Chlororespiration

Organ specific analysis of the anaerobic primary metabolism in rice and wheat seedlings II: light exposure reduces needs for fermentation and extends survival during anaerobiosis

Low oxygen stress in plants can occur during flooding and compromise the availability and utilization of carbohydrates in root and shoot tissues. Low-oxygen-tolerant rice and -sensitive wheat plants were analyzed under anaerobiosis in light to evaluate main factors of the primary metabolism that affect sensitivity against oxygen deprivation: activity of glycolysis and the rate of photosynthesis. Relatively stable ATP contents (93 and 58% of aerated control levels after 24 h anaerobiosis) in illuminated shoot tissues account for enhanced tolerance of rice and wheat seedlings to anaerobiosis upon light exposure in comparison to anoxia in darkness. Although the photosynthetic process was inhibited during low oxygen stress, which was partly due to CO(2) deficiency, more light-exposed than dark-incubated seedlings survived. Illuminated plants could tolerate a 70% lower anaerobic ethanol production in shoots in comparison to darkness, although still an 18-times higher ethanol production rate was determined in rice than in wheat leaves. In conclusion, light-exposed plants grown under anaerobiosis may recycle low amounts of generated oxygen between photosynthesis and dissimilation and generate additional energy not only from substrate phosphorylation during glycolysis but also from other sources like cyclic electron transport.

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q(B)-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.

Imaging viral infection: studies on Nicotiana benthamiana plants infected with the pepper mild mottle tobamovirus

We have studied by kinetic Chl-fluorescence imaging (Chl-FI) Nicotiana benthamiana plants infected with the Italian strain of the pepper mild mottle tobamovirus (PMMoV-I). We have mapped leaf photosynthesis at different points of the fluorescence induction curve as well as at different post-infection times. Images of different fluorescence parameters were obtained to investigate which one could discriminate control from infected leaves in the absence of symptoms. The non-photochemical quenching (NPQ) of excess energy in photosystem II (PSII) seems to be the most adequate chlorophyll fluorescence parameter to assess the effect of tobamoviral infection on the chloroplast. Non-symptomatic mature leaves from inoculated plants displayed a very characteristic time-varying NPQ pattern. In addition, a correlation between NPQ amplification and virus localization by tissue-print was found, suggesting that an increase in the local NPQ values is associated with the areas invaded by the pathogen. Changes in chloroplast ultrastructure in non-symptomatic leaf areas showing different NPQ levels were also investigated. A gradient of ultrastructural modifications was observed among the different areas.

Changes in plant mitochondrial electron transport alter cellular levels of reactive oxygen species and susceptibility to cell death signaling molecules

Transgenic tobacco (Nicotiana tabacum) lacking mitochondrial alternative oxidase (AOX) have been compared with wild-type (Wt) tobacco using two different systems, either suspension cell cultures or leaves. In both systems, a lack of AOX was accompanied by an increase in some anti-oxidant defenses, consistent with the hypothesis that a lack of AOX increases the mitochondrial generation of reactive oxygen species (ROS). In most cases, this increase in anti-oxidant defenses could more than offset the presumed increased rate of ROS generation, resulting paradoxically in a lower steady-state level of ROS than was found in Wt leaves or suspension cells. We also found that the amount of cell death induced by salicylic acid or nitric oxide correlated strongly with the level of ROS (irrespective of the level of AOX), while death induced by azide was dependent upon the presence or absence of AOX. These results suggest that susceptibility to cell death by signaling molecules (salicylic acid and nitric oxide) is dependent upon the steady-state cellular level of ROS and that AOX levels clearly contribute to this steady state, perhaps by influencing the rate of mitochondrial-generated ROS and hence the cellular level of anti-oxidant defenses.

IMMUTANS does not act as a stress-induced safety valve in the protection of the photosynthetic apparatus of Arabidopsis during steady-state photosynthesis

IMMUTANS (IM) encodes a thylakoid membrane protein that has been hypothesized to act as a terminal oxidase that couples the reduction of O(2) to the oxidation of the plastoquinone (PQ) pool of the photosynthetic electron transport chain. Because IM shares sequence similarity to the stress-induced mitochondrial alternative oxidase (AOX), it has been suggested that the protein encoded by IM acts as a safety valve during the generation of excess photosynthetically generated electrons. We combined in vivo chlorophyll fluorescence quenching analyses with measurements of the redox state of P(700) to assess the capacity of IM to compete with photosystem I for intersystem electrons during steady-state photosynthesis in Arabidopsis (Arabidopsis thaliana). Comparisons were made between wild-type plants, im mutant plants, as well as transgenics in which IM protein levels had been overexpressed six (OE-6 x) and 16 (OE-16 x) times. Immunoblots indicated that IM abundance was the only major variant that we could detect between these genotypes. Overexpression of IM did not result in increased capacity to keep the PQ pool oxidized compared to either the wild type or im grown under control conditions (25 degrees C and photosynthetic photon flux density of 150 micromol photons m(-2) s(-1)). Similar results were observed either after 3-d cold stress at 5 degrees C or after full-leaf expansion at 5 degrees C and photosynthetic photon flux density of 150 micromol photons m(-2) s(-1). Furthermore, IM abundance did not enhance protection of either photosystem II or photosystem I from photoinhibition at either 25 degrees C or 5 degrees C. Our in vivo data indicate that modulation of IM expression and polypeptide accumulation does not alter the flux of intersystem electrons to P(700)(+) during steady-state photosynthesis and does not provide any significant photoprotection. In contrast to AOX1a, meta-analyses of published Arabidopsis microarray data indicated that IM expression exhibited minimal modulation in response to myriad abiotic stresses, which is consistent with our functional data. However, IM exhibited significant modulation in response to development in concert with changes in AOX1a expression. Thus, neither our functional analyses of the IM knockout and overexpression lines nor meta-analyses of gene expression support the model that IM acts as a safety valve to regulate the redox state of the PQ pool during stress and acclimation. Rather, IM appears to be strongly regulated by developmental stage of Arabidopsis.

C3 photosynthesis in silico

A computer model comprising light reactions, electron-proton transport, enzymatic reactions, and regulatory functions of C3 photosynthesis has been developed as a system of differential budget equations for intermediate compounds. The emphasis is on electron transport through PSII and PSI and on the modeling of Chl fluorescence and 810 nm absorptance signals. Non-photochemical quenching of PSII excitation is controlled by lumenal pH. Alternative electron transport is modeled as the Mehler type O2 reduction plus the malate-oxaloacetate shuttle based on the chloroplast malate dehydrogenase. Carbon reduction enzymes are redox-controlled by the ferredoxin-thioredoxin system, sucrose synthesis is controlled by the fructose 2,6-bisphosphate inhibition of cytosolic FBPase, and starch synthesis is controlled by ADP-glucose pyrophosphorylase. Photorespiratory glycolate pathway is included in an integrated way, sufficient to reproduce steady-state rates of photorespiration. Rate-equations are designed on principles of multisubstrate-multiproduct enzyme kinetics. The parameters of the model were adopted from literature or were estimated from fitting the photosynthetic rate and pool sizes to experimental data. The model provided good simulations for steady-state photosynthesis, Chl fluorescence, and 810 nm transmittance signals under varying light, CO2 and O2 concentrations, as well as for the transients of post-illumination CO2 uptake, Chl fluorescence induction and the 810 nm signal. The modeling shows that the present understanding of photosynthesis incorporated in the model is basically correct, but still insufficient to reproduce the dark-light induction of photosynthesis, the time kinetics of non-photochemical quenching, 'photosynthetic control' of plastoquinone oxidation, cyclic electron flow around PSI, oscillations in photosynthesis. The model may find application for predicting the results of gene transformations, the analysis of kinetic experimental data, the training of students.

