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Chlorophyll fluorescence has been widely used for the estimation of photosynthesis or its regulatory mechanisms. Chlorophyll fluorescence measurements are the methods with non-destructive nature and do not require contact between plant materials and fluorometers. Furthermore, the measuring process is very rapid. These characteristics of chlorophyll fluorescence measurements make them a suitable tool to screen mutants of photosynthesis-related genes. Furthermore, it has been shown that genes with a wide range of functions can be also analyzed by chlorophyll fluorescence through metabolic interactions. In this short review, we would like to first introduce the basic principle of the chlorophyll fluorescence measurements, and then explore the advantages and limitation of various screening methods. The emphasis is on the possibility of chlorophyll fluorescence measurements to screen mutants defective in metabolisms other than photosynthesis.

In cyanobacteria, the PII protein (the glnB gene product) regulates a number of proteins involved in nitrogen assimilation including PipX, the coactivator of the global nitrogen regulator protein NtcA. In Synechococcus elongatus PCC 7942, construction of a PII-less mutant retaining the wild-type pipX gene is difficult because of the toxicity of uncontrolled action of PipX and the other defect(s) resulting from the loss of PIIper se, but the nature of the PipX toxicity and the PipX-independent defect(s) remains unclear. Characterization of a PipX-less glnB mutant (PD4) in this study showed that the loss of PII increases the sensitivity of PSII to ammonium. Ammonium was shown to stimulate the formation of reactive oxygen species in the mutant cells. The ammonium-sensitive growth phenotype of PD4 was rescued by the addition of an antioxidant α-tocopherol, confirming that photo-oxidative damage was the major cause of the growth defect. A targeted PII mutant retaining wild-type pipX was successfully constructed from the wild-type S. elongatus strain (SPc) in the presence of α-tocopherol. The resulting mutant (PD1X) showed an unusual chlorophyll fluorescence profile, indicating extremely slow reduction and re-oxidation of QA, which was not observed in mutants defective in both glnB and pipX. These results showed that the aberrant action of uncontrolled PipX resulted in an impairment of the electron transport reactions in both the reducing and oxidizing sides of QA.

The response regulator RpaB plays a central role in transcriptional regulation of photosynthesis-related genes in cyanobacteria. RpaB is phosphorylated by its cognate histidine kinase Hik33 and functions as both an activator and a repressor under low-light conditions, whereas its phosphorylation level and DNA-binding activity promptly decrease upon the upshift of photon flux density, causing changes in the gene expression profile. In this study, we assessed the possibility of redox regulation of the DNA-binding activity of RpaB in Synechocystis sp. PCC 6803 by the addition of inhibitors of photosynthetic electron transport, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, or the reducing agent dithiothreitol under different photon flux densities. Analysis of the phosphorylation level of RpaB revealed that reduction of QA and increase in the availability of reducing equivalents at the acceptor side of photosystem I (PSI) can independently trigger dephosphorylation. The redox-state-dependent regulation by an unidentified thiol other than Cys59 of RpaB is prerequisite for the phosphorylation-dependent regulation of the DNA-binding activity. Environmental signals, recognized by Hik33, and metabolic signals recognized as the availability of reducing equivalents, must be integrated at the master regulator RpaB, in order to attain the flexible regulation of acclimatory responses.

We discovered that pigments including carotenoids and (bacterio)chlorophylls in pigment-protein complexes, membrane fragments, and chlorosomes suspended in water could be injected directly into C18 HPLC and analyzed without any other treatments. We applied this method to LH1-RC and chromatophores of purple bacteria, chlorosomes of green sulfur bacteria, thylakoid membranes of cyanobacteria, and PSII and thylakoid membranes of spinach. HPLC elution profiles and pigment composition were the same as those of the conventional extraction method. The principle of this method might be that samples are first trapped on top of column, followed by the immediate extraction of the pigments with the HPLC eluent and their separation using the C18 column, as usual. In the conventional extraction method, pigments are first extracted with organic solvents, followed by evaporation of the solvents. The dried pigments are then dissolved in organic solvents and injected into C18 HPLC after filtration. The advantages of this method include the preventions of pigment isomerization and oxidation and the possibility of injecting all samples. Its drawbacks include the accumulation of denatured proteins at the top of column, causing increased HPLC pressure. The use of a guard column might solve this problem. Many factors, such as samples, column, and HPLC systems, may affect this method. Nevertheless, we think that some samples can be analyzed using this method.

