The requirement for carotenoids in the assembly and function of the photosynthetic complexes in Chlamydomonas reinhardtii

Overexpressing Carotenoid Biosynthetic Genes in Synechocystis sp. PCC 6803 Improved Intracellular Pigments and Antioxidant Activity, Which Can Decrease the Viability and Proliferation of Lung Cancer Cells In Vitro

In the antioxidant system in cyanobacteria, non-enzymatic antioxidants, such as carotenoids, are considered good candidates for coping with oxidative stress, particularly light stress, and pharmaceutical therapeutic applications. A significant amount of carotenoid accumulation has been recently improved by genetic engineering. In this study, to achieve higher carotenoid production with higher antioxidant activity, we successfully constructed five Synechocystis sp. PCC 6803 strains overexpressing (OX) native genes related to the carotenoids biosynthetic pathway, including OX_CrtB, OX_CrtP, OX_CrtQ, OX_CrtO, and OX_CrtR. All of the engineered strains maintained a significant quantity of myxoxanthophyll, while increasing zeaxanthin and echinenone accumulation. In addition, higher components of zeaxanthin and echinenone were noted in all OX strains, ranging from 14 to 19% and from 17 to 22%, respectively. It is worth noting that the enhanced echinenone component responded to low light conditions, while the increased β-carotene component contributed to a high light stress response. According to the higher antioxidant activity of all OX strains, the carotenoid extracts presented lower IC₅₀ in lung cancer cell lines H460 and A549, with values less than 157 and 139 µg/mL, respectively, when compared with those of WTc, particularly OX_CrtR and OX_CrtQ. A higher proportion of zeaxanthin and β-carotene in OX_CrtR and OX_CrtQ, respectively, may considerably contribute to the ability to treat lung cancer cells with antiproliferative and cytotoxic effects.

Temperature-induced zeaxanthin overproduction in Synechococcus elongatus PCC 7942

The exogenous crtZ gene from Brevundimonas sp. SD212, coding for a 3,3' β-car hydroxylase, was expressed in Synechococcus elongatus PCC 7942 under the control of a temperature-inducible promoter in an attempt to engineer the carotenoid metabolic pathway, to increase the content of zeaxanthin and its further hydroxylated derivatives caloxanthin and nostoxanthin. These molecules are of particular interest due to their renowned antioxidant properties. Cultivation of the engineered strain S7942Z-Ti at 35 °C, a temperature which is well tolerated by the wild-type strain and at which the inducible expression system is activated, led to a significant redistribution of the relative carotenoid content. β-Carotene decreased to about 10% of the pool that is an excess of a threefold decrease with respect to the control, and concomitantly, zeaxanthin became the dominant carotenoid accounting for about half of the pool. As a consequence, zeaxanthin and its derivatives caloxanthin and nostoxanthin collectively accounted for about 90% of the accumulated carotenoids. Yet, upon induction of CrtZ expression at 35 °C the S7942Z-Ti strain displayed a substantial growth impairment accompanied, initially, by a relative loss of carotenoids and successively by the appearance of chlorophyll degradation products which can be interpreted as markers of cellular stress. These observations suggest a limit to the exploitation of Synechococcus elongatus PCC 7942 for biotechnological purposes aimed at increasing the production of hydroxylated carotenoids.

Engineering astaxanthin accumulation reduces photoinhibition and increases biomass productivity under high light in Chlamydomonas reinhardtii

Background

Astaxanthin is a highly valuable ketocarotenoid with strong antioxidative activity and is natively accumulated upon environmental stress exposure in selected microorganisms. Green microalgae are photosynthetic, unicellular organisms cultivated in artificial systems to produce biomass and industrially relevant bioproducts. While light is required for photosynthesis, fueling carbon fixation processes, application of high irradiance causes photoinhibition and limits biomass productivity.

Results

Here, we demonstrate that engineered astaxanthin accumulation in the green alga Chlamydomonas reinhardtii conferred high light tolerance, reduced photoinhibition and improved biomass productivity at high irradiances, likely due to strong antioxidant properties of constitutively accumulating astaxanthin. In competitive co-cultivation experiments, astaxanthin-rich Chlamydomonas reinhardtii outcompeted its corresponding parental background strain and even the fast-growing green alga Chlorella vulgaris.

