Mechanisms Underlying the C3-CAM Photosynthetic Shift in Facultative CAM Plants

Crassulacean acid metabolism (CAM), one of three kinds of photosynthesis, is a water-use efficient adaptation to an arid environment. CAM is characterized by CO2 uptake via open stomata during the nighttime and refixation CO2 via the Calvin cycle during the daytime. Facultative CAM plants can shift the photosynthesis from C-3 to CAM and exhibit greater plasticity in CAM expression under different environments. Though leaf thickness is an important anatomical feature of CAM plants, there may be no anatomical feature changes during the C-3-CAM transition for all facultative CAM plants. The shift from C-3 photosynthesis to CAM in facultative CAM plants is accompanied by significant changes in physiology including stomata opening, CO2 gas exchange and organic acid fluxes; the activities of many decarboxylating enzymes increase during the shift from C-3 to CAM; the molecular changes occur during the photosynthesis C-3-CAM shift involved DNA hypermethylation, transcriptional regulation, post-transcriptional regulation and protein level regulation. Recently, omics approaches were used to discover more proceedings underling the C-3-CAM transition. However, there are few reviews on the mechanisms involved in this photosynthetic shift in facultative CAM plants. In this paper, we summarize the progress in the comparative analysis of anatomical, physiological, metabolic and molecular properties of facultative CAM plants between C-3 and CAM photosynthesis. Facultative CAM plants also show the potential for sustainable food crop and biomass production. We also discuss the implications of the photosynthesis transition from C-3 to CAM on horticultural crops and address future directions for research.