Signal Reception
Receptors bind to specific molecules (hormones/environmental stimuli)
Initiates the signaling process
Signal Transduction
Messengers spread the signal within the cell
Activates proteins and enzymes that further relay the signal
Response
Includes processes like:
Phototropism (growth towards light)
Stress responses (e.g. closing stomata during drought)
Immune defense activation to combat pathogens
Perception Mechanism
Light is perceived by phytochromes, which absorb light
Conformational change in phytochromes activates plant response
Responses
De-etiolation (Greening): Stops elongation, develops chlorophyll, prepares for photosynthesis
Phototropism: Plant bends towards light as auxin accumulates on the shaded side, promoting cell elongation
Key Experiments
Darwin's observation (1880): Plant shoots bend toward light indicating a tip-sensed signal
Boysen-Jensen (1913): Showed a chemical signal from the tip was involved in bending using barriers (agar/mica)
New Experimental Setup
Plant with light from one side bends towards the light due to auxin accumulation on the shaded side.
Plant with light from all directions does not bend as auxin distribution is even.
Auxin:
Produced in apical meristem, young leaves, seeds
Promotes elongation, root formation, phototropism, gravitropism, maintains apical dominance
Cytokinins:
Produced in roots, seeds, fruits, growing tissues
Stimulate cell division, promote lateral bud growth, delay leaf aging
Gibberellin (GA):
Produced in seeds, young leaves, roots
Stimulates stem elongation, seed germination, flowering, fruit development
Abscisic Acid (ABA):
Produced in leaves, roots, seeds (under stress)
Inhibits growth, promotes seed dormancy, induces stomatal closure during drought
Ethylene:
Produced in ripening fruit, wounded tissues, aging leaves
Regulates fruit ripening, leaf abscission, stress responses
Phototropism:
Growth towards light controlled by auxin, causing cell elongation
Seed Germination:
Light can trigger or inhibit germination; acts as a signal to break dormancy, regulated by phytochromes
Shade Avoidance:
Plants elongate towards light when shaded, competing for sunlight to maximize photosynthesis
Phototropism
Positive: Growth towards light for photosynthesis
Negative: Growth away from light to absorb nutrients
Gravitropism
Positive: Growth towards gravity for anchorage and nutrient access
Negative: Growth against gravity to reach light and air
Thigmotropism:
Not detailed, but generally refers to plant responses to touch/environmental mechanical stimuli
Early agricultural practices indicated plants need specific soil substances
Jan van Helmont's Experiment (1600s): Highlighted factors beyond water for plant growth
Obtaining Cations from Soil:
Ion exchange process: Roots release hydrogen ions to capture cations
Acidic soils' high proton concentration reduces nutrient availability and solubility of toxic metals
19th Century Discoveries:
Justus Von Liebig identified macronutrients (N, O, P, K) essential for plant growth
1930s hydroponic experiments revealed the need for micronutrients (Fe, Cl, Zn), made through controlled experiments and observations
Causes:
Results from atmospheric pollution, primarily from coal combustion
Effects:
Damages plant tissues, affects human health, causes soil nutrient deficiencies, leads to leaching of nutrients from soils.