Mesembryanthemum crystallinum, a facultative halophyte and C₃–Crassulacean acid metabolism (CAM) intermediate plant, has become a favoured plant for studying stress response mechanisms during C₃–CAM shifts. One hour of exposure to excess light (EL) caused inhibition of photosynthetic electron transport in M.crystallinum leaves as indicated by chlorophyll a fluorescence measurements. This was accompanied by an increase in NADP-malic enzyme (ME), one of the key cytosolic enzymes involved in CAM, and by a general increase in superoxide dismutase (SOD) activity. In contrast, NAD-ME activity (the mitochondrial form of ME) was not affected by EL. Exposure to EL and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) treatment of a whole plant in low light induced hydrogen peroxide (H₂O₂) and C₃ to CAM transition. In contrast, treatment with 3-3,4-dichlorophenyl-1,1-dimethyl urea (DCMU) has blocked high light-induced H₂O₂ accumulation and C₃–CAM transition. Moreover, the abundance of transcripts encoding different SODs, ascorbate peroxidase and SOD activity was differently regulated by DCMU and DBMIB. Results of applying EL or high light, H₂O₂ and photosynthetic electron transport inhibitors suggest that the redox events in the vicinity of PSII and/or PSI and photo-produced H₂O₂ play a major role in the regulation of C₃–CAM transition and photooxidative stress responses in M. crystallinum.

Enhanced antioxidant defence plays an essential role in plant survival under stress conditions. However, excessive antioxidant activity sometimes suppresses the signal necessary for the initiation of the desired biological reactions. One such example is microspore embryogenesis (ME)—a process of embryo-like structure formation triggered by stress in immature male gametophytes. The study focused on the role of reactive oxygen species and antioxidant defence in triticale (×Triticosecale Wittm.) and barley (Hordeum vulgare L.) microspore reprogramming. ME was induced through various stress treatments of tillers and its effectiveness was analysed in terms of ascorbate and glutathione contents, total activity of low molecular weight antioxidants and activities of glutathione–ascorbate cycle enzymes. The most effective treatment for both species was a combination of low temperature and exogenous application of 0.3 M mannitol, with or without 0.3 mM reduced glutathione. The applied treatments induced genotype-specific defence responses. In triticale, both ascorbate and glutathione were associated with ME induction, though the role of glutathione did not seem to be related to its function as a reducing agent. In barley, effective ME was accompanied by an accumulation of ascorbate and high activity of enzymes regulating its redox status, without direct relation to glutathione content.

Key message H ₂ O ₂ is necessary to elicit rhizogenic action of auxin. Activities of specific catalase and manganese superoxide dismutase forms mark roots development. Abstract 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.