Redox of plastoquinone pool regulates the expression and activity of NADPH dehydrogenase supercomplex in Synechocystis sp. strain PCC 6803

Microcystin aids in cold temperature acclimation: Differences between a toxic Microcystis wildtype and non-toxic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26 °C to 19 °C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, reactive oxygen species damage, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 ΔmcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19 °C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents.

Cool temperature acclimation in toxigenic Microcystis aeruginosa PCC 7806 and its non-toxigenic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26° C to 19° C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 ΔmcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19° C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents.

Cyanobacterial NDH-1 Complexes

Light reaction of photosynthesis is efficiently driven by protein complexes arranged in an orderly in the thylakoid membrane. As the 5th complex, NAD(P)H dehydrogenase complex (NDH-1) is involved in cyclic electron flow around photosystem I to protect plants against environmental stresses for efficient photosynthesis. In addition, two kinds of NDH-1 complexes participate in CO2 uptake for CO2 concentration in cyanobacteria. In recent years, great progress has been made in the understanding of the assembly and the structure of NDH-1. However, the regulatory mechanism of NDH-1 in photosynthesis remains largely unknown. Therefore, understanding the regulatory mechanism of NDH-1 is of great significance to reveal the mechanism of efficient photosynthesis. In this mini-review, the author introduces current progress in the research of cyanobacterial NDH-1. Finally, the author summarizes the possible regulatory mechanism of cyanobacterial NDH-1 in photosynthesis and discusses the research prospect.

Plastoquinone and Ubiquinone in Plants: Biosynthesis, Physiological Function and Metabolic Engineering

Plastoquinone (PQ) and ubiquinone (UQ) are two important prenylquinones, functioning as electron transporters in the electron transport chain of oxygenic photosynthesis and the aerobic respiratory chain, respectively, and play indispensable roles in plant growth and development through participating in the biosynthesis and metabolism of important chemical compounds, acting as antioxidants, being involved in plant response to stress, and regulating gene expression and cell signal transduction. UQ, particularly UQ₁₀, has also been widely used in people's life. It is effective in treating cardiovascular diseases, chronic gingivitis and periodontitis, and shows favorable impact on cancer treatment and human reproductive health. PQ and UQ are made up of an active benzoquinone ring attached to a polyisoprenoid side chain. Biosynthesis of PQ and UQ is very complicated with more than thirty five enzymes involved. Their synthetic pathways can be generally divided into two stages. The first stage leads to the biosynthesis of precursors of benzene quinone ring and prenyl side chain. The benzene quinone ring for UQ is synthesized from tyrosine or phenylalanine, whereas the ring for PQ is derived from tyrosine. The prenyl side chains of PQ and UQ are derived from glyceraldehyde 3-phosphate and pyruvate through the 2-C-methyl-D-erythritol 4-phosphate pathway and/or acetyl-CoA and acetoacetyl-CoA through the mevalonate pathway. The second stage includes the condensation of ring and side chain and subsequent modification. Homogentisate solanesyltransferase, 4-hydroxybenzoate polyprenyl diphosphate transferase and a series of benzene quinone ring modification enzymes are involved in this stage. PQ exists in plants, while UQ widely presents in plants, animals and microbes. Many enzymes and their encoding genes involved in PQ and UQ biosynthesis have been intensively studied recently. Metabolic engineering of UQ₁₀ in plants, such as rice and tobacco, has also been tested. In this review, we summarize and discuss recent research progresses in the biosynthetic pathways of PQ and UQ and enzymes and their encoding genes involved in side chain elongation and in the second stage of PQ and UQ biosynthesis. Physiological functions of PQ and UQ played in plants as well as the practical application and metabolic engineering of PQ and UQ are also included.

Cyanobacterial phytochrome2 regulates the heterotrophic metabolism and has a function in the heat and high-light stress response

