Convergence and divergence of the photoregulation of pigmentation and cellular morphology in Fremyella diplosiphon

Photosynthetic pigment accumulation and cellular and filament morphology are regulated reversibly by green light (GL) and red light (RL) in the cyanobacterium Fremyella diplosiphon during complementary chromatic adaptation (CCA). The photoreceptor RcaE (regulator of chromatic adaptation), which appears to function as a light-responsive sensor kinase, controls both of these responses. Recent findings indicate that downstream of RcaE, the signaling pathways leading to light-dependent changes in morphology or pigment synthesis and/or accumulation branch, and utilize distinct molecular components. We recently reported that the regulation of the accumulation of the GL-absorbing photosynthetic accessory protein phycoerythrin (PE) and photoregulation of cellular morphology are largely independent, as many mutants with severe PE accumulation defects do not have major disruptions in the regulation of cellular morphology. Furthermore, morphology can be disrupted under GL without impacting GL-dependent PE accumulation. Most recently, however, we determined that the disruption of the cpeR gene, which encodes a protein that is known to function as an activator of PE synthesis under GL, results in disruption of cellular morphology under GL and RL. Thus, apart from RcaE, CpeR is only the second known regulator to impact morphology under both light conditions in F. diplosiphon.     

Temporal dynamics of changes in reactive oxygen species (ROS) levels and cellular morphology are coordinated during complementary chromatic acclimation in Fremyella diplosiphon

Fremyella diplosiphon alters the phycobiliprotein composition of its light-harvesting complexes, i.e., phycobilisomes, and its cellular morphology in response to changes in the prevalent wavelengths of light in the external environment in a phenomenon known as complementary chromatic acclimation (CCA). The organism primarily responds to red light (RL) and green light (GL) during CCA to maximize light absorption for supporting optimal photosynthetic efficiency. Recently, we found that RL-characteristic spherical cell morphology is associated with higher levels of reactive oxygen species (ROS) compared to growth under GL where lower ROS levels and rectangular cell shape are observed. The RL-dependent association of increased ROS levels with cellular morphology was demonstrated by treating cells with a ROS-scavenging antioxidant which resulted in the observation of GL-characteristic rectangular morphology under RL. To gain additional insights into the involvement of ROS in impacting cellular morphology changes during CCA, we conducted experiments to study the temporal dynamics of changes in ROS levels and cellular morphology during transition to growth under RL or GL. Alterations in ROS levels and cell morphology were found to be correlated with each other at early stages of acclimation of low white light-grown cells to growth under high RL or cells transitioned between growth in RL and GL. These results provide further general evidence that significant RL-dependent increases in ROS levels are temporally correlated with changes in morphology toward spherical. Future studies will explore the light-dependent mechanisms by which ROS levels may be regulated and the direct impacts of ROS on the observed morphology changes.     

Photoregulation of morphogenesis of tobacco plants transformed with the gene of human interlenkin-18

The effects of blue light (BL), green light (GL), and red light (RL) on morphogenesis photoregulation of transgenic tobacco (Nicotiana tabacum L.) plants containing a copy of human interleukin-18 gene were studied. Wild-type and transgenic (Il1 No.87–1 and Il18 No.7–11) lines and derived calluses were used. Photomorphogenesis of transgenic lines under GL and RL differed substantially from that of initial line at early stages of morphogenesis; this was expressed in hypocotyl lengthening under GL and cotyledon enhanced expansion under RL. Growth responses to light quality were related to changes in the levels of zeatin, IAA, and ABA. Thus, hypocotyl lengthening and increased cotyledon area under GL occurred at the elevated level of IAA and reduced level of ABA. Growth of callus cells in transgenic lines during zero passage differed from wild-type calluses only in darkness. The data obtained indicate that tobacco plant transformation with the human interleukin-18 gene changed functioning of the photoregulation systems at early developmental stages. This phenomenon is evidently explained by the pleiotropic effects of the gene inserted.     

Phototropin 1 and cryptochrome action in response to green light in combination with other wavelengths

Genetic studies have shown the effects of various photoreceptors on early photomorphogenic processes, defining the precise time course of red (RL), far-red (FrL) and blue light (BL) action. In this study, the effect of green wavebands in conjunction with these responses is examined. Longer-term (end point; 24–96 h) analysis of hypocotyl elongation in enriched green environments shows an increase in growth compared to seedlings under blue, red or both together. The effect was only observed at lower fluence rates (<10 μmol/m² s). Genetic analyses demonstrate that cryptochromes are required for this GL effect, consistent with earlier findings, and that the phy receptors have no influence. However, analysis of early (minutes to hours) stem growth kinetics indicates that GL cannot reverse the cryptochrome-mediated BL effect during early stem growth inhibition, and instead acts additively with BL to drive cryptochrome-mediated inhibition. Green light (GL) treatments antagonize RL and FrL-mediated hypocotyl inhibition. The GL opposition of RL responses persists in phyA, phyB, cry1cry2 and phot2 mutants. The response requires phot1 and NPH3, suggesting that this is not a GL response, but instead a response to extremely low-fluence rate BL. Tests with dim BL (<0.1 μmol/m² s) confirm a previously uncharacterized phot1-dependent promotion of stem growth, opposing the effects of RL. These findings demonstrate how enriched green environments may adjust RL and BL photomorphogenic responses through both the crys and phot1 receptors, and define a new role for phot1 in stem growth promotion.     

