As right as rain: deciphering drought-related metabolic flexibility in the C-4-CAM Portulaca

Optimization of carbon and water usage in plants is a widespread strategy to survive in hot and dry environments. Based on the ancestral C-3 photosynthesis, two major carbon-concentrating mechanisms (CCMs) evolved, allowing spatial (C-4) or temporal (Crassulacean acid metabolism, CAM) segregation of carbon-fixing activities. While C-4 and CAM have generally been considered mutually exclusive, they share most of their biochemical machinery. An exception to the rule is found among the Caryophyllaceae, in the Portulaca genus. Described as an `inducible' CAM, the C-4 species Portulaca oleracea accumulates malate transiently under drought stress and in a reversible manner. Concentrating mostly at the transcriptome level, Ferrari et al. try deciphering the complex interplay between C-4 and CAM under various drought conditions. A better understanding of how the two carbon-fixing mechanisms are coordinated could shed light on key regulatory mechanisms necessary to improve C-4 crops under changing environments.