Natively oxidized amino acid residues in the spinach PS I-LHC I supercomplex

Dysfunction of Chloroplast Protease Activity Mitigates pgr5 Phenotype in the Green Algae Chlamydomonas reinhardtii

Researchers have described protection mechanisms against the photoinhibition of photosystems under strong-light stress. Cyclic Electron Flow (CEF) mitigates electron acceptor-side limitation, and thus contributes to Photosystem I (PSI) protection. Chloroplast protease removes damaged protein to assist with protein turn over, which contributes to the quality control of Photosystem II (PSII). The PGR5 protein is involved in PGR5-dependent CEF. The FTSH protein is a chloroplast protease which effectively degrades the damaged PSII reaction center subunit, D1 protein. To investigate how the PSI photoinhibition phenotype in pgr5 would be affected by adding the ftsh mutation, we generated double-mutant pgr5ftsh via crossing, and its phenotype was characterized in the green algae Chlamydomonas reinhardtii. The cells underwent high-light incubation as well as low-light incubation after high-light incubation. The time course of Fv/Fm values in pgr5ftsh showed the same phenotype with ftsh1-1. The amplitude of light-induced P700 photo-oxidation absorbance change was measured. The amplitude was maintained at a low value in the control and pgr5ftsh during high-light incubation, but was continuously decreased in pgr5. During the low-light incubation after high-light incubation, amplitude was more rapidly recovered in pgr5ftsh than pgr5. We concluded that the PSI photoinhibition by the pgr5 mutation is mitigated by an additional ftsh1-1 mutation, in which plastoquinone pool would be less reduced due to damaged PSII accumulation.

Impact of growth light environment on oxygen sensitivity in rice: Pseudo-first-order response of photosystem I photoinhibition to O2 partial pressure

Photosynthetic organisms generate reactive oxygen species (ROS) during photosynthetic electron transport reactions on the thylakoid membranes within both photosystems (PSI and PSII), leading to the impairment of photosynthetic activity, known as photoinhibition. In PSI, ROS production has been suggested to follow Michaelis-Menten- or second-order reaction-dependent kinetics in response to changes in the partial pressure of O2 . However, it remains unclear whether ROS-mediated PSI photoinhibition follows the kinetics mentioned above. In this study, we aimed to elucidate the ROS production kinetics from the aspect of PSI photoinhibition in vivo. For this research objective, we investigated the O2 dependence of PSI photoinhibition by examining intact rice leaves grown under varying photon flux densities. Subsequently, we found that the degree of O2 -dependent PSI photoinhibition linearly increased in response to the increase in O2 partial pressure. Furthermore, we observed that the higher photon flux density on plant growth reduced the O2 sensitivity of PSI photoinhibition. Based on the obtained data, we investigated the O2 -dependent kinetics of PSI photoinhibition by model fitting analysis to elucidate the mechanism of PSI photoinhibition in leaves grown under various photon flux densities. Remarkably, we found that the pseudo-first-order reaction formula successfully replicated the O2 -dependent PSI photoinhibition kinetics in intact leaves. These results suggest that ROS production, which triggers PSI photoinhibition, occurs by an electron-leakage reaction from electron carriers within PSI, consistent with previous in vitro studies.

Superoxide Anion Radical Generation in Photosynthetic Electron Transport Chain

This review analyzes data available in the literature on the rates, characteristics, and mechanisms of oxygen reduction to a superoxide anion radical at the sites of photosynthetic electron transport chain where this reduction has been established. The existing assumptions about the role of the components of these sites in this process are critically examined using thermodynamic approaches and results of the recent studies. The process of O2 reduction at the acceptor side of PSI, which is considered the main site of this process taking place in the photosynthetic chain, is described in detail. Evolution of photosynthetic apparatus in the context of controlling the leakage of electrons to O2 is explored. The reasons limiting application of the results obtained with the isolated segments of the photosynthetic chain to estimate the rates of O2 reduction at the corresponding sites in the intact thylakoid membrane are discussed.

The wheat leaf delayed virescence of mutant dv4 is associated with the abnormal photosynthetic and antioxidant systems

