Here’s a comprehensive overview of bioreactor systems for plant cell suspension and callus culture, tailored for applications in secondary metabolite production and large-scale plant propagation:
1. Introduction
Bioreactors are controlled vessels designed for mass cultivation of plant cells, tissues, or organs under sterile, monitored conditions. They enable scaling up of callus cultures, cell suspensions, and somatic embryos for research, pharmaceutical, and commercial applications.
Advantages over flask cultures:
- High biomass yield
- Controlled aeration, pH, and nutrient supply
- Reduced labor and contamination risk
- Suitable for secondary metabolite production
2. Cell Suspension vs. Callus Culture
| Feature | Callus Culture | Cell Suspension Culture |
|---|---|---|
| Form | Undifferentiated cell mass, semi-solid | Single cells or small aggregates in liquid medium |
| Medium | Solid or semi-solid | Liquid (agitated) |
| Growth Rate | Slower | Faster; higher growth potential |
| Applications | Induction of organs, embryogenesis | Secondary metabolite production, large-scale propagation, biotransformation |
Transition: Callus tissue is often used as the starter material for suspension cultures, where cells are released into liquid medium and grown in bioreactors.
3. Bioreactor Systems for Plant Cultures
A. Types of Bioreactors
1. Stirred Tank Bioreactors
- Cylindrical vessels with mechanical agitation (impeller) and aeration.
- Advantages: Good mixing, oxygen transfer, scalable.
- Challenges: Shear stress can damage fragile plant cells.
2. Air-Lift Bioreactors
- Circulation is driven by air bubbles instead of mechanical stirring.
- Advantages: Low shear stress, good aeration.
- Applications: Sensitive plant cell suspensions.
3. Bubble Column Bioreactors
- Simple vertical vessels with air pumped from the bottom.
- Advantages: Low cost, low shear stress, easy scale-up.
- Disadvantages: Less efficient mixing than stirred tanks.
4. Temporary Immersion Bioreactors (TIB)
- Explants or embryos are intermittently immersed in nutrient medium.
- Advantages: Reduces hyperhydricity, improves gas exchange.
- Applications: Somatic embryos, organ cultures, micropropagation.
5. Wave Bioreactors
- Flexible plastic bags on a rocking platform; gentle agitation.
- Advantages: Low shear stress, disposable, scalable.
- Applications: Suspension cultures, virus-free plant propagation.
B. Key Parameters for Bioreactor Culture
| Parameter | Importance | Typical Range / Notes |
|---|---|---|
| Medium composition | Provides nutrients & hormones | MS, B5, WPM, with auxins/cytokinins for desired growth |
| pH | Affects enzyme activity & cell growth | Usually 5.5–6.0 |
| Temperature | Optimizes metabolism | 24–28°C (species dependent) |
| Dissolved Oxygen (DO) | Essential for aerobic metabolism | 20–50% saturation |
| Agitation | Prevents sedimentation, ensures nutrient mixing | Stirring speed: 50–120 rpm (low shear preferred) |
| Inoculum density | Initial cell concentration | 0.5–2 g/L fresh weight (callus) |
| Light | For photomixotrophic cultures | 16 h light / 8 h dark (if photosynthetic cells) |
4. Applications in Plant Biotechnology
- Secondary Metabolite Production
- Examples: paclitaxel (Taxus), ginsenosides (Panax), agarwood resin compounds (Aquilaria)
- Bioreactors allow controlled elicitation (methyl jasmonate, fungal elicitors) to increase metabolite yield.
- Mass Propagation
- Somatic embryos and organ cultures can be scaled up in TIBs or wave bioreactors.
- Genetic Transformation
- Suspension cultures can be transformed using Agrobacterium or particle bombardment.
- Synthetic Seed Production
- Somatic embryos grown in bioreactors are harvested for encapsulation.
5. Challenges
- Shear sensitivity of plant cells → requires gentle mixing
- Foam formation → needs antifoaming agents
- Hyperhydricity (vitrification) → controlled immersion & aeration needed
- High cost of large-scale operation
- Medium optimization is species-specific
6. Future Prospects
- Automated monitoring of pH, oxygen, and metabolites
- Elicitor-fed bioreactors for enhanced secondary metabolite production
- Integration with bioprocess engineering and synthetic biology for valuable compounds
- Use in commercial agarwood resin precursor production