Mesembryanthemum crystallinum, which switches the mode of photosynthesis from C3 to crassulacean acid metabolism (CAM) upon high salt stress, was shown here to exhibit diurnal changes in not only the CO2 fixation pathway but also Chl fluorescence parameters under CAM-induced conditions. We conducted comprehensive time course measurements of M. crystallinum leaf Chl fluorescence using the same leaf throughout the CAM induction period. By doing so, we were able to distinguish the effect of CAM induction from that of photoinhibition and avoid the possible effects of differences in foliar age. We found that the diurnal change in the status of electron transfer could be ascribed to the formation of a proton gradient across thylakoid membranes presumably resulting from diurnal changes in the ATP/ADP ratio reported earlier. The electron transport by actinic illumination thus became limited at the step of plastoquinol oxidation by the Cyt b6/f complex in the ‘night’ period upon CAM induction, resulting in high levels of non-photochemical quenching. The actinically induced non-photochemical quenching in the ‘night’ period correlated well with the degree of CAM induction. Chl fluorescence parameters, such as NPQ or qN, could be used as a simple indexing system for the CAM induction.

The pmgA-disrupted (ΔpmgA) mutant in the cyanobacterium Synechocystis sp. PCC 6803 suffers severe growth inhibition under photomixotrophic conditions. In order to elucidate the key factors enabling the cells to grow under photomixotrophic conditions, we isolated spontaneous suppressor mutants from the ΔpmgA mutant derived from a single colony. When the ΔpmgA mutant was spread on a BG11 agar plate supplemented with glucose, colonies of suppressor mutants appeared after the bleaching of the background cells. We identified the mutation site of these suppressor mutants and found that 11 mutants out of 13 had a mutation in genes related to the type 1 NAD(P)H dehydrogenase (NDH-1) complex. Among them, eight mutants had mutations within the ndhF3 (sll1732) gene: R32stop, W62stop, V147I, G266V, G354W, G586C, and deletion of 7 bp within the coding region. One mutant had one base insertion in the putative -10 box of the ndhC (slr1279) gene, leading to the decrease in the transcripts of the ndhCKJ operon. Two mutants had one base insertion and deletion in the coding region of cupA (sll1734), which is co-transcribed with ndhF3 and ndhD3 and comprises together a form of NDH-1 complex (NDH-1MS complex) involved in inducible high-affinity CO2 uptake. The results indicate that the loss of the activity of this complex effectively rescues the ΔpmgA mutant under photomixotrophic condition with 1 % CO2. However, little difference among WT and mutants was observed in the activities ascribed to the NDH-1MS complex, i.e., CO2 uptake and cyclic electron transport. This may suggest that the NDH-1MS complex has the third, currently unknown function under photomixotrophic conditions.

The photoinhibition of Photosystem I (PSI) drew less attention compared with that of Photosystem II (PSII). This could be ascribed to several reasons, e.g. limited combinations of plant species and environmental conditions that cause PSI photoinhibition, the non-regulatory aspect of PSI photoinhibition, and methodological difficulty to determine the accurate activity of PSI under stress conditions. However, the photoinhibition of PSI could be more dangerous than that of PSII because of the very slow recovery rate of PSI. This article is intended to introduce such characteristics of PSI photoinhibition with special emphasis on the relationship between two photosystems as well as the protective mechanism of PSI in vivo. Although the photoinhibition of PSI could be induced only in specific conditions and specific plant species in intact leaves, PSI itself is quite susceptible to photoinhibition in isolated thylakoid membranes. PSI seems to be well protected from photoinhibition in vivo in many plant species and many environmental conditions. This is quite understandable because photoinhibition of PSI is not only irreversible but also the potential cause of many secondary damages. This point would be different from the case of PSII photoinhibition, which could be regarded as one of the regulatory mechanisms under stressed as well as non-stressed conditions.

