Visible light induced biohydrogen production from sucrose using the photosensitization of Mg chlorophyll-a

A photoinduced hydrogen production system, coupling sucrose degradation with invertase and glucose dehydrogenase (GDH) and hydrogen production with colloidal platinum as a catalyst using the visible light-induced photosensitization of Mg chlorophyll-a (Mg Chl-a), has been developed. Continuous hydrogen gas production was observed when the reaction mixture containing sucrose, invertase, GDH, nicotinamide adenine dinucleotide (NAD(+)), Mg Chl-a, methyl viologen (MV(2+), an electron relay reagent), and colloidal platinum was irradiated by visible light.

Ginkgo biloba retains functions of both type I and type II flowering plant phytochrome

While the photoreceptor systems of flowering plants have been well studied, the origins of these gene families and their functions are only partially understood. To begin to resolve the evolutionary origins of angiosperm photoreceptor function, we have studied the photomorphogenic responses of the early diverging gymnosperm Ginkgo biloba. Here, we describe the effects of continuous white light, red light, far-red light, and blue light on stem length, chlorophyll accumulation, Lhcb mRNA accumulation, and plastid development. Differences in the efficacy of these light regimes on de-etiolation in Ginkgo suggest separate but complementary roles for red and blue light-sensing systems. Additionally, the unique manner in which developmental regulation occurs in Ginkgo reveals a far-red high irradiance response different from both angiosperm and other gymnosperm species. We conclude from these data that Ginkgo contains a functional complement to both flowering plant type I and type II phytochromes, as well as independent blue light-sensing system(s). The implications of these findings are discussed with respect to the evolution of higher plant photoreceptors.

Unique fluorescence properties of a cyanobacterium Gloeobacter violaceus PCC 7421: reasons for absence of the long-wavelength PSI Chl a fluorescence at -196 degrees C

We investigated the reason for the absence of the long-wavelength PSI Chl a fluorescence at -196 degrees C in the cyanobacterium Gloeobacter violaceus using two methods: p-nitrothiophenol (p-NTP) treatment and time-resolved fluorescence spectra. The p-NTP treatment showed that PSII Chl a fluorescence was specifically affected in a manner similar to that for Synechocystis sp. PCC 6803 and spinach chloroplasts, although there were no components modified by the p-NTP treatment, indicating an absence of the long-wavelength PSI Chl a fluorescence. The time-resolved fluorescence spectra with a time resolution of 1.3 ps and spectral resolution of 1.0 nm gave no indication of the presence of the long-wavelength PSI fluorescence in the wavelength region between 700 nm and 760 nm, indicating that a very fast energy transfer among Chl a molecules could not account for the absence of the long-wavelength PSI fluorescence. From these data, it seems that the absence of the long-wavelength PSI fluorescence is due to a lack of the formation of a component responsible for the fluorescence at -196 degrees C, which may originate from a difference in the amino acid sequence. We discuss the significance of this phenomenon and interpret our findings in terms of the evolution of cyanobacteria.

Random mutagenesis targeted to the psbAII gene of Synechocystis sp. PCC 6803 to identify functionally important residues in the D1 protein of the photosystem II reaction center

More than one hundred mutants of Synechocystis sp. PCC 6803 impaired in photoautotrophic growth were generated by in vitro random PCR mutagenesis targeted to a region of the psbAII gene corresponding to a 210 amino acid (Ser148-Ala357) segment of the D1 protein. The 90 random mutants that could translate the full-length D1 protein carried 1-9 (on average 3.0) amino acid substitutions in the targeted region. Mutations were often found in the obligate photoheterotrophic strains at specific residues that have been reported or speculated to be important in the function of PSII, such as Y161, H198, H272, E333 and H337. This verifies the usefulness of the present method to identify functionally important residues in PSII. Other residues that were often mutated in the strains with impaired photoautotrophy included non-charged residues around the lumenal edges of transmembrane helices C, D and E, such as I192 and N296. Eleven mutants carried a single-point mutation in residues, such as Q165, Q187, W278, A294 and N298, and these identified the functional importance of these residues, most of which were on the donor side of PSII. A preliminary characterization of some of the mutants obtained in this study is provided.

