Unit 5: Plant growth regulators

Plant growth regulators (PGRs) are organic compounds, other than nutrients, that in small quantities promote, inhibit, or otherwise modify plant physiological processes. In plant tissue culture (an outgrowth of Haberlandt's 1902 totipotency hypothesis, realised experimentally in the 1950s), PGRs are the principal levers used to steer a mass of undifferentiated cells toward defined organs. This unit deals specifically with how PGRs direct organogenesis in vitro — the de novo formation of shoots and roots from cultured explants.

Defining properties this unit relies on:

• Totipotency: Every living plant cell retains the full genome and can, given the right signals, regenerate a whole plant.
• Two hormone classes dominate organogenesis: Auxins and cytokinins act as the master switches; their ratio, not absolute amount, decides organ fate.
• Dedifferentiation → redifferentiation: Differentiated explant cells first revert to a meristematic state (often a callus), then reorganise into new organ primordia.
• Concentration-dependence: The same hormone can promote at low doses and inhibit at high doses (a biphasic dose response).
• Basal medium context: PGRs act against a defined nutrient background (e.g. Murashige & Skoog, 1962) supplying macro/micronutrients, vitamins, and a carbon source (usually 3% sucrose).

II. Auxins — The Root-Inducing and Callus-Inducing Regulators

Auxins are PGRs derived from or mimicking indole-3-acetic acid (IAA); they govern cell elongation, vascular differentiation, and — critically here — root initiation.

A. Definition and mode of action

Auxins promote cell wall loosening and DNA synthesis, priming cells for division and expansion.

• Natural auxin: IAA (indole-3-acetic acid) — endogenous but light- and heat-labile, so rarely relied on alone in culture.
• Synthetic auxins: IBA (indole-3-butyric acid), NAA (α-naphthaleneacetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid) — more stable and potent.
• Cellular effect: Activate proton pumps → acidify cell wall → activate expansins → wall loosening (the "acid growth" mechanism).
• Typical working range: 0.1–5 mg L⁻¹, prepared as stocks in dilute NaOH or ethanol because auxins are poorly water-soluble.

B. Effect on rooting (in vitro rooting)

Auxin is the dominant signal for adventitious root formation from shoot bases or explants.

• Root induction: A high auxin : low cytokinin ratio triggers root primordia. IBA and NAA are preferred over IAA for stability.
• Three-phase rooting sequence:
  • Induction phase: Requires auxin; competent cells dedifferentiate into root founder cells.
  • Initiation phase: Root meristemoids organise — auxin still needed.
  • Elongation/expression phase: Auxin becomes inhibitory; roots emerge better after transfer to auxin-free or reduced-auxin medium.
• 2,4-D contrast: Strongly favours callus and somatic embryogenesis rather than organised roots; NAA/IBA favour discrete roots.
• Worked example: Microshoots on ½-strength MS + 1.0 mg L⁻¹ IBA for 5–7 days (induction), then transferred to PGR-free ½ MS to allow root elongation — a common two-step protocol that separates the auxin-dependent and auxin-inhibited phases.

C. Limitations in rooting

Auxin overdose is a frequent cause of failure.

• Callus at the base: Excess auxin produces a basal callus that blocks vascular continuity between shoot and root.
• Species recalcitrance: Woody/mature-phase explants root poorly regardless of auxin dose.
• Vitrification risk: High auxin plus poor aeration yields hyperhydric (glassy) roots that fail on transplanting.

III. Cytokinins — The Shoot-Inducing Regulators

Cytokinins are adenine-derived PGRs that promote cell division and, in organogenesis, drive shoot bud formation and release of axillary buds from apical dominance.

A. Definition and mode of action

Cytokinins stimulate cytokinesis and shoot meristem identity.

• Natural cytokinins: Zeatin, 2iP (isopentenyladenine) — adenine derivatives.
• Synthetic cytokinins: BAP/BA (6-benzylaminopurine), kinetin (6-furfurylaminopurine), TDZ (thidiazuron, a phenylurea with high cytokinin activity).
• Cellular effect: Promote G2→M transition and reactivate quiescent axillary meristems by counteracting auxin-mediated apical dominance.
• Typical working range: 0.5–5 mg L⁻¹; TDZ is active at far lower doses (0.01–1 mg L⁻¹) and can substitute for auxin+cytokinin together.

B. Effect on shooting (in vitro shooting)

Cytokinin is the dominant signal for shoot multiplication and adventitious bud formation.

• Shoot induction: A high cytokinin : low auxin ratio promotes shoot bud regeneration and breaks apical dominance for multiple-shoot proliferation.
• Axillary vs adventitious shoots:
  • 1. Axillary bud proliferation: Cytokinin (e.g. BAP 1–2 mg L⁻¹) releases pre-existing axillary buds — genetically stable, preferred for micropropagation.
  • 2. Adventitious shoot regeneration: Shoots form de novo from callus or explant surface — higher throughput but greater somaclonal-variation risk.
• TDZ note: At low concentration it induces prolific shooting but can inhibit shoot elongation, requiring transfer to BAP for elongation.
• Worked example: Nodal explant on MS + 2.0 mg L⁻¹ BAP + 0.1 mg L⁻¹ NAA yields a cluster of 6–10 shoots in 4 weeks; the small auxin supplement supports growth without overriding the shoot-promoting ratio.

