Introduction

Micropropagation is a method of plant propagation using extremely small pieces of plant tissue taken from a carefully chosen and prepared mother plant, and growing these under laboratory conditions to produce new plants. It is widely used in commercial horticulture.

Plant tissue culture refers to the practice of growing plant material in laboratories in all forms, including micropropagation, and also other techniques which although not always of immediate practical use in horticulture are very important in other fields such as plant science research and plant breeding.

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In theory all plants can be micropropagated but in practice there are not always defined laboratory protocols (exact methods with closely defined chemical and other conditions) for many plants. Considerable research is often required to find out how to micropropagate particular plants. Despite this there are five areas where micropropagation is relevant to home gardeners:

1. ‘Bulking’ up new plants: When a breeding programme results in just one or even a few plants, it can take years to ‘bulk up’ supplies and bring the plant to market. By using micropropagation the rate of increase can be speeded up by as much as ten times. The labour intensive nature of micropropagation has led to out-sourcing of micropropagation from European nurseries to contractors in low wage, typically Asian, countries, with the plantlets being returned for growing on.
2. Propagating plants that are hard to propagate by other means: Orchids are an example where the current low cost of these houseplants is due in part at least to the use of micropropagtion of these slow growing plants.
3. Producing disease free plants: Disease can be eliminated from micropropagated plants by heat treatment of cultures (plant material growing in containers such as honeypot type jars), culturing from tested known virus-free plants and by selecting the actively dividing tips (meristems) of plants for culture. Meristems are typically free of virus unlike older plants which can be infected. The availability of mini-tubers of ‘heritage’ potatoes for gardeners has been accomplished by applying these methods to the original virus infested stocks.
4. Producing plants with desirable horticultural properties: Micropropagation is sometimes used to produce plants with desirable characteristics such as speed of growth (perhaps due to lower disease levels found in plants originating from artificially cultured plant material) or bushier habit. On the other hand plants from micropropagation can be subtly different from the true type. Some plants can be very different due to genetic instability in culture and this may not be apparent until the plant flowers.
5. Conserving rare plants: The UK has a rich resource of garden plants introduced or bred decades or even centuries ago. These are at risk from introduced diseases, Phytophthora ramorum dieback for example, neglect or just age. Micropropagation of those irreplaceable subjects that are difficult to propagate by other means, can be successful in conserving them.

Micropropagation methods

A typical micropropagation laboratory will have at least the following equipment;

• a protective cabinet to allow work on plant material in clean conditions
• a gas burner to sterilise equipment
• a pressure cooker (autoclave) to sterilise media
• scalpels and tweezers
• pH meter
• heating apparatus (microwave for example) to prepare media
• suitable culture containers
• a warm, brightly lit growing area
• a clean well lit, warm, weaning greenhouse
• cold storage of media
• measuring equipment, pipettes, measuring cylinders and an accurate balance are also essential
• a dissecting microscope might also be required

The chemicals and ready-made media involved are widely available. Kits are sometimes offered for specific plants. Some suppliers may be reluctant to sell to private individuals for health and safety and for liability reasons.

Although micropropagation is technically demanding, requires scrupulous attention to detail and is potentially expensive, home micropropagation is not beyond the capabilities of determined and resourceful gardeners.

There are five stages to micropropagation:

1. Mother plant preparation. Without careful preparation the mother plants from which material for micropropagation is taken can be contaminated with viruses, fungi and bacteria that contaminate subsequent cultures. For best results mother plants are grown under cover, watered from the base and kept free of pests and diseases.
2. Culture initiation. Explants (tiny, sometimes microscopic, portions of plant material taken for culture) are taken, cleaned and disinfected and placed into the appropriate growing media in a suitable container generally of plastic and similar to a honey jar. The medium will contain selected concentrations of plant hormones (typically low or no auxins and higher cytokinin levels) that induce good quality shoots.
3. Multiplication. By altering plant hormones levels (typically low or no auxins and raised and cytokinin levels, plus higher levels of mineral supplements) shoots from the initiation stage are induced to ‘proliferate’ into clumps of tiny shoots. These clumps may be subcultured or divided into separate shoots and placed in fresh media and separate containers, the process being continued until sufficient shoots have been obtained, off-types occur or the culture loses its ability to be subcultured. This process can be seen as taking cuttings in miniature.
4. Rooting. The shoots from the multiplication stage, usually lack roots until transferred to a medium that induces root formation (typically low or no cytokinin and raised auxin levels). Rooting can also be achieved in more familiar horticultural soilless cutting media, perlite/coir mixtures for example, in humid conditions as with conventional softwood cuttings.
5. Weaning. Rooted ‘micro-plantlets’ are vulnerable to the brighter, colder, drier conditions outside the laboratory and also to the presence of pests and diseases. Skilled weaning by reducing protection and raising light levels is required if viable plants are to be obtained.

For fuller details see list of literature and online resources below.

Simple micropropagation guides for home or educational use are also published.

Problems

In practice microbial contamination of cultures can easily occur unless the skilled and scrupulous aseptic (microbe free) technique is followed.

Weaning plantlets is notoriously difficult, much more demanding than weaning ordinary rooted cuttings, but patience, practice and attention to detail brings success.

References

Books

Plants from Test Tubes: An Introduction to Micropropagation, 4th Edition (2013), by H Scoggins & M Bridgen, Timber Press; ISBN-10: 1604692065, ISBN-13: 978-1604692068

Plant Tissue Culture, Techniques and Experiments 3rd Edition by R H Smith, Academic Press, (2012),  ISBN-10: 0124159206, ISBN-13: 978-0124159204

Hartmann and Kester's Plant Propagation: Principles and Practices 8th Edition by H Hartmann, D E. Kester, FT. Davies  & R Geneve, Prentice Hall, (2010), ISBN-10: 0135014492, ISBN-13: 978-0135014493

The RHS libraries hold some micropropagation books, available to visitors. These can also be borrowed, by post if necessary, by RHS members. There is an online library catalogue.

Articles

"Micropropagation in Horticulture", by S Gunn The Plantsman (new series)  Vol. 5 (2006) pp238–243

"Saving historic Cornish plants [using micropropagation]", by B Robson The Plantsman (new series) Vol. 6 (2007) pp18–23

The RHS libraries can, for a fee, provide photocopies, by post if necessary.

On-line literature

Micropropagation for schools using African Violet leaves.

Micropropagation at home.

Hellebore micropropagation.

Videos

Many videos are available on-line, such as:

Micropropagation at Royal Botanic Gardens Kew