🪴 Plant Growth Regulators

Introduction

Plant growth or elongation is measured using an auxanometer. Plant growth follows an S-shaped (sigmoid) curve with three distinct phases:

1. Lag phase (Slow growth): The initial phase with the least growth — cells are preparing for division, synthesizing proteins and nucleic acids
2. Log phase (Rapid growth): The phase of maximum growth rate — cells are actively dividing and elongating at their fastest
3. Stationary phase: Growth plateaus and there is no further net change — the organism has reached its maximum size or the environment limits further growth

• The word "hormone" was coined by Starling (derived from Greek hormao meaning "to excite" or "to stimulate")
• The term "phytohormone" (plant hormone) was given by Thimann

Classification of Plant Hormones

Plant hormones are broadly divided into two categories based on their primary effect:

Category	Examples	Key Function
Growth Promoters	Auxin, Gibberellin, Cytokinin	Promote growth, cell division, and development
Growth Inhibitors	Ethylene (partial inhibitor), ABA (Abscisic acid)	Inhibit or regulate growth; promote senescence and dormancy

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1. Auxins

Discovery

• Charles Darwin and Francis Darwin first observed phototropism in coleoptiles — the tip of the coleoptile bends toward light, suggesting some chemical signal moves from the tip downward
• F.W. Went discovered auxin and demonstrated its role using the famous Avena coleoptile curvature test (Went's oat coleoptile experiment) — he placed coleoptile tips on agar blocks, then placed the agar on decapitated coleoptiles and observed bending proportional to auxin concentration
• Auxin is a bipolar (bidirectional) hormone — it can move both upward and downward in the plant
• Auxin causes phototropism (bending toward light) — this property led to its discovery

Types of Auxins

Natural Auxins	Synthetic Auxins
IAA (Indole-3-Acetic Acid) — the primary natural auxin	IBA (Indole Butyric Acid)
	NAA (Naphthalene Acetic Acid)
	2,4-D (2,4-Dichlorophenoxyacetic acid) — powerful weed killer
	2,4,5-T (2,4,5-Trichlorophenoxyacetic acid) — weed killer
	IPA (Indole Propionic Acid)
	PAA (Phenylacetic Acid)

Biosynthesis: Auxin (IAA) is synthesized from the amino acid tryptophan via zinc (Zn) as a cofactor, primarily in growing tips (shoot apices, young leaves)

• Kegel and Haennecke discovered heteroauxin — the first naturally occurring auxin identified
• Kogl discovered heteroauxin (Auxin-A) in human urine — a surprising finding that helped advance auxin research

Physiological Effects of Auxins

(1) Apical Dominance

• Auxin produced in the shoot apex is transported downward and suppresses the growth of lateral (side) buds
• The main shoot grows taller while lateral branches remain dormant — this gives many plants their characteristic "Christmas tree" shape
• Removing the apical bud releases lateral buds from inhibition — this is why pruning encourages bushier growth

(2) Cell Elongation

• Auxin promotes cell enlargement by loosening cell walls — this is explained by the acid growth hypothesis (auxin activates proton pumps that acidify the cell wall, activating expansins that loosen cellulose fibers)
• Works on cells below the apex — the apex produces auxin but the elongation occurs in cells beneath it

(3) Root Formation

• In natural and in vitro conditions, auxin promotes adventitious root formation (roots arising from unusual places like stems or leaves)
• Used commercially for root initiation in cuttings — dipping stem cuttings in auxin solution dramatically improves rooting success

(4) Parthenocarpy

• Auxin can induce seedless fruit development — this is called parthenocarpy
• Fruits that develop without fertilization and seeds are called parthenocarpic fruits
• Examples: Grape, banana, watermelon

(5) Flower Thinning

• NAA reduces the number of flowers on the plant, thereby reducing fruit number but increasing individual fruit size — used commercially in apple orchards

(6) Abscission Control

• When fruits ripen, auxin levels naturally drop, leading to abscission (separation and falling off of leaves, flowers, and fruits)
• Application of NAA, IBA, 2,4-D can delay abscission — keeping fruits on the tree longer

(7) Weed Killing

• 2,4-D and 2,4,5-T are used as selective herbicides to kill broad-leaved dicot weeds without harming monocot crops (like wheat, rice, corn)
• They cause uncontrolled, lethal growth in dicots — the weeds literally grow themselves to death

(8) Phototropism and Geotropism

• Auxin causes differential growth — when auxin accumulates on one side of a stem, that side elongates more, causing the plant to bend toward light (phototropism) or respond to gravity (geotropism)

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2. Gibberellins (GA)

