Propagation by all methods of indirect organogenesis carries a risk that the regenerated plants will differ genetically from each other and from the stock plant. Propagation by indirect organogenesis is described here for the sake of completeness; because of its potential as a propagation method, if the occurrence of genetic variation can be controlled; and because it is necessary for the regeneration and propagation of plants, which have been genetically transformed.
Because they are not formed on tissues of the original mother plant, shoots (or other organs) are said to be regenerated indirectly when they are formed on previously unorganised callus, or in cell cultures. Separate root and shoot initials are characteristically formed in callus cultures and are only observed occasionally in suspensions where they are typically produced in large cell aggregates. Somatic embryogenesis occurs in both callus and suspension cultures. Adjustment of the growth regulators in the culture medium can bring about shoot or root formation in callus from a very large number of species. Inception of roots and shoots is most frequent in tissues that have been recently isolated, and morphogenic capacity generally declines with time as the tissues are subcultured. Nevertheless, some callus cultures maintain their regenerative ability over long periods.
As explained in Chapter 10, callus cultures vary in their morphogenic potential or competence. Because of this, the callus which originates from some plants, or from some kinds of explant, may not be responsive to techniques and media which frequently result in morphogenesis. The tissue may be non-morphogenic, or may only produce roots, from which plants cannot be regenerated. In some cases callus lines with different appearances (texture, colour, etc) and/or morphogenic capacities can be isolated from the same explant (Fig 2.5). These differences may reflect the epigenetic potential of the cells, or be caused by the appearance of genetic variability amongst the cells of the culture (Volume 2) and support Street's (1979) suggestion that primary explants may be composed of cells or tissues capable of morphogenesis (competent cells) and others that are incapable (non-competent cells). Another possibility is that the operator is not able to create the appropriate conditions to express the full potential of the plant material he is working with.
Morphogenic and non-morphogenic callus lines, selected from primary callus, can retain their characteristics over many years (Reuther, 1990). Special treatments, such as, a change of medium, an altered cultural environment, or an adjustment of the growth regulators added to the medium, may induce shoots or roots to form in some apparently non-morphogenic calluses; but generally, treatments to reverse a non-regenerative condition are unsuccessful.
In practice, the speed and efficiency with which plantlets can be regenerated from callus depends upon:
• the interval between culture initiation and the onset of morphogenesis;
• the choice of the appropriate type of callus;
• the frequency and rate of shoot bud initiation;
• whether shoot regeneration can be readily re-induced when the callus is subcultured;
• the number of subcultures that are possible without loss of morphogenesis;
• whether newly-initiated buds can be grown into shoots capable of being isolated and subsequently rooted.
Normal callus cultures produce shoots relatively slowly, but from some plants, and certain explants, under conditions that are not yet fully understood, callus can be initiated which has an especially high ability to regenerate shoots or somatic embryos.
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