Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of Light and Heat Stresses are Closely Linked to Membrane Fluidity of the Thylakoids

Exogenous ascorbic acid induces systemic heat stress tolerance in tomato seedlings: transcriptional regulation mechanism

The current study was devoted to assessing the impact of exogenous ascorbic acid (AsA) in inducing systemic thermotolerance against acute heat stress in tomato (Solanum lycopersicum) seedlings. There were four treatment groups including untreated control (CK), heat-stressed tomato (HS: exposure to 40 °C for 8 h), and treated with ascorbic acid (0.5 mM AsA), and the last group includes both the exogenous application of ascorbic acid and heat stress (AsA + HS). The HS led to leaf curling and mild wilting while plants treated with AsA displayed similar phenotype with control plants, approving that AsA eliminated the injurious effects of the heat stress. The oxidative damage to cell components was confirmed by higher levels of hydrogen peroxide, lipid peroxidation, electrolyte leakage, total oxidant status, and oxidative stress index. Moreover, acute heat stress significantly reduced the photosynthetic pigment contents, and nutrient contents in tomato seedling leaves. In contrast, ascorbic acid postulated a priming effect on tomato roots and, substantially, alleviated heat stress effects on seedlings through reducing the oxidative damage and increasing the contents of ascorbic acid, proline, photosynthetic pigments, and upregulation of heat shock proteins in leaves. Ascorbic acid seems to be a key signaling molecule which enhanced the thermotolerance of tomato plants.

Lipid Dependence of Xanthophyll Cycling in Higher Plants and Algae

The xanthophyll cycles of higher plants and algae represent an important photoprotection mechanism. Two main xanthophyll cycles are known, the violaxanthin cycle of higher plants, green and brown algae and the diadinoxanthin cycle of Bacillariophyceae, Xanthophyceae, Haptophyceae, and Dinophyceae. The forward reaction of the xanthophyll cycles consists of the enzymatic de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin or diadinoxanthin to diatoxanthin during periods of high light illumination. It is catalyzed by the enzymes violaxanthin or diadinoxanthin de-epoxidase. During low light or darkness the back reaction of the cycle, which is catalyzed by the enzymes zeaxanthin or diatoxanthin epoxidase, restores the epoxidized xanthophylls by a re-introduction of the epoxy groups. The de-epoxidation reaction takes place in the lipid phase of the thylakoid membrane and thus, depends on the nature, three dimensional structure and function of the thylakoid lipids. As the xanthophyll cycle pigments are usually associated with the photosynthetic light-harvesting proteins, structural re-arrangements of the proteins and changes in the protein-lipid interactions play an additional role for the operation of the xanthophyll cycles. In the present review we give an introduction to the lipid and fatty acid composition of thylakoid membranes of higher plants and algae. We introduce the readers to the reaction sequences, enzymes and function of the different xanthophyll cycles. The main focus of the review lies on the lipid dependence of xanthophyll cycling. We summarize the current knowledge about the role of lipids in the solubilization of xanthophyll cycle pigments. We address the importance of the three-dimensional lipid structures for the enzymatic xanthophyll conversion, with a special focus on non-bilayer lipid phases which are formed by the main thylakoid membrane lipid monogalactosyldiacylglycerol. We additionally describe how lipids and light-harvesting complexes interact in the thylakoid membrane and how these interactions can affect the structure of the thylakoids. In a dedicated chapter we offer a short overview of current membrane models, including the concept of membrane domains. We then use these concepts to present a model of the operative xanthophyll cycle as a transient thylakoid membrane domain which is formed during high light illumination of plants or algal cells.

Influence of Brevibacterium linens RS16 on foliage photosynthetic and volatile emission characteristics upon heat stress in Eucalyptus grandis

