Plant growth regulators

In short

Plant growth regulators (PGRs) are organic substances, natural or synthetic, that in very low concentration (micromolar or less) promote, inhibit or otherwise modify a physiological process in plants at a site away from or at the site of production. The five classical groups are auxins, gibberellins, cytokinins, abscisic acid and ethylene; brassinosteroids, jasmonates, salicylic acid, strigolactones and polyamines are the newer classes. PGRs control cell division and elongation, apical dominance, rooting, flowering, fruit set and ripening, dormancy, abscission and stress responses, so they underpin horticulture, plant tissue culture, seed technology and herbicide chemistry. Examined in B.Sc Botany plant physiology papers, in CSIR-UGC NET / GATE Life Sciences (XL) and Biotechnology (BT), ICAR-JRF/NET and IARI entrance; applied by Indian employers in seed and agrochemical companies, tissue-culture labs, floriculture and post-harvest firms. In India PGRs are regulated as "plant growth regulators" under the Insecticides Act, 1968 and the Insecticides Rules, 1971 — they need CIB&RC registration before manufacture or sale.

Explained

Definition, nomenclature and general properties

• Phytohormone (plant hormone): a naturally occurring organic compound produced by the plant, active at very low concentration, that regulates growth or development. PGR is the wider term and includes synthetic analogues (NAA, 2,4-D, kinetin, ethephon, CCC).
• Plant hormones differ from animal hormones: no ductless glands, no single target organ, the same molecule has many effects, and the response depends on concentration, tissue sensitivity and hormone ratio, not on the hormone alone.
• Typical physiological range: 10⁻⁸ to 10⁻⁵ M. Above the optimum, promoters often become inhibitory (auxin is the classic example).
• Classification by action: promoters (auxins, gibberellins, cytokinins, brassinosteroids), inhibitors (ABA, jasmonic acid), gaseous (ethylene), retardants/anti-gibberellins (CCC/chlormequat, paclobutrazol, maleic hydrazide).
• Hormone crosstalk: development is decided by ratios — auxin:cytokinin (organogenesis), GA:ABA (dormancy vs germination), auxin:ethylene (abscission).

Auxins

• Discovery: Darwin and Darwin (1880) on coleoptile phototropism; Boysen-Jensen and Paál showed a diffusible stimulus; F. W. Went (1928) isolated it from *Avena* coleoptile tips in agar; Kögl named it auxin (Greek *auxein*, to grow). Natural auxin = indole-3-acetic acid (IAA).
• Types: natural — IAA, indole-3-butyric acid (IBA, also in plants), phenylacetic acid, 4-chloro-IAA; synthetic — NAA (naphthalene acetic acid), 2,4-D, 2,4,5-T, MCPA, picloram, IBA (commercial rooting hormone).
• Biosynthesis: mainly from tryptophan by the two-step indole-3-pyruvic acid (IPyA) route — TAA1 aminotransferase then YUCCA flavin monooxygenase. Synthesised in shoot apical meristems, young leaves, developing seeds and fruits.
• Transport: polar basipetal (apex → base) in the shoot, cell to cell, by PIN efflux carriers (basal membrane) and AUX1/LAX influx carriers; energy-dependent, blocked by TIBA and NPA. Chemiosmotic model: IAAH enters passively, IAA⁻ ions leave only through PIN. Long-distance acropetal movement also occurs in phloem.
• Signalling: auxin binds the TIR1/AFB F-box receptor, which ubiquitinates Aux/IAA repressors for 26S-proteasome degradation, releasing ARF transcription factors → gene expression. Rapid effects via acid growth hypothesis: auxin activates plasma-membrane H⁺-ATPase, wall acidification activates expansins, wall loosens, cell elongates.
• Effects: cell elongation; apical dominance (apical bud auxin suppresses lateral buds); phototropism and gravitropism (lateral redistribution, Cholodny–Went theory); adventitious root initiation in cuttings; prevention of abscission at low concentration; parthenocarpy; xylem differentiation; fruit set; at high dose it is a selective herbicide for dicot weeds (2,4-D); induces ethylene at high concentration.
• Limitation: narrow effective range; overdose causes epinasty, stem twisting and death — which is exactly how 2,4-D works.