Imaging of photo-oxidative stress responses in leaves

High resolution digital imaging was used to identify sites of photo-oxidative stress responses in Arabidopsis leaves non-invasively, and to demonstrate the potential of using a suite of imaging techniques for the study of oxidative metabolism in planta. Tissue-specific photoinhibition of photosynthesis in individual chloroplasts in leaves was imaged by chlorophyll fluorescence microscopy. Singlet oxygen production was assessed by imaging the quenching of the fluorescence of dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole (DanePy) that results from its reaction with singlet oxygen. Superoxide and hydrogen peroxide accumulation were visualized by the reduction of nitroblue tetrazolium (NBT) to formazan deposits and by polymerization with 3,3'-diaminobenzidine (DAB), respectively. Stress-induced expression of a gene involved with antioxidant metabolism was imaged from the bioluminescence from leaves of an Arabidopsis APX2-LUC transformant, which co-expresses an ascorbate peroxidase (APX2) with firefly luciferase. Singlet oxygen and superoxide production were found to be primarily located in mesophyll tissues whereas hydrogen peroxide accumulation and APX2 gene expression were primarily localized in the vascular tissues.

Imaging of photo-oxidative stress responses in leaves

High resolution digital imaging was used to identify sites of photo-oxidative stress responses in Arabidopsis leaves non-invasively, and to demonstrate the potential of using a suite of imaging techniques for the study of oxidative metabolism in planta. Tissue-specific photoinhibition of photosynthesis in individual chloroplasts in leaves was imaged by chlorophyll fluorescence microscopy. Singlet oxygen production was assessed by imaging the quenching of the fluorescence of dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole (DanePy) that results from its reaction with singlet oxygen. Superoxide and hydrogen peroxide accumulation were visualized by the reduction of nitroblue tetrazolium (NBT) to formazan deposits and by polymerization with 3,3'-diaminobenzidine (DAB), respectively. Stress-induced expression of a gene involved with antioxidant metabolism was imaged from the bioluminescence from leaves of an Arabidopsis APX2-LUC transformant, which co-expresses an ascorbate peroxidase (APX2) with firefly luciferase. Singlet oxygen and superoxide production were found to be primarily located in mesophyll tissues whereas hydrogen peroxide accumulation and APX2 gene expression were primarily localized in the vascular tissues.

UV-B-induced formation of reactive oxygen species and oxidative damage of the cyanobacterium Anabaena sp.: protective effects of ascorbic acid and N-acetyl-L-cysteine

Reactive oxygen species (ROS) are involved in the oxidative damage of the cyanobacterium Anabaena sp. caused by UV-B (280-315 nm) radiation. UV-B-induced overproduction of ROS as well as the oxidative stress was detected in vivo by using the ROS-sensitive probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA). Thiobarbituric acid reactive substances (TBARS) and fluorometric analysis of DNA unwinding (FADU) methods were adapted to measure lipid peroxidation and DNA strand breaks in Anabaena sp. Moderate UV-B radiation causes an increase of ROS production, enhanced lipid peroxidation and DNA strand breaks, yielding a significantly decreased survival. In contrast, the supplementation of UV-A in our work only showed a significant increase in total ROS levels and DNA strand breaks while no significant effect on lipid peroxidation, chlorophyll bleaching or survival was observed. The presence of ascorbic acid and N-acetyl-L-cysteine (NAC) reversed the oxidative stress and protected the organisms from chlorophyll bleaching and the damage of photosynthetic apparatus induced by UV-B significantly, resulting in a considerably higher survival rate. Ascorbic acid also exhibited a significant protective effect on lipid peroxidation and DNA strand breaks while NAC did not show a substantial effect. These results suggest that ascorbic acid exhibited significantly higher protective efficiency with respect to DNA strand breaks and survival than NAC while NAC appears to be especially effective in defending the photosynthetic apparatus from oxidative damage.

The negatively acting factors EID1 and SPA1 have distinct functions in phytochrome A-specific light signaling

EID1 (empfindlicher im dunkelroten Licht) and SPA1 (suppressor of phytochrome A[phyA]-105) function as negatively acting components in phyA-specific light signaling. Mutants in the respective genes led to very similar phenotypes under weak-light conditions. To examine whether both genes are functionally redundant, detailed physiological and genetic analyses were performed with eid1 and spa1 mutants isolated from the same wild-type background. Measurements of hypocotyl elongation, anthocyanin accumulation, and Lhcb1-transcript accumulation under different light treatments demonstrated that SPA1 has a strong influence on the regulation of very low fluence responses and a weaker influence on high-irradiance responses. In contrast, EID1 severely altered high-irradiance responses and caused almost no change on very low fluence responses. Analyses on eid1 phyA-105 double mutants demonstrated that EID1 could not suppress the phenotype of the weak phyA allele under continuous far-red light. Measurements on eid1 spa1 double mutants exhibited a strong interference of both genes in the regulation of hypocotyl elongation. These results indicate that EID1 and SPA1 are involved in different but interacting phyA-dependent signal transduction chains.

