Increased sensitivity of photosynthesis to antimycin A induced by inactivation of the chloroplast ndhB gene. Evidence for a participation of the NADH-dehydrogenase complex to cyclic electron flow around photosystem I

Comparative analyses of complete chloroplast genomes reveal interspecific difference and intraspecific variation of Tripterygium genus

The genus Tripterygium was of great medicinal value and attracted much attention on the taxonomic study using morphological and molecular methods. In this study, we assembled 12 chloroplast genomes of Tripterygium to reveal interspecific difference and intraspecific variation. The sequence length (156,692-157,061 bp) and structure of Tripterygium were conserved. Comparative analyses presented abundant variable regions for further study. Meanwhile, we determined the ndhB gene under positive selection through adaptive evolution analysis. And the phylogenetic analyses based on 15 chloroplast genomes supported the monophyly of Tripterygium hypoglaucum and the potential sister relationship between Tripterygium wilfordii and Tripterygium regelii. Molecular dating analysis indicated that the divergence time within Tripterygium was approximately 5.99 Ma (95% HPD = 3.11-8.68 Ma). The results in our study provided new insights into the taxonomy, evolution process, and phylogenetic construction of Tripterygium using complete plastid genomes.

Loss of plastid ndh genes in an autotrophic desert plant

Plant plastid genomes are highly conserved with most flowering plants having the same complement of essential plastid genes. Here, we report the loss of five of the eleven NADH dehydrogenase subunit genes (ndh) in the plastid of a desert plant jojoba (Simmondsia chinensis). The plastid genome of jojoba was 156,496 bp with one large single copy region (LSC), a very small single copy region (SSC) and two expanded inverted repeats (IRA + IRB). The NADH dehydrogenase (NDH) complex is comprised of several protein subunits, encoded by the ndh genes of the plastome and the nucleus. The ndh genes are critical to the proper functioning of the photosynthetic electron transport chain and protection of plants from oxidative stress. Most plants are known to contain all eleven ndh genes. Plants with missing or defective ndh genes are often heterotrophs either due to their complete or holo- or myco- parasitic nature. Plants with a defective NDH complex, caused by the deletion/pseudogenisation of some or all the ndh genes, survive in milder climates suggesting the likely extinction of plant lineages lacking these genes under harsh climates. Interestingly, some autotrophic plants do exist without ndh gene/s and can cope with high or low light. This implies that these plants are protected from oxidative stress by mechanisms excluding ndh genes. Jojoba has evolved mechanisms to cope with a non-functioning NDH complex and survives in extreme desert conditions with abundant sunlight and limited water.

Comparative and phylogenetic analysis of Asparagus meioclados Levl. and Asparagus munitus Wang et S. C. Chen plastomes and utility of plastomes mutational hotspots

Tiandong is a vital traditional Chinese herbal medicine. It is derived from the tuber root of the Asparagus cochinchinensis according to the Pharmacopoeia of the people's republic of China (2020 Edition). On account of the similar morphology, Asparagus meioclados and Asparagus munitus were used as Tian-Dong in southwest China. Chloroplast (cp) genomes are highly active genetic components of plants and play an extremely important role in improving the efficiency of the identification of plant species. To differentiate the medicinal plants belonging to the genus Asparagus, we sequenced and analyzed the complete plastomes (plastid genomes) of A. meioclados and A. munitus and obtained two plastomes whose length changed to 156,515 bp and 156,381 bp, respectively. A total of 111 unique genes have been detected in plastome, which included 78 protein-coding genes, 29 tRNA genes and 4 rRNA genes. In plastomes of A. meioclados and A. munitus, 14,685 and 14,987 codons were detected, among which 9942 and 10,207 had the relative synonymous codon usage (RSCU) values higher than 1, respectively. A. meioclados and A. munitus have 26 SSRs patterns, among which A. meioclados was 25 and A. munitus 21. The average Ka/Ks value was 0.36, and positive selection was detected in genes of the photosynthetic system (ndhF and rbcL) in Asparagus species. To perform the comparative analysis of plastomes, the two newly sequenced plastomes of the A. meioclados and A. munitus species were compared with that of A. cochinchinensis, and 12 hotspots, including 5 coding regions and 7 inter-genomic regions, were identified. Based on the whole plastome of Asparagus, 2 divergent hotspots (accD and rpl32-trnL-UAG) and 1 international barcode fragment (rbcL) were screened, which may be used as particular molecular markers for the identification of Asparagus species. In addition, we determined the phylogenetic relationship between A. meioclados and A. munitus in the genus Asparagus. This study enriches our knowledge of the molecular evolutionary relationships of the Asparagus genus and provides treasured data records for species identification, molecular breeding, and evolutionary analysis of this genus.

Comparative plastomes of Pueraria montana var. lobata (Leguminosae: Phaseoleae) and closely related taxa: insights into phylogenomic implications and evolutionary divergence

BACKGROUND

Pueraria montana var. lobata (kudzu) is an important food and medicinal crop in Asia. However, the phylogenetic relationships between Pueraria montana var. lobata and the other two varieties (P. montana var. thomsonii and P. montana var. montana) remain debated. Although there is increasing evidence showing that P. montana var. lobata adapts to various environments and is an invasive species in America, few studies have systematically investigated the role of the phylogenetic relationships and evolutionary patterns of plastomes between P. montana var. lobata and its closely related taxa.

RESULTS

26 newly sequenced chloroplast genomes of Pueraria accessions resulted in assembled plastomes with sizes ranging from 153,360 bp to 153,551 bp. Each chloroplast genome contained 130 genes, including eight rRNA genes, 37 tRNA genes, and 85 protein-coding genes. For 24 newly sequenced accessions of these three varieties of P. montana, we detected three genes and ten noncoding regions with higher nucleotide diversity (π). After incorporated publically available chloroplast genomes of Pueraria and other legumes, 47 chloroplast genomes were used to construct phylogenetic trees, including seven P. montana var. lobata, 14 P. montana var. thomsonii and six P. montana var. montana. Phylogenetic analysis revealed that P. montana var. lobata and P. montana var. thomsonii formed a clade, while all sampled P. montana var. montana formed another cluster based on cp genomes, LSC, SSC and protein-coding genes. Twenty-six amino acid residues were identified under positive selection with the site model. We also detected six genes (accD, ndhB, ndhC, rpl2, rpoC2, and rps2) that account for among-site variation in selective constraint under the clade model between accessions of the Pueraria montana var. lobata clade and the Pueraria montana var. montana clade.

CONCLUSION

Our data provide novel comparative plastid genomic insights into conservative gene content and structure of cp genomes pertaining to P. montana var. lobata and the other two varieties, and reveal an important phylogenetic clue and plastid divergence among related taxa of P. montana come from loci that own moderate variation and underwent modest selection.