Gibberellins

• Discovery: from *Gibberella fujikuroi* (now *Fusarium fujikuroi*), the fungus causing bakanae ("foolish seedling") disease of rice; Kurosawa (1926), Yabuta and Sumiki isolated gibberellin. Over 130 GAs known; GA₁, GA₃ (gibberellic acid), GA₄ and GA₇ are the bioactive ones; GA₃ is the commercial form.
• Chemistry: diterpenoid acids with the *ent*-gibberellane skeleton; C₂₀ and C₁₉ GAs (C₁₉ forms are the active ones).
• Biosynthesis: GGPP → *ent*-kaurene (CPS, KS) → GA₁₂-aldehyde → GA₁₂ → bioactive GA by GA20ox and GA3ox; inactivated by GA2ox. Inhibitors of synthesis: paclobutrazol, CCC (chlormequat), ancymidol, AMO-1618.
• Signalling: GA binds soluble receptor GID1 → the complex recruits DELLA proteins (growth repressors) for degradation → growth genes expressed. GA-insensitive dwarfs have stabilised DELLA.
• Effects: internode elongation and bolting in rosette plants (cabbage, beet); reversal of genetic dwarfism (dwarf maize, pea — classic bioassay); seed germination — GA from the embryo induces α-amylase in barley aleurone, mobilising starch; breaking seed and bud dormancy; promotes flowering in long-day plants; parthenocarpy; enlarges grape berries; delays senescence of citrus rind; induces maleness in some cucurbits.
• Limitation: excess causes weak, spindly growth and lodging; GA-treated seedless grapes need careful dosing.

Cytokinins

• Discovery: Skoog and Miller — autoclaved herring sperm DNA yielded kinetin (6-furfurylaminopurine), a synthetic/artefact cytokinin; the first natural cytokinin zeatin came from immature maize (*Zea mays*) kernels.
• Types: natural — zeatin, dihydrozeatin, isopentenyl adenine (iP); synthetic — kinetin, BAP/6-benzylaminopurine, and the non-purine phenylureas thidiazuron (TDZ) and CPPU. All are adenine derivatives except the phenylureas.
• Site and transport: root apical meristem chiefly; also developing seeds and fruits. Moves acropetally in the xylem with the transpiration stream; synthesis step = isopentenyl transferase (IPT); degraded by cytokinin oxidase.
• Signalling: two-component system — receptor histidine kinases AHK2/3/4 → phosphotransfer via AHPs → type-B ARR transcription factors.
• Effects: cell division (cytokinesis) in presence of auxin; shoot bud initiation; release of lateral buds from apical dominance; delay of leaf senescence (Richmond–Lang effect); chloroplast development and chlorophyll retention; nutrient mobilisation; counteracts apical dominance and promotes axillary branching; breaks dormancy in some seeds.
• Skoog–Miller organogenesis rule (tissue culture): high auxin:cytokinin → root formation; low auxin:cytokinin (cytokinin-rich) → shoot formation; roughly equal → callus.

Abscisic acid

• Discovery: isolated as "abscisin II" from cotton fruit (Addicott) and as "dormin" from dormant birch/sycamore (Wareing); a sesquiterpenoid (C₁₅), made in plastids from carotenoid (zeaxanthin → violaxanthin → xanthoxin) by the NCED cleavage step — hence an indirect isoprenoid route, not from mevalonate directly in the cytosol.
• Signalling: ABA binds PYR/PYL/RCAR receptors → inhibits PP2C phosphatases (ABI1/ABI2) → SnRK2 kinases stay active → phosphorylate SLAC1 anion channels and ABF transcription factors.
• Effects: the stress hormone — closes stomata under water deficit by driving K⁺ and anion efflux from guard cells; induces and maintains seed dormancy; promotes synthesis of storage proteins and LEA proteins; induces bud dormancy and leaf senescence; inhibits GA-induced α-amylase; antitranspirant.
• Named for abscission but ethylene, not ABA, is the main abscission hormone; ABA acts partly by inducing ethylene.
• GA:ABA balance decides dormancy versus germination; a high ABA level enforces dormancy, while in viviparous (*vp*) maize mutants ABA is low, so seeds germinate on the cob.

