1. The Chapter 31 Homework is due on Monday, April 1st
The Chapter 31 Test will be on Tuesday, April 2nd.

2. Chapter 31
Auxin

3. You Must Know
The role of auxins in plants. (This is the only plant hormone you will be expected to remember.)
How phototropism use changes in the environment to modify plant growth and behavior.

4. Concept 31.1: Plant hormones help coordinate growth, development, and responses to stimuli
chemical signals that modify or control one or more specific physiological processes within a plant.
produced in very low concentration.
Most aspects of plant growth and development are under hormonal control.
Ex. Auxin: effects include cell elongation and root formation 4

5. A plant’s response to light
Tropism:
Any response resulting in curvature of organs toward or away from a stimulus
Phototropism:
A plant’s response to light
Video clip

6. Coleoptile: The covering of the young shoot of the embryo of a grass seed.

7. Auxin and Phototropism
Cell elongation .
Auxin is produced in shoot tips and is transported down the stem.
Auxin transporter proteins move the hormone from the basal (bottom) end of one cell into the apical (top) end of the neighboring cell.
Light

8. Notice the relative size of the cells.

9. What part of a grass coleoptile senses light?
Opaque
cap
Trans-
parent
cap
Opaque
shield
over
curvature
Light
In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light.
They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present. They postulated that a signal was transmitted from the tip to the elongating region.
Tip
removed
Darwin and Darwin: Phototropism occurs only when the tip is illuminated. 9

10. Auxin Plants grown in the dark. Results Excised tip on agar cube
Frits Went
Growth-promoting
chemical diffuses
into agar cube
Control
Control
(agar cube
lacking
chemical)
Offset
cubes
Plants grown in the dark.
In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments. 10

11. 4 3 H 2 H H H H H H H 1 Plasma membrane H
Figure 31.5a 4
Cell wall-loosening enzymes cleave cross-linking
polysaccharides.
CELL WALL 3
Low pH
activates
expansins. H 2
Acidity
increases. H H H H H H
The role of auxin in cell elongation: Polar transport of auxin stimulates proton pumps in the plasma membrane.
According to the acid growth hypothesis, the proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric. With the cellulose loosened, the cell can elongate.
Wedge-shaped expansins (red), activated by low pH, separate cellulose microfibrils (brown) from cross-linking polysaccharides (green). The exposed cross-linking polysaccharides are now more accessible to cell wall-loosening enzymes (purple). H
Proton
pump
activity
increases. 1
Plasma membrane
ATP H
CYTOPLASM 11

12. Sliding cellulose microfibrils allow cell to elongate.
Figure 31.5b
H2O
Cell
wall
Plasma
membrane
Vacuole 5
Sliding cellulose microfibrils
allow cell to elongate. 12

13. At high concentrations, auxin may inhibit cell elongation.
Auxin’s role in plant development: Polar transport of auxin controls the spatial organization of the developing plant.
At high concentrations, auxin may inhibit cell elongation.
Auxin also alters gene expression and stimulates a sustained growth response.
Reduced auxin flow from the shoot of a branch stimulates growth in lower branches. Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem. 13

14. You Must Know
The three components of a signal transduction pathway and how changes could alter cellular responses. (This is review from Chapter 5)

15. The Three Stages of Cell Signaling: A Review
Cells receiving signals undergo three processes
Reception
Transduction
Response
© 2014 Pearson Education, Inc. 15

16. Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane
Reception
Receptor
Signaling
molecule
Cell Signals are detected by receptors that undergo changes in shape in response to a specific stimulus. 16

17. Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane
Reception
Transduction
Receptor
Relay molecules
Signaling
molecule
Transduction is a multistep pathway that amplifies the signal. This allows a small number of signal molecules to produce a large cellular response 17

18. Figure
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane
Reception
Transduction
Response
Receptor
Activation
Relay molecules
Figure Overview of cell signaling (step 3)
Signaling
molecule
Cellular response is primarily accomplished by two mechanisms: (1) turning genes on or off and thereby increasing or decreasing mRNA production, or (2) activating existing enzyme molecules. 18