Anthocyanin accumulation in the illuminated surface of maize leaves enhances protection from photo-inhibitory risks at low temperature, without further limitation to photosynthesis

At suboptimal temperatures, anthocyanins accumulate in the illuminated leaf surface of some maize genotypes and, if the anthocyanins shade chloroplasts, they can effectively reduce the risk of photo‐inhibition but also photo‐synthesis. To investigate this phenomenon, gas exchange, fluorescence, superoxide dismutase activity and xantho‐phyll composition of anthocyanin‐containing HOPI and anthocyanin‐deficient W22 maize genotypes were measured in either white or red light, where the latter is not absorbed by anthocyanins. Despite differences in light absorption in chloroplasts, photosynthesis did not differ between HOPI and W22 under either light source, suggesting that neither CO₂ supply nor photochemistry were more limiting in red leaves than in green leaves. In fact, no major differences in transpiration were detected. The ΔF/F_m (photosystem II quantum yield) of HOPI in white light was higher than in red light and higher than ΔF/F_m of W22 with either light source. This probably compensated for the lower white light absorption of HOPI chloroplasts compared with W22 because of the presence of anthocyanins and led to similar rates of calculated electron transport for both genotypes. After exposure to high white light at 5 °C, xanthophyll de‐epoxidation and superoxide dismutase activity were lower in HOPI than in W22. Further, HOPI could be exposed to a much higher irradiance than W22 before F_v/F_m was reduced to that of W22.     

The effect of temperature,light-spectrum,desiccation and salinity gradients on the photosynthetic performance of a subtidal brown alga,Sargassum macrocarpum,from Japan

The effect of temperature, light-spectrum, desiccation and salinity gradients on the photosynthesis of a Japanese subtidal brown alga, Sargassum macrocarpum (Fucales), was determined using a pulse amplitude modulation-chlorophyll fluorometer and dissolved oxygen sensors. Temperature responses of the maximum (F_v/F_m in darkness) and effective (ΔF/F_m′ at 50 μmol photons m⁻² s⁻¹; = Φ_PSII) quantum yields during 6-day culture (4–36°C) remained high at 12–28°C, but decreased at higher temperatures. Nevertheless, ΔF/F_m′ also dropped at temperatures below 8°C, suggesting light sensitivity under chilling temperatures because F_v/F_m remained high. Photosynthesis–irradiance responses at 24°C under red (660 nm), green (525 nm), blue (450 nm) and white light (metal halide lamp) showed that maximum net photosynthesis under blue and white light was greater than under red and green light, indicating the sensitivity and photosynthetic availability of blue light in the subtidal light environment. In the desiccation experiment, samples under aerial exposure of up to 8 h under dim-light at 24°C and 50% humidity showed that ΔF/F_m′ quickly declined after more than 45 min of emersion; furthermore, ΔF/F_m′ also failed to recover to initial levels even after 1 day of rehydration in seawater. Under the emersion state, the ΔF/F_m′ remained high when the relative water content (RWC) was greater than 50%; in contrast, it quickly dropped when the RWC was less than 50%. When the RWC was reduced below 50%, ΔF/F_m′ did not return to initial levels, regardless of subsequent re-hydration, suggesting a low capacity of photosynthesis to recover from desiccation. The stenohaline response of photosynthesis under 3-day culture is evident, given that ΔF/F_m′ declined when salinity was beyond 20–40 psu. Adaptation to subtidal environments in temperate waters of Japan can be linked to these traits.     

Light-inducible flux control of triosephosphate isomerase on glycolysis in Escherichia coli

An engineering tool for controlling flux distribution on metabolic pathways to an appropriate state is highly desirable in bioproduction. An optogenetic switch, which regulates gene expression by light illumination is an attractive on/off switchable system, and is a promising way for flux control with an external stimulus. We demonstrated a light-inducible flux control between glycolysis and the methylglyoxal (MGO) pathway in Escherichia coli using a CcaS/CcaR system. CcaR is phosphorylated by green light and is dephosphorylated by red light. Phosphorylated CcaR induces gene expression under the cpcG2 promoter. The tpiA gene was expressed under the cpcG2 promoter in a genomic tpiA deletion strain. The strain was then cultured with glucose minimum medium under green or red light. We found that tpiA messenger RNA level under green light was four times higher than that under red light. The repression of tpiA expression led to a decrease in glycolytic flux, resulting in slower growth under red light (0.25 hr ⁻¹) when compared to green light (0.37 hr ⁻¹). The maximum extracellular MGO concentration under red light (0.2 mM) was higher than that under green light (0.05 mM). These phenotypes confirm that the MGO pathway flux was enhanced under red light.     