We developed here the quantitative and objective method to analyze chlorophyll fluorescence from the cyanobacterium Synechocystis sp. PCC 6803 in the aim of systematic examination of gene function. The overall similarity of the chlorophyll fluorescence induction kinetics was evaluated for 499 mutants. Mutants of 333 genes showed the difference in the fluorescence kinetics from that of wild type, indicating the wide interaction of photosynthesis with other metabolisms. Hierarchical clustering of the similarity of the mutants enables us to group together the mutants having defect in the regulation of photosystem stoichiometry as well as those having defects in respiration or other functions. Furthermore, wild-type cells treated with inhibitors of respiration and mutants of genes involved in respiration shared similar induction kinetics. Apparently, quantitative comparison of the induction kinetics could be useful to analyze the function of genes as well as to predict the target sites of various chemicals.

The water-water cycle is the electron flow through scavenging enzymes for the reactive species of oxygen in chloroplasts, and is proposed to play a role in alternative electron sink in photosynthesis. Here we showed that the water-water cycle is impaired in the T-DNA insertion mutant of AtHMA1 gene encoding a Cu transporting ATPase in chloroplasts. Chlorophyll fluorescence under steady state was not affected in hma1, indicating that photosynthetic electron transport under normal condition was not impaired. Under electron acceptor limited conditions, however, hma1 showed distinguished phenotype in chlorophyll fluorescence characteristics. The most severe phenotype of hma1 could be observed in high (0.1%) CO(2) concentrations, indicating that hma1 has the defect other than photorespiration. The transient increase of chlorophyll fluorescence upon the cessation of the actinic light as well as the NPQ induction of chlorophyll fluorescence revealed that the two pathways of cyclic electron flow around PSI, NDH-pathway and FQR-pathway, are both intact in hma1. Based on the NPQ induction under 0% oxygen condition, we conclude that the water-water cycle is impaired in hma1, presumably due to the decreased level of Cu/Zn SOD in the mutant. Under high CO(2) condition, hma1 exhibited slightly higher NPQ induction than wild type plants, while this increase of NPQ in hma1 was suppressed when hma1 was crossed with crr2 having a defect in NDH-mediated PSI cyclic electron flow. We propose that the water-water cycle and NDH-mediated pathways might be regulated compensationally with each other especially when photorespiration is suppressed.

Downregulation of photosystem I (PSI) content is an essential process for cyanobacteria to grow under high-light (HL) conditions. In a pmgA (sll 968) mutant of Synechocystis sp. PCC 6803, the levels of PSI content, chlorophyll and transcripts of the psaAB genes encoding reaction-centre subunits of PSI could not be maintained low during HL incubation, although the causal relationship among these phenotypes remains unknown. In this study, we modulated the activity of psaAB transcription or that of chlorophyll synthesis to estimate their contribution to the regulation of PSI content under HL conditions. Analysis of the psaAB-OX strain, in which the psaAB genes were overexpressed under HL conditions, revealed that the amount of psaAB transcript could not affect PSI content by itself. Suppression of chlorophyll synthesis by an inhibitor, laevulinic acid, in the pmgA mutant revealed that chlorophyll availability could be a determinant of PSI content under HL. It was also suggested that chlorophyll content under HL conditions is mainly regulated at the level of 5-aminolaevulinic acid synthesis. We conclude that, upon the shift to HL conditions, activities of psaAB transcription and of 5-aminolaevulinic acid synthesis are strictly downregulated by regulatory mechanism(s) independent of PmgA during the first 6 h, and then a PmgA-mediated regulatory mechanism becomes active after 6 h onward of HL incubation to maintain these activities at a low level., This is a pre-copy-editing, author-produced PDF of an article accepted for publication in MICROBIOLOGY-SGM following peer review. The definitive publisher-authenticated version American Society for Microbiology, MICROBIOLOGY-SGM 155 (2009), 989-996; DOI 10.1099/mic.0.024018-0 online at: http://mic.sgmjournals.org/cgi/content/abstract/155/3/989

