Light-induced Accumulation Research Articles

Constructing C-rich polymeric carbon nitride homojunctions for enhanced storage capacity of photo-rechargeable batteries

The light-induced accumulation of long-lived trapped electrons within the conjugated heptazine backbone enables polymeric carbon nitrides the dual ability of light absorption and charge storage. However, the storage capacity has still been plagued by the severe recombination of photoinduced electrons and holes. Carbon-rich polymeric carbon nitrides (CPCN) with the conjugated backbone combining heptazine motifs and individual aromatic nucleus have tunable energy band structure. Herein, in order to enhance the photo-rechargeable capacity, we present a type-II homojunction constructed by two CPCN monolayer materials with different bandgaps and band-edges to build the band bending for charge separation. The desirable hole-extraction route for the CPCN photo-rechargeable solar cell is revealed and a remarkable high record of photo-charging capacity of 486 C g−1 is achieved. The microstructure of the CPCN materials are characterized by XPS, NMR, UPS and UV-Vis. The photoelectrochemical properties of the CPCN monolayer and homojunction photoelectrodes are investigated using cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS) and Mott-Schottky method.

Dynamic Changes in the Antioxidative Defense System in the Tea Plant Reveal the Photoprotection-Mediated Temporal Accumulation of Flavonoids under Full Sunlight Exposure.

To reveal the mechanisms underlying how light affects flavonoid metabolism and the potential role of flavonoids in protecting against photooxidative stress in tea leaves, tea plants adapted to low-light conditions were exposed to full sunlight over 48 h. There was an increase in the activities of catalase (CAT) and superoxide dismutase (SOD) as well as greater accumulation of reactive oxygen species, lutein, tocopherols, ascorbate and malondialdehyde, suggestive of a time-dependent response to photooxidative stress in tea leaves. Analysis of the time dependency of each element of the antioxidant system indicated that carotenoids and tocopherols exhibited the fastest response to light stress (within 3 h), followed by SOD, CAT and catechin, which peaked at 24 h. Meanwhile, flavonols, vitamin C and glutathione showed the slowest response. Subsequent identification of the main phytochemicals involved in protecting against oxidative stress using untargeted metabolomics revealed a fast and initial accumulation of nonesterified catechins that preceded the increase in flavonol glycosides and catechin esters. Gene expression analysis suggested that the light-induced accumulation of flavonoids was highly associated with the gene encoding flavonol synthase. Ultraviolet B (UV-B) irradiation further validated the time-dependent and collaborative effects of flavonoids in photoprotection in tea plants. Intriguingly, the dynamics of the metabolic response are highly distinct from those reported for Arabidopsis, suggesting that the response to light stress is not conserved across plants. This study additionally provides new insights into the functional role of flavonoids in preventing photooxidative stress and may contribute to further improving tea quality through the control of light intensity.

Control of electronic band profiles through depletion layer engineering in core\u2013shell nanocrystals

Fermi level pinning in doped metal oxide (MO) nanocrystals (NCs) results in the formation of depletion layers, which affect their optical and electronic properties, and ultimately their application in smart optoelectronics, photocatalysis, or energy storage. For a precise control over functionality, it is important to understand and control their electronic bands at the nanoscale. Here, we show that depletion layer engineering allows designing the energetic band profiles and predicting the optoelectronic properties of MO NCs. This is achieved by shell thickness tuning of core–shell Sn:In2O3–In2O3 NCs, resulting in multiple band bending and multi-modal plasmonic response. We identify the modification of the band profiles after the light-induced accumulation of extra electrons as the main mechanism of photodoping and enhance the charge storage capability up to hundreds of electrons per NC through depletion layer engineering. Our experimental results are supported by theoretical models and are transferable to other core-multishell systems as well.

Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

Radical-Triggered Reaction Mechanism of the Green-to-Red Photoconversion of EosFP.

