swell, bending the stem towards light.

Stem continues to respond to direction of light during the day, and different cells swell, or relax changing direction.

Similar process cause an opening / closing response to flowers (poppies) or Leaves (prayer plants).

Can be under circadian rhythm.

Shoot apical meristem produces auxin which moves down stem and inhibits auxiliary bud from growing.

Roots produce cytokinins that move up and stimulate buds to grow.

Pinching back tops makes plants bushier Pruning sends a surge of cytokinins up to

remaining buds- fast growth in spring. Limiting root growth can stunt plants.

Many annual’s have abscisic acid (ABA) in seed coat.

ABA keeps embryo, seed dormant Rains wash out ABA Embryo swells produces gibberillins

which cause seed to germinate Ensures germination after soil is wet

enough. Other seeds respond to cold, light etc.

Fig. 39-12Early germination in red mangrove

Early germination in maize mutant

Coleoptile

Ethylene is the only gaseous hormone. May spread to other plants Causes fruit to ripen

“one bad apple…” Positive feedback loop

Organic acids convert to sugars, pectin in middle lamella breaks down

Ethylene sensitive fruit can be stored green under carbon dioxide for months Apples, bananas not strawberries, mangoes

Gassed before sending to market Potential area for biotechnology

Growing tips meets on object

Secrete ethylene

Causes stem to 1) slow

elongation 2) thicken 3) grow sideways

Until around object and resumes upward growth.

Fig. 39-3

CELLWALL

CYTOPLASM

Reception Transduction Response

Relay proteins and

second messengers

Activationof cellularresponses

Hormone orenvironmental stimulus

Receptor

Plasma membrane

1 2 3

Ethylene mutants

Testing in lab dwarfism in

many plants Bolting- &

flowering Fruit set Stimulate cell

division & elongation

Promotes seed germination

Fig. 39-10

(a)Gibberellin-induced stem growth

(b) Gibberellin-induced fruit growth

Leaf abscission cause by balance of ethylene and auxin

Apoptosis cell death- recycles many essential nutrients to plant, stimulated by burst of ethylene

How do plants detect when this should happen?

Not covered

Table 39-1

Not covered

Auxins: growth, cell elongation in stem root, Apical Dominance, seedless fruit

Cytokinins: (roots) root growth, stimulates cell differentiation & growth retards senescence (fruit, flower life), stimulates germination

Gibberellins; stimulate cell division & elongation, fruit set, bolting, promotes seed germination

Ethylene: fruit ripening, opposes some auxin affects

Absicisc acid; inhibits growth, closes stomata, dormancy in seeds

Oligosaccharins- Trigger defense mechanisms short sugar chains released from cell wall by

enzymatic breakdown of cellulose and pectin. Brassinosteroids- steroids required for normal

growth and development. Studied mostly by mutations lacking these

compounds.

Fig. 39-1

Phototropism responds to blue light levels

Many responses to light detected by phytochrome sensitive to red light.

Seed germination Need light to germinate

Shade avoidance Higher PR ratio in shade Plants grows taller to reach brighter

light Flower response – Florigen Other photoreceptors sense blue

light: phototropism

In light Pr converts rapidly to Pfr In dark, Pfr slowly reverts to Pr Used to time amount of darkness, or dawn Resets internal biological clock

Actually refer to length of darkness Many plants are day neutral

Far red light counteracts red light,

erasing “day” signal

Fig. 39-23

24 hours

Graft

Short-dayplant

24 hours 24 hours

Long-day plantgrafted to

short-day plant

Long-dayplant

Flowering hormone ? Structure still not

discovered may be a macro

molecule - CONSTANS protein

Can be induced in one plant and move to another

Moves cell to cell, slower than phloem

Economic significance?

Induced by photoperiod Not induced

When underground (in darkness) young stems etiolate grow long internodes no leaves produced yellow, no chlorophyll expressed

Light detected by phytochrome reverses etiolation, plant sprouts leaves

Fig. 39-3

CELLWALL

CYTOPLASM

Reception Transduction Response

Relay proteins and

second messengers

Activationof cellularresponses

Hormone orenvironmental stimulus

Receptor

Plasma membrane

1 2 3

Fig. 39-4-3

CYTOPLASM

Reception

Plasmamembrane

Cellwall

Phytochromeactivated by light

Light

Transduction

Second messenger produced

cGMPSpecific protein kinase 1 activated

NUCLEUS

1 2

Specific protein kinase 2 activated

Ca2+ channel opened

Ca2+

Response3

Transcriptionfactor 1

Transcriptionfactor 2

NUCLEUS

Transcription

Translation

De-etiolation(greening)responseproteins

P

P

Fig. 39-26

(a) Unstimulated state

Leafletsafter stimulation

Pulvinus(motororgan)

(c) Cross section of a leaflet pair in the stimulated state (LM)

(b) Stimulated state

Side of pulvinus With flaccid cells

Side of pulvinus With turgid cells

Vein

0.5

µm