C. Limitations in shooting

Cytokinin excess degrades shoot quality.

• Stunting: High cytokinin gives many but short, rosette-like shoots.
• Hyperhydricity: Excess BAP/TDZ causes translucent, water-soaked shoots.
• Carry-over inhibition: Residual cytokinin in tissue suppresses subsequent rooting, so shoots are usually excised and moved to auxin medium.

IV. The Auxin : Cytokinin Ratio — The Master Switch of Organogenesis

Organ fate in culture is decided chiefly by the relative concentration of auxin to cytokinin, the principle established by Skoog and Miller (1957) in tobacco pith.

A. The Skoog–Miller principle

The ratio, not the total hormone level, programmes organ identity.

• High cytokinin : low auxin → shoot bud formation (caulogenesis).
• Low cytokinin : high auxin → root formation (rhizogenesis).
• Balanced / intermediate ratio → unorganised callus proliferation.
• Anchor: In tobacco pith, raising kinetin against a fixed IAA level shifted cultures from roots → callus → shoots along a continuous gradient.

B. Effect on organogenesis: shooting vs rooting compared

Organogenesis is direct (from explant) or indirect (via callus), and the ratio steers both.

Auxin ↑  Cytokinin ↓   →  ROOTS
Auxin ↓  Cytokinin ↑   →  SHOOTS
Auxin ≈  Cytokinin      →  CALLUS (unorganised)

• 1. In vitro shooting pathway:
  • Signal: Cytokinin-dominant medium (e.g. BAP-rich MS).
  • Sequence: Explant → shoot meristemoids → leafy microshoots → elongation on reduced cytokinin.
  • Genetic fidelity: Highest via axillary route; adventitious/callus route risks somaclonal variation.
• 2. In vitro rooting pathway:
  • Signal: Auxin-dominant medium (IBA/NAA), often on ½-strength MS with lowered sucrose.
  • Sequence: Excised microshoot → basal root primordia → adventitious roots → hardening.
  • Practical split: Short auxin pulse for induction, then auxin-free medium for elongation to avoid basal callus.
• Symbol/parameter key:
  • MS: Murashige & Skoog basal medium.
  • ½ MS: Macronutrients halved — lowers salts/nitrogen that otherwise suppress rooting.
  • mg L⁻¹: Milligrams of PGR per litre of medium (working concentration unit).

C. Sequential culture strategy

Because shoots and roots need opposite ratios, organogenesis is staged, not simultaneous.

• Stage I – initiation: Establish aseptic explant on balanced medium.
• Stage II – multiplication: Cytokinin-dominant medium for shoot proliferation.
• Stage III – rooting: Transfer excised shoots to auxin-dominant medium.
• Stage IV – acclimatisation: Move rooted plantlets to soil under high humidity, gradually hardened (ex vitro adaptation).
• Reason for staging: Cytokinin carried into Stage III inhibits rooting; auxin carried into Stage II suppresses shoot buds — hence physical transfer between media.

V. Supporting and Modulating Regulators in Organogenesis

Beyond the auxin–cytokinin pair, other PGRs fine-tune shooting and rooting outcomes.

A. Gibberellins (GA₃)

Gibberellins promote elongation and can rescue stunted cultures.

• Effect on shooting: GA₃ (0.1–1 mg L⁻¹) elongates short cytokinin-induced shoots and breaks bud/seed dormancy in culture.
• Effect on rooting: Generally inhibitory — GA₃ suppresses adventitious root initiation, so it is withheld during the rooting stage.

B. Abscisic acid (ABA)

ABA is chiefly a stress and maturation regulator used sparingly.

• Effect: Promotes maturation of somatic embryos and improves stress tolerance for acclimatisation.
• Organogenesis role: Generally antagonises both shoot and root proliferation; used to condition plantlets, not to induce organs.

C. Ethylene

Ethylene is a gaseous regulator that accumulates in sealed vessels.

• Effect: Often inhibits shoot regeneration and can promote or inhibit rooting depending on species and level.
• Management: Ventilated closures or silver nitrate (an ethylene-action inhibitor) counter its build-up in tightly sealed cultures.

D. Significance for in vitro organogenesis

Getting PGR choice and ratio right underpins the whole applied value of tissue culture.

• Clonal propagation: Cytokinin-driven shoot multiplication enables rapid production of genetically uniform plants.
• Rooting and hardening: Auxin-driven rhizogenesis converts microshoots into transplantable plantlets, completing the cycle.
• Germplasm and crop improvement: Controlled organogenesis supports virus-free stock production, conservation, and regeneration of transformed cells into whole plants.