Reaction intermediates in the green-to-red photoconversion of the photochromic fluorescent protein EosFP have been observed using high-intensity continuous blue illumination. An intermediate was identified through light-induced accumulation that continues to convert the green form in subsequent darkness, putatively containing a tyrosyl radical, albeit with anomalously shifted features in both the electronic and FTIR spectra. Lowering the pH to 5.5 significantly delays the decay of this tyrosyl intermediate, which is accompanied by Stark-shifted features in the electronic spectra of reactants and products. Vibrational mode assignments for the high-frequency and fingerprint FTIR spectral regions of the reaction intermediates support a proposed sequence of events where the newly formed Cα═Cβ ethylenic bond precedes modifications on the His-62 imidazole ring and confirms a C═O(NH2) product group on Phe-61. We propose a reaction mechanism that involves tyrosyl generation via singlet excited-state-mediated oxidation which subsequently triggers the covalent reactions by oxidation of the green chromophore.

SUMO E3 Ligase SIZ1 stabilizes MYB75 to regulate anthocyanin accumulation under high light conditions in Arabidopsis

Sumoylation is one of post-translational modification (PTM) in which SUMO (small ubiquitin-like modifier) are covalently conjugated to protein substrates through a range of biochemical steps. This paper presents evidence that SUMO E3 ligase SIZ1 positively regulates anthocyanin accumulation. Loss-of-function siz1 mutant seedlings exhibit anthocyanin accumulation-reduced phenotype under high light conditions. Moreover, SIZ1 interacts and sumoylates MYB75/PAP1, a key transcription factor in anthocyanin accumulation. Loss-of-function siz1 or K246R substitution in MYB75 blocked SIZ1-mediated sumoylation in vitro and in vivo. Anthocyanin accumulation in mutant myb75-c can not be rescued by expressing MYB75K246R, but expression of wild-type MYB75WT complements the mutant phenotype. It suggested that sumoylation is important for MYB75 function. We further prove that sumoylation is essential for MYB75 protein stability. And SIZ1 is involved in the light-induced accumulation of anthocyanins. Our findings reveal an important role for sumoylation of MYB in regulation of anthocyanin accumulation in plants.

Light-Induced Vitamin C Accumulation in Tomato Fruits is Independent of Carbohydrate Availability.

L-ascorbate (ASC) is essential for human health. Therefore, there is interest in increasing the ASC content of crops like tomato. High irradiance induces accumulation of ASC in green tomato fruits. The D-mannose/L-galactose biosynthetic pathway accounts for the most ASC in plants. The myo-inositol and galacturonate pathways have been proposed to exist but never identified in plants. The D-mannose/L-galactose starts from D-glucose. In a series of experiments, we tested the hypothesis that ASC levels depend on soluble carbohydrate content when tomato fruits ripen under irradiances that stimulate ASC biosynthesis. We show that ASC levels considerably increased when fruits ripened under light, but carbohydrate levels did not show a parallel increase. When carbohydrate levels in fruits were altered by flower pruning, no effects on ASC levels were observed at harvest or after ripening under irradiances that induce ASC accumulation. Artificial feeding of trusses with sucrose increased carbohydrate levels, but did not affect the light-induced ASC levels. We conclude that light-induced accumulation of ASC is independent of the carbohydrate content in tomato fruits. In tomato fruit treated with light, the increase in ASC was preceded by a concomitant increase in myo-inositol.

Abstract P118: Tissue Transglutaminase Contributes to Pro-Inflammatory Cytokine LIGHT-Induced Ang II Sensitization

