Lecture 20 Plasticity

5th lecture in the plant0biology series; focus: plant plasticity0—the capacity of plants to alter form, physiology & behavior in response to environmental variation.
Emphasis on hormonal regulation, historical discoveries, adaptive significance, and human applications.

Darwin & Darwin (late 1800s)- Observed oat coleoptiles bending toward a candle (phototropism).
- Experimental manipulations:
- Control (intact): curved toward light.
- Tip removed 0→0 no bending.
- Tip covered with opaque cap 0→0 no bending.
- Tip covered with transparent cap 0→0 bending.
- Base covered (tip still exposed) 0→0 bending.
- Inference: stimulus perceived at tip; signal transmitted downward.
Boysen0Jensen & Paál (early 1900s)- Separated coleoptile tip using permeable agar vs. impermeable mica.
- Bending occurred only when an agar bridge allowed a diffusible chemical to pass, confirming a mobile signal.

Frits Went (≈1930s)- Isolated the first plant hormone: auxin (indole-3-acetic acid, IAA).
- Empirical formula \text{C}{10}\text{H}{9}\text{NO}_{2}.
- Agar block experiments quantified curvature proportional to auxin dose.
- Identified blue-light receptor phototropin 0→0 lateral auxin transport to dark side 0→0 differential cell elongation.

Synthesised in shoot apical meristems & young leaves.
Major roles- Cell elongation, vascular differentiation, adventitious rooting, apical dominance.
- Tropisms: phototropism & gravitropism (shoots negative, roots positive at different [IAA]).
Adaptive example- Shade-grown vs. sun-grown Acacia seedlings display contrasting architectures mediated by auxin distribution.
Human use: rooting powders; pruning to break apical dominance for bushier ornamentals or higher cannabis yields.

Formula (GA₃) ≈ \text{C}{19}\text{H}{22}\text{O}_{6}.
Produced in apical buds, young leaves, embryos.
Functions- Stem elongation, seed germination, flowering induction (bolting), fruit growth.
Agricultural uses- Spraying grape inflorescences enlarges berries & pedicels0—reduces fungal rot, enhances market appeal.
- Synchronising germination in labs by GA soaking.
Mutant insights: GA-deficient Brassica dwarfs; GA application restores normal stature.

Adenine derivatives (e.g., zeatin).
Synthesised mainly in roots; transported via xylem.
Roles- Cell division, shoot initiation in tissue culture, delay of senescence, antagonism to auxin (releasing axillary buds).
Example: treated bean leaf remained green longer than control.

Formula \text{C}{15}\text{H}{20}\text{O}_{4}.
Originally misnamed for presumed role in abscission; key actual roles:- Inducing seed dormancy; washed off by rain to trigger germination (ABA↓ : GA↑ ratio).
- Root-to-shoot drought signalling 0→0 stomatal closure.
Knock-out tomato mutant lacking ABA shows rapid wilting when soil dries.

Simplest hormone; gaseous \text{C}{2}\text{H}{4}.
Produced during mechanical stress, senescence, ripening.
Positive feedback loop0—small stimulus 0→0 autocatalytic surge.
Triple response in seedlings- Inhibition of elongation.
- Radial stem expansion (thickening).
- Horizontal growth (apical hook exaggeration).
Uses- Commercial fruit ripening (bananas, tomatoes, avocados) after storage.
- Explains lunch-bag browning & banana 0“overnight”0 ripening.
Experiments: wild-type vs. ethylene-insensitive Arabidopsis under wind; insensitive lines grow tall & flop.

Recently discovered; steroidal; implicated in immune signalling & growth synergy with auxin/GA. Not covered in depth.

Auxin gradient from apex suppresses lateral buds.
Shade saplings (e.g., sugar maple) benefit from strong apical dominance 0→0 vertical escape from canopy.
Open-grown plants favour weak dominance 0→0 branched crowns intercept multi-directional light.
Fire-resprouting eucalypts: loss of shoot tips (auxin source) triggers epicormic bud burst 0→0 fuzzy regrowth; dominance re-established over time.

Blue light absorbed by phototropin 0→0 auxin relocation 0→0 stem curvature toward light.

Statolith hypothesis: starch-filled amyloplasts settle to cell bottoms, re-orienting auxin flow.
Shoots exhibit negative gravitropism; roots positive.

Climbing vines (beans) circle; contact induces localized ethylene 0→0 differential growth 0→0 coiling around support.

Arctic poppies track sun; floral cups heat to attract pollinators.
Sunflower myth: inflorescences fix east; leaves actually exhibit diurnal solar tracking and pre-dawn re-orientation.

Seismonasty in Mimosa pudica: leaflet closure via rapid turgor loss on inner pulvini cells; possible anti-herbivory signal.
Nyctinasty: nightly leaf drooping to conserve water/turgor.

Photosynthetically Active Radiation (PAR) quality:- Red (~660 nm) highly useful; far-red (>700 nm) poor.
Phytochrome photoreceptor interconverts:- P{R} (absorbs red) \rightleftharpoons P{FR} (absorbs far-red & drives growth responses).
High \frac{R}{FR} in open sun 0→0 P_{FR} predominates; growth normal.
Low \frac{R}{FR} under canopy/reflection 0→0 P_{R} accumulates 0→0 SAS:- Enhanced auxin, stem elongation, reduced lateral branching, earlier flowering.
Demonstrated with green-light filters producing elongated Arabidopsis.

Two-stage trigger: any of the three sensory hairs touched twice within ~20 s.
Mechanism: rapid water flux reverses pre-stressed lobes (snap-buckling, like toy 0“poppers”)0 0→0 closure in ~0.1 s.
Ecological context: nutrient-poor, acidic wetlands; carnivory supplies nitrogen & phosphorus.

Horticulture manipulation (auxin pruning, GA sprays, cytokinin anti-aging) boosts yield & aesthetics.
Ethylene-regulated storage reduces spoilage; raises consumer concerns but proven safe.
Genetic knockouts & hormone mutants illuminate adaptive value; raise GMO debate on crop improvement.
Understanding plasticity critical for climate resilience (drought-induced ABA, wind/heat ethylene responses).

Phototropin, phytochrome, statolith, apical dominance, triple response, shade0avoidance, nyctinasty, seismonasty.
Hormone interactions often governed by ratios (e.g., \frac{GA}{ABA} in germination) rather than absolute amounts.