Micropropagation, Multiple Shoot Induction & Ex Vitro Acclimatization of Nodal Explants
Micropropagation is the true-to-type, clonal in vitro propagation of plants under controlled aseptic conditions. Formulated systematically by Toshio Murashige in 1974, commercial micropropagation proceeds through five defined stages:
- Stage 0 (Donor Plant Preparation): Cultivating mother plants in regulated greenhouses with controlled hygiene and fungicidal regimens to minimize endogenous systemic endophytes.
- Stage 1 (Culture Initiation & Surface Asepsis): Surface disinfection of explants (nodal segments, shoot apices) with chemical sterilants ($HgCl_2$, $NaOCl$) and establishment on nutrient medium without microbial contamination.
- Stage 2 (Shoot Multiplication): Inducing release of axillary bud dormancy and multiple adventitious shoot bud proliferation by manipulating the Cytokinin-to-Auxin ratio. A high cytokinin concentration (e.g., 1.5 to 2.5 mg/L 6-benzylaminopurine, BAP) suppresses apical dominance, stimulating 5 to 12 shoots per explant per subculture cycle.
- Stage 3 (In Vitro Rooting / Rhizogenesis): Transferring elongated micro-shoots (≥2 cm) to low-salt medium (1/2 MS) with an optimal auxin concentration (e.g., 0.5 to 1.0 mg/L IBA or NAA) to trigger adventitious root formation.
- Stage 4 (Acclimatization & Hardening): Transferring plantlets from high-humidity, heterotrophic culture vessels to ex vitro potting substrates. In vitro plantlets lack epicuticular wax layers and have non-functional stomata; gradual humidity reduction (95% to 60%) inside mist chambers induces functional stomatal regulation and autotrophy.
The Skoog and Miller (1957) Hormonal Organogenesis Model:
Morphogenic organogenesis is dictated by the quantitative ratio of auxin to cytokinin:
$$text{High Cytokinin : Low Auxin} longrightarrow text{Shoot Bud Proliferation (Caulogenesis)}$$
$$text{High Auxin : Low Cytokinin} longrightarrow text{Adventitious Rooting (Rhizogenesis)}$$
$$text{Balanced Auxin : Cytokinin} longrightarrow text{Unorganized Callus Proliferation}$$
- Explant Material: 1.0 to 1.5 cm juvenile nodal segments bearing axillary buds from actively growing shoots of *Bacopa monnieri* or *Mentha*.
- Surface Sterilants: 70% (v/v) ethanol; 0.1% (w/v) Mercuric chloride ($HgCl_2$) or 1.5% Sodium hypochlorite ($NaOCl$); Tween-20 surfactant.
- Nutrient Media Formulations:
– Initiation Medium (Stage 1): MS basal salts + 30 g/L sucrose + 0.5 mg/L BAP, pH 5.8, 0.8% agar.
– Multiplication Medium (Stage 2): MS basal salts + 30 g/L sucrose + 2.0 mg/L BAP + 0.2 mg/L NAA, pH 5.8, 0.8% agar.
– Rooting Medium (Stage 3): 1/2 MS basal salts + 15 g/L sucrose + 1.0 mg/L Indole-3-butyric acid (IBA), pH 5.8, 0.8% agar. - Acclimatization Substrates: Autoclaved soilrite : perlite : vermiculite mixture (1:1:1 by volume) saturated with 1/4 MS macronutrients.
Micropropagation Equations:
1. Multiplication Rate ($M$):
$$M = frac{text{Total usable shoots harvested}}{text{Total explants subcultured}}$$
2. Theoretical Annual Multiplication Potential ($N_t$):
$$N_t = N_0 times (M)^k quad (text{where } k = text{number of 4-week subculture cycles per year} = 12)$$
3. Hardening Survival Rate (%):
$$text{Survival Rate (%)} = frac{text{Number of viable autotrophic plants at Day 30}}{text{Total plantlets transferred ex vitro}} times 100$$
| Hormone Concentration (mg/L) | Shoots / Explant | Mean Shoot Length (cm) | Rooting Response (%) | Roots / Shoot |
|---|---|---|---|---|
| MS0 (Control, No Hormones) | $1.2 pm 0.2$ | $3.8 pm 0.4$ | 15.0% | $1.5 pm 0.3$ |
| 1.0 BAP + 0.1 NAA | $4.5 pm 0.6$ | $3.2 pm 0.3$ | 0.0% | 0 |
| 2.0 BAP + 0.2 NAA (Optimal Mult.) | $8.4 pm 0.9$ | $2.8 pm 0.3$ | 0.0% | 0 |
| 1/2 MS + 1.0 IBA (Rooting) | $1.0 pm 0.0$ | $4.6 pm 0.5$ | 95.0% | $7.8 pm 0.8$ |
- Vitrification (Hyperhydricity): Shoots cultured under high cytokinin or in tightly sealed vessels may develop glassy, translucent, brittle, water-soaked leaves that fail to survive hardening. Remediate by increasing gelling agent concentration (from 0.8% to 1.0% agar) and using vented vessel lids.
- Complete Agar Removal During Deflasking: Incomplete rinsing of agar from delicate root hairs is the single leading cause of post-transplant damping-off fungal infection in greenhouses. Clean every trace under lukewarm water before potting.
Ans: 6-Benzylaminopurine (BAP) is a synthetic adenine-type cytokinin that is significantly more stable, resistant to enzymatic degradation by plant cytokinin oxidase/dehydrogenase (CKX), and orders of magnitude less expensive than natural zeatin, providing highly cost-effective and reproducible axillary shoot bud break.
Ans: In vitro culture vessels have nearly 100% relative humidity, low light, and external sucrose. Consequently, plantlets develop (1) a thin or absent epicuticular wax layer, (2) abnormal, non-closing stomata that remain locked open, and (3) reduced photosynthetic Rubisco activity. Sudden exposure to ambient dry air causes immediate fatal desiccation.
Ans: Full-strength MS medium contains high levels of inorganic nitrogen salts ($NH_4NO_3$ and $KNO_3$) which inhibit root induction and promote leafy callus overgrowth at the stem base. Halving the salt concentration reduces nitrogen inhibition while providing adequate micronutrients for adventitious root primordia.
Ans: Hyperhydricity is a physiological malformation caused by excessive water retention in intercellular spaces, triggered by high relative humidity, low agar concentration (high water matrix potential), excess cytokinin levels, ammonium toxicity, or ethylene accumulation in sealed containers.
