Phytohormones

Introduction to Plant Hormones (Phytohormones)

  • Definition: Small, endogenous chemical messengers that coordinate cellular and whole-plant functions.
  • Key distinctions from animal hormones:
    • Animals: many hormones, each triggers a specific, narrow response (e.g., insulin → blood-glucose control).
    • Plants: comparatively few hormones; each affects most cells and elicits diverse, context-dependent responses.
  • Modulating factors of hormone action
    • Environmental cues (light, gravity, temperature, water, nutrients).
    • Developmental stage (seed → vegetative → reproductive).
    • Crosstalk with other phytohormones (synergistic or antagonistic effects) – e.g., auxin vs cytokinin on branching.
  • Primary vs. secondary metabolites
    • Primary: essential molecules (e.g., amino acids, sugars) directly tied to growth and survival.
    • Secondary: non-essential but adaptive molecules (phenolics, terpenes, alkaloids) – defense, attraction, etc.
  • Economic importance
    • Crop productivity, food security, pharmaceuticals, growth regulators in fruits/vegetables/cereals/legumes.

Natural Phytohormones vs. Phytoregulators (PGRs)

  • Phytohormones
    • Origin: synthesized naturally inside plant tissues.
    • Function: regulate intrinsic processes (growth, development, stress responses).
    • Scope: operate within the natural plant system.
  • Phytoregulators (Plant Growth Regulators, Bioregulators)
    • Origin: synthetic, external, or produced via biotechnology.
    • Function: applied by humans to achieve specific agricultural or lab goals (e.g., rooting powders, herbicides).
    • Examples: \text{IBA} rooting gel, 2,4\text{-D} herbicide, paclobutrazol (GA inhibitor).
    • If a hormone is made synthetically and used for a purpose different from its native role, call it a phytoregulator.

General Steps of Hormone Action

  • Synthesis & Accumulation
    • Tightly regulated in space/time; often linked to developmental or stress cues.
    • Newly synthesized molecules may be chemically modified → inactive storage forms.
    • Activation = reversible de-conjugation; degradation controls steady-state levels.
  • Transport
    • Long-distance: xylem & phloem.
    • Short-distance: carrier proteins, diffusion, plasmodesmata.
    • Auxin transport is best-characterized (polar transport via PIN proteins).
  • Perception
    • Receptors can be membrane-bound (e.g., His-kinase cytokinin receptors, BRI1 for BRs) or intracellular (GA–GID1, ABA–PYR/PYL).
    • Hormone binding → conformational (allosteric) shift → signal initiation.
  • Signal Transduction
    • Kinase/phosphatase cascades, second messengers (Ca²⁺, ROS, cGMP), ubiquitin–26S proteasome (e.g., auxin & GA pathways).
    • End-points: changes in gene expression (activation/repression via transcription factors) and non-genomic responses (ion fluxes, enzyme activity).

Major Natural Phytohormones

Auxins

  • Name from Greek auxein (“to grow”). Primary molecule: Indole-3-acetic acid (IAA), structurally similar to tryptophan.
  • Key roles
    • Cell elongation (acid-growth hypothesis).
    • Apical dominance: inhibits axillary shoot buds → fewer branches.
    • Phototropism & gravitropism: differential distribution → curvature toward light/against gravity.
    • Embryonic patterning, vascular differentiation, lateral root initiation, maintenance of root stem cells.
  • Transport
    • Polar auxin transport: basipetal (shoot → root) via PIN, AUX1/LAX, ABCB carriers.
  • Molecular mechanism
    • Auxin binds TIR1/AFB (F-box proteins) → part of SCF^{TIR1} E3 ubiquitin ligase.
    • Aux/IAA repressors ubiquitinated → degraded by 26S proteasome.
    • Freed ARF (Auxin Response Factor) transcription factors activate AuxRE-containing genes.
  • Synthetic & natural analogues
    • Natural: \text{IAA}, \text{PAA} (phenylacetic acid).
    • Synthetic: \text{IBA}, \text{NAA}, 2,4\text{-D} (herbicide at high dose).
  • Practical uses
    • Rooting powders/gels for cuttings.
    • Fruit thinning (NAA + IBA in apples/pears).
    • Selective broad-leaf weed control (2,4-D).
  • Classic experiments
    • Charles & Francis Darwin (1880): coleoptile tips perceive light → mobile signal (auxin) causes bending.
    • Boysen-Jensen & Went: agar block “diffusible growth substance” = auxin.

