Plant tissue culture

In short

Plant tissue culture is the aseptic growth of plant cells, tissues or organs on a defined nutrient medium under controlled light and temperature, exploiting totipotency — the ability of a single somatic plant cell to regenerate a whole plant. It underpins micropropagation of banana, sugarcane, potato, orchids and teak, virus-free planting material, haploid breeding, protoplast fusion and production of secondary metabolites. In exams it is a core unit of B.Sc Biotechnology/Botany Techniques papers and appears in GATE-BT, CSIR-UGC NET (Life Sciences), ICAR-ARS and IIT-JAM. In jobs it is the daily work of commercial tissue culture labs recognised by DBT under the National Certification System for Tissue Culture Raised Plants (NCS-TCP), and of agri-biotech R&D at ICAR institutes, IARI, NBPGR, Jain Irrigation, Godrej Agrovet and KF Bioplants.

Explained

Basic principles and terminology

• Totipotency: every living plant cell carries the full genome and can, under the right hormonal signal, dedifferentiate and regenerate a whole plant. Haberlandt proposed it in 1902; F. C. Steward (carrot) and Reinert demonstrated somatic embryogenesis in 1958.
• Explant: the piece of tissue excised from the mother plant (shoot tip, nodal segment, leaf disc, anther, embryo, root tip). Juvenile, meristematic tissue responds best.
• Dedifferentiation → redifferentiation: a differentiated explant cell loses its specialised state to divide as callus, then redifferentiates into shoots, roots or embryos.
• Callus: an unorganised, dividing mass of parenchymatous cells formed on a solid medium; suspension culture is free cells and small aggregates in agitated liquid medium.
• Competence and induction: a cell must first become competent to respond, then receive the inductive hormonal signal — hence the fixed sequence of induction, multiplication and rooting media.
• Aseptic condition is non-negotiable: plant media feed bacteria and fungi far better than plants, so all work is done in a laminar air flow (LAF) cabinet — typically a horizontal clean bench delivering HEPA-filtered Class 100 (ISO Class 5) air over the work surface — with autoclaved media and flamed instruments.

Culture medium and its components

• Macronutrients (mM range): N as both NO₃⁻ and NH₄⁺, P, K, Ca, Mg, S. Micronutrients (µM): Fe (as FeSO₄–Na₂EDTA chelate), Mn, Zn, Cu, Mo, B, Co, I.
• Carbon source: sucrose 2–3% (20–30 g/L) — cultures are heterotrophic because photosynthesis is minimal in vitro.
• Vitamins: thiamine (B1, essential), nicotinic acid, pyridoxine, myo-inositol (100 mg/L in MS); amino acid/organic N: glycine, casein hydrolysate, glutamine.
• Gelling agent: agar 0.6–0.8%; alternatives Phytagel, gellan gum. Liquid medium needs a support (filter paper bridge) or agitation.
• pH: adjusted to 5.6–5.8 before autoclaving (agar will not set below ~4.5 and nutrient availability shifts).
• Sterilisation: autoclave at 121 °C, 15 psi (≈1.05 kg/cm²), 15–20 min. Thermolabile additives (GA₃, zeatin, urea, antibiotics, some vitamins) are filter-sterilised through a 0.22 µm membrane and added to cooled (≈45–50 °C) medium.
• Media formulations: MS (Murashige & Skoog, 1962) — high salt, the default for most dicots and monocots; B5 (Gamborg) — low ammonium, for suspension and protoplast cultures; White's — low salt, root culture; Nitsch & Nitsch — anther culture; N6 (Chu) — cereals; WPM (Lloyd & McCown) — woody plants.
• Culture room conditions: 25 ± 2 °C, 16 h photoperiod, 1,000–3,000 lux (cool white fluorescent or LED), 50–60% RH.

