Lecture 12 Plant Responses (External)

Overview of Plant Responses to External Signals

  • Plants constantly interpret and respond to external cues; light is the most influential, but gravity, touch, pathogens, herbivores, temperature, water and salt stress are also crucial.
  • Central idea: signal perception → transduction → response (morphological, physiological, biochemical).

Photomorphogenesis & Light Perception

  • Photomorphogenesis = light-regulated development (germination, de-etiolation, shade avoidance, flowering, etc.).
  • Two receptor families dominate:
    • Blue-light receptors (cryptochromes, phototropins, ZEITLUPE‐family proteins, etc.).
    • Phytochromes (red/far-red receptors).
  • Action spectra experiments (e.g.
    • coleoptile curvature maximal at ≤ 500 nm500\,\text{nm}, peak ≈436 nm\approx 436\,\text{nm}) identify the wavelengths most effective and therefore the receptor involved.

Blue-Light Receptors

  • Key roles
    • Phototropism (directional growth towards light).
    • Stomatal opening / guard-cell osmoregulation.
    • Leaf expansion, chloroplast movement, inhibition of stem elongation in seedlings.
  • Phototropins mediate differential cell elongation by redistributing auxin to the shaded side of the organ.
  • Blue light is
    • Most effective at ≈450 nm\approx 450\,\text{nm}.
    • Active at intensities below which red light has little influence, explaining why twilight still orients shoots.

Phytochrome System (Red/Far-red)

  • Exists in two reversible forms: Pr→ far red red lightPfr\text{Pr}\xrightarrow[~far\,red~]{red\,light}\text{Pfr} and Pfr→far red lightPr\text{Pfr}\xrightarrow[]{far\,red\,light}\text{Pr}.
  • Pfr = biologically active, triggers:
    • Seed germination (lettuce classics).
    • Shade-avoidance (elongation under low R:FR + reduced branching).
    • Inhibition of stem elongation & promotion of branching in full sun.
    • Circadian clock entrainment.
    • Initiation / repression of flowering, depending on species.
  • Dark reversion and enzymatic degradation slowly remove Pfr, creating a physiological ‘timer’ for night length.

Biological Clocks & Circadian Rhythms

  • Circadian rhythm = internal ≈24 h\approx 24\,\text{h} oscillation, persists without external cues (free-running) but is entrained (phase-locked) by light signals perceived by phytochromes and cryptochromes.
  • Coordinates photosynthesis, hormone synthesis, leaf movements, scent emission and gene expression with the day/night cycle.

Photoperiodism (Day-length Sensing)

  • Photoperiodism = physiological response triggered by relative lengths of day and night; enables seasonal timing of:
    • Bud break, flowering, seed set, leaf fall, winter dormancy.
  • Critical factor is uninterrupted DARK period, not daylight duration.
  • Categories
    • Short-day (long-night) plants (SDP): flower only when night ≥ critical threshold. Example: tobacco, mānuka, kiwifruit, cannabis.
    • Long-day (short-night) plants (LDP): flower when night ≤ threshold. Example: lettuce.
    • Day-neutral plants: insensitive to photoperiod.
  • Night-break Experiment: A short flash of red light during the night prevents SDP flowering but promotes LDP flowering; subsequent far-red flash cancels the effect ⇒ phytochrome is the sensor.
Grafting & the Florigen Concept
  • Florigen = mobile flowering hormone (protein FT or a complex) produced in photoperiod-sensitive leaves, transported via phloem to shoot apical meristem (SAM).
  • Grafting LDP leaf onto SDP shoot can induce flowering in SDP under non-inductive photoperiod, proving systemic signal.
Vernalization (Temperature Memory)
  • Chilling requirement fulfills or releases flowering blocks (e.g., kiwifruit, many temperate perennials). Works in concert with photoperiod pathways.

Responses to Other Physical Stimuli

Gravity (Gravitropism)
  • Statolith hypothesis: Dense, starch-filled amyloplasts sediment within columella cells (root) or endodermis (shoot) acting as gravity sensors.
  • Auxin redistribution → differential growth: roots curve downward (positive), shoots upward (negative).
Mechanical Stimuli
  • Thigmomorphogenesis: chronic mechanical perturbation (wind, handling) → shorter, thicker stems, altered wood anatomy (Krumholtz forms on alpine ridges).
  • Thigmotropism: rapid, directional growth in response to touch (e.g., tendril twining around support). Under study for unique biomechanical patterns.
  • Rapid leaf movements (Mimosa pudica)
    • Triggered by touch, heat, or electrical/chemical signals.
    • Driven by pulvinar motor cells’ turgor changes controlled by action potentials analogous to animal nerve impulses.
    • Mimosa shows habituation/‘learning’ in experiments (controversial but illustrates plant neurobiology claims).

