Lecture 11 Plant Responses (Internal)

Administrative & Assessment Information

  • Remaining schedule for the “Plants” module
    • One more lecture tomorrow (covers external signalling)
    • No lecture Friday morning → gap before test
    • In-course test: Tuesday, 1 pm (normal lecture rooms)
    • Tauranga: same theatre
    • Hamilton: LG 03
  • Test format (material = all plant lectures, incl. today & tomorrow)
    • 25 multi-choice questions (MCQ)
    • 15 marks / ≈15 min of short-answer questions
  • Revision material on Moodle
    • Full practice test from a previous year
    • Extra MCQ & short-answer practice (often mirrors Learning Outcomes)
  • Exam technique reminders
    • MCQ: read twice, rule out obviously wrong options, compare remaining choices
    • Short answer: note mark-value (e.g. “List three …” → give exactly three); definition ≈ one sentence; no essays for 1-mark items

Core Concepts of Plant Signalling

  • Every biological signal pathway can be dissected into three obligatory components
    1. Reception – stimulus is detected by a receptor (membrane-bound or intracellular)
    2. Transduction – relay/amplification via one or more second messengers
    3. Response – altered gene expression, enzyme activity, ion flux, growth etc.
  • Second messengers commonly used in plants
    • \text{cGMP}
    • \text{Ca^{2+}} pulses
    • \text{IP_3}, reactive oxygen species, pH shifts
  • Signal integration = extensive crosstalk; same stimulus or hormone often triggers multiple downstream cascades

Case Study ➔ Parasitic Plants & Environmental Cues

  • Manuka in N. NZ often draped with native parasitic vine resembling “cooked spaghetti”
  • Dodder (genus Cuscuta) – serious agricultural weed; eradication attempts in Whangamarino Wetland use drones + ground teams
    • Seedling exhibits helical search pattern; “sniffs” volatile cues from host + senses red : far-red ratio (shading)
    • Once haustoria attach, dodder severs own root → full parasite
    • Demonstrates chemical, photic and circadian signalling
  • Convergent evolution: unrelated parasitic families developed almost identical spaghetti morphology

Signal Transduction Example – Potato Etiolation/De-etiolation

  • Dark-grown tuber shoots = etiolated
    • Long internodes, no chlorophyll, unexpanded leaves
  • Exposure to light ➔ de-etiolation (greening, leaf expansion)
  • Key photoreceptor = phytochrome
    • Mutants in phytochrome fail to de-etiolate
    • Pathway (simplified):
    • Light → phytochrome conformational change →
    • ↑\text{cGMP} & \text{[Ca^{2+}]_{cyt}} →
    • Activation of protein kinases → phosphorylation of transcription factors →
    • Differential expression of \sim100s–1000s genes → chlorophyll, plastid & cell-wall modifications

Plant Hormones (“Plant Growth Regulators”)

  • General definition: extremely potent organic molecules produced endogenously that act as chemical messengers to regulate plant physiology & development
  • Key contrasts with animal hormones
    • Often synthesised in many tissues, not one specialised gland
    • Frequently exert multiple effects on diverse organs
    • Strong synergistic/antagonistic interactions; effects context-dependent

Auxins (e.g. Indole-3-acetic acid, IAA)

  • Primary production sites: shoot apical meristem, young leaves, developing seeds
  • Polar transport (basipetal & laterally) via PIN efflux carriers
    • PIN proteins localised to basal membranes → create auxin gradients
  • Major functions
    • Cell elongation via Acid-Growth Hypothesis
    • Auxin → H(^+)-ATPase pumps protons into cell wall space
    • \downarrow\text{pH} (to ≈4.5) → expansin activation + cross-link cleavage ⇒ wall loosens ⇒ turgor-driven expansion
    • Apical dominance (suppresses axillary buds)
    • Phototropism, gravitropism (differential distribution)
    • Embryonic root–shoot polarity; vascular differentiation
  • Applied uses
    • Rooting powders/gels (IBA, NAA)
    • Tipit-Gel in kiwifruit “gel-pruning” – reduced shoot regrowth but inadvertently enlarged watery fruit, leading to quality issues
  • Classic discovery timeline
    • Darwin & Darwin: oat coleoptile experiments (tip senses light)
    • Boysen-Jensen: gelatin vs mica inserts reveal diffusible chemical
    • Frits Went: agar block assay quantified dose–response ⇒ named “auxin”

Cytokinins (CK)

  • Synthesised mainly in root apical meristems; transported upward in xylem
  • Promote cell division, shoot initiation, lateral bud outgrowth; delay senescence
  • Tissue-culture rule of thumb
    • \text{High CK} : \text{Low Auxin} \Rightarrow shoot formation
    • \text{Low CK} : \text{High Auxin} \Rightarrow root formation
    • Balanced ⇒ callus proliferation
  • Interact antagonistically with auxin in apical dominance

