Distinct responses to copper stress in the halophyte Mesembryanthemum crystallinum

Selective gene expression allows the halophyte Mesembryanthemum crystallinum to survive a salt stress. To broaden our understanding of the environmental cues initiating diverse stress responses in this higher plant, unstressed and 0.4 M NaCl-stressed plants were compared to plants treated with several concentrations of copper (CuSO4), an increasingly relevant environmental heavy metal pollutant, Comparisons of control and copper-stressed plants included germination, chlorophyll content, accumulation of proline, heat shock protein (HSP) 60 and a Crassulacean acid metabolism (CAM)-specific marker enzyme phosphoenolpyruvate carboxylase (PEPCase). In germination and whole plant tests, M. crystallinum was significantly more tolerant to copper than Arabidopsis thaliana. Mature M. crystallinum plants stressed with 50 ppm CuSO4 for 48 h became dehydrated. These plants produced a ii-Fold increase in proline concentration and accumulated both the CAM-specific PEFCase and HSP 60 compared to controls. Higher levels of copper stress resulted in a 10-fold increase in leaf proline content, 10-fold HSP 60 accumulation but Ilo detectable PEPCase protein compared to unstressed controls. HSP 60 did not accumulate under NaCl stress. Concurrent with copper-induced generic responses to stress, copper was accumulated and concentrated in leaves (3 500 ppm). Together, these results suggest that this halophyte copes with copper metal exposure through distinct genetic mechanisms.