Abstract
H2O2 is necessary to elicit rhizogenic action of auxin. Activities of specific catalase and manganese superoxide dismutase forms mark roots development. Hypocotyl explants of Mesembryanthemum crystallinum regenerated roots on medium containing 2,4-dichlorophenoxyacetic acid. Explants became competent to respond to the rhizogenic action of auxin on day 3 of culture, when hydrogen peroxide content in cultured tissue was the highest. L-Ascorbic acid added to the medium at 5 μM lowered the H2O2 level, inhibited rhizogenesis and induced non-regenerative callus, suggesting that certain level of H2O2 is required to promote root initiation. Coincident with the onset of rhizogenic determination, meristemoids formed at the periphery of the hypocotyl stele and the activity of the manganese form of superoxide dismutase, MnSOD-2 was induced. Once induced, MnSOD-2 activity was maintained through the post-determination phase of rooting, involving root growth. MnSOD-2 activity was not found in non-rhizogenic explants maintained in the presence of AA. Analyses of the maximum photochemical efficiency of photosystem II and the oxygen uptake rate revealed that the explants were metabolically arrested during the predetermination stage of rhizogenesis. Respiratory and photosynthetic rates were high during root elongation and maturation. Changes in catalase and peroxidase activities correlated with fluctuations of endogenous H2O2 content throughout rhizogenic culture. Expression of a specific CAT-2 form accompanied the post-determination stage of rooting and a high rate of carbohydrate metabolism during root growth. On the other hand, the occurrence of MnSOD-2 activity did not depend on the metabolic status of explants. The expression of MnSOD-2 activity throughout root development seems to relate it specifically to root metabolism and indicates it as a molecular marker of rhizogenesis in M. crystallinum.