SummarySigma factors encoded by the nucleus of plants confer promoter specificity on the bacterial-type RNApolymerase in chloroplasts. We previously showed that transcripts of OsSIG1, which encodes one such sigmafactor in rice, accumulaterelatively late during leaf development. We havenow isolated and characterizedtwoallelic mutants of OsSIG1, in which OsSIG1 is disrupted by insertion of the retrotransposon Tos17, in order tocharacterize the functions of OsSIG1. The OsSIG1 )/ plants were found to be fertile but they manifested anapproximately one-third reduction in the chlorophyll content of mature leaves. Quantitative RT-PCR andnorthern blot analyses of chloroplast gene expression revealed that the abundance of transcripts derived fromthe psaA operon was markedly reduced in OsSIG1 )/) plants compared with that in wild-type homozygotes.This effect was accompanied by a reduction in the abundance of the core protein complex (PsaA–PsaB) ofphotosystemI.Analysisofchlorophyllfluorescencealsorevealedasubstantialreductionintherateofelectrontransfer from photosystem II to photosystem I in the OsSIG1 mutants. Our results thus indicate that OsSIG1plays an important role in the maintenance of photosynthetic activity in mature chloroplasts of rice byregulating expression of chloroplast genes for components of photosystem I.Keywords: sigma factor, photosystem I, chloroplast, RNA polymerase, transcription, rice.IntroductionGene expression in plastids is mediated by at least two dif-ferent transcriptional systems based on a plastid-encodedRNA polymerase (PEP) and a nucleus-encoded RNApolymerase (NEP; Shiina et al., 2005; Hajdukiewicz et al.,1997; Hedtke et al., 1997). Plastid-encoded RNA polymeraseis a multisubunit eubacterial-type RNA polymerase, with thecore subunits being encoded by the plastid genes rpoA,rpoB, rpoC1 and rpoC2. Nucleus-encoded RNA polymeraseis a single-subunit bacteriophage-type enzyme that isencoded by a nuclear gene and is similar to a mitochondrialRNA polymerase. Analysis of PEP-deficient plants hassuggested that PEP functions predominantly in the expres-sion of photosynthetic genes and that NEP transcribesnon-photosynthetic genes in plastids (Liere and Maliga,1999). Plastid-encoded RNA polymerase requires sigmafactors, which are encoded by the nuclear genome, forpromoter recognition and initiation of transcription atspecific genes. To date, six sigma factors (SIG1, SIG2,SIG3, SIG4, SIG5 and SIG6) have been identified andcharacterized in the model dicotyledonous plant Arabid-opsis thaliana (Fujiwara et al., 2000; Isono et al., 1997;Tanaka et al., 1997). For the model monocotyledonousplant rice (Oryza sativa), six sigma factor genes, OsSIG1(Os-SigA), OsSIG2A, OsSIG2B, OsSIG3, OsSIG5 and Os-SIG6, have also been isolated (Kasai et al., 2004; Kubotaet al., 2007; Tozawa et al., 1998) or predicted from the