Plant growth regulators and the control of morphogenesis

• Auxins: IAA (natural, light-labile), IBA, NAA, 2,4-D (strongest callus inducer, used for somatic embryogenesis and suspension cultures). Promote cell division, callus, root initiation, elongation.
• Cytokinins: BAP (6-benzylaminopurine), kinetin, zeatin, TDZ (thidiazuron, very potent in woody species). Promote cell division, shoot bud multiplication, break apical dominance.
• Skoog and Miller (1957) ratio rule — the single most examined concept:
  • high auxin : low cytokinin → root formation
  • low auxin : high cytokinin → shoot formation
  • roughly equal, moderate-to-high both → callus proliferation
• Gibberellin (GA₃): shoot elongation, breaking dormancy of in vitro buds. ABA: maturation and desiccation tolerance of somatic embryos, used in synthetic seed and cryopreservation protocols. Ethylene: usually harmful accumulation in sealed vessels, controlled by AgNO₃ or vented closures.
• Organogenesis: shoots/roots form from callus or directly from the explant — direct (no callus, genetically safer) or indirect (via callus, higher risk of somaclonal variation).
• Somatic embryogenesis: bipolar embryos (shoot + root pole) form from somatic cells, passing globular → heart → torpedo → cotyledonary stages; usually induced with 2,4-D and matured after its removal. Basis of synthetic seeds (somatic embryos encapsulated in 3% sodium alginate + CaCl₂ beads).

Micropropagation — the five standard stages

1. Stage 0 — mother plant selection and pre-treatment: choose elite, true-to-type, disease-indexed mother stock; keep it under cover and reduce overhead watering to lower contamination.
2. Stage I — initiation/establishment: explant washed with detergent (Tween-20), surface-sterilised with 70% ethanol 30 s then 0.1% HgCl₂ 2–5 min or 1–2% sodium hypochlorite 10–15 min, rinsed 3–5 times in sterile distilled water, inoculated. (HgCl₂ is highly toxic and mercury waste must be handled under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016; many labs have moved to NaOCl.)
3. Stage II — multiplication: repeated subculture every 3–4 weeks on cytokinin-rich medium; shoot numbers increase geometrically. This stage decides the commercial economics.
4. Stage III — rooting/elongation: transfer to low-cytokinin, auxin-containing (IBA/NAA) or hormone-free half-strength MS medium; some species root ex vitro directly in the nursery.
5. Stage IV — hardening and acclimatisation: in vitro plants have poor cuticle, non-functional stomata and no root hairs. Transfer to a sterile potting mix (cocopeat : perlite : vermiculite) in a mist chamber/polyhouse, high humidity reduced gradually, light raised over 3–6 weeks, then primary and secondary nursery. Mortality here is the commonest cause of commercial loss.
6. Meristem/shoot-tip culture for virus elimination: excise a 0.2–1.0 mm apical dome (virus titre is lowest in the meristem as it is not vascularised); combine with thermotherapy (35–38 °C) and chemotherapy (ribavirin) for stubborn viruses. Mandatory for potato (seed potato programme), banana (BBTV, BSV) and sugarcane.
7. Indexing: regenerated plants are tested virus-free by ELISA, PCR/RT-PCR or immuno-capture PCR before mass multiplication.

Specialised culture techniques

• Anther and microspore culture: immature anthers/microspores at the uninucleate stage are cultured to give haploids; colchicine doubling gives homozygous doubled haploids in one generation, shortening breeding by 4–6 years. Guha and Maheshwari (Delhi University, 1964) first produced haploids from *Datura innoxia* anthers — a landmark Indian contribution.
• Ovule, ovary and embryo culture; embryo rescue: rescues hybrid embryos that would abort after a wide cross; also breaks seed dormancy and tests seed viability.
• Endosperm culture: gives triploid plants (useful for seedless fruits such as watermelon, apple, citrus).
• Protoplast isolation and somatic hybridisation: cell wall removed enzymatically with cellulase + macerozyme/pectinase in a mannitol/sorbitol osmoticum (0.4–0.7 M); fusion by PEG (polyethylene glycol) with high pH/Ca²⁺ or electrofusion. Fusion of two complete protoplasts gives a somatic hybrid (e.g. *Nicotiana* hybrids, pomato); fusion with an irradiated/enucleate donor gives a cybrid (cytoplasmic hybrid, used to transfer CMS and herbicide resistance). Protoplasts regenerate a wall in 2–4 days and then divide.
• Cell suspension and bioreactor culture: for secondary metabolites and biomass; measured by packed cell volume, fresh/dry weight and viability (FDA or Evans blue). Hairy root culture uses *Agrobacterium rhizogenes* (rol genes) for stable, fast, hormone-free roots producing root-specific metabolites.
• Elicitation and precursor feeding raise metabolite yield: methyl jasmonate, salicylic acid, chitosan, yeast extract.
• Germplasm conservation: slow-growth storage (4–8 °C, mannitol/ABA added, 1–2 year subculture interval) and cryopreservation in liquid nitrogen at −196 °C by vitrification/encapsulation-dehydration using PVS2 solution. NBPGR, New Delhi, runs India's National Cryogenebank.
• Protoplast/cell-level selection screens for salt, drought, herbicide or disease tolerance from large populations in a small space.