Double mutation cpSRP43--/cpSRP54-- is necessary to abolish the cpSRP pathway required for thylakoid targeting of the light-harvesting chlorophyll proteins

Biochemical and genetic studies have established that the light-harvesting chlorophyll proteins (LHCPs) of the photosystems use the cpSRP (chloroplast signal recognition particle) pathway for their targeting to thylakoids. Previous analyses of single cpSRP mutants, chaos and ffc, deficient in cpSRP43 and cpSRP54, respectively, have revealed that half of the LHCPs are still integrated into the thylakoid membranes. Surprisingly, the effects of both mutations are additive in the double mutant ffc/chaos described here. This mutant has pale yellow leaves at all stages of growth and drastically reduced levels of all the LHCPs except Lhcb 4. Although the chloroplasts have a normal shape, the thylakoid structure is affected by the mutation, probably as a consequence of reduction of all the LHCPs. ELIPs (early light-inducible proteins), nuclear-encoded proteins related to the LHCP family and inducible by light stress, were also drastically reduced in the double mutant. However, proteins targeted by other chloroplastic targeting pathways (DeltapH, Sec and spontaneous pathways) accumulated to similar levels in the wild-type and the double mutant. Therefore, the near total loss of LHCPs and ELIPs in the double mutant suggests that cpSRP is the predominant, if not exclusive, targeting pathway for these proteins. Phenotypic analysis of the double mutant, compared to the single mutants, suggests that the cpSRP subunits cpSRP43 and cpSRP54 contribute to antenna targeting in an independent but additive way.

Enhanced susceptibility of photosynthesis to low-temperature photoinhibition due to interruption of chill-induced increase of S-adenosylmethionine decarboxylase activity in leaves of spinach (Spinacia oleracea L.)

The possible involvement of polyamines in the chilling tolerance of spinach (Spinacia oleracea L.) was investigated focusing on photosynthesis. During chilling at 8/5C (day/night) for 6 d, S-adenosylmethionine decarboxylase (SAMDC) activity increased significantly in leaves in parallel with the increase in putrescine and spermidine (Spd) content in leaves and chloroplasts. Treatment of leaves with methylglyoxal-bis(guanylhydrazone) (MGBG), an SAMDC inhibitor, resulted in the deterioration of plant growth and photosynthesis under chilling conditions, which was reversed by the concomitant treatment with Spd through the roots. Plants treated with MGBG showed lower photochemical efficiency of PSII than either the control or plants treated with MGBG plus Spd during chilling and even after transfer to warm conditions, suggesting an increase of photoinhibition due to low Spd in chloroplasts. Indeed, MGBG-treated plants had much lower activities of thylakoid electron transport and enzymes in carbon metabolism as well as higher degrees of lipid peroxidation of thylakoid membranes compared to the control. These results indicate that the enhanced activity of SAMDC with a consequential rise of Spd in chloroplasts is crucial for the cold acclimation of the photosynthetic apparatus in spinach leaves.

Signal transduction in response to excess light: getting out of the chloroplast

Plants are continually in danger of absorbing more light energy than they can use productively for their metabolism. Acclimation to environmental conditions therefore includes the development of mechanisms for dissipating or avoiding the accumulation of such excess excitation energy. Acclimation could be controlled by many signal transduction pathways that would be initiated by the perception of excess excitation energy both inside and outside the chloroplast. Recent studies in related areas provide models of how these signalling pathways could operate in acclimation to excess light. Components of photosynthetic electron transport chains, reactive oxygen species, redox-responsive protein kinases, thiol-regulated enzymes, chlorophyll precursors and chloroplast-envelope electron transport chains all have roles in these models.