In order to avoid the photodamage, cyanobacteria regulate the distribution of light energy absorbed by phycobilisome antenna either to photosystem II (PSII) or to photosystem I (PSI) upon high-light acclimation by the process so called state transition. We found that an alternative PSI subunit, PsaK2 (sll0629 gene product), is involved in this process in the cyanobacterium Synechocystis sp. PCC 6803. Examination of the subunit composition of the purified PSI reaction center complexes revealed that PsaK2 subunit was absent in the PSI complexes under low-light condition, but was incorporated into the complexes during acclimation to high light. The growth of the psaK2 mutant on solid medium was inhibited under high-light condition. We determined the photosynthetic characteristics of the wild type strain and the two mutants, the psaK1 (ssr0390) mutant and the psaK2 mutant, using pulse amplitude modulation (PAM) fluorometer. Non-photochemical quenching (qN), which reflects the energy transfer from phycobilisome to PSI in cyanobacteria, was higher in high-light grown cells than in low-light grown cells, both in the wild type and the psaK1 mutant. However, this change of qN during acclimation to high light was not observed in the psaK2 mutant. Thus, PsaK2 subunit is involved in the energy transfer from phycobilisome to PSI under high-light condition. The role of PsaK2 in state transition under high-light condition was also confirmed by chlorophyll fluorescence emission spectra determined at 77 K. The results suggest that PsaK2-dependent state transition is essential for the growth of this cyanobacterium under high-light condition., Copyright notice. c2002 Journal of Biological Chemistry. All rights reserved. Source published by American Physical Society.

Oxygen evolution is inhibited when leaves of chilling-sensitive plants like cucumber are treated at 0 degrees C in the dark. The activity is restored by moderate illumination at room temperature. We examined the changes in the redox state of the Mn-cluster in cucumber leaves in the processes of dark-chilling inhibition and subsequent light-induced reactivation by means of thermoluminescence (TL). A TL B-band arising from S(2)Q(B)(-) charge recombination in PSII was observed upon single-flash illumination of untreated leaves, whereas four flashes were required to yield the B-band after dark-chilling treatment for 24 h. This three-step delay indicates that over-reduced states of the Mn-cluster such as the S(-2) state were formed during the treatment. Fitting analysis of the flash-number dependence of the TL intensities showed that the Mn-cluster was more reduced with a longer period of the treatment and that S(-3) was the lowest S-state detectable in the dark-chilled leaves. Measurements of the Mn content by atomic absorption spectroscopy showed that Mn atoms were gradually released from PSII during the dark-chilling treatment but re-bound to PSII by illumination at 30 degrees C. Thus, dark-chilling inhibition of oxygen evolution can be ascribed to the disintegration of the Mn-cluster due to its over-reduction. The observation of the S(-3) state in the present in vivo system strongly suggests that S(-3), which has been observed only by addition of exogenous reductants into in vitro preparations, is indeed a redox intermediate of the Mn-cluster in the processes of its disintegration and photoactivation.

To examine the role of sulfoquinovosyl diacylglycerol (SQDG) in thylakoid membranes, we compared the structural and functional properties of photosystem II (PSII) between a mutant of Chlamydomonas reinhardtii defective in SQDG ( hf-2) and the wild type. The PSII core complex of hf-2, as compared with that of the wild type, showed structural fragility when solubilized with a detergent, dodecyl beta- d-maltoside, suggesting that the physical properties of the PSII complex were altered by the loss of SQDG. On the other hand, exposure of the cells to 41 degrees C for 120 min in the dark decreased the PSII activity to 70% and 50% of the initial levels in the wild type and hf-2, respectively, which implies that the PSII activity, in the absence of SQDG, becomes less stable under heat-stress conditions. PSII inactivated to 60% of the initial level by dark incubation at 41 degrees C was reactivated by following illumination even at 41 degrees C to more than 90% in the wild type, but only to 70% in hf-2. These results suggest that PSII inactivated by heat recovers through some mechanism dependent on light, and that SQDG participates in functioning of the mechanism. The conformational disorder of PSII caused by the defect in SQDG might be correlated with the increased susceptibility of its activity to heat-stress.

