Plant Hormones (Phytohormones)

Plant hormones are small, organic molecules produced in low concentrations that regulate plant growth, development, and responses to stimuli. They act at sites often distant from their synthesis and coordinate processes like cell division, elongation, senescence, and tropic responses.

Classification of Plant Hormones

Plant hormones are classified into five major classical groups, along with some newly discovered growth regulators:

1. Auxins

2. Gibberellins (GAs)

3. Cytokinins

4. Abscisic Acid (ABA)

5. Ethylene

New/Other growth regulators:

• Brassinosteroids

• Jasmonates (JAs)

• Salicylic acid (SA)

• Strigolactones

• Polyamines

• Nitric oxide (NO)

1. Auxins

Structure and Natural Forms:

• Most common natural auxin: Indole-3-acetic acid (IAA)

• Synthesized mainly in shoot apices, young leaves, and developing seeds

Biosynthesis:

• Derived from Tryptophan

• Main pathways: Indole-3-pyruvic acid (IPA) and Tryptamine pathway

Transport:

• Polar transport (basipetal in stem, acropetal in roots)

• Uses PIN proteins (efflux carriers), AUX1/LAX (influx carriers)

Functions:

• Cell elongation (acid growth hypothesis)

• Apical dominance

• Root initiation

• Vascular differentiation

• Fruit development (parthenocarpy)

• Tropic responses (phototropism, gravitropism)

Experimental Evidence:

• Darwin's Phototropism Experiment

• Went’s Avena curvature test

2. Gibberellins (GAs)

Structure:

• Diterpenoid acids, over 130 identified

• Biologically active: GA1, GA3 (gibberellic acid), GA4, GA7

Biosynthesis:

• Synthesized in young tissues, embryos, and seeds

• Precursor: Geranylgeranyl pyrophosphate (GGPP)

Transport:

• Non-polar transport, via xylem and phloem

Functions:

• Stem elongation (especially in rosette plants)

• Seed germination (stimulates α-amylase)

• Breaking seed and bud dormancy

• Fruit setting and growth (e.g., grape elongation)

• Bolting in rosette plants (e.g., cabbage)

Experimental Evidence:

• Foolish Seedling Disease caused by Gibberella fujikuroi in rice due to excess GAs

3. Cytokinins

Structure:

• Derivatives of adenine, active forms include zeatin (natural cytokinin)

Biosynthesis:

• Synthesized mainly in root apices, also in developing seeds and fruits

Transport:

• Non-polar transport via xylem

Functions:

• Promote cell division (cytokinesis)

• Delay leaf senescence

• Promote lateral bud growth (overcoming apical dominance)

• Morphogenesis in tissue culture (high cytokinin: low auxin = shoot formation)

• Chloroplast development

Experimental Evidence:

• Skoog and Miller’s Tissue Culture Experiment

4. Abscisic Acid (ABA)

Structure:

• A sesquiterpenoid compound

Biosynthesis:

• Derived from carotenoids, mainly in chloroplasts

• Precursor: Violaxanthin

Transport:

• Moves through phloem, xylem, and symplastically

Functions:

• Induces stomatal closure during water stress

• Promotes seed dormancy

• Inhibits seed germination and growth

• Acts as a stress hormone (cold, drought, salinity)

Experimental Evidence:

• Stomatal closure in response to drought, mediated by ABA-induced Ca²⁺ signaling

5. Ethylene

Structure:

• A simple gaseous hydrocarbon (C₂H₄)

Biosynthesis:

• Methionine → S-adenosyl methionine (SAM) → ACC (1-aminocyclopropane-1-carboxylic acid) → Ethylene

• Enzyme: ACC synthase and ACC oxidase

Transport:

• Diffuses as a gas

Functions:

• Promotes fruit ripening (climacteric fruits)

• Stimulates senescence and abscission

• Triple response in seedlings: short, thick hypocotyl, inhibition of elongation, and exaggerated apical hook

• Root hair and root hair zone initiation

• Stress responses (flooding, mechanical injury)

Experimental Evidence:

• Triple response in pea seedlings

• Use of ethephon (ethylene-releasing compound)

6. Brassinosteroids

Structure:

• Polyhydroxylated sterol-like molecules

• Common: Brassinolide

Functions:

• Cell expansion and elongation

• Pollen tube growth

• Xylem differentiation

• Stress tolerance

7. Jasmonates

Structure:

• Derivatives of fatty acids like linolenic acid

Functions:

• Wound response and defense against herbivores

• Inhibition of seed and pollen germination

• Root growth regulation

8. Salicylic Acid

Functions:

• Defense signaling (Systemic Acquired Resistance – SAR)

• Thermogenesis in some plants

• Regulation of ion uptake and flowering

9. Strigolactones

Functions:

• Inhibition of lateral branching

• Promote symbiosis with mycorrhiza

• Seed germination of parasitic plants like Striga

Hormonal Interactions (Cross-talk)

Process	Hormones Involved	Type of Interaction
Apical dominance	Auxin (+), Cytokinin (−), Strigolactone (+)	Synergistic/Antagonistic
Seed dormancy	ABA (+), GA (−)	Antagonistic
Fruit ripening	Ethylene (+), ABA (+), Auxin/GA (−)	Synergistic
Root initiation	Auxin (+), Cytokinin (−)	Antagonistic
Stomatal closure	ABA (+), Cytokinin (−)	Antagonistic

Signal Transduction Pathways (Important for CSIR-NET)

1. Auxin Signaling (TIR1 Pathway):

• Receptor: TIR1 (F-box protein)

• Auxin binds TIR1 → promotes degradation of Aux/IAA repressors → activation of ARF (Auxin Response Factors)

2. Gibberellin Signaling:

• Receptor: GID1

• GA-GID1 complex → degradation of DELLA proteins (growth repressors) → promotes gene expression

3. Cytokinin Signaling:

• Involves two-component system (Histidine kinase receptor like CRE1 → AHP → ARR)

4. ABA Signaling:

• Receptor: PYR/PYL/RCAR

• ABA binding inhibits PP2C, activating SnRK2 kinases, which trigger ABA-responsive gene expression

5. Ethylene Signaling:

• Receptor: ETR1

• In absence of ethylene: ETR1 activates CTR1 (a kinase) → suppresses EIN2

• In presence of ethylene: CTR1 inactive → EIN2 activated → stabilization of EIN3 → gene expression

Practical Applications in Agriculture:

Hormone	Agricultural Use
Auxin	Rooting hormone, parthenocarpic fruit, weed control (2,4-D)
Gibberellin	Malting in barley, fruit elongation (grapes), bolting
Cytokinin	Tissue culture, delay senescence in leafy vegetables
ABA	Drought resistance, seed dormancy control
Ethylene	Fruit ripening (ethephon), flowering in pineapple

I. EXPERIMENTAL EVIDENCE FOR PLANT HORMONE ACTION

A. Auxin

1. Darwin’s Phototropism Experiment (1880)

• Observation: Coleoptiles of canary grass bend toward light.

• Conclusion: The tip of the coleoptile is responsible for sensing light and sending a signal (auxin) to the elongating zone.

2. Boysen-Jensen Experiment (1913)

• Placed a mica block or gelatin block between the tip and the rest of the coleoptile.

• Result: Bending occurred with gelatin (permeable) but not with mica (impermeable).

• Conclusion: A chemical (auxin) diffuses from the tip.

3. F.W. Went's Avena Curvature Test (1926)

• Removed coleoptile tip, placed it on agar, then transferred agar to one side of decapitated coleoptile.

• Result: Curvature towards the opposite side.

• Conclusion: Demonstrated the existence and activity of Indole-3-acetic acid (IAA).

4. Paál’s Experiment

• Asymmetric placement of coleoptile tips caused curvature, showing unequal auxin distribution promotes differential cell elongation.

B. Gibberellin

1. Foolish Seedling Disease in Rice (Bakanae Disease)

• Caused by Gibberella fujikuroi, a fungus that secretes gibberellins.

• Observation: Infected rice seedlings grew abnormally tall.

• Conclusion: Identified gibberellins as growth-promoting substances.

2. GA-deficient Dwarf Mutants

• In Arabidopsis and pea, GA-deficient mutants are dwarfed.

• Application of GA restores normal growth.

3. Barley Aleurone Layer Assay

• GA stimulates α-amylase synthesis in germinating seeds.

• Enzyme breaks down starch to sugars needed for embryo growth.

C. Cytokinin

1. Skoog and Miller (1957)

• Experiment: In tobacco callus cultures, varying auxin:cytokinin ratios.

• Result:

  • High cytokinin : auxin = shoot formation

  • High auxin : cytokinin = root formation

  • Equal ratio = callus proliferation

2. Delayed Senescence

• Detached leaves stay green longer in cytokinin-containing solution (e.g., kinetin).

D. Abscisic Acid (ABA)

1. Stomatal Closure Experiment

• ABA triggers increase in cytosolic Ca²⁺, leading to efflux of K⁺ and Cl⁻, causing guard cells to lose turgor.