Comparative chloroplast genome and phylogenetic analyses of Chinese Polyspora

Polyspora Sweet (Theaceae) are winter ornamental landscape plants native to southern and southeastern Asia, some of which have medicinal value. The chloroplast (cp) genome data of Polyspora are scarce, and the gene evolution and interspecific relationship are still unclear. In this study, we sequenced and annotated Polyspora chrysandra cp genome and combined it with previously published genomes for other Chinese Polyspora species. The results showed that cp genomes of six Chinese Polyspora varied in length between 156,452 bp (P. chrysandra) and 157,066 bp (P. speciosa), but all contained 132 genes, with GC content of 37.3%, and highly similar genes distribution and codon usage. A total of eleven intergenic spacer regions were found having the highest levels of divergence, and eight divergence hotspots were identified as molecular markers for Phylogeography and genetic diversity studies in Polyspora. Gene selection pressure suggested that five genes were subjected to positive selection. Phylogenetic relationships among Polyspora species based on the complete cp genomes were supported strongly, indicating that the cp genomes have the potential to be used as super barcodes for further analysis of the phylogeny of the entire genus. The cp genomes of Chinese Polyspora species will provide valuable information for species identification, molecular breeding and evolutionary analysis of genus Polyspora.

Comparative chloroplast genome analyses of Paraboea (Gesneriaceae): Insights into adaptive evolution and phylogenetic analysis

Paraboea (Gesneriaceae) distributed in the karst areas of South and Southwest China and Southeast Asia, is an ideal genus to study the phylogeny and adaptive evolution of karst plants. In this study, the complete chloroplast genomes of twelve Paraboea species were sequenced and analyzed. Twelve chloroplast genomes ranged in size from 153166 to 154245 bp. Each chloroplast genome had a typical quartile structure, and relatively conserved type and number of gene components, including 131 genes which are composed of 87 protein coding genes, 36 transfer RNAs and 8 ribosomal RNAs. A total of 600 simple sequence repeats and 389 non-overlapped sequence repeats were obtained from the twelve Paraboea chloroplast genomes. We found ten divergent regions (trnH-GUG-psbA, trnM-CAU, trnC-GCA, atpF-atpH, ycf1, trnK-UUU-rps16, rps15, petL, trnS-GCU-trnR-UCU and psaJ-rpl33) among the 12 Paraboea species to be potential molecular markers. In the phylogenetic tree of 31 Gesneriaceae plants including twelve Paraboea species, all Paraboea species clustered in a clade and confirmed the monophyly of Paraboea. Nine genes with positive selection sites were detected, most of which were related to photosynthesis and protein synthesis, and might played crucial roles in the adaptability of Paraboea to diverse karst environments. These findings are valuable for further study of the phylogeny and karst adaptability of Gesneriaceae plants.

The Complementary Roles of Chloroplast Cyclic Electron Transport and Mitochondrial Alternative Oxidase to Ensure Photosynthetic Performance

Chloroplasts use light energy and a linear electron transport (LET) pathway for the coupled generation of NADPH and ATP. It is widely accepted that the production ratio of ATP to NADPH is usually less than required to fulfill the energetic needs of the chloroplast. Left uncorrected, this would quickly result in an over-reduction of the stromal pyridine nucleotide pool (i.e., high NADPH/NADP⁺ ratio) and under-energization of the stromal adenine nucleotide pool (i.e., low ATP/ADP ratio). These imbalances could cause metabolic bottlenecks, as well as increased generation of damaging reactive oxygen species. Chloroplast cyclic electron transport (CET) and the chloroplast malate valve could each act to prevent stromal over-reduction, albeit in distinct ways. CET avoids the NADPH production associated with LET, while the malate valve consumes the NADPH associated with LET. CET could operate by one of two different pathways, depending upon the chloroplast ATP demand. The NADH dehydrogenase-like pathway yields a higher ATP return per electron flux than the pathway involving PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1). Similarly, the malate valve could couple with one of two different mitochondrial electron transport pathways, depending upon the cytosolic ATP demand. The cytochrome pathway yields a higher ATP return per electron flux than the alternative oxidase (AOX) pathway. In both Arabidopsis thaliana and Chlamydomonas reinhardtii, PGR5/PGRL1 pathway mutants have increased amounts of AOX, suggesting complementary roles for these two lesser-ATP yielding mechanisms of preventing stromal over-reduction. These two pathways may become most relevant under environmental stress conditions that lower the ATP demands for carbon fixation and carbohydrate export.

The Influence of Metabolic Inhibitors, Antibiotics, and Microgravity on Intact Cell MALDI-TOF Mass Spectra of the Cyanobacterium Synechococcus Sp. UPOC S4

The aim and novelty of this paper are found in assessing the influence of inhibitors and antibiotics on intact cell MALDI-TOF mass spectra of the cyanobacterium Synechococcus sp. UPOC S4 and to check the impact on reliability of identification. Defining the limits of this method is important for its use in biology and applied science. The compounds included inhibitors of respiration, glycolysis, citrate cycle, and proteosynthesis. They were used at 1–10 μM concentrations and different periods of up to 3 weeks. Cells were also grown without inhibitors in a microgravity because of expected strong effects. Mass spectra were evaluated using controls and interpreted in terms of differential peaks and their assignment to protein sequences by mass. Antibiotics, azide, and bromopyruvate had the greatest impact. The spectral patterns were markedly altered after a prolonged incubation at higher concentrations, which precluded identification in the database of reference spectra. The incubation in microgravity showed a similar effect. These differences were evident in dendrograms constructed from the spectral data. Enzyme inhibitors affected the spectra to a smaller extent. This study shows that only a long-term presence of antibiotics and strong metabolic inhibitors in the medium at 10⁻⁵ M concentrations hinders the correct identification of cyanobacteria by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF).

Cytochrome b6f - Orchestrator of photosynthetic electron transfer

Cytochrome b₆f (cytb₆f) lies at the heart of the light-dependent reactions of oxygenic photosynthesis, where it serves as a link between photosystem II (PSII) and photosystem I (PSI) through the oxidation and reduction of the electron carriers plastoquinol (PQH₂) and plastocyanin (Pc). A mechanism of electron bifurcation, known as the Q-cycle, couples electron transfer to the generation of a transmembrane proton gradient for ATP synthesis. Cytb₆f catalyses the rate-limiting step in linear electron transfer (LET), is pivotal for cyclic electron transfer (CET) and plays a key role as a redox-sensing hub involved in the regulation of light-harvesting, electron transfer and photosynthetic gene expression. Together, these characteristics make cytb₆f a judicious target for genetic manipulation to enhance photosynthetic yield, a strategy which already shows promise. In this review we will outline the structure and function of cytb₆f with a particular focus on new insights provided by the recent high-resolution map of the complex from Spinach.

M-Type Thioredoxins Regulate the PGR5/PGRL1-Dependent Pathway by Forming a Disulfide-Linked Complex with PGRL1

In addition to linear electron transport, photosystem I cyclic electron transport (PSI-CET) contributes to photosynthesis and photoprotection. In Arabidopsis (Arabidopsis thaliana), PSI-CET consists of two partially redundant pathways, one of which is the PROTON GRADIENT REGULATION5 (PGR5)/PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1)-dependent pathway. Although the physiological significance of PSI-CET is widely recognized, the regulatory mechanism behind these pathways remains largely unknown. Here, we report on the regulation of the PGR5/PGRL1-dependent pathway by the m-type thioredoxins (Trx m). Genetic and phenotypic characterizations of multiple mutants indicated the physiological interaction between Trx m and the PGR5/PGRL1-dependent pathway in vivo. Using purified Trx proteins and ruptured chloroplasts, in vitro, we showed that the reduced form of Trx m specifically decreased the PGR5/PGRL1-dependent plastoquinone reduction. In planta, Trx m4 directly interacted with PGRL1 via disulfide complex formation. Analysis of the transgenic plants expressing PGRL1 Cys variants demonstrated that Cys-123 of PGRL1 is required for Trx m4-PGRL1 complex formation. Furthermore, the Trx m4-PGRL1 complex was transiently dissociated during the induction of photosynthesis. We propose that Trx m directly regulates the PGR5/PGRL1-dependent pathway by complex formation with PGRL1.

