What is plant tissue culture?
Plant tissue culture means growing plant parts outside the plant, on a clean food mix, in a closed container. It is also called in vitro culture ("in glass").
It works because of totipotency: almost every living plant cell has the full set of instructions (genes) and can grow into a whole plant if given the right food and signals.
The piece we start with is the explant. It can be a shoot tip, a bit of leaf or stem, a root tip, an embryo, an anther or even a single cell.
Steps of plant tissue culture
- Choose and clean the explant: take it from a healthy plant. Wash it in mild bleach or alcohol, then rinse with sterile water (surface sterilisation).
- Prepare the medium: a nutrient medium (the best known is MS medium) has sucrose for energy, macro- and micro-nutrient salts, vitamins, amino acids, plant hormones (growth regulators) and agar to make it a jelly. It is autoclaved.
- Inoculate: place the explant on the medium inside a laminar air-flow cabinet, which blows filtered, germ-free air.
- Incubate: about 25 °C with about 16 hours of light a day.
- Callus and organ formation: cells divide into a callus, then form shoots and roots (organogenesis) or tiny embryos (somatic embryogenesis).
- Multiply (subculture): shoots are cut and moved to fresh medium again and again.
- Hardening (acclimatisation): plantlets are moved to pots with high humidity, then slowly to normal air, then to the field.
Auxin and cytokinin: the steering wheel
Auxin (like IAA, NAA, 2,4-D) and cytokinin (like kinetin, BAP) are plant hormones. Their ratio decides what the callus becomes:
- High cytokinin, low auxin → shoots
- High auxin, low cytokinin → roots
- Both about equal → callus keeps growing, or both shoots and roots form
This idea was shown in tobacco tissue in the 1950s by Skoog and Miller.
Types and uses
- Micropropagation: making thousands of identical plants (clones, called somaclones) quickly in a small space, all year. Used for banana, sugarcane, potato, orchids, teak, bamboo.
- Meristem culture → virus-free plants: the very tip of a shoot (the meristem) usually has no virus, so plants grown from it are healthy. Used for banana, potato, sugarcane.
- Anther / pollen culture: gives haploid plants, useful to breed pure lines faster.
- Embryo rescue: growing an embryo that would die in the seed (for example in some crosses).
- Somatic hybridisation: cell walls are removed with enzymes to make protoplasts; two protoplasts are fused to make a hybrid, like the pomato (potato + tomato).
- Cell suspension culture: cells grow in liquid in bioreactors to make useful chemicals (medicines, colours).
- Transgenic plants: a new gene is put into cultured cells, and whole plants are grown from them (for example, insect-resistant Bt cotton). Such plants are tested for biosafety (effects on health and nature) before farmers may grow them.
- Conservation: rare plants are saved as cultures or frozen (cryopreservation in liquid nitrogen at −196 °C).
Advantages and limits
Advantages: many plants from one piece; all identical; disease-free; needs little space; works all year; saves rare plants.
Limits: needs a clean lab, skill and money; one germ can spoil a whole batch; all plants are the same, so one new disease can hit them all (low genetic variety); sometimes small random changes appear (somaclonal variation).
Try it at home
A kitchen version of the same idea (cloning without a lab): put a mint or money-plant cutting in a glass of water. Count the roots every two days for two weeks. Then stand a carrot top in a saucer of water and watch new leaves grow. In the 3D, use the auxin–cytokinin slider in step 4 to see roots or shoots.
Key formulas and definitions
- Totipotency: one plant cell can grow into a whole plant
- High cytokinin : auxin → shoots; high auxin : cytokinin → roots
- Plantlets after n cycles = (shoots per cycle)ⁿ
- Medium = sucrose + mineral salts + vitamins + hormones + agar
- Incubation: about 25 °C, 16 h light
Worked examples
1. Each shoot gives 4 new shoots per 4-week cycle. How many plantlets from one explant after 5 cycles?
4⁵ = 1024 plantlets in about 20 weeks.
2. A lab needs 10 000 banana plantlets. Each cycle multiplies by 5. How many cycles from one explant?
5⁵ = 3125 (not enough), 5⁶ = 15 625 (enough). So 6 cycles.
3. A potato crop is full of virus. How can a farmer get healthy plants of the same variety?
Take the shoot meristem (very tip, usually virus-free), culture it, and multiply. The new plants are the same variety but free of the virus.
Common mistakes
- Thinking callus is a disease or a seed. Callus is just a lump of dividing, unspecialised plant cells.
- Swapping the hormones: more cytokinin gives shoots, more auxin gives roots, not the other way.
- Forgetting hardening. Plantlets from a jar die if planted straight in the field, because the jar air is very humid.
- Saying tissue-culture plants have new variety. Micropropagated plants are clones, the same as the parent.