CR Specification focus:
‘Explain how artificial plant clones are produced by micropropagation and tissue culture, and evaluate arguments for and against their use.’
Micropropagation and tissue culture enable rapid, large-scale cloning of genetically identical plants using small samples of tissue. These methods revolutionise agriculture, horticulture, and conservation through precise propagation.
The Principles of Artificial Plant Cloning
Artificial cloning replicates plants asexually, producing genetically identical offspring (clones) from a single parent plant. This avoids genetic variation caused by sexual reproduction and maintains desirable traits such as high yield, disease resistance, or ornamental qualities.
Micropropagation is a form of tissue culture—the in vitro (in glass) growth of plant tissues under controlled conditions. It allows the production of thousands of identical plants from minimal starting material.
Tissue Culture: The Basis of Micropropagation
Tissue culture involves growing plant cells on a nutrient medium under sterile laboratory conditions. The medium typically contains:
Mineral ions (e.g. nitrates, phosphates, potassium)
Vitamins and carbohydrates (often sucrose as an energy source)
Plant growth regulators (PGRs) such as auxins and cytokinins to control development.
Tissue culture: The technique of growing plant cells, tissues, or organs in a sterile, nutrient-rich medium under controlled conditions to produce new plants.
By manipulating the concentrations of auxins and cytokinins, scientists control whether cells form roots, shoots, or undifferentiated masses.
The Process of Micropropagation
Micropropagation progresses through several distinct stages, each critical to success:
Simplified flow diagram of micropropagation, from explant through callus formation, shoot multiplication, and rooting, to ex vitro transfer. The labels emphasise the asexual cloning route that preserves desirable traits. The diagram includes generic nutrient-medium context but avoids unnecessary detail. Source
1. Selection and Preparation of Explant
A small piece of tissue, the explant, is taken from a parent plant (often a meristem, leaf, or stem tip).
Plants chosen are typically disease-free and display desirable genetic traits.
Explants are sterilised using dilute bleach or alcohol to eliminate microbial contamination.
Explant: A small sample of plant tissue removed from a parent plant for use in tissue culture or micropropagation.
2. Callus Formation
The explant is placed on a nutrient agar medium containing high concentrations of auxins to encourage cell division.
Cells divide by mitosis, forming a callus, which is an undifferentiated mass of totipotent cells.
Callus developing on a Robusta coffee explant cultured on MS medium. The pale, undifferentiated tissue bulges from the explant margin, illustrating the totipotent callus stage described in tissue culture. The specific crop (coffee) is extra detail beyond the syllabus but does not change the process shown. Source
Callus: A mass of undifferentiated plant cells formed when explant tissue divides during the early stages of tissue culture.
Totipotency, the ability of a single plant cell to regenerate into a complete organism, underpins this process.
3. Shoot Induction
The callus is transferred to a medium with a higher cytokinin-to-auxin ratio, promoting shoot formation.
Tiny shoots, or plantlets, develop over several weeks.
Relationship between auxin–cytokinin ratios and developmental outcomes in tissue culture: shoots at high cytokinin, roots at high auxin, and callus at intermediate levels. This visual distils the practical hormone manipulation central to micropropagation. The hosting page also shows process photos not required by the syllabus; the chart itself is the relevant element. Source
4. Root Formation
Shoots are transferred again to a medium with a higher auxin-to-cytokinin ratio to stimulate root growth.
Once roots and shoots have formed, the plantlets resemble miniature plants.
5. Acclimatisation
The plantlets are washed to remove agar and transferred to sterile compost or soil.
Initially, they are kept in humid conditions to prevent water loss through transpiration, as their cuticles and stomata are still underdeveloped.
Gradually, they are exposed to normal greenhouse or outdoor conditions.
Applications of Micropropagation
Micropropagation is used in various sectors for different goals:
Horticulture and Commercial Agriculture
Rapid production of ornamental plants, fruit crops, and vegetables (e.g. orchids, potatoes, bananas).
Ensures uniformity of traits such as flower colour or fruit quality.
Enables year-round plant availability, independent of seasonal limits.
Conservation Biology
Propagation of rare or endangered species when natural reproduction is slow or seeds are difficult to germinate.
Preserves genetic stock of valuable or wild plant varieties.
Genetic Research
Provides genetically identical material for experiments, ensuring results are due to treatments rather than genetic differences.
Facilitates development of genetically modified plants by incorporating transgenes into cultured tissues.
Advantages of Micropropagation
Micropropagation offers several significant benefits:
Speed and scale: Thousands of clones can be produced from minimal tissue in a short time.
Genetic uniformity: All offspring possess the parent’s desirable traits.
Disease-free plants: Starting with meristem tissue, often free of viruses, produces clean stock.