Gibberellins (GA, esp. GA$_3$)

  • Produced in young shoots, seeds
  • Functions
    • Internode elongation / bolting (lettuce, spinach)
    • Breaking seed & bud dormancy, counteracting \text{ABA}
    • Stimulating \alpha-amylase production in cereals during germination ⇒ releases sugars
    • Fruit enlargement (e.g. Thompson seedless grapes) and pasture growth ("ProGibb" on ryegrass)
  • Malting process in brewing exploits GA-induced enzyme cascade

Abscisic Acid (ABA)

  • Misnamed (not primary abscission trigger) – think stress hormone
  • Synthesised in mature leaves, roots; transported via phloem & xylem
  • Core roles
    • Induces & maintains seed dormancy
    • Antagonises GA during germination
    • Drought response: root-derived \text{ABA} → guard-cells → closes stomata, conserves water
    • Promotes cold & salt tolerance pathways

Ethylene (C(2)H(4))

  • Small gaseous hydrocarbon; synthesis stimulated by auxin, wounding, flooding, senescence
  • Classical Triple Response in etiolated seedlings encountering barrier
    1. Slowed elongation
    2. Radial swelling (thickening)
    3. Horizontal growth (apical hook accentuation)
  • Major effects
    • Fruit ripening in climacteric species (banana, apple, tomato)
    • Ripple-effect autocatalytic burst of ethylene & respiration
    • Leaf & fruit abscission, flower senescence
    • Stress & pathogen responses
  • Practical manipulation
    • Commercial ripening rooms inject ppm-level ethylene (bananas)
    • Storage uses ethylene blockers (e.g. 1!\text{-}\text{MCP}) to extend shelf-life

Recently Characterised Hormones / Regulators

Brassinosteroids (BR)

  • Poly-hydroxylated steroids; structurally animal-like yet plant-specific
  • Promote cell expansion, vascular differentiation, pollen tube growth, stress tolerance
  • Short-distance signalling; mutants = dwarf, dark-green, delayed senescence

Jasmonates (JA, Methyl-jasmonate)

  • Derived from linolenic acid; volatile versions act airborne
  • Central to wound & defence signalling – herbivory, pathogen attack
  • Induce proteinase inhibitors, secondary metabolites; inhibit growth, root elongation

Strigolactones (SL)

  • Carotenoid-derived, discovered via Striga parasitic weed research
  • Exuded by roots
    • Stimulate germination of parasitic Striga/Orobanche
    • Recruit arbuscular mycorrhizal fungi
  • Inside plant: inhibit axillary bud outgrowth (works with auxin); regulate root architecture

Hormone Interactions & Crosstalk

  • Few responses are mono-hormonal; key antagonisms/synergies
    • Auxin ↑ vs Cytokinin ↓ lateral buds (apical dominance)
    • GA ↑ vs ABA ↓ seed germination
    • Ethylene often modulates auxin transport; BR amplifies auxin effects
  • Signalling networks include shared second messengers & phosphorylation cascades; mutations in one pathway often reveal hidden roles of others (e.g. ethylene-insensitive dwarfs)

Laboratory & Real-World Applications Mentioned

  • Bean-plant lab: exogenous Auxin, CK, GA treatments to observe stem internode & leaf responses
  • Drone-guided eradication of dodder infestations in wetlands
  • “Gel-pruning” of gold kiwifruit with auxin gel – illustrated unintended quality impacts (large, low-dry-matter fruit)
  • Dairy pasture early-spring boost with GA spray (“ProGibb” signs on fence posts)
  • Post-harvest industry: controlled ethylene for ripening; 1\text{-MCP} or low- \text{O}_2 storage to block ethylene response in apples

Key Terminology & Study Checklist

  • Etiolation / De-etiolation
  • Phototropism, Gravitropism, Apical dominance, Abscission, Senescence
  • Haustorium, Convergent evolution, Climacteric fruit
  • Receptor → Second messenger (cGMP, \text{Ca^{2+}}) → Kinase → Transcription factor
  • Acid-Growth hypothesis; PIN transporters; Triple Response
  • Auxin, Cytokinin, Gibberellin, Abscisic Acid, Ethylene (+ BR, JA, SL)
  • Practical: rooting gel, tissue culture A:CK ratios, GA in barley malting, ethylene in ripening rooms

Revise Learning Outcomes for every plant lecture; test is short – focus on identifying keywords & core mechanisms rather than exhaustive detail.