Hypertension is a severe cardiovascular complication with elevated inflammation and pressor sensitivity. Activated enzymatically and transcriptionally by inflammatory cytokines, tissue transglutaminase (TG2), a ubiquitous crosslinking enzyme, is known to stabilize the Ang II receptor AT1 and contribute to hypertension and cardio-renal syndrome.We hypothesized that TG2 play an essential role in the inflammation-induced Ang II sensitization in hypertension via its posttranslational modifications. To test this, we found a significant TG2-dependent increase in renal transglutaminase activity in animals infused with inflammatory cytokine LIGHT/TNFSF14 (4ng/day) for 14 days (PBS=0.5±0.04, LIGHT=0.8±0.03, LIGHT+TG2 inhibitor ERW1041E=0.6±0.05, LIGHT+TG2-/-=0.4±0.01mU/mg protein, n=4-5, p<0.05). Western blot further determined an ~9 fold increase in renal TG2 and ~4 fold in AT1 in animals infused with LIGHT only (n=3, p<0.05). Mutagenesis study on AT1 confirmed the requirement of TG2 modification in LIGHT-induced receptor accumulation. Following this, we identified an ~80% pronouncement in Ang II-induced calcium response in luciferase reporter cells co-treated with LIGHT but not LIGHT plus ERW1041E (n=3, p<0.01). Taken together, our studies may shed light on the intrinsic role of TG2 as the pressor sensitizer downstream of the cytokines. Our results could also be a proof of concept for the use of TG2 inhibitors as novel antihypertensives.

Photosynthesis supported by a chlorophyll f-dependent, entropy-driven uphill energy transfer in Halomicronema hongdechloris cells adapted to far-red light.

The phototrophic cyanobacterium Halomicronema hongdechloris shows far-red light-induced accumulation of chlorophyll (Chl) f, but the involvement of the pigment in photosynthetic energy harvesting by photosystem (PS) II is controversially discussed. While H. hongdechloris contains negligible amounts of Chl f in white-light culture conditions, the ratio of Chl f to Chl a is reversibly changed up to 1:8 under illumination with far-red light (720-730nm). We performed UV-Vis absorption spectroscopy, time-integrated and time-resolved fluorescence spectroscopy for the calculation of decay-associated spectra (DAS) to determine excitation energy transfer (EET) processes between photosynthetic pigments in intact H. hongdechloris filaments. In cells grown under white light, highly efficient EET occurs from phycobilisomes (PBSs) to Chl a with an apparent time constant of about 100ps. Charge separation occurs with a typical apparent time constant of 200-300ps from Chl a. After 3-4 days of growth under far-red light, robust Chl f content was observed in H. hongdechloris and EET from PBSs reached Chl f efficiently within 200ps. It is proposed based on mathematical modeling by rate equation systems for EET between the PBSs and PSII and subsequent electron transfer (ET) that charge separation occurs from Chl a and excitation energy is funneled from Chl f to Chl a via an energetically uphill EET driven by entropy, which is effective because the number of Chl a molecules coupled to Chl f is at least eight- to tenfold larger than the corresponding number of Chl f molecules. The long lifetime of Chl f molecules in contact to a tenfold larger pool of Chl a molecules allows Chl f to act as an intermediate energy storage level, from which the Gibbs free energy difference between Chl f and Chl a can be overcome by taking advantage from the favorable ratio of degeneracy coefficients, which formally represents a significant entropy gain in the Eyring formulation of the Arrhenius law. Direct evidence for energetically uphill EET and charge separation in PSII upon excitation of Chl f via anti-Stokes fluorescence in far-red light-adapted H. hongdechloris cells was obtained: Excitation by 720nm laser light resulted in robust Chl a fluorescence at 680nm that was distinctly temperature-dependent and, notably, increased upon DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) treatment in far-red light-adapted cells. Thus, rather than serving as an excitation energy trap, Chl f in far-red light-adapted H. hongdechloris cells is directly contributing to oxygenic photosynthesis at PSII.

Suppression of the phytoene synthase gene (EgcrtB) alters carotenoid content and intracellular structure of Euglena gracilis

