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

Chapter 31 Auxin

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.

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

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

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

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

Notice the relative size of the cells.

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

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

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

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

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

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

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

Figure 5.20-1 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

Figure 5.20-2 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

Figure 5.20-3 EXTRACELLULAR FLUID CYTOPLASM Plasma membrane Reception Transduction Response Receptor Activation Relay molecules Figure 5.20-3 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