Structure of the Photosystem II protein complex form Arabidopsis thaliana created using cryo-electron microscopy. Global resolution: 2.44Å; local resolution illustrated by colour: 2.0 violet, 2.5 blue, 3.0 green, 3.5 yellow. Illustration: Jack Forsman, J. Messinger & W. Schröder group).
Plants use sunlight to turn water and carbon dioxide into energy stored in biomolecules such as sugars, a process that also produces the oxygen in the air. But exactly how water reaches the part of the system where the initial steps of this reaction happen – the active site for water splitting - has remained unclear. Now, researchers have revealed a detailed structure of this system in plants, uncovering what they describe as a “water valve” that helps control the flow of water during photosynthesis.
Two years ago, researchers from Umeå University revealed the detailed structure of Photosystem II, the molecular machinery that drives photosynthesis, in cyanobacteria. For the first time, they were able to see this structure at very high resolution using cryo-electron microscopy, even identifying individual water molecules and hydrogen atoms inside the system.
Now, the research team led by Johannes Messinger has taken the next step by resolving the same structure in plants, specifically in Arabidopsis or thale cress. The study, published in New Phytologist, compares Photosystem II in plants and photosynthetic bacteria, revealing how it has evolved independently over almost one billion years.
Key parts of photosystem II are conserved across species
“Comparing the structures of Photosystem II from Arabidopsis and cyanobacteria showed us which areas are the same, and therefore functionally important,” explains Johannes Messinger, professor at Umeå University and group leader at Umeå Plant Science Centre. “We assume that those areas that are different are less critical, as they can change without affecting photosynthesis.”
An early step in photosynthesis is the splitting of water, a reaction that releases oxygen and provides the electrons and part of the energy needed to convert carbon dioxide into sugars. This is the process the researchers focused on.
Johannes Messinger and his team study how water is split at the molecular level during photosynthesis (photo: Mattias Pettersson).
The team was particularly interested in how water molecules move through Photosystem II and how they interact with the manganese-containing catalytic centre, the part of the system where water is split.
“We were looking at water molecules and water channels in both structures. Around the catalytic centre, they were almost identical, suggesting that the arrangement of the water molecules is very important for the function of photosystem II,” says Jack Forsman, one of the two shared first authors of the study. “However, further away, the picture was very different and the channels deviated significantly.”
The researchers identified a narrow bottleneck in one of these channels, which they call ‘the water valve’. This point sits just before the catalytic centre and likely plays a key role in controlling how water is delivered to it.
A “water valve” controls how water reaches the reaction centre
“Before ‘the water valve’, the only requirement is that water can easily reach this point, which is why the channels can vary between plants and cyanobacteria without affecting function,” explains Wolfgang Schröder, one of the authors and leader of the previous study. “After the bottleneck, however, water molecules need to be in very specific positions to interact correctly with the catalytic centre.”
Understanding how water is transported and positioned in this system could help scientists design materials for artificial photosynthesis, technologies that aim to produce fuels from water, carbon dioxide and sunlight. Today, such reactions often rely on rare and expensive metals, but insights from plants could help enable the use of more abundant elements such as manganese, opening up new possibilities for developing more sustainable energy technologies in the future.
“Our data clearly show that it is important not only to design the metal catalyst itself, but also the surrounding water network,” says Johannes Messinger. “In future, we will focus on how these bottlenecks affect water flow and water-splitting, as well as study Photosystem II in additional species.”
Jack Forsman (right), André T. Graça (left) and Wolfgang Schröder (middle). Forsman and Graça share first authorship of the study, led by Johannes Messinger and Wolfgang Schröder (photo: André T. Graça).
About the study in New Phytologist
Jack Forsman, André T. Graça, Abuzer Orkun Aydin, Michael Hall, Rana Hussein, Wolfgang P. Schröder and Johannes Messinger, The structure of intact and active Photosystem II from Arabidopsis thaliana at 2.44 Å resolution, New Phytologist 2026, DOI: https://doi.org/10.1111/nph.71085.
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