Plants use sunlight to convert water and carbon dioxide into energy-rich sugars and oxygen in various ways (photosynthesis). Drought is a major challenge in this process. A research team led by Wolfram Weckwerth at the University of Vienna has now demonstrated how a particularly water-efficient variant of this process (CAM) has evolved in diverse ways within a single tropical tree genus. By analysing the genomes of three species of the genus Clusia, the researchers were able to trace how genome duplication and subsequent genetic rearrangement contribute to the diversity of different CAM traits. The findings have recently been published in Nature Communications.
Around 1800, Alexander von Humboldt made an unusual observation. He dipped the leaf of a tropical tree into water and noted that, despite sunlight, no oxygen bubbles formed – as had previously been the case. This plant keeps its stomata – which normally serve to absorb CO2 and release oxygen during the day – closed during daylight hours, thereby preventing water loss through evaporation. CO2 is then absorbed at night, chemically bound and stored in the form of malic acid. This principle is known as ‘CAM photosynthesis’ (Crassulacean Acid Metabolism). How this strategy has evolved within the genus Clusia and why it occurs in different forms has, until now, been poorly understood.
About the study
As part of the study, the genomes of three Clusia species representing different CAM phenotypes were analysed: Clusia rosea, Clusia minor and Clusia major. The research team combined molecular data with physiological measurements under realistic environmental conditions.
From Genomes to Photosynthesis
The genus Clusia comprises the only known trees that practise CAM and exhibits an extraordinary range of photosynthetic strategies – from classical C₃ photosynthesis, in which plants absorb carbon dioxide during the day, to highly pronounced CAM. This diversity makes them an ideal research model for evolutionary transitions between different forms of photosynthesis. The analyses showed that all three Clusia species are ancient polyploids – their genomes were multiplied over the course of evolution (polyploidisation) and subsequently restructured over long periods of time (diploidisation). “In the process, gene copies are lost, deactivated or take on new functions,” explains lead author Hannes Kramml from the Division of Molecular Systems Biology, Department of Functional and Evolutionary Ecology, at the University of Vienna. Second lead author Johannes Herpell adds: “Genes crucial for nocturnal CO₂ storage in CAM metabolism are particularly affected.” Study leader Wolfram Weckwerth goes on to explain: “The genomes have not simply multiplied; over millions of years, they have been reorganised, reduced and functionally rewired. This enormous plasticity explains the physiological diversity of CAM in the genus Clusia.”
CAM under realistic environmental conditions
To investigate the effects of these genetic differences, the team analysed the plants throughout the day under near-natural greenhouse conditions with varying water availability. They combined physiological measurements with analyses of gene activity, proteins and metabolic products. Clusia rosea exhibits strong CAM with pronounced nocturnal storage of carbon dioxide in the form of malic acid. Clusia minor activates CAM primarily under stress conditions, whilst Clusia major displays a hybrid form of C₃ photosynthesis and CAM. These differences are consistently reflected in gene activity and metabolic profiles and can be linked to the genetic changes now identified. Here, CAM does not appear as a one-off evolutionary event, but as the result of repeated genomic reorganisation, which enabled the species to adapt to very different ecological niches.
Implications for agriculture and climate resilience
CAM plants require significantly less water and are therefore considered potential models for climate-resilient crops. The new genomic data enable the identification of metabolic processes associated with efficient CO₂ fixation and high water-use efficiency. In the long term, these findings could help to adapt crops more specifically to arid environmental conditions.
Summary
- In addition to conventional photosynthesis, plants can also utilise a water-saving variant in which CO₂ is absorbed mainly at night (CAM)
- The tropical tree genus Clusia exhibits an exceptional diversity of such photosynthetic forms and is the only tree-like genus known to date to possess CAM
- Genome duplication and subsequent genetic rearrangements are linked to this diversity
- Genes involved in starch breakdown and energy supply are particularly affected
- The results provide insights into how to better understand water-efficient metabolic processes and utilise them for climate-adapted plants