Ethylene

• The only gaseous hormone, C₂H₄; Neljubow (1901) traced the "triple response" of pea seedlings (shortened, thickened hypocotyl, exaggerated apical hook, horizontal growth) to illuminating gas.
• Biosynthesis (Yang cycle): methionine → SAM (S-adenosylmethionine) → ACC by ACC synthase (ACS, the rate-limiting step) → ethylene by ACC oxidase (ACO); 5′-methylthioadenosine recycles methionine. Inhibitors: AVG, AOA (block ACS), Co²⁺ (blocks ACO); action inhibitors silver thiosulphate (STS), 1-MCP (1-methylcyclopropene), CO₂.
• Signalling: ETR1 receptors in the ER are negative regulators — ethylene binding switches them off, inactivating CTR1 kinase, stabilising EIN3/EIL1 → ERF genes. Hence ethylene-insensitive mutants are dominant (etr1-1).
• Effects: climacteric fruit ripening (banana, mango, tomato, papaya, apple — autocatalytic rise in respiration and ethylene); abscission of leaves, flowers and fruits; epinasty; senescence of flowers; root hair formation; triple response; breaks dormancy in potato; promotes femaleness in cucurbits; promotes flowering in pineapple and mango; induces aerenchyma and rapid internode elongation in deepwater rice (with GA).
• Ethephon (2-chloroethylphosphonic acid) is the liquid source that releases ethylene in planta — used for rubber latex yield, pineapple and mango flowering, tomato ripening, boll opening in cotton.
• Non-climacteric fruits (citrus, grape, pineapple pulp, strawberry) do not show a ripening ethylene burst, so they must be harvested ripe.

Newer regulators, bioassays and Indian regulation

• Brassinosteroids (brassinolide, 24-epibrassinolide): steroidal; from *Brassica napus* pollen; cell elongation and division, xylem differentiation, stress tolerance; perceived at the cell surface by BRI1 receptor kinase. Dwarf, dark-green, de-etiolated phenotypes in biosynthesis mutants.
• Jasmonic acid / methyl jasmonate: from linolenic acid by the octadecanoid pathway; wound and herbivore defence, proteinase inhibitor induction, tendril coiling, tuber formation, senescence; receptor COI1.
• Salicylic acid: phenolic; systemic acquired resistance (SAR) and PR-protein induction against biotrophic pathogens; thermogenesis in *Arum* inflorescences; receptor NPR1.
• Strigolactones: carotenoid-derived; inhibit shoot branching (with auxin), stimulate arbuscular mycorrhizal hyphal branching, and also germinate the parasitic weeds *Striga* and *Orobanche*; receptor D14/MAX2.
• Others: polyamines (putrescine, spermidine), nitric oxide, systemin and other peptide hormones, florigen = FT protein (not a classical PGR), triacontanol.
• Bioassays (frequently asked): auxin — Avena coleoptile curvature test, split pea stem curvature, *Avena* straight-growth test; gibberellin — dwarf pea/maize elongation, barley α-amylase (half-seed) assay, lettuce hypocotyl; cytokinin — tobacco pith callus, radish cotyledon expansion, retention of chlorophyll in detached leaves, *Amaranthus* betacyanin; ABA — stomatal closure, inhibition of α-amylase, wheat embryo germination; ethylene — pea triple response, gas chromatography.
• Indian regulation and practice: PGRs fall under the Insecticides Act, 1968 and Insecticides Rules, 1971, administered by the Central Insecticides Board and Registration Committee (CIB&RC), Faridabad; registration under section 9(3) is required, with label-approved crops and doses. Unapproved use of calcium carbide for mango ripening is prohibited under the Food Safety and Standards (Prohibition and Restrictions on Sales) Regulations, 2011 (ethylene ripening within prescribed limits is permitted); residues are checked against FSSAI tolerance limits. Organic certification under NPOP restricts synthetic PGRs. A Pesticide Management Bill to replace the 1968 Act has been under consideration for years but, as of this writing, the 1968 Act remains in force — verify current status before quoting. GST on plant growth regulators falls in the 5% slab under the rates effective 22 September 2025; confirm the current rate notification for the exact HSN.