BackgroundPhotosynthetic organisms utilize carotenoids for photoprotection as well as light harvesting. Our previous study revealed that high-intensity light increases the expression of the gene for phytoene synthase (EgcrtB) in Euglena gracilis (a unicellular phytoflagellate), the encoded enzyme catalyzes the first committed step of the carotenoid biosynthesis pathway. To examine carotenoid synthesis of E. gracilis in response to light stress, we analyzed carotenoid species and content in cells grown under various light intensities. In addition, we investigated the effect of suppressing EgcrtB with RNA interference (RNAi) on growth and carotenoid content.ResultsAfter cultivation for 7 days under continuous light at 920 μmol m−2 s−1, β-carotene, diadinoxanthin (Ddx), and diatoxanthin (Dtx) content in cells was significantly increased compared with standard light intensity (55 μmol m−2 s−1). The high-intensity light (920 μmol m−2 s−1) increased the pool size of diadinoxanthin cycle pigments (i.e., Ddx + Dtx) by 1.2-fold and the Dtx/Ddx ratio from 0.05 (control) to 0.09. In contrast, the higher-intensity light treatment caused a 58% decrease in chlorophyll (a + b) content and diminished the number of thylakoid membranes in chloroplasts by approximately half compared with control cells, suggesting that the high-intensity light-induced accumulation of carotenoids is associated with an increase in both the number and size of lipid globules in chloroplasts and the cytoplasm. Transient suppression of EgcrtB in this alga by RNAi resulted in significant decreases in cell number, chlorophyll, and total major carotenoid content by 82, 82 and 86%, respectively, relative to non-electroporated cells. Furthermore, suppression of EgcrtB decreased the number of chloroplasts and thylakoid membranes and increased the Dtx/Ddx ratio by 1.6-fold under continuous illumination even at the standard light intensity, indicating that blocking carotenoid synthesis increased the susceptibility of cells to light stress.ConclusionsOur results indicate that suppression of EgcrtB causes a significant decrease in carotenoid and chlorophyll content in E. gracilis accompanied by changes in intracellular structures, suggesting that Dtx (de-epoxidized form of diadinoxanthin cycle pigments) contributes to photoprotection of this alga during the long-term acclimation to light-induced stress.

MYB75 Phosphorylation by MPK4 Is Required for Light-Induced Anthocyanin Accumulation in Arabidopsis.

Light is a major environmental cue affecting various physiological and metabolic processes in plants. Although plant photoreceptors are well characterized, the mechanisms by which light regulates downstream responses are less clear. In Arabidopsis thaliana, the accumulation of photoprotective anthocyanin pigments is light dependent, and the R2R3 MYB transcription factor MYB75/PAP1 regulates anthocyanin accumulation. Here, we report that MYB75 interacts with and is phosphorylated by MAP KINASE4 (MPK4). Their interaction is dependent on MPK4 kinase activity and is required for full function of MYB75. MPK4 can be activated in response to light and is involved in the light-induced accumulation of anthocyanins. We show that MPK4 phosphorylation of MYB75 increases its stability and is essential for light-induced anthocyanin accumulation. Our findings reveal an important role for a MAPK pathway in light signal transduction.

Storage of fresh-cut lettuce at light levels below the light compensation point increases sugar levels and greatly prolongs the shelf-life

The quality of fresh-cut butterhead lettuce held at 12°C was greatly improved when the product was stored under low levels of light, provided by either fluorescent tubes, or red, blue or green LEDs. Although the applied light level (5 μmol m -2 s -1 photosynthetically active radiation) was well below the light compensation point, significant accumulation of carbohydrates (sugars and starch) was observed. The level of carbohydrates positively correlated with the shelf-life. Photosynthetic activity was low and net photosynthesis during storage was negative and cannot account for the observed accumulation of carbohydrates. Treating the leaf pieces with an inhibitor of photosynthetic electron transport, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), diminished the light-induced sugar accumulation and the beneficial effect of light on fresh-cut quality. Currently, it is not clear which processes are responsible for the sugar accumulation in light-stored samples. We hypothesize that, under low light conditions, sugars may be produced through the processing of chloroplast degradation products in the glyoxysome, subsequent production of malate and oxaloacetate and production of glucose through reversal of the glycolysis pathway (gluconeogenesis).

Participation of Two Carbonic Anhydrases of the Alpha Family in Photosynthetic Reactions in Arabidopsis thaliana.