Oxygen-evolving photosystem II (PSII) particles were purified from Chlamydomonas reinhardtii having His-tag extension at the C terminus of the CP47 protein, by a single-step Ni(2+)-affinity column chromatography after solubilization of thylakoid membranes with sucrose monolaurate. The PSII particles consisted of, in addition to intrinsic proteins, three extrinsic proteins of 33, 23 and 17 kDa. The preparation showed a high oxygen-evolving activity of 2,300-2,500 micro mol O(2) (mg Chl)(-1) h(-1) in the presence of Ca(2+) using ferricyanide as the electron acceptor, while its activity was 680-720 micro mol O(2) (mg Chl)(-1) h(-1) in the absence of Ca(2+) and Cl(-) ions. The activity was 710-820 micro mol O(2) (mg Chl)(-1) h(-1) independent of the presence or absence of Ca(2+) and Cl(-) when 2,6-dichloro-p-benzoquinone was used as the acceptor. These activities were scarcely inhibited by DCMU. The kinetics of flash-induced fluorescence decay revealed that the electron transfer from Q(A)(-) to Q(B) was significantly inhibited, and the electron transfer from Q(A)(-) to ferricyanide was largely stimulated in the presence of Ca(2+). These results indicate that the acceptor side, Q(B) site, was altered in the PSII particles but its donor side remained intact. Release-reconstitution experiments revealed that the extrinsic 23 and 17 kDa proteins were released only partially by NaCl-wash, while most of the three extrinsic proteins were removed when treated with urea/NaCl, alkaline Tris or CaCl(2). The 23 and 17 kDa proteins directly bound to PSII independent of the other extrinsic proteins, and the 33 kDa protein functionally re-bound to CaCl(2)-treated PSII which had been reconstituted with the 23 and 17 kDa proteins. These binding properties were largely different from those of the extrinsic proteins in higher plant PSII, and suggest that each of the three extrinsic proteins has their own binding sites independent of the others in the green algal PSII.

The recovery process after chilling-induced photoinhibition of photosystem I (PSI) was studied in leaves of a chilling-sensitive plant, cucumber (Cucumis sativus L. cv. Nanshin). Determination of chlorophyll content, photosystem (PS) activities in vivo and in vitro, and the amount of reaction-center subunits of PSI revealed that: (i) The content of chlorophyll decreased to 70% of the original level gradually from 1 to 3 days after exposure to a low temperature. (ii) The amount of functional PSI per unit leaf area was reduced to 30% of the initial level by the chilling treatment. The amount of functional PSI gradually increased during the next 6 days but only to 50% of the original level. (iii) When expressed on a chlorophyll basis, however, the amount of functional PSI recovered to 90% of the original level 6 days after the treatment. (iv) The residual amount of chlorophyll on the third day after the treatment closely correlated with the amount of functional PSI at that point. These results indicate that the decrease in chlorophyll content at a normal growth temperature after chilling treatment is a consequence of the degradation of irreversibly damaged PSI complexes. Immunoblot analysis confirmed that PsaAB protein, the reaction-center subunits of PSI, was degraded during the 3 days after chilling treatment. Some characteristics of the chilling injury frequently reported, i.e. irreversibility of the injury and development of visible symptoms at room temperature, can be explained as a consequence of the chilling-induced photoinhibition of PSI.

We characterized the photosynthetic properties of the pmgA mutant of Synechocystis PCC 6803, which cannot change its photosystem stoichiometry under a high-light condition (200 micromol x m(-2) x s(-1)), in order to clarify the physiological significance of the regulation of photosystem stoichiometry. We found that (1) PSII activity was inhibited more in wild-type cells on the first day under the high-light conditions than in mutant cells. (2) The growth of the mutants following the initial imposition of high light was faster than that of wild-type cells. (3) However, growth was severely inhibited in the mutants after the third day of exposure to high light. (4) The growth inhibition in the mutants under the extended high-light conditions was reversed by the addition of sublethal concentrations of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which seemed to mimic photoinhibition of PSII. These results suggest that the main role of adjusting the photosystem stoichiometry with respect to light intensity is not to maintain efficient photosynthesis, but to down regulate electron transfer. Failure to down regulate electron flow leads to cell death under prolonged exposure to high light in this cyanobacterium.

