Unit 4: In vitro propagation

Callus culture is the aseptic cultivation of an unorganized, dividing mass of parenchymatous cells (callus) derived from a differentiated plant part (explant) on a nutrient medium supplemented with growth regulators. It is the foundational technique of plant tissue culture, exploiting the property of totipotency — the capacity of a single somatic plant cell to regenerate a whole plant.

• Explant: the excised piece of living plant tissue (leaf, stem, root, embryo, node) placed on medium to initiate culture.
• Callus: an amorphous, friable or compact mass of unspecialized cells arising through wound-induced dedifferentiation.
• Dedifferentiation: reversion of a mature, differentiated cell to a meristematic, actively dividing state.
• Totipotency: the genetic potential of each cell to express the full genome and regenerate a complete organism.
• Aseptic condition: all work carried out in a laminar air-flow cabinet with sterilized media, glassware and instruments to exclude microbial contamination.
• Growth regulator balance: callus proliferation depends on an auxin : cytokinin ratio that favours cell division without organized differentiation (typically high or balanced auxin).

II. Materials, Media and Growth Regulators — the working system

The reagents and their concentrations determine whether an explant divides, greens, browns or dies; this section fixes the components later steps depend on.

A. Basal culture medium

The nutrient base supplies inorganic salts, an energy source, vitamins and a gelling agent.

• Murashige and Skoog (MS) medium (1962): the standard basal medium, rich in nitrate and ammonium salts; the reference formulation for most callus work.
• Macronutrients: N, P, K, Ca, Mg, S — e.g. KNO₃ and NH₄NO₃ supply nitrogen.
• Micronutrients: Fe (as Fe-EDTA chelate), Mn, Zn, B, Cu, Mo, Co, I in trace amounts.
• Carbon source: sucrose at 30 g L⁻¹ (3%), since cultured cells are largely heterotrophic.
• Vitamins: thiamine (B₁), nicotinic acid, pyridoxine and myo-inositol (100 mg L⁻¹).
• Gelling agent: agar at 0.6–0.8% to solidify the medium; omitted for suspension culture.
• pH: adjusted to 5.6–5.8 before autoclaving, because agar sets poorly and salts precipitate outside this range.

B. Plant growth regulators

The hormone combination is the single most decisive variable for callus induction.

• Auxins: promote cell division and enlargement; common ones are 2,4-D (2,4-dichlorophenoxyacetic acid), NAA and IAA.
  • 2,4-D: the most potent callus-inducing auxin, used at 0.5–2.0 mg L⁻¹; drives sustained dedifferentiation and suppresses organogenesis.
• Cytokinins: promote cell division and, at higher ratios, shoot formation; e.g. BAP and kinetin.
• Auxin : cytokinin ratio:
  • High auxin, low cytokinin: favours callus and root initiation.
  • Balanced or high cytokinin: shifts callus toward shoot regeneration — hence for pure callus a division-favouring, non-organogenic balance is chosen.

C. Sterilization of media and glassware

Sterility prevents fast-growing microbes from outcompeting the slow explant.

• Autoclaving: medium and glassware at 121 °C, 15 psi, 15–20 min.
• Heat-labile compounds: growth regulators like GA₃ are filter-sterilized (0.22 µm) and added to cooled medium (~45 °C) to avoid thermal breakdown.

III. Selection and Surface Sterilization of the Explant

The choice and cleaning of the starting tissue governs both contamination rate and callusing response.

A. Selecting the explant

The physiological state of the source tissue dictates callus vigour.

• Juvenility: young, actively growing tissues (seedling parts, shoot tips, immature embryos) dedifferentiate more readily than mature tissue.
• Meristematic content: regions with dividing cells — nodes, cambium, procambium — callus fastest.
• Health of donor plant: disease-free, well-nourished mother plants reduce endogenous contamination.

B. Surface sterilization

The goal is to kill surface microbes while sparing the plant cells.

• Sequence: running-tap-water wash → 70% ethanol for 30–60 s → sodium hypochlorite (0.5–1% available chlorine) or 0.1% HgCl₂ for 5–10 min → three to four rinses in sterile distilled water.
• Tween-20: a drop added to the hypochlorite acts as a wetting agent, improving contact with the tissue surface.
• Trimming: cut surfaces damaged by the sterilant are excised inside the hood before inoculation, exposing healthy cells.

IV. Preparation of Callus from Various Explants

The generic protocol is fixed while the explant type is varied; each tissue has a characteristic response, medium tweak and callusing behaviour.

A. General callus induction protocol

One standard workflow underlies all explant types.