Conclusions

Metabolic engineering inducing astaxanthin and ketocarotenoids accumulation caused improved high light tolerance and increased biomass productivity in the model species for microalgae Chlamydomonas reinhardtii. Thus, engineering microalgal pigment composition represents a powerful strategy to improve biomass productivities in customized photobioreactors setups. Moreover, engineered astaxanthin accumulation in selected strains could be proposed as a novel strategy to outperform growth of other competing microalgal strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02173-3.

Life cycle and functional genomics of the unicellular red alga Galdieria for elucidating algal and plant evolution and industrial use

Sexual reproduction has not been observed in unicellular red algae and Glaucophyceae, early branching groups in Archaeplastida, in which red algae and Viridiplantae independently evolved multicellular sexual life cycles. The finding of sexual reproduction in the unicellular red alga Galdieria provides information on the missing link of life cycle evolution in Archaeplastida. In addition, the metabolic plasticity, the polyextremophilic features, a relatively small genome, transcriptome data for the diploid and haploid, and the genetic modification tools developed here provide a useful platform for understanding the evolution of Archaeplastida, photosynthesis, metabolism, and environmental adaptation. For biotechnological use of the information and tools of Galdieria, the newly found cell wall–less haploid makes cell disruption less energy/cost intensive than the cell-walled diploid.

Sexual reproduction is widespread in eukaryotes; however, only asexual reproduction has been observed in unicellular red algae, including Galdieria, which branched early in Archaeplastida. Galdieria possesses a small genome; it is polyextremophile, grows either photoautotrophically, mixotrophically, or heterotrophically, and is being developed as an industrial source of vitamins and pigments because of its high biomass productivity. Here, we show that Galdieria exhibits a sexual life cycle, alternating between cell-walled diploid and cell wall–less haploid, and that both phases can proliferate asexually. The haploid can move over surfaces and undergo self-diploidization or generate heterozygous diploids through mating. Further, we prepared the whole genome and a comparative transcriptome dataset between the diploid and haploid and developed genetic tools for the stable gene expression, gene disruption, and selectable marker recycling system using the cell wall–less haploid. The BELL/KNOX and MADS-box transcription factors, which function in haploid-to-diploid transition and development in plants, are specifically expressed in the haploid and diploid, respectively, and are involved in the haploid-to-diploid transition in Galdieria, providing information on the missing link of the sexual life cycle evolution in Archaeplastida. Four actin genes are differently involved in motility of the haploid and cytokinesis in the diploid, both of which are myosin independent and likely reflect ancestral roles of actin. We have also generated photosynthesis-deficient mutants, such as blue-colored cells, which were depleted in chlorophyll and carotenoids, for industrial pigment production. These features of Galdieria facilitate the understanding of the evolution of algae and plants and the industrial use of microalgae.

Suppression of the Lycopene Cyclase Gene Causes Downregulation of Ascorbate Peroxidase Activity and Decreased Glutathione Pool Size, Leading to H2O2 Accumulation in Euglena gracilis

Carotenoids are photosynthetic pigments and hydrophobic antioxidants that are necessary for the survival of photosynthetic organisms, including the microalga Euglena gracilis. In the present study, we identified an uncharacterized gene encoding the E. gracilis β-carotene synthetic enzyme lycopene cyclase (EgLCY) and discovered a relationship between EgLCY-mediated carotenoid synthesis and the reactive oxygen species (ROS) scavenging system ascorbate-glutathione cycle. The EgLCY cDNA sequence was obtained via homology searching E. gracilis transcriptome data. An enzyme assay using Escherichia coli demonstrated that EgLCY converts lycopene to β-carotene. E. gracilis treated with EgLCY double-stranded RNA (dsRNA) produced colorless cells with hypertrophic appearance, inhibited growth, and marked decrease in carotenoid and chlorophyll content, suggesting that EgLCY is essential for the synthesis of β-carotene and downstream carotenoids, which are abundant and physiologically functional. In EgLCY dsRNA-treated cells, the ascorbate-glutathione cycle, composed of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDAR), and glutathione reductase (GR), was unusually modulated; APX and GR activities significantly decreased, whereas DHAR and MDAR activities increased. Ascorbate content was significantly increased and glutathione content significantly decreased in EgLCY dsRNA-treated cells and was correlated with their recycling enzyme activities. Fluorescent imaging demonstrated that EgLCY dsRNA-treated cells accumulated higher levels of H2O2 compared to wild-type cells. Taken together, this study revealed that EgLCY-mediated synthesis of β-carotene and downstream carotenoid species upregulates APX activity and increases glutathione pool size for H2O2 scavenging. Our study suggests a possible relationship between carotenoid synthesis and the ascorbate-glutathione cycle for ROS scavenging in E. gracilis.