Environmental (Abiotic) Stresses

  • ‘Stress’ = any environmental condition that reduces plant growth, reproduction, or survival below genetically determined potential.
Drought
  • Immediate response: ABA synthesis in roots/leaves → stomatal closure.
  • Long-term: reduced leaf area, deeper roots, osmotic adjustment, expression of stress-proteins and late-embryogenesis abundant (LEA) proteins, altered metabolism.
Flooding / Waterlogging
  • Hypoxic/anoxic root zone → ethylene accumulation triggers:
    • Aerenchyma (gas-filled air tubes) formation in cortex (e.g., maize roots).
    • Adventitious root growth, shoot elongation in rice.
Salinity
  • Dual effect: osmotic stress (reduced water uptake) + ionic toxicity (Na+,Cl−\text{Na}^+, \text{Cl}^-).
  • Strategies: exclusion at root, tissue compartmentalisation, synthesis of compatible solutes, salt glands (NZ iceplant research model).
Heat Stress
  • Protein denaturation and membrane fluidity increase.
  • Heat-shock proteins (HSPs) act as molecular chaperones, preventing irreversible aggregation.
Cold Stress
  • Decreased membrane fluidity, risk of intracellular ice.
  • Plants adjust lipid composition (more unsaturated fatty acids), accumulate anti-freeze proteins and osmolytes.

Biotic Interactions: Pathogens

Two-Tiered Innate Immunity
  1. PAMP-Triggered Immunity (PTI)
    • Pattern-recognition receptors (PRRs) detect conserved microbial signatures (flagellin, chitin).
    • Activates basal defences: cell-wall reinforcement, ROS burst, antimicrobial phytoalexins.
  2. Effector-Triggered Immunity (ETI)
    • Many pathogens secrete effectors that suppress PTI.
    • Intracellular NLR (nucleotide-binding leucine-rich repeat) resistance (R) proteins recognize specific effectors → stronger, rapid response.
    • Coevolutionary ‘arms race’ between plant R genes and pathogen effector genes.
Hypersensitive Response (HR) & Systemic Acquired Resistance (SAR)
  • HR: Localised programmed cell death around infection site; deprives biotrophs of nutrients.
  • SAR: Signal (methyl salicylate) moves through phloem → priming of distal tissues, expression of pathogenesis-related (PR) genes → broad-spectrum resistance lasting days–weeks.
Case Study: Psa (Pseudomonas syringae pv. actinidiae) in Kiwifruit
  • Ca. 38 effectors/toxins encoded.
  • Cultivar resistance requires matching R gene for each effector; absence leads to uncontrolled infection and leaf-spotting disease.
  • HR lesions visible; illustrates need for multi-gene breeding strategies.

Biotic Interactions: Herbivores

  • Defences organised across multiple biological levels (Fig. 39.27):
    • Molecular: toxic secondary metabolites (alkaloids, cyanogenic glycosides), proteinase inhibitors.
    • Cellular: laticifers secreting latex, calcium oxalate crystals.
    • Tissue: tough sclerenchyma, raphides.
    • Organ: spines, stinging hairs (Urtica ferox – NZ tree nettle contains formic acid and other irritants).
    • Organismal: whole-plant timing of defences, induced volatiles to attract parasitoids/predators.
    • Population & community: synchronous masting, associational resistance, mutualistic bodyguards (ants–acacias).

Agricultural & Real-World Connections

  • Manipulating photoperiod and vernalization critical for horticulture (e.g., controlled-environment cannabis production with blackout curtains).
  • Climate-change scenario: compound stresses (heat × drought × high CO2_2) projected to reduce global yields; integrative problem-solving exercise invites use of textbook quantitative skills.

Key Terminology Checklist (self-test)

  • photomorphogenesis • blue-light receptor • phytochrome • action spectrum • circadian rhythm • photoperiodism • short-day plant • long-day plant • gravitropism • statolith • thigmomorphogenesis • thigmotropism • tendril • stress • PAMP • effector • R gene • hypersensitive response • systemic acquired resistance • evolutionary arms race.

Suggested Study Strategy

  • Map each signal (light, gravity, pathogen, etc.) to its receptor, early signalling components, hormones, and final response; create flow-charts.
  • Practice explaining night-break and red/far-red reversal experiments.
  • Use real crop/forest examples (kiwifruit, mānuka, maize, iceplant) to anchor memory.
  • Apply concepts to the climate-change case study on page 889 of Campbell Biology (11th ed.).