Salt induced programmed cell death in rice: evidence from chloroplast proteome signature

Soil salinity, depending on its intensity, drives a challenged plant either to death, or survival with compromised productivity. On exposure to moderate salinity, plants can often survive by sacrificing some of their cells 'in target' following a route called programmed cell death (PCD). In animals, PCD has been well characterised, and involvement of mitochondria in the execution of PCD events has been unequivocally proven. In plants, mechanistic details of the process are still in grey area. Previously, we have shown that in green tissues of rice, for salt induced PCD to occur, the presence of active chloroplasts and light are equally important. In the present work, we have characterised the chloroplast proteome in rice seedlings at 12 and 24 h after salt exposure and before the time point where the signature of PCD was observed. We identified almost 100 proteins from chloroplasts, which were divided in to 11 categories based on the biological functions in which they were involved. Our results concerning the differential expression of chloroplastic proteins revealed involvement of some novel candidates. Moreover, we observed maximum phosphorylation pattern of chloroplastic proteins at an early time point (12 h) of salt exposure.

Photoprotection by mitochondrial alternative pathway is enhanced at heat but disabled at chilling

The mitochondrial alternative pathway (AP) represents an important photoprotective mechanism for the chloroplast, but the temperature sensitivity of its photoprotective role is unknown. In this study, using the aox1a Arabidopsis mutant, the photoprotective role of the AP was verified under various temperatures, and the mechanism underlying the temperature sensitivity of the AP's photoprotective role was clarified. It was observed that the photoprotective role of the AP increased with rising temperature but was absent at low temperature. The photoprotective role of the AP was severely reduced under non-photorespiratory conditions. Disturbance of the AP inhibited the conversion of glycine to serine in mitochondria, which may restrain upstream photorespiratory metabolism and aggravate photoinhibition. With rising temperatures, photorespiration accelerated and the restraint of photorespiration caused by disturbance of the AP also increased, determining the temperature sensitivity of the AP's photoprotective role. We also verified that not only the AP but also the cytochrome pathway in mitochondria contributes to photoprotection by maintaining photorespiration.

The Role of Chloroplast Gene Expression in Plant Responses to Environmental Stress

Chloroplasts are plant organelles that carry out photosynthesis, produce various metabolites, and sense changes in the external environment. Given their endosymbiotic origin, chloroplasts have retained independent genomes and gene-expression machinery. Most genes from the prokaryotic ancestors of chloroplasts were transferred into the nucleus over the course of evolution. However, the importance of chloroplast gene expression in environmental stress responses have recently become more apparent. Here, we discuss the emerging roles of the distinct chloroplast gene expression processes in plant responses to environmental stresses. For example, the transcription and translation of psbA play an important role in high-light stress responses. A better understanding of the connection between chloroplast gene expression and environmental stress responses is crucial for breeding stress-tolerant crops better able to cope with the rapidly changing environment.

Signaling interactions between mitochondria and chloroplasts in Nicotiana tabacum leaf

Research has begun to elucidate the signal transduction pathway(s) that control cellular responses to changes in mitochondrial status. Important tools in such studies are chemical inhibitors used to initiate mitochondrial dysfunction. This study compares the effect of different inhibitors and treatment conditions on the transcript amount of nuclear genes specifically responsive to mitochondrial dysfunction in leaf of Nicotiana tabacum L. cv. Petit Havana. The Complex III inhibitors antimycin A (AA) and myxothiazol (MYXO), and the Complex V inhibitor oligomycin (OLIGO), each increased the transcript amount of the mitochondrial dysfunction genes. Transcript responses to OLIGO were greater during treatment in the dark than in the light, and the dark treatment resulted in cell death. In the dark, transcript responses to AA and MYXO were similar to one another, despite MYXO leading to cell death. In the light, transcript responses to AA and MYXO diverged, despite cell viability remaining high with either inhibitor. This divergent response may be due to differential signaling from the chloroplast because only AA also inhibited cyclic electron transport, resulting in a strong acceptor-side limitation in photosystem I. In the light, chemical inhibition of chloroplast electron transport reduced transcript responses to AA, while having no effect on the response to MYXO, and increasing the response to OLIGO. Hence, when studying mitochondrial dysfunction signaling, different inhibitor and treatment combinations differentially affect linked processes (e.g. chloroplast function and cell fate) that then contribute to measured responses. Therefore, inhibitor and treatment conditions should be chosen to align with specific study goals.

On a Cold Night: Transcriptomics of Grapevine Flower Unveils Signal Transduction and Impacted Metabolism

Low temperature is a critical environmental factor limiting plant productivity, especially in northern vineyards. To clarify the impact of this stress on grapevine flower, we used the Vitis array based on Roche-NimbleGen technology to investigate the gene expression of flowers submitted to a cold night. Our objectives were to identify modifications in the transcript levels after stress and during recovery. Consequently, our results confirmed some mechanisms known in grapes or other plants in response to cold stress, notably, (1) the pivotal role of calcium/calmodulin-mediated signaling; (2) the over-expression of sugar transporters and some genes involved in plant defense (especially in carbon metabolism), and (3) the down-regulation of genes encoding galactinol synthase (GOLS), pectate lyases, or polygalacturonases. We also identified some mechanisms not yet known to be involved in the response to cold stress, i.e., (1) the up-regulation of genes encoding G-type lectin S-receptor-like serine threonine-protein kinase, pathogen recognition receptor (PRR5), or heat-shock factors among others; (2) the down-regulation of Myeloblastosis (MYB)-related transcription factors and the Constans-like zinc finger family; and (3) the down-regulation of some genes encoding Pathogen-Related (PR)-proteins. Taken together, our results revealed interesting features and potentially valuable traits associated with stress responses in the grapevine flower. From a long-term perspective, our study provides useful starting points for future investigation.

Faster photosynthetic induction in tobacco by expressing cyanobacterial flavodiiron proteins in chloroplasts

Plants grown in the field experience sharp changes in irradiation due to shading effects caused by clouds, other leaves, etc. The excess of absorbed light energy is dissipated by a number of mechanisms including cyclic electron transport, photorespiration, and Mehler-type reactions. This protection is essential for survival but decreases photosynthetic efficiency. All phototrophs except angiosperms harbor flavodiiron proteins (Flvs) which relieve the excess of excitation energy on the photosynthetic electron transport chain by reducing oxygen directly to water. Introduction of cyanobacterial Flv1/Flv3 in tobacco chloroplasts resulted in transgenic plants that showed similar photosynthetic performance under steady-state illumination, but displayed faster recovery of various photosynthetic parameters, including electron transport and non-photochemical quenching during dark-light transitions. They also kept the electron transport chain in a more oxidized state and enhanced the proton motive force of dark-adapted leaves. The results indicate that, by acting as electron sinks during light transitions, Flvs contribute to increase photosynthesis protection and efficiency under changing environmental conditions as those found by plants in the field.

Electron Transfer through the Acceptor Side of Photosystem I: Interaction with Exogenous Acceptors and Molecular Oxygen

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A_1A/A_1B in the symmetric redox cofactor branches of PS I and iron-sulfur clusters F_X, F_A, and F_B have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A₁ site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.