Growth and production of buckwheat (Fagopyrum esculentum) treated with reduced, ambient, and enhanced UV-B radiation

The effect of enhanced UV-B radiation on buckwheat (Fagopyrum esculentum Moench. variety 'Darja'), an important high elevation crop, was studied in order to estimate its vulnerability in changing UV-B environment. Plants were grown in outdoor experiments from July to October under reduced and ambient UV-B levels, and an UV-B level simulating 17% ozone depletion in Ljubljana. During the development the following parameters were monitored: light saturated photosynthetic activity, transpiration, potential and effective photochemical efficiencies of photosystem II, the contents of photosynthetic pigments and methanol soluble UV-B absorbing compounds. At the end of the experiment, growth rate and production of seeds were estimated. In the following growth season the seeds collected from plants exposed to different UV-B treatments were tested for germination capacity. Total UV-B absorbing compounds during plant development were increased by UV-B radiation, photosynthetic pigments (chlorophyll a and b and carotenoids) decreased. Photosynthetic rate was lowered in an early stage of development. UV-B treatment resulted in the increase in the transpiration rate and consequently the decrease in water use efficiency (WUE). The disturbances in water economy and in photosynthesis affected the reproduction potential negatively; the production of seeds in plants cultivated under ambient and enhanced UV-B was 57 and 39% of the production of specimens treated with reduced UV-B, respectively. The germination of seeds collected from treated plants revealed on average about 95% success, independently of the treatment, but the time needed for germination was the shortest for seeds developed under enhanced UV-B level treatment. Enhanced UV-B radiation affected water relations and production of buckwheat, but not the potential of seeds for germination.

The influence of enhanced UV-B radiation on Batrachium trichophyllum and Potamogeton alpinus -- aquatic macrophytes with amphibious character

The responses of two amphibious species, Batrachium trichophyllum and Potamogeton alpinus to different UV-B environments were studied. Plant material from natural environments, as well as from outdoor treatments was examined. In long-term outdoor experiments plants were grown under three different levels of UV-B radiation: reduced and ambient UV-B levels, and a UV-B level simulating 17% ozone depletion. The following parameters were monitored: contents of total methanol soluble UV-absorbing compounds and chlorophyll a, terminal electron transport system (ETS) activity and optimal and effective quantum yield of photosystem II. No effect of the different UV-B levels on the measured parameters was observed. The amount of UV-B absorbing compounds seems to be saturated, since no differences were observed between treatments and no increase was found in peak season, when natural UV-B levels were the highest. Physiological measurements revealed no harmful effects; neither on potential and actual photochemical efficiency, nor on terminal ETS activity. The contents of UV-B absorbing compounds were examined also in plant material sampled in low and high altitude environments during the growth season. Both species exhibited no seasonal dynamics of production of UV-absorbing compounds. The contents were variable and showed no significant differences between high and low altitude populations.

Increasing rice photosynthesis by manipulation of the acclimation and adaptation to light

There are three important considerations in assessing the interaction of crop plants with light: (a) how does the plant respond to the light environment both in the short-term (regulation) and in the long-term (acclimation), (b) under what conditions are these responses inadequate, leading to photoinhibition, and (c) are the responses optimally adapted for maximum agricultural yield? Despite a wealth of knowledge about these processes in model plant species, it is impossible to predict how significant they are in influencing the yield of rice. Therefore, in collaboration with IRRI, we have undertaken a study of photoinhibition and photoacclimation of rice under field conditions. The results of this study are presented, along with an assessment of the implications for improvement of rice yield.

Responses of photosynthetic electron transport in stomatal guard cells and mesophyll cells in intact leaves to light, CO2, and humidity

High-resolution images of the chlorophyll fluorescence parameter Fq'/Fm' from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. Fq'/Fm' estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic photosynthetic electron transport. Over a range of light intensities, values of Fq'/Fm' were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of Fq'/Fm' of guard and mesophyll cells to changes of CO2 and O2 concentration were very similar. There were similar reductions of Fq'/Fm' of guard and mesophyll cells over a wide range of CO2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, Fq'/Fm' of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO2 was low, but no change when ambient CO2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO2 concentration.

Responses of photosynthetic electron transport in stomatal guard cells and mesophyll cells in intact leaves to light, CO2, and humidity

High-resolution images of the chlorophyll fluorescence parameter Fq'/Fm' from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. Fq'/Fm' estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic photosynthetic electron transport. Over a range of light intensities, values of Fq'/Fm' were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of Fq'/Fm' of guard and mesophyll cells to changes of CO2 and O2 concentration were very similar. There were similar reductions of Fq'/Fm' of guard and mesophyll cells over a wide range of CO2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, Fq'/Fm' of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO2 was low, but no change when ambient CO2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO2 concentration.