Conservation value: Allows multiplication of rare or endangered plants without removing many individuals from the wild.
Year-round propagation: Independent of weather and seasonal constraints.
Space efficiency: Requires far less growing area than field propagation.
Long-term storage: Cultured tissues can be cryopreserved for future use.
Disadvantages and Ethical Considerations
Despite its advantages, micropropagation raises biological, economic, and ethical issues.
Loss of genetic diversity: Cloning reduces variation, making populations more vulnerable to disease or environmental changes.
High cost and technical expertise: Requires specialised equipment and skilled staff, limiting use by small-scale growers.
Contamination risk: A single infection in culture may destroy thousands of developing clones.
Acclimatisation failures: Many plantlets fail to survive the transition from sterile to natural conditions.
Intellectual property issues: Cloning of patented plant varieties may infringe breeder rights and raise ethical concerns about ownership of living material.
Longevity and Performance
Clones may accumulate somaclonal variation (genetic mutations occurring in culture), which can alter traits or reduce viability over generations.
Somaclonal variation: Genetic changes that arise in plant tissue cultures, resulting in clones that differ slightly from the parent plant.
While sometimes undesirable, somaclonal variation can also be exploited for crop improvement by generating new genetic diversity.
Evaluation: For and Against Micropropagation
For:
Ensures rapid, large-scale, disease-free propagation of elite plants.
Enables restoration of endangered species and preservation of genetic resources.
Supports global agriculture through predictable and uniform yields.
Against:
Reduces genetic variability and adaptability.
Involves expensive laboratory procedures.
Raises ethical concerns over ownership and manipulation of living organisms.
Micropropagation thus represents a powerful but double-edged tool—essential in modern biotechnology, yet requiring careful ethical and ecological management to ensure sustainability and genetic security.
FAQ
Micropropagation is a laboratory-based technique that uses sterile, controlled environments and nutrient media to grow plants from small tissue samples or even single cells.
Traditional vegetative propagation, such as taking cuttings or grafting, occurs in soil or compost without sterile conditions.
Micropropagation can produce thousands of identical plants rapidly and is ideal for species with poor seed viability or limited natural propagation methods, whereas traditional methods are slower and more prone to disease transmission.
Meristem tissue, found in shoot or root tips, is composed of actively dividing, undifferentiated cells.
These cells are virus-free because pathogens rarely penetrate this region, ensuring that cloned plants remain healthy.
Additionally, meristem cells have high totipotency, meaning they can differentiate into all types of plant tissues, leading to higher success rates in tissue culture compared to mature, specialised cells.
Sucrose acts as an energy source for cells in culture since photosynthesis cannot occur effectively under artificial lighting or in opaque media.
It provides the carbohydrate required for respiration and biosynthesis during cell division and differentiation.
Without an external energy supply like sucrose, cultured tissues would lack the energy to maintain metabolism and growth, halting development before callus or shoot formation can occur.
Sterility is essential to stop bacteria or fungi from outcompeting plant tissues.
Key precautions include:
Sterilising explants using alcohol or sodium hypochlorite.
Autoclaving glassware, instruments, and media to eliminate microorganisms.
Working within a laminar flow hood that filters airborne contaminants.
Using gloves and sterilised tools when transferring cultures.
Even a single contaminated sample can spread pathogens throughout a culture batch, destroying hundreds of developing clones.
Although the process is designed to produce clones, somaclonal variation can arise due to mutations during extended periods in culture.
These genetic changes often occur in the callus stage when cells divide rapidly under artificial hormone conditions.
Some variations may alter plant traits such as size, colour, or yield. While usually undesirable, breeders occasionally exploit this variation to create new cultivars with beneficial characteristics.
Practice Questions
Question 1 (2 marks)
Explain why micropropagation produces genetically identical plants.
Mark Scheme:
1 mark for stating that micropropagation uses cells or tissues from a single parent plant.
1 mark for explaining that cell division occurs by mitosis, producing clones with identical genetic material (DNA).
Question 2 (5 marks)
Describe the main stages involved in micropropagation and explain one advantage and one disadvantage of using this technique in commercial agriculture.
Mark Scheme:
1 mark: Identification of taking an explant (small piece of tissue) from a parent plant.
1 mark: Description of callus formation from the explant when placed on a nutrient medium containing plant growth regulators (auxins).
1 mark: Reference to manipulation of hormone ratios (cytokinins and auxins) to promote shoot and root formation.
1 mark: Mention of transfer of plantlets to soil or compost after rooting (acclimatisation).
1 mark: Explanation of one advantage (e.g. rapid production of disease-free or genetically uniform plants) and one disadvantage (e.g. reduced genetic diversity or high cost).