The expression of genes of two carbonic anhydrases (CA) belonging to the α-family, α-CA2 and α-CA4 (according to the nomenclature in N. Fabre et al. (2007) Plant Cell Environ., 30, 617-629), was studied in arabidopsis (Arabidopsis thaliana, var. Columbia) leaves. The expression of the At2g28210 gene coding α-CA2 decreased under increase in plant illumination, while the expression of the At4g20990 gene coding α-CA4 increased. Under conditions close to optimal for photosynthesis, in plants with gene At2g28210 knockout, the effective quantum yield of photosystem 2 and the light-induced accumulation of hydrogen peroxide in leaves were lower than in wild type plants, while the coefficient of non-photochemical quenching of leaf chlorophyll a fluorescence and the rate of CO2 assimilation in leaves were higher. In plants with At4g20990 gene knockout, the same characteristics changed in opposite ways relative to wild type. Possible mechanisms of the participation of α-CA2 and α-CA4 in photosynthetic reactions are discussed, taking into account that protons can be either consumed or released in the reactions they catalyze.

Probing molecular events associated with early development of thylakoid membranes by comparative proteomics and low temperature fluorescence

A comparison of protein profiles between prolamellar bodies from dark-grown etioplasts and thylakoid membranes from de-etioplasts illuminated respectively for 1, 5 and 9h revealed 155 differentially expressed CBB-stained spots. Clear results showed that the nonphototransformable Pchlide627–632 was the dominant pigment form in the PLBs of rice etioplasts during plant development in dark and transformed slowly to chlorophyllide in rice etioplasts when exposed to light. The light-induced accumulation of ACC oxidase, which catalyzes the final step of ethylene synthesis using ACC as substrate, would facilitate chlorophyll synthesis by inducing PORa/b expression via ethylene signaling. It could be also suggested that cyclic electron transport might play an important role in generation of ATP for carbon fixation and photoprotection of photosystems from excessive light in prothylakoid. Furthermore, the overproduction of ClpC1, which targets proteins to the ClpPR core complex for degradation, was observed only in Stage 1, during which period PLBs disrupted and converted into prothylakoids, suggesting that ClpC1 was of particular importance for disassembly of PLBs of etioplasts when exposed to light. This study revealed the possible biochemical and physiological processes lead to the formation of functional thylakoid membranes. Biological significanceIn this study, we monitored the light-induced transformation of prolamellar bodies into thylakoid membranes, which is correlated to the biogenesis of photosynthetic apparatus involving a complex cascade of biochemical and structural events. Three stages of thylakoid development classified according to the thylakoid development status (Adam et al., 2011) were studied for biogenesis of photosynthetic apparatus: Stage 1, prothylakoids emerge from the disrupted PLBs; Stage 2, prothylakoids converted into primary thylakoids which were dispersed in the stroma; Stage 3, the continuous grana and stroma thylakoids are formed. The development stage-dependent changes in the proteomic profile of the thylakoids were analyzed by two-dimensional electrophoresis (2-DE). This information was complemented with the steady-state 77K chlorophyll fluorescence of thylakoids at the corresponding development stage. Together, these analyses allowed us to further understand the molecular processes connected to the formation of functional thylakoid membranes.

The bioactive monoterpene indole alkaloid N,β-d-glucopyranosyl vincosamide is regulated by irradiance quality and development in Psychotria leiocarpa

Leaves of Psychotria leiocarpa accumulate a major shoot-specific indole alkaloid, N,β-d-glucopyranosyl vincosamide (GPV), which has antioxidant, but no apparent antifeedant or allelopathic activity. The species is tolerant to high doses of UV-B, possibly due to constitutive GPV accumulation in leaves. In seedlings, GPV accumulation is induced by white light in a photosynthesis independent fashion. To better understand the regulation of the in planta GPV pool, detailed alkaloid profiles were examined. Light shift tests confirmed that GPV accumulation is light dependent. GPV was tightly regulated in its organ distribution, with highest concentrations in reproductive structures. Far-red and blue wavelengths promoted GPV accumulation without clear correlation with carbohydrate or protein concentrations or dry biomass in seedlings, thus indicating direct effects of light. Since light conditions are often associated with higher oxidative stress, light-induced accumulation of GPV may contribute to maintain redox balance. In vitro, GPV was an overall more effective antioxidant compared to closely related alkaloids. Indirect evidence of potential antioxidant properties of GPV in vivo was obtained by application of GPV on leaves of UV sensitive species exposed to high doses of UV-B, which significantly improved tolerance to this stress. In contrast to the known constitutive accumulation of GPV in leaves, the alkaloid concentration proved to be highly dynamic, changing during development, reproductive organogenesis and seedling irradiance treatments. Data further support a role for GPV as an oxidative stress protectant and provide a means to improve alkaloid yields in plant biomass for pharmacological applications.

