Both external and internal factors induce heterogeneity in senescing leaves of deciduous trees

Autumn senescence is characterised by spatial and temporal heterogeneity. We show that senescing birch (Betula spp.) leaves had lower PSII activity (probed by the F V /F M chlorophyll a fluorescence parameter) in late autumn than in early autumn. We confirmed that PSII repair slows down with decreasing temperature, while rates of photodamage and recovery, measured under laboratory conditions at 20°C, were similar in these leaves. We propose that low temperatures during late autumn hinder repair and lead to accumulation of non-functional PSII units in senescing leaves. Fluorescence imaging of birch revealed that chlorophyll preferentially disappeared from inter-veinal leaf areas. These areas showed no recovery capacity and low non-photochemical quenching while green veinal areas of senescing leaves resembled green leaves. However, green and yellow leaf areas showed similar values of photochemical quenching. Analyses of thylakoids isolated from maple (Acer platanoides ) leaves showed that red, senescing leaves contained high amounts of carotenoids and α-tocopherol, and our calculations suggest that α-tocopherol was synthesised during autumn. Thylakoids isolated from red maple leaves produced little singlet oxygen, probably due to the high antioxidant content. However, the rate of PSII photodamage did not decrease. The data show that the heterogeneity of senescing leaves must be taken into account to fully understand autumn senescence.

Transcriptomic and proteomic analyses of distinct Arabidopsis organs reveal high PSI-NDH complex accumulation in stems

In addition to leaves, the main site of photosynthetic reactions, active photosynthesis also takes place in stems, siliques and tree trunks. Although non-foliar photosynthesis has a marked effect on plant growth and yield, only limited information on the expression patterns of photosynthesis-related genes and the structure of photosynthetic machinery in different plant organs has been available. Here, we report the results of transcriptomic analysis of various organs of Arabidopsis thaliana and compare the gene expression profiles of young and mature leaves with a special focus on photosynthetic genes. Further, we analyzed the composition and organization of the photosynthetic electron transfer machinery in leaves, stems and green siliques at the protein level using BN-PAGE. RNA-Seq analysis revealed unique gene expression profiles in different plant organs and showed major differences in the expression of photosynthesis-related genes in young as compared to mature rosettes. Gel-based proteomic analysis of the thylakoid protein complex organization further showed that all studied plant organs contain the necessary components of the photosynthetic electron transfer chain. Intriguingly, stems accumulate high amounts of PSI-NDH complex, which has previously been implicated in cyclic electron transfer.

Biophysical and molecular characteristics of senescing leaves of two Norway maple varieties differing in anthocyanin content

Disassembly and degradation of the photosynthetic protein complexes during autumn senescence, a vital step to ensure efficient nutrient relocalization for winter storage, is poorly understood. Concomitantly with the degradation, anthocyanins are often synthesized. However, as to why leaves accumulate red pigments, no consensus exists. One possibility is that anthocyanins protect senescing leaves from excess light. In this study, we investigated the pigment composition, photosynthetic performance, radical production, and degradation of the photosynthetic protein complexes in Norway maple (Acer platanoides) and in its highly pigmented, purple-colored variety (Faassen's black) during autumn senescence, to dissect the possible roles of anthocyanins in photoprotection. Our findings show that senescing Faassen's black was indeed more resistant to Photosystem II (PSII) photoinhibition, presumably due to its high anthocyanin content, than the green maple. However, senescing Faassen's black exhibited low photosynthetic performance, probably due to a poor capacity to repair PSII. Furthermore, an analysis of photosynthetic protein complexes demonstrated that in both maple varieties, the supercomplexes consisting of PSII and its antenna were disassembled first, followed by the degradation of the PSII core, Photosystem I, Cytochrome b6 f, and ATP synthase. Strikingly, the degradation process appeared to proceed faster in Faassen's black, possibly explaining its poor PSII repair capacity. The results suggest that tolerance against PSII photoinhibition may not necessarily translate to a better fitness. Finally, thylakoids isolated from senescing and non-senescing leaves of both maple varieties accumulated very little carbon-centered radicals, suggesting that thylakoids may not be a major source of reactive oxygen species in senescing leaves.

Red pigments in autumn leaves of Norway maple do not offer significant photoprotection but coincide with stress symptoms

The reasons behind autumn colors, a striking manifestation of anthocyanin synthesis in plants, are poorly understood. Usually, not all leaves of an anthocyanic plant turn red or only a part of the leaf blade turns red. In the present study, we compared green, red and yellow sections of senescing Norway maple leaves, asking if red pigments offer photoprotection, and if so, whether the protection benefits the senescing tree. Green and senescing maple leaves were illuminated with strong white, green or red light in the absence or presence of lincomycin which blocks photosystem II (PSII) repair. Irrespective of the presence of anthocyanins, senescing leaves showed weaker capacity to repair PSII than green leaves. Furthermore, the rate of photoinhibition of PSII did not significantly differ between red and yellow sections of senescing maple leaves. We also followed pigment contents and photosynthetic reactions in individual leaves, from the end of summer until abscission of the leaf. In maple, red pigments accumulated only during late senescence, but light reactions stayed active until most of the chlorophyll had been degraded. PSII activity was found to be lower and non-photochemical quenching higher in red leaf sections, compared with yellow sections of senescing leaves. Red leaf sections were also thicker. We suggest that the primary function of anthocyanin synthesis is not to protect senescing leaves from excess light but to dispose of carbohydrates. This would relieve photosynthetic control, allowing the light reactions to produce energy for nutrient translocation at the last phase of autumn senescence when carbon skeletons are no longer needed.

Singlet oxygen production by photosystem II is caused by misses of the oxygen evolving complex

Singlet oxygen (¹ O₂ ) is a harmful species that functions also as a signaling molecule. In chloroplasts, ¹ O₂ is produced via charge recombination reactions in photosystem II, but which recombination pathway(s) produce triplet Chl and ¹ O₂ remains open. Furthermore, the role of ¹ O₂ in photoinhibition is not clear. We compared temperature dependences of ¹ O₂ production, photoinhibition, and recombination pathways. ¹ O₂ production by pumpkin thylakoids increased from -2 to +35°C, ruling out recombination of the primary charge pair as a main contributor. S₂ Q_A ^- or S₂ Q_B ^- recombination pathways, in turn, had too steep temperature dependences. Instead, the temperature dependence of ¹ O₂ production matched that of misses (failures of the oxygen (O₂ ) evolving complex to advance an S-state). Photoinhibition in vitro and in vivo (also in Synechocystis), and in the presence or absence of O₂ , had the same temperature dependence, but ultraviolet (UV)-radiation-caused photoinhibition showed a weaker temperature response. We suggest that the miss-associated recombination of P₆₈₀ ⁺ Q_A ^- is the main producer of ¹ O₂ . Our results indicate three parallel photoinhibition mechanisms. The manganese mechanism dominates in UV radiation but also functions in white light. Mechanisms that depend on light absorption by Chls, having ¹ O₂ or long-lived P₆₈₀ ⁺ as damaging agents, dominate in red light.

