Plant and Cell Physiology最新文献

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.

Photosynthetic bacteria provide an excellent model for investigating the primary processes of photosynthesis due to their relatively simple photochemical systems and ease of biochemical sample preparation. While light-harvesting (LH) complexes containing bacteriochlorophyll (Bchl) a have been extensively studied, much less is known about Bchl b-based pigment-protein complexes. The purple photosynthetic bacterium Blastochloris (Blc.) viridis is unusual in possessing only an LH1-reaction center (RC) core complex. Its LH1 complex incorporates Bchl b dimers along with two distinct carotenoids-1,2-dihydroneurosporene and 1,2-dihydrolycopene. Unlike Bchl a-containing systems, this complex features a remarkably red-shifted Qy absorption band located at 1010 nm, enabling efficient LH in the near-infrared region. Beyond their role in energy transfer, carotenoids in LH1-RC complexes serve as crucial photoprotective agents, mitigating oxidative stress by quenching triplet states that could otherwise generate harmful reactive oxygen species. However, the triplet energy transfer and quenching reactions in Bchl b-containing systems remain largely unexplored. In this study, we employed sub-nanosecond time-resolved absorption spectroscopy to investigate the excitation energy transfer dynamics and photoprotective mechanisms in the purified LH1-RC and RC complexes of Blc. viridis. Our findings reveal previously uncharacterized triplet-triplet energy transfer processes from Bchl b to carotenoids. These results not only advance our understanding of Bchl b-based light-harvesting systems but also provide key insights for the development of artificial photosynthetic platforms optimized for near-infrared light utilization.

Photosystem II catalyzes the light-driven oxidation of water, progressing via sequential oxidation states (S-states) of the Mn4CaO5 cluster. Among structural snapshots of intermediate S-states obtained using X-ray free-electron laser (XFEL) crystallography, two-flash XFEL structures assigned to the S3 state reveal an additional oxygen atom (O6) near the O5 site of the cluster, leading to proposals that O6 is incorporated as a new substrate water molecule during the S2 to S3 transition. However, recent re-analyses of the XFEL data highlight potential complications, including conformational heterogeneity, refinement bias, and possible radiation-induced artifacts. In addition, many proposals have been put forwarded without evaluating associated proton and electron transfer processes, despite the fact that water oxidation involves the stepwise removal of protons and electrons. Here, we shed light on electron and proton transfer events during the photocycle by summarizing mechanistic proposals, including those in which O6 is not incorporated. If the remaining reduced site, Mn1(III), is oxidized during the S2 to S3 transition, this step encounters difficulties due to its high redox potential and poor electronic coupling with the electron acceptor, D1-Tyr161 (TyrZ). Efficient proton transfer requires pre-existing H-bond networks, which are absent near O5 and O6, imposing kinetic penalties on proton release. Assigning O6 as a substrate oxygen would imply that O5 is the other substrate, requiring its deprotonation earlier in the Kok cycle.

The chloroplast NADH dehydrogenase (NDH)-like complex facilitates the ferredoxin-dependent reduction of plastoquinone, coupled with proton translocation across the thylakoid membrane, thereby mediating cyclic electron transport around photosystem I (PSI). The NDH complex evolved from the cyanobacterial counterpart and it forms a large supercomplex with two copies of the PSI complex in angiosperms. In Arabidopsis, NDH-deficient mutants exhibit impaired oxidation of PSI during low-light phases under fluctuating light conditions. Despite its important physiological function clarified in angiosperms, the NDH complex has been lost in certain lineages of eukaryotic phototrophs, including some green and red algae, as well as specific gymnosperms. This loss is likely to be compensated by alternative regulatory mechanisms involving the PROTON GRADIENT REGULATION 5 protein or flavodiiron proteins (Flv). This review article provides an overview of the current knowledge on the evolution of the NDH-PSI supercomplex through the acquisition of new subunits. We also summarize the evolutional loss of the NDH complex, primarily focusing on monocotyledonous plants to extensively investigate the loss of the NDH complex in angiosperms, which had lost Flv genes early in their evolution. In monocots, loss of the NDH complex is relatively rare and occurred mostly in Orchidaceae (Asparagales) and among submerged aquatic plants in Alismatales. These findings support the idea that the NDH complex is crucial for maintaining optimal photosynthetic activity in terrestrial angiosperms exposed to harsh light environments.

The chemical properties of the primary (QA) and secondary (QB) plastoquinone electron acceptors of Photosystem II (PS II) depend on their protein environments. The DE loop of the D2 protein (residues 222-262) contributes to the QA-binding site while the DE loop of the D1 protein (residues 233-266) contributes to the QB-binding environment. The roles of the invariant D2-Met246 and D2-Asn250 residues in the vicinity of the QA-binding site have been investigated in the cyanobacterium Synechocystis sp. PCC 6803 using mutants targeting both residues. The M246F strain was phenotypically similar to control cells; however, the M246A, N250A, and N250H strains had slowed photoautotrophic growth and were sensitive to high light and the addition of formate. In addition, the M246K and N250N strains were unable to assemble PS II. Chlorophyll a fluorescence measurements indicated electron transfer between QA and QB was modified in the M246A, N250A, and N250H strains, and the exchange of plastoquinol between the QB-binding site and the plastoquinone pool in the thylakoid membrane was impaired. Modified electron transfer in these mutants in the presence or absence of formate was restored by the addition of bicarbonate. In addition, thermoluminescence measurements showed a down shift in the redox midpoint potential of the QA/QA- couple in the N250A and N250H strains. These results demonstrate that Met246 and Asn250 play indispensable roles in the quinone-iron-acceptor complex, influencing both QA binding and the binding of the bicarbonate ligand to the non-heme iron that is located between QA and QB.

