Plant Responses to Internal and External Signals

Signal Transduction Pathways• Link internal and environmental signals to

cellular responses

Greening Response in plants• Light activates a

G-protein• 2 transduction

pathways are stimulated

• 2nd messengers– cGMP

• Triggers protein kinase cascasde

– Ca2+• Activates

specific kinase

• Transcription factor activated

• Translation• Greening protein

Hormones• A compound produced by one part of the body

and then transported to other parts of the body, where it binds to a specific receptor and triggers responses in target cells and tissues

Darwin/Darwin Experiment

• Conclusion:– The tip of the coleoptile, rather than the curving region, was

sensitive to light.

Went Experiments• Conclusion:– Agar blocks had

absorbed a substance from the coleoptile tips that promoted the growth of cells

– Substance causing the curvature of the coleoptile was a hormone

Plant Hormones• Auxins

– Cell elongation• Coleoptile bends toward light

– New root growth on cuttings– Fruit development

• Seedless tomatoes with synthetic auxins

• Gibberellins– Growth of stems and leaves– Promotes seed germination– Fruit development

• Seedless grapes with synthetic gibberellins

• Cytokinins– Stimulates cell division and

differentiation– Anti-aging

• Ethylene– Promotes falling of leaves– Gas produced by fruit that causes

ripening (one rotten apple spoils the bunch)

• Abscisic Acid– Drought stress: controls stomata (K+

channels)– Seed dormancy: inhibits conversion of

starch to sugar until conditions are optimal

• Apical dominance (interplay of hormones)– Cytokinins from root promote growth

of axillary buds– Auxins from apical meristem of shoot

inhibits growth of axillary buds– Results in plant with relatively few

side branches

Phytochromes

• Photoreceptors enabling plants to respond to light– Seed germination– Shade avoidance– Flowering

Short-day and Long-day plants• Photoperiodism– Any response of a plant that is linked to day length

• Flowering as a response– Short-day plants• Bloom in late summer, autumn or winter when days are short

– Long-day plants• Bloom in late spring or early summer when days are long

– Plants respond to hours of continuous darkness rather than to hours of sunlight

Response to external stimuli other than light

• Positive Gravitropism– Roots grow down

• Negative Gravitropism– Shoots grow up

• Thigmotropism–Mechanical stimuli (touch) stunt growth– Directional growth in response to touch

Plant Response to Stress• Drought

– Close stomata– Decrease new leaf growth (decrease SA for transpiration)– Leaves curl up (decrease SA)– Roots proliferate to maximize exposure to water

• Flooding– Decrease oxygen in air spaces of soil– Increase ethylene, causing root cell apoptosis (death) to create “air” tubes (snorkel)

• Salt– Decreases water potential which inhibits water uptake– Too much is toxic– Makes solutes (sugars) to bring water potential down in cells– Salt glands pump out salt in leaf epidermis (halophytes)

• Heat– Heat-shock proteins help prevent denaturing of enzymes

• Cold– If fluidity is lost, lipids become locked in place altering solute transport and membrane

proteins– Increase unsaturated fatty acids (may take days) to keep lipids from solidifying– Increase solutes that are tolerable (sugars) to reduce water loss when freezing

Physical and Chemical Defense• Physical– Thorns

• Chemical– Toxins (canavanine replaces arginine)– Recruit predatory animals– Systematic acquired resistance (SAR)• Non-specific response providing protection against a

diversity of pathogens• Release salicylic acid (aspirin)

– Hypersensitive response• Localized