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.

Single-walled carbon nanotubes protect photosynthetic reactions in Chlamydomonas reinhardtii against photoinhibition

Single-walled carbon nanotubes (SWCNTs) are among the most exploited carbon allotropes in nanosensing, bioengineering, and photobiological applications, however, the interactions of nanotubes with the photosynthetic process and structures are still poorly understood. We found that SWCNTs are not toxic to the photosynthetic apparatus of the model unicellular alga Chlamydomonas reinhardtii and demonstrate that this carbon nanomaterial can protect algal photosynthesis against photoinhibition. The results show that the inherent phytotoxicity of the nanotubes may be overcome by an intentional selection of nanomaterial characteristics. A low concentration (2 μg mL-1) of well-dispersed, purified and small SWCNTs did not alter the growth and pigment accumulation of the cultures. Indeed, under the photoinhibitory conditions of our experiments, SWCNT-enriched samples were characterized by a lower rate of PSII inactivation, reduced excitation pressure in PSII, a higher rate of photosynthetic electron transport, and an increased non-photochemical quenching in comparison with the controls. In addition, SWCNTs change the distribution of energy between the photosystems in favour of PSII (state 1). The underlying mechanism of this action is not yet understood but possibly, electrons or energy can be exchanged between the redox active nanotubes and photosynthetic components, and probably other redox active intra-chloroplast constituents. Alternatively, nanotubes may promote the formation of an NPQ conformation of PSII. Our results provided evidence for such electron/energy transfer from photosynthetic structures toward the nanotubes. The discovered photoprotective effects can potentially be used in photobiotechnology to maintain the photosynthetic activity of microorganisms under unfavourable conditions.

Light-induced damage to photosystem II at a very low temperature (195 K) depends on singlet oxygen

Photosynthetic organisms, like evergreen plants, may encounter strong light at low temperatures. Light, despite being the energy source of photosynthesis, irreversibly damages photosystem II (PSII). We illuminated plant thylakoid membranes and intact cyanobacterial cells at -78.5°C and assayed PSII activity with oxygen evolution or chlorophyll fluorescence, after thawing the sample. Both UV radiation and visible light damaged PSII of pumpkin (Cucurbita maxima) thylakoids at -78.5°C, but visible-light-induced photoinhibition at -78.5°C, unlike at +20°C, proceeded only in the presence of oxygen. A strong magnetic field that would decrease triplet chlorophyll formation by recombination of the primary radical pair slowed down photoinhibition at -78.5°C, suggesting that singlet oxygen produced via recombination of the primary pair is a major contributor to photoinhibition at -78.5°C. However, a magnetic field did not affect singlet oxygen production at +25°C. Thylakoids of winter leaves of an evergreen plant, Bergenia, were less susceptible to photoinhibition both at -78.5°C and +20°C, contained high amounts of carotenoids and produced little singlet oxygen (measured at +20°C), compared to thylakoids of summer leaves. In contrast, high carotenoid amount and low singlet oxygen yield did not protect a Synechocystis mutant from photoinhibition at -78.5°C. Thylakoids isolated from Arabidopsis thaliana grown under high light, which reduces PSII antenna size, were more resistant than control plants against photoinhibition at -78.5°C but not at +20°C, although carotenoid amounts were similar. The results indicate that visible-light-induced photoinhibition at -78.5°C depends on singlet oxygen, whereas photoinhibition at +20°C is largely independent of oxygen.

Ultraviolet screening by slug tissue and tight packing of plastids protect photosynthetic sea slugs from photoinhibition

One of the main mysteries regarding photosynthetic sea slugs is how the slug plastids handle photoinhibition, the constant light-induced damage to Photosystem II of photosynthesis. Recovery from photoinhibition involves proteins encoded by both the nuclear and plastid genomes, and slugs with plastids isolated from the algal nucleus are therefore expected to be incapable of constantly repairing the damage as the plastids inside the slugs grow old. We studied photoinhibition-related properties of the sea slug Elysia timida that ingests its plastids from the green alga Acetabularia acetabulum. Spectral analysis of both the slugs and the algae revealed that there are two ways the slugs use to avoid major photoinhibition of their plastids. Firstly, highly photoinhibitory UV radiation is screened by the slug tissue or mucus before it reaches the plastids. Secondly, the slugs pack the plastids tightly in their thick bodies, and therefore plastids in the outer layers protect the inner ones from photoinhibition. Both properties are expected to greatly improve the longevity of the plastids inside the slugs, as the plastids do not need to repair excessive amounts of damage.

Foliar element distributions in Guadua bamboo, a major forest dominant in southwestern Amazonia

Silica is the best-known component filling the spaces that form phytoliths in many plants, but phytoliths may also contain other elements. We used scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX) to map element distributions in the leaves of Guadua bamboo, which is a successful competitor in southwestern Amazonian forests. We emphasize immobile elements that can be mineralized (silicon and calcium) as well as potassium, an abundant mobile nutrient with many vital functions. We discovered high silicon (Si) content with little or no calcium (Ca) or potassium (K) in bulliform cells, bilobate shaped short cells and stomata, all of which can form phytoliths, and moderately high Si content in the bundle sheet, prickle tips and papillae. K often surrounded Si-loaded cells, Si and K had overlapping distributions in the intercostal areas near vein margins, and Ca showed abundant spotted distribution in the intercostal areas. The dark inside content of the costal zones of some samples in light microscopy showed no mineralization but prominently contained sulfur. Adjacent fusoid cells showed different Si, K and Ca combinations, which may suggest potentially variable functions of these cells. Widespread Si deposition strengthens Guadua bamboo leaves and may help it to outcompete tree species during periods of drought.

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.

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.

