Dear Colleagues,

Photosystems I (PSI) and II (PSII) are large membrane pigment–protein complexes that convert solar energy into the chemical energy that powers life on Earth. Over the last decade, we have witnessed an explosion of high-resolution structures of PSI and PSII from diverse photosynthetic organisms, sometimes in different oligomerization states and even from organisms grown in different light conditions. This has been the result of exceptional advances in the development of the instrumentation and software adopted for the structural techniques used to determine these challenging structures. Among these techniques, single particle cryo-electron microscopy and serial femtosecond crystallography have recently emerged to flank traditional X-ray crystallography.

Knowledge of the structure of PSI and PSII in diverse photosynthetic organisms allows new ideas to be developed on the evolution of the reaction centers, the functional role of the oligomerization state of PSI, and the diversification of the light-harvesting antenna components of PSII within supercomplexes. These structures also update our understanding on many aspects of the PSI and PSII function, such as light harvesting and energy transfer from outer antenna systems to the photosystem cores, charge separation and acceptor side mechanisms of PSII, mechanistic description of the water oxidation catalytic cycle, state transitions in plants, and acclimation and non-photochemical quenching processes.

These high-resolution structures pave the way for a comprehensive understanding of the dynamic molecular mechanisms by which photosynthetic organisms adapt to different light and nutrient conditions. Complementary structural information obtained by electron microscopy at lower resolution together with cryo-electron tomography help in elucidating the PSI and PSII arrangement within the thylakoid membrane system. Furthermore, in the near future, it is expected that these high-resolution structures will increasingly become key players for investigations on protein–protein interactions and the docking of specific molecules of interest to the agri-food industry. They will constitute the basis for molecular dynamic simulations and structure-based computational analysis on excitation energy transfer and photochemistry. They will become the blueprint for targeted biotechnological applications.

This Special Issue of Plants aims to highlight recent advances in our understanding of the structure–function relationship of PSI and PSII, stemming from the outstanding advances currently available on their structures. New insights at the molecular, physiological, and evolutionary level will be showcased.

Dr. Cristina Pagliano
Dr. Andrea Fantuzzi
Guest Editors

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• Photosystems I and II
• supercomplexes
• evolution of photosystems
• photosynthetic molecular mechanism
• cryo-electron microscopy
• X-ray crystallography
• serial femtosecond crystallography
• computational analysis on excitation energy/electron transfer and charge separation
• molecular dynamics simulation