Plant and Cell Physiology最新文献

Burgeoning global demand for crop products and the negative impact of climate change on crop production are driving the need to improve yield by developing new elite crop varieties without expanding planted area or increasing agronomic inputs. Improvement in photosynthesis is critical for enhancing crop productivity. Even though leaf photosynthesis is well-studied, the photosynthetic potential of non-foliar green tissues like pods in Brassicaceae and Fabaceae species remains underexplored. This review emphasizes pod photosynthesis in determining seed yield and quality in Brassicaceae and Fabaceae crops. At present, accurate and efficient phenotyping methods are unavailable, limiting understanding and genetic improvement of pod photosynthesis. Novel approaches like chlorophyll fluorescence and hyperspectral reflectance are promising for high-throughput phenotyping of pod photosynthetic traits. This review further discusses genetic targets and regulatory mechanisms for enhancing pod photosynthesis, including transcription factors like GOLDEN2-LIKE and GATA that may regulate photosynthetic capacity in pods, suggesting potential genetic manipulation strategies to boost crop productivity. In conclusion, unlocking the genetic and physiological bases of pod photosynthesis offers opportunities for advancing crop breeding to ensure sustainable food security amidst climate change and increasing global population pressures. Future research should focus on developing high-throughput phenotyping tools and elucidating genetic pathways to maximize pod photosynthesis in crops.

The organization of thylakoid supercomplexes are important for the efficiency of photosynthesis; however, the organization of the photoprotective pigments in supercomplexes of thylakoids in Pisum sativum under drought stress have not previously been studied. Here, we examined the photoprotective pigments, protein-pigment interactions, and macromolecular structural organization from sucrose density gradient (SDG) fractions. Solubilized thylakoid membranes were separated from SDG, in which four fractions were obtained: light-harvesting complexes (LHC)II monomer (F1), LHCII trimer (F2), photosystem (PS)II core (F3), and PSI-LHCI (F4). Circular dichroism data revealed that LHCII trimer complexes marginally changed under drought stress. In addition, significant alterations were observed in PSI-LHCI complexes compared with the PSII complexes. Under drought stress, lutein and β-carotene levels increased in the PSII core, suggesting a protective function of these pigments against drought stress. In contrast, xanthophylls, lutein, and β-carotene concentrations were reduced in PSI-LHCI, suggesting that the reduction of these pigments and of the pigment-protein complexes is not important in drought stress. Further, zeaxanthin was enhanced in LHCII trimeric complexes, which induced non-photochemical quenching due to the dissipation of excess energy absorbed by chlorophylls through Chlorophyll-Carotenoid interactions. Additionally, under drought stress, carotenoid levels were significantly enhanced in the PSII core, while lutein levels increased in PSII-LHCII complexes. The levels of photoprotective pigments are in agreement with the data obtained from the differential expression of genes involved in the production of carotenoids. Furthermore, zeaxanthin-dependent genes and proteins accumulated under drought stress, as shown by real-time PCR and western blot data, suggesting that violaxanthin is converted to zeaxanthin in drought stress. Taken together, we show that the presence of zeaxanthin and the differential expression of lutein and violaxanthin probably lead to remedial structural changes in thylakoid supercomplexes.

The oxygen-evolving center (OEC) of photosystem II (PSII) is a unique Mn4CaO5 cluster that catalyzes the water-splitting reaction to produce electrons, protons, and dioxygen. Recently, the detailed structures of the OEC in different S-states have been revealed by X-ray free-electron laser (XFEL). To facilitate understanding the structure-function relationship of the OEC, a series of artificial Mn4CaO4 clusters have been synthesized, which closely mimic the main metal-oxide core and peripheral ligands, as well as the redox properties of the OEC. Herein, we have systematically analyzed the oxidation states of all Mn ions in the structural data of the OEC revealed by XFEL and artificial Mn4CaO4 clusters. It shows that the oxidation states of some Mn ions in structural data of OEC are significantly lower than the expected values in native PSII, suggesting the occurrence of the reduction of high-valent Mn ions induced by XFEL, whereas all Mn ions in artificial Mn4CaO4 clusters have the same oxidation states as those in the S1 state OEC in native PSII. Furthermore, for the first time, we have observed that the missing μ2-O bridge in the artificial Mn4CaO4 cluster can be generated in solution, forming an unstable Mn4CaO5 cluster, which supports that this μ2-O bridge (O4) is exchangeable and may serve as the active site for O-O bond formation in the cluster. These results provide new insights into the catalytic mechanism of the oxygen-evolving reaction in both natural and artificial photosynthesis.

Bacteriochlorophylls (BChls) c and e are responsible for the main part of the light-harvesting process in chlorosome antenna systems of green sulfur bacteria, and contain a methyl group at the peripheral C-20 position of their core chlorin rings. This study performed in vitro and in vivo analysis of the C-20 methyltransferase BchU derived from the green sulfur bacterium Chlorobaculum tepidum, which synthesizes BChl c, to clarify the role of this enzyme in the biosynthetic pathway. Although the reaction step of BchU in the biosynthesis could not be determined by genetic analysis, enzymatic assays using various substrates showed that BchU reacts primarily with substrates after hydration of BchF and BchV at the C-3 position. The results in this study allow the proposition of a biosynthetic pathway for BChl c and e involving this enzyme.

Light-harvesting complexes (LHCs) play crucial roles in efficient photoenergy conversion and photoprotection of photosynthetic systems. In LHCs, functional pigments such as chlorophylls (Chls), bacteriochlorophylls (BChls), and carotenoids are sophisticatedly assembled with the help of polypeptides. The pigment assemblies in LHCs control the site-energy of each pigment, excitonic interactions among pigments, and excitation energy gradient in the protein matrix, as well as the formation and stability of the protein structure. In vitro reconstitution of LHCs is promising in understanding these structural and functional mechanisms of LHCs. In this review, we summarize two strategies of pigment reconstitution of LHCs; one is the formation of LHCs from a mixture of photosynthetic pigments and denatured polypeptides by their self-assembly, and the other is pigment substitution by the insertion of exogenous pigments into apoproteins partially lacking bound pigments. Next, we overview reconstitution studies of major LHC II derived from oxygenic photosynthetic organisms and core and peripheral antenna proteins of purple photosynthetic bacteria. Here, we focus on substituting Chls and BChls, key pigments in photosynthesis, in LHCs by the reconstitution. (B)Chl reconstitution of LHCs has allowed us to change essential parameters for the pigment-protein interactions and photofunctions, deepening our understanding of the molecular basis of the efficient light-harvesting functions. Reconstitution of LHCs will also be helpful for the modification and design of pigment-protein complexes toward utilization of sunlight energy for global problems on agricultural productivity and bioenergy production.

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.