Root-type ferredoxin-NADP+ oxidoreductase isoforms in Arabidopsis thaliana: Expression patterns, location and stress responses

In Arabidopsis, two leaf-type ferredoxin-NADP⁺ oxidoreductase (LFNR) isoforms function in photosynthetic electron flow in reduction of NADP⁺ , while two root-type FNR (RFNR) isoforms catalyse reduction of ferredoxin in non-photosynthetic plastids. As the key to understanding, the function of RFNRs might lie in their spatial and temporal distribution in different plant tissues and cell types, we examined expression of RFNR1 and RFNR2 genes using β-glucuronidase (GUS) reporter lines and investigated accumulation of distinct RFNR isoforms using a GFP approach and Western blotting upon various stresses. We show that while RFNR1 promoter is active in leaf veins, root tips and in the stele of roots, RFNR2 promoter activity is present in leaf tips and root stele, epidermis and cortex. RFNR1 protein accumulates as a soluble protein within the plastids of root stele cells, while RFNR2 is mainly present in the outer root layers. Ozone treatment of plants enhanced accumulation of RFNR1, whereas low temperature treatment specifically affected RFNR2 accumulation in roots. We further discuss the physiological roles of RFNR1 and RFNR2 based on characterization of rfnr1 and rfnr2 knock-out plants and show that although the function of these proteins is partly redundant, the RFNR proteins are essential for plant development and survival.

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.

Action spectrum of the redox state of the plastoquinone pool defines its function in plant acclimation

The plastoquinone (PQ) pool mediates electron flow and regulates photoacclimation in plants. Here we report the action spectrum of the redox state of the PQ pool in Arabidopsis thaliana, showing that 470-500, 560 or 650-660 nm light favors Photosystem II (PSII) and reduces the PQ pool, whereas 420-440, 520 or 690 nm light favors Photosystem I (PSI) and oxidizes PQ. These data were used to construct a model predicting the redox state of PQ from the spectrum of any polychromatic light source. Moderate reduction of the PQ pool induced transition to light state 2, whereas state 1 required highly oxidized PQ. In low-intensity PSI light, PQ was more oxidized than in darkness and became gradually reduced with light intensity, while weak PSII light strongly reduced PQ. Natural sunlight was found to favor PSI, which enables plants to use the redox state of the PQ pool as a measure of light intensity.

Photosynthetic sea slugs induce protective changes to the light reactions of the chloroplasts they steal from algae

Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own cells, and mechanisms that allow preservation of the chloroplasts are unknown. We found that the slug Elysia timida induces changes to the photosynthetic light reactions of the chloroplasts it steals from the alga Acetabularia acetabulum. Working with a large continuous laboratory culture of both the slugs (>500 individuals) and their prey algae, we show that the plastoquinone pool of slug chloroplasts remains oxidized, which can suppress reactive oxygen species formation. Slug chloroplasts also rapidly build up a strong proton-motive force upon a dark-to-light transition, which helps them to rapidly switch on photoprotective non-photochemical quenching of excitation energy. Finally, our results suggest that chloroplasts inside E. timida rely on oxygen-dependent electron sinks during rapid changes in light intensity. These photoprotective mechanisms are expected to contribute to the long-term functionality of the chloroplasts inside the slugs.

Plants, algae and a few other organisms rely on a process known as photosynthesis to fuel themselves, as they can harness cellular structures called chloroplasts to convert light into usable energy. Animals typically lack chloroplasts, making them unable to use photosynthesis to power themselves. The sea slug Elysia timida, however, can steal whole chloroplasts from the cells of the algae it consumes: the stolen structures then become part of the cells in the gut of the slug, allowing the animal to gain energy from sunlight.

Once they are in the digestive system of the slug, the chloroplasts survive and keep working for longer than expected. Indeed, these structures are often harmed as a side effect of photosynthesis, but the sea slug does not have the right genes to help repair this damage. In addition, conditions inside animal cells are widely different to the ones found inside algae and plants. It is not clear then how the sea slug extends the lifespan of its chloroplasts by preventing damage caused by sunlight.

To investigate this question, Havurinne and Tyystjärvi compared photosynthesis in sea slugs and the algae they eat. A range of methods, including measuring fluorescence from the chloroplasts, was used: this revealed that the slug changes the inside of the stolen chloroplasts, making them more resistant to damage.

First, when exposed to light the stolen chloroplasts can quickly switch on a mechanism that dissipates light energy to heat, which is less damaging. Second, a molecule that serves as an intermediate during photosynthesis is kept in a ‘safe’ state which prevents it from creating harmful compounds. And finally, additional safeguard molecules ‘deactivate’ compounds that could otherwise mediate damaging reactions. Overall, these measures may reduce the efficiency of the chloroplasts but allow them to keep working for much longer.

Early chloroplasts were probably independent bacteria that were captured and ‘domesticated’ by other cells for their ability to extract energy from the sun. Photosynthesizing sea slugs therefore provide an interesting way to understand some of the challenges of early life. The work by Havurinne and Tyystjärvi may also reveal new ways to harness biological processes such as photosynthesis for energy production in other contexts.

Effects of low temperature on photoinhibition and singlet oxygen production in four natural accessions of Arabidopsis

Low temperature decreases PSII damage in vivo, confirming earlier in vitro results. Susceptibility to photoinhibition differs among Arabidopsis accessions and moderately decreases after 2-week cold-treatment. Flavonols may alleviate photoinhibition. The rate of light-induced inactivation of photosystem II (PSII) at 22 and 4 °C was measured from natural accessions of Arabidopsis thaliana (Rschew, Tenela, Columbia-0, Coimbra) grown under optimal conditions (21 °C), and at 4 °C from plants shifted to 4 °C for 2 weeks. Measurements were done in the absence and presence of lincomycin (to block repair). PSII activity was assayed with the chlorophyll a fluorescence parameter F_v/F_m and with light-saturated rate of oxygen evolution using a quinone acceptor. When grown at 21 °C, Rschew was the most tolerant to photoinhibition and Coimbra the least. Damage to PSII, judged from fitting the decrease in oxygen evolution or F_v/F_m to a first-order equation, proceeded more slowly or equally at 4 than at 22 °C. The 2-week cold-treatment decreased photoinhibition at 4 °C consistently in Columbia-0 and Coimbra, whereas in Rschew and Tenela the results depended on the method used to assay photoinhibition. The rate of singlet oxygen production by isolated thylakoid membranes, measured with histidine, stayed the same or slightly decreased with decreasing temperature. On the other hand, measurements of singlet oxygen from leaves with Singlet Oxygen Sensor Green suggest that in vivo more singlet oxygen is produced at 4 °C. Under high light, the PSII electron acceptor Q_A was more reduced at 4 than at 22 °C. Singlet oxygen production, in vitro or in vivo, did not decrease due to the cold-treatment. Epidermal flavonols increased during the cold-treatment and, in Columbia-0 and Coimbra, the amount correlated with photoinhibition tolerance.