Key points

• PGR — organic compound active at very low concentration that modifies a plant physiological process; includes synthetics.
• Auxin (IAA) — made at shoot tip from tryptophan; polar basipetal transport by PIN carriers; causes cell elongation and apical dominance.
• Acid growth hypothesis — auxin pumps H⁺ out, wall pH falls, expansins loosen cellulose–hemicellulose network, cell elongates.
• TIR1–Aux/IAA–ARF — auxin signalling by targeted destruction of repressors.
• Gibberellin (GA₃) — diterpenoid from *Fusarium fujikuroi*; internode elongation, bolting, α-amylase induction, dwarf reversal.
• GID1–DELLA — GA works by degrading DELLA growth repressors.
• Cytokinin (zeatin, kinetin) — adenine derivative from root tip, moves acropetally in xylem, drives cell division and shoot formation.
• Richmond–Lang effect — cytokinin delays leaf senescence and retains chlorophyll.
• Skoog–Miller ratio — high auxin:cytokinin → roots; low → shoots; intermediate → callus.
• ABA — sesquiterpenoid from carotenoid cleavage (NCED); stress hormone; closes stomata and enforces seed dormancy.
• PYR/PYL–PP2C–SnRK2 — ABA signalling core; PP2C inhibition keeps SnRK2 active.
• Ethylene — gaseous; Yang cycle via SAM and ACC; ACC synthase is rate-limiting; ripens climacteric fruits and causes abscission.
• Triple response — short thick hypocotyl, exaggerated hook, horizontal growth in dark-grown pea with ethylene.
• 1-MCP and STS — block ethylene receptors; used to extend shelf and vase life.
• Brassinosteroid, jasmonate, salicylic acid, strigolactone — elongation/stress, wound defence, SAR, branching inhibition and mycorrhizal signalling respectively.
• Antagonisms — GA vs ABA (germination), auxin vs cytokinin (dominance and organogenesis), auxin vs ethylene (abscission).
• Herbicidal auxin — 2,4-D kills broadleaf dicots selectively in cereal fields because monocots metabolise and translocate it differently.
• CIB&RC registration under the Insecticides Act, 1968 is mandatory for selling any PGR formulation in India.

Formulas and examples

Example 1 — preparing an IAA stock and working solution. Required: 100 mL of 10⁻⁵ M IAA. M(IAA, C₁₀H₉NO₂) = 175.18 g/mol.

• Mass needed = 10⁻⁵ mol/L × 0.1 L × 175.18 g/mol = 1.7518 × 10⁻⁴ g = 0.175 mg — too small to weigh accurately.
• So make a stock: 17.5 mg in 100 mL (dissolve in a few drops of 1 N NaOH or ethanol first) = 175 mg/L = 10⁻³ M.
• Dilute: C₁V₁ = C₂V₂ → 10⁻³ × V₁ = 10⁻⁵ × 100 → V₁ = 1 mL stock made up to 100 mL. 10⁻⁵ M = 1.75 mg/L ≈ 1.75 ppm.

Example 2 — ppm to molarity for GA₃. A grape grower sprays 40 ppm GA₃. M(GA₃, C₁₉H₂₂O₆) = 346.37 g/mol.

• 40 ppm = 40 mg/L = 0.040 g/L.
• Molarity = 0.040 / 346.37 = 1.1548 × 10⁻⁴ ≈ 1.15 × 10⁻⁴ M (115 µM).
• For 500 L of spray: 40 mg/L × 500 L = 20,000 mg = 20 g GA₃. Using a 0.001% (10 ppm) ready formulation would need an impractical volume, so growers use technical-grade GA₃ 90% w/w: 20/0.90 = 22.2 g of product.