Origins, evolutionary history, and taxonomic distribution of alternative oxidase and plastoquinol terminal oxidase

Alternative oxidase (AOX) and plastoquinol terminal oxidase (PTOX) are related quinol oxidases associated with respiratory and photosynthetic electron transport chains, respectively. Contrary to previous belief, AOX is present in numerous animal phyla, as well as heterotrophic and marine phototrophic proteobacteria. PTOX appears limited to organisms capable of oxygenic photosynthesis, including cyanobacteria, algae and plants. We propose that both oxidases originated in prokaryotes from a common ancestral di-iron carboxylate protein that diversified to AOX within ancient proteobacteria and PTOX within ancient cyanobacteria. Each then entered the eukaryotic lineage separately; AOX by the endosymbiotic event that gave rise to mitochondria and later PTOX by the endosymbiotic event that gave rise to chloroplasts. Both oxidases then spread through the eukaryotic domain by vertical inheritance, as well as by secondary and potentially tertiary endosymbiotic events.

The role of chloroplast NAD(P)H dehydrogenase in protection of tobacco plant against heat stress

After incubation at 42 degrees C for more than 48 h, brown damages occurred on the stems of tobacco (Nicotiana tabacum L.) ndhC-ndhK-ndhJ deletion mutant (deltandhCKJ), followed by wilt of the leaves, while less the phenotype was found in its wild type (WT). Analysis of the kinetics of post-illumination rise in chlorophyll fluorescence indicated that the PSI cyclic electron flow and the chlororespiration mediated by NAD(P)H dehydrogenase (NDH) was significantly enhanced in WT under the high temperature. After leaf disks were treated with methyl viologen (MV), photosynthetic apparatus of deltandhCKJ exhibited more severe photo-oxidative damage, even bleaching of chlorophyll. Analysis of P700 oxidation and reduction showed that the NDH mediated cyclic electron flow probably functioned as an electron competitor with Mehler reaction, to reduce the accumulation of reactive oxygen species (ROS). When leaf disks were heat stressed at 42 degrees C for 6 h, the photochemical activity declined more markedly in deltandhCKJ than in WT, accompanied with more evident decrease in the amount of soluble Rubisco activase. In addition, the slow phase of millisecond-delayed light emission (ms-DLE) of chlorophyll fluorescence indicated that NDH was involved in the building-up of transthylakoid proton gradient (deltapH), while the consumption of deltapH was highly inhibited in deltandhCKJ after heat stress. Based on the results, we supposed that the cyclic electron flow mediated by NDH could be stimulated under the heat stressed conditions, to divert excess electrons via chlororespiration pathway, and sustain CO2 assimilation by providing extra deltapH, thus reducing the photooxidative damage.

Stimulation of chlororespiration by heat and high light intensity in oat plants

High irradiance and moderate heat inhibit the activity of the photosynthetic apparatus of oat (Avena sativa L.) leaves. The incubation of oat leaves under high light intensity in conjunction with high temperatures strongly decreased the maximal quantum yield of photosystem (PS) II, indicating the close synergistic effect of both stress factors on PS II inhibition and the subsequent irreversible damage to the photosynthetic apparatus. The PS I A/B protein levels remained similar to control values in leaves incubated under high light intensity or moderate heat, and decreased only when both stress factors were simultaneously applied. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in response to high light intensity and/or heat treatments. In addition, these stress treatments were seen to stimulate the activity of electron donation by NADPH and ferredoxin to plastoquinone, the PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes. Incubation with n-propyl gallate (an inhibitor of PTOX) inhibited the increase of NDH-K and PTOX levels under high light intensity and heat, and slightly stimulated the activity of electron donation by NADPH and ferredoxin to plastoquinone. Antimycin A (an inhibitor of cyclic electron flow) increased the NADH DH activity and preserved the levels of NDH-K and PTOX in thylakoid membranes from leaves incubated under high light intensity and heat. The up-regulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the protection against high irradiance and moderate heat.

Agrobacterium tumefaciens type II NADH dehydrogenase

Type II NADH dehydrogenases (NDH-2) are monomeric enzymes that catalyse quinone reduction and allow electrons to enter the respiratory chain in different organisms including higher plant mitochondria, bacteria and yeasts. In this study, an Agrobacterium tumefaciens gene encoding a putative alternative NADH dehydrogenase (AtuNDH-2) was isolated and expressed in Escherichia coli as a (His)6-tagged protein. The purified 46 kDa protein contains FAD as a prosthetic group and oxidizes both NADH and NADPH with similar Vmax values, but with a much higher affinity for NADH than for NADPH. AtuNDH-2 complements the growth (on a minimal medium) of an E. coli mutant strain deficient in both NDH-1 and NDH-2, and is shown to supply electrons to the respiratory chain when incubated with bacterial membranes prepared from this mutant. By measuring photosystem II chlorophyll fluorescence on thylakoid membranes prepared from the green alga Chlamydomonas reinhardtii, we show that AtuNDH-2 is able to stimulate NADH-dependent reduction of the plastoquinone pool. We discuss the possibility of using heterologous expression of NDH-2 enzymes to improve nonphotochemical reduction of plastoquinones and H2 production in C. reinhardtii.

Chloroplastic NAD(P)H dehydrogenase in tobacco leaves functions in alleviation of oxidative damage caused by temperature stress

In this study, the function of the NAD(P)H dehydrogenase (NDH)-dependent pathway in suppressing the accumulation of reactive oxygen species in chloroplasts was investigated. Hydrogen peroxide accumulated in the leaves of tobacco (Nicotiana tabacum) defective in ndhC-ndhK-ndhJ (DeltandhCKJ) at 42 degrees C and 4 degrees C, and in that of wild-type leaves at 4 degrees C. The maximum quantum efficiency of PSII decreased to a similar extent in both strains at 42 degrees C, while it decreased more evidently in DeltandhCKJ at 4 degrees C. The parameters linked to CO(2) assimilation, such as the photochemical efficiency of PSII, the decrease of nonphotochemical quenching following the initial rise, and the photosynthetic O(2) evolution, were inhibited more significantly in DeltandhCKJ than in wild type at 42 degrees C and were seriously inhibited in both strains at 4 degrees C. While cyclic electron flow around PSI mediated by NDH was remarkably enhanced at 42 degrees C and suppressed at 4 degrees C. The proton gradient across the thylakoid membranes and light-dependent ATP synthesis were higher in wild type than in DeltandhCKJ at either 25 degrees C or 42 degrees C, but were barely formed at 4 degrees C. Based on these results, we suggest that cyclic photophosphorylation via the NDH pathway might play an important role in regulation of CO(2) assimilation under heat-stressed condition but is less important under chilling-stressed condition, thus optimizing the photosynthetic electron transport and reducing the generation of reactive oxygen species.