Antagonistic effect of indole-3-acetic acid on kinetin-stimulated amaranthin accumulation in the cotyledons of Amaranthus mangostanus seedlings

ABSTRACTLight triggered the initiation of amaranthin biosynthesis in cotyledons of Amaranthus mangostanus L. seedlings. Cytokinin induced amaranthin synthesis in the dark and increased the accumulation of amaranthin under light irradiation. No studies have explored whether indole-3-acetic acid (IAA) can affect kinetin-induced amaranthin accumulation in seedlings of A. mangostanus L. In this study, we found that IAA inhibited both the kinetin- and light-induced synthesis of amaranthin. In the dark, 10.0 mg l−1 IAA caused a 68% reduction in amaranthin production after induction by 5.0 mg l−1 kinetin. In the presence of light, 10.0 mg l−1 IAA resulted in a 50% decrease in amaranthin synthesis following induction by 5.0 mg l−1 kinetin. In addition, IAA could reverse kinetin-induced amaranthin accumulation under red, blue, or far-red light conditions. Our results suggest that IAA had an antagonistic effect on the light-induced or cytokinin-stimulated accumulation of amaranthin in the cotyledons of A. mangostanus L. seedlings.

Quantification of superoxide radical production in thylakoid membrane using cyclic hydroxylamines

Applicability of two lipophilic cyclic hydroxylamines (CHAs), CM-H and TMT-H, and two hydrophilic CHAs, CAT1-H and DCP-H, for detection of superoxide anion radical (O2∙−) produced by the thylakoid photosynthetic electron transfer chain (PETC) of higher plants under illumination has been studied. ESR spectrometry was applied for detection of the nitroxide radical originating due to CHAs oxidation by O2∙−. CHAs and corresponding nitroxide radicals were shown to be involved in side reactions with PETC which could cause miscalculation of O2∙− production rate. Lipophilic CM-H was oxidized by PETC components, reducing the oxidized donor of Photosystem I, P700+, while at the same concentration another lipophilic CHA, TMT-H, did not reduce P700+. The nitroxide radical was able to accept electrons from components of the photosynthetic chain. Electrostatic interaction of stable cation CAT1-H with the membrane surface was suggested. Water-soluble superoxide dismutase (SOD) was added in order to suppress the reaction of CHA with O2∙− outside the membrane. SOD almost completely inhibited light-induced accumulation of DCP∙, nitroxide radical derivative of hydrophilic DCP-H, in contrast to TMT∙ accumulation. Based on the results showing that change in the thylakoid lumen pH and volume had minor effect on TMT∙ accumulation, the reaction of TMT-H with O2∙− in the lumen was excluded. Addition of TMT-H to thylakoid suspension in the presence of SOD resulted in the increase in light-induced O2 uptake rate, that argued in favor of TMT-H ability to detect O2∙− produced within the membrane core. Thus, hydrophilic DCP-H and lipophilic TMT-H were shown to be usable for detection of O2∙− produced outside and within thylakoid membranes.