Example 3 — ethephon dose for a field. Label dose: ethephon 39% SL at 500 g a.i. ha⁻¹ on 2 ha (illustrative; follow the CIB&RC-approved label).

• Total a.i. = 500 × 2 = 1,000 g.
• Product = 1,000 / 0.39 = 2,564 g ≈ 2.56 kg (about 2.2 L at ~1.15 kg/L).
• At 500 L ha⁻¹ spray volume, total water = 1,000 L; concentration = 1,000 g a.i. / 1,000 L = 1 g/L = 1,000 ppm a.i.

Example 4 — RGR with and without cytokinin. Control seedlings: 0.85 g → 2.10 g dry weight in 14 days. BAP-treated: 0.85 g → 2.90 g.

• Control RGR = (ln 2.10 − ln 0.85)/14 = (0.7419 − (−0.1625))/14 = 0.9044/14 = 0.0646 g g⁻¹ d⁻¹.
• Treated RGR = (ln 2.90 − ln 0.85)/14 = (1.0647 + 0.1625)/14 = 1.2272/14 = 0.0877 g g⁻¹ d⁻¹.
• Growth promotion on final weight = (2.90 − 2.10)/2.10 × 100 = 38.1%.

Example 5 — tissue-culture medium ratio. MS medium with 2.0 mg/L NAA + 0.2 mg/L BAP versus 0.2 mg/L NAA + 2.0 mg/L BAP.

• First: auxin:cytokinin = 10:1 → root (rhizogenesis). Second: 1:10 → shoot (caulogenesis). Equal (say 1.0 + 1.0) → undifferentiated callus.
• Preparing 1 L of the shoot medium from a 100 mg/L BAP stock: need 2.0 mg → V = 2.0/0.1 mg/mL = 20 mL of stock per litre.

Where it is used

• Horticulture and plantation crops: GA₃ on Thompson Seedless grapes in Maharashtra and Karnataka for berry size and cluster elongation; ethephon for uniform mango and pineapple flowering and for rubber latex stimulation in Kerala; NAA to prevent pre-harvest fruit drop in citrus and mango; maleic hydrazide to suppress onion and potato sprouting in store.
• Plant tissue culture and micropropagation: banana, sugarcane, orchid, bamboo and *Eucalyptus* labs run on MS medium with defined auxin:cytokinin ratios; TDZ and BAP for shoot multiplication, IBA/NAA for rooting; employers include NRCB Tiruchirappalli, state agri-universities, and private hardening nurseries.
• Agrochemical industry: 2,4-D, ethephon, chlormequat (CCC) and paclobutrazol formulation, registration dossiers and bioefficacy trials at UPL, Coromandel, Dhanuka, Rallis, Godrej Agrovet — roles in product development and regulatory affairs under CIB&RC.
• Seed technology: GA₃ and KNO₃ priming to break dormancy for germination tests; ABA assays for pre-harvest sprouting tolerance in wheat; hybrid seed production uses ethephon/GA for sex expression in cucurbits and for synchronising flowering.
• Post-harvest and cold chain: ethylene-based ripening chambers approved for mango and banana (replacing banned calcium carbide), 1-MCP (SmartFresh-type) treatments for apples, STS pulsing for cut flowers in Hosur, Pune and Bengaluru floriculture units.
• Research and teaching roles: JRF/SRF positions at ICAR institutes, NIPGR, NBRI, Bose Institute and university departments, where PGR bioassays, mutant screening and hormone-signalling work are standard.