Fibrillin expression is regulated by abscisic acid response regulators and is involved in abscisic acid-mediated photoprotection

Fibrillins are lipid-binding proteins of plastids that are induced under abiotic stress conditions. In response to environmental stress, plants generate abscisic acid (ABA) as an endogenous signal. We show that ABA treatment and fibrillin accumulation enhance the tolerance of photosystem II toward light stress-triggered photoinhibition in Arabidopsis. ABA induces fibrillin accumulation, and the ABA response regulators ABI1 and ABI2 regulate fibrillin expression. The abundance of fibrillin transcripts was specifically reduced in the ABA-insensitive abi1 mutant but not in the abi2 mutant. However, leaves of abi2 revealed in comparison to WT and abi1 enhanced fibrillin levels, pointing to a posttranscriptional control mechanism. Protein interaction analysis identified the protein phosphatase ABI2 to target the preprotein of fibrillin. Interaction was abrogated either by deleting the signal peptide of prefibrillin or by the single amino acid exchange present in the phosphatase-deficient abi2 protein. Thus, ABI1 and ABI2 seem to control fibrillin expression that is involved in mediating ABA-induced photoprotection.

Genome-based approaches to understanding phosphorus deprivation responses and PSR1 control in Chlamydomonas reinhardtii

The Chlamydomonas reinhardtii transcription factor PSR1 is required for the control of activities involved in scavenging phosphate from the environment during periods of phosphorus limitation. Increased scavenging activity reflects the development of high-affinity phosphate transport and the expression of extracellular phosphatases that can cleave phosphate from organic compounds in the environment. A comparison of gene expression patterns using microarray analyses and quantitative PCRs with wild-type and psr1 mutant cells deprived of phosphorus has revealed that PSR1 also controls genes encoding proteins with potential "electron valve" functions--these proteins can serve as alternative electron acceptors that help prevent photodamage caused by overexcitation of the photosynthetic electron transport system. In accordance with this finding, phosphorus-starved psr1 mutants die when subjected to elevated light intensities; at these intensities, the wild-type cells still exhibit rapid growth. Acclimation to phosphorus deprivation also involves a reduction in the levels of transcripts encoding proteins involved in photosynthesis and both cytoplasmic and chloroplast translation as well as an increase in the levels of transcripts encoding stress-associated chaperones and proteases. Surprisingly, phosphorus-deficient psr1 cells (but not wild-type cells) also display expression patterns associated with specific responses to sulfur deprivation, suggesting a hitherto unsuspected link between the signal transduction pathways involved in controlling phosphorus and sulfur starvation responses. Together, these results demonstrate that PSR1 is critical for the survival of cells under conditions of suboptimal phosphorus availability and that it plays a key role in controlling both scavenging responses and the ability of the cell to manage excess absorbed excitation energy.

Isolation, subunit composition and interaction of the NDH-1 complexes from Thermosynechococcus elongatus BP-1

NDH (NADH-quinone oxidoreductase)-1 complexes in cyanobacteria have specific functions in respiration and cyclic electron flow as well as in active CO2 uptake. In order to isolate NDH-1 complexes and to study complex-complex interactions, several strains of Thermosynechococcus elongatus were constructed by adding a His-tag (histidine tag) to different subunits of NDH-1. Two strains with His-tag on CupA and NdhL were successfully used to isolate NDH-1 complexes by one-step Ni2+ column chromatography. BN (blue-native)/SDS/PAGE analysis of the proteins eluted from the Ni2+ column revealed the presence of three complexes with molecular masses of about 450, 300 and 190 kDa, which were identified by MS to be NDH-1L, NDH-1M and NDH-1S respectively, previously found in Synechocystis sp. PCC 6803. A larger complex of about 490 kDa was also isolated from the NdhL-His strain. This complex, designated 'NDH-1MS', was composed of NDH-1M and NDH-1S. NDH-1L complex was recovered from WT (wild-type) cells of T. elongatus by Ni2+ column chromatography. NdhF1 subunit present only in NDH-1L has a sequence of -HHDHHSHH- internally, which appears to have an affinity for the Ni2+ column. NDH-1S or NDH-1M was not recovered from WT cells by chromatography of this kind. The BN/SDS/PAGE analysis of membranes solubilized by a low concentration of detergent indicated the presence of abundant NDH-1MS, but not NDH-1M or NDH-1S. These results clearly demonstrated that NDH-1S is associated with NDH-1M in vivo.

Photosystem II Heterogeneity of in hospite Zooxanthellae in Scleractinian Corals Exposed to Bleaching Conditions

Increased ocean temperatures are thought to be triggering mass coral bleaching events around the world. The intracellular symbiotic zooxanthellae (genus Symbiodinium) are expelled from the coral host, which is believed to be a response to photosynthetic damage within these symbionts. Several sites of impact have been proposed, and here we probe the functional heterogeneity of Photosystem II (PSII) in three coral species exposed to bleaching conditions. As length of exposure to bleaching conditions (32 degrees C and 350 micromol photons m(-2) s(-1)) increased, the QA- reoxidation kinetics showed a rise in the proportion of inactive PSII centers (PSIIx), where QB was unable to accept electrons. PSIIx contributed up to 20% of the total PSII centers in Pocillopora damicornis, 35% in Acropora nobilis and 14% in Cyphastrea serailia. Changes in Fv/Fm and amplitude of the J step along fast induction curves were found to be highly dependent upon the proportion of PSIIx centers within the total pool of PSII reaction centers. Determination of PSII antenna size revealed that under control conditions in the three coral species up to 60% of PSII centers were lacking peripheral light-harvesting complexes (PSIIbeta). In P. damicornis, the proportion of PSIIbeta increased under bleaching conditions and this could be a photoprotective mechanism in response to excess light. The rapid increases in PSIIx and PSIIbeta observed in these corals under bleaching conditions indicates these physiological processes are involved in the initial photochemical damage to zooxanthellae.

Identification of a novel protein, CRR7, required for the stabilization of the chloroplast NAD(P)H dehydrogenase complex in Arabidopsis

An Arabidopsis thaliana mutant, crr7 (chlororespiratory reduction), was isolated using chlorophyll fluorescence imaging to detect reduced activity in NAD(P)H dehydrogenase (NDH). The chloroplast NDH complex is considered to have originated from cyanobacteria in which the NDH complex is involved in respiration, photosystem I (PSI) cyclic electron transport and CO2 uptake. In higher plants the NDH complex functions in PSI cyclic electron transport within the chloroplast. Despite exhaustive biochemical approaches, the entire subunit composition of the NDH complex is unclear in both cyanobacteria and chloroplasts. In crr7 accumulation of the NDH complex was specifically impaired. In vivo analysis of electron transport supported the specific loss of the NDH complex in crr7. CRR7 (At5g39210) encodes a protein of 156 amino acids, including a putative plastid target signal, and does not contain any known motifs. In contrast to CRR2 and CRR4, involved in the expression of chloroplast ndh genes, CRR7 is conserved in cyanobacterial genomes. Although CRR7 did not contain any transmembrane domains, it localized to the membrane fraction of the chloroplast. CRR7 was unstable in the crr2-2 mutant background, in which the expression of ndhB was impaired. These results strongly suggest that CRR7 is a novel subunit of the chloroplast NDH complex.

