Plant research often involves growing new plants in a controlled environment. These may be plants that we have genetically altered in some way or may be plants of which we need many copies all exactly alike. These things can be accomplished through tissue culture of small tissue pieces from the plant of interest. These small pieces may come from a single mother plant or they may be the result of genetic transformation of single plant cells which are then encouraged to grow and to ultimately develop into a whole plant. Tissue culture techniques are often used for commercial production of plants as well as for plant research.
Tissue culture involves the use of small pieces of plant tissue (explants) which are cultured in a nutrient medium under sterile conditions. Using the appropriate growing conditions for each explant type, plants can be induced to rapidly produce new shoots, and, with the addition of suitable hormones new roots. These plantlets can also be divided, usually at the shoot stage, to produce large numbers of new plantlets. The new plants can then be placed in soil and grown in the normal manner.
Many types of plants are suitable for use in the classroom. Cauliflower, rose cuttings, African violet leaves and carnation stems will all easily produce clones (exact genetic copies) through tissue culture. Cauliflower florets in particular give excellent results since they can be grown into a complete plant in the basic tissue culture media, without the need for additional growth or root hormones. Green shoots are generally observable within three weeks, and roots develop within six weeks.
The most important part of this activity, however, is to maintain as sterile an environment as possible. Even one fungal spore or bacterial cell that comes into contact with the growth medium will rapidly reproduce and soon completely overwhelm the small plant piece that you are trying to clone.
1 vial of Murashige Skoog (MS) medium. (If you wish to make up your own growing medium you could use the recipe for the Murashige medium given at the end of this section.)
1 L sterile distilled water
8 g of agar/L
30 g sucrose/L
1.5-L or 2-L container in which to prepare the growth medium
Small amounts of 1M NaOH and 1M HCl to adjust the pH of the medium
30 flat-bottom culture tubes with closures
Glass aquarium or box lined with plastic
Plastic sheet to cover the top of the aquarium
10% bleach in a spray bottle
70% alcohol in a spray bottle
Forceps or tweezers
Cutting equipment such as a scalpel blade or razor blade
2 bottles of sterile distilled water (purchase at the grocery store)
Your chosen plant (cauliflower, rose, African violet, or carnation)
Paper towel for cutting on, or sterile petri dishes if available
Beaker or container in which to wash the plant material
Detergent-water mixture - 1 ml detergent per liter of water
Bleach sterilizing solution - dilute commercial bleach (5-6% sodium hypochlorite) to a final concentration of 1-2% sodium hypochlorite in distilled water in a large beaker or jar.
2 or 3 beakers or containers of sterile water
A well-lit area away from direct sunlight or use full-spectrum gro-lights
Hormones such as BAP (benzylaminopurine) and NAA (naphthalene acetic acid) to stimulate growth and root development, respectively. (Commercial rooting hormone solutions and powders are also available from hardware stores.)
Preperation and sterilization of growing medium (when not provided pre-poured
These steps will make 1 L of growth medium, which is enough to prepare about 65 growing tubes.
Note:If you wish to use plants other than cauliflower, you need to prepare two different media which contain plant hormones necessary to stimulate development of differentiated tissues. The first one should contain a cytokinin such as BAP which promotes shoot formation and the second one a rooting hormone such as NAA or store bought rooting hormone. To do this, prepare the mixture up until the end of step 2. Keeping the mixture warm so that it does not solidify, divide it equally into two pre-warmed containers. Each container can be used to prepare about 30 tubes as above. The first container should have BAP added at the rate of 2.0 mg/l. The second container should have the NAA hormone added at the rate of 0.1 mg/L. To do this, it is necessary to make concentrated solutions of both BAP (2.0 mg/ml) and NAA (1.0 mg/ml). Add 1 ml of the concentrated BAP stock or 100 µl of the NAA concentrated stock to each 1 liter of medium that you prepare. If you use rooting hormone purchased from your local hardware or nursery supply store instead of NAA, then just follow the directions before adding to your medium.
A classroom transfer chamber can be made from a clean glass aquarium turned on its side. Scrub the aquarium thoroughly with a 30% bleach solution, making sure that you wear gloves and do not inhale the fumes. Rinse with sterile distilled water, turn upside down on a clean counter or paper towels and allow to dry. Cut holes in a clean plastic sheet to allow arms to reach into the chamber and reinforce the cut edges with tape if necessary. Tape the clean plastic sheet over the open side of the aquarium making sure that the arm holes are located at a convenient height. Plastic sleeves could also be fitted to these holes if you wish to make it easier to prevent the entry of airborne spores into the chamber. The finished aquarium chamber can be sterilized by spraying with 10% chlorox bleach just prior to each use and drying with sterile paper towel.
Wrap the forceps, scalpels, razor blades, paper towel and gloves (rubber or surgical) in aluminum foil, seal with tape and sterilize by processing them in a pressure cooker for twenty minutes. These items can also be sterilized by placing in an oven at 350°F for 15 minutes. You can wrap each item separately or put together a "kit" so that each student will have their own sterile equipment to use.
Alternatively the forceps and blades can be sterilized by dipping in 10% bleach and then rinsing in sterile water, or dipping in alcohol and then placing in a flame, although this is not recommended for use in crowded classrooms. If you choose to dip in bleach and rinse in sterile water, it is best if fresh solutions are available for each 3-4 students since the water can easily be contaminated if care is not used. These liquid containers should only be opened once they are inside of the sterile chamber.
Your plant material must first be surface-sterilized to remove any bacteria or fungal spores present. We aim to kill all microorganisms, but at the same time not cause any adverse damage to the plant material.
Note 1: At this point, the tissue is considered sterile. All subsequent rinses should be done with sterile water, and all manipulations of the tissue performed with sterile instruments and supplies. Open one container at a time and never leave the lid off longer than necessary.
Note 2: Many students will not fully appreciate the importance of carefully sterilizing explants and so there will be some cultures that become contaminated with bacterial or fungal growth. If you do not wish to emphasize this aspect of the laboratory, students can be provided with plant materials that the instructor has already sterilized prior to use by the class.
Use the sterile gloves and equipment for all of these steps.
Figure 1 - Click to Enlarge
Once roots are well formed the plants are ready to be transferred into soil.
Figure 2 - Click to Enlarge
Sourced from Biotechnology Australia http://www.biotechnologyonline.gov.au/
A similar protocol for propagating strawberry runners can be found at: www.accessexcellence.org/LC/ST/st2bgplantprot.html