Micropropagation

In vitro culture of plants in a controlled, sterile environment

Micropropagation is the practice of rapidly multiplying stock plant material to produce a large number of progeny plants, using modern plant tissue culture methods.

Micropropagation is used to multiply novel plants, such as those that have been genetically modified or bred through conventional plant breeding methods. It is also used to provide a sufficient number of plantlets for planting from a stock plant which does not produce seeds, or does not respond well to vegetative propagation.

Methods

Establishment

Micropropagation begins with the collection of explant(s), that are then sterilized on their surfaces. This small portion of plant tissue, which may be as small as a cell, is placed on a growth medium, typically a medium containing sucrose as an energy source and one or more plant growth regulators (plant hormones). Usually the medium is thickened with agar to create a gel which supports the explant during growth.

The plant tissue should now begin to grow and differentiate into new tissues. For example, media containing cytokinins are used to create branched shoots from plant buds.

Multiplication

Following the successful growth of plant tissue, the establishment stage may be repeated, by taking tissue samples from the plantlets produced in the first stage. Through repeated cycles of this process, a single cell sample may be magnified to hundreds or thousands of plants.

Pretransplant

This stage involves treating the plantlets/shoots produced to encourage root growth and "hardening." It is performed in vitro, or in a sterile "test tube" environment.

Root growth does not always occur in the earlier stages in plant cell culture, and is of course a requirement for successful plant growth after the micropropagation procedure. It is performed in vitro by transferring the plantlets to a growth medium containing auxin(s).

"Hardening" refers to the preparation of the plants for a natural growth environment. Until this stage, the plantlets have been grown in "ideal" conditions, designed to encourage rapid growth. Due to lack of necessity, the plants are likely to be highly susceptible to disease and will be inefficient in their use of water and energy.

Hardening typically involves slowly weaning the plantlets from a high-humidity, low light, warm environment to what would be considered a normal growth environment for the species in question.

This stage (pretransplant) is not always performed, instead being incorporated into the last stage by encouraging root growth and hardening ex vitro, or in nonsterile plant media.

Transfer from culture

In the final stage of plant micropropagation, the plantlets are removed from the plant media and transferred to soil or (more commonly) potting compost for continued growth by conventional methods.

This stage is often combined with the "pretransplant" stage.

Advantages

Micropropagation has a number of advantages over traditional plant propagation techniques:

• Micropropagation produces disease-free plants
• Micropropagation produces rooted plantlets ready for growth, rather than seeds or cuttings
• It has an extraordinarily high fecundity, producing thousands of propagules in the same time it would take a conventional technique to produce tens or hundreds
• It is the only viable method of regenerating genetically modified cells or cells after protoplast fusion
• It is a good way of multiplying plants which produce seeds in uneconomical amounts (if at all) or whose seed can't be stored (vgr. recalcitrant seeds)
• Micropropagation often produces more robust plants, leading to accelerated growth compared to similar plants produced by conventional methods

Disadvantages

Micropropagation may appear to be the perfect means of multiplying plants, but it has associated problems:

• It is very expensive, and can have a labour cost of more than 70%
• An infected plant sample can produce infected progeny. This is uncommon, as stocks are usually carefully vetted to prevent this.
• Not all plants can be successfully tissue cultured.
• Sometimes plants or cultivars do not come true to type after being tissue cultured, this is often dependent on the type of explant material utilized during the initiation phase into culture or the result of the age of the cell or propagule line.

The greatest limitation is the cost. Most plants will naturally produce seeds, which are normally disease free and will readily grow under good conditions. The number of seeds varies, but is normally acceptable for multiplication and the seeds are relatively low cost. For this reason, many plant breeders will never resort to micropropagation because of the prohibitive cost.

Mechanisation of the process would eliminate most of the labour cost associated, but this has proven difficult so far despite active attempts to develop this technology.

See also

• Bioreactor
• Floriculture
• Fragmentation (biology)
• Plant propagation
• Plant tissue culture
• Vegetative reproduction

External links

• Micropropagation with diagrams