🎒 Techniques — Meristem Callus Somatic Embryogenesis

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

---

Techniques — Meristem Callus Somatic Embryogenesis

Meristem Culture

Meristem culture involves the excision and in vitro culture of the apical meristem (the dome-shaped group of actively dividing cells at the shoot tip) with one or two leaf primordia. The explant size is typically 0.1 to 0.5 mm.

Procedure

1. Select healthy, vigorously growing shoots as source material
2. Surface sterilize the shoot tips with sodium hypochlorite (1%) for 10 to 15 minutes followed by mercuric chloride (0.1%) for 3 to 5 minutes
3. Rinse 3 to 4 times with sterile distilled water
4. Under a stereomicroscope in the laminar flow cabinet, carefully remove outer leaves to expose the meristem dome
5. Excise the meristem with 1 to 2 leaf primordia using a fine scalpel
6. Place on MS medium supplemented with BAP (0.5 to 1.0 mg/L) and NAA (0.1 mg/L)
7. Incubate under controlled light and temperature conditions

Applications

• Virus elimination: Viruses cannot penetrate the actively dividing meristematic cells due to the absence of vascular tissue
• Genetic stability: Plants regenerated from meristems are true-to-type
• Crops: Potato, strawberry, garlic, sugarcane, banana, and sweet potato

Callus Culture

Callus is an unorganized mass of undifferentiated cells formed when plant tissue is placed on medium containing auxins. Callus culture is the foundation for many downstream applications.

Induction and Maintenance

• Explant sources: Leaf segments, stem internodes, root segments, cotyledons, immature embryos
• Auxins used: 2,4-D (most common, 0.5 to 5.0 mg/L), NAA, picloram
• Subculture interval: Every 3 to 4 weeks onto fresh medium to maintain active growth
• Types of callus: Compact (hard, nodular) and friable (soft, easily dispersible)

Organogenesis from Callus

By manipulating the auxin-cytokinin ratio in the medium:

• Shoot regeneration: Transfer callus to medium with high cytokinin (BAP 2 to 5 mg/L) and low auxin
• Root regeneration: Transfer shoots to medium with auxin (IBA or NAA 0.5 to 1.0 mg/L)
• Indirect organogenesis: Callus forms first, then organs differentiate from callus cells

Somatic Embryogenesis

Somatic embryogenesis is the formation of embryo-like structures (somatic embryos) from somatic (non-reproductive) cells without fertilization. These embryos develop through stages resembling zygotic embryo development: globular, heart, torpedo, and cotyledonary.

Two Pathways

Feature	Direct Somatic Embryogenesis	Indirect Somatic Embryogenesis
Callus phase	No callus intermediate	Embryos arise from callus
Genetic fidelity	High (true-to-type)	Lower (somaclonal variation risk)
Time required	Shorter	Longer
Explants	Immature zygotic embryos, leaf cells	Various tissues via callus

Protocol (General)

1. Induction: Place explants on medium with auxin (2,4-D at 1 to 5 mg/L) for embryogenic callus formation
2. Development: Transfer to medium with reduced or no auxin for embryo maturation
3. Maturation: Supplement with ABA or high sucrose concentration for proper development
4. Germination: Place mature somatic embryos on hormone-free or low-cytokinin medium
5. Conversion: Embryos develop into complete plantlets with shoot and root

Advantages of Somatic Embryogenesis

• Bipolar structure: Each somatic embryo has both shoot and root meristems, eliminating the separate rooting step
• Scale-up potential: Suspension cultures can produce millions of somatic embryos in bioreactors
• Synthetic seed technology: Somatic embryos encapsulated in sodium alginate beads for storage and transport

---

Summary Cheat Sheet

Topic	Key takeaway
Main focus	Techniques — Meristem Callus Somatic Embryogenesis.
Section context	Revise this lesson with the rest of Techniques & Protocols for stronger conceptual continuity.