Antioxidative response of Mesembryanthemum crystallinum plants to exogenous SO2 application

The facultative halophyte, Mesembryanthemum crystallinum shifts its mode of carbon assimilation from the C3 pathway to crassulacean acid metabolism (CAM) in response to many factors that lead to the generation of reactive oxygen species (ROS) at the cellular level. Reactive oxygen species have been suggested to be involved in the initiation of CAM induction. In our experiment we would like to test whether the space/place of ROS production could play a role in CAM induction. In the present studies we applied exogenously sulphur dioxide to M. crystallinum plants as an oxidative stress factor that accumulates mainly in chloroplasts. On the basis of our data, one can suggest that oxidative stress in the cellular space, mainly in chloroplasts, is not sufficient to induce functional CAM in M. crystallinum plants. The second aim was to evaluate the influence of SO2 fumigation/sulphite incubation on the activity and mRNA transcript level of SOD isoenzymes, especially FeSOD, that is pointed out as a one of the first indicators correlating with the C3/CAM transformation. The data indicate that the activity of FeSOD and CuZnSOD isoforms increase under SO2/sulphite stress, despite of no induction of functional CAM. The increase of the activity level both of these enzymes were not accompanied by the induction of their mRNA transcript levels, suggesting a post-transcriptional regulation of activity of these enzymes. The pattern of FeSOD and CuZnSOD induction suggests that CuZnSOD might take over the role of FeSOD in conditions of severe oxidative stresses. The induction of CuZnSOD is probably due to the action of sulphite per se.