Problems, limitations and quality control

• Contamination: fungal/bacterial (visible in 2–7 days) and endophytic/latent bacteria (appear after several subcultures); controlled by LAF discipline, periodic fumigation, indexing on nutrient-rich test medium, and antibiotics only as a last resort.
• Browning/phenolic exudation: common in woody and tropical species; controlled by activated charcoal (0.1–0.3%), PVP, ascorbic/citric acid, frequent early subculture, dark incubation.
• Vitrification (glassiness)/hyperhydricity: water-logged, translucent, brittle shoots from high humidity and liquid media; reduced by vented caps, agar concentration increase, lower cytokinin.
• Shoot-tip necrosis: calcium and boron deficiency at the apex.
• Recalcitrance: cereals, legumes and mature woody tissues regenerate poorly — genotype-dependent protocols are needed.
• Somaclonal variation: genetic/epigenetic change from in vitro culture (polyploidy, aneuploidy, point mutation, transposon activation, DNA methylation change). It increases with 2,4-D, callus phase and number of subcultures, so commercial protocols cap subcultures (often 10–12 cycles for banana) and verify trueness-to-type by RAPD/SSR/ISSR or flow cytometry. Variation is a defect in micropropagation but a source of novelty in breeding (e.g. salt-tolerant and disease-tolerant somaclones; 'Pusa Jai Kisan' mustard was developed through somaclonal variation at IARI).
• Costs: labour and electricity dominate; the technique is justified only for high-value or vegetatively difficult species.
• Indian regulatory and quality framework:
  • NCS-TCP (National Certification System for Tissue Culture Raised Plants), notified under the Seeds Act, 1966, administered by DBT with BCIL as Management Agency — recognises production facilities (recognition valid 2 years, renewable) and accredits test laboratories (ATLs) for virus indexing and genetic fidelity. Recognition is voluntary - it is not a legal precondition for selling tissue-culture plants - but most leading producers hold it, and government schemes and institutional buyers usually insist on certified material.
  • Seeds Act, 1966 and Seeds (Control) Order, 1983 for sale; Protection of Plant Varieties and Farmers' Rights Act, 2001 (PPV&FRA) for variety registration; Biological Diversity Act, 2002 (as amended in 2023) and NBA approval for access to Indian biological resources.
  • Genetically modified cultures fall under the Rules, 1989 of the Environment (Protection) Act, 1986, with IBSC/RCGM/GEAC oversight; a MoEFCC order of 30 March 2022 exempts SDN-1/SDN-2 genome-edited plants free of foreign DNA from Rules 7-11 (so no GEAC clearance), with IBSC verification under DBT's 2022 genome-edited plant guidelines.
  • Mission for Integrated Development of Horticulture (MIDH) and NHB give capital subsidy for commercial tissue culture units; exact rates are revised periodically and should be checked against the current scheme guidelines.