Heat stress induces secondary metabolic changes in plants, channeling photosynthetic carbon and energy, away from primary metabolic processes, including, growth. Use of ACC (1-aminocyclopropane-1-carboxylate) deaminase containing plant growth promoting bacteria (PGPB) in conferring heat resistance in plants and the role of PGPB, in altering net carbon assimilation, constitutive and stress volatile emissions has not been studied yet. We exposed leaves of Eucalyptus grandis inoculated and non-inoculated with PGPB Brevibacterium linens RS16 to two levels of heat stress (37 °C and 41 °C for 5 min) and quantified temporal changes in foliage photosynthetic characteristics and volatile emission rates at 0.5 h, day 1 and day 5 after the stress application. Heat stress resulted in immediate reductions in dark-adapted photosystem II (PSII) quantum yield (F_v/F_m), net assimilation rate (A), stomatal conductance to water vapor (g_s), and enhancement of stress volatile emissions, including enhanced emissions of green leaf volatiles (GLV), mono- and sesquiterpenes, light weight oxygenated volatile organic compounds (LOC), geranyl-geranyl diphosphate pathway volatiles (GGDP), saturated aldehydes, and benzenoids, with partial recovery by day 5. Changes in stress-induced volatiles were always less in leaves inoculated with B. linens RS16. However, net assimilation rate was enhanced by bacterial inoculation only in the 37 °C treatment and overall reduction of isoprene emissions was observed in bacterially-treated leaves. Principal component analysis (PCA), correlation analysis and partial least squares discriminant analysis (PLS-DA) indicated that different stress applications influenced specific volatile organic compounds. In addition, changes in the expression analysis of heat shock protein 70 gene (DnaK) gene in B. linens RS16 upon exposure to higher temperatures further indicated that B. linens RS16 has developed its own heat resistance mechanism to survive under higher temperature regimes. Taken together, this study demonstrates that foliar application of ACC deaminase containing PGPB can ameliorate heat stress effects in realistic biological settings.

Effects of nanoTiO2 on tomato plants under different irradiances

In this study, we investigated the physiological and photochemical influences of nanoTiO₂ exposure on tomato plants (Lycopersicum esculentum Mill.). Tomato plants were exposed to 100 mg L^-1 of nanoTiO₂ for 90 days in a hydroponic system. Light irradiances of 135 and 550 μmol_photon m^-2 s^-1 were applied as environmental stressors that could affect uptake of nanoTiO₂. To quantify nanoTiO₂ accumulation in plant bodies and roots, we used transmission electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, and X-ray powder diffraction. Phenotypic and physiological influences such as color change, growth rate, fruit productivity, pigment concentration, and enzyme activity (SOD, CAT, APX) were monitored. We observed numerous effects caused by high irradiance and nanoTiO₂ exposure, such as rapid chlorophyll decrease, increased anthocyanin and carotenoid concentrations, high enzymatic activity, and an approximately eight-fold increase in fruit production. Moreover, light absorption in the nanoTiO₂-treated tomato plants, as measured by a ultraviolet-visible light spectrometer, increased by a factor of approximately 19, likely due to natural pigments that worked as sensitizers, and this resulted in an ∼120% increase in photochemical activities on A, ФPSII, ФCO₂, g_sw, and E.

Transcriptomic and metabolomic profiling of Camellia sinensis L. cv. 'Suchazao' exposed to temperature stresses reveals modification in protein synthesis and photosynthetic and anthocyanin biosynthetic pathways

To determine the mechanisms in tea plants responding to temperature stresses (heat and cold), we examined the global transcriptomic and metabolomic profiles of the tea plant cultivar 'Suchazao' under moderately low temperature stress (ML), severely low temperature stress (SL), moderately high temperature stress (MH) and severely high temperature stress (SH) using RNA-seq and high performance liquid chromatography tandem mass spectrometry/mass spectrometry (HPLC-MS/MS), respectively. The identified differentially expressed genes indicated that the synthesis of stress-resistance protein might be redirected to cope with the temperature stresses. We found that heat shock protein genes Hsp90 and Hsp70 played more critical roles in tea plants in adapting to thermal stress than cold, while late embryogenesis abundant protein genes (LEA) played a greater role under cold than heat stress, more types of zinc finger genes were induced under cold stress as well. In addition, energy metabolisms were inhibited by SH, SL and ML. Furthermore, the mechanisms of anthocyanin synthesis were different under the cold and heat stresses. Indeed, the CsUGT75C1 gene, encoding UDP-glucose:anthocyanin 5-O-glucosyl transferase, was up-regulated in the SL-treated leaves but down-regulated in SH. Metabolomics analysis also showed that anthocyanin monomer levels increased under SL. These results indicate that the tea plants share certain foundational mechanisms to adjust to both cold and heat stresses. They also developed some specific mechanisms for surviving the cold or heat stresses. Our study provides effective information about the different mechanisms tea plants employ in surviving cold and heat stresses, as well as the different mechanisms of anthocyanin synthesis, which could speed up the genetic breeding of heat- and cold-tolerant tea varieties.

'Birth defects' of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water-oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane-inlet mass spectrometry and O₂ -polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these 'PSII birth defects' in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de-etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂ -polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B -inhibitor binding, and thermoluminescence studies indicate that the decline of the high-light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A ^-  → Q_B during de-etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer-range energy transfer.