Phycobilisome (PBS) is a water-soluble light-harvesting supercomplex found in cyanobacteria, glaucophytes, and rhodophytes. PBS interacts with photosynthetic reaction centers, specifically photosystems II and I (PSII and PSI), embedded in the thylakoid membrane. It is widely accepted that PBS predominantly associates with PSII, which functions as the initial complex in the linear electron transport chain. Structures of various types of PBSs with different morphologies and/or absorption properties have been reported using cryo-electron microscopy and X-ray crystallography. However, the detailed energy transfer process between PBS and PSII remains to be elucidated due to the lack of a reliable preparation method for PBS-PSII megacomplexes, in which PBS and PSII interact with each other. In this study, we established a new method for isolating the PBS-PSII megacomplex using ammonium sulfate and dodecyl-α-D-maltoside as a stabilizing reagent and a detergent, respectively. In addition, we evaluated the detailed energy transfer mechanism in the PBS-PSII megacomplex, revealing the rate constants of the funnel-type excitation energy transfer from PBS to PSII. The method will enhance our understanding of the biochemical properties and energy transfer dynamics of diverse PBS-PSII megacomplexes.

F1FO-ATP synthase, the enzyme complex responsible for adenosine triphosphate (ATP) production, is universally conserved and central to cellular energy metabolism in bacteria as well as in mitochondria and plastids-organelles derived from ancestral bacteria. Although its basic structure and rotational catalytic mechanism are conserved, F1FO-ATP synthase exhibits remarkable regulatory diversity, which is evident in its structural variations, tissue-specific isoforms, and ATP synthesis and hydrolysis mechanisms, reflecting the metabolic demands and environmental contexts of different organisms and organelles. Among the diverse F1FO-ATP synthase isoforms, the plastid F1FO-ATP synthase exhibits unique regulatory features, including redox-dependent modulation, which adjusts enzyme activity in response to light availability. Certain angiosperms possess two isoforms of the γ subunit, encoded by ATPC1 and ATPC2, which give rise to redox-sensitive and redox-insensitive forms of the enzyme, respectively. The latter is active in the dark and may contribute to the maintenance of the proton motive force regulation, thereby supporting stress adaptation in non-photosynthetic tissues. In this review, through a phylogenetic analysis of the γ subunit, we integrate structural, physiological, and evolutionary aspects of plastid F1FO-ATP synthase and discuss how the diversification of ATP synthases, especially within plastid, underpins their broader physiological significance beyond ATP production. Furthermore, we discuss why the chloroplast ATP synthase must be redox-regulated.

The photochemical reflectance index (PRI) is a normalized reflectance index that is expected to be useful for estimating photosynthetic activity based on remote-sensing images. Experimental and theoretical studies have examined how the PRI is related to photosynthesis, but they have been based on observations under steady-state light conditions. Photosynthetic systems display differential temporal responsiveness when exposed to variation in light intensity. Here, we examined the responses of the CO2 assimilation rate (A), quantum yield of PSII photochemistry (ФP), nonphotochemical quenching (NPQ), and the PRI in two poplar species, one being a hybrid that does not close stomata in the dark (nonclosing type). When dark-adapted leaves were exposed to strong light (induction phase), the response time was ФP = NPQ < PRI < A for the normal type and ФP = NPQ < PRI = A for the nonclosing type. Consequently, the PRI-NPQ and the PRI-A relationships differed between the steady-state and induction phase. On the other hand, when the light-adapted leaves were transferred from dark to light, the time response was similar among ФP, NPQ, and the PRI. Therefore, the PRI can be used to assess ФP and NPQ even under dynamic light conditions if light-adapted leaves are used. Our results imply that, following sudden increases in light intensity, CO2 assimilation in the normal type poplar is limited by stomatal conductance, and that PSII-related parameters, including the PRI, are temporally decoupled from A. Estimates of A based on the PRI would be overestimates under dynamic conditions, which needs to be taken into account when interpreting remote-sensing data.

The thylakoid membrane (TM), a defining feature for almost all oxygen-evolving photosynthetic organisms, serves as the structural foundation for light-driven energy conversion. In vascular plants, the TM evolved into a complex architecture composed of single-layered stroma thylakoids and stacked grana thylakoids, enabling the spatial organization of two photosystems (PSII and PSI) to optimize light capture and energy transfer. In addition, two membrane regions, one connecting these two compartments (grana margin) and the other corresponding to the curvature domain in grana, function in dissipating excess energy, balancing electron transfer, and maintaining functional PSII. Recent advances in electron microscopy imaging and proteome analysis of membrane subcompartments have provided new insights into the structure and dynamic adaptations of the TM in response to diverse environmental conditions. To describe the mechanisms that govern TM architecture, dynamics, and integrity, I am introducing the concept of "thylakostasis" (thylakoid homeostasis). Here, I provide an overview of the molecular components and processes central to thylakostasis, including the biosynthesis of lipids, chlorophyll, and proteins. I focus particularly on the membrane remodeling proteins whose functions have been elucidated recently, such as VIPP1, a member of the evolutionarily conserved PspA/ESCRT-III superfamily; FZL, a dynamin-like GTPase; and CURT1, a curvature-inducing protein unique to photosynthetic organisms. Together, these factors orchestrate TM biogenesis, remodeling, and adaptive flexibility that is essential for photosynthetic efficiency.