Discovery

• Discovered by Japanese scientist Kurosawa while studying the "bakanae" (foolish seedling) disease in rice — infected rice seedlings grew abnormally tall and spindly, then fell over
• The disease was caused by the fungus Gibberella fujikuroi (now Fusarium moniliforme) — the fungus produced a growth-promoting substance
• The active compound isolated was named Gibberellic acid (GA₃)
• It is a non-photosensitive hormone (its activity is not affected by light conditions)
• Gibberellin is a secondary metabolite produced from the mevalonic acid pathway — synthesized via the terpenoid pathway

Key Facts

• First isolated gibberellin: GA₃ (most commonly studied and used commercially)
• Over 130 types of gibberellins are known in plants
• Gibberellins are synthesized in young developing tissues (seeds, young leaves, root tips)

Physiological Effects of Gibberellins

(1) Stem Elongation / Bolting

• Causes dramatic internode elongation in rosette plants — plants that normally grow as flat, compact rosettes (e.g., cabbage, radish) suddenly shoot up tall flowering stalks
• This phenomenon is called bolting
• Examples: Phaseolus (bean), Beta (beet)

(2) Seed Germination

• GA promotes the synthesis of hydrolytic enzymes (especially α-amylase) in the aleurone layer of cereal seeds
• The aleurone layer normally stores protein
• GA₃ stimulates amylase production → amylase breaks down starch into sugars → sugars provide energy for embryo growth

Process in seed germination: Seed → Embryo produces GA₃ → GA₃ reaches Aleurone layer → Amylase enzyme synthesized → Starch converted to Sugar → Energy for germination

(3) Cytokinin-like Effects

• Skoog and Miller discovered cytokinins from yeast DNA, but GA can also promote some cell division in certain tissues

(4) Parthenocarpy

• Like auxin, GA can also induce seedless fruit formation — used commercially in grape production to produce larger, seedless grapes

(5) Substitute for Vernalization

• GA can replace the cold treatment requirement (vernalization) for flowering in some plants — biennial plants that normally need a cold winter can be made to flower by applying GA

(6) Breaking Dormancy

• GA breaks seed dormancy and bud dormancy
• Promotes germination even without proper cold treatment — very useful in agriculture

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3. Cytokinins

Discovery

• Discovered by Skoog and Miller from yeast DNA — the first cytokinin found was kinetin (actually a degradation product of DNA, not naturally occurring in plants)
• Letham named kinetin as cytokinin (because it promotes cytokinesis — cell division)
• Letham and Miller discovered zeatin (natural cytokinin) from corn (Zea mays) endosperm — this was the first naturally occurring cytokinin discovered in plants

Physiological Effects

(1) Cell Division (Cytokinesis)

• Primary function: promotes cell division in all plant parts
• Acts by stimulating mitosis (nuclear division followed by cytokinesis)

(2) Delay of Senescence (Anti-aging)

• Inhibits leaf senescence (yellowing and aging) — this is called the Richmond-Lang effect
• Promotes green retention in leaves — cytokinin-treated leaves stay green longer than untreated ones

(3) Breaking Dormancy

• Promotes growth of lateral buds — directly opposes auxin's apical dominance
• Increases the number of lateral shoots and branches, making plants bushier

(4) Callus and Organ Formation

• The ratio of Auxin to Cytokinin determines what type of growth occurs in tissue culture — this is a critical concept for plant biotechnology:

Ratio	Result
High auxin, low cytokinin	Root formation
Low auxin, high cytokinin	Shoot formation
Equal auxin and cytokinin	Callus (undifferentiated mass) formation
Medium cytokinin, low auxin	Callus formation

(5) Counters Apical Dominance

• Cytokinin promotes lateral bud growth, directly opposing the inhibitory effect of auxin from the shoot apex

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4. Ethylene (C₂H₄)

Discovery

• It is the only gaseous hormone in plants — a simple two-carbon molecule (H₂C=CH₂)
• Discovered by Burg (who established its role as a hormone)
• Natural ethylene source: produced by ripening fruits, stressed tissues, and aging organs
• Commercial form: Ethephon (a liquid that breaks down to release ethylene gas in plant tissues)

Physiological Effects

(1) Fruit Ripening

• Ethylene is the master fruit-ripening hormone — it promotes ripening in climacteric fruits (fruits that show a burst of respiration and ethylene production during ripening)
• When unripe fruits are kept in a closed room, the ethylene they produce accumulates and causes all fruits to ripen — this is why one rotten apple can spoil the whole barrel!
• Climacteric fruits: Mango, banana, apple, tomato, papaya
• Commercially, ethephon is sprayed on fruits for synchronized ripening

(2) Inhibition of Flowering

• Generally inhibits flowering in most plants
• However, it promotes flowering in pineapple and mango — this is commercially exploited to induce uniform flowering