Drought and Elevated CO2 Impacts Photosynthesis and Biochemicals of Basil (Ocimum basilicum L.)

Drought-induced reduction in crop growth and productivity can be compensated by increasing atmospheric carbon dioxide (CO2), a significant contributor to climate change. Drought stress (DS) affects crops worldwide due to dwindling water resources and irregular rainfall patterns. The experiment was set up under a randomized complete block design within a three-by-two factorial arrangement. Six SPAR chambers represent three blocks (10 replications each), where each chamber has 30 pots in three rows. Each chamber was maintained with 30/22 (day/night) °C temperature, with either ambient (aCO2; 420 ppm) or elevated CO2 (eCO2; 720 ppm) concentrations. This experiment was designed to address the impact of DS on the physiological and biochemical attributes and study how the eCO2 helps alleviate the adversity of DS in basil. The study demonstrated that DS + eCO2 application highly accelerated the decrease in all forms of carotene and xanthophylls. eCO2 positively influenced and increased anthocyanin (Antho) and chlorophyll (LChl). eCO2 supplementation increased LChl content in basil under DS. Furthermore, DS significantly impeded the photosynthetic system in plants by decreasing CO2 availability and causing stomatal closure. Although eCO2 did not increase net photosynthesis (Pn) activity, it decreased stomatal conductance (gs) and leaf transpiration rate (E) under DS, showing that eCO2 can improve plant water use efficiency by lowering E and gs. Peroxidase and ascorbate activity were higher due to the eCO2 supply to acclimate the basil under the DS condition. This study suggests that the combination of eCO2 during DS positively impacts basil’s photosynthetic parameters and biochemical traits than aCO2.

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

The genes crucial to carotenoid metabolism under elevated CO2 levels in carrot (Daucus carota L.)

The CO₂ saturation point can reach as high as 1819 μmol· mol^-1 in carrot (Daucus carota L.). In recent years, carrot has been cultivated in out-of-season greenhouses, but the molecular mechanism of CO₂ enrichment has been ignored, and this is a missed opportunity to gain a comprehensive understanding of this important process. In this study, it was found that CO₂ enrichment increased the aboveground and belowground biomasses and greatly increased the carotenoid contents. Twenty genes related to carotenoids were discovered in 482 differentially expressed genes (DEGs) through RNA sequencing (RNA-Seq.). These genes were involved in either carotenoid biosynthesis or the composition of the photosystem membrane proteins, most of which were upregulated. We suspected that these genes were directly related to quality improvement and increases in biomass under CO₂ enrichment in carrot. As such, β-carotene hydroxylase activity in carotenoid metabolism and the expression levels of coded genes were determined and analysed, and the results were consistent with the observed change in carotenoid content. These results illustrate the molecular mechanism by which the increase in carotenoid content after CO₂ enrichment leads to the improvement of quality and biological yield. Our findings have important theoretical and practical significance.

Effect of Long-Term of He-Ne Laser Light Irradiation on Selected Physiological Processes of Triticale

In agriculture, the bio-stimulating properties of laser light increase the yielding capacity of crop species. The experiment aimed to determine the pre-sowing effect of irradiation time with laser He-Ne red light of triticale grains (×Triticosecale Wittm. ex A.Camus) on germination and selected morphological and physiological parameters of seedlings and plants grown from them. The highest values of germination indexes were found for grains irradiated with laser for 3 h. In relation to the control, the elongation growth of seedlings was stimulated in grains irradiated with light for 3 h and inhibited for 24 h. The values of the fresh and dry mass of seedlings changed depending on the exposure time. He-Ne light did not significantly affect the degree of destabilization of seedling cell membranes. Biometric analysis of plants grown from irradiated grains showed different reactions of triticale organs to the irradiation time. Red light clearly stimulated the increase in the value of organ mass. Chlorophyll content in leaves was higher in plants grown from grains irradiated for 3 h. Photosynthetic activity did not change significantly relative to the control. The fluorescence emission indexes were mostly lower than in the control, which indicated a positive effect of the laser. In general, the red light of the laser stimulated the morphology and physiology of seedlings and plants, although, for some features, long exposure to red light caused a slight reduction effect.

Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteria Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyll a complexes (exciplexes) was monitored by measuring the electrochromic shift of β-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I from T. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P₇₀₀⁺A₁^- at 40 ps was limited by energy transfer from LWC to the primary donor P₇₀₀ and accompanied by carotenoid bleach at 498 nm. In PS I from Synechocystis 6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state P_B⁺Chl_2B^-, where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl_2B. The following decrease of carotenoid signal at ~ 5 ps was ascribed to electron transfer to the more distant molecule Chl_3B. The reduction of phylloquinone in the sites A_1A and A_1B was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state P_B⁺A_1A^-.

Photosynthesis without β-carotene

Carotenoids are essential in oxygenic photosynthesis: they stabilize the pigment-protein complexes, are active in harvesting sunlight and in photoprotection. In plants, they are present as carotenes and their oxygenated derivatives, xanthophylls. While mutant plants lacking xanthophylls are capable of photoautotrophic growth, no plants without carotenes in their photosystems have been reported so far, which has led to the common opinion that carotenes are essential for photosynthesis. Here, we report the first plant that grows photoautotrophically in the absence of carotenes: a tobacco plant containing only the xanthophyll astaxanthin. Surprisingly, both photosystems are fully functional despite their carotenoid-binding sites being occupied by astaxanthin instead of β-carotene or remaining empty (i.e. are not occupied by carotenoids). These plants display non-photochemical quenching, despite the absence of both zeaxanthin and lutein and show that tobacco can regulate the ratio between the two photosystems in a very large dynamic range to optimize electron transport.

Chloroplast Sec14-like 1 (CPSFL1) is essential for normal chloroplast development and affects carotenoid accumulation in Chlamydomonas

Carotenoids are essential molecules in oxygenic photoautotrophs, and they fulfill essential requirements for human and animal nutrition. How carotenoid accumulation is regulated in the chloroplast, a cyanobacterium-derived organelle, remains poorly understood, despite significant advancements in identifying enzymes of the carotenoid biosynthetic pathway. This study identifies a role of chloroplast Sec14-like 1 (CPSFL1), a CRAL-TRIO protein of eukaryotic origin, in modulation of carotenoid biosynthesis and accumulation in the chloroplast. The CPSFL1 protein represents an isoprenoid- and carotenoid-binding protein that associates with membranes through interactions with the phospholipid phosphatidic acid. These findings have implications for understanding carotenoid biosynthesis and optimizing algal carotenoid nutritional quality.

Plastid isoprenoid-derived carotenoids serve essential roles in chloroplast development and photosynthesis. Although nearly all enzymes that participate in the biosynthesis of carotenoids in plants have been identified, the complement of auxiliary proteins that regulate synthesis, transport, sequestration, and degradation of these molecules and their isoprenoid precursors have not been fully described. To identify such proteins that are necessary for the optimal functioning of oxygenic photosynthesis, we screened a large collection of nonphotosynthetic (acetate-requiring) DNA insertional mutants of Chlamydomonas reinhardtii and isolated cpsfl1. The cpsfl1 mutant is extremely light-sensitive and susceptible to photoinhibition and photobleaching. The CPSFL1 gene encodes a CRAL-TRIO hydrophobic ligand-binding (Sec14) domain protein. Proteins containing this domain are limited to eukaryotes, but some may have been retargeted to function in organelles of endosymbiotic origin. The cpsfl1 mutant showed decreased accumulation of plastidial isoprenoid-derived pigments, especially carotenoids, and whole-cell focused ion-beam scanning-electron microscopy revealed a deficiency of carotenoid-rich chloroplast structures (e.g., eyespot and plastoglobules). The low carotenoid content resulted from impaired biosynthesis at a step prior to phytoene, the committed precursor to carotenoids. The CPSFL1 protein bound phytoene and β-carotene when expressed in Escherichia coli and phosphatidic acid in vitro. We suggest that CPSFL1 is involved in the regulation of phytoene synthesis and carotenoid transport and thereby modulates carotenoid accumulation in the chloroplast.

