From explant to plantlet

The starting piece of plant tissue is the explant: a piece of leaf, an apical meristem (growing tip), an embryo, a cotyledon or a hypocotyl. Immature embryos usually respond best, then apical meristems, then leaves.

An explant can regenerate a plant in two ways:

• Organogenesis: the explant's specialised cells first lose their specialisation and divide into an undifferentiated mass (dedifferentiation). Then, guided by auxin and cytokinin, they specialise again and form shoot or root buds (redifferentiation).
• Somatic embryogenesis: embryos form from ordinary body (somatic) cells, going through the same stages as embryos formed from a fertilised egg.

A tissue culture laboratory needs a washing area, space for preparing media, equipment to sterilise and store them, and an aseptic culture room where light, temperature and humidity are controlled.

The steps, for example with tomato:

• Sterilise the medium: in an autoclave, or by passing heat-sensitive parts through a micropore filter.
• Choose the explant.
• Surface-sterilise it, usually with sodium hypochlorite, then rinse it in sterile water. (Explants from seedlings grown in sterile conditions need not be treated.)
• Inoculate: place it on the medium.
• Let it grow into a callus; move the callus to a shoot-forming medium; cut off the shoots and place them on a rooting medium.
• Harden the young plantlets: grow them in sterile soil in a greenhouse, so they adjust gradually to normal conditions, before planting them in the field.