The Cytochrome b 6 f Complex: Biophysical Aspects of Its Functioning in Chloroplasts

This chapter presents an overview of structural properties of the cytochrome (Cyt) b ₆ f complex and its functioning in chloroplasts. The Cyt b ₆ f complex stands at the crossroad of photosynthetic electron transport pathways, providing connectivity between Photosystem (PSI) and Photosysten II (PSII) and pumping protons across the membrane into the thylakoid lumen. After a brief review of the chloroplast electron transport chain, the consideration is focused on the structural organization of the Cyt b ₆ f complex and its interaction with plastoquinol (PQH₂, reduced form of plastoquinone), a mediator of electron transfer from PSII to the Cyt b ₆ f complex. The processes of PQH₂ oxidation by the Cyt b ₆ f complex have been considered within the framework of the Mitchell's Q-cycle. The overall rate of the intersystem electron transport is determined by PQH₂ turnover at the quinone-binding site Q_o of the Cyt b ₆ f complex. The rate of PQH₂ oxidation is controlled by the intrathylakoid pH_in, which value determines the protonation/deprotonation events in the Q_o-center. Two other regulatory mechanisms associated with the Cyt b ₆ f complex are briefly overviewed: (i) redistribution of electron fluxes between alternative (linear and cyclic) pathways, and (ii) "state transitions" related to redistribution of solar energy between PSI and PSII.

Chilling stress and hydrogen peroxide accumulation in Chrysanthemum morifolium and Spathiphyllum lanceifolium. Involvement of chlororespiration

Plants of Chrysanthemum morifolium (sun species) and Spathiphyllum lanceifolium (shade species) were used to study the effects of chilling stems under high illumination. The stress conditions resulted in a greater accumulation of H₂O₂ in C. morifolium than in S. lanceifolium, and in the down-regulation of photosynthetic linear electron transport in both species. However, only a slight decrease in the maximal quantum yield of PSII was observed under unfavorable conditions in both species, suggesting that mechanisms exist in the chloroplasts that dissipate excess excitation energy and prevent damage to the photosynthetic apparatus. Additionally, changes were observed in the PGR5 polypeptide involved in cyclic electron flow around PSI and in chlororespiratory enzymes (plastidial NDH complex and PTOX). The level of PGR5 increased significantly only in chilled plants of C. morifolium, whereas the levels of the PTOX and NDH-H polypeptide of the plastidial NDH complex and the NDH activity increased significantly only in chilled plants of S. lanceifolium. These findings suggest that the cyclic electron flow involving PGR5 is more active in C. morifolium, while in S. lanceifolium, other mechanisms involving chlororespiratory enzymes are stimulated in response to chilling and high light, resulting in less H₂O₂ being accumulated in leaves.

Physiological Functions of Cyclic Electron Transport Around Photosystem I in Sustaining Photosynthesis and Plant Growth

The light reactions in photosynthesis drive both linear and cyclic electron transport around photosystem I (PSI). Linear electron transport generates both ATP and NADPH, whereas PSI cyclic electron transport produces ATP without producing NADPH. PSI cyclic electron transport is thought to be essential for balancing the ATP/NADPH production ratio and for protecting both photosystems from damage caused by stromal overreduction. Two distinct pathways of cyclic electron transport have been proposed in angiosperms: a major pathway that depends on the PROTON GRADIENT REGULATION 5 (PGR5) and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1) proteins, which are the target site of antimycin A, and a minor pathway mediated by the chloroplast NADH dehydrogenase-like (NDH) complex. Recently, the regulation of PSI cyclic electron transport has been recognized as essential for photosynthesis and plant growth. In this review, we summarize the possible functions and importance of the two pathways of PSI cyclic electron transport.

NDH-1 and NDH-2 Plastoquinone Reductases in Oxygenic Photosynthesis

Oxygenic photosynthesis converts solar energy into chemical energy in the chloroplasts of plants and microalgae as well as in prokaryotic cyanobacteria using a complex machinery composed of two photosystems and both membrane-bound and soluble electron carriers. In addition to the major photosynthetic complexes photosystem II (PSII), cytochrome b6f, and photosystem I (PSI), chloroplasts also contain minor components, including a well-conserved type I NADH dehydrogenase (NDH-1) complex that functions in close relationship with photosynthesis and likewise originated from the endosymbiotic cyanobacterial ancestor. Some plants and many microalgal species have lost plastidial ndh genes and a functional NDH-1 complex during evolution, and studies have suggested that a plastidial type II NADH dehydrogenase (NDH-2) complex substitutes for the electron transport activity of NDH-1. However, although NDH-1 was initially thought to use NAD(P)H as an electron donor, recent research has demonstrated that both chloroplast and cyanobacterial NDH-1s oxidize reduced ferredoxin. We discuss more recent findings related to the biochemical composition and activity of NDH-1 and NDH-2 in relation to the physiology and regulation of photosynthesis, particularly focusing on their roles in cyclic electron flow around PSI, chlororespiration, and acclimation to changing environments.

Lower leaf gas-exchange and higher photorespiration of treated wastewater irrigated Citrus trees is modulated by soil type and climate

Water quality, soil and climate can interact to limit photosynthesis and to increase photooxidative damage in sensitive plants. This research compared diffusive and non-diffusive limitations to photosynthesis as well as photorespiration of leaves of grapefruit trees in heavy clay and sandy soils having a previous history of treated wastewater (TWW) irrigation for >10 years, with different water qualities [fresh water (FW) vs TWW and sodium amended treated wastewater (TWW + Na)] in two arid climates (summer vs winter) and in orchard and lysimeter experiments. TWW irrigation increased salts (Na(+) and Cl(-) ), membrane leakage, proline and soluble sugar content, and decreased osmotic potentials in leaves of all experiments. Reduced leaf growth and higher stomatal and non-stomatal (i.e. mesophyll) limitations were found in summer and on clay soil for TWW and TWW + Na treatments in comparison to winter, sandy soil and FW irrigation, respectively. Stomatal closure, lower chlorophyll content and altered Rubisco activity are probable causes of higher limitations. On the other hand, non-photochemical quenching, an alternative energy dissipation pathway, was only influenced by water quality, independent of soil type and season. Furthermore, light and CO2 response curves were investigated for other possible causes of higher non-stomatal limitation. A higher proportion of non-cyclic electrons were directed to the O2 dependent pathway, and a higher proportion of electrons were diverted to photorespiration in summer than in winter. In conclusion, both diffusive and non-diffusive limitations contribute to the lower photosynthetic performance of leaves following TWW irrigation, and the response depends on soil type and environmental factors.