Genomic Survey of Salt Acclimation-Related Genes in the Halophilic Cyanobacterium Euhalothece sp. Z-M001

Like other halophilic cyanobacterial genomes, the de novo-assembled genome of Euhalothece sp. Z-M001 lacks genes encoding keto-carotenoid biosynthesis enzymes, despite the presence of genes encoding carotenoid-binding proteins (CBPs). Consistent with this, HPLC analysis of carotenoids identified β-carotene and zeaxanthin as the dominant carotenoids. CBPs coexpressed with the zeaxanthin biosynthesis gene increased the survival rates of Escherichia coli strains by preventing antibiotic-induced accumulation of reactive oxygen species (ROS). RNA-seq analysis of Euhalothece revealed that among various salt resistance-related genes, those encoding the Na⁺ transporting multiple resistance and pH adaptation (Mrp) systems, glycine betaine biosynthesis enzymes, exopolysaccharide metabolic enzymes, and CBPs were highly upregulated, suggesting their importance in hypersaline habitats. During the early phase of salt deprivation, the amounts of β-carotene and zeaxanthin showed a negative correlation with ROS content. Overall, we propose that in some halophilic cyanobacteria, β-carotene and zeaxanthin, rather than keto-carotenoids, serve as the major chromophores for CBPs, which in turn act as effective antioxidants.

UV-B Physiological Changes Under Conditions of Distress and Eustress in Sweet Basil

UV-B radiation has been previously reported to induce protective or deleterious effects on plants depending on the UV-B irradiation doses. To elucidate how these contrasting events are physiologically coordinated, we exposed sweet basil plants to two UV-B doses: low (8.5 kJ m-2 day-1, 30 min exposure) and high (68 kJ m-2 day-1, 4 h exposure), with the plants given both doses once continuously in a single day. Physiological tests during and after both UV-B exposures were performed by comparing the stress-induced damage and adverse effects on photosynthetic activity, the concentration and composition of photosynthetic and non-photosynthetic pigments, and stress-related hormones biosynthesis in basil plants. Our results showed that upon receiving a high UV-B dose, a severe inactivation of oxygen evolving complex (OEC) activity at the PSII donor side and irreversible PSII photodamage caused primarily by limitation of the acceptor side occurred, which overloaded protective mechanisms and finally led to the death of the plants. In contrast, low UV-B levels did not induce any signs of UV-B stress injuries. The OEC partial limitation and the inactivation of the electron transport chain allowed the activation of photoprotective mechanisms, avoiding irreversible damage to PSII. Overall results indicate the importance of a specific response mechanisms regulating photoprotection vs irreversible photoinhibition in basil that were modulated depending on the UV-B doses.

Non-endogenous ketocarotenoid accumulation in engineered Synechocystis sp. PCC 6803

The cyanobacterium Synechocystis sp. PCC 6803 is a model species commonly employed for biotechnological applications. It is naturally able to accumulate zeaxanthin (Zea) and echinenone (Ech), but not astaxanthin (Asx), which is the highest value carotenoid produced by microalgae, with a wide range of applications in pharmaceutical, cosmetics, food and feed industries. With the aim of finding an alternative and sustainable biological source for the production of Asx and other valuable hydroxylated and ketolated intermediates, the carotenoid biosynthetic pathway of Synechocystis sp. PCC 6803 has been engineered by introducing the 4,4' β-carotene oxygenase (CrtW) and 3,3' β-carotene hydroxylase (CrtZ) genes from Brevundimonas sp. SD-212 under the control of a temperature-inducible promoter. The expression of exogenous CrtZ led to an increased accumulation of Zea at the expense of Ech, while the expression of exogenous CrtW promoted the production of non-endogenous canthaxanthin and an increase in the Ech content with a concomitant strong reduction of β-carotene (β-car). When both Brevundimonas sp. SD-212 genes were coexpressed, significant amounts of non-endogenous Asx were obtained accompanied by a strong decrease in β-car content. Asx accumulation was higher (approximately 50% of total carotenoids) when CrtZ was cloned upstream of CrtW, but still significant (approximately 30%) when the position of genes was inverted. Therefore, the engineered strains constitute a useful tool for investigating the ketocarotenoid biosynthetic pathway in cyanobacteria and an excellent starting point for further optimisation and industrial exploitation of these organisms for the production of added-value compounds.