• Measured with patch-clamp and fluorescent dyes.

2. Seed Dormancy Studies

• Application of ABA delays germination; ABA-deficient mutants germinate prematurely.

E. Ethylene

1. Triple Response in Etiolated Seedlings

• Components:

  • Inhibition of stem elongation

  • Radial swelling (stem thickening)

  • Apical hook exaggeration

• Seen in Arabidopsis when exposed to ethylene or its precursors (e.g., ACC).

2. Fruit Ripening

• Application of ethylene gas to climacteric fruits (e.g., bananas, tomatoes) speeds up ripening.

• Ethephon used commercially to induce ripening.

3. Abscission in Leaf/Flowers

• Ethylene promotes cell separation at abscission zones.

II. BIOSYNTHETIC PATHWAYS OF PLANT HORMONES

A. Auxin (Indole-3-acetic acid – IAA)

Precursors:

• Tryptophan-dependent pathways

• Tryptophan-independent pathways

Main Pathways:

Pathway	Intermediate	Enzyme
Indole-3-pyruvic acid (IPA)	IPA → IAA	TAA1 (Tryptophan aminotransferase) and YUC (YUCCA family monooxygenases)
Tryptamine pathway	Tryptamine → IAA	Tryptophan decarboxylase, monoamine oxidase

Key Points:

• Synthesized in shoot apices, young leaves, developing seeds

• Transported via polar auxin transport, involving PIN, AUX1, P-glycoproteins

B. Gibberellin

Precursor:

• Geranylgeranyl diphosphate (GGPP) – A C20 compound

Biosynthetic Pathway Stages:

Stage	Location	Key Steps
Stage 1	Plastid	GGPP → ent-copalyl diphosphate (ent-CDP) → ent-kaurene
Stage 2	ER membrane	ent-kaurene → ent-kaurenoic acid → GA12
Stage 3	Cytosol	GA12 → active GAs (e.g., GA1, GA3) by 2-oxoglutarate-dependent dioxygenases (GA 20-oxidase, GA 3-oxidase)

Sites of Synthesis:

• Young leaves, seeds, root tips

C. Cytokinin

Precursor:

• Adenosine monophosphate (AMP) derivatives

Main Pathway:

• Isopentenyl transferase (IPT) enzyme catalyzes:

  • ATP/ADP/AMP + isopentenyl pyrophosphate (IPP) → isopentenyladenine (iP)

• iP → Zeatin via hydroxylation at the isopentenyl side chain.

Sites of Synthesis:

• Root apical meristems, developing seeds, young fruits

Transport:

• Mainly through xylem

D. Abscisic Acid (ABA)

Precursor:

• Carotenoids (C40) → Xanthophylls (C40) → Abscisic acid (C15)

Biosynthesis Steps:

Step	Molecule	Enzyme
1	Violaxanthin → Neoxanthin	Epoxidase
2	Neoxanthin → Xanthoxin (C15)	9-cis-epoxycarotenoid dioxygenase (NCED)
3	Xanthoxin → ABA	Short-chain dehydrogenase & ABA-aldehyde oxidase

Site of Synthesis:

• Plastids (early steps) and cytosol (final steps)

Key Enzyme:

• NCED – Rate-limiting in ABA biosynthesis

E. Ethylene

Precursor:

• Methionine (Met)

Steps:

Step	Intermediate	Enzyme
1	Methionine → S-adenosyl methionine (SAM)	SAM synthetase
2	SAM → 1-aminocyclopropane-1-carboxylic acid (ACC)	ACC synthase
3	ACC → Ethylene + CO₂ + HCN	ACC oxidase

Site of Synthesis:

• All tissues, especially ripening fruits, senescing tissues, stressed organs

Regulation:

• ACC synthase is the rate-limiting enzyme.

• Ethylene biosynthesis is autocatalytic (increases its own synthesis).

Table: Hormone Biosynthesis & Evidence

Hormone	Precursor	Key Enzyme	Evidence
Auxin	Tryptophan	TAA1, YUC	Darwin, Went, Boysen-Jensen
GA	GGPP → ent-kaurene	GA 20-oxidase, GA 3-oxidase	Foolish seedling, aleurone assay
Cytokinin	AMP + IPP	IPT	Skoog & Miller’s experiment
ABA	Violaxanthin	NCED	Stomatal closure, dormancy
Ethylene	Methionine → ACC	ACC synthase, ACC oxidase	Triple response, fruit ripening