Molecular architecture for state transition: insights from structural biology and evolutionary trajectories

Photosynthetic state transitions rapidly reallocate excitation energy between PSI and PSII to maintain redox poise in the thylakoid electron transport chain. This process relies on reversible phosphorylation of LHCII, allowing its transient association with PSI. Cryo-electron microscopy has resolved the structural interface between phosphorylated LHCII and PSI, revealing a conserved RRpT motif that docks to a site formed by PsaH and PsaL proteins. Strikingly, analogous PSI supercomplexes have now been identified in early diverging green lineages, including the bryophyte Physcomitrium patens and the marine prasinophyte Ostreococcus tauri, each displaying lineage-specific adaptations involving the moss-specific antenna protein Lhcb9 and the prasinophyte-specific antenna protein Lhcp, respectively. These findings suggest that the core molecular architecture for state transitions originated early in green plant evolution and was subsequently remodeled in distinct lineages to support adaptation to freshwater and terrestrial habitats. LHCII phosphorylation is primarily regulated by the redox state of the plastoquinone pool and its interaction with the cytochrome b6f complex. Conserved Ser/Thr kinases (Stt7/STN7) and PP2C-type phosphatases (TAP38/PPH1) mediate this process, integrating redox signaling into photosynthetic regulation. The kinase is further modulated by thioredoxin reduced downstream of PSI, adding an additional layer of redox-dependent control. This review synthesizes recent structural, biochemical, and phylogenetic insights, reframing state transition as a photoregulatory strategy that coordinates environmental light sensing with the optimization of energy capture, photoprotection, and adaptive plasticity.