Responses of photosynthetic electron transport in stomatal guard cells and mesophyll cells in intact leaves to light, CO2, and humidity

High-resolution images of the chlorophyll fluorescence parameter Fq'/Fm' from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. Fq'/Fm' estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic photosynthetic electron transport. Over a range of light intensities, values of Fq'/Fm' were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of Fq'/Fm' of guard and mesophyll cells to changes of CO2 and O2 concentration were very similar. There were similar reductions of Fq'/Fm' of guard and mesophyll cells over a wide range of CO2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, Fq'/Fm' of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO2 was low, but no change when ambient CO2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO2 concentration.

The Antarctic psychrophile, Chlamydomonas subcaudata, is deficient in state I-state II transitions

State I-State II transitions were monitored in vivo and in vitro in the Antarctic, psychrophillic, green alga, Chlamydomonas subcaudata, as changes in the low-temperature (77 K) chlorophyll fluorescence emission maxima at 722 nm (F722) relative to 699 nm (F699). As expected, the control mesophillic species, Chlamydomonas reinhardtii, was able to modulate the light energy distribution between photosystem II and photosystem I in response to exposure to four different conditions: (i) dark/anaerobic conditions, (ii) a change in Mg2+ concentration, (iii) red light, and (iv) increased incubation temperature. This was correlated with the ability to phosphorylate both of its major light-harvesting polypeptides. In contrast, exposure of C. subcaudata to the same four conditions induced minimum alterations in the 77 K fluorescence emission spectra, which was correlated with the ability to phosphorylate only one of its major light-harvesting polypeptides. Thus, C. subcaudata appears to be deficient in the ability to undergo a State I-State II transition. Functionally, this is associated with alterations in the apparent redox status of the intersystem electron transport chain and with higher rates of photosystem I cyclic electron transport in the psychrophile than in the mesophile, based on in vivo P700 measurements. Structurally, this deficiency is associated with reduced levels of Psa A/B relative to D1, the absence of specific photosystem I light-harvesting polypeptides [R.M. Morgan et al. (1998) Photosynth Res 56:303-314] and a cytochrome b6/f complex that exhibits a form of cytochrome f that is approximately 7 kDa smaller than that observed in C. reinhardtii. We conclude that the Antarctic psychrophile, C. subcaudata, is an example of a natural variant deficient in State I-State II transitions.

Effects of natural intensities of visible and ultraviolet radiation on epidermal ultraviolet screening and photosynthesis in grape leaves

Grape (Vitis vinifera cv Silvaner) vine plants were cultivated under shaded conditions in the absence of ultraviolet (UV) radiation in a greenhouse, and subsequently placed outdoors under three different light regimes for 7 d. Different light regimes were produced by filters transmitting natural radiation, or screening out the UV-B (280-315 nm), or screening out the UV-A (315-400 nm) and the UV-B spectral range. During exposure, synthesis of UV-screening phenolics in leaves was quantified using HPLC: All treatments increased concentrations of hydroxycinnamic acids but the rise was highest, reaching 230% of the initial value, when UV radiation was absent. In contrast, UV-B radiation specifically increased flavonoid concentrations resulting in more than a 10-fold increase. Transmittance in the UV of all extracted phenolics was lower than epidermal UV transmittance determined fluorimetrically, and the two parameters were curvilinearly related. It is suggested that curvilinearity results from different absorption properties of the homogeneously dissolved phenolics in extracts and of the non-homogeneous distribution of phenolics in the epidermis. UV-B-dependent inhibition of maximum photochemical yield of photosystem II (PSII), measured as variable fluorescence of dark-adapted leaves, recovered in parallel to the buildup of epidermal screening for UV-B radiation, suggesting that PSII is protected against UV-B damage by epidermal screening. However, UV-B inhibition of CO(2) assimilation rates was not diminished by efficient UV-B screening. We propose that protection of UV-B inactivation of PSII is observed because preceding damage is efficiently repaired while those factors determining UV-B inhibition of CO(2) assimilation recover more slowly.