Sequences required for the activity of PTOX (IMMUTANS), a plastid terminal oxidase: in vitro and in planta mutagenesis of iron-binding sites and a conserved sequence that corresponds to Exon 8

The thylakoid membranes of most photosynthetic organisms contain a terminal oxidase (PTOX, the product of the Arabidopsis IMMUTANS gene) that functions in the oxidation of the plastoquinone pool. PTOX and AOX are diiron carboxylate proteins, and based on crystal structures of other members of this protein class, a structural model of PTOX has been proposed in which the ligation sphere of the diiron center is composed of six conserved histidine and glutamate residues. We tested the functional significance of these residues by site-directed mutagenesis of PTOX in vitro and in planta, taking advantage null immutans alleles for the latter studies. These experiments showed that the six iron-binding sites do not tolerate change, even conservative ones. We also examined the significance of a conserved sequence in (or near) the PTOX active site that corresponds precisely to Exon 8 of the IM gene. In vitro and in planta mutagenesis revealed that conserved amino acids within this domain can be altered but that deletion of all or part of the domain abolishes activity. Because protein accumulates normally in the deletion mutants, the data suggest that the conformation of the Exon 8 sequence is important for PTOX activity. An allele of immutans (designated 3639) was identified that lacks the Exon 8 sequence; it does not accumulate PTOX protein. Chloroplast import assays revealed that mutant enzymes lacking Exon 8 have enhanced turnover. We conclude that the Exon 8 domain is required not only for PTOX activity but also for its stability.

Specific function of a plastid sigma factor for ndhF gene transcription

The complexity of the plastid transcriptional apparatus (two or three different RNA polymerases and numerous regulatory proteins) makes it very difficult to attribute specific function(s) to its individual components. We have characterized an Arabidopsis T-DNA insertion line disrupting the nuclear gene coding for one of the six plastid sigma factors (SIG4) that regulate the activity of the plastid-encoded RNA polymerase PEP. This mutant shows a specific diminution of transcription of the plastid ndhF gene, coding for a subunit of the plastid NDH [NAD(P)H dehydrogenase] complex. The absence of another NDH subunit, i.e. NDHH, and the absence of a chlorophyll fluorescence transient previously attributed to the activity of the plastid NDH complex indicate a strong down-regulation of NDH activity in the mutant plants. Results suggest that plastid NDH activity is regulated on the transcriptional level by an ndhF-specific plastid sigma factor, SIG4.

Probing the FQR and NDH activities involved in cyclic electron transport around Photosystem I by the 'afterglow' luminescence

Far-red illumination of plant leaves for a few seconds induces a delayed luminescence rise, or afterglow, that can be measured with the thermoluminescence technique as a sharp band peaking at around 40-45 degrees C. The afterglow band is attributable to a heat-induced electron flow from the stroma to the plastoquinone pool and the PSII centers. Using various Arabidopsis and tobacco mutants, we show here that the electron fluxes reflected by the afterglow luminescence follow the pathways of cyclic electron transport around PSI. In tobacco, the afterglow signal relied mainly on the ferredoxin-quinone oxidoreductase (FQR) activity while the predominant pathway responsible for the afterglow in Arabidopsis involved the NAD(P)H dehydrogenase (NDH) complex. The peak temperature T(m) of the afterglow band varied markedly with the light conditions prevailing before the TL measurements, from around 30 degrees C to 45 degrees C in Arabidopsis. These photoinduced changes in Tm followed the same kinetics and responded to the same light stimuli as the state 1-state 2 transitions. PSII-exciting light (leading to state 2) induced a downward shift while preillumination with far-red light (inducing state 1) caused an upward shift. However, the light-induced downshift was strongly inhibited in NDH-deficient Arabidopsis mutants and the upward shift was cancelled in plants durably acclimated to high light, which can perform normal state transitions. Taken together, our results suggest that the peak temperature of the afterglow band is indicative of regulatory processes affecting electron donation to the PQ pool which could involve phosphorylation of NDH. The afterglow thermoluminescence band provides a new and simple tool to investigate the cyclic electron transfer pathways and to study their regulation in vivo.

Down the slippery slope: plastid genome evolution in Convolvulaceae

Cuscuta (dodder) is the only parasitic genus found in Convolvulaceae (morning-glory family). We used long PCR approach to obtain large portions of plastid genome sequence from Cuscuta sandwichiana in order to determine the size, structure, gene content, and synteny in the plastid genome of this Cuscuta species belonging to the poorly investigated holoparasitic subgenus Grammica. These new sequences are compared with the tobacco chloroplast genome, and, where data are available, with corresponding regions from taxa in the other Cuscuta subgenera. When all known plastid genome structural rearrangements in parasitic and nonparasitic Convolvulaceae are considered in a molecular phylogenetic framework, three categories of rearrangements in Cuscuta are revealed: plesiomorphic, autapomorphic, and synapomorphic. Many of the changes in Cuscuta, previously attributed to its parasitic mode of life, are better explained either as plesiomorphic conditions within the family, i.e., conditions shared with the rest of the Convolvulaceae, or, in most cases, autapomorphies of particular Cuscuta taxa, not shared with the rest of the species in the genus. The synapomorphic rearrangements are most likely to correlate with the parasitic lifestyle, because they represent changes found in Cuscuta exclusively. However, it appears that most of the affected regions, belonging to all of these three categories, have probably no function (e.g., introns) or are of unknown function (a number of open reading frames, the function of which, if any, has yet to be discovered).

Inhibitor studies on non-photochemical plastoquinone reduction and H(2) photoproduction in Chlamydomonas reinhardtii

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O(2) uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H(2) in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H(2) production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H(2) production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.

Open Reading Frame ssr2016 is Required for Antimycin A-sensitive Photosystem I-driven Cyclic Electron Flow in the Cyanobacterium Synechocystis sp. PCC 6803

Open reading frame ssr2016 encodes a protein with substantial sequence similarities to PGR5 identified as a component of the antimycin A-sensitive ferredoxin:plastoquinone reductase (FQR) in PSI cyclic photophosphorylation in Arabidopsis thaliana. We studied cyclic electron flow in Synechocystis sp. PCC 6803 in vivo in ssr2016 deletion mutants generated either in a wild-type background or in a ndhB deletion mutant. Our results indicate that ssr2016 is required for FQR and that it operates in a parallel pathway to the NDH1 complex. The ssr2016 deletion mutants are high light sensitive, suggesting that FQR might be important in controlling redox poise under adverse conditions.