Response of Chloroplast NAD(P)H Dehydrogenase-Mediated Cyclic Electron Flow to a Shortage or Lack in Ferredoxin-Quinone Oxidoreductase-Dependent Pathway in Rice Following Short-Term Heat Stress

Cyclic electron flow (CEF) around photosystem I (PSI) can protect photosynthetic electron carriers under conditions of stromal over-reduction. The goal of the research reported in this paper was to investigate the responses of both PSI and photosystem II (PSII) to a short-term heat stress in two rice lines with different capacities of cyclic electron transfer, i.e., Q4149 with a high capacity (hcef) and C4023 with a low capacity (lcef). The absorbance change at 820 nm (ΔA820) was used here to assess the charge separation in the PSI reaction center (P700). The results obtained show that short-term heat stress abolishes the ferredoxin-quinone oxidoreductase (FQR)-dependent CEF in rice and accelerates the initial rate of P700 (+) re-reduction. The P700 (+) amplitude was slightly increased at a moderate heat-stress (35°C) because of a partial restriction of FQR but it was decreased following high heat-stress (42°C). Assessment of PSI and PSII activities shows that PSI is more susceptible to heat stress than PSII. Under high temperature, FQR-dependent CEF was completely removed and NDH-dependent CEF was up-regulated and strengthened to a higher extent in C4023 than in Q4149. Specifically, under normal growth temperature, hcef (Q4149) was characterized by higher FQR- and chloroplast NAD(P)H dehydrogenase (NDH)-dependent CEF rates than lcef (C4023). Following thermal stress, the activation of NDH-pathway was 130 and 10% for C4023 and Q4149, respectively. Thus, the NDH-dependent CEF may constitute the second layer of plant protection and defense against heat stress after the main route, i.e., FQR-dependent CEF, reaches its capacity. We discuss the possibility that under high heat stress, the NDH pathway serves as a safety valve to dissipate excess energy by cyclic photophosphorylation and overcome the stroma over-reduction following inhibition of CO2 assimilation and any shortage or lack in the FQR pathway. The potential role of the NDH-dependent pathway during the evolution of C4 photosynthesis is briefly discussed.

The Effects of Cold Stress on Photosynthesis in Hibiscus Plants

The present work studies the effects of cold on photosynthesis, as well as the involvement in the chilling stress of chlororespiratory enzymes and ferredoxin-mediated cyclic electron flow, in illuminated plants of Hibiscus rosa-sinensis. Plants were sensitive to cold stress, as indicated by a reduction in the photochemistry efficiency of PSII and in the capacity for electron transport. However, the susceptibility of leaves to cold may be modified by root temperature. When the stem, but not roots, was chilled, the quantum yield of PSII and the relative electron transport rates were much lower than when the whole plant, root and stem, was chilled at 10°C. Additionally, when the whole plant was cooled, both the activity of electron donation by NADPH and ferredoxin to plastoquinone and the amount of PGR5 polypeptide, an essential component of the cyclic electron flow around PSI, increased, suggesting that in these conditions cyclic electron flow helps protect photosystems. However, when the stem, but not the root, was cooled cyclic electron flow did not increase and PSII was damaged as a result of insufficient dissipation of the excess light energy. In contrast, the chlororespiratory enzymes (NDH complex and PTOX) remained similar to control when the whole plant was cooled, but increased when only the stem was cooled, suggesting the involvement of chlororespiration in the response to chilling stress when other pathways, such as cyclic electron flow around PSI, are insufficient to protect PSII.

Functional characterization of mutants affected in the carbonic anhydrase domain of the respiratory complex I in Arabidopsis thaliana

The NADH-ubiquinone oxidoreductase complex (complex I) (EC 1.6.5.3) is the main entrance site of electrons into the respiratory chain. In a variety of eukaryotic organisms, except animals and fungi (Opisthokonta), it contains an extra domain comprising trimers of putative γ-carbonic anhydrases, named the CA domain, which has been proposed to be essential for assembly of complex I. However, its physiological role in plants is not fully understood. Here, we report that Arabidopsis mutants defective in two CA subunits show an altered photorespiratory phenotype. Mutants grown in ambient air show growth retardation compared to wild-type plants, a feature that is reversed by cultivating plants in a high-CO2 atmosphere. Moreover, under photorespiratory conditions, carbon assimilation is diminished and glycine accumulates, suggesting an imbalance with respect to photorespiration. Additionally, transcript levels of specific CA subunits are reduced in plants grown under non-photorespiratory conditions. Taken together, these results suggest that the CA domain of plant complex I contributes to sustaining efficient photosynthesis under ambient (photorespiratory) conditions.

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.

Induction events and short-term regulation of electron transport in chloroplasts: an overview

Regulation of photosynthetic electron transport at different levels of structural and functional organization of photosynthetic apparatus provides efficient performance of oxygenic photosynthesis in plants. This review begins with a brief overview of the chloroplast electron transport chain. Then two noninvasive biophysical methods (measurements of slow induction of chlorophyll a fluorescence and EPR signals of oxidized P700 centers) are exemplified to illustrate the possibility of monitoring induction events in chloroplasts in vivo and in situ. Induction events in chloroplasts are considered and briefly discussed in the context of short-term mechanisms of the following regulatory processes: (i) pH-dependent control of the intersystem electron transport; (ii) the light-induced activation of the Calvin-Benson cycle; (iii) optimization of electron transport due to fitting alternative pathways of electron flow and partitioning light energy between photosystems I and II; and (iv) the light-induced remodeling of photosynthetic apparatus and thylakoid membranes.

Energetic coupling between plastids and mitochondria drives CO2 assimilation in diatoms

Diatoms are one of the most ecologically successful classes of photosynthetic marine eukaryotes in the contemporary oceans. Over the past 30 million years, they have helped to moderate Earth's climate by absorbing carbon dioxide from the atmosphere, sequestering it via the biological carbon pump and ultimately burying organic carbon in the lithosphere. The proportion of planetary primary production by diatoms in the modern oceans is roughly equivalent to that of terrestrial rainforests. In photosynthesis, the efficient conversion of carbon dioxide into organic matter requires a tight control of the ATP/NADPH ratio which, in other photosynthetic organisms, relies principally on a range of plastid-localized ATP generating processes. Here we show that diatoms regulate ATP/NADPH through extensive energetic exchanges between plastids and mitochondria. This interaction comprises the re-routing of reducing power generated in the plastid towards mitochondria and the import of mitochondrial ATP into the plastid, and is mandatory for optimized carbon fixation and growth. We propose that the process may have contributed to the ecological success of diatoms in the ocean.

Oxidation of plastohydroquinone by photosystem II and by dioxygen in leaves

In sunflower leaves linear electron flow LEF=4O2 evolution rate was measured at 20 ppm O2 in N2. PSII charge separation rate CSRII=aII∙PAD∙(Fm-F)/Fm, where aII is excitation partitioning to PSII, PAD is photon absorption density, Fm and F are maximum and actual fluorescence yields. Under 630 nm LED+720 nm far-red light (FRL), LEF was equal to CSRII with aII=0.51 to 0.58. After FRL was turned off, plastoquinol (PQH2) accumulated, but LEF decreased more than accountable by F increase, indicating PQH2-oxidizing cyclic electron flow in PSII (CEFII). CEFII was faster under conditions requiring more ATP, consistent with CEFII being coupled with proton translocation. We propose that PQH2 bound to the QC site is oxidized, one e- moving to P680+, the other e- to Cyt b559. From Cyt b559 the e- reduces QB- at the QB site, forming PQH2. About 10-15% electrons may cycle, causing misses in the period-4 flash O2 evolution and lower quantum yield of photosynthesis under stress. We also measured concentration dependence of PQH2 oxidation by dioxygen, as indicated by post-illumination decrease of Chl fluorescence yield. After light was turned off, F rapidly decreased from Fm to 0.2 Fv, but further decrease to F0 was slow and O2 concentration dependent. The rate constant of PQH2 oxidation, determined from this slow phase, was 0.054 s(-1) at 270 μM (21%) O2, decreasing with Km(O2) of 60 μM (4.6%) O2. This eliminates the interference of O2 in the measurements of CEFII.