• Inoculation: the sterilized explant is placed, cut-surface down, in firm contact with the medium in a laminar air-flow cabinet.
• Orientation: contact between the wounded surface and medium is essential, as wound signalling and nutrient uptake occur there.
• Incubation: cultures held at 25 ± 2 °C, usually in the dark or dim light for induction, since darkness discourages chlorophyll synthesis and organized greening.
• Callus appearance: visible proliferation of soft, whitish-to-cream tissue at the cut edges in 2–4 weeks.
• Subculturing: callus transferred to fresh medium every 3–4 weeks to replenish nutrients and remove toxic metabolites; repeated subculture maintains the callus line.

B. Callus from leaf explants

Leaf lamina and midrib segments are convenient, abundant and highly responsive.

• Preparation: young expanded leaves cut into ~0.5–1 cm² squares, ensuring each piece includes a vein for a dividing cell source.
• Medium: MS with 2,4-D (1–2 mg L⁻¹), sometimes with a low cytokinin.
• Response: callus emerges along cut edges and the midrib; useful in species such as tobacco and Datura.
• Note: the lower (abaxial) surface placed on the medium often calluses better owing to stomatal and mesophyll access.

C. Callus from stem/internode and nodal explants

Stem segments carry cambial and pith tissue that dedifferentiate freely.

1. Internodal segment: a ~1 cm length lacking a bud; the cambial ring and parenchyma callus vigorously on 2,4-D-containing MS.
2. Nodal segment: carries an axillary bud; if strong auxin is used the bud is suppressed and the tissue calluses rather than shoots.
   • Response: compact-to-friable callus from cut ends; widely used in sugarcane, potato and many woody species.

D. Callus from root explants

Root tissue, though less commonly used, calluses well from young lateral or seedling roots.

• Preparation: aseptically raised seedling roots cut into ~1 cm segments to avoid soil-borne contamination.
• Medium: MS with auxin (NAA or 2,4-D); root explants are often auxin-sensitive.
• Response: callus from the pericycle and vascular region — the same cell layers that naturally form lateral roots.

E. Callus from cotyledon and hypocotyl explants

Seedling-derived explants are aseptic from the outset and highly juvenile.

• Source: seeds surface-sterilized and germinated in vitro; cotyledons and hypocotyl segments excised from 5–10 day-old seedlings.
• Advantage: no external microbial load, so contamination is minimal.
• Response: rapid, prolific callus on MS + 2,4-D + low BAP; standard in Brassica, legumes and many dicots.

F. Callus from embryo and endosperm explants

Reproductive tissues serve special purposes beyond routine callusing.

1. Immature embryo: dedifferentiates readily and is favoured for cereals (wheat, rice, maize) where other tissues are recalcitrant.
2. Endosperm: being triploid (3n), its callus regenerates triploid plantlets — valuable for producing seedless fruit.

G. Callus from anther and pollen explants

Male reproductive explants yield haploid callus.

• Anther culture: anthers at the correct microspore stage cultured so that microspores divide into haploid callus and, ultimately, haploid plants.
• Use: haploids doubled with colchicine give instant homozygous diploid lines for breeding.

H. Factors affecting callus induction across explants

A common set of variables tunes the response of every explant type.

• Genotype: species and cultivar differ sharply; some are recalcitrant, callusing poorly regardless of medium.
• Explant age and position: younger, more meristematic tissue calluses faster than mature tissue.
• Growth-regulator type and level: the auxin:cytokinin balance selected in Section II governs induction versus organogenesis.
• Light and temperature: dark and 25 ± 2 °C favour undifferentiated growth.
• Wounding: the cut surface releases signals and phenolics that both trigger division and, if excessive, cause browning.

I. Common problems and their control

Two recurring failures must be managed for a usable callus line.

1. Microbial contamination: bacterial slime or fungal mycelium on the medium — controlled by rigorous surface sterilization, aseptic technique and, where justified, low-dose antibiotics.
2. Phenolic browning: oxidation of released phenols blackens tissue and medium and can kill the explant — countered by antioxidants (ascorbic acid, citric acid, activated charcoal) and frequent subculturing to fresh medium.

J. Significance of callus culture

Callus is the gateway to most downstream tissue-culture applications.

• Regeneration: callus gives rise to plantlets via organogenesis (shoot/root formation) or somatic embryogenesis.
• Micropropagation and cell lines: friable callus seeds suspension cultures for scaled cell growth.
• Secondary metabolites: callus and its suspensions produce alkaloids, pigments and other industrially useful compounds.
• Genetic manipulation: callus cells serve as targets for somaclonal variation selection and for genetic transformation.