Cytokinins (CKs)

  • Adenine derivatives; most active form: trans-zeatin.
  • Functions
    • Stimulate cell division (cytokinesis).
    • Delay leaf senescence (stay-green trait → higher photosynthetic duration).
    • Regulate nutrient allocation; enhance N and K uptake (N:P:K = 12:12:12 fertilizer synergy).
    • Root nodule formation in legumes; modulation of root/shoot architecture.
    • Stem-cell maintenance at shoot apical meristem; antagonistic/synergistic interplay with auxin.
  • Signaling
    • Perception by membrane His-kinase receptors (AHK2, AHK3, AHK4/CRE1, CKI1).
    • Phosphorelay: His → Asp (receiver domain) → AHP phosphotransfer proteins → type-B ARR TFs (activate CK-responsive genes); type-A ARRs act as negative feedback regulators.
  • Applications / Case studies
    • Elevated CK rice → more grains per panicle (altered inflorescence architecture).
    • Tobacco overexpressing CK biosynthesis gene → delayed senescence, enhanced drought tolerance.

Strigolactones (SLs)

  • Carotenoid-derived diterpenoids synthesized mainly in roots.
  • Ecological roles
    • Signal to arbuscular mycorrhizal fungi (symbiosis establishment).
    • Germination stimulant for parasitic Striga spp.
  • Developmental roles
    • Inhibit axillary bud outgrowth (antagonistic to CK, synergistic with auxin in apical dominance).
    • Mutation (e.g., rice dwarf27) → excess tillering.
  • Long-distance coordination
    • Shoot-derived auxin → root SL biosynthesis → SL transported upward → suppress lateral buds.

Gibberellins (GAs)

  • Family of >130 diterpenoid acids; only some are bioactive (e.g., \text{GA}1, GA3, GA4, GA7).
  • Historical note: “foolish seedling” disease in rice caused by Gibberella fujikuroi overproducing GA.
  • Functions
    • Stem elongation (bolting), seed germination (α-amylase in cereals), flowering induction (many long-day plants), sex determination, fruit set and growth.
    • \text{GA}_4 often breaks seed dormancy; paclobutrazol inhibits GA biosynthesis → dwarf plants.
  • Signaling
    • Bioactive GA binds soluble receptor GID1 → conformational change enabling interaction with DELLA repressor proteins.
    • GA-GID1-DELLA complex → DELLA ubiquitination by SCF^{SLY1} (E3 ligase) → 26S proteasomal degradation → growth gene activation.
  • Agronomic implications
    • “Green Revolution” semi-dwarf wheat & rice carry GA metabolism/signaling mutations (e.g., wheat Rht DELLA, rice sd1 GA20-oxidase) → shorter, lodging-resistant, high-yield cultivars.
    • GA sprays: parthenocarpic pears, cherry & citrus fruit sizing; GA_4/7 for apple quality.

Brassinosteroids (BRs)

  • Steroidal hormones (e.g., brassinolide) synthesized from campesterol.
  • Roles
    • Cell elongation/expansion (cell-wall loosening enzymes → increased turgor-driven expansion).
    • Pollen tube growth, vascular differentiation, root hair formation, seed germination, stress tolerance.
  • Signaling
    • Perception by LRR receptor-like kinase BRI1 (plasma membrane).
    • BRI1-BAK1 heterodimer → phosphorylation cascade → inhibition of BIN2 kinase → de-phosphorylation & activation of BES1/BZR1 TFs → expression of BR-responsive genes.
  • Phenotype of BR mutants: severe dwarfism (Arabidopsis bri1, pea lka, tomato dwarf).

Ethylene (C₂H₄)

  • Small gaseous hydrocarbon produced from methionine via enzymes ACC synthase & ACC oxidase.
  • Functions
    • Fruit ripening (cl