Maleimides stimulate oxygen reduction in illuminated thylakoids

AbstractN-ethylmaleimide (NEM) and N,N′-(1,4-phenylene)dimaleimide (PDM) were discovered to stimulate light-induced oxygen uptake in isolated thylakoids, and PDM provided the same stimulation at one order less concentrations. Oxygen uptake rate increased promptly after NEM or PDM addition to thylakoids. The inhibitors of photosynthetic electron transport as well as catalase decreased this rate close to zero, whereas ascorbate increased it almost three-fold. Dithiothreitol suppressed oxygen uptake stimulated by NEM. NEM stimulated light-induced reduction of cytochrome c, and this stimulation was suppressed by superoxide dismutase. It was concluded that NEM and PDM being reduced can effectively reduce molecules O2 producing superoxide radicals

Acclimation of Chlamydomonas reinhardtii to extremely strong light

Most photosynthetic organisms are sensitive to very high light, although acclimation mechanisms enable them to deal with exposure to strong light up to a point. Here we show that cultures of wild-type Chlamydomonas reinhardtii strain cc124, when exposed to photosynthetic photon flux density 3000 μmol m−2 s−1 for a couple of days, are able to suddenly attain the ability to grow and thrive. We compared the phenotypes of control cells and cells acclimated to this extreme light (EL). The results suggest that genetic or epigenetic variation, developing during maintenance of the population in moderate light, contributes to the acclimation capability. EL acclimation was associated with a high carotenoid-to-chlorophyll ratio and slowed down PSII charge recombination reactions, probably by affecting the pre-exponential Arrhenius factor of the rate constant. In agreement with these findings, EL acclimated cells showed only one tenth of the 1O2 level of control cells. In spite of low 1O2 levels, the rate of the damaging reaction of PSII photoinhibition was similar in EL acclimated and control cells. Furthermore, EL acclimation was associated with slow PSII electron transfer to artificial quinone acceptors. The data show that ability to grow and thrive in extremely strong light is not restricted to photoinhibition-resistant organisms such as Chlorella ohadii or to high-light tolerant mutants, but a wild-type strain of a common model microalga has this ability as well. </p

Oxygen and ROS in photosynthesis

Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.</p

Genetic autonomy and low singlet oxygen yield support kleptoplast functionality in photosynthetic sea slugs

The kleptoplastic sea slug Elysia chlorotica consumes Vaucheria litorea, stealing its plastids, which then photosynthesize inside the animal cells for months. We investigated the properties of V. litorea plastids to understand how they withstand the rigors of photosynthesis in isolation. Transcription of specific genes in laboratory-isolated V. litorea plastids was monitored for 7 days. The involvement of plastid-encoded FtsH, a key plastid maintenance protease, in recovery from photoinhibition in V. litorea was estimated in cycloheximide-treated cells. In vitro comparison of V. litorea and spinach thylakoids was applied to investigate reactive oxygen species formation in V. litorea. In comparison to other tested genes, the transcripts of ftsH and translation elongation factor EF-Tu (tufA) decreased slowly in isolated V. litorea plastids. Higher levels of FtsH were also evident in cycloheximide-treated cells during recovery from photoinhibition. Charge recombination in PSII of V. litorea was found to be fine-tuned to produce only small quantities of singlet oxygen, and the plastids also contained reactive oxygen species-protective compounds. Our results support the view that the genetic characteristics of the plastids are crucial in creating a photosynthetic sea slug. The plastid’s autonomous repair machinery is likely enhanced by low singlet oxygen production and elevated expression of FtsH.</p

Oxygen reduction in chloroplast thylakoids results in production of hydrogen peroxide inside the membrane

AbstractHydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 μmol quanta m−2 s−1 it reached 60% of all electrons entering the electron transport chain. Neither the presence of a superoxide dismutase inhibitor, potassium cyanide or sodium azide, in the thylakoid suspension, nor unstacking of the thylakoids appreciably affected the partitioning of electrons to hydrogen peroxide production. Also, osmolarity-induced changes in the thylakoid lumen volume, as well as variation of the lumen pH induced by the presence of Gramicidin D, had negligible effects on such partitioning. The flow of electrons participating in lumen hydrogen peroxide production was found to be near 10% of the total electron flow from water. It is concluded that a considerable amount of hydrogen peroxide is generated inside thylakoid membranes, and a possible mechanism, as well as the significance, of this process are discussed

Experimental evidence suggesting that H2O2 is produced within the thylakoid membrane in a reaction between plastoquinol and singlet oxygen

AbstractPlastoquinol (PQH2-9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH2-9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH2-9 to singlet oxygen (1O2) and analyzed the products. The main reaction of 1O2 with PQH2-9 in methanol was found to result in formation of PQ-9 and H2O2, and the amount of H2O2 produced was essentially the same as the amount of oxidized PQH2-9. Formation of H2O2 in the reaction between 1O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane

Measurement of the redox state of the plastoquinone pool in cyanobacteria

Here, we developed a method for measuring the in vivo redox state of the plastoquinone (PQ) poolin the cyanobacteria Synechocystis sp. PCC 6803. Cells were illuminated on a glass fiber filter, PQwas extracted with ethyl acetate and determined with HPLC. Control samples with fully oxidizedand reduced photoactive PQ pool were prepared by far-red and high light treatments, respectively,or by blocking the photosynthetic electron transfer chemically before or after PQ in moderatelight. The photoactive pool comprised 50 % of total PQ. We find that the PQ poolof cyanobacteria behaves under light treatments qualitatively similarly as in plant chloroplasts, isless reduced during growth under high than under ambient CO2 and remains partly reduced indarkness.</p

Reactive Oxygen Species and Antioxidants Associated with Photosynthesis

光合成に伴い生成する活性酸素種は，細胞のレドックスシグナルとして作用すると同時に環境ストレス障害の要因ともなる．活性酸素には化学的特性の異なるいくつかの分子種があり，それぞれ特異的な検出法がある．活性酸素種およびそれを消去するアスコルビン酸やグルタチオンなどの還元剤は，寿命が短い，相互の酸化還元反応が容易に進行する，といった特性をもつため，定量的な抽出・測定に注意を要する．この節では，チラコイド膜を用いる活性酸素生成機構解析，組織抽出液の過酸化物，酸化タンパク質，抗酸化物の定量法を中心に解説する．Photosynthesis reactions are intrinsically accompanied by the production of reactive oxygen species (ROS) such as superoxide anion radical(O2·-), hydrogen peroxide, singlet oxygen (1O2) and hydroxyl radical (HO·), which influence the productivity and stress responses of plants. Selected methods for determining individual ROS in vitro and in vivo and for evaluating the redox status of ascorbate and glutathione in plant tissue extracts are described, with specific precautions.3章 単離・精製・活性測定 1. 代謝産物量の定量