Differential expression of RubisCO and PEPC during the salinity stress recovery of facultative CAM plants

Our previous work demonstrated that the model facultative CAM plant (Mesembryanthemum crystallinum L.) requires less than 48 h to fully rearrange its photosynthetic apparatus and complete recovery of photosynthetic quantum efficiency following salinity stress withdrawal. Our latest findings show that not all aspects of this highly plastic photosynthetic apparatus undergo such rapid changes. Although significantly lower than in salinity-sustained (+NaCl) plants, the stress-dependent PEPC1 protein remained expressed in salinity stress-withdrawn (-NaCl) plants for two consecutive days after stress removal, whereas its expression was undetectable in control (C3) plants. Limited PEPC activity in salinity-withdrawn plants was reflected in a significantly higher ¹³C discrimination than in salinity-sustained (+NaCl) plants. Interestingly, changes in RubisCO expression at the protein level did not appear to be part of the rapid modifications following stress withdrawal. For two consecutive days after stress removal, the large RubisCO subunit remained expressed at similar levels in both salinity-stressed (+NaCl) and stress-withdrawn (-NaCl) plants, while being significantly lower than in unstressed (C3) control plants. On the other hand, one day after stress withdrawal (-NaCl), proline levels dropped to those observed in salinity-untreated (C3) control plants. Conversely, stressor removal had only a minor impact on oxidative stress-related damage in recovering plants. Moreover, withdrawing osmotic stress had little effect on the growth rate. Despite a significantly higher relative water content (RWC) in stress-withdrawn (-NaCl) plants compared to those that remained under salinity stress (+NaCl), growth did not noticeably accelerate. Overall, this study provides new insights into the recovery process from salinity stress.