Different toxicities of nanoscale titanium dioxide particles in the roots and leaves of wheat seedlings

Despite previous studies on exploring the environmental effects of titanium dioxide nanoparticles particle (nTiO₂) on plants, the detailed impacts of nTiO₂on the antioxidant system and photosynthesis of plants is still not well understood.

Assessing plant performance in the Enviratron

BACKGROUND

Assessing the impact of the environment on plant performance requires growing plants under controlled environmental conditions. Plant phenotypes are a product of genotype × environment (G × E), and the Enviratron at Iowa State University is a facility for testing under controlled conditions the effects of the environment on plant growth and development. Crop plants (including maize) can be grown to maturity in the Enviratron, and the performance of plants under different environmental conditions can be monitored 24 h per day, 7 days per week throughout the growth cycle.

RESULTS

The Enviratron is an array of custom-designed plant growth chambers that simulate different environmental conditions coupled with precise sensor-based phenotypic measurements carried out by a robotic rover. The rover has workflow instructions to periodically visit plants growing in the different chambers where it measures various growth and physiological parameters. The rover consists of an unmanned ground vehicle, an industrial robotic arm and an array of sensors including RGB, visible and near infrared (VNIR) hyperspectral, thermal, and time-of-flight (ToF) cameras, laser profilometer and pulse-amplitude modulated (PAM) fluorometer. The sensors are autonomously positioned for detecting leaves in the plant canopy, collecting various physiological measurements based on computer vision algorithms and planning motion via "eye-in-hand" movement control of the robotic arm. In particular, the automated leaf probing function that allows the precise placement of sensor probes on leaf surfaces presents a unique advantage of the Enviratron system over other types of plant phenotyping systems.

CONCLUSIONS

The Enviratron offers a new level of control over plant growth parameters and optimizes positioning and timing of sensor-based phenotypic measurements. Plant phenotypes in the Enviratron are measured in situ-in that the rover takes sensors to the plants rather than moving plants to the sensors.

Pivotal roles of environmental sensing and signaling mechanisms in plant responses to climate change

Climate change reshapes the physiology and development of organisms through phenotypic plasticity, epigenetic modifications, and genetic adaptation. Under evolutionary pressures of the sessile lifestyle, plants possess efficient systems of phenotypic plasticity and acclimation to environmental conditions. Molecular analysis, especially through omics approaches, of these primary lines of environmental adjustment in the context of climate change has revealed the underlying biochemical and physiological mechanisms, thus characterizing the links between phenotypic plasticity and climate change responses. The efficiency of adaptive plasticity under climate change indeed depends on the realization of such biochemical and physiological mechanisms, but the importance of sensing and signaling mechanisms that can integrate perception of environmental cues and transduction into physiological responses is often overlooked. Recent progress opens the possibility of considering plant phenotypic plasticity and responses to climate change through the perspective of environmental sensing and signaling. This review aims to analyze present knowledge on plant sensing and signaling mechanisms and discuss how their structural and functional characteristics lead to resilience or hypersensitivity under conditions of climate change. Plant cells are endowed with arrays of environmental and stress sensors and with internal signals that act as molecular integrators of the multiple constraints of climate change, thus giving rise to potential mechanisms of climate change sensing. Moreover, mechanisms of stress-related information propagation lead to stress memory and acquired stress tolerance that could withstand different scenarios of modifications of stress frequency and intensity. However, optimal functioning of existing sensors, optimal integration of additive constraints and signals, or memory processes can be hampered by conflicting interferences between novel combinations and novel changes in intensity and duration of climate change-related factors. Analysis of these contrasted situations emphasizes the need for future research on the diversity and robustness of plant signaling mechanisms under climate change conditions.

Photosystem II Extrinsic Proteins and Their Putative Role in Abiotic Stress Tolerance in Higher Plants

Abiotic stress remains one of the major challenges in managing and preventing crop loss. Photosystem II (PSII), being the most susceptible component of the photosynthetic machinery, has been studied in great detail over many years. However, much of the emphasis has been placed on intrinsic proteins, particularly with respect to their involvement in the repair of PSII-associated damage. PSII extrinsic proteins include PsbO, PsbP, PsbQ, and PsbR in higher plants, and these are required for oxygen evolution under physiological conditions. Changes in extrinsic protein expression have been reported to either drastically change PSII efficiency or change the PSII repair system. This review discusses the functional role of these proteins in plants and indicates potential areas of further study concerning these proteins.

