In the tropical forests of the Americas, a small group of trees has a peculiar habit: they hold their breath all day.
Most plants open the tiny pores on their leaves, called stomata, during daylight to take in carbon dioxide for photosynthesis. Trees in the genus Clusia can do the opposite. They keep their pores sealed under the hot midday sun, then open up after dark to take in CO2 through the night, storing it as malic acid to use once the sun is back up. It is a slow, patient way of avoiding losing precious water to evaporation in the heat — and scientists have now worked out how these trees came to do it, adding a new chapter to a story that began more than two centuries ago.
A puzzle dating back to Humboldt
Around 1800, the explorer and naturalist Alexander von Humboldt noticed something odd about a tropical tree leaf he had dipped in water. Even in sunlight, it produced none of the oxygen bubbles he expected from a photosynthesising plant. The explanation, worked out much later, is that the tree's pores were closed for the day — it was doing its gas exchange overnight instead.
This strategy, known as Crassulacean Acid Metabolism, or CAM, has long been known in cacti and other succulents. But Clusia is unusual: it contains the only trees known to use it, and its species span an extraordinary range, from ordinary daytime photosynthesis to strong CAM, with stress-triggered and hybrid versions in between. How CAM evolved within the genus, and why it takes so many forms, had remained poorly understood.
Solving it species by species
A team led by Wolfram Weckwerth at the University of Vienna has now traced the answer to the trees' genomes. In a study published this year in Nature Communications, the researchers compared three Clusia species: Clusia rosea, which relies on strong CAM and stores large amounts of carbon dioxide overnight; Clusia minor, which mainly switches CAM on under drought and other stress; and Clusia major, which runs a hybrid of CAM and ordinary daytime photosynthesis.
All three turn out to be ancient polyploids — at some point in their history, their entire genomes were multiplied. Over millions of years, those enlarged genomes were gradually reshuffled, with some gene copies lost, switched off, or repurposed for entirely new jobs.
“In the process, gene copies are lost, deactivated or take on new functions,” said Hannes Kramml, the study's lead author, from the University of Vienna's Division of Molecular Systems Biology. “Genes crucial for nocturnal CO2 storage in CAM metabolism are particularly affected,” added Johannes Herpell, the paper's second lead author.
Weckwerth, who led the study, summed it up: “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.”
To connect those genetic changes to how the trees actually behave, the team grew the plants in near-natural greenhouse conditions, varied how much water they received, and tracked their physiology, gene activity, proteins and metabolism throughout the day. The differences between the species showed up consistently at every level.
Rather than a single evolutionary switch flipping CAM on once, the research suggests that repeated rounds of genomic reorganisation produced different versions of the trick, helping each species adapt to its own ecological niche.
A hopeful, if distant, payoff for farming
Because CAM plants need substantially less water than plants relying on conventional photosynthesis, they have long interested scientists hunting for ways to help agriculture cope with a hotter, drier climate. The researchers say the new genomic data could help identify the processes behind efficient carbon capture and high water-use efficiency, and in the longer term help adapt crops to arid conditions.
That does not mean drought-proof wheat or maize is around the corner — engineering something as intricate as CAM into a food crop remains a long-term ambition rather than an imminent breakthrough. But more than two centuries after Humboldt watched a leaf refuse to bubble, the Clusia trees may yet have something useful to teach the crops of the future.
