FULLY HABITUATED SUGAR-BEET CALLUS - UNDER PERMANENT STRESS

A fully habituated nonorganogenic (HNO) sugarbeet callus line, compared to a normal (hormone dependent) one originated from the same plant exhibits many characteristics of a vitrified tissue and several traits common to animal cancer cells. Four types of biochemical or metabolic characteristics of HNO callus [deficiency of tetrapyrrole-containing compounds; lipid (per)oxidation and malondialdehyde formation; high activity of enzyme protective systems; proline, glutamate, and polyamine accumulation] may be interpreted as responses to stress. The deficiency of tetrapyrrole-containing compounds can be considered an indirect protection against activated forms of oxygen as well as the higher activity of the antioxidant defense mechanisms; This fits with suggestions in the literature that the autonomy associated with plant cancer tissue is explained on the basis of antioxidants as stimulators of cell division and corresponding inhibitors of cell differentiation. Such changes occurring in HNO cells may in turn be responsible for a greater absorption and sensitivity to ammonium ions. An altered nitrogen metabolism leads to proline, glutamate, and polyamine accumulation. Lipid peroxidation and malondialdehyde accumulation rapidly occur in this very sensitive HNO callus in a prolonged culture cycle, which might be related to the appearance of necrosis bands. Thus there are arguments allowing us to consider HNO cells as mutants adapted to some stresses, but resulting changes in their structure might have rendered them still more sensitive to other factors.