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 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.