Interview questions

• Q: How does a plant hormone differ from an animal hormone? A: Plants have no glands or dedicated target organs; the same hormone produces many effects depending on tissue, concentration and the ratio to other hormones, and transport may be polar and cell-to-cell rather than through a circulatory system.
• Q: Why is auxin transport called polar? A: PIN efflux carriers sit on the basal face of the cell, so IAA leaves only downward — giving energy-dependent basipetal movement in the shoot, blocked by TIBA or NPA.
• Q: Explain apical dominance and how you would demonstrate it. A: Auxin from the apical bud suppresses axillary buds; remove the apex and laterals grow, but apply auxin paste to the cut stump and suppression returns, while cytokinin application releases the laterals.
• Q: What is the acid growth hypothesis? A: Auxin stimulates the plasma-membrane H⁺-ATPase, the wall acidifies, expansins and wall enzymes loosen cellulose–hemicellulose links, and the cell elongates under turgor.
• Q: How does gibberellin trigger starch mobilisation in germinating barley? A: GA released by the embryo reaches the aleurone layer and induces transcription of α-amylase, which hydrolyses endosperm starch to sugars; ABA opposes this, which is the basis of the half-seed bioassay.
• Q: State the Skoog–Miller rule and its use. A: A high auxin-to-cytokinin ratio gives roots, a low ratio gives shoots, and a near-equal ratio gives callus — the design principle for every micropropagation protocol.
• Q: Which hormone closes stomata and how? A: ABA — through PYR/PYL receptors it inhibits PP2C phosphatases, SnRK2 kinases stay active and phosphorylate SLAC1 anion channels, K⁺ and anions leave the guard cell, turgor falls and the pore closes.
• Q: Why are ethylene receptor mutants like etr1-1 dominant? A: ETR1 is a negative regulator; a mutant receptor that cannot bind ethylene stays permanently active and keeps signalling off even when the normal allele is present.
• Q: What is the difference between climacteric and non-climacteric fruits? A: Climacteric fruits (banana, mango, tomato) show an autocatalytic ethylene and respiration burst and can ripen after harvest; non-climacteric fruits (grape, citrus, strawberry) do not, so they must be harvested ripe.
• Q: Why is calcium carbide ripening banned in India, and what is the legal alternative? A: Carbide generates acetylene along with arsenic and phosphorus impurities and is prohibited under the FSS (Prohibition and Restrictions on Sales) Regulations, 2011; ripening with ethylene gas or approved ethephon treatment within prescribed limits is the permitted route.
• Q: How can one chemical, 2,4-D, be both a growth promoter and a herbicide? A: At micromolar levels it mimics IAA and promotes elongation, but at high dose it causes uncontrolled growth, massive ethylene and ABA production and vascular collapse, killing dicots while cereals detoxify and compartmentalise it.
• Q: What are strigolactones and why do they matter to Indian agriculture? A: Carotenoid-derived hormones that suppress shoot branching and attract mycorrhizal fungi, but they also stimulate germination of the parasitic weeds *Striga* on sorghum and millets and *Orobanche* on mustard and tobacco.
• Q: Which approval is needed to sell a PGR formulation in India? A: Registration with the Central Insecticides Board and Registration Committee under the Insecticides Act, 1968 and Insecticides Rules, 1971, with an approved label claim for crop, dose and waiting period.

Common mistakes

• Calling kinetin a natural plant hormone — it is a DNA-degradation product/synthetic cytokinin; the first natural one is zeatin.
• Writing that auxin transport is only basipetal — it is polar basipetal in the shoot through PIN carriers, while cytokinin moves acropetally in the xylem and GA moves non-polarly.
• Saying ABA is the hormone of abscission because of its name — ethylene is the principal abscission hormone.
• Treating ABA as a mevalonate-pathway sesquiterpene made like other cytosolic terpenes; it is cleaved from a plastid carotenoid by NCED.
• Confusing ethylene biosynthesis inhibitors (AVG, AOA, Co²⁺) with action inhibitors (STS, 1-MCP) in post-harvest questions.
• Assuming a promoter always promotes: auxin at supra-optimal dose inhibits root growth and kills dicot weeds; concentration decides the response.
• Reporting ppm and molarity interchangeably, or forgetting that IAA and GA₃ must be dissolved in alkali or ethanol before making up with water, and that IAA solutions are light-sensitive and degrade.
• Quoting PGR doses from a textbook rather than the CIB&RC-approved label, and ignoring the waiting period before harvest.

Related topics

Photoperiodism, vernalisation and flowering · Seed dormancy and germination · Plant tissue culture and micropropagation · Senescence and abscission · Photomorphogenesis and phytochrome signalling · Stress physiology and signal transduction