Explants Foundation of Plant Tissue Culture and Modern Propagation Techniques
Introduction
The field of plant biotechnology has witnessed significant growth with the advancement of tissue culture techniques, where explants serve as the foundational material. An explant is a piece of plant tissue taken from a parent plant and used to grow a new plant under controlled in vitro conditions. Depending on the purpose, explants may be derived from various parts of the plant such as leaves, stems, roots, or meristems.
Explants play a crucial role in the mass propagation of plants, conservation of rare species, genetic modification, and production of disease-free plants. This article delves into the types, preparation, sterilization, and applications of explants, as well as the challenges associated with their use in plant biotechnology.
Definition of Explants
In plant tissue culture, an explant refers to any small piece of plant tissue or organ that is excised and cultured in a nutrient-rich medium under sterile conditions to initiate the growth of a new plant. The success of in vitro culture depends heavily on the type, condition, and physiological age of the explant.
Types of Explants
Explants can be classified based on their origin on the parent plant:
1. Shoot Tip and Meristem Explants
- Taken from the apical or axillary meristem.
- Commonly used to produce virus-free plants in crops like banana and potato.
2. Leaf Explants
- Used in species like tobacco and strawberry.
- Ideal for callus formation and regeneration studies.
3. Root Explants
- Used to regenerate whole plants in some herbaceous species.
- Often involved in the production of secondary metabolites.
4. Stem Explants
- Include nodal segments and internodal sections.
- Useful for clonal propagation.
5. Floral Explants
- Taken from flower parts like petals, stamens, or ovules.
- Employed in embryo rescue and hybridization studies.
6. Seed and Embryo Explants
- Used in the germination of recalcitrant seeds and somatic embryogenesis.
Selection Criteria for Explants
The choice of explant significantly affects the success of tissue culture. Ideal explants should be:
- Young and actively dividing
- Free from disease and pests
- Responsive to in vitro conditions
- Genetically stable
Preparation and Sterilization
Because contamination is a major challenge in tissue culture, explants must undergo careful preparation and surface sterilization.
Steps:
- Selection and Collection: Healthy mother plants are selected, and explants are excised using sterile tools.
- Washing: Explants are washed under running water to remove dust and debris.
- Surface Sterilization:
- Treated with 70% ethanol for 30 seconds.
- Followed by a disinfectant like sodium hypochlorite (0.1–1%) for 10–15 minutes.
- Rinsed several times with sterile distilled water.
Some explants may also require treatment with antibiotics or fungicides depending on microbial load.
Culture Media and Growth Conditions
Explants are cultured in Murashige and Skoog (MS) medium or other nutrient-rich formulations. The medium is supplemented with:
- Macronutrients and micronutrients
- Vitamins
- Plant growth regulators (e.g., auxins, cytokinins)
- Carbon source (usually sucrose)
Culture conditions such as temperature (22–28°C), light intensity (16-hour photoperiod), and humidity are optimized for specific species.
Stages of Tissue Culture from Explants
- Initiation: The explant is placed on the culture medium to initiate growth.
- Multiplication: Induced shoot or callus proliferation occurs through growth regulators.
- Rooting: Shoots are transferred to a medium with auxins to promote root development.
- Hardening: Acclimatization in a greenhouse before transfer to soil.
Applications of Explants
1. Clonal Propagation
- Rapid production of genetically identical plants from elite mother plants.
2. Disease Elimination
- Meristem tip culture removes viruses and bacteria.
3. Germplasm Conservation
- Explants can be cryopreserved for long-term storage of endangered species.
4. Genetic Engineering
- Explants serve as target tissue for gene transformation techniques using Agrobacterium tumefaciens or biolistics.
5. Secondary Metabolite Production
- Cultured explants can be used to produce bioactive compounds such as alkaloids, flavonoids, and terpenoids.
Challenges in Using Explants
- Contamination: Despite sterilization, microbial contamination remains a major issue.
- Somaclonal Variation: Genetic instability may occur during long-term culture.
- Recalcitrance: Some species or explants are difficult to regenerate in vitro.
- Explant Browning: Phenolic exudation leads to tissue necrosis and culture loss.
Recent Advances
Recent innovations in explant culture include:
- Use of nanoparticles for improved sterilization.
- Microfluidics for precision nutrient delivery.
- Artificial intelligence in predicting explant responsiveness.
Conclusion
Explants are the cornerstone of modern plant tissue culture and biotechnology. Their proper selection, preparation, and culture are critical for the successful propagation and genetic improvement of plant species. Despite challenges, continuous innovation in explant-based research holds great promise for sustainable agriculture, conservation, and pharmaceutical production.
References
- George, E. F., Hall, M. A., & De Klerk, G. J. (2008). Plant Propagation by Tissue Culture: Volume 1. The Background. Springer.
- Bhojwani, S. S., & Razdan, M. K. (1996). Plant Tissue Culture: Theory and Practice. Elsevier.
- Murashige, T., & Skoog, F. (1962). A revised medium for rapid growth and bio assays with tobacco tissue cultures. Physiologia Plantarum, 15(3), 473-497.
- Thorpe, T. A. (2007). History of plant tissue culture. Molecular Biotechnology, 37(2), 169-180.
- Rout, G. R., & Mohapatra, A. (2006). Tissue culture of ornamental pot plant: A critical review on present scenario and future prospects. Biotechnology Advances, 24(6), 531–560.
