Chapter 39 Plant Responses to Internal and External Signals p. 802

Signal-transduction pathways link internal and environmental signals to cellular responses p. 803

• Figure 39.2 p. 803 Review of a model for Signal-Transduction Pathways
• 3 steps for signal-transduction
• 1. Reception
• 2. Signal Transduction
• 3. Response
• Greening: seedlings germinated in the dark look different than the adult plant; the germinating seedling will have pale stems and unexpanded leaves. When exposed to light the plant undergoes a change: elongation of stems slows, leaves expand, chlorophyll production begins…the plant begins “greening”.  Below is a summary of signal-transduction for greening
• Reception: Signals are detected by proteins called receptors. Receptors are mostly found on the plasma membrane and undergo a conformation change when exposed to specific signals. Phytochrome is receptor for the greening process and light is the signal.
• Signal Transduction: The receptor (Phytochrome in this example) sends the signal to a second messenger in the cell. The second messenger amplifies the signal. The second messenger causes a chain reaction of events (transduction).
• Response: The transduction process eventually leads to a cellular response to the initial signal. The response typically triggers the nucleus to transcribe and translate a DNA segment. In this example the nucleus will respond to the greening process by producing greening proteins…causing the seedling to grow, taking on the adult form.

 

Plant responses to hormones p. 806

• Hormone- a compound produced by one part of the body and then transported to other parts, where it binds to a specific receptor and triggers a response. The target for hormones are called target cells or target tissues
• Tropism- plant response toward or away from a stimulus

·        Auxin (aka IAA..indoleacetic acid) p. 809

§         Produced in embryo of seed,  the tips of stems (meristems) and young leaves

§         Stimulates fruit development, cell differentiation and root growth

§         Suppresses leaf and fruit drop; inhibits bud growth

§         Used as a growth promoting hormone

§         Influenced by light, auxin migrates from the lighted side of a stem to the darkened side. As a result the darkened side of stem elongates faster and bends toward the light

§         Used in agent orange

·        Cytokinins p. 810

§         Produced in root tip

§         Promotes lateral bud growth and inhibits leaf drop

§         Promotes cell division and differentiation

§         Stimulates seed germination

§         Delays senescence (aging of the plant)

·        Gibberelins (GA3) p. 812

§         Produced in developing shoots and seeds

§         Stimulates stem elongation and cell division

§         Stimulates  seed germination and fruit development

§         Used to culture seedless plants

·        Abscisic acid p. 813

§         Produced in leaves, stems, roots and green fruits

§         Inhibits leaf abscission (drop or loss p. 815)  and promotes buds and seed dormancy

§         Closes stomata during water stress

·        Ethylene p. 814

§         A gaseous compound produced in most tissues

§         Promotes fruit ripening, leaf, flower and fruit abscission

§          Promotes Senescence- aging of a plant

§         Ripening fruit releases ethylene…stimulates nearby fruits to ripen

§         Ancient China…Burning incense (which contains ethylene) to ripen fruit

§         Pineapples in Hawaii are sprayed with ethylene to induce the plants in a field to flower at the same time.

 

Photoperiodism synchronizes many plant repsonses to changes of season p. 821

Photoperiodism- response (reproductive triggers) to changes in the amount of daylight and darkness. Determines the time of year a plant will bloom.

·        Circadian rhythm- physiological cycles with a 24 hour frequency; triggered by environmental cues.

 3 Main Groups

Plants bloom according to the amount of darkness

·        Short-day plants

§         Require long periods of darkness

§         Bloom late summer or fall

§         Example: Poinsettias are subjected to artificial darkness in the greenhouse around September. Must be exposed to 10hr days for 8 weeks.

·        Long-day plants

§         Require short periods of darkness

§         Bloom late spring or summer

·        Day-neutral plants

§         Do not flower in response to daylight changes

§         Use changes in temperature and water

§         Example: Tulips are produced from bulbs (modified underground stems). Tulips are refrigerated before grown because the bud within requires cold treatment to overcome dormancy… called forcing.

·        Phytochrome: light receptor involved in  Photoperiodism

§         Inhibits flowering in short-day plants

§         Induces flowering in long-day plants

 

 

 

Critical Night Length p. 821

·        Figure 39.22 and 39.23 p. 822

·        Critical night length the amount of darkness a plant needs in order to bloom.

·        Red light is the most effective color in interrupting the night time portion of the photoperiod (lengths of day and night)

1.      Phytochrome- receives the red light (P_r)

2.      When plants detect red light it will interrupt the critical night length ..the plant will not bloom

3.      Far Red Light (P_fr) will cancel the impact of red light. A plant exposed to a flash of red followed by a flash of far red light will bloom.

·         

Plants respond to environmental stimuli through a combination of developmental and physiological mechanisms p. 823

 

 

Tropism- a directional growth response determined by the direction of the stimulus. Auxin is usually responsible

·        Positive tropism- growth response is toward the stimuli

·        Negative tropism- away from stimuli

§         Phototropism- response to light

§         Graviotropism- response to gravity

·        Positive gravitropism- roots

·        Negative – stems and shoots

§         Thigmotropism- response to touch.  Pea tendrils attach to objects that are touched.

Environmental Factors affecting growth

• Light
• Temperature
• Water and humidity
• Plant nutrition