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E. F. George et al. (eds.), Plant Propagation by Tissue Culture 3rd Edition, 1-28. © 2008 Springer.

• Embryo cultures, where fertilised or unfertilised zygotic (seed) embryos are dissected out of developing seeds or fruits and cultured in vitro until they have grown into seedlings. Embryo culture is quite distinct from somatic embryogenesis (see below).

These types of cultures are described in more detail later in this chapter.

2.2. CULTURES OF UNORGANISED TISSUES

'Tissue culture' is commonly used as a collective term to describe all kinds of in vitro plant cultures although strictly it should refer only to cultures of unorganised aggregates of cells. In practice the following kinds of cultures are most generally recognised:

• Callus (or tissue) cultures. The growth and maintenance of largely unorganised cell masses, which arise from the uncoordinated and disorganised growth of small plant organs, pieces of plant tissue, or previously cultured cells.

• Suspension (or cell) cultures. Populations of plant cells and small cell clumps, dispersed in an agitated, that is aerated, liquid medium.

• Protoplast cultures. The culture of plant cells that have been isolated without a cell wall.

• Anther cultures. The culture of complete anthers containing immature pollen microspores. The objective is usually to obtain haploid plants by the formation of somatic embryos (see below) directly from the pollen, or sometimes by organogenesis via callus. Pollen cultures are those initiated from pollen that has been removed from anthers.

2.3. USING TISSUE CULTURES FOR PLANT PROPAGATION

The objective of plant propagation via tissue culture, termed micropropagation, is to propagate plants true-to-type, that is, as clones. Plants obtained from tissue culture are called microplants and can be derived from tissue cultures in three ways:

• from pre-existing shoot buds or primordial buds (meristems) which are encouraged to grow and proliferate;

• following shoot morphogenesis when new shoots are induced to form in unorganised tissues or directly upon explanted tissues of the mother plant;

• through the formation of somatic embryos which resemble the seed embryos of intact plants, and which can grow into seedlings in the same way. This process is called somatic embryogenesis.

To obtain plants by the first two of these methods, it is necessary to treat shoots of an adequate size as miniature cuttings and induce them to produce roots.

The derivation of new plants from cells, which would not normally have taken part in the process of regeneration, shows that living, differentiated plant cells may express totipotency, i.e. they each retain a latent capacity to produce a whole plant. Totipotency is a special characteristic of cells in young tissues and meristems. It can be exhibited by some differentiated cells, e.g. cambial cells and leaf palisade cells but not those which have developed into terminally differentiated structures (e.g. sieve tubes or tracheids).

Theoretically, plant cells, organs, or plants, can all be cloned, i.e., produced in large numbers as a population where all the individuals have the same genetic constitution as the parent. Present tissue culture techniques do not permit this in every case and irregularities do sometimes occur, resulting in 'somaclonal variants' (Larkin and Scowcroft, 1981). Nevertheless, as will be described in the chapters, which follow, a very large measure of success can be achieved and cultures of various kinds can be used to propagate plants.

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