Leaf-age dependent response of carotenoid accumulation to elevated CO2 in Arabidopsis

Carotenoids contribute to photosynthesis, photoprotection, phytohormone and apocarotenoid biosynthesis in plants. Carotenoid-derived metabolites control plant growth, development and signalling processes and their accumulation can depend upon changes in the environment. Elevated carbon dioxide (eCO₂) often enhances carbon assimilation, early growth patterns and overall plant biomass, and may increase carotenoid accumulation due to higher levels of precursors from isoprenoid biosynthesis. Variable effects of eCO₂ on carotenoid accumulation in leaves have been observed for different plant species. Here, we determined whether the variable response of carotenoids to eCO₂ was potentially a function of leaf age and the impact of eCO₂ on leaf development by growing Arabidopsis in ambient CO₂ (400 ppm) and eCO₂ (800 ppm). eCO₂ increased plant leaf number, rosette area, biomass, seed yield and net photosynthesis. In addition, eCO₂ increased carotenoid content by 10-20% in younger emerging leaves, but not in older mature leaves. Older leaves contained approximately 60% less total carotenoids compared to younger leaves. The age-dependent effect on carotenoid content was observed for cotyledon, juvenile and adult phase leaves. We conclude that younger leaves utilize additional carbon from enhanced photosynthesis in eCO₂ to increase carotenoid content, yet older leaves have less capacity to store additional carbon into carotenoids.

Response of Chlorophyll, Carotenoid and SPAD-502 Measurement to Salinity and Nutrient Stress in Wheat (Triticum aestivum L.)

Abiotic stress can alter key physiological constituents and functions in green plants. Improving the capacity to monitor this response in a non-destructive manner is of considerable interest, as it would offer a direct means of initiating timely corrective action. Given the vital role that plant pigments play in the photosynthetic process and general plant physiological condition, their accurate estimation would provide a means to monitor plant health and indirectly determine stress response. The aim of this work is to evaluate the response of leaf chlorophyll and carotenoid (Ct) content in wheat (Triticum aestivum L.) to changes in varying application levels of soil salinity and fertilizer applied over a complete growth cycle. The study also seeks to establish and analyze relationships between measurements from a SPAD-502 instrument and the leaf pigments, as extracted at the anthesis stage. A greenhouse pot experiment was conducted in triplicate by employing distinct treatments of both soil salinity and fertilizer dose at three levels. Results showed that higher doses of fertilizer increased the content of leaf pigments across all levels of soil salinity. Likewise, increasing the level of soil salinity significantly increased the chlorophyll and Ct content per leaf area at all levels of applied fertilizer. However, as an adaptation process and defense mechanism under salinity stress, leaves were found to be thicker and narrower. Thus, on a per-plant basis, increasing salinity significantly reduced the chlorophyll (Chlt) and Ct produced under each fertilizer treatment. In addition, interaction effects of soil salinity and fertilizer application on the photosynthetic pigment content were found to be significant, as the higher amounts of fertilizer augmented the detrimental effects of salinity. A strong positive (R2 = 0.93) and statistically significant (p < 0.001) relationship between SPAD-502 values and Chlt and between SPAD-502 values and Ct content (R2 = 0.85) was determined based on a large (n = 277) dataset. We demonstrate that the SPAD-502 readings and plant photosynthetic pigment content per-leaf area are profoundly affected by salinity and nutrient stress, but that the general form of their relationship remains largely unaffected by the stress. As such, a generalized regression model can be used for Chlt and Ct estimation, even across a range of salinity and fertilizer gradients.