Dissecting the interaction of photosynthetic electron transfer with mitochondrial signalling and hypoxic response in the Arabidopsis rcd1 mutant

The Arabidopsis mutant rcd1 is tolerant to methyl viologen (MV). MV enhances the Mehler reaction, i.e. electron transfer from Photosystem I (PSI) to O₂, generating reactive oxygen species (ROS) in the chloroplast. To study the MV tolerance of rcd1, we first addressed chloroplast thiol redox enzymes potentially implicated in ROS scavenging. NADPH-thioredoxin oxidoreductase type C (NTRC) was more reduced in rcd1. NTRC contributed to the photosynthetic and metabolic phenotypes of rcd1, but did not determine its MV tolerance. We next tested rcd1 for alterations in the Mehler reaction. In rcd1, but not in the wild type, the PSI-to-MV electron transfer was abolished by hypoxic atmosphere. A characteristic feature of rcd1 is constitutive expression of mitochondrial dysfunction stimulon (MDS) genes that affect mitochondrial respiration. Similarly to rcd1, in other MDS-overexpressing plants hypoxia also inhibited the PSI-to-MV electron transfer. One possible explanation is that the MDS gene products may affect the Mehler reaction by altering the availability of O₂. In green tissues, this putative effect is masked by photosynthetic O₂ evolution. However, O₂ evolution was rapidly suppressed in MV-treated plants. Transcriptomic meta-analysis indicated that MDS gene expression is linked to hypoxic response not only under MV, but also in standard growth conditions. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Testing the Potential of Regulatory Sigma Factor Mutants for Wastewater Purification or Bioreactor Run in High Light

It is shown that a freshly inoculated culture of the model cyanobacterium Synechocystis sp. PCC 6803 consumed almost all phosphate and 50% of nitrate within 6 days from the nutrient-rich BG-11 growth medium, indicating potential of cyanobacteria to purify wastewaters. Synechocystis sp. PCC 6803 control strain also collected nutrients efficiently from a landfill leachate wastewater KA2 (5.9-6.9 mM ammonium and 0.073-0.077 mM phosphate). Wastewaters might induce oxidative stress to microalgae, which prompted us to test growth of sigma factor inactivation strains, as ΔsigBCE and ΔsigCDE strains show superior growth in chemically induced oxidative stress. All cyanobacterial strains, including a stress-sensitive strain ΔsigBCDE, grew well in KA2 for four days, indicating that KA2 did not cause immediate oxidative stress. Completely arrested growth and bleaching of ΔsigBCDE cells after one week in KA2 wastewater point to the importance of group 2 sigma factor-mediated changes in gene expression during wastewater treatment. The growth of ΔsigBCD was arrested early in un-buffered and Hepes buffered (pH 7.5) KA2. In ΔsigBCD, all phosphate transporter genes are upregulated in standard conditions, and ΔsigBCD cells showed growth defects in low-phosphate BG-11 medium. ΔsigBCD cells removed phosphate slower from KA2 than the control strain, but phosphate supplementation of KA2 did not improve growth of ΔsigBCD. The ΔsigBCE strain showed superior growth in a laboratory-scale bioreactor in bright light and removed phosphate even slightly more efficiently than the control strain if KA2 was Hepes buffered although ΔsigBCE grew slowly in un-buffered KA2 and in low-phosphate BG-11 medium. The results indicate that engineering expression of regulatory group 2 sigma factor(s) might be useful for practical applications.

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.

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) pool in the cyanobacteria Synechocystis sp. PCC 6803. Cells were illuminated on a glass fiber filter, PQ was extracted with ethyl acetate and determined with HPLC. Control samples with fully oxidized and 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 moderate light. The photoactive pool comprised 50% of total PQ. We find that the PQ pool of cyanobacteria behaves under light treatments qualitatively similarly as in plant chloroplasts, is less reduced during growth under high than under ambient CO₂ and remains partly reduced in darkness.

Unresolved quenching mechanisms of chlorophyll fluorescence may invalidate MT saturating pulse analyses of photosynthetic electron transfer in microalgae

Chlorophyll a fluorescence is a powerful tool for estimating photosynthetic efficiency, but there are still unanswered questions that hinder the use of its full potential. The present results describe a caveat in estimation of photosynthetic performance with so-called rapid light curves (RLCs) with pulse amplitude modulation fluorometers. RLCs of microalgae show a severe decrease in photosynthetic performance in high light, although a similar decrease cannot be seen with other methods. We show that this decrease cannot be assigned to energy-dependent non-photochemical quenching or photoinhibition or to the geometry of the algal sample. The measured decrease in electron transfer rate is small in the tested siphonaceuous algae and higher plants, but very notable in all planktonic species, exhibiting species-dependent variation in extent and reversibility. We performed in-depth analysis of the phenomenon in the diatom Phaeodactylum tricornutum, in which the decrease is the most pronounced and reversible among the tested organisms. The results suggest that quenching of fluorescence by oxidized plastoquinone alone cannot explain the phenomenon, and alternative quenching mechanisms within PSII need to be considered.

Arabidopsis RCD1 coordinates chloroplast and mitochondrial functions through interaction with ANAC transcription factors

Reactive oxygen species (ROS)-dependent signaling pathways from chloroplasts and mitochondria merge at the nuclear protein RADICAL-INDUCED CELL DEATH1 (RCD1). RCD1 interacts in vivo and suppresses the activity of the transcription factors ANAC013 and ANAC017, which mediate a ROS-related retrograde signal originating from mitochondrial complex III. Inactivation of RCD1 leads to increased expression of mitochondrial dysfunction stimulon (MDS) genes regulated by ANAC013 and ANAC017. Accumulating MDS gene products, including alternative oxidases (AOXs), affect redox status of the chloroplasts, leading to changes in chloroplast ROS processing and increased protection of photosynthetic apparatus. ROS alter the abundance, thiol redox state and oligomerization of the RCD1 protein in vivo, providing feedback control on its function. RCD1-dependent regulation is linked to chloroplast signaling by 3'-phosphoadenosine 5'-phosphate (PAP). Thus, RCD1 integrates organellar signaling from chloroplasts and mitochondria to establish transcriptional control over the metabolic processes in both organelles.

Most plant cells contain two types of compartments, the mitochondria and the chloroplasts, which work together to supply the chemical energy required by life processes. Genes located in another part of the cell, the nucleus, encode for the majority of the proteins found in these compartments.