Cyanobacteria combine the photosynthetic and respiratory electron transport in one membrane system, the thylakoid membrane. This feature requires an elaborate regulation mechanism to maintain a certain redox status of the electron transport chain, hence allowing proper photosynthetic and respiratory energy metabolism. In this context, metabolic adaptations, as seen in the light-to-dark and dark-to-light transitions, are particularly challenging. However, the molecular basis of the underlying regulatory mechanisms is not well-understood. Here, we describe a function of cyanobacterial phytochrome2 (Cph2), a phytochrome of the cyanobacterial model system Synechocystis sp. PCC 6803, in regulation of the primary energy metabolism. When cells are shifted from photoautotrophic planktonic growth to light-activated heterotrophic growth and biofilm initiation, knockout of Cph2 results in impaired growth, a decrease in the activity of Glc-6-P dehydrogenase, a decrease of the transcript abundance/activity of cytochrome-c-oxidase, and slower phycocyanin degradation. Measurements of the plastoquinone reduction confirm an impaired heterotrophic metabolism in the cph2 knockout. When cells that were adapted to heterotrophic metabolism are shifted back to light conditions, the knockout of Cph2 results in an altered photosystem II chlorophyll fluorescence induction curve, which is indicative of an impaired redox balance of the electron transport chain. Moreover, Cph2 plays a role in the heat and high-light stress response, particularly under photomixotrophic conditions. Our results show a function of Cph2 in the adaptation of the primary energy metabolism to changing trophic conditions. The physiological role of Cph2 in biofilm formation is discussed.

Genetic identification of a high-affinity Ni transporter and the transcriptional response to Ni deprivation in Synechococcus sp. strain WH8102

One biological need for Ni in marine cyanobacteria stems from the utilization of the Ni metalloenzyme urease for the assimilation of urea as a nitrogen source. In many of the same cyanobacteria, including Synechococcus sp. strain WH8102, an additional and obligate nutrient requirement for Ni results from usage of a Ni superoxide dismutase (Ni-SOD), which is encoded by sodN. To better understand the effects of Ni deprivation on WH8102, parallel microarray-based analysis of gene expression and gene knockout experiments were conducted. The global transcriptional response to Ni deprivation depends upon the nitrogen source provided for growth; fewer than 1% of differentially expressed genes for Ni deprivation on ammonium or urea were concordantly expressed. Surprisingly, genes for putative Ni transporters, including one colocalized on the genome with sodN, sodT, were not induced despite an increase in Ni transport. Knockouts of the putative Ni transporter gene sodT appeared to be lethal in WH8102, so the genes for sodT and sodN in WH8102 were interrupted with the gene for Fe-SOD, sodB, and its promoter from Synechococcus sp. strain WH7803. The sodT::sodB exconjugants were unable to grow at low Ni concentrations, confirming that SodT is a Ni transporter. The sodN::sodB exconjugants displayed higher growth rates at low Ni concentrations than did the wild type, presumably due to a relaxed competition between urease and Ni-SOD for Ni. Both sodT::sodB and sodN::sodB lines exhibited an impaired ability to grow at low Fe concentrations. We propose a posttranslational allosteric SodT regulation involving the binding of Ni to a histidine-rich intracellular protein loop.

SHORT-TERM EFFECTS OF ACETATE AND MICROAEROBIC CONDITIONS ON PHOTOSYNTHESIS AND RESPIRATION IN CHLORELLA SOROKINIANA GXNN 01 (CHLOROPHYTA)(1)

The culture of microalgae using organic carbon sources decreases the cost of operation in closed systems. The effect of carbon sources on microalgae is thus an interesting problem in not only theoretical research but also practical production. The short-term effects of acetate and microaerobic conditions on the growth, photosynthesis, and respiration of the green microalga Chlorella sorokiniana I. Shihira & R.W. Krauss GXNN 01 were described after acetate addition to autotrophic cultures. As the acetate concentration increased, cells needed a longer lag phase to grow, and 243.8 mM acetate completely inhibited growth. Acetate addition induced an immediate response in photosynthesis and respiration. The activity of PS II and PS I were impaired and declined with different rates, and then recovered compared with autotrophic cells. Carbonic anhydrase and Rubisco activities were also inhibited at the beginning, and respiration was increased. We propose that ATP consumption for acetate assimilation results in surplus NADPH, and then accumulated reducing power over-reduces inter-photosystem components and raises the transthylakoid proton gradient, which redistributes energy between PS I and PS II, and leads to a decrease in the PS II/PS I ratio and O2 evolution. An apparent cyclic electron flow was also observed, which may be mainly mediated by NAD(P)H dehydrogenase-dependent pathway since NADPH was in excess. These observations pointed to an acclimation process after acetate addition, and suggested the interaction between photosynthesis and respiration involving ATP and reducing power.