Chromatic variation of the abundance of PSII complexes observed with the red alga Prophyridium cruentum

Chromatic regulation of photosystem stoichiometry in cyanophytes, green algae and probably vascular plants is achieved by regulation of the abundance of PSI in response to thylakoid electron transport state at least under our experimental conditions [cf. Fujita (1997) Photosyn. Res. 53: 83]. However, variation of not only PSI but also PSII, in reverse of each other, is characteristic of the stoichiometry regulation in red algae and some of marine cyanophytes. Our previous study with the red alga Porphyridium cruentum has revealed that PSII is inactivated by 50% upon a light shift from the light absorbed by Chl a, PSI light, to that mainly absorbed by phycobilisomes (PBS), PSII light [Fujita (1999) Plant Cell Physiol. 40: 924]. To evaluate the contribution of the photoinactivation to the chromatic variation of PSII, variation of the abundance of PSI, PSII and PBS, together with the fluorescence parameter and the activity of PSII, was followed after a light shift from PSI light to PSII light. Upon a light shift to PSII light, PSII, determined as Cyt b(559) per PBS, decreased rapidly, following the photoinactivation, down to the level a half of that before the light shift, and remained constant. Since the increase in PBS was not significant during this period, a rapid decrease of PSII/PBS led us to tentatively conclude that the degradation of PSII is a main cause for variation of the abundance of PSII. Photoinactivation of PSII, and also decrease in Cyt b(559), was accelerated, but only slightly, by the addition of chloramphenicol (CAP) at a moderate concentration while CAP at the same concentration significantly suppressed the increment of PSI determined as P700. A selective effect of CAP supports the above conclusion.

Dissection of the light signal transduction pathways regulating the two early light-induced protein genes in Arabidopsis

The expression of light-regulated genes in plants is controlled by different classes of photoreceptors that act through a variety of signaling molecules. During photomorphogenesis, the early light-induced protein (Elip) genes are among the first to be induced. To understand the light signal transduction pathways that regulate Elip expression, the two Elip genes, Elip1 and Elip2, in Arabidopsis were studied, taking advantage of the genetic tools available for studying light signaling in Arabidopsis. Using two independent quantitative reverse transcriptase-PCR techniques, we found that red, far-red, and blue lights positively regulate expression of the Elip genes. Phytochrome A and phytochrome B are involved in this signaling. The cryptochrome or phototropin photoreceptors are not required for blue-light induction of either Elip gene, suggesting the involvement of an additional, unidentified, blue-light receptor. Although the COP9 signalosome, a downstream regulator, is involved in dark repression of both Elips, Elip1 and Elip2 show different expression patterns in the dark. The transcription factor HY5 promotes the light induction of Elip1, but not Elip2. A defect in photosystem II activity in greening of hy5 seedlings may result from the loss of Elip1. Heat shock positively controlled Elip1 and Elip2 in a light-independent fashion. This induction is independent of HY5, indicating that heat shock and light activate transcription of the Elip genes through independent pathways.

Senescence is induced in individually darkened Arabidopsis leaves, but inhibited in whole darkened plants

It has long been known that leaf senescence can be induced in many plant species by detaching leaves and placing them in the darkness. It recently has been shown that entire Arabidopsis plants placed in the darkness are not induced to senesce, as judged by visible yellowing and certain molecular markers. Here, we show that when individual Arabidopsis leaves are darkened, but not when entire plants are darkened, senescence is induced in the covered leaves. This induction of senescence is highly localized. The phenomenon is leaf age dependent in that it occurs more rapidly and strongly in older leaves than in younger ones, as is the case with many forms of induced senescence. Whole adult plants placed in darkness, in contrast, show delayed senescence, although seedlings lacking primary leaves do not. These observations imply that the light status of the entire plant affects the senescence of individual leaves. A model summarizing the results is presented.

Inhibition of plastocyanin to P(700)(+) electron transfer in Chlamydomonas reinhardtii by hyperosmotic stress

Oxygen electrode and fluorescence studies demonstrate that linear electron transport in the freshwater alga Chlamydomonas reinhardtii can be completely abolished by abrupt hyperosmotic shock. We show that the most likely primary site of inhibition of electron transfer by hyperosmotic shock is a blockage of electron transfer between plastocyanin (PC) or cytochrome c(6) and P(700). The effects on this reaction were reversible upon dilution of the osmolytes and the stability of plastocyanin or photosystem (PS) I was unaffected. Electron micrographs of osmotically shocked cells showed a significant decrease in the thylakoid lumen volume. Comparison of estimated lumenal width with the x-ray structures of plastocyanin and PS I suggest that lumenal space contracts during HOS so as to hinder the movement of docking to PS I of plastocyanin or cytochrome c(6).