Redox changes in the chloroplast and hydrogen peroxide are essential for regulation of C3-CAM transition and photooxidative stress responses in the facultative CAM plant Mesembryanthemum crystallinum L.

Abstract

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.