Photoprotection vs. Photoinhibition of Photosystem II in Transplastomic Lettuce (Lactuca sativa) Dominantly Accumulating Astaxanthin

Transplastomic (chloroplast genome-modified; CGM) lettuce that dominantly accumulates astaxanthin grows similarly to a non-transgenic control with almost no accumulation of naturally occurring photosynthetic carotenoids. In this study, we evaluated the activity and assembly of PSII in CGM lettuce. The maximum quantum yield of PSII in CGM lettuce was <0.6; however, the quantum yield of PSII was comparable with that in control leaves under higher light intensity. CGM lettuce showed a lower ability to induce non-photochemical quenching (NPQ) than the control under various light intensities. The fraction of slowly recovering NPQ in CGM lettuce, which is considered to be photoinhibitory quenching (qI), was less than half that of the control. In fact, 1O2 generation was lower in CGM than in control leaves under high light intensity. CGM lettuce contained less PSII, accumulated mostly as a monomer in thylakoid membranes. The PSII monomers purified from the CGM thylakoids bound echinenone and canthaxanthin in addition to β-carotene, suggesting that a shortage of β-carotene and/or the binding of carbonyl carotenoids would interfere with the photophysical function as well as normal assembly of PSII. In contrast, high accumulation of astaxanthin and other carbonyl carotenoids was found within the thylakoid membranes. This finding would be associated with the suppression of photo-oxidative stress in the thylakoid membranes. Our observation suggests the importance of a specific balance between photoprotection and photoinhibition that can support normal photosynthesis in CGM lettuce producing astaxanthin.

Effects of marine actinomycete on the removal of a toxicity alga Phaeocystis globose in eutrophication waters

Phaeocystis globosa blooms in eutrophication waters can cause severely damage in marine ecosystem and consequently influence human activities. This study investigated the effect and role of an algicidal actinomycete (Streptomyces sp. JS01) on the elimination process of P. globosa. JS01 supernatant could alter algal cell membrane permeability in 4 h when analyzed with flow cytometry. Reactive oxygen species (ROS) levels were 7.2 times higher than that at 0 h following exposure to JS01 supernatant for 8 h, which indicated that algal cells suffered from oxidative damage. The Fv/Fm value which could reflect photosystem II (PS II) electron flow status also decreased. Real-time PCR showed that the expression of the photosynthesis related genes psbA and rbcS were suppressed by JS01 supernatant, which might induce damage to PS II. Our results demonstrated that JS01 supernatant can change algal membrane permeability in a short time and then affect photosynthesis process, which might block the PS II electron transport chain to produce excessive ROS. This experiment demonstrated that Streptomyces sp. JS01 could eliminate harmful algae in marine waters efficiently and may be function as a harmful algal bloom controller material.

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Comprehensive insights into the response of Alexandrium tamarense to algicidal component secreted by a marine bacterium

Harmful algal blooms occur throughout the world, threatening human health, and destroying marine ecosystems. Alexandrium tamarense is a globally distributed and notoriously toxic dinoflagellate that is responsible for most paralytic shellfish poisoning incidents. The culture supernatant of the marine algicidal bacterium BS02 showed potent algicidal effects on A. tamarense ATGD98-006. In this study, we investigated the effects of this supernatant on A. tamarense at physiological and biochemical levels to elucidate the mechanism involved in the inhibition of algal growth by the supernatant of the strain BS02. Reactive oxygen species (ROS) levels increased following exposure to the BS02 supernatant, indicating that the algal cells had suffered from oxidative damage. The levels of cellular pigments, including chlorophyll a and carotenoids, were significantly decreased, which indicated that the accumulation of ROS destroyed pigment synthesis. The decline of the maximum photochemical quantum yield (Fv/Fm) and relative electron transport rate (rETR) suggested that the photosynthesis systems of algal cells were attacked by the BS02 supernatant. To eliminate the ROS, the activities of antioxidant enzymes, including superoxide dismutase (SOD) and catalase (CAT), increased significantly within a short period of time. Real-time PCR revealed changes in the transcript abundances of two target photosynthesis-related genes (psbA and psbD) and two target respiration-related genes (cob and cox). The transcription of the respiration-related genes was significantly inhibited by the treatments, which indicated that the respiratory system was disturbed. Our results demonstrate that the BS02 supernatant can affect the photosynthesis process and might block the PS II electron transport chain, leading to the production of excessive ROS. The increased ROS can further destroy membrane integrity and pigments, ultimately inducing algal cell death.

