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Publisher Summary Plant cells cultured in vitro normally need an exogenous supply of plant hormones, namely an auxin and/or a cytokinin, for continued growth. Sometimes the cells lose this requirement during subculturing and become able to grow on hormone-free medium or on medium that lacks one or other of the hormones. This phenomenon is called as auxin habituation. The chapter presents the phemomena of habituation and genetic tumors as systems that allow the interconversion of two different states (normal and tumorous) without any apparent genetic modification. With respect to reversion from a tumorous to a normal state, there are a few reports in the case of crown gall and hairy root induced by Agrobacterium infection. In these cases, evidence for methylation and subsequent inactivation of genes in the T-DNA have been presented. Similar modifications of genes may occur in genetic tumors, but the modification reactions must be much more rapidly reversible than methylation. In the case of habituation, reversibility to a normal, hormone-requiring state is reduced during subcultures. This is the case in decreases of organforming capacity and also in increases in the variability of regenerated plants. |