(3) Senescence and Abscission

• Promotes leaf fall (abscission) and aging of plant parts
• Promotes yellowing of leaves through chlorophyll destruction

(4) Stomatal Closure

• When water is scarce, ethylene promotes stomatal closure (working alongside ABA)

(5) Triple Response

• In etiolated (dark-grown) seedlings, ethylene causes a characteristic triple response:
  1. Inhibition of stem elongation (shorter stems)
  2. Thickening of stem (radial expansion — stems become fatter)
  3. Horizontal growth (diageotropism — stems grow sideways instead of upward)

(6) Lateral Growth

• Promotes lateral (horizontal) growth in stems
• Inhibits longitudinal (vertical) growth — making plants shorter and stockier

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5. Abscisic Acid (ABA) — Stress Hormone

Discovery

• Discovered by Okhuma
• Wareing discovered it in sycamore leaves and named it dormin (because it induced dormancy)
• Addicott and associates found it near the abscission zone of cotton bolls and named it abscissin
• Both dormin and abscissin turned out to be the same compound — ABA (Abscisic Acid)
• ABA is a carotenoid derivative — produced from the breakdown of carotenoid pigments
• Found in stressed plants — hence universally called the stress hormone

Physiological Effects

(1) Leaf Abscission

• Promotes abscission (separation and dropping) of leaves, fruits, and flowers by antagonizing (working against) auxin
• When ABA levels rise relative to auxin, the abscission zone is activated and fruit/leaf drop occurs

(2) Dormancy Induction

• Promotes bud dormancy and seed dormancy — prepares plants for unfavorable conditions (winter, drought)
• Opposes the growth-promoting effects of GA and cytokinins

(3) Stomatal Closure

• Most important function of ABA: Causes guard cells to lose K⁺ ions → water exits guard cells by osmosis → guard cells become flaccid → stomata close
• This is the primary mechanism of drought response — when soil water is scarce, ABA accumulates in leaves and closes stomata to conserve water

(4) Senescence Promotion

• Promotes yellowing and aging of leaves (working alongside ethylene)

(5) Growth Inhibition

• Inhibits stem elongation and overall growth
• Acts as a brake on growth processes — allowing the plant to redirect resources to stress survival

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Comparison of All Five Hormone Groups

Property	Auxin (IAA)	Gibberellin (GA₃)	Cytokinin (Kinetin/Zeatin)	Ethylene (C₂H₄)	ABA
Chemical nature	Indole compound	Terpenoid (diterpene)	Purine derivative (adenine)	Simple gas (C₂H₄)	Terpenoid (sesquiterpene)
Discovery	Went (1928)	Kurosawa (1926)	Skoog & Miller (1955)	Burg	Okhuma, Wareing
Synthesis site	Shoot tips, young leaves	Young tissues, seeds	Root tips, developing seeds	Ripening fruits, nodes	Leaves, root caps
Transport	Polar (basipetal)	Non-polar	Through xylem	Diffusion (gas)	Through xylem & phloem
Cell elongation	Promotes	Promotes strongly	Little effect	Inhibits (lateral)	Inhibits
Cell division	Promotes	Some effect	Strongly promotes	Little effect	Inhibits
Apical dominance	Promotes	No direct effect	Opposes (promotes lateral)	No direct effect	No direct effect
Fruit ripening	No	No	No	Yes (promotes)	Promotes senescence
Dormancy	Breaks (sometimes)	Breaks	Breaks	No major role	Induces dormancy
Senescence	Delays (in some tissues)	No major role	Delays (anti-senescence)	Promotes	Promotes
Stomatal closure	No	No	No	Yes (partial)	Yes (primary role)
Abscission	Delays/prevents	No major role	No major role	Promotes	Promotes
Parthenocarpy	Yes	Yes	No	No	No
Bolting	No	Yes	No	No	No
Weed killing	Yes (2,4-D)	No	No	No	No
Root formation	Promotes	No major role	Inhibits	No major role	Inhibits