Carotenoid triplet states in photosystem II: coupling with low-energy states of the core complex

The photo-excited triplet states of carotenoids, sensitised by triplet-triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII-LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet ((3)Car) populations were identified in PSII-LHCII, and four in the PSII core complex. Thus, four (3)Car populations are attributed to β-carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII-LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D|=0.0370-0.0373 cm(-1)/|E|=0.00373-0.00375 cm(-1) and |D|=0.0381-0.0385 cm(-1)/|E|=0.00393-0.00389 cm(-1), are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth (3)Car component is observed only in the PSII-LHCII supercomplex and is then assigned to the peripheral light harvesting system.

Photosynthesis-related quantities for education and modeling

A quantitative understanding of the photosynthetic machinery depends largely on quantities, such as concentrations, sizes, absorption wavelengths, redox potentials, and rate constants. The present contribution is a collection of numbers and quantities related mainly to photosynthesis in higher plants. All numbers are taken directly from a literature or database source and the corresponding reference is provided. The numerical values, presented in this paper, provide ranges of values, obtained in specific experiments for specific organisms. However, the presented numbers can be useful for understanding the principles of structure and function of photosynthetic machinery and for guidance of future research.

Understanding chloroplast biogenesis using second-site suppressors of immutans and var2

Chloroplast biogenesis is an essential light-dependent process involving the differentiation of photosynthetically competent chloroplasts from precursors that include undifferentiated proplastids in leaf meristems, as well as etioplasts in dark-grown seedlings. The mechanisms that govern these developmental processes are poorly understood, but entail the coordinated expression of nuclear and plastid genes. This coordination is achieved, in part, by signals generated in response to the metabolic and developmental state of the plastid that regulate the transcription of nuclear genes for photosynthetic proteins (retrograde signaling). Variegation mutants are powerful tools to understand pathways of chloroplast biogenesis, and over the years our lab has focused on immutans (im) and variegated2 (var2), two nuclear gene-induced variegations of Arabidopsis. im and var2 are among the best-characterized chloroplast biogenesis mutants, and they define the genes for plastid terminal oxidase (PTOX) and the AtFtsH2 subunit of the thylakoid FtsH metalloprotease complex, respectively. To gain insight into the function of these proteins, forward and reverse genetic approaches have been used to identify second-site suppressors of im and var2 that replace or bypass the need for PTOX and AtFtsH2 during chloroplast development. In this review, we provide a brief update of im and var2 and the functions of PTOX and AtFtsH2. We then summarize information about second-site suppressors of im and var2 that have been identified to date, and describe how they have provided insight into mechanisms of photosynthesis and pathways of chloroplast development.

A reaction center-dependent photoprotection mechanism in a highly robust photosystem II from an extremophilic red alga, Cyanidioschyzon merolae

Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extremely low pH levels and moderately high temperatures. The photosynthetic apparatus of the red alga Cyanidioschyzon merolae represents an intermediate type between cyanobacteria and higher plants, suggesting that this alga may provide the evolutionary link between prokaryotic and eukaryotic phototrophs. Although we now have a detailed structural model of photosystem II (PSII) from cyanobacteria at an atomic resolution, no corresponding structure of the eukaryotic PSII complex has been published to date. Here we report the isolation and characterization of a highly active and robust dimeric PSII complex from C. merolae. We show that this complex is highly stable across a range of extreme light, temperature, and pH conditions. By measuring fluorescence quenching properties of the isolated C. merolae PSII complex, we provide the first direct evidence of pH-dependent non-photochemical quenching in the red algal PSII reaction center. This type of quenching, together with high zeaxanthin content, appears to underlie photoprotection mechanisms that are efficiently employed by this robust natural water-splitting complex under excess irradiance. In order to provide structural details of this eukaryotic form of PSII, we have employed electron microscopy and single particle analyses to obtain a 17 Å map of the C. merolae PSII dimer in which we locate the position of the protein mass corresponding to the additional extrinsic protein stabilizing the oxygen-evolving complex, PsbQ'. We conclude that this lumenal subunit is present in the vicinity of the CP43 protein, close to the membrane plane.