Plant tissue culture, the growth of plant cells outside an intact plant, is a
technique essential in many areas of the plant sciences. Cultures of individual
or groups of plant cells, and whole organs, contribute to understanding both
fundamental and applied science.
It relies on maintaining plant cells in aseptic conditions on a suitable
nutrient medium. The culture can be sustained as a mass of undifferentiated
cells for an extended period of time, or regenerated into whole plants.
Designing a strategy to culture cells from a plant for the first time can still
seem like a matter of trial and error, and luck. However, the commercial
production of valuable horticulture crops by micropropagation, which relies on
tissue culture, shows that it exists in the routine, as well as experimental world.
Plant cells can be grown in isolation from intact plants in tissue culture
systems. The cells have the characteristics of callus cells, rather than other plant
cell types. These are the cells that appear on cut surfaces when a plant is
wounded, and which gradually cover and seal the damaged area.
Pieces of plant tissue will slowly divide and grow into a colorless mass of
cells if they are kept in special conditions. These are:
- initiated from the most appropriate plant tissue for the particular plant
- the presence of a high concentration of auxin and cytokinin growth
regulators in the growth media
- a growth medium containing organic and inorganic compounds to sustain
- aseptic conditions during culture to exclude competition from microorganisms.
The plant cells can grow on a solid surface as friable, pale-brown lumps
(called callus), or as individual or small clusters of cells in a liquid medium
called a suspension culture. These cells can be maintained indefinitely, provided
they are subcultured regularly into fresh growth medium.
Tissue culture cells generally lack the distinctive features of most plant
cells. They have a small vacuole, lack chloroplasts and photosynthetic pathways,
and the structural or chemical features that distinguish so many cell types
within the intact plant are absent. They are most similar to the undifferentiated
cells found in meristematic regions that become fated to develop into each cell
type as the plant grows. Tissue culture cells can also be induced to redifferentiate
into whole plants by alterations to the growth media.
Plant tissue cultures can be initiated from almost any part of a plant. The
physiological state of the plant does have an influence on its response to
attempts to initiate tissue culture. The parent plant must be healthy and free
from obvious signs of disease or decay. The source, termed explant, may be
dictated by the reason for carrying out the tissue culture. Younger tissue contains
a higher proportion of actively dividing cells and is more responsive to a callusinitiation
program. The plants themselves must be actively growing, and not
about to enter a period of dormancy.
The exact conditions required to initiate and sustain plant cells in culture,
or to regenerate intact plants from cultured cells, are different for each plant
species. Each variety of a species will often have a particular set of cultural
requirements. Despite all the knowledge that has been obtained about plant
tissue culture during the twentieth century, these conditions have to be identified
for each variety through experimentation.