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In green plants, photosystem I (PSI) and photosystem II (PSII) bind to their respective light-harvesting complexes (LHCI and LHCII) to form the PSI−LHCI supercomplex and the PSII−LHCII supercomplex, respectively. These supercomplexes further form megacomplexes, like PSI−PSII and PSII−PSII in Arabidopsis (Arabidopsis thaliana) and spinach to modulate their light-harvesting properties, but not in the green alga Chlamydomonas reinhardtii. Here, we fractionated and characterized the stable rice PSI−PSII megacomplex. The delayed fluorescence from PSI (lifetime ∼25 ns) indicated energy transfer capabilities between the two photosystems (energy spillover) in the rice PSI−PSII megacomplex. Fluorescence lifetime analysis revealed that the slow PSII to PSI energy transfer component was more dominant in the rice PSI−PSII supercomplexes than in Arabidopsis ones, suggesting that PSI and PSII in rice form a megacomplex not directly but through LHCII molecule(s), which was further confirmed by the negatively stained electron microscopy analysis. Our results suggest species diversity in the formation and stability of photosystem megacomplexes, and the stable PSI–PSII supercomplex in rice may reflect its structural adaptation. [ABSTRACT FROM AUTHOR]

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy in chemical forms. The light-harvesting, excitation transfer, charge separation and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledge about the architectures and assemblies of plant and green algal PSII–light harvesting complex II (LHCII) supercomplexes, with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII–LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution and about the energy transfer pathways that define their light-harvesting and photoprotective properties. [ABSTRACT FROM AUTHOR]

Photosystem II (PSII) splits water and drives electron transfer to plastoquinone via photochemical reactions using light energy. It is surrounded by light‐harvesting complex II (LHCII) to form the PSII–LHCII supercomplex. Complete characterization of its structure and function has, however, been hampered due to instability of the complex in the presence of detergent. To overcome this problem, we developed a new procedure for purifying the PSII–LHCII supercomplexes of Chlamydomonas reinhardtii employing amphipol A8‐35. The obtained supercomplexes showed little LHCII dissociation even 4 days after purification. Oxygen‐evolving activity was retained within amphipol if the extrinsic polypeptides were kept associated by betaine. Electron microscopy revealed that this method also improved structural uniformity and that the major organization was C2S2M2L2. [ABSTRACT FROM AUTHOR]

Nonphotochemical quenching (NPQ) is a crucial mechanism for fine-tuning light harvesting and protecting the photosystem II (PSII) reaction centres from excess light energy in plants and algae. This process is regulated by photoprotective proteins LHCSR1, LHCSR3, and PsbS in green algae, such as Chlamydomonas reinhardtii. The det 1–2 phot mutant, which overexpresses these photoprotective proteins, resulting in a significantly higher NPQ response, has been recently discovered in C. reinhardtii. Here, we analysed the physiological impact of this response on algal cells and found that det 1–2 phot was capable of efficient growth under high light intensities, where wild-type (WT) cells were unable to survive. The mutant exhibited a smaller PSII cross-section in the dark and showed a detachment of the peripheral light-harvesting complex II (LHCII) antenna in the NPQ state, as suggested by a rise in the chlorophyll fluorescence parameter of photochemical quenching in the dark (qPd > 1). Furthermore, fluorescence decay-associated spectra demonstrated a decreased excitation pressure on PSII, with excess energy being directed toward PSI. The amount of LHCSR1, LHCSR3, and PsbS in the mutant correlated with the magnitude of the protective NPQ response. Overall, the study suggests the mechanism by which the overexpression of photoprotective proteins in det 1–2 phot brings about an efficient and effective photoprotective response, enabling the mutant to grow and survive under high light intensities that would otherwise be lethal for WT cells. • Constitutive overexpression of LHCSR and PsbS increases photoprotective NPQ. • There is a concomitant detachment of the PSII antenna, with energy rerouted to PSI. • Cells overexpressing LHCSR and PsbS can thus readily survive high light conditions. [ABSTRACT FROM AUTHOR]

Photosystem I (PSI) from the green alga Chlamydomonas reinhardtii , with various numbers of membrane bound antenna complexes (LHCI), has been described in great detail. In contrast, structural characterization of soluble binding partners is less advanced. Here, we used X-ray crystallography and single particle cryo-EM to investigate three structures of the PSI-LHCI supercomplex from Chlamydomonas reinhardtii. An X-ray structure demonstrates the absence of six chlorophylls from the luminal side of the LHCI belts, suggesting these pigments were either physically absent or less stably associated with the complex, potentially influencing excitation transfer significantly. CryoEM revealed extra densities on luminal and stromal sides of the supercomplex, situated in the vicinity of the electron transfer sites. These densities disappeared after the binding of oxidized ferredoxin to PSI-LHCI. Based on these structures, we propose the existence of a PSI-LHCI resting state with a reduced active chlorophyll content, electron donors docked in waiting positions and regulatory binding partners positioned at the electron acceptor site. The resting state PSI-LHCI supercomplex would be recruited to its active form by the availability of oxidized ferredoxin. [Display omitted] • Algal PSI-LHCI needs to adapt to changing light conditions. • An X-ray structure of PSI-LHCI at 3.4 Å shows the absence of six chlorophylls. • A cryo-EM structure shows densities of unknown molecular identity on the lumenal and stromal side of PSI-LHCI. • Binding of oxidized ferredoxin releases all extra densities from PSI-LHCI. • Re-activation of PSI-LHCI via ferredoxin binding is proposed. [ABSTRACT FROM AUTHOR]