Researchers led by Wolfram Weckworth at the University of Vienna have shown how a particularly water-efficient form of photosynthesis, known as CAM, evolved in multiple ways within a single genus of tropical trees. By comparing the genomes of three Crucia species, the research team traced how ancient genome duplication and large-scale genetic rearrangements helped produce different forms of CAM photosynthesis. The findings were recently published in Nature Communications.

The 200-year-old mystery behind CAM photosynthesis

Around 1800, Alexander von Humboldt noticed something unusual while studying tropical trees. He placed one of the leaves in water and found that, even in sunlight, it did not produce the oxygen bubbles he had observed earlier.

The explanation lies in the plant’s unusual daily schedule. Stomata are tiny openings that normally allow carbon dioxide (CO₂) to enter leaves. In plants that use CAM photosynthesis, the stomata open at night rather than during the hottest part of the day. This reduces water loss through evaporation. The plants absorb carbon dioxide at night, chemically convert it, and store it as malic acid.

This strategy is known as CAM, or crassulacean acid metabolism. Scientists have long understood how CAM works, but it remained unclear exactly how it evolved within the genus Crucia and why different species use the process in different ways.

Comparing the genomes of three tropical tree species

To investigate the evolutionary history of CAM, researchers analyzed the genomes of three Crucia species with different CAM phenotypes: Crucia rosea, Crucia minor, and Crucia major. They combined molecular data with measurements of how the plants functioned under realistic environmental conditions.

The genus Crucia is particularly valuable for studying the evolution of photosynthesis because it contains the only known trees that use CAM. Its species also display a broad range of photosynthetic strategies, from conventional C₃ photosynthesis, in which carbon dioxide is absorbed during the day, to highly active CAM.

This diversity gives scientists a unique opportunity to study how plants can transition between different forms of photosynthesis.

Ancient genome duplication rewired photosynthesis

Genome analysis showed that all three Crucia species are ancient polyploids. At some point in their evolutionary history, their genomes were duplicated. Over millions of years, these expanded genomes were rearranged and reshaped through a process known as diploidization.

“During this process, gene copies are lost, become inactive, or take on new functions,” explains lead author Hannes Krumml from the Department of Functional, Evolutionary Ecology and Molecular Systems Biology at the University of Vienna.

Second lead author Johannes Herpel added that genes involved in nighttime carbon dioxide storage during CAM metabolism were particularly affected.

Rather than simply retaining extra copies of the same genes, the plants gradually transformed their duplicated genomes into functionally different versions.

“Rather than simply multiplying, the genome has been rearranged, reduced, and functionally rewired over millions of years. This enormous plasticity explains the physiological diversity of CAM in this genus,” explains study leader Wolfram Weckworth.

Different trees use CAM in different ways

The researchers then examined how these genetic differences affected the plants. They monitored the trees throughout the day in near-natural greenhouse conditions while varying the amount of water available.

The research team combined measurements of plant physiology with analyses of gene activity, proteins, and metabolites.

The three species showed markedly different strategies. Crucia rosea uses strong CAM and stores significant amounts of carbon dioxide as malic acid during the night. Crucia minor primarily activates CAM under stressful conditions. Crucia major, meanwhile, uses a hybrid strategy that combines C₃ photosynthesis and CAM.

These physiological differences were also reflected in patterns of gene activity and metabolism, allowing the researchers to connect plant behavior with the genomic changes they identified.

The findings suggest that CAM in Crucia did not arise through a single evolutionary event. Instead, repeated genome rearrangements appear to have produced different water-saving strategies, helping individual species adapt to distinct ecological niches.

Could CAM research help create more drought-resistant crops?

The results could ultimately have implications beyond tropical trees.

CAM plants require significantly less water than plants that rely entirely on conventional photosynthesis, making them a potentially useful model for developing climate-resilient crops.

New genomic information may help researchers identify the metabolic processes involved in efficient carbon dioxide fixation and high water-use efficiency. In the long term, this knowledge could help scientists develop crops that are better adapted to dry environmental conditions.