Extremophiles as a Model of a Natural Ecosystem: Transcriptional Coordination of Genes Reveals Distinct Selective Responses of Plants Under Climate Change Scenarios

The goal of this research was to generate networks of co-expressed genes to explore the genomic responses of Rhizophora mangle L. populations to contrasting environments and to use gene network analysis to investigate their capacity for adaptation in the face of historical and future perturbations and climatic changes. RNA sequencing data were generated for R. mangle samples collected under field conditions from contrasting climate zones in the equatorial and subtropical regions of Brazil. A gene co-expression network was constructed using Pearson's correlation coefficient, showing correlations among 78,364 transcriptionally coordinated genes. Each region exhibited two distinct network profiles; genes correlated with the oxidative stress response showed higher relative expression levels in subtropical samples than in equatorial samples, whereas genes correlated with the hyperosmotic salinity response, heat response and UV response had higher expression levels in the equatorial samples than in the subtropical samples. In total, 992 clusters had enriched ontology terms, which suggests that R. mangle is under higher stress in the equatorial region than in the subtropical region. Increased heat may thus pose a substantial risk to species diversity at the center of its distribution range in the Americas. This study, which was performed using trees in natural field conditions, allowed us to associate the specific responses of genes previously described in controlled environments with their responses to the local habitat where the species occurs. The study reveals the effects of contrasting environments on gene expression in R. mangle, shedding light on the different abiotic variables that may contribute to the genetic divergence previously described for the species through the use of simple sequence repeats (SSRs). These effects may result from two fundamental processes in evolution, namely, phenotypic plasticity and natural selection.

Modeling the effects of Irgarol 1051 on coral using lipidomic methodology for environmental monitoring and assessment

Coral is commonly selected as a bioindicator of detecting a variety of adverse factors such as photosystem II herbicide Irgarol 1051, through measuring pan-type biomarkers. To improve the effectiveness of biomonitoring, omic technologies have recently been applied to model the systemic changes in an organism. Membrane lipids create a dynamic cell structure based on the physiological state, which offers a distinct lipid profile to specifically detect environmental threats and assess the associated health risk. To demonstrate the potential of a lipidomic methodology for biomonitoring, the glycerophosphocholine (GPC) profiles of the coral Seriatopora caliendrum were observed during 3 days of Irgarol (0.1-2.0 μg/L) exposure. The lipid profile variations were modeled based on the Irgarol dose and the coral photoinhibition levels to develop an excellent quantitative model. The predominant changes correlated with the photoinhibition, decreasing the lyso-GPCs and GPCs with lower unsaturated chains and increasing GPCs with highly polyunsaturated chains, can be related to the consequence of blocking the photosynthetic electron flow based on the associated physiological roles. Other dose-specific lipid changes led to the partial exchange of PC(O-16:0/20:5) for PC(16,0/20:5) as a first-line response to counteract the membrane opening caused by Irgarol. Increased levels of the GPCs with 20:4 or 22:6 chains, which can promote mitochondrial functionality, confirmed an elevated respiration level in the coral exposed to Irgarol levels of >0.5 μg/L. Notably, plasmanylcholines with 20:4 or 22:6 chains and phosphatidylcholines with 22:6 or 22:5 chains, which can alter their membrane material properties to mitigate organelle pre-swelling and swelling in different ways, formed in the coral exposed to the 0.5 and 2.0 μg/L Irgarol levels. Such coral adaptations further predict the health risks associated with altered physiological conditions. In this study, the lipidomic methodology is demonstrated as a potential tool for environmental monitoring and assessment.

To defend or to grow: lessons from Arabidopsis C24

The emergence of Arabidopsis as a model species and the availability of genetic and genomic resources have resulted in the identification and detailed characterization of abiotic stress signalling pathways. However, this has led only to limited success in engineering abiotic stress tolerance in crops. This is because there needs to be a deeper understanding of how to combine resistances to a range of stresses with growth and productivity. The natural variation and genomic resources of Arabidopsis thaliana (Arabidopsis) are a great asset to understand the mechanisms of multiple stress tolerances. One natural variant in Arabidopsis is the accession C24, and here we provide an overview of the increasing research interest in this accession. C24 is highlighted as a source of tolerance for multiple abiotic and biotic stresses, and a key accession to understand the basis of basal immunity to infection, high water use efficiency, and water productivity. Multiple biochemical, physiological, and phenological mechanisms have been attributed to these traits in C24, and none of them constrains productivity. Based on the uniqueness of C24, we postulate that the use of variation derived from natural selection in undomesticated species provides opportunities to better understand how complex environmental stress tolerances and resource use efficiency are co-ordinated.