The interrelationship between the lower oxygen limit, chlorophyll fluorescence and the xanthophyll cycle in plants

The lower oxygen limit (LOL) in plants may be identified through the measure of respiratory gases [i.e. the anaerobic compensation point (ACP) or the respiratory quotient breakpoint (RQB)], but recent work shows it may also be identified by a sudden rise in dark minimum fluorescence (F(o)). The interrelationship between aerobic respiration and fermentative metabolism, which occur in the mitochondria and cytosol, respectively, and fluorescence, which emanates from the chloroplasts, is not well documented in the literature. Using spinach (Spinacia oleracea), this study showed that F(o) and photochemical quenching (q(P)) remained relatively unchanged until O(2) levels dropped below the LOL. An over-reduction of the plastoquinone (PQ) pool is believed to increase F(o) under dark + anoxic conditions. It is proposed that excess cytosolic reductant due to inhibition of the mitochondria's cytochrome oxidase under low-O(2), may be the primary reductant source. The maximum fluorescence (F(m)) is largely unaffected by low-O(2) in the dark, but was severely quenched, mirroring changes to the xanthophyll de-epoxidation state (DEPS), under even low-intensity light (≈4 μmol m(-2) s(-1)). In low light, the low-O(2)-induced increase in F(o) was also quenched, likely by non-photochemical and photochemical means. The degree of quenching in the light was negatively correlated with the level of ethanol fermentation in the dark. A discussion detailing the possible roles of cyclic electron flow, the xanthophyll cycle, chlororespiration and a pathway we termed 'chlorofermentation' were used to interpret fluorescence phenomena of both spinach and apple (Malus domestica) over a range of atmospheric conditions under both dark and low-light.

The role of carotenoid isomerase in maintenance of photosynthetic oxygen evolution in rice plant

Carotenoid isomerase (CRTISO) has been suggested to protect photosystem II (PS II) from photodamage, probably through its product lutein. However, the mechanism of the photoprotection still remains to be further elucidated. In this work, we cloned a point mutated gene reported to encode a CRTISO which is responsible for the accumulation of lutein in rice mutant zel1 by a map-based cloning approach. The mutant phenotype was rescued by transformation with the corresponding gene of the wild type (WT). The activity of photosynthetic oxygen evolution was evidently suppressed in zel1. The amount of the core protein of PS II CP47 was much lower in all the PS II complexes especially in the LHCII-PS II supercomplexes and CP43-free PS II of zel1 than that of WT. On the other hand, the amount of another core protein of PS II CP43 of zel1 was decreased in the higher supercomplexes, whereas it was increased in the lower ones and PS II monomer. The immunodetection displayed that CP43, CP47, and the oxygen-evolving extrinsic proteins PsbO and PsbP were reduced, but the amount of reaction center protein D1 did not show significant change in zel1. Northern blot analysis showed that the transcriptional level of CP43 was down-regulated but not that of CP47 or D1 in zel1. In addition, the plastoquinone (PQ) Q(A) was in a reduced state in zel1. On the basis of the results, we suggest that CRTISO might function in regulating the transcription of CP43 and the translation of CP47 by affecting the redox state of the PQ to stabilize the extrinsic proteins of oxygen evolution complexes in the rice plant.

The response of electron transport mediated by active NADPH dehydrogenase complexes to heat stress in the cyanobacterium Synechocystis 6803

The electron-transport machinery in photosynthetic membranes is known to be very sensitive to heat. In this study, the rate of electron transport (ETR) driven by photosystem I (PSI) and photosystem II (PSII) during heat stress in the wild-type Synechocystis sp. strain PCC 6803 (WT) and its ndh gene inactivation mutants DeltandhB (M55) and DeltandhD1/ndhD2 (D1/D2) was simultaneously assessed by using the novel Dual-PAM-100 measuring system. The rate of electron transport driven by the photosystems (ETR(PSs)) in the WT, M55, and D1/D2 cells incubated at 30 degrees C and at 55 degrees C for 10 min was compared. Incubation at 55 degrees C for 10 min significantly inhibited PSII-driven ETR (ETR(PSII)) in the WT, M55 and D1/D2 cells, and the extent of inhibition in both the M55 and D1/D2 cells was greater than that in the WT cells. Further, PSI-driven ETR (ETR(PSI)) was stimulated in both the WT and D1/D2 cells, and this rate was increased to a greater extent in the D1/D2 than in the WT cells. However, ETR(PSI) was considerably inhibited in the M55 cells. Analysis of the effect of heat stress on ETR(PSs) with regard to the alterations in the 2 active NDH-1 complexes in the WT, M55, and D1/D2 cells indicated that the active NDH-1 supercomplex and mediumcomplex are essential for alleviating the heat-induced inhibition of ETR(PSII) and for accelerating the heat-induced stimulation of ETR(PSI), respectively. Further, it is believed that these effects are most likely brought about by the electron transport mediated by each of these 2 active NDH-1 complexes.