Susceptibility of green leaves and green flower petals of CAM orchid <Emphasis Type="Italic">Dendrobium</Emphasis> cv. Burana Jade to high irradiance under natural tropical conditions

Photosynthetic rates of green leaves (GL) and green flower petals (GFP) of the CAM plant Dendrobium cv. Burana Jade and their sensitivities to different growth irradiances were studied in shade-grown plants over a period of 4 weeks. Maximal photosynthetic O₂ evolution rates and CAM acidities [dawn/dusk fluctuations in titratable acidity] were higher in leaves exposed to intermediate sunlight [a maximal photosynthetic photon flux density (PPFD) of 500–600 μmol m⁻² s⁻¹] than in leaves grown under full sunlight (a maximal PPFD of 1 000–1 200 μmol m⁻² s⁻¹) and shade (a maximal PPFD of 200–250 μmol m⁻² s⁻¹). However, these two parameters of GFP were highest in plants grown under the shade and lowest in full sun-grown plants. Both GL and GFP of plants exposed to full sunlight had lower predawn F_v/F_m [dark adapted ratio of variable to maximal fluorescence (the maximal photosystem 2 yield without actinic irradiation)] than those of shade-grown plants. When exposed to intermediate sunlight, however, there were no significant changes in predawn F_v/F_m in GL whereas a significant decrease in predawn F_v/F_m was found in GFP of the same plant. GFP exposed to full sunlight exhibited a greater decrease in predawn F_v/F_m compared to those exposed to intermediate sunlight. The patterns of changes in total chlorophyll (Chl) content of GL and GFP were similar to those of F_v/F_m. Although midday F_v/F_m fluctuated with prevailing irradiance, changes of midday F_v/F_m after exposure to different growth irradiances were similar to those of predawn F_v/F_m in both GL and GFP. The decreases in predawn and midday F_v/F_m were much more pronounced in GFP than in GL under full sunlight, indicating greater sensitivity in GFP to high irradiance (HI). In the laboratory, electron transport rate and photochemical and non-photochemical quenching of Chl fluorescence were also determined under different irradiances. All results indicated that GFP are more susceptible to HI than GL. Although the GFP of Dendrobium cv. Burana Jade require a lower amount of radiant energy for photosynthesis and this plant is usually grown in the shade, is not necessarily a shade plant.     

Influence of Spectral Quality of Light on Pollen Tube Elongation in Arachis hypogaea

The effect of light of different wave lengths (red RL, blueBL, yellow YL, green GL and far-red FR) on pollen tube elongationhas been studied in Arachis hypogaea. Short exposure of pollento RL caused enhanced tube elongation and this effect was reducedby FR and BL. The effects of RL and FR were mutually reversible.Similarly, the inhibitory effect of blue light could be overcomeby IAA and riboflavin separately whereas acetylcholine and GA₃mimicked the RL effect. The activities of peroxidase (PO) andmalate dehydrogenase (MDH) were increased by RL and IAA whereasBL increased the activity of IAA-oxidase. The regulatory roleof light of different spectral composition in controlling pollentube elongation is discussed. Arachis hypogaea, pollen tube elongation, light     

Engineering of a green‐light inducible gene expression system in Synechocystis sp. PCC6803

In order to construct a green‐light‐regulated gene expression system for cyanobacteria, we characterized a green‐light sensing system derived from Synechocystis sp. PCC6803, consisting of the green‐light sensing histidine kinase CcaS, the cognate response regulator CcaR, and the promoter of cpcG2 (P_cpcG2). CcaS and CcaR act as a genetic controller and activate gene expression from P_cpcG2 with green‐light illumination. The green‐light induction level of the native P_cpcG2 was investigated using GFPuv as a reporter gene inserted in a broad‐host‐range vector. A clear induction of protein expression from native P_cpcG2 under green‐light illumination was observed; however, the expression level was very low compared with P_trc, which was reported to act as a constitutive promoter in cyanobacteria. Therefore, a Shine‐Dalgarno‐like sequence derived from the cpcB gene was inserted in the 5′ untranslated region of the cpcG2 gene, and the expression level of CcaR was increased. Thus, constructed engineered green‐light sensing system resulted in about 40‐fold higher protein expression than with the wild‐type promoter with a high ON/OFF ratio under green‐light illumination. The engineered green‐light gene expression system would be a useful genetic tool for controlling gene expression in the emergent cyanobacterial bioprocesses.