Involvement of chlororespiration in chilling stress in the tropical species Spathiphyllum wallisii

Spathiphyllum wallisii plants were used to study the effect of chilling stress under high illumination on photosynthesis and chlororespiration. Leaves showed different responses that depended on root temperature. When stem, but not root, was chilled, photosystem II (PSII) was strongly photoinhibited. However, when the whole plant was chilled, the maximal quantum yield of PSII decreased only slightly below the normal values and cyclic electron transport was stimulated. Changes were also observed in the chlororespiration enzymes and PGR5. In whole plants chilled under high illumination, the amounts of NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX) remained similar to control and increased when only stem was chilled. In contrast, the amount of PGR5 polypeptide was higher in plants when both root and stem were chilled than in plants in which only stem was chilled. The results indicated that the contribution of chlororespiration to regulating photosynthetic electron flow is not relevant when the whole plant is chilled under high light, and that another pathway, such as cyclic electron flow involving PGR5 polypeptide, may be more important. However, when PSII activity is strongly photoinhibited in plants in which only stem is chilled, chlororespiration, together with other routes of electron input to the electron transfer chain, is probably essential.

Antimycin A sensitive pathway independent from PGR5 cyclic electron transfer triggers non-photochemical reduction of PQ pool and state transitions in Arabidopsis thaliana

We investigated the mechanism involved in triggering state transitions at 40°C in Arabidopsis thaliana. Leaves (1-6 week old) exposed to 40°C exhibited state II transition indicating its role as one of the earliest stress responsive mechanism apart from regulation of light energy distribution between photosystem (PS)II and PSI. Post illumination transients (rise in Fo') revealed that non-photochemical reduction of PQ pool at 40°C in dark is responsible for activation of STN7 kinase, consequently light harvesting complex (LHC)II phosphorylation leading to state II condition. Later, in pgr5 mutant, non-photochemical reduction of PQ pool was observed indicating the involvement of alternative electron transfer routes. In chlororespiratory mutant crr2-2, state II transition occurred signifying that the reduction of PQ pool is independent from NDH mediated cyclic electron transfer. Further, antimycin A inhibitor studies in wt and mutants revealed its inhibitory action on non-photochemical reduction of PQ pool affecting both LHCII phosphorylation and migration to PSI which leads to state I. Thus, our study showed that antimycin A sensitive pathway independent from PGR5 dependent cyclic electron transfer, is responsible for inducing non-photochemical reduction of PQ pool and state transitions at 40°C.

A security network in PSI photoprotection: regulation of photosynthetic control, NPQ and O2 photoreduction by cyclic electron flow

Cyclic electron flow (CEF) around PSI regulates acceptor-side limitations and has multiple functions in the green alga, Chlamydomonas reinhardtii. Here we draw on recent and historic literature and concentrate on its role in Photosystem I (PSI) photoprotection, outlining causes and consequences of damage to PSI and CEF's role as an avoidance mechanism. We outline two functions of CEF in PSI photoprotection that are both linked to luminal acidification: firstly, its action on Photosystem II with non-photochemical quenching and photosynthetic control and secondly, its action in poising the stroma to overcome acceptor-side limitation by rebalancing NADPH and ATP ratios for carbon fixation.

High root temperature affects the tolerance to high light intensity in Spathiphyllum plants

Spathiphyllum wallisii plants were sensitive to temperature stress under high illumination, although the susceptibility of leaves to stress may be modified by root temperature. Leaves showed higher tolerance to high illumination, in both cold and heat conditions, when the roots were cooled, probably because the chloroplast were protected by excess excitation energy dissipation mechanisms such as cyclic electron transport. When the roots were cooled both the activity of electron donation by NADPH and ferredoxin to plastoquinone and the amount of PGR5 polypeptide, an essential component of cyclic electron flow around PSI, increased. However, when the stems were heated or cooled under high illumination, but the roots were heated, the quantum yield of PSII decreased considerably and neither the electron donation activity by NADPH and ferredoxin to plastoquinone nor the amount of PGR5 polypeptide increased. In such conditions, the cyclic electron flow cannot be enhanced by high light and PSII is damaged as a result of insufficient dissipation of excess light energy. Additionally, the damage to PSII induced the increase in both chlororespiratory enzymes, NDH complex and PTOX.

Specificity in ROS signaling and transcript signatures

SIGNIFICANCE

Reactive oxygen species (ROS), important signaling molecules in plants, are involved in developmental control and stress adaptation. ROS production can trigger broad transcriptional changes; however, it is not clear how specificity in transcriptional regulation is achieved.

RECENT ADVANCES

A large collection of public transcriptome data from the model plant Arabidopsis thaliana is available for analysis. These data can be used for the analysis of biological processes that are associated with ROS signaling and for the identification of suitable transcriptional indicators. Several online tools, such as Genevestigator and Expression Angler, have simplified the task to analyze, interpret, and visualize this wealth of data.

CRITICAL ISSUES

The analysis of the exact transcriptional responses to ROS requires the production of specific ROS in distinct subcellular compartments with precise timing, which is experimentally difficult. Analyses are further complicated by the effect of ROS production in one subcellular location on the ROS accumulation in other compartments. In addition, even subtle differences in the method of ROS production or treatment can lead to significantly different outcomes when various stimuli are compared.

FUTURE DIRECTIONS

Due to the difficulty of inducing ROS production specifically with regard to ROS type, subcellular localization, and timing, we propose that the concept of a "ROS marker gene" should be re-evaluated. We suggest guidelines for the analysis of transcriptional data in ROS signaling. The use of "ROS signatures," which consist of a set of genes that together can show characteristic and indicative responses, should be preferred over the use of individual marker genes.

The cytochrome b6f complex at the crossroad of photosynthetic electron transport pathways

Regulation of photosynthetic electron transport at the level of the cytochrome b6f complex provides efficient performance of the chloroplast electron transport chain (ETC). In this review, after brief overview of the structural organization of the chloroplast ETC, the consideration of the problem of electron transport control is focused on the plastoquinone (PQ) turnover and its interaction with the b6f complex. The data available show that the rates of plastoquinol (PQH2) formation in PSII and its diffusion to the b6f complex do not limit the overall rate of electron transfer between photosystem II (PSII) and photosystem I (PSI). Analysis of experimental and theoretical data demonstrates that the rate-limiting step in the intersystem chain of electron transport is determined by PQH2 oxidation at the Qo-site of the b6f complex, which is accompanied by the proton release into the thylakoid lumen. The acidification of the lumen causes deceleration of PQH2 oxidation, thus impeding the intersystem electron transport. Two other mechanisms of regulation of the intersystem electron transport have been considered: (i) "state transitions" associated with the light-induced redistribution of solar energy between PSI and PSII, and (ii) redistribution of electron fluxes between alternative pathways (noncyclic electron transport and cyclic electron flow around PSI).