Key points

• Totipotency — a single somatic plant cell can regenerate an entire plant; the foundation of all tissue culture.
• Explant — excised tissue used to start a culture; juvenile and meristematic tissue is most responsive.
• MS medium — Murashige & Skoog 1962, high-salt general-purpose medium; pH 5.6–5.8, 3% sucrose, 0.8% agar.
• Autoclaving — 121 °C, 15 psi, 15–20 min; heat-labile hormones instead filter-sterilised at 0.22 µm.
• Auxin:cytokinin high — roots; low — shoots; intermediate — callus (Skoog–Miller).
• 2,4-D — the strongest callus and somatic embryogenesis inducer; also the biggest cause of somaclonal variation.
• Callus — unorganised dividing cell mass; suspension culture — cells in agitated liquid medium.
• Organogenesis — shoot/root formation, direct or indirect (via callus); somatic embryogenesis gives bipolar embryos.
• Micropropagation stages — 0 mother plant, I initiation, II multiplication, III rooting, IV hardening.
• Meristem culture — 0.2–1.0 mm apical dome gives virus-free plants; confirmed by ELISA/PCR indexing.
• Anther culture — haploids (Guha & Maheshwari, 1964, *Datura*); colchicine doubling gives doubled haploids.
• Endosperm culture — triploids; embryo rescue — saves aborting wide-cross hybrids.
• Protoplast — wall-less cell from cellulase + pectinase in osmoticum; PEG or electrofusion gives somatic hybrids, cybrids with donor cytoplasm.
• Hairy roots — *A. rhizogenes*-transformed roots, hormone-independent, used for secondary metabolites.
• Synthetic seed — somatic embryo encapsulated in sodium alginate–CaCl₂ beads for storage and transport.
• Cryopreservation — liquid nitrogen at −196 °C with PVS2 vitrification; NBPGR runs India's cryogenebank.
• Somaclonal variation — in vitro genetic/epigenetic change; limit subcultures and check fidelity by SSR/ISSR or flow cytometry.
• NCS-TCP — DBT/BCIL certification of tissue-culture plant production facilities and test labs under the Seeds Act, 1966; voluntary, but the industry standard.

Formulas and examples

Example 1 — Banana micropropagation output. A lab starts with 50 sword-sucker shoot-tip explants. Multiplication rate is 4 per 4-week cycle, and 10 cycles are allowed before fidelity risk.

• Theoretical yield = 50 × 4¹⁰ = 50 × 1,048,576 = 52,428,800 shoots in 40 weeks.
• Real labs never run this out: Stage I establishment is 60% (50 → 30 usable lines), and loss at each cycle makes the effective rate about 3. Yield = 30 × 3¹⁰ = 30 × 59,049 = 1,771,470 shoots.
• If rooting is 90% and hardening survival 85%, saleable plants = 1,771,470 × 0.90 × 0.85 = 1,355,174.
• Lesson: the exponent magnifies small per-cycle losses — 4 → 3 costs a factor of ~17.8 over 10 cycles.

Example 2 — Target-driven planning. A contract needs 2,00,000 hardened sugarcane plants in 9 months (39 weeks). Starting material 200 shoots, M = 3.2 per 3-week cycle, combined rooting + hardening survival 80%.

• Shoots needed in vitro = 2,00,000 / 0.80 = 2,50,000.
• n = log(2,50,000/200)/log 3.2 = log(1,250)/log(3.2) = 3.0969/0.5051 = 6.13 → 7 cycles.
• Time = 7 × 3 = 21 weeks of multiplication + ~4 weeks Stage I + ~8 weeks hardening/nursery = 33 weeks — inside the 39-week deadline.
• Check with 7 cycles: 200 × 3.2⁷ = 200 × 3,435.97 = 6,87,195 shoots → 5,49,756 plants, comfortably above target (so fewer vessels can be run in the last cycle).

Example 3 — Preparing BAP and NAA from stock solutions. Need 1 L of MS medium with 2.0 mg/L BAP and 0.5 mg/L NAA. Stocks: BAP 1 mg/mL (dissolved in a few drops of 1 N NaOH, made up with water), NAA 0.5 mg/mL (dissolved in a little ethanol).

• BAP: C₁V₁ = C₂V₂ → V₁ = (2.0 mg × 1 L)/(1 mg/mL) = 2.0 mL of stock.
• NAA: V₁ = (0.5 mg)/(0.5 mg/mL) = 1.0 mL of stock.
• Molar check for BAP (M = 225.25 g/mol): 2.0 mg/L ÷ 225,250 mg/mol = 8.88 × 10⁻⁶ mol/L = 8.9 µM.
• Both are reasonably heat-stable and can be added before autoclaving; GA₃ in the same medium would have to be filter-sterilised and added at ~45 °C.

