Root, shoot and leaves (but not embryo) are the organs that are induced in plant tissue culture. Since embryo is an independent structure and does not have vascular supply, it is not supposed to be the plant organ. Organogenesis (i.e.
development of organs) starts with stimulation caused by the chemicals of medium, substances carried over from the original explants and endogenous compounds produced by the culture (Thomas and Davey, 1975).
Skoog (1944) for the first time indicated that the organogenesis could be chemically controlled. He observed root initiation (rhizogenesis) and shoot inhibition (caulogenesis) after addition of auxin to the medium. Further, Skoog and co-workers gave the concept of regulation of organogenesis by a balance between cytokinin and auxin. Skoog and Miller (1957) demonstrated that a high ratio of auxin: cytokinin stimulated the formation of root in tobacco callus, but a low ratio of the same induced shoot formation. The hypothesis of organogenesis was advanced by Torrey (1966) who propounded that organogenesis in callus starts with the development of a group of meristematic cells i.e.
meristemoids that can respond to the factors within the system to initiate a primordium which, depending on kinds of factors, induces either root, shoot or embryoid. (Table 8.2).
Table 8.2 In vitro control of organogenesis by auxins and cytokinins
|| No growth
Nandi and Palni (1992)