Shifting photosynthesis between the fast and slow lane: Facultative CAM and water-deficit stress

Five decades ago, the first report of a shift from C₃ to CAM (crassulacean acid metabolism) photosynthesis following the imposition of stress was published in this journal. The annual, Mesembryanthemum crystallinum (Aizoaceae), was shown to be a C₃ plant when grown under non-saline conditions, and a CAM plant when exposed to high soil salinity. This observation of environmentally triggered CAM eventually led to the introduction of the term facultative CAM, which categorises CAM that is induced or upregulated in response to water-deficit stress and is lost or downregulated when the stress is removed. Reversibility of C₃-to-CAM shifts distinguishes stress-driven facultative-CAM responses from purely ontogenetic increases of CAM activity. We briefly review how the understanding of facultative CAM has developed, evaluate the current state of knowledge, and highlight questions of continuing interest. We demonstrate that the long-lived leaves of a perennial facultative-CAM arborescent species, Clusia pratensis, can repeatedly switch between C₃ and CAM in response to multiple wet-dry-wet cycles. Undoubtedly, this is a dedicated response to environment, independent of ontogeny. We highlight the potential for engineering facultative CAM into C₃ crops to provide a flexible capacity for drought tolerance.