Chlorophyll (Chl) is a key pigment for wheat (Triticum aestivum L.) photosynthesis, consequently impacts grain yield. A wheat mutant named as delayed virescence 4 (dv4) was obtained from cultivar Guomai 301 (wild type, WT) treated with ethyl methane sulfonate (EMS). The seedling leaves of dv4 were shallow yellow, apparently were chlorophyll deficient. They started to turn green at the jointing stage and returned to almost ordinary green at the heading stage. Leaf transcriptome comparison of Guomai 301 and dv4 at the jointing stage showed that most differentially expressed genes (DEGs) of transcription and translation were highly expressed in dv4, one key gene nicotianamine aminotransferase A (NAAT-A) involved in the synthesis and metabolism pathways of tyrosine, methionine and phenylalanine was significantly lowly expressed. The expression levels of the most photosynthesis related genes, such as photosystem I (PS I), ATPase and light-harvesting chlorophyll protein complex-related homeotypic genes, and protochlorophyllide reductase A (PORA) were lower; but macromolecule degradation and hypersensitivity response (HR) related gene heat shock protein 82 (HSP82) was highly expressed. Compared to WT, the contents of macromolecules such as proteins and sugars were reduced; the contents of Chl a, Chl b, total Chl, and carotenoids in leaves of dv4 were significantly less at the jointing stage, while the ratio of Chl a / Chl b was the same as that of WT. The net photosynthetic rate, stomatal conductance and transpiration rate of dv4 were significantly lower. The H₂O₂ content were higher, while the contents of total phenol and malondialdehyde (MDA), antioxidant enzyme activities were lower in leaves of dv4. In conclusion, the reduced contents of macromolecules and photosynthetic pigments, the abnormal photosynthetic and antioxidant systems were closely related to the phenotype of dv4.

Oxidative modification of LHC II associated with photosystem II and PS I-LHC I-LHC II membranes

Under aerobic conditions the production of Reactive Oxygen Species (ROS) by electron transport chains is unavoidable, and occurs in both autotrophic and heterotrophic organisms. In photosynthetic organisms both Photosystem II (PS II) and Photosystem I (PS I), in addition to the cytochrome b₆/f complex, are demonstrated sources of ROS. All of these membrane protein complexes exhibit oxidative damage when isolated from field-grown plant material. An additional possible source of ROS in PS I and PS II is the distal, chlorophyll-containing light-harvesting array LHC II, which is present in both photosystems. These serve as possible sources of ¹O₂ produced by the interaction of ³O₂ with ³chl* produced by intersystem crossing. We have hypothesized that amino acid residues close to the sites of ROS generation will be more susceptible to oxidative modification than distant residues. In this study, we have identified oxidized amino acid residues in a subset of the spinach LHC II proteins (Lhcb1 and Lhcb2) that were associated with either PS II membranes (i.e. BBYs) or PS I-LHC I-LHC II membranes, both of which were isolated from field-grown spinach. We identified oxidatively modified residues by high-resolution tandem mass spectrometry. Interestingly, two different patterns of oxidative modification were evident for the Lhcb1 and Lhcb2 proteins from these different sources. In the LHC II associated with PS II membranes, oxidized residues were identified to be located on the stromal surface of Lhcb1 and, to a much lesser extent, Lhcb2. Relatively few oxidized residues were identified as buried in the hydrophobic core of these proteins. The LHC II associated with PS I-LHC I-LHC II membranes, however, exhibited fewer surface-oxidized residues but, rather a large number of oxidative modifications buried in the hydrophobic core regions of both Lhcb1 and Lhcb2, adjacent to the chlorophyll prosthetic groups. These results appear to indicate that ROS, specifically ¹O₂, can modify the Lhcb proteins associated with both photosystems and that the LHC II associated with PS II membranes represent a different population from the LHC II associated with PS I-LHC I-LHC II membranes.

Low N level increases the susceptibility of PSI to photoinhibition induced by short repetitive flashes in leaves of different rice varieties

The recovery from photoinhibition is much slower in photosystem (PS) I than in PSII; therefore, the susceptibility of PSI to photoinhibition is important with respect to photosynthetic production under special physiological conditions. Previous studies have shown that repetitive short-pulse (rSP) illumination selectively induces PSI photoinhibition. Depending on the growth light intensity or the variety/species of the plant, PSI photoinhibition is different, but the underlying mechanisms remain unknown. Here, we aimed to clarify whether the differences in the susceptibility of PSI to photoinhibition depend on environmental factors or on rice varieties and which physiological properties of the plant are related to this susceptibility. We exposed mature leaves of rice plants to rSP illumination. We examined the effects of elevated CO₂ concentration and low N during growth on the susceptibility of PSI to photoinhibition and compared it in 12 different varieties. We fitted the decrease in the quantum yield of PSI during rSP illumination and estimated a parameter indicating susceptibility. Low N level increased susceptibility, whereas elevated CO₂ concentration did not. The susceptibility differed among different rice varieties, and many indica varieties showed higher susceptibility than the temperate japonica varieties. Susceptibility was negatively correlated with the total chlorophyll content and N content. However, the decrease in P m ' value, an indicator of damaged PSI, was positively correlated with chlorophyll content. This suggests that in leaves with a larger electron transport capacity, the overall PSI activity may be less susceptible to photoinhibition, but more damaged PSI may accumulate during rSP illumination.