Example 4 — Macronutrient weighing for MS. MS requires KNO₃ 1,900 mg/L and NH₄NO₃ 1,650 mg/L. Prepare 500 mL of a 10× macro stock and then 2 L of medium.

• For 10× in 500 mL: KNO₃ = 1,900 × 10 × 0.5 = 9,500 mg = 9.5 g; NH₄NO₃ = 1,650 × 10 × 0.5 = 8.25 g.
• To make 2 L of 1× medium: take 2 L / 10 = 200 mL of the 10× stock.
• Nitrogen check: KNO₃ (101.1 g/mol) at 1,900 mg/L = 18.79 mM NO₃⁻; NH₄NO₃ (80.04 g/mol) at 1,650 mg/L = 20.61 mM, giving 20.61 mM NH₄⁺ and 20.61 mM NO₃⁻ → total NO₃⁻ = 39.4 mM, NH₄⁺ = 20.6 mM, total N ≈ 60 mM — the "high salt" signature of MS.

Example 5 — Cost per plant. A batch of 10,000 hardened plants costs: media and chemicals ₹28,000; labour ₹70,000; electricity (AC, light, autoclave) ₹45,000; vessels, closures, cocopeat, trays ₹22,000; overheads and certification ₹15,000.

• Total = 28,000 + 70,000 + 45,000 + 22,000 + 15,000 = ₹1,80,000.
• Cost per saleable plant = 1,80,000 / 10,000 = ₹18.00.
• If hardening survival falls from 85% to 70%, saleable plants = 10,000 × (70/85) = 8,235 and cost = 1,80,000 / 8,235 = ₹21.86 — a 21% rise in unit cost from one stage.

Where it is used

• Commercial micropropagation industry: banana (G-9 tissue-culture plants dominate Indian plantations), sugarcane, potato minitubers, Bt cotton seed parents, orchids, anthurium, gerbera, bamboo, teak, eucalyptus and sandalwood clones — Jain Irrigation (Jalgaon), KF Bioplants (Pune), Godrej Agrovet, Rise 'n' Shine, ITC and state horticulture mission units.
• Public R&D and certification: ICAR–IARI, NBPGR (cryogenebank, germplasm), ICAR–SBI Coimbatore (sugarcane), CPRI Shimla (seed potato), NRC Banana Trichy, CSIR–NBRI/CIMAP/IHBT, TERI, and DBT's NCS-TCP accredited test laboratories doing virus indexing and genetic fidelity testing.
• Plant genetic engineering pipelines: regeneration protocols are the bottleneck step for *Agrobacterium*-mediated or biolistic transformation and for CRISPR genome editing in rice, cotton, chickpea and brinjal.
• Phytopharmaceutical and nutraceutical production: cell suspension and hairy root cultures for withanolides (*Withania*), bacosides (*Bacopa*), artemisinin, azadirachtin, podophyllotoxin and taxol; supplies the Ayurvedic and herbal extract industry.
• Forestry and plantation clonal programmes: ITC Bhadrachalam and WIMCO clonal eucalyptus/poplar; bamboo missions; TNAU and state forest development corporations.
• Job roles: Tissue Culture Technician/Supervisor (inoculation, media kitchen, hardening), Production Manager of a TC facility, QC analyst (ELISA/PCR indexing), R&D Associate in plant transformation, Nursery/Agri-business Manager; also Agriculture Development Officer and ARS-NET academic routes.