At any given time, the mitochondria and the chloroplasts send specific, ‘retrograde’ signals to the nucleus to turn on or off the genes they need. For example, mitochondria produce molecules known as reactive oxygen species (ROS) if they are having problems generating energy. These molecules activate several regulatory proteins that move into the nucleus and switch on MDS genes, a set of genes which helps to repair the mitochondria. Chloroplasts also produce ROS that can act as retrograde signals. It is still unclear how the nucleus integrates signals from both chloroplasts and mitochondria to ‘decide’ which genes to switch on, but a protein called RCD1 may play a role in this process. Indeed, previous studies have found that Arabidopsis plants that lack RCD1 have defects in both their mitochondria and chloroplasts. In these mutant plants, the MDS genes are constantly active and the chloroplasts have problems making ROS.

To investigate this further, Shapiguzov, Vainonen et al. use biochemical and genetic approaches to study RCD1 in Arabidopsis. The experiments confirm that this protein allows a dialog to take place between the retrograde signals of both mitochondria and chloroplasts. On one hand, RCD1 binds to and inhibits the regulatory proteins that usually activate the MDS genes under the control of mitochondria. This explains why, in the absence of RCD1, the MDS genes are always active, which is ultimately disturbing how these compartments work.

On the other hand, RCD1 is also found to be sensitive to the ROS that chloroplasts produce. This means that chloroplasts may be able to affect when mitochondria generate energy by regulating the protein. Finally, further experiments show that MDS genes can affect both mitochondria and chloroplasts: by influencing how these genes are regulated, RCD1 therefore acts on the two types of compartments.

Overall, the work by Shapiguzov, Vainonen et al. describes a new way Arabidopsis coordinates its mitochondria and chloroplasts. Further studies will improve our understanding of how plants regulate these compartments in different environments to produce the energy they need. In practice, this may also help plant breeders create new varieties of crops that produce energy more efficiently and which better resist to stress.

Photosystem-II D1 protein mutants of Chlamydomonas reinhardtii in relation to metabolic rewiring and remodelling of H-bond network at QB site

Photosystem II (PSII) reaction centre D1 protein of oxygenic phototrophs is pivotal for sustaining photosynthesis. Also, it is targeted by herbicides and herbicide-resistant weeds harbour single amino acid substitutions in D1. Conservation of D1 primary structure is seminal in the photosynthetic performance in many diverse species. In this study, we analysed built-in and environmentally-induced (high temperature and high photon fluency – HT/HL) phenotypes of two D1 mutants of Chlamydomonas reinhardtii with Ala250Arg (A250R) and Ser264Lys (S264K) substitutions. Both mutations differentially affected efficiency of electron transport and oxygen production. In addition, targeted metabolomics revealed that the mutants undergo specific differences in primary and secondary metabolism, namely, amino acids, organic acids, pigments, NAD, xanthophylls and carotenes. Levels of lutein, β-carotene and zeaxanthin were in sync with their corresponding gene transcripts in response to HT/HL stress treatment in the parental (IL) and A250R strains. D1 structure analysis indicated that, among other effects, remodelling of H-bond network at the Q_B site might underpin the observed phenotypes. Thus, the D1 protein, in addition to being pivotal for efficient photosynthesis, may have a moonlighting role in rewiring of specific metabolic pathways, possibly involving retrograde signalling.

Chloroplast Acetyltransferase NSI Is Required for State Transitions in Arabidopsis thaliana

The amount of light energy received by the photosynthetic reaction centers photosystem II (PSII) and photosystem I (PSI) is balanced through state transitions. Reversible phosphorylation of a light-harvesting antenna trimer (L-LHCII) orchestrates the association between L-LHCII and the photosystems, thus adjusting the amount of excitation energy received by the reaction centers. In this study, we identified the enzyme NUCLEAR SHUTTLE INTERACTING (NSI; AT1G32070) as an active lysine acetyltransferase in the chloroplasts of Arabidopsis thaliana Intriguingly, nsi knockout mutant plants were defective in state transitions, even though they had a similar LHCII phosphorylation pattern as the wild type. Accordingly, nsi plants were not able to accumulate the PSI-LHCII state transition complex, even though the LHCII docking site of PSI and the overall amounts of photosynthetic protein complexes remained unchanged. Instead, the nsi mutants showed a decreased Lys acetylation status of specific photosynthetic proteins including PSI, PSII, and LHCII subunits. Our work demonstrates that the chloroplast acetyltransferase NSI is needed for the dynamic reorganization of thylakoid protein complexes during photosynthetic state transitions.

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH₂-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH₂-9 is oxidized by the cytochrome b₆/f complex (Cyt b₆/f) but some PQH₂-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH₂-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH₂-9. We examined the oxidation of PQH₂-9 via dismutation of PQH₂-9 and PQ-9 and scavenging of the superoxide anion radical (O₂^-) and hydrogen peroxide (H₂O₂) by PQH₂-9. Oxidation of PQH₂-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH₂-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b₅₅₉. In the light, oxidation of PQH₂-9 by singlet oxygen and by O₂^- formed in PSI contribute significantly. In addition, Cyt b_6/f forms H₂O₂ with a PQH₂-9 dependent mechanism. Measurements of the reaction of O₂^- with PQH₂-9 and PQ-9 in acetonitrile showed that O₂^- oxidizes PQH₂-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH₂-9 and O₂^- was found to be 10⁴ M^-1 s^-1. H₂O₂ was found to oxidize PQH₂-9 to PQ-9, but failed to oxidize all PQH₂-9, suggesting that the oxidation of PQH₂-9 by H₂O₂ proceeds via deprotonation mechanisms producing PQH^--9, PQ^2--9 and the protonated hydrogen peroxide cation, H₃O₂⁺.

Inactivation of group 2 σ factors upregulates production of transcription and translation machineries in the cyanobacterium Synechocystis sp. PCC 6803

We show that the formation of the RNAP holoenzyme with the primary σ factor SigA increases in the ΔsigBCDE strain of the cyanobacterium Synechocystis sp. PCC 6803 lacking all group 2 σ factors. The high RNAP-SigA holoenzyme content directly induces transcription of a particular set of housekeeping genes, including ones encoding transcription and translation machineries. In accordance with upregulated transcripts, ΔsigBCDE contain more RNAPs and ribosomal subunits than the control strain. Extra RNAPs are fully active, and the RNA content of ΔsigBCDE cells is almost tripled compared to that in the control strain. Although ΔsigBCDE cells produce extra rRNAs and ribosomal proteins, functional extra ribosomes are not formed, and translation activity and protein content remained similar in ΔsigBCDE as in the control strain. The arrangement of the RNA polymerase core genes together with the ribosomal protein genes might play a role in the co-regulation of transcription and translation machineries. Sequence logos were constructed to compare promoters of those housekeeping genes that directly react to the RNAP-SigA holoenzyme content and those ones that do not. Cyanobacterial strains with engineered transcription and translation machineries might provide solutions for construction of highly efficient production platforms for biotechnical applications in the future.

