Crassulacean Acid Metabolism in Plant Physiology

Crassulacean acid metabolism (CAM) is an alternative photosynthetic pathway that enables plants to conserve water in arid and semi-arid environments by temporally separating carbon uptake and fixation. During the night, stomata open to admit CO₂, which is fixed by phosphoenolpyruvate carboxylase into oxaloacetate and subsequently converted to malate for storage in the vacuole. With daylight, stomata close to minimise transpirational loss and malate is decarboxylated, releasing CO₂ around Rubisco for the Calvin cycle. CAM occurs in diverse lineages—succulents, epiphytic orchids, agaves and some ferns—and exhibits substantial plasticity, ranging from obligate to facultative and CAM-cycling forms. Integration with the circadian clock ensures synchronised enzyme activities and stomatal movements, while specialised leaf anatomy, including large chlorenchyma and hydrenchyma cells, supports acid storage and hydraulic buffering. Beyond its evolutionary interest, CAM has drawn attention for its high intrinsic water-use efficiency and potential to enhance drought resilience and carbon sequestration in crops, offering strategies for agriculture on marginal lands under climate change.

Research from Nature Portfolio

Comparative genomics of an obligate CAM species has revealed molecular signatures of convergent evolution across independent CAM lineages. A high-quality genome assembly and transcriptome time series demonstrated that proteins involved in nocturnal CO₂ uptake, stomatal motion, carbohydrate metabolism, heat tolerance and circadian regulation have undergone parallel sequence changes and diel re-scheduling of gene expression. These insights define core genetic building blocks for CAM engineering into C₃ crops to improve water-use efficiency. In parallel, de novo transcriptome analysis of a drought-tolerant Agave species under well-watered versus drought conditions identified thousands of stress-responsive genes. Key pathways included hormonal signalling, antioxidant activity, wax biosynthesis and reactive oxygen species homeostasis, shedding light on the molecular basis of desert adaptation and offering genetic resources for breeding water-efficient cultivars.

Crassulacean Acid Metabolism in Plant Physiology publication trend

The graph below shows the total number of articles in crassulacean acid metabolism in plant physiology across all publications each year (not limited to Nature Index journals).

Technical terms

Crassulacean acid metabolism (CAM): A photosynthetic adaptation in which stomata open at night to fix CO₂ into organic acids, which are decarboxylated by day for the Calvin cycle.

Phosphoenolpyruvate carboxylase (PPC): The primary enzyme that catalyses nocturnal fixation of CO₂ into oxaloacetate in CAM and C₄ plants.

Malate: The four-carbon organic acid formed from oxaloacetate, stored in the vacuole overnight and decarboxylated by day to release CO₂.

Hydraulic capacitance: The capacity of plant tissues to store and release water, buffering against fluctuations in water availability.

Facultative CAM: A form of CAM in which plants switch between C₃ and CAM pathways in response to environmental stress, particularly drought.

Diel cycle: The 24-hour cycle of light and dark periods that governs temporal separation of metabolic processes in CAM.

References

1. The Kalanchoë genome provides insights into convergent evolution and building blocks of crassulacean acid metabolism. Nature Communications (2017).
2. De novo assembly of Agave sisalana transcriptome in response to drought stress provides insight into the tolerance mechanisms. Scientific Reports (2019).
3. A Synthetic Facultative CAM‐Like Shuttle in C3 Rice Plants. Advanced Science (2025).
4. Dissecting succulence: Crassulacean acid metabolism and hydraulic capacitance are independent adaptations in Clusia leaves. Plant Cell & Environment (2023).