Continuous wetness of leaves in the light causes a reduction in the carbon exchange rate (CER) in Phaseolus vulgaris L. [Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431]. In this study, we investigated the initial cause of photoinhibition upon application of water, designated rain treatment, and we found a large decrease in the rate of electron transport through the whole chain from water to methyl viologen via PSII and PSI. In spite of the decrease in the rate of electron transport, there was no decrease in the activity of either PSI or PSII when these activities were measured separately. The intactness of PSI was also confirmed by the absence of any change in the photooxidizable amount of P-700, the reaction centre of PSI, and the intactness of PSII was confirmed by measurements of Chi fluorescence. The results suggest that the inhibition by the rain treatment, which occurs at the site between PSI and PSII, might be a novel type of photoinhibition, unlike the conventional types of photoinhibition that involve PSI and PSII.

We examined photosynthetic properties of Eupatorium makinoi leaves infected by a geminivirus. Since a major symptom of the geminivirus infection is variegation or yellowing of leaves, Chl content was used as an index of disease severity. As leaf Chl content was lowered, leaf absorptance, maximal quantum yield of photosynthesis on an absorbed quantum basis (φo2,max) and light-saturated rate of photosynthesis (Pmax) decreased. The share of energy allocated to PS II, which can be estimated from fluorescence parameters and oxygen evolution rate, was about 30% lower in the infected yellow leaves than in uninfected leaves. Analyses of the composition of thylakoid polypeptides by gel electrophoresis showed preferential loss of LHC II. The lower φo2,maxin the infected leaves was, thus, attributed to the decreased energy allocation to PS II. These features were largely consistent with those of b-less mutants, but lowered Pmaxhas been never reported for b-less mutants. Possible mechanisms causing these changes in photosynthetic properties to the infected leaves are discussed.

Photoinhibition was denned originally as the decrease in photosynthetic activity that occurs upon excess illumination. The site of photoinhibiti on has generally been considered to be located in PSII. However, a novel type of photoinhibition has recently been characterized in chillingsensitive plants. This photoinhibition occurs under relatively weak illumination at chilling temperatures and the main site of damage is in PSI. The photoinhibition of PSI is initiated by the inactivation of the acceptor side, with the subsequent destruction of the reaction center and the degradation of the product of the psaB gene, which is one of the two major subunit polypeptides of the PSI reaction center complex. Chilling and oxidative stress (the presence of reactive species of oxygen) are characteristic requirements for the photoinhibition of PSI in vivo.

To clarify the way in which the light available for growth affects respiration in leaves of sun and shade plants, we examined the respiratory properties of mature leaves of Spinacia oleracea L., a sun species, and of Alocasia macrorrhiza (L.) G. Don., a shade species, that had been grown at various irradiances. In leaves of S. oleracea, the respiratory rates, on a dry mass basis, decreased with time during the night, and the higher was the growth irradiance during the day, the higher was the respiratory rate. The marked decreases in the respiratory rate during the night were accompanied by decreases in the concentration of carbohydrates in the leaves. By contrast, the respiratory rates of leaves of A. macrorrhiza were virtually constant throughout the night and the absolute rates were lower than those of S. oleracea even though the absolute value of the concentration of carbohydrates and its decrease at night resembled to those in S. oleracea. The maximum activities of respiratory enzymes were also similar to those in S. oleracea. However, the leaves of A. macrorrhiza contained less soluble protein than those of S. oleracea. These results suggest that, in S. oleracea, the concentration of carbohydrates might determine the respiratory rate while such is not the case in A. macrorrhiza. The lower respiratory rates in A. macrorrhiza might be due to a lower demand for ATP.