Combined increases in mitochondrial cooperation and oxygen photoreduction compensate for deficiency in cyclic electron flow in Chlamydomonas reinhardtii

During oxygenic photosynthesis, metabolic reactions of CO2 fixation require more ATP than is supplied by the linear electron flow operating from photosystem II to photosystem I (PSI). Different mechanisms, such as cyclic electron flow (CEF) around PSI, have been proposed to participate in reequilibrating the ATP/NADPH balance. To determine the contribution of CEF to microalgal biomass productivity, here, we studied photosynthesis and growth performances of a knockout Chlamydomonas reinhardtii mutant (pgrl1) deficient in PROTON GRADIENT REGULATION LIKE1 (PGRL1)-mediated CEF. Steady state biomass productivity of the pgrl1 mutant, measured in photobioreactors operated as turbidostats, was similar to its wild-type progenitor under a wide range of illumination and CO2 concentrations. Several changes were observed in pgrl1, including higher sensitivity of photosynthesis to mitochondrial inhibitors, increased light-dependent O2 uptake, and increased amounts of flavodiiron (FLV) proteins. We conclude that a combination of mitochondrial cooperation and oxygen photoreduction downstream of PSI (Mehler reactions) supplies extra ATP for photosynthesis in the pgrl1 mutant, resulting in normal biomass productivity under steady state conditions. The lower biomass productivity observed in the pgrl1 mutant in fluctuating light is attributed to an inability of compensation mechanisms to respond to a rapid increase in ATP demand.

Continuous ECS-indicated recording of the proton-motive charge flux in leaves

Technical features and examples of application of a special emitter-detector module for highly sensitive measurements of the electrochromic pigment absorbance shift (ECS) via dual-wavelength (550-520 nm) transmittance changes (P515) are described. This device, which has been introduced as an accessory of the standard, commercially available Dual-PAM-100 measuring system, not only allows steady-state assessment of the proton motive force (pmf) and its partitioning into ΔpH and ΔΨ components, but also continuous recording of the overall charge flux driven by photosynthetic light reactions. The new approach employs a double-modulation technique to derive a continuous signal from the light/dark modulation amplitude of the P515 signal. This new, continuously measured signal primarily reflects the rate of proton efflux via the ATP synthase, which under quasi-stationary conditions corresponds to the overall rate of proton influx driven by coupled electron transport. Simultaneous measurements of charge flux and CO2 uptake as a function of light intensity indicated a close to linear relationship in the light-limited range. A linear relationship between these two signals was also found for different internal CO2 concentrations, except for very low CO2, where the rate of charge flux distinctly exceeded the rate of CO2 uptake. Parallel oscillations in CO2 uptake and charge flux were induced by high CO2 and O2. The new device may contribute to the elucidation of complex regulatory mechanisms in intact leaves.

Antimycin A-like molecules inhibit cyclic electron transport around photosystem I in ruptured chloroplasts

Antimycin A₃ (AA) is used as an inhibitor of cyclic electron transport around photosystem I. However, the high concentrations of AA that are needed for inhibition have secondary effects, even in chloroplasts. Here, we screened for chemicals that inhibited ferredoxin-dependent plastoquinone reduction in ruptured chloroplasts at lower concentrations than those required for AA. We identified two AA-like compounds: AAL1 and AAL2. AAL1 likely shares an inhibitory site with AA, most probably in the PGR5–PGRL1 protein complex, and enhances O₂ evolution in photosystem II, most likely via an uncoupler-like effect. AAL1 and AAL2 are unlikely to penetrate intact leaves. In ruptured chloroplasts, AALs are superior to AA as inhibitors of cyclic electron transport.

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Antimycin A₃ (AA) inhibits PSI cyclic electron transport.

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AA-like compounds inhibit PSI cyclic electron transport at lower concentrations than AA.

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AAL1 targets the same site as AA to inhibit PSI cyclic electron transport.

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V3K alteration in PGR5 does not confer resistance to AAL2.

Variations in efficiency of plastidial RNA editing within ndh transcripts of perennial ryegrass (Lolium perenne) are not linked to differences in drought tolerance

Projected climate change is likely to subject key temperate grassland species, such as perennial ryegrass (Lolium perenne) to drought stress. Previous studies have shown that the NADH dehydrogenase complex (NDH) is involved with countering oxidative stress during environmental stresses like drought. We studied RNA editing within plastidial transcripts of the NDH complex in relation to the drought response of several accessions of perennial ryegrass. We found dramatic and reproducible differences in RNA editing efficiency between accessions, but efficiency was not influenced by imposition of drought stress, and a direct relationship between editing behaviour and drought response was not detected.

Maintenance of healthy grasslands is essential for efficient livestock production, yet projected climate change is likely to place a heavy drought stress burden on key grassland species, such as perennial ryegrass (Lolium perenne). It is therefore important to gather an in-depth knowledge of the underlying plant response to this stress. The present study is focused on RNA editing (post-transcriptional nucleotide modifications resulting in altered transcripts) within plastidial transcripts of the NADH:ubiquinone oxidoreductase (NDH) complex (NADH dehydrogenase complex) in relation to the drought response of several accessions of perennial ryegrass. Previous studies have shown that the NDH complex is involved in countering oxidative stress during environmental stresses like drought. Owing to the nature of RNA editing within this complex, the RNA editing machinery could play a potential role in regulating the activity of the NDH complex. The investigation revealed dramatic and reproducible differences in RNA editing efficiency between accessions, but efficiency was not influenced by imposition of drought stress, and a direct relationship between editing behaviour and drought response was not detected.

Antimycin A effect on the electron transport in chloroplasts of two Chlamydomonas reinhardtii strains

The effects of antimycin A on the redox state of plastoquinone and on electron donation to photosystem I (PS I) were studied in sulfur-deprived Chlamydomonas reinhardtii cells of the strains cc406 and 137c. We found that this reagent suppresses cyclic electron flow around PS I in the cc406 strain, whereas this inhibitory effect was completely absent in the 137c strain. In the latter strain, antimycin A induced rapid reduction of plastoquinone in the dark and considerably enhanced the rate of electron donation to P700 (+) in the dark. Importantly, neither myxothiazol, an inhibitor of mitochondrial respiration, FCCP, a protonophore, nor propyl gallate, an inhibitor of the plastid terminal oxidase, induced such a strong effect like antimycin A. The results indicate that in the chloroplast of the 137c strain, antimycin A has a site of action outside of the machinery of cyclic electron flow.

Near isohydric grapevine cultivar displays higher photosynthetic efficiency and photorespiration rates under drought stress as compared with near anisohydric grapevine cultivar

Drought stress is known to limit photosynthesis rates and to inflict photo-oxidative damage in grapevines. Grapevines, which are considered drought-tolerant plants, are characterized by diverse hydraulic and photosynthetic behaviors, depending on the cultivar. This research compared the photosynthesis and the photorespiration of Cabernet Sauvignon (Cs) (isohydric) and Shiraz (anisohydric) in an attempt to acquire a wider perspective on the iso/anisohydric phenomenon and its implications. Shiraz and Cs were subjected to terminal drought in the greenhouse. Soil water content (θ), leaf water potential (Ψl ) and stomata conductance (gs ) were measured to determine the cultivars' hydraulic behavior. Gas exchange and fluorometry measurements were taken at 21 and 2% O2 to acquire photosynthesis and photorespiration characteristics. Cs was found to behave in a near isohydric manner whereas Shiraz behaved in a near anisohydric manner. Compared to Shiraz, the reduced stomata conductance values of Cs were accompanied by higher water use efficiency and photorespiration rates, as well as photosystem II photochemical potential (Fv /Fm ). As compared with Shiraz, Cs compensated for lower stomata conductance by higher photosynthesis and photorespiration. These two processes contributed to higher electron flow rates that might have a role in photoinhibition avoidance, which was observed in the stability of Fv /Fm under drought stress.