FesM, a membrane iron-sulfur protein, is required for cyclic electron flow around photosystem I and photoheterotrophic growth of the cyanobacterium Synechococcus sp. PCC 7002

While it is known that cyclic electron flow around photosystem I is an important pathway of photosynthetic electron transfer for converting light energy to chemical energy, some components of cyclic electron flow remain to be revealed. Here, we show that fesM, encoding a novel membrane iron-sulfur protein, is essential to cyclic electron flow in the cyanobacterium Synechococcus sp. PCC 7002. The FesM protein is predicted to have a cAMP-binding domain, an NtrC-like domain, a redox sensor motif, and an iron-sulfur (4Fe-4S) motif. Deletion of fesM (fesM-D) led to an inability for Synechococcus cells to grow in the presences of glycerol and 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Photoheterotrophic growth was restored by a complete fesM gene present on a replicable plasmid. A mutant fesM gene encoding a truncated FesM protein lacking the cAMP domain failed to restore the phenotype, suggesting this domain is important to the function of FesM. Measurements of reduction of P700(+) and the redox state of interphotosystem electron carriers showed that cells had a slower rate of respiratory electron donation to the interphotosystem electron transport chain, and cyclic electron flow around photosystem I in fesM-D was impaired, suggesting that FesM is a critical protein for respiratory and cyclic electron flow. Phosphatase fusion analysis showed that FesM contains nine membrane-spanning helices, and all functional domains of FesM are located on the cytoplasmic face of the thylakoid membranes.

Chloroplasts as source and target of cellular redox regulation: a discussion on chloroplast redox signals in the context of plant physiology

During the evolution of plants, chloroplasts have lost the exclusive genetic control over redox regulation and antioxidant gene expression. Together with many other genes, all genes encoding antioxidant enzymes and enzymes involved in the biosynthesis of low molecular weight antioxidants were transferred to the nucleus. On the other hand, photosynthesis bears a high risk for photo-oxidative damage. Concomitantly, an intricate network for mutual regulation by anthero- and retrograde signals has emerged to co-ordinate the activities of the different genetic and metabolic compartments. A major focus of recent research in chloroplast regulation addressed the mechanisms of redox sensing and signal transmission, the identification of regulatory targets, and the understanding of adaptation mechanisms. In addition to redox signals communicated through signalling cascades also used in pathogen and wounding responses, specific chloroplast signals control nuclear gene expression. Signalling pathways are triggered by the redox state of the plastoquinone pool, the thioredoxin system, and the acceptor availability at photosystem I, in addition to control by oxolipins, tetrapyrroles, carbohydrates, and abscisic acid. The signalling function is discussed in the context of regulatory circuitries that control the expression of antioxidant enzymes and redox modulators, demonstrating the principal role of chloroplasts as the source and target of redox regulation.

The photosynthesis of individual algal cells during the cell cycle of Scenedesmus quadricauda studied by chlorophyll fluorescence kinetic microscopy

A microscope for imaging of chlorophyll fluorescence kinetics was equipped with a chamber that allows the growth of an immobilised population of algae and their study under well-defined conditions. Single cells of the chlorococcal alga Scenedesmus quadricauda were grown and recorded for periods of whole cell cycles (up to 48 h) displaying a normal course of cell development. Heterogeneity in fluorescence yield among individual coenobia in the population and among different cells in one coenobium were analysed. Differences were observed both in the shape of Kautsky transients and in the modulation of fluorescence parameter values during the progress of the cell cycle. The extent of heterogeneity in fluorescence parameters was cell cycle dependent - in some phases of the cycle, the population was almost homogeneous, while distinct heterogeneity was observed, in particular between the protoplast division and the release of the daughter coenobia. The heterogeneity was not random but reflected developmental processes.

Non-photochemical quenching of chlorophyll a fluorescence by oxidised plastoquinone: new evidences based on modulation of the redox state of the endogenous plastoquinone pool in broken spinach chloroplasts

Twenty-five years ago, non-photochemical quenching of chlorophyll fluorescence by oxidised plastoquinone (PQ) was proposed to be responsible for the lowering of the maximum fluorescence yield reported to occur when leaves or chloroplasts were treated in the dark with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of electron flow beyond the primary quinone electron acceptor (Q(A)) of photosystem (PS) II. Since then, the notion of PQ-quenching has received support but has also been put in doubt, due to inconsistent experimental findings. In the present study, the possible role of the native PQ-pool as a non-photochemical quencher was reinvestigated, employing measurements of the fast chlorophyll a fluorescence kinetics (from 50 micros to 5 s). The about 20% lowering of the maximum fluorescence yield F(M), observed in osmotically broken spinach chloroplasts treated with DCMU, was eliminated when the oxidised PQ-pool was non-photochemically reduced to PQH(2) by dark incubation of the samples in the presence of NAD(P)H, both under anaerobic and aerobic conditions. Incubation under anaerobic conditions in the absence of NAD(P)H had comparatively minor effects. In DCMU-treated samples incubated in the presence of NAD(P)H fluorescence quenching started to develop again after 20-30 ms of illumination, i.e., the time when PQH(2) starts getting reoxidized by PS I activity. NAD(P)H-dependent restoration of F(M) was largely, if not completely, eliminated when the samples were briefly (5 s) pre-illuminated with red or far-red light. Addition to the incubation medium of HgCl(2) that inhibits dark reduction of PQ by NAD(P)H also abolished NAD(P)H-dependent restoration of F(M). Collectively, our results provide strong new evidence for the occurrence of PQ-quenching. The finding that DCMU alone did not affect the minimum fluorescence yield F(0) allowed us to calculate, for different redox states of the native PQ-pool, the fractional quenching at the F(0) level (Q(0)) and to compare it with the fractional quenching at the F(M) level (Q(M)). The experimentally determined Q(0)/Q(M) ratios were found to be equal to the corresponding F(0)/F(M) ratios, demonstrating that PQ-quenching is solely exerted on the excited state of antenna chlorophylls.

Photosynthetic activity of Lolium perenne as a function of endophyte status and zinc nutrition

Grass infection by endophyte fungi can confer host resistance to different types of stress, but limited evidence is available on the related effects on the photosynthetic mechanism of the grasses. Zinc has direct and indirect effects on this mechanism and is one of the more important environmental pollutants. To measure whether photosynthesis of the host plant is affected by fungal infection when endophyte-free and endophyte-infected grasses contain similar excess zinc concentrations in their leaves, two batches of Lolium perenne L. cv. Apollo plants were established, one batch was infected with Neotyphodium lolii, the other was not. Both batches were then treated for 8 d with a nutrient solution containing 0, 1, 5, 10 or 20 mm ZnSO₄. The increase in zinc concentration induced some reduction in photosystem II (PSII) activity but not enough to account for the total drop in the net photosynthetic rate. Endophyte fungus favoured maintenance of the PSII activity, but did not significantly modify the net photosynthesis and similar zinc concentration levels were observed in leaves of both types of plant. Interactive effects of zinc and light induced less photodamage to the PSII of the host, which is able to react to an increase in photon flux density (PFD). In endophyte-free plants, the reaction centre contributed more than antenna complexes to energy dissipation. In endophyte-infected plants, the quenching of the reaction centre and antenna complexes rose simultaneously and at a constant rate, as zinc concentrations increased.

