Crassulacean acid metabolism (CAM) is a specialised mode of photosynthesis that improves atmospheric CO2 assimilation in water-limited terrestrial and epiphytic habitats and in CO2-limited aquatic environments. In contrast with C-3 and C-4 plants, CAM plants take up CO2 from the atmosphere partially or predominantly at night. CAM is taxonomically widespread among vascular plants and is present in many succulent species that occupy semiarid regions, as well as in tropical epiphytes and in some aquatic macrophytes. This water-conserving photosynthetic pathway has evolved multiple times and is found in close to 6% of vascular plant species from at least 35 families. Although many aspects of CAM molecular biology, biochemistry and ecophysiology are well understood, relatively little is known about the evolutionary origins of CAM. This review focuses on five main topics: (1) the permutations and plasticity of CAM, (2) the requirements for CAM evolution, (3) the drivers of CAM evolution, (4) the prevalence and taxonomic distribution of CAM among vascular plants with emphasis on the Orchidaceae and (5) the molecular underpinnings of CAM evolution including circadian clock regulation of gene expression.