Water deficit and heat affect the tolerance to high illumination in hibiscus plants

This work studies the effects of water deficit and heat, as well as the involvement of chlororespiration and the ferredoxin-mediated cyclic pathway, on the tolerance of photosynthesis to high light intensity in Hibiscus rosa-sinensis plants. Drought and heat resulted in the down–regulation of photosynthetic linear electron transport in the leaves, although only a slight decrease in variable fluorescence (F_v)/maximal fluorescence (F_m) was observed, indicating that the chloroplast was protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus. The incubation of leaves from unstressed plants under high light intensity resulted in an increase of the activity of electron donation by nicotinamide adenine dinucleotide phosphate (NADPH) and ferredoxin to plastoquinone, but no increase was observed in plants exposed to water deficit, suggesting that cyclic electron transport was stimulated by high light only in control plants. In contrast, the activities of the chlororespiration enzymes (NADH dehydrogenase (NDH) complex and plastid terminal oxidase (PTOX)) increased after incubation under high light intensity in leaves of the water deficit plants, but not in control plants, suggesting that chlororespiration was stimulated in stressed plants. The results indicate that the relative importance of chlororespiration and the cyclic electron pathway in the tolerance of photosynthesis to high illumination differs under stress conditions. When plants were not subjected to stress, the contribution of chlororespiration to photosynthetic electron flow regulation was not relevant, and another pathway, such as the ferredoxin-mediated cyclic pathway, was more important. However, when plants were subjected to water deficit and heat, chlororespiration was probably essential.

Stimulation of chlororespiration by drought under heat and high illumination in Rosa meillandina

Rosa meillandina plants were used to study the effects of water deficit on photosynthesis and chlororespiration. Plants showed high tolerance to heat and high illumination in controlled conditions that ensured that there was no water deficit. However, when heat and high illumination were accompanied by low watering photosynthetic linear electron transport was down regulated, as indicated by the reduced photochemistry efficiency of PS II, which was associated with an increase in the non-photochemical quenching of fluorescence. In addition to the effects on the photosynthetic activity, changes were also observed in the plastidial NDH complex, PTOX and PGR5. In plants exposed to heat and high illumination without water deficit, the activities and amounts of the chlororespiration enzymes, NDH complex and PTOX, remained similar to the control and only increased in response to drought, high light and heat stress, applied together. In contrast, both the PS I activity and the amount of PGR5 polypeptide were higher in plants exposed to heat and high illumination without water deficit than in those with water deficit. The results indicated that in the conditions studied, the contribution of chlororespiration to regulating photosynthetic electron flow is not relevant when there is no water deficit, and another pathway, such as cyclic electron flow involving PGR5 polypeptide, may be more important. However, when PS II activity is inhibited by drought, chlororespiration, together with other routes of electron input to the electron transfer chain, is probably essential.

The enhancement of cyclic electron flow around photosystem I improves the recovery of severely desiccated Porphyra yezoensis (Bangiales, Rhodophyta)

Porphyra yezoensis, a representative species of intertidal macro-algae, is able to withstand periodic desiccation at low tide but is submerged in seawater at high tide. In this study, changes in photosynthetic electron flow in P. yezoensis during desiccation and re-hydration were investigated. The results suggested that the cyclic electron flow around photosystem I (PSI) increased significantly during desiccation, continued to operate at times of severe desiccation, and showed greater tolerance to desiccation than the electron flow around PSII. In addition, PSI activity in desiccated blades recovered faster than PSII activity during re-hydration. Even though linear electron flow was suppressed by DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea], cyclic electron flow could still be restored. This process was insensitive to antimycin A and could be suppressed by dibromothymoquinone (DBMIB). The prolonged dark treatment of blades reduced the speed in which the cyclic electron flow around PSI recovered, suggesting that stromal reductants, including NAD(P)H, played an important role in the donation of electrons to PSI and were the main cause of the rapid recovery of cyclic electron flow in desiccated blades during re-hydration. These results suggested that cyclic electron flow in P. yezoensis played a significant physiological role during desiccation and re-hydration and may be one of the most important factors allowing P. yezoensis blades to adapt to intertidal environments.

Transient MPK6 activation in response to oxygen deprivation and reoxygenation is mediated by mitochondria and aids seedling survival in Arabidopsis

Mitogen-activated protein kinases (MPKs) are regulated by diverse stresses with a reactive oxygen species (ROS) component. Here, we report the rapid and transient activation of MPK3, MPK4 and MPK6 upon oxygen deprivation as well as reoxygenation in seedlings of Arabidopsis thaliana. MPK activation peaked within 2 h of oxygen deprivation and again at a higher magnitude within 5 min of reoxygenation. MPK6 was the predominant kinase regulated by oxygen availability in both aerial and root tissue, except in mpk6 mutants, which displayed compensatory activation of MPK3. A universal consequence of oxygen deprivation in eukaryotes is inhibition of the terminal step of the mitochondrial electron transport chain (mETC). We demonstrate that treatment of seedlings with the mETC inhibitors antimycin A and potassium cyanide under normoxia promotes transient MPK6 and MPK3 activation. Confocal imaging of seedlings provided evidence that both oxygen deprivation and mETC inhibitors stimulate mitochondria-associated ROS production. We found that seedling survival of prolonged oxygen deprivation was improved in transgenics that ectopically overexpress MPK3, MPK4 and MPK6, but the induction of mRNAs associated with low oxygen acclimation responses were not markedly altered in MPK6 overexpression lines or mpk6 loss-of-function mutants. However, distinctions in MPK6 activation potential were correlated with other differences in mRNAs accumulation. Our findings suggest that oxygen deprivation and reoxygenation trigger mitochondrial ROS production to activate MPK signaling, which in turn regulate reversible processes that aid survival of transient oxygen deprivation.

Interplay between non-photochemical plastoquinone reduction and re-oxidation in pre-illuminated Chlamydomonas reinhardtii: a chlorophyll fluorescence study

In photosynthetic eukaryotes, the redox state of the plastoquinone (PQ) pool is an important sensor for mechanisms that regulate the photosynthetic electron transport. In higher plants, a multimeric nicotinamide adenine dinucleotide (phosphate) (NAD(P))H dehydrogenase (NDH) complex and a plastid terminal oxidase (PTOX) are involved in PQ redox homeostasis in the dark. We recently demonstrated that in the microalgae Chlamydomonas reinhardtii, which lacks the multimeric NDH complex of higher plants, non-photochemical PQ reduction is mediated by a monomeric type-II NDH (Nda2). In this study, we further explore the nature and the importance of non-photochemical PQ reduction and oxidation in relation to redox homeostasis in this alga by recording the 'dark' chlorophyll fluorescence transients of pre-illuminated algal samples. From the observation that this fluorescence transient is modified by addition of propyl gallate, a known inhibitor of PTOX, and in a Nda2-deficient strain we conclude that it reflects post-illumination changes in the redox state of PQ resulting from simultaneous PTOX and Nda2 activity. We show that the post-illumination fluorescence transient can be used to monitor changes in the relative rates of the non-photochemical PQ reduction and reoxidation in response to different physiological situations. We study this fluorescence transient in algae acclimated to high light and in a mutant deficient in mitochondrial respiration. Some of our observations indicate that the chlororespiratory pathway participates in redox homeostasis in C. reinhardtii.