Interaction of pigment-protein complexes within aggregates stimulates dissipation of excess energy

Pigment-protein complexes in photosynthetic membranes exist mainly as aggregates that are functionally active as monomers but more stable due to their ability to dissipate excess energy. Dissipation of energy in the photosystem I (PSI) trimers of cyanobacteria takes place with a contribution of the long-wavelength chlorophylls whose excited state is quenched by cation radical of P700 or P700 in its triplet state. If P700 in one of the monomer complexes within a PSI trimer is oxidized, energy migration from antenna of other monomer complexes to cation radical of P700 via peripherally localized long-wavelength chlorophylls results in energy dissipation, thus protecting PSI complex of cyanobacteria against photodestruction. It is suggested that dissipation of excess absorbed energy in aggregates of the light-harvesting complex LHCII of higher plants takes place with a contribution of peripherally located chlorophylls and carotenoids.

State transitions in Chlamydomonas reinhardtii. The role of the Mehler reaction in state 2-to-state 1 transition

The light intensity-dependent transition to state 1 of dark-adapted anaerobic state 2 Chlamydomonas reinhardtii cells is stimulated by oxygen and by other electron acceptors for photosystem I, such as oxaloacetate and methylviologen. This suggests that the transition to state 1 requires the oxidation of the intersystem chain by photosystem I photochemistry. On the other hand, the mere oxidation in the dark of the chain-by addition of O2-leads only to a slow and incomplete transition. The light-driven stimulation by O2 of the state 1 transition is saturated at an O2 concentration of 15 to 20 microm, definitely higher than that of respiration. We suggest that this may represent the affinity for oxygen of the Mehler reaction, a conclusion that is confirmed by the observations that mitochondrial respiration is apparently not involved in modulating state 2-to-state 1 transition. The catalysis of the state 2-to-state 1 transition upon illumination of anaerobically adapted algae might represent, therefore, a relevant physiological role of this process in C. reinhardtii.

Functional flexibility and acclimation of the thylakoid membrane

Light is an elusive substrate for the function of photosynthetic light reactions of photosynthesis in the thylakoid membrane. Therefore structural and functional dynamics, which occur in the timescale from seconds to several days, are required both at low and high light conditions. The best characterized short-time regulation mechanism at low light is a rapid state transition, resulting in higher absorption cross section of PSI at the expense of PSII. If the low light conditions continue, activation of the lhcb-genes and synthesis of the light-harvesting proteins will occur to optimize the functions of PSII and PSI. At high light, the transition to state 2 is completely inhibited, but the feedback de-excitation of absorbed energy as heat, known as the energy-dependent quenching (q(E)), is rapidly set up. It requires, at least, the DeltapH-dependent activation of violaxanthin de-epoxidase and involvement of the PsbS protein. Another crucial mechanism for protection against the high light stress is the PSII repair cycle. Furthermore, the water-water cycle, cyclic electron transfer around PSI and chlororespiration are important means induced under high irradiation, functioning mainly to avoid an excess production of reactive oxygen species.

New subunits NDH-M, -N, and -O, encoded by nuclear genes, are essential for plastid Ndh complex functioning in higher plants

In higher plants, the Ndh complex reduces plastoquinones and is involved in cyclic electron flow around photosystem I, supplying extra-ATP for photosynthesis, particularly under environmental stress conditions. Based on plastid genome sequences, the Ndh complex would contain 11 subunits (NDH-A to -K), but homologies with bacterial complex indicate the probable existence of additional subunits. To identify missing subunits, tobacco (Nicotiana tabacum) NDH-H was His tagged at its N terminus using plastid transformation. A functional Ndh subcomplex was purified by Ni(2+) affinity chromatography and its subunit composition analyzed by mass spectrometry. Five plastid encoded subunits (NDH-A, -H, -I, -J, and -K) were identified as well as three new subunits (NDH-M, -N, and -O) homologous to cyanobacterial and higher plant proteins. Arabidopsis thaliana mutants missing one of these new subunits lack a functional Ndh complex, and NDH-M and NDH-N are not detected in a tobacco transformant lacking the Ndh complex. We discuss the involvement of these three nuclear-encoded subunits in the functional integrity of the plastidial complex.

Identification of NdhL and Ssl1690 (NdhO) in NDH-1L and NDH-1M complexes of Synechocystis sp. PCC 6803

The subunit compositions of two types of NAD(P)H dehydrogenase complexes of Synechocystis sp. PCC 6803, NDH-1L and NDH-1M, were studied by two-dimensional blue-native/SDS-PAGE followed by electrospray tandem mass spectrometry. Fifteen proteins were observed in NDH-1L including hydrophilic subunits (NdhH, -K, -I, -J, -M, and -N) and hydrophobic subunits (NdhA, -B, -E, -G, -D1, and -F1). In addition, NdhL and a novel subunit, Ssl1690 (designated NdhO), were shown to be components of this complex. All subunits mentioned above were present in the NDH-1M complex except NdhD1 and NdhF1. NdhL and Ssl1690 (NdhO) were homologous to hypothetical proteins encoded by genomic DNA in higher plants, suggesting that chloroplast NDH-1 complexes contain related subunits. Diagnostic sequence motifs were found for both NdhL and NdhO homologous proteins. Analysis of ndhL deletion mutant (M9) revealed the presence of assembled NDH-1L and NDH-1M complexes, but these complexes appear to be functionally impaired in the absence of NdhL. Both NDH-1 complexes were absent in the ndhB deletion mutant (M55).

Photosynthetic evolution in parasitic plants: insight from the chloroplast genome

Despite the enormous diversity in plant form, structure and growth environment across the seed-bearing plants (angiosperms and gymnosperms), the chloroplast genome has, with few exceptions, remained remarkably conserved. This conservation suggests the existence of universal evolutionary selection pressures associated with photosynthesis-the primary function of chloroplasts. The stark exceptions to this conservation occur in parasitic angiosperms, which have escaped the dominant model by evolving the capacity to obtain some or all of their carbon (and nutrients) from their plant hosts. The consequence of this evolution to parasitism is a relaxation of the evolutionary constraints associated with the need to maintain photosynthetic function, the very function that drove early stages of the ancient symbiotic relationship that produced the contemporary chloroplast. Extreme examples of reductionism among parasitic angiosperms reveals major alterations in chloroplast function with the loss of photosynthetic capacity and, with that, massive alterations in chloroplast genome content. This review highlights emerging patterns in reported gene loss and gene retention in the chloroplast genomes of parasitic plants. Some gene losses appear to occur in the early stages of parasitic evolution, even before the loss of photosynthetic capacity, like the chlororespiratory (ndh) genes. This contrasts with unexpected gene retentions, like that of the rbcL gene responsible for photosynthetic carbon dioxide fixation, and belies current understanding of gene function. The review relates gene retention to current knowledge of protein function and gene processing that has implications to broader aspects of genome conservation in organelles.

