1.  Plant Responses to Internal and External Signals Chapter 39

2. Signals  A signal is something that causes a response.  The 3 stages of signal transduction pathways:  1. Reception: a signal binds to a receptor  2. Transduction: The signal is relayed through the cell/body  3. Response: Activation of cellular responses

3. Tropism  Tropism is when a plant curves towards or away from a stimuli.  Phototropism is when a shoot grows towards a light source (the cells on the dark side elongate)

4. Plant Hormones  Hormones are chemical signals that coordinate the different parts of an organism.  You need to know 6 plant hormones: auxin, cytokinins, gibberellins, brassinosteroids, abscisic acid, and ethylene.

5. Auxin  Found in seed embryos, meristems of apical buds, and young leaves  Simulates stem elongation, root growth, cell differentiation, branching; regulates fruit development, functions in phototropism, promotes xylem differentiation.

6. Cytokinins  Synthesized in roots, transported to other organs  Affect root growth and differentiation, stimulates cell growth and division, germination

7. Gibberellins  Found in meristems of apical buds and roots, young leaves, and embryos  Promotes seed and bud germination, stem elongation, leaf growth; stimulates flower and fruit development; affects root growth

8. Brassinosteroids  Found in seeds, fruit, shoots, leaves, and buds  Inhibits root growth, keeps leaves on plants, promotes xylem differentiation

9. Abscisic acid  Found in leaves, stems, roots, and green fruit  Inhibits growth, closes stomata during water stress, promotes seed dormancy

10. Ethylene  Found in tissues of ripening fruit, nodes of stems, aging leaves and flowers  Promotes fruit ripening, opposes auxin effects, promotes or inhibits growth and development of roots, leaves, and flowers

11. Responses to light  Photomorphogenesis is the effect of light on plant morphology.  There are two major classes of light receptors: blue-light photoreceptors and phytochromes.

12. Blue-light Photoreceptors  Blue light causes phototropism, the opening of stomata, and the growth of plant seedlings.

13. Phytochromes  Phytochromes cause:  Seed germination  Avoidance of shady areas

14. Circadian Rhythms  Circadian rhythms are 24 hour cycles common to all eukaryotes. They include:  Pulse, blood pressure, alertness, urine composition, metabolic rate, and sex drive  In plants, it controls opening and closing of flowers and stomata

15. Flowering  Short-day plants flower when daylight is short, such as spring or late fall.  Long-day plants flower in late spring or early summer, when days are 12-14 hours long.  Day neutral plants are unaffected by photoperiod and flower whenever they mature.  Flowering is also affected by length of dark hours and temperature.  The flowering hormone in plants is called florigen.

16. Stimuli other than light  Gravity: plants grow up towards the sun  Wind: plants grow out of windy areas  Drought: plants conserve water in droughts  Floods: Plant roots undergo apoptosis so they don’t get too much water  Salt: Plants absorb less water  Heat: stomata close  Cold: increase lipid concentration in cell membranes

17. Plant defenses  Plants defend themselves against herbivores by producing thorns, chemicals, and distasteful or toxic compounds  Plants defend themselves from pathogens by having thick cell walls and developing genetic resistance.