FULLY HABITUATED SUGAR-BEET CALLUS - UNDER PERMANENT STRESS

A fully habituated nonorganogenic (HNO) sugarbeet callus line, compare d to a normal (hormone dependent) one originated from the same plant e xhibits 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 c ompounds; lipid (per)oxidation and malondialdehyde formation; high act ivity of enzyme protective systems; proline, glutamate, and polyamine accumulation] may be interpreted as responses to stress. The deficienc y of tetrapyrrole-containing compounds can be considered an indirect p rotection against activated forms of oxygen as well as the higher acti vity of the antioxidant defense mechanisms; This fits with suggestions in the literature that the autonomy associated with plant cancer tiss ue is explained on the basis of antioxidants as stimulators of cell di vision and corresponding inhibitors of cell differentiation. Such chan ges occurring in HNO cells may in turn be responsible for a greater ab sorption and sensitivity to ammonium ions. An altered nitrogen metabol ism leads to proline, glutamate, and polyamine accumulation. Lipid per oxidation and malondialdehyde accumulation rapidly occur in this very sensitive HNO callus in a prolonged culture cycle, which might be rela ted to the appearance of necrosis bands. Thus there are arguments allo wing us to consider HNO cells as mutants adapted to some stresses, but resulting changes in their structure might have rendered them still m ore sensitive to other factors.