Cyclic electron flow around photosystem I is essential for photosynthesis

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

In-depth analysis of the thylakoid membrane proteome of Arabidopsis thaliana chloroplasts: new proteins, new functions, and a plastid proteome database

An extensive analysis of the Arabidopsis thaliana peripheral and integral thylakoid membrane proteome was performed by sequential extractions with salt, detergent, and organic solvents, followed by multidimensional protein separation steps (reverse-phase HPLC and one- and two-dimensional electrophoresis gels), different enzymatic and nonenzymatic protein cleavage techniques, mass spectrometry, and bioinformatics. Altogether, 154 proteins were identified, of which 76 (49%) were alpha-helical integral membrane proteins. Twenty-seven new proteins without known function but with predicted chloroplast transit peptides were identified, of which 17 (63%) are integral membrane proteins. These new proteins, likely important in thylakoid biogenesis, include two rubredoxins, a potential metallochaperone, and a new DnaJ-like protein. The data were integrated with our analysis of the lumenal-enriched proteome. We identified 83 out of 100 known proteins of the thylakoid localized photosynthetic apparatus, including several new paralogues and some 20 proteins involved in protein insertion, assembly, folding, or proteolysis. An additional 16 proteins are involved in translation, demonstrating that the thylakoid membrane surface is an important site for protein synthesis. The high coverage of the photosynthetic apparatus and the identification of known hydrophobic proteins with low expression levels, such as cpSecE, Ohp1, and Ohp2, indicate an excellent dynamic resolution of the analysis. The sequential extraction process proved very helpful to validate transmembrane prediction. Our data also were cross-correlated to chloroplast subproteome analyses by other laboratories. All data are deposited in a new curated plastid proteome database (PPDB) with multiple search functions (http://cbsusrv01.tc.cornell.edu/users/ppdb/). This PPDB will serve as an expandable resource for the plant community.

Control of chloroplast redox by the IMMUTANS terminal oxidase

Variegation mutants offer excellent opportunities to study interactions between the nucleus-cytoplasm, the chloroplast, and the mitochondrion. Variegation in the immutans (im) mutant of Arabidopsis is induced by a nuclear recessive gene and the extent of variegation can be modulated by light and temperature. Whereas the green sectors have morphologically normal chloroplasts, the white sectors are devoid of pigments and accumulate a colourless carotenoid, phytoene. The green sectors are hypothesized to arise from cells that have avoided irreversible photooxidative damage whereas the white sectors originate from cells that are photooxidized. Cloning of the IMMUTANS (IM) gene has revealed that IMMUTANS (IM) is a plastid homologue of the mitochondrial alternative oxidase. This finding suggested a model in which IM functions as a redox component of the phytoene desaturation pathway, which requires phytoene desaturase activity. Consistent with this idea, IM has quinol oxidase activity in vitro. Recent studies have revealed that IM plays a more global role in plastid metabolism. For example, it appears to be the elusive terminal oxidase of chlororespiration and also functions as a light stress protein.

Metabolic Flexibility of the Green Alga Chlamydomonas reinhardtii as Revealed by the Link between State Transitions and Cyclic Electron Flow

In this Review we focus on the conversion of linear photosynthetic electron transport from water to NADP to the cyclic pathway around Photosystem I in the green alga Chlamydomonas reinhardtii. We discuss the strict relationship that exists between the changes in pathways of electron transport and state transitions, i.e., the reversible functional association of light harvesting proteins with one of the two photosystems of oxygenic photosynthesis. Such a link has not been reported in the case of other photosynthetic organisms, where the state transitions do not affect the pathway of electron transport. Rather, they provide a tool to optimise the rate of linear flow. We propose a kinetic-structural model that explains the mechanism of this particular relationship in Chlamydomonas, and discuss the advantages that this peculiar situation gives to the energetic metabolism of this alga.

Photosynthesis and state transitions in mitochondrial mutants of Chlamydomonas reinhardtii affected in respiration

Photosynthetic activities were analyzed in Chlamydomonas reinhardtii mitochondrial mutants affected in different complexes (I, III, IV, I + III, and I + IV) of the respiratory chain. Oxygen evolution curves showed a positive relationship between the apparent yield of photosynthetic linear electron transport and the number of active proton-pumping sites in mitochondria. Although no significant alterations of the quantitative relationships between major photosynthetic complexes were found in the mutants, 77 K fluorescence spectra showed a preferential excitation of photosystem I (PSI) compared with wild type, which was indicative of a shift toward state 2. This effect was correlated with high levels of phosphorylation of light-harvesting complex II polypeptides, indicating the preferential association of light-harvesting complex II with PSI. The transition to state 1 occurred in untreated wild-type cells exposed to PSI light or in 3-(3,4-dichlorophenyl)-1,1-dimethylureatreated cells exposed to white light. In mutants of the cytochrome pathway and in double mutants, this transition was only observed in white light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. This suggests higher rates of nonphotochemical plastoquinone reduction through the chlororespiratory pathway, which was confirmed by measurements of the complementary area above the fluorescence induction curve in dark-adapted cells. Photo-acoustic measurements of energy storage by PSI showed a stimulation of PSI-driven cyclic electron flow in the most affected mutants. The present results demonstrate that in C. reinhardtii mutants, permanent defects in the mitochondrial electron transport chain stabilize state 2, which favors cyclic over linear electron transport in the chloroplast.

Plastoquinones are effectively reduced by ferredoxin:NADP+ oxidoreductase in the presence of sodium cholate micelles. Significance for cyclic electron transport and chlororespiration

The effect of sodium cholate and other detergents (Triton X-100, sodium dodecyl sulphate, octyl glucoside, myristyltrimethylammonium bromide) on the reduction of plastoquinones (PQ) with a different length of the side-chain by spinach ferredoxin:NADP(+) oxidoreductase (FNR) in the presence of NADPH has been studied. Both NADPH oxidation and oxygen uptake due to plastosemiquinone autoxidation were highly stimulated only in the presence of sodium cholate among the used detergents. Sodium cholate at the concentration of 20 mM was found to be the most effective on both PQ-4 and PQ-9-mediated oxygen uptake. The FNR-dependent reduction of plastoquinones incorporated into sodium cholate micelles was stimulated by spinach ferredoxin but inhibited by Mg(2+) ions. It was concluded that the structure of sodium cholate micelles facilitates contact of plastoquinone molecules with the enzyme and creates favourable conditions for the reaction similar to those found in thylakoid membranes for PQ-9 reduction. The obtained results were discussed in terms of the function of FNR as a ferredoxin:plastoquinone reductase both in cyclic electron transport and chlororespiration.

A nucleus-encoded factor, CRR2, is essential for the expression of chloroplast ndhB in Arabidopsis

The chloroplast NDH complex, NAD(P)H dehydrogenase, reduces the plastoquinone pool non-photochemically and is involved in cyclic electron flow around photosystem I (PSI). A transient increase in chlorophyll fluorescence after turning off actinic light is a result of NDH activity. We focused on this subtle change in chlorophyll fluorescence to isolate nuclear mutants affected in chloroplast NDH activity in Arabidopsis by using chlorophyll fluorescence imaging. crr2-1 and crr2-2 (chlororespiratory reduction) are recessive mutant alleles in which accumulation of the NDH complex is impaired. Except for the defect in NDH activity, photosynthetic electron transport was unaffected. CRR2 encodes a member of the plant combinatorial and modular protein (PCMP) family consisting of more than 200 genes in Arabidopsis. CRR2 functions in the intergenic processing of chloroplast RNA between rps7 and ndhB, which is possibly essential for ndhB translation. We have determined the function of a PCMP family member, indicating that the family is closely related to pentatrico-peptide PPR proteins involved in the maturation steps of organellar RNA.