Dynamic interplay between photodamage and photoprotection in photosystem II

Photoinhibition is the light-induced reduction in photosynthetic efficiency and is usually associated with damage to the D1 photosystem II (PSII) reaction centre protein. This damage must either be repaired, through the PSII repair cycle, or prevented in the first place by nonphotochemical quenching (NPQ). Both NPQ and D1 repair contribute to light tolerance because they ensure the long-term maintenance of the highest quantum yield of PSII. However, the relative contribution of each of these processes is yet to be elucidated. The application of a pulse amplitude modulation fluorescence methodology, called protective NPQ, enabled us to evaluate of the protective effectiveness of the processes. Within this study, the contribution of NPQ and D1 repair to the photoprotective capacity of Arabidopsis thaliana was elucidated by using inhibitors and mutants known to affect each process. We conclude that NPQ contributes a greater amount to the maintenance of a high PSII yield than D1 repair under short periods of illumination. This research further supports the role of protective components of NPQ during light fluctuations and the value of protective NPQ and q_Pd as unambiguous fluorescence parameters, as opposed to q_I and F_v /F_m , for quantifying photoinactivation of reaction centre II and light tolerance of photosynthetic organisms.

Fine-Tuning of Photosynthesis Requires CURVATURE THYLAKOID1-Mediated Thylakoid Plasticity

The thylakoid membrane system of higher plant chloroplasts consists of interconnected subdomains of appressed and nonappressed membrane bilayers, known as grana and stroma lamellae, respectively. CURVATURE THYLAKOID1 (CURT1) protein complexes mediate the shape of grana stacks in a dosage-dependent manner and facilitate membrane curvature at the grana margins, the interface between grana and stroma lamellae. Although grana stacks are highly conserved among land plants, the functional relevance of grana stacking remains unclear. Here, we show that inhibiting CURT1-mediated alteration of thylakoid ultrastructure in Arabidopsis (Arabidopsis thaliana) reduces photosynthetic efficiency and plant fitness under adverse, controlled, and natural light conditions. Plants that lack CURT1 show less adjustment of grana diameter, which compromises regulatory mechanisms like the photosystem II repair cycle and state transitions. Interestingly, CURT1A suffices to induce thylakoid membrane curvature in planta and thylakoid hyperbending in plants overexpressing CURT1A. We suggest that CURT1 oligomerization is regulated at the posttranslational level in a light-dependent fashion and that CURT1-mediated thylakoid plasticity plays an important role in fine-tuning photosynthesis and plant fitness during challenging growth conditions.

Physiological and molecular characterisation for high temperature stress in Lens culinaris

In the present study, 11 lentil (Lens culinaris Medik) genotypes including heat tolerant and heat sensitive genotypes identified after a screening of 334 accessions of lentil for traits imparting heat tolerance, were characterised based on physiological traits and molecular markers. Results showed a higher reduction in pollen viability among sensitive genotypes (up to 52.3%) compared with tolerant genotypes (up to 32.4%) at 43°C. Higher photosynthetic electron transport rate was observed among heat tolerant genotypes and two heat tolerant lentil genotypes, IG 4258 (0.43) and IG 3330 (0.38) were having highest Fv/Fm values. However, membrane stability was significantly higher in only one heat tolerant genotype, ILL 10712, indicating that different mechanisms are involved to control heat tolerance in lentil. The molecular characterisation of lentil genotypes with 70 polymorphic SSR and genic markers resulted into distinct clusters in accordance with their heat stress tolerance. A functional marker ISM11257 (intron spanning marker) amplifying an allele of 205bp in size was present only among heat tolerant genotypes, and could be further used in a breeding program to identify heat tolerant lentil genotypes. The findings of this study will contribute to the development of heat tolerant lentil cultivars.

Chlorophyll fluorescence analysis revealed essential roles of FtsH11 protease in regulation of the adaptive responses of photosynthetic systems to high temperature

BACKGROUND

Photosynthetic systems are known to be sensitive to high temperature stress. To maintain a relatively "normal" level of photosynthetic activities, plants employ a variety of adaptive mechanisms in response to environmental temperature fluctuations. Previously, we reported that the chloroplast-targeted AtFtsH11 protease played an essential role for Arabidopsis plants to survive at high temperatures and to maintain normal photosynthetic efficiency at moderately elevated temperature. To investigate the factors contributing to the photosynthetic changes in FtsH11 mutant, we performed detailed chlorophyll fluorescence analyses of dark-adapted mutant plants and compared them to Col-0 WT plants under normal, two moderate high temperatures, and a high light conditions.