Degradation of chlorophyll and synthesis of flavonols during autumn senescence-the story told by individual leaves

Autumn senescence of deciduous trees is characterized by chlorophyll degradation and flavonoid synthesis. In the present study, chlorophyll and flavonol contents were measured every morning and evening during the whole autumn with a non-destructive method from individual leaves of Sorbus aucuparia, Acer platanoides, Betula pendula and Prunus padus. In most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. The fast phase lasted only ~1 week and ended with abscission. In S. aucuparia, contrary to the other species, the chlorophyll content of leaflets slowly but steadily decreased during the whole autumn, but rapid chlorophyll degradation commenced only prior to leaflet abscission also in this species. An increase in flavonols commonly accompanied the rapid degradation of chlorophyll. The results may suggest that each individual tree leaf retains its photosynthetic activity, reflected by a high chlorophyll content, until a rapid phase of chlorophyll degradation and flavonoid synthesis begins. Therefore, in studies of autumn senescence, leaves whose chlorophyll content is decreasing and leaves with summertime chlorophyll content (i.e. the leaves that have not yet started to degrade chlorophyll) should be treated separately.

Action Spectrum of Photoinhibition in the Diatom Phaeodactylum tricornutum

Light-dependent electron transfer is necessary for photosynthesis, but light also damages PSII. Light-induced damage to PSII is called photoinhibition, and the damaging reactions of photoinhibition are still under debate. Diatoms possess an exotic combination of light-harvesting pigments, Chls a/c and fucoxanthin, making them an interesting platform for studying the photoreceptors of photoinhibition. We first confirmed the direct proportionality of photoinhibition to the photon flux density of incident light in the diatom Phaeodactylum tricornutum. Phaeodactylum is known for its efficient non-photochemical quenching, and the effect of this photoprotective mechanism on photoinhibition was tested. Photoinhibition proceeded essentially at the same rate in blue-light-grown Phaeodactylum cells that are capable of non-photochemical quenching and in red-light-grown, non-photochemical quenching-deficient cells. To obtain more insight into how the pigment composition of diatoms affects photoinhibition, we measured the action spectrum of photoinhibition in Phaeodactylum. In visible light, the action spectrum resembled the absorption spectrum of Phaeodactylum, and UV radiation caused much more photoinhibition than visible light. Comparison of the action spectrum of photoinhibition with the absorption spectrum and the excitation spectrum of 77 K PSII fluorescence emission confirmed that photosynthetic pigments are involved in photoinhibition, but the photoinhibitory efficiency of red light is weak, suggesting that the role of light-harvesting pigments as light receptors of photoinhibition is secondary. Finally, we compared photoinhibition in Phaeodactylum with that in other photosynthetic organisms, and our data indicate that the PSII reaction centers of Phaeodactylum are not particularly well protected against the primary damage of photoinhibition.

Photoinhibition in marine picocyanobacteria

Marine Synechococcus and Prochlorococcus cyanobacteria have different antenna compositions although they are genetically near to each other, and different strains thrive in very different illumination conditions. We measured growth and photoinhibition of PSII in two low-light and one high-light Prochlorococcus strains and in one Synechococcus strain. All strains were found to be able to shortly utilize moderate or even high light, but the low-light strains bleached rapidly in moderate light. Measurements of photoinhibition in the presence of the antibiotic lincomycin showed that a low-light Prochlorococcus strain was more sensitive than a high-light strain and both were more sensitive than the marine Synechococcus. The action spectrum of photoinhibition showed an increase from blue to ultraviolet wavelengths in all strains, suggesting contribution of manganese absorption to photoinhibition, but blue light caused less photoinhibition in marine cyanobacteria than expected on the basis of earlier results from plants and cyanobacteria. The visible-light part of the action spectrum resembled the absorption spectrum of the organism, suggesting that photosynthetic antenna pigments, especially divinyl chlorophylls, have a more important role as photoreceptors of visible-light photoinhibition in marine cyanobacteria than in other photoautotrophs.

Oxygen produced by cyanobacteria in simulated Archaean conditions partly oxidizes ferrous iron but mostly escapes-conclusions about early evolution

The Earth has had a permanently oxic atmosphere only since the great oxygenation event (GOE) 2.3-2.4 billion years ago but recent geochemical research has revealed short periods of oxygen in the atmosphere up to a billion years earlier before the permanent oxygenation. If these "whiffs" of oxygen truly occurred, then oxygen-evolving (proto)cyanobacteria must have existed throughout the Archaean aeon. Trapping of oxygen by ferrous iron and other reduced substances present in Archaean oceans has often been suggested to explain why the oxygen content of the atmosphere remained negligible before the GOE although cyanobacteria produced oxygen. We tested this hypothesis by growing cyanobacteria in anaerobic high-CO₂ atmosphere in a medium with a high concentration of ferrous iron. Microcystins are known to chelate iron, which prompted us also to test the effects of microcystins and nodularins on iron tolerance. The results show that all tested cyanobacteria, especially nitrogen-fixing species grown in the absence of nitrate, and irrespective of the ability to produce cyanotoxins, were iron sensitive in aerobic conditions but tolerated high concentrations of iron in anaerobicity. This result suggests that current cyanobacteria would have tolerated the high-iron content of Archaean oceans. However, only 1 % of the oxygen produced by the cyanobacterial culture was trapped by iron, suggesting that large-scale cyanobacterial photosynthesis would have oxygenated the atmosphere even if cyanobacteria grew in a reducing ocean. Recent genomic analysis suggesting that ability to colonize seawater is a secondary trait in cyanobacteria may offer a partial explanation for the sustained inefficiency of cyanobacterial photosynthesis during the Archaean aeon, as fresh water has always covered a very small fraction of the Earth's surface. If oxygenic photosynthesis originated in fresh water, then the GOE marks the adaptation of cyanobacteria to seawater, and the late-Proterozoic increase in oxygen concentration of the atmosphere is caused by full oxidation of the oceans.

