Organogenesis

Organogenesis

Somatic embryogenesis relies on plant regeneration through a process analogous to zygotic embryo germination. Organogenesis relies on the production of organs, either directly from an explant or from a callus culture. There are three methods of plant regeneration via organogenesis.

The first two methods depend on adventitious organs arising either from a callus culture or directly from an explant. Alternatively, the third method is by axillary bud formation and growth, which can also be used to regenerate whole plants from some types of tissue culture.

Organogenesis relies on the inherent plasticity of plant tissues, and is regulated by altering the components of the medium. In particular, it is the auxin to cytokinin ratio of the medium that determines which developmental pathway the regenerating tissue will take.

It is usual to induce shoot formation by increasing the cytokinin to auxin ratio of the culture medium. These shoots can then be rooted relatively simply.



Organogenesis in tobacco (Nicotiana tabacum)

Organogenesis from tobacco pith callus is the classical example of how varying plant growth regulator regimes can be used to manipulate the pattern of regeneration from plant tissue cultures.

When cultured on a medium containing both auxin and cytokinin, callus will proliferate. If the auxin to cytokinin ratio is increased, adventitious roots will form from the callus by organogenesis.

If the auxin to cytokinin ratio is decreased, adventitious shoots will be formed. If the explants are cultured on medium containing only a cytokinin, shoots can be produced directly.

Tobacco plants can also be easily regenerated from tobacco leaf pieces. Leaves are cut into approximate 1-cm squares with a sterile scalpel (avoiding large leaf veins and any damaged areas). The leaf pieces are then transferred (right side up) to gelled MS medium supplemented with 1 mg l−1 BAP and 0.1 mg l−1 NAA . Over the next few weeks, callus forms on the explants, particularly around the cut surfaces. After 3–5 weeks, shoots emerge directly from the explants or from callus derived from the explants. When these shoots are about 1 cm long, they can be cut at the base and placed on to solid MS medium without any plant growth regulators. The shoots will form roots and plantlets that will grow in this medium and can subsequently be transferred to soil.