Interview questions

• Q: What is totipotency, and why is it central to tissue culture? A: It is the capacity of a single somatic plant cell to divide and regenerate a complete plant because it retains the whole genome. Every regeneration protocol — callus, organogenesis, somatic embryogenesis — depends on it.
• Q: Which hormonal ratio gives shoots and which gives roots? A: A low auxin-to-cytokinin ratio favours shoot formation, a high ratio favours roots, and intermediate levels of both keep the tissue as proliferating callus (Skoog and Miller, 1957).
• Q: Why is medium pH set to 5.6–5.8 before autoclaving? A: It keeps nutrients, especially iron and phosphate, available, suits plant cell uptake, and allows agar to gel properly; pH also falls slightly during autoclaving and culture, so it is adjusted in advance.
• Q: How do you obtain a virus-free plant from an infected mother plant? A: Culture a 0.2–1 mm meristem tip, where the virus titre is lowest because the dome lacks vascular connections, optionally with thermotherapy at 35–38 °C, then confirm freedom by ELISA or RT-PCR indexing.
• Q: Why do you add some growth regulators after autoclaving? A: GA₃, zeatin, ABA, urea and antibiotics are heat-labile and degrade at 121 °C, so they are filter-sterilised through a 0.22 µm membrane and added to medium cooled to about 45–50 °C.
• Q: What is somaclonal variation and how do you control it in a commercial lab? A: It is genetic and epigenetic change arising in culture, promoted by 2,4-D, a callus phase and repeated subculture. Labs use direct organogenesis, cap subcultures (often 10–12 for banana) and verify trueness-to-type by SSR/ISSR markers or flow cytometry.
• Q: Why do micropropagated plants die when moved straight to the field? A: In vitro they grow in high humidity with sugar supplied, so they have a thin cuticle, non-functional stomata and poor root hairs. They need staged hardening in a mist chamber or polyhouse with humidity reduced and light increased gradually.
• Q: How are haploids produced, and what is their breeding value? A: By anther or isolated microspore culture at the uninucleate stage; doubling with colchicine gives completely homozygous doubled haploids, so a pure line is obtained in one generation instead of six or more selfing cycles.
• Q: Distinguish a somatic hybrid from a cybrid. A: A somatic hybrid results from fusing two intact protoplasts, combining both nuclear and cytoplasmic genomes. A cybrid carries the nucleus of one parent with cytoplasm (mitochondria/chloroplasts) of the other, obtained by inactivating the donor nucleus — used to transfer cytoplasmic male sterility.
• Q: Why are hairy root cultures preferred over cell suspensions for secondary metabolites? A: *A. rhizogenes*-transformed roots grow fast without exogenous hormones, are genetically and biochemically stable over long periods, and express root-specific biosynthetic pathways that dedifferentiated suspension cells often switch off.
• Q: What is hyperhydricity and how do you fix it? A: A water-soaked, glassy, brittle shoot condition caused by high in-vessel humidity, liquid media and excess cytokinin; it is reduced with vented closures, higher agar concentration, lower cytokinin and better gas exchange.
• Q: What Indian certification applies to selling tissue-culture plants? A: The National Certification System for Tissue Culture Raised Plants (NCS-TCP), set up under the Seeds Act, 1966, and run by DBT with BCIL — it recognises production facilities and uses accredited test laboratories for virus indexing and genetic fidelity before certified, labelled material is sold. Recognition is voluntary, not a legal precondition for sale, but most leading producers and government-scheme buyers rely on it.

Common mistakes

• Writing that auxin alone makes shoots — the ratio decides the organ, and it is a high auxin:cytokinin ratio that gives roots.
• Autoclaving heat-labile additives such as GA₃, zeatin or antibiotics along with the medium, then reporting a "no response" result.
• Confusing callus (unorganised mass) with somatic embryo (organised, bipolar) or calling indirect organogenesis somatic embryogenesis.
• Forgetting the osmoticum in protoplast isolation and wash solutions — naked protoplasts burst immediately in plain water or standard medium.
• Expecting a published MS + BAP protocol to work across genotypes; response is strongly genotype-, explant- and season-dependent, so protocols must be optimised.
• Quoting theoretical multiplication (N₀ × Mⁿ) as production capacity without applying establishment, rooting and hardening survival — the examiner looks for the survival-corrected figure.
• Treating virus indexing and genetic fidelity testing as optional; NCS-TCP certification is voluntary in law, but scheme buyers and serious customers expect it, and untested stock spreads viruses.
• Skipping hardening or shortening it, and overlooking that in vitro plants are heterotrophic on sucrose, not photosynthetically self-sufficient.

Related topics

Recombinant DNA technology · Cell culture techniques · PCR and DNA sequencing · Plant breeding and genetics · Secondary metabolites and phytochemistry · Agricultural biotechnology and transgenic crops