Chapter 7
cytokinin oxidase enzyme (see Chapter 4), and can induce cytokinin autonomous growth, for example in some Phaseolus lunatus genotypes (Mok et al., 1982). However, this is certainly not the whole story. Thus, Lambe et al., (1997) have presented evidence that there are marked changes in DNA methylation during long-term callus culture and suggest that auxin has a role in this process (and see also below under ‘Effects of habituation’). 17.4. GENETIC CONTROL
Although habituation of cultured tissues is usually reversible, it is now known that heterotrophic cells can also become permanently autotrophic as a result of somatic mutation in the genes governing the production of endogenous growth factors. Evidence that this might be the case was discovered by several workers; callus capable of growing on media without the addition of growth substances was, for example, obtained by exposing cultures to gamma radiation
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(Pandey and Sabharwal, 1979); the ability of some genotypes of Phaseolus lunatus to grow in vitro without the presence of cytokinin appeared to be under genetic control (Mok and Mok, 1979). Delbreil and Jullien (1994) using different genotypes of Asparagus officinalis demonstrated the random occurrence of highly embryogenic tissue, which could be continuously subcultured on hormone-free medium. Genetic analysis demonstrated that this was due to a dominant monogenic mutation. Callus, re-initiated from plants derived from habituated sugarbeet callus, was again found to be habituated (Coumans et al., 1982), as were plants regenerated from auxin- habituated callus of Nicotiana bigelovii var. quadrivalvis (Bennici, 1983). Examples of stable and sexually transmissible ‘habituations’ are provided by Meins et al., (1983) and Meins and Foster (1986).
Fig. 7.5 Two possible explanations of growth regulator habituation in plant cultures. 17.5. EFFECTS OF HABITUATION
Habituated cultures show many differences in metabolism to normal cultures. This can take the
form of changes not only in the growth regulators to which the callus has become habituated but also of others. Thus, Bisbis et al., (2000a) showed that fully