Impacts of simulated drought stress and artificial damage on concentrations of flavonoids in Jatropha curcas (L.), a biofuel shrub

We studied the possible roles of flavonoids in the antioxidant and antiherbivore chemistry in Jatropha curcas (L.), a Latin American shrub that holds great potential as a source of biofuel. Changes in flavonoid concentrations in the leaves of J. curcas seedlings exposed to artificial damage and to different rainfall patterns were assessed by applying a 3²-factorial experiment in a greenhouse. The concentrations of different flavonoids in the leaves of seedlings were significantly affected by interaction effects of artificial damage, drought stress and age of the seedling. The highest flavonoid concentrations were obtained in seedlings imposed to the highest percentage of artificial damage (50 %) and grown under extreme drought stress (200 mm year^-1). In this treatment combination, flavonoid concentrations were three-fold as compared to seedlings exposed to the same level of artificial damage but grown in 1900 mm year^-1 rainfall application. Without artificial damage, the concentration of flavonoids in the seedlings grown in 200 mm year^-1 rainfall application was still two-fold compared to seedlings grown in higher (>800 mm year^-1) rainfall applications. Thus, the observed flavonoid concentration patterns in the leaves of J. curcas seedlings were primarily triggered by drought stress and light rather than by artificial damage, suggesting that drought causes oxidative stress in J. curcas.

Roles of Group 2 Sigma Factors in Acclimation of the Cyanobacterium Synechocystis sp. PCC 6803 to Nitrogen Deficiency

Acclimation of cyanobacteria to environmental conditions is mainly controlled at the transcriptional level, and σ factors of the RNA polymerase have a central role in this process. The model cyanobacterium Synechocystis sp. PCC 6803 has four non-essential group 2 σ factors (SigB, SigC, SigD and SigE) that regulate global metabolic responses to various adverse environmental conditions. Here we show that although none of the group 2 σ factors is essential for the major metabolic realignments induced by a short period of nitrogen starvation, the quadruple mutant without any group 2 σ factors and triple mutants missing both SigB and SigD grow slowly in BG-11 medium containing only 5% of the nitrate present in standard BG-11. These ΔsigBCDE, ΔsigBCD and ΔsigBDE strains lost PSII activity rapidly in low nitrogen and accumulated less glycogen than the control strain. An abnormally high glycogen content was detected in ΔsigBCE (SigD is active), while the carotenoid content became high in ΔsigCDE (SigB is active), indicating that SigB and SigD regulate the partitioning of carbon skeletons in low nitrogen. Long-term survival and recovery of the cells after nitrogen deficiency was strongly dependent on group 2 σ factors. The quadruple mutant and the ΔsigBDE strain (only SigC is active) recovered more slowly from nitrogen deficiency than the control strain, and ΔsigBCDE in particular lost viability during nitrogen starvation. Nitrogen deficiency-induced changes in the pigment content of the control strain recovered essentially in 1 d in nitrogen-replete medium, but little recovery occurred in ΔsigBCDE and ΔsigBDE.

Subunits B'γ and B'ζ of protein phosphatase 2A regulate photo-oxidative stress responses and growth in Arabidopsis thaliana

Plants survive periods of unfavourable conditions with the help of sensory mechanisms that respond to reactive oxygen species (ROS) as signalling molecules in different cellular compartments. We have previously demonstrated that protein phosphatase 2A (PP2A) impacts on organellar cross-talk and associated pathogenesis responses in Arabidopsis thaliana. This was evidenced by drastically enhanced pathogenesis responses and cell death in cat2 pp2a-b'γ double mutants, deficient in the main peroxisomal antioxidant enzyme CATALASE 2 and PP2A regulatory subunit B'γ (PP2A-B'γ). In the present paper, we explored the impacts of PP2A-B'γ and a highly similar regulatory subunit PP2A-B'ζ in growth regulation and light stress tolerance in Arabidopsis. PP2A-B'γ and PP2A-B'ζ display high promoter activities in rapidly growing tissues and are required for optimal growth under favourable conditions. Upon acclimation to a combination of high light, elevated temperature and reduced availability of water, however, pp2a-b'γζ double mutants grow similarly to the wild type and show enhanced tolerance against photo-oxidative stress. We conclude that by controlling ROS homeostasis and signalling, PP2A-B'γ and PP2A-B'ζ may direct acclimation strategies upon environmental perturbations, hence acting as important determinants of defence responses and light acclimation in plants.

Shifting the Sun: Solar Spectral Conversion and Extrinsic Sensitization in Natural and Artificial Photosynthesis

Solar energy harvesting is largely limited by the spectral sensitivity of the employed energy conversion system, where usually large parts of the solar spectrum do not contribute to the harvesting scheme, and where, of the contributing fraction, the full potential of each photon is not efficiently used in the generation of electrical or chemical energy. Extrinsic sensitization through photoluminescent spectral conversion has been proposed as a route to at least partially overcome this problem. Here, we discuss this approach in the emerging context of photochemical energy harvesting and storage through natural or artificial photosynthesis. Clearly contrary to application in photovoltaic energy conversion, implementation of solar spectral conversion for extrinsic sensitization of a photosynthetic machinery is very straightforward, and-when compared to intrinsic sensitization-less-strict limitations with regard to quantum coherence are seen. We now argue the ways in which extrinsic sensitization through photoluminescent spectral converters will-and will not-play its role in the area of ultra-efficient photosynthesis, and also illustrate how such extrinsic sensitization requires dedicated selection of specific conversion schemes and design strategies on system scale.

Photobiological hydrogen production and artificial photosynthesis for clean energy: from bio to nanotechnologies

Global energy demand is increasing rapidly and due to intensive consumption of different forms of fuels, there are increasing concerns over the reduction in readily available conventional energy resources. Because of the deleterious atmospheric effects of fossil fuels and the uncertainties of future energy supplies, there is a surge of interest to find environmentally friendly alternative energy sources. Hydrogen (H2) has attracted worldwide attention as a secondary energy carrier, since it is the lightest carbon-neutral fuel rich in energy per unit mass and easy to store. Several methods and technologies have been developed for H2 production, but none of them are able to replace the traditional combustion fuel used in automobiles so far. Extensively modified and renovated methods and technologies are required to introduce H2 as an alternative efficient, clean, and cost-effective future fuel. Among several emerging renewable energy technologies, photobiological H2 production by oxygenic photosynthetic microbes such as green algae and cyanobacteria or by artificial photosynthesis has attracted significant interest. In this short review, we summarize the recent progress and challenges in H2-based energy production by means of biological and artificial photosynthesis routes.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Cultivation of Nannochloropsis for eicosapentaenoic acid production in wastewaters of pulp and paper industry

The eicosapentaenoic acid (EPA) containing marine microalga Nannochloropsis oculata was grown in an effluent from anaerobic digestion of excess activated sludge from a wastewater treatment plant serving a combination of a pulp and a paper mill and a municipality (digester effluent, DE), mixed with the effluent of the same wastewater treatment plant. The maximum specific growth rate and photosynthesis of N. oculata were similar in the DE medium and in artificial sea water medium (ASW) but after 7 days, algae grown in the DE medium contained seven times more triacylglycerols (TAGs) per cell than cells grown in ASW, indicating mild stress in the DE medium. However, the volumetric rate of EPA production was similar in the ASW and DE media. The results suggest that N. oculata could be used to produce EPA, utilizing the nutrients available after anaerobic digestion of excess activated sludge of a pulp and paper mill.