Phytochrome-induced SIG2 expression contributes to photoregulation of phytochrome signalling and photomorphogenesis in Arabidopsis thaliana

Chloroplast-localized sigma factor (SIG) proteins promote specificity of the plastid-encoded RNA polymerase. SIG2 function appears to be necessary for light-grown Arabidopsis thaliana plants. Specific photoreceptors or light-dependent factors that impact the light-induced accumulation of SIG2 have not been reported. A molecular link between phytochromes and nuclear-encoded SIG2, which impacts photomorphogenesis specifically under red (R) and far-red (FR) light, is described here. Both phyA and phyB promote SIG2 transcript accumulation. Disruption of SIG2 results in R- and FR-specific defects in the inhibition of hypocotyl elongation and cotyledon expansion, although no impairments in these responses are detected for sig2 mutants under blue (B) or white (W) light. SIG2 also impacts root elongation under W and R, and the R-dependent expression of PIF4, encoding a phytochrome-interacting factor, and HY2, which encodes a phytochrome chromophore biosynthetic enzyme. Whereas SIG2 apparently impacts the accumulation of the phytochromobilin (PΦB) phytochrome chromophore, sig2 mutants differ significantly from PΦB mutants, primarily due to wavelength-specific defects in photomorphogenesis and disruption of a distinct subset of phytochrome-dependent responses. The molecular link between phytochromes and SIG2 is likely to be an important part of the co-ordination of gene expression to maintain stoichiometry between the nuclear-encoded phytochrome apoprotein and plastid-derived PΦB, which combine to form photoactive phytochromes, and/or light-dependent SIG2 accumulation is involved in an inductive light signalling pathway co-ordinating components between nucleus and plastids.

Crag Is a GEF for Rab11 Required for Rhodopsin Trafficking and Maintenance of Adult Photoreceptor Cells

Author SummaryAnimals sense light through receptors called Rhodopsins. These proteins are typically localized to stacked membranes in photoreceptors. In flies, upon light exposure, Rhodopsin undergoes conformational changes and becomes active as metarhodopsin. Metarhodopsin then initiates a signaling cascade that activates the photoreceptor cell. To deactivate the light response, metarhodopsin is converted back into Rhodopsin by absorption of another photon of light. Under certain conditions, metarhodopsin cannot be converted back to Rhodopsin, and it is then endocytosed and degraded. Rhodopsin then needs to be synthesized and delivered back to the membrane stacks. Here, we show that the Calmodulin-binding protein Crag is required for the delivery of newly made Rhodopsin to the membrane stacks. Loss of Crag leads to the accumulation of Rhodopsin in the cytosol, followed by shrinkage of membrane stack volume, and, eventually, photoreceptor cell degeneration. We also show that Crag activates a target protein, Rab11, which mediates the vesicular transport of Rhodospin to the membrane. Finally, we document that the human homolog of Crag, DENND4A, is able to rescue the loss of Crag in flies, suggesting that DENND4A functions in a similar process in vertebrates.

Constituents of bile, bilirubin and TUDCA, protect against oxidative stress-induced retinal degeneration

Two constituents of bile, bilirubin and tauroursodeoxycholic acid (TUDCA), have antioxidant activity. However, bilirubin can also cause damage to some neurons and glial cells, particularly immature neurons. In this study, we tested the effects of bilirubin and TUDCA in two models in which oxidative stress contributes to photoreceptor cell death, prolonged light exposure and rd10+/+ mice. In albino BALB/c mice, intraperitoneal injection of 5 mg/kg of bilirubin or 500 mg/kg of TUDCA prior to exposure to 5000 lux of white light for 8 h significantly reduced loss of rod and cone function assessed by electroretinograms. Both treatments also reduced light-induced accumulation of superoxide radicals in the outer retina, rod cell death assessed by outer nuclear layer thickness, and disruption of cone inner and outer segments. In rd10+/+ mice, intraperitoneal injections of 5 or 50 mg/kg of bilirubin or 500 mg/kg of TUDCA every 3 days starting at postnatal day (P) 6, caused significant preservation of cone cell number and cone function at P50. Rods were not protected at P50, but both bilirubin and TUDCA provided modest preservation of outer nuclear layer thickness and rod function at P30. These data suggest that correlation of serum bilirubin levels with rate of vision loss in patients with retinitis pigmentosa could provide a useful strategy to test the hypothesis that cones die from oxidative damage in patients with retinitis pigmentosa. If proof-of-concept is established, manipulation of bilirubin levels and administration of TUDCA could be tested in interventional trials.