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 2 nd 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