Potential of carbon nanotubes in algal biotechnology

A critical mass of knowledge is emerging on the interactions between plant cells and engineered nanomaterials, revealing the potential of plant nanobiotechnology to promote and support novel solutions for the development of a competitive bioeconomy. This knowledge can foster the adoption of new methodological strategies to empower the large-scale production of biomass from commercially important microalgae. The present review focuses on the potential of carbon nanotubes (CNTs) to enhance photosynthetic performance of microalgae by (i) widening the spectral region available for the energy conversion reactions and (ii) increasing the tolerance of microalgae towards unfavourable conditions occurring in mass production. To this end, current understanding on the mechanisms of uptake and localization of CNTs in plant cells is discussed. The available ecotoxicological data were used in an attempt to assess the feasibility of CNT-based applications in algal biotechnology, by critically correlating the experimental conditions with the observed adverse effects. Furthermore, main structural and physicochemical properties of single- and multi-walled CNTs and common approaches for the functionalization and characterization of CNTs in biological environment are presented. Here, we explore the potential that nanotechnology can offer to enhance functions of algae, paving the way for a more efficient use of photosynthetic algal systems in the sustainable production of energy, biomass and high-value compounds.

Multiple regulatory mechanisms in the chloroplast of green algae: relation to hydrogen production

A complex regulatory network in the chloroplast of green algae provides an efficient tool for maintenance of energy and redox balance in the cell under aerobic and anaerobic conditions. In this review, we discuss the structural and functional organizations of electron transport pathways in the chloroplast, and regulation of photosynthesis in the green microalga Chlamydomonas reinhardtii. The focus is on the regulatory mechanisms induced in response to nutrient deficiency stress and anoxia and especially on the role of a hydrogenase-mediated reaction in adaptation to highly reducing conditions and ATP deficiency in the cell.

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

Plastoquinol (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 ((1)O2) and analyzed the products. The main reaction of (1)O2 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 (1)O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane.

SASP, a Senescence-Associated Subtilisin Protease, is involved in reproductive development and determination of silique number in Arabidopsis

Senescence involves increased expression of proteases, which may participate in nitrogen recycling or cellular signalling. 2D zymograms detected two protein species with increased proteolytic activity in senescing leaves of Arabidopsis thaliana. A proteomic analysis revealed that both protein species correspond to a subtilisin protease encoded by At3g14067, termed Senescence-Associated Subtilisin Protease (SASP). SASP mRNA levels and enzyme activity increase during leaf senescence in leaves senescing during both the vegetative or the reproductive phase of the plant life cycle, but this increase is more pronounced in reproductive plants. SASP is expressed in all above-ground organs, but not in roots. Putative AtSASP orthologues were identified in dicot and monocot crop species. A phylogenetic analysis shows AtSASP and its putative orthologues clustering in one discrete group of subtilisin proteases in which no other Arabidospsis subtilisin protease is present. Phenotypic analysis of two knockout lines for SASP showed that mutant plants develop more inflorescence branches during reproductive development. Both AtSASP and its putative rice orthologue (OsSASP) were constitutively expressed in sasp-1 to complement the mutant phenotype. At maturity, sasp-1 plants produced 25% more inflorescence branches and siliques than either the wild-type or the rescued lines. These differences were mostly due to an increased number of second and third order branches. The increased number of siliques was compensated for by a small decrease (5.0%) in seed size. SASP downregulates branching and silique production during monocarpic senescence, and its function is at least partially conserved between Arabidopsis and rice.

Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

A tandem mass spectrometric method for singlet oxygen measurement

Singlet oxygen, a harmful reactive oxygen species, can be quantified with the substance 2,2,6,6-tetramethylpiperidine (TEMP) that reacts with singlet oxygen, forming a stable nitroxyl radical (TEMPO). TEMPO has earlier been quantified with electron paramagnetic resonance (EPR) spectroscopy. In this study, we designed an ultra-high-performance liquid chromatographic-tandem mass spectrometric (UHPLC-ESI-MS/MS) quantification method for TEMPO and showed that the method based on multiple reaction monitoring (MRM) can be used for the measurements of singlet oxygen from both nonbiological and biological samples. Results obtained with both UHPLC-ESI-MS/MS and EPR methods suggest that plant thylakoid membranes produce 3.7 × 10(-7) molecules of singlet oxygen per chlorophyll molecule in a second when illuminated with the photosynthetic photon flux density of 2000 μmol m(-2 ) s(-1).

The omega subunit of the RNA polymerase core directs transcription efficiency in cyanobacteria

The eubacterial RNA polymerase core, a transcription machinery performing DNA-dependent RNA polymerization, consists of two α subunits and β, β' and ω subunits. An additional σ subunit is recruited for promoter recognition and transcription initiation. Cyanobacteria, a group of eubacteria characterized by oxygenic photosynthesis, have a unique composition of the RNA polymerase (RNAP) core due to splitting of the β' subunit to N-terminal γ and C-terminal β' subunits. The physiological roles of the small ω subunit of RNAP, encoded by the rpoZ gene, are not yet completely understood in any bacteria. We found that although ω is non-essential in cyanobacteria, it has a major impact on the overall gene expression pattern. In ΔrpoZ strain, recruitment of the primary σ factor into the RNAP holoenzyme is inefficient, which causes downregulation of highly expressed genes and upregulation of many low-expression genes. Especially, genes encoding proteins of photosynthetic carbon concentrating and carbon fixing complexes were down, and the ΔrpoZ mutant showed low light-saturated photosynthetic activity and accumulated photoprotective carotenoids and α-tocopherol. The results indicate that the ω subunit facilitates the association of the primary σ factor with the RNAP core, thereby allowing efficient transcription of highly expressed genes.

Photosynthesis-related quantities for education and modeling

A quantitative understanding of the photosynthetic machinery depends largely on quantities, such as concentrations, sizes, absorption wavelengths, redox potentials, and rate constants. The present contribution is a collection of numbers and quantities related mainly to photosynthesis in higher plants. All numbers are taken directly from a literature or database source and the corresponding reference is provided. The numerical values, presented in this paper, provide ranges of values, obtained in specific experiments for specific organisms. However, the presented numbers can be useful for understanding the principles of structure and function of photosynthetic machinery and for guidance of future research.