RESULTS

We found that mutation of FtsH11 gene caused significant decreases in photosynthetic efficiency of photosystems when environmental temperature raised above optimal. Under moderately high temperatures, the FtsH11 mutant showed significant 1) decreases in electron transfer rates of photosystem II (PSII) and photosystem I (PSI), 2) decreases in photosynthetic capabilities of PSII and PSI, 3) increases in non-photochemical quenching, and a host of other chlorophyll fluorescence parameter changes. We also found that the degrees of these negative changes for utilizing the absorbed light energy for photosynthesis in FtsH11 mutant were correlated with the level and duration of the heat treatments. For plants grown under normal temperature and subjected to the high light treatment, no significant difference in chlorophyll fluorescence parameters was found between the FtsH11 mutant and Col-0 WT plants.

CONCLUSIONS

The results of this study show that AtFtsH11 is essential for normal photosynthetic function under moderately elevated temperatures. The results also suggest that the network mediated by AtFtsH11 protease plays critical roles for maintaining the thermostability and possibly structural integrity of both photosystems under elevated temperatures. Elucidating the underlying mechanisms of FtsH11 protease in photosystems may lead to improvement of photosynthetic efficiency under heat stress conditions, hence, plant productivity.

Thermoacclimation and genome adaptation of the membrane lipidome in marine Synechococcus

The marine cyanobacteria of the genus Synechococcus are important primary producers, displaying a wide latitudinal distribution that is underpinned by diversification into temperature ecotypes. The physiological basis underlying these ecotypes is poorly known. In many organisms, regulation of membrane fluidity is crucial for acclimating to variations in temperature. Here, we reveal the detailed composition of the membrane lipidome of the model strain Synechococcus sp. WH7803 and its response to temperature variation. Unlike freshwater strains, membranes are almost devoid of C18, mainly containing C14 and C16 chains with no more than two unsaturations. In response to cold, we observed a rarely observed process of acyl chain shortening that likely induces membrane thinning, along with specific desaturation activities. Both of these mechanisms likely regulate membrane fluidity, facilitating the maintenance of efficient photosynthetic activity. A comprehensive examination of 53 Synechococcus genomes revealed clade-specific gene sets regulating membrane lipids. In particular, the genes encoding desaturase enzymes, which is a key to the temperature stress response, appeared to be temperature ecotype-specific, with some of them originating from lateral transfers. Our study suggests that regulation of membrane fluidity has been among the important adaptation processes for the colonization of different thermal niches by marine Synechococcus.

Identification of core subunits of photosystem II as action sites of HSP21, which is activated by the GUN5-mediated retrograde pathway in Arabidopsis

Photosystem II (PSII) is the most thermolabile photosynthetic complex. Physiological evidence suggests that the small chloroplast heat-shock protein 21 (HSP21) is involved in plant thermotolerance, but the molecular mechanism of its action remains largely unknown. Here, we have provided genetic and biochemical evidence that HSP21 is activated by the GUN5-dependent retrograde signaling pathway, and stabilizes PSII by directly binding to its core subunits such as D1 and D2 proteins under heat stress. We further demonstrate that the constitutive expression of HSP21 sufficiently rescues the thermosensitive stability of PSII and survival defects of the gun5 mutant with dramatically improving granal stacking under heat stress, indicating that HSP21 is a key chaperone protein in maintaining the integrity of the thylakoid membrane system under heat stress. In line with our interpretation based on several lines of in vitro and in vivo protein-interaction evidence that HSP21 interacts with core subunits of PSII, the kinetics of HSP21 binding to the D1 and D2 proteins was determined by performing an analysis of microscale thermophoresis. Considering the major role of HSP21 in protecting the core subunits of PSII from thermal damage, its heat-responsive activation via the heat-shock transcription factor HsfA2 is critical for the survival of plants under heat stress. Our findings reveal an auto-adaptation loop pathway that plant cells optimize particular needs of chloroplasts in stabilizing photosynthetic complexes by relaying the GUN5-dependent plastid signal(s) to activate the heat-responsive expression of HSP21 in the nucleus during adaptation to heat stress in plants.