Arabidopsis genes encoding mitochondrial type II NAD(P)H dehydrogenases have different evolutionary origin and show distinct responses to light

In addition to proton-pumping complex I, plant mitochondria contain several type II NAD(P)H dehydrogenases in the electron transport chain. The extra enzymes allow the nonenergy-conserving electron transfer from cytoplasmic and matrix NAD(P)H to ubiquinone. We have investigated the type II NAD(P)H dehydrogenase gene families in Arabidopsis. This model plant contains two and four genes closely related to potato (Solanum tuberosum) genes nda1 and ndb1, respectively. A novel homolog, termed ndc1, with a lower but significant similarity to potato nda1 and ndb1, is also present. All genes are expressed in several organs of the plant. Among the nda genes, expression of nda1, but not nda2, is dependent on light and circadian regulation, suggesting separate roles in photosynthesis-associated and other respiratory NADH oxidation. Genes from all three gene families encode proteins exclusively targeted to mitochondria, as revealed by expression of green fluorescent fusion proteins and by western blotting of fractionated cells. Phylogenetic analysis indicates that ndc1 affiliates with cyanobacterial type II NADH dehydrogenase genes, suggesting that this gene entered the eukaryotic cell via the chloroplast progenitor. The ndc1 should then have been transferred to the nucleus and acquired a signal for mitochondrial targeting of the protein product. Although they are of different origin, the nda, ndb, and ndc genes carry an identical intron position.

Photosynthesis of overwintering evergreen plants

In this review we focus on photosynthetic behavior of overwintering evergreens with an emphasis on both the acclimative responses of photosynthesis to cold and the winter behavior of photosynthesis in conifers. Photosynthetic acclimation is discussed in terms of the requirement for a balance between the energy absorbed through largely temperature-insensitive photochemical processes and the energy used for temperature-sensitive biochemical processes and growth. Cold acclimation transforms the xanthophyll-mediated nonphotochemical antenna quenching of absorbed light from a short-term dynamic response to a long-term sustained quenching for the whole winter period. This acclimative response helps protect the evergreen foliage from photooxidative damage during the winter when photosynthesis is restricted or prevented by low temperatures. Although the molecular mechanisms behind the sustained winter excitation quenching are largely unknown, it does involve major alterations in the organization and composition of the photosystem II antenna. In addition, photosystem I may play an important role in overwintering evergreens not only by quenching absorbed light photochemically via its support of cyclic electron transport at low temperatures, but also by nonphotochemical quenching of absorbed light irrespective of temperature. The possible role of photosystem II reaction centers in nonphotochemical quenching of absorbed energy in overwintering evergreens is also discussed. Processes like chlororespiration and cyclic electron transport may also be important for maintaining the functional integrity of the photosynthetic apparatus of overwintering evergreens both during periods of thawing in winter and during recovery from winter stress in spring. We suggest that the photosynthetic acclimation responses of overwintering evergreens represent specific evolutionary adaptations for plant species that invest in the long-term maintenance of leaf structure in cold climatic zones as exemplified by the boreal forests of the Northern Hemisphere.

Membrane-bound diiron carboxylate proteins

Four proteins have been identified recently as diiron carboxylate proteins on the basis of conservation of six amino acids (four carboxylate residues and two histidines) constituting an iron-binding motif. Unlike previously identified proteins with this motif, biochemical studies indicate that each of these proteins is membrane bound, although homology modeling rules out a transmembrane mode of binding. Therefore, the predicted structure of each protein [the alternative oxidase (AOX), the plastid terminal oxidase (PTOX), the diiron 5-demethoxyquinone hydroxylase (DMQ hydroxylase), and the aerobic Mg-protoporphyrin IX monomethylester hydroxylase (MME hydroxylase)] is that of a protein bound monotopically to one leaflet of the membrane bilayer. Three of these enzymes utilize a quinol substrate, with two oxidizing the quinol (AOX and PTOX) and one hydroxylating it (DMQ hydroxylase). MME hydroxylase is involved in synthesis of the isocyclic ring of chlorophyll. Two enzymes are involved in respiration (AOX and, indirectly, the diiron DMQ hydroxylase through ubiquinone biosynthesis) and two in photosynthesis, through their roles in carotenoid and chlorophyll biosynthesis (PTOX and MME hydroxylase, respectively). We discuss what is known about each enzyme as well as our expectations based on their identification as interfacially bound proteins with a diiron carboxylate active site.

Involvement of a plastid terminal oxidase in plastoquinone oxidation as evidenced by expression of the Arabidopsis thaliana enzyme in tobacco

Chlororespiration has been defined as a respiratory electron transport chain in interaction with photosynthetic electron transport involving both non-photochemical reduction and oxidation of plastoquinones. Different enzymatic activities, including a plastid-encoded NADH dehydrogenase complex, have been reported to be involved in the non-photochemical reduction of plastoquinones. However, the enzyme responsible for plasquinol oxidation has not yet been clearly identified. In order to determine whether the newly discovered plastid oxidase (PTOX) involved in carotenoid biosynthesis acts as a plastoquinol oxidase in higher plant chloroplasts, the Arabidopsis thaliana PTOX gene (At-PTOX) was expressed in tobacco under the control of a strong constitutive promoter. We showed that At-PTOX is functional in tobacco chloroplasts and strongly accelerates the non-photochemical reoxidation of plastoquinols; this effect was inhibited by propyl gallate, a known inhibitor of PTOX. During the dark to light induction phase of photosynthesis at low irradiances, At-PTOX drives significant electron flow to O(2), thus avoiding over-reduction of plastoquinones, when photo- synthetic CO(2) assimilation was not fully induced. We proposed that PTOX, by modulating the redox state of intersystem electron carriers, may participate in the regulation of cyclic electron flow around photosystem I.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

In vivo interactions between photosynthesis, mitorespiration, and chlororespiration in Chlamydomonas reinhardtii

Interactions between photosynthesis, mitochondrial respiration (mitorespiration), and chlororespiration have been investigated in the green alga Chlamydomonas reinhardtii using flash illumination and a bare platinum electrode. Depending on the physiological status of algae, flash illumination was found to induce either a fast (t(1/2) approximately 300 ms) or slow (t(1/2) approximately 3 s) transient inhibition of oxygen uptake. Based on the effects of the mitorespiratory inhibitors myxothiazol and salicyl hydroxamic acid (SHAM), and of propyl gallate, an inhibitor of the chlororespiratory oxidase, we conclude that the fast transient is due to the flash-induced inhibition of chlororespiration and that the slow transient is due to the flash-induced inhibition of mitorespiration. By measuring blue-green fluorescence changes, related to the redox status of the pyridine nucleotide pool, and chlorophyll fluorescence, related to the redox status of plastoquinones (PQs) in C. reinhardtii wild type and in a photosystem I-deficient mutant, we show that interactions between photosynthesis and chlororespiration are favored when PQ and pyridine nucleotide pools are reduced, whereas interactions between photosynthesis and mitorespiration are favored at more oxidized states. We conclude that the plastid oxidase, similar to the mitochondrial alternative oxidase, becomes significantly engaged when the PQ pool becomes highly reduced, and thereby prevents its over-reduction.