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Key Facts for Exam Revision

Fact	Detail
Auxin discoverer	F.W. Went (coleoptile experiment)
First natural auxin	IAA (Indole-3-Acetic Acid)
Gibberellin discovered from	Gibberella fujikuroi (bakanae disease, Kurosawa)
First cytokinin	Kinetin (from yeast DNA, Skoog & Miller)
First natural cytokinin	Zeatin (from corn, Letham & Miller)
Only gaseous hormone	Ethylene (C₂H₄)
Stress hormone	ABA (Abscisic Acid)
Bolting caused by	Gibberellin
Apical dominance caused by	Auxin
Stomatal closure by	ABA (primary), Ethylene (partial)
Fruit ripening hormone	Ethylene
Anti-senescence hormone	Cytokinin (Richmond-Lang effect)
Weed killer hormones	2,4-D and 2,4,5-T (synthetic auxins)
Parthenocarpy by	Auxin and Gibberellin
Auxin:Cytokinin for root	High auxin, low cytokinin
Auxin:Cytokinin for shoot	Low auxin, high cytokinin
Auxin:Cytokinin for callus	Equal ratio
Auxin synthesis from	Tryptophan (amino acid) via Zn
Ethylene commercial form	Ethephon
ABA derived from	Carotenoid degradation
Hormone word coined by	Starling
Phytohormone word by	Thimann
Auxanometer measures	Plant growth/elongation
S-shaped growth curve	Lag → Log → Stationary phase
IAA detected by	Ehrlich reagent (color test)
Gibberellin promotes in seeds	Amylase enzyme production in aleurone layer

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Practice Questions (from PDF)

Practice Questions with Answers (Click to expand)

1. Which promotes bolting (stem elongation)? — (d) Gibberellin
2. Maximum growth occurs in: — (d) Log phase
3. Richmond-Lang effect is shown by: — (c) Cytokinin (delays senescence)
4. Auxin is transported by: — (d) Polar (basipetal) transport
5. Which hormone increases flower number? — (b) Auxin (NAA)
6. Which hormone causes fruit drop? — (d) 2,4-D (at high concentration)
7. IAA bioassay is done by: — (a) Avena coleoptile curvature test
8. Auxin inhibits: — (a) Lateral bud growth (apical dominance)
9. Natural hormone form is: — GA₃ (gibberellic acid is a naturally produced hormone)
10. Which hormone inhibits vernalization? — (d) GA₃ (substitutes for cold treatment, effectively bypassing vernalization requirement)
11. If auxin concentration increases in xylem sap, water absorption will: — (d) Increase
12. Which hormone is found in coconut water? — (d) Cytokinin (zeatin)

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Summary Cheat Sheet

Concept / Topic	Key Details / Explanation
Plant growth measurement	Auxanometer; growth follows S-shaped (sigmoid) curve: Lag → Log → Stationary
Hormone word coined by	Starling; phytohormone by Thimann
Growth promoters	Auxin, Gibberellin, Cytokinin
Growth inhibitors	Ethylene (partial), ABA (Abscisic acid)
Auxin discovery	Charles & Francis Darwin (phototropism); F.W. Went (Avena coleoptile test)
Natural auxin	IAA (Indole-3-Acetic Acid); synthesized from tryptophan via Zn
Synthetic auxins	IBA, NAA, 2,4-D (weed killer), 2,4,5-T, IPA, PAA
Auxin — Apical dominance	Shoot apex auxin suppresses lateral buds; removing apex → bushy growth
Auxin — Parthenocarpy	Induces seedless fruit (grape, banana, watermelon)
Auxin — Weed killing	2,4-D and 2,4,5-T kill dicot weeds selectively
Auxin — Root formation	Promotes adventitious roots in cuttings
Gibberellin discovery	By Kurosawa; from fungus Gibberella fujikuroi (bakanae disease in rice)
GA₃	First isolated gibberellin; over 130 types known
Gibberellin — Bolting	Dramatic internode elongation in rosette plants
Gibberellin — Seed germination	Promotes α-amylase synthesis in aleurone layer → starch → sugar
Gibberellin — Vernalization substitute	GA₃ replaces cold treatment requirement for flowering
Cytokinin discovery	By Skoog & Miller (kinetin from yeast DNA); first natural: Zeatin from corn by Letham & Miller
Cytokinin — Cell division	Primary function: promotes cytokinesis (mitosis)
Cytokinin — Anti-senescence	Richmond-Lang effect — delays leaf yellowing/aging
Auxin:Cytokinin ratio	High auxin = roots; High cytokinin = shoots; Equal = callus
Ethylene	Only gaseous hormone (C₂H₄); discovered by Burg; commercial form: Ethephon
Ethylene — Fruit ripening	Master ripening hormone; climacteric fruits: mango, banana, apple, tomato
Ethylene — Triple response	Inhibits elongation, thickens stem, horizontal growth (in etiolated seedlings)
Ethylene — Promotes flowering	In pineapple and mango (exception to general inhibition)
ABA discovery	By Okhuma; dormin by Wareing; abscissin by Addicott; = stress hormone
ABA — Stomatal closure	Causes K⁺ loss from guard cells → flaccid → stomata close (primary drought response)
ABA — Dormancy	Promotes bud and seed dormancy; derived from carotenoid degradation
ABA — Senescence	Promotes leaf yellowing and aging alongside ethylene