Evolution along the crassulacean acid metabolism continuum

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 C3 and C4 plants, CAM plants take up CO2 from the atmosphere partially or predominantly at night. CAM is taxonomically widespreadamongvascular plants and is present inmanysucculent species that occupy semiarid regions, aswell as in tropical epiphytes and in some aquatic macrophytes. This water-conserving photosynthetic pathway has evolvedmultiple times and is found in close to 6%of vascular plant species fromat least 35 families.Althoughmany aspects ofCAMmolecular 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 theOrchidaceae and (5) themolecular underpinnings ofCAMevolution including circadian clock regulation of gene expression. Additional keywords: phosphoenolpyruvate carboxylase, photosynthesis, dC.