The psaI gene encoding the 5.2 kDa protein component (PsaI) of the photosystem I complex was cloned from the cyanobacterium Anabaena 29413. The gene is present in single copy in this cyanobacterial genome. The nucleotide sequence of a 500 bp region of the cloned DNA revealed the presence of an open reading frame encoding a 46 amino acid long polypeptide. The N-terminal 11 residues are absent in the mature polypeptide and thus represents the first identified cleavable presequence on the PsaI protein. We suggest that this presequence directs the N-terminus of the protein to the thylakoid lumen.

Sigma factors encoded by the nucleus of plants confer promoter specificity on the bacterial-type RNA polymerase in chloroplasts. We previously showed that transcripts of OsSIG1, which encodes one such sigma factor in rice, accumulate relatively late during leaf development. We have now isolated and characterized two allelic mutants of OsSIG1, in which OsSIG1 is disrupted by insertion of the retrotransposon Tos17, in order to characterize the functions of OsSIG1. The OsSIG1-/- plants were found to be fertile but they manifested an approximately one-third reduction in the chlorophyll content of mature leaves. Quantitative RT-PCR and northern blot analyses of chloroplast gene expression revealed that the abundance of transcripts derived from the psaA operon was markedly reduced in OsSIG1-/- plants compared with that in wild-type homozygotes. This effect was accompanied by a reduction in the abundance of the core protein complex (PsaA-PsaB) of photosystem I. Analysis of chlorophyll fluorescence also revealed a substantial reduction in the rate of electron transfer from photosystem II to photosystem I in the OsSIG1 mutants. Our results thus indicate that OsSIG1 plays an important role in the maintenance of photosynthetic activity in mature chloroplasts of rice by regulating expression of chloroplast genes for components of photosystem I.

The physiological role of sulfoquinovosyl diacylglycerol (SQDG) in photosynthesis was investigated with a SQDG defective mutant (hf-2) of Chlamydomonas reinhardtii that did not have any detectable amount of SQDG. The mutant showed a lower rate of photosystem II (PSII) activity by approximately 40% and also a lower growth rate than those of the wild-type. Results of genetical analysis of hf-2 strongly suggest that the SQDG defect and the lowered PSII activity are due to a single gene mutation. The supplementation of SQDG to hf-2 cells restored the lowered PSII activity to the same level as that of wild-type cells, and also enabled the mutant to grow even in the presence of 135 nm 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Moreover, the incubation of isolated thylakoid membranes of hf-2 with SQDG raised the lowered PSII activity. Chemical modifications of SQDG impaired the recovery of PSII activity. The results suggest that SQDG is indispensable for PSII activity in Chlamydomonas by maintaining PSII complexes in their proper state.

Background Most organisms, especially photoautotrophs, alter their behaviours in response to day–night alternations adaptively because of their great reliance on light. Upon light-to-dark transition, dramatic and universal decreases in transcription level of the majority of the genes in the genome of the unicellular cyanobacterium, Synechococcus elongatus PCC 7942 are observed. Because Synechococcus is an obligate photoautotroph, it has been generally assumed that repression of the transcription in the dark (dark repression) would be caused by a nocturnal decrease in photosynthetic activities through the reduced availability of energy (e.g. adenosine triphosphate (ATP)) needed for mRNA synthesis. Results However, against this general assumption, we obtained evidence that the rapid and dynamic dark repression is an active process. Although the addition of photosynthesis inhibitors to cells exposed to light mimicked transcription profiles in the dark, it did not significantly affect the cellular level of ATP. By contrast, when ATP levels were decreased by the inhibition of both photosynthesis and respiration, the transcriptional repression was attenuated through inhibition of RNA degradation. This observation indicates that Synechococcus actively downregulates genome-wide transcription in the dark. Even though the level of total mRNA dramatically decreased in the dark, Synechococcus cells were still viable, and they do not need de novo transcription for their survival in the dark for at least 48 hours. Conclusions Dark repression appears to enable cells to enter into nocturnal dormancy as a feed-forward process, which would be advantageous for their survival under periodic nocturnal conditions. Electronic supplementary material The online version of this article (doi:10.1186/s12915-015-0144-2) contains supplementary material, which is available to authorized users.