DOES ALTERED NITROGEN-METABOLISM AND H2O2 ACCUMULATION EXPLAIN THE VITRIFIED STATUS OF THE FULLY HABITUATED CALLUS OF BETA-VULGARIS (L)

The habituated callus is a vitrified tissue which has two main biochemical characteristics both leading to production of toxic forms of oxygen: first the blockage of the porphyrin pathway and a lack of H2O2 detoxifying enzymes (catalase and peroxidases); secondly a deviation of the nitrogen metabolism induced by NH3 accumulation. Ammonia detoxification is ensured by increased glutamate dehydrogenase activity and accumulation of both proline and polyamines. A putative linkage between proline synthesis and the HMP pathway, as proposed for animal proliferating tissues (Phang 1985), might explain a high purine biosynthesis and cytokinin autonomy.