Revealing the structure of land plant photosystem II: the journey from negative-stain EM to cryo-EM.

Photosystem II (PSII) is a multi-subunit pigment-protein complex in plant chloroplasts that drives water oxidation and oxygen evolution in photosynthesis. In land plants, PSII forms dimeric core complexes associated with multiple light-harvesting complex II (LHCII) antenna proteins (LHCB1-6), assembling into large PSII-LHCII supercomplexes within thylakoid membranes. Over the past few decades, our understanding of PSII structure has advanced dramatically, evolving from low-resolution electron microscopy (EM) data to near-atomic models. This review highlights key milestones in PSII structural research, focusing on how progressive advances in EM from early negative-stain imaging to high-resolution single-particle cryo-EM, have revealed the organization of PSII supercomplexes in various plant species, ranging from angiosperms (e.g., Arabidopsis thaliana, spinach, pea) to gymnosperms (e.g., Norway spruce). These developments have led to the most detailed structural models of plant PSII to date. Future work will aim not only to determine PSII structures from a broader range of organisms, but also to explore the organization and dynamics of larger assemblies such as PSII megacomplexes and PSII-partner complexes, both in isolation and in situ within the thylakoid membrane.