Scientists Trace Multiple Ways a Water-Saving Photosynthesis Evolved

Plants normally take in carbon dioxide and release oxygen through open pores called stomata during the day. A water-saving alternative called CAM photosynthesis — short for Crassulacean Acid Metabolism — instead keeps the stomata shut in daylight to limit water loss, and opens them at night to absorb carbon dioxide, which the plant stores as malic acid until sunrise. A research group led by Wolfram Weckwerth at the University of Vienna has now shown how this strategy evolved in several distinct ways within the tropical tree genus Clusia, publishing the findings in Nature Communications.

The team analyzed the genomes of three Clusia species with different photosynthesis types — Clusia rosea, Clusia minor and Clusia major — alongside physiological measurements taken under realistic greenhouse conditions with varying water supply. All three species turned out to be ancient polyploids, meaning their genomes were duplicated during evolution and then reorganized over millions of years, a process called diploidization. "In the process, gene copies are lost, deactivated or take on new functions," said lead author Hannes Kramml of the university's Division of Molecular Systems Biology. Co-author Johannes Herpell added that genes crucial for nighttime carbon dioxide storage were particularly affected by this reshaping.

The three species now use CAM very differently. Clusia rosea shows strong CAM with clear nighttime storage of carbon dioxide as malic acid, Clusia minor switches on CAM mainly under drought stress while otherwise using ordinary daytime photosynthesis, and Clusia major uses a mixed strategy of both. "The genomes have not simply multiplied; over millions of years, they have been reorganized, reduced and functionally rewired," Weckwerth said, adding that this explains the physiological diversity of CAM within the genus.

Because CAM plants use much less water, the researchers say the new genomic data could help identify the metabolic processes behind efficient carbon dioxide fixation and water efficiency, which could eventually help scientists adapt food crops to hotter, drier conditions.