Crassulacean acid metabolism (CAM) is a specialized type of photosynthesis: stomata close during the day, enhancing water conservation, and open in the night, allowing CO2 uptake. Therefore, water use efficiency (WUE) of CAM plants is much higher than C3 and C4 plants under comparable growth conditions. Mesembryanthemum crystallinum (common ice plant) is a facultative CAM species that can switch from C3 to CAM under salt or drought stresses. However, the molecular mechanisms underlying the transition from C3 photosynthesis to CAM remain unknown.
Common ice plant leaves were used to determine the transition period from C3 to CAM photosynthesis after salt or drought stresses. Leaf diurnal changes in stomatal movement, carbon assimilation, vacuolar acidity, gene expression of key CAM enzymes (e.g., phosphoenolpyruvate carboxylase (PEPC) and PEP carboxykinase (PEPCK)), as well as their activities were measured to determine the critical transition time points of CAM initiation from C3. Leaves, stomatal guard cells, and mesophyll cells were collected for proteomics and metabolomics analyses. Bioinformatics and machine learning tools were used to determine molecular switches at different levels of regulation.
We have determined the transition time from C3 to CAM in M. crystallinum under drought and salt stress. Our previous transcriptomics work identified 495 differential transcripts between control and salt-treated samples during the C3 to CAM transition, including seven CAM-related genes, 18 transcription factors, and 285 known guard cell expressed genes. PEPC1 and PPCK1, which encode key enzymes of the CAM photosynthesis, were up-regulated in guard cells after seven days of salt treatment, indicating that guard cells themselves can transit from C3 to CAM. Label-free quantitative proteomics and metabolomics data have been acquired, and they will be presented after quality control and statistical/bioinformatic analyses.
This study unravels novel molecular switches and provides important information towards introducing CAM into crops to enhance WUE and yield.