Alternative oxidase involvement in cold stress response of Arabidopsis thaliana fad2 and FAD3+ cell suspensions altered in membrane lipid composition

The OsTIL1 lipocalin protects cell membranes from reactive oxygen species damage and maintains the 18:3-containing glycerolipid biosynthesis under cold stress in rice

Lipocalins constitute a conserved protein family that binds to and transports a variety of lipids while fatty acid desaturases (FADs) are required for maintaining the cell membrane fluidity under cold stress. Nevertheless, it remains unclear whether plant lipocalins promote FADs for the cell membrane integrity under cold stress. Here, we identified the role of OsTIL1 lipocalin in FADs-mediated glycerolipid remodeling under cold stress. Overexpression and CRISPR/Cas9 mediated gene edition experiments demonstrated that OsTIL1 positively regulated cold stress tolerance by protecting the cell membrane integrity from reactive oxygen species damage and enhancing the activities of peroxidase and ascorbate peroxidase, which was confirmed by combined cold stress with a membrane rigidifier dimethyl sulfoxide or a H2 O2 scavenger dimethyl thiourea. OsTIL1 overexpression induced higher 18:3 content, and higher 18:3/18:2 and (18:2 + 18:3)/18:1 ratios than the wild type under cold stress whereas the gene edition mutant showed the opposite. Furthermore, the lipidomic analysis showed that OsTIL1 overexpression led to higher contents of 18:3-mediated glycerolipids, including galactolipids (monoglactosyldiacylglycerol and digalactosyldiacylglycerol) and phospholipids (phosphatidyl glycerol, phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl serine and phosphatidyl inositol) under cold stress. RNA-seq and enzyme linked immunosorbent assay analyses indicated that OsTIL1 overexpression enhanced the transcription and enzyme abundance of four ω-3 FADs (OsFAD3-1/3-2, 7, and 8) under cold stress. These results reveal an important role of OsTIL1 in maintaining the cell membrane integrity from oxidative damage under cold stress, providing a good candidate gene for improving cold tolerance in rice.

Overexpression of the Purple Perilla (Perilla frutescens (L.)) FAD3a Gene Enhances Salt Tolerance in Soybean

The increasingly serious trend of soil salinization inhibits the normal growth and development of soybeans, leading to reduced yields and a serious threat to global crop production. Microsomal ω-3 fatty acid desaturase encoded by the FAD3 gene is a plant enzyme that plays a significant role in α-linolenic acid synthesis via regulating the membrane fluidity to better accommodate various abiotic stresses. In this study, PfFAD3a was isolated from perilla and overexpressed in soybeans driven by CaMV P35S, and the salt tolerance of transgenic plants was then evaluated. The results showed that overexpression of PfFAD3a increased the expression of PfFAD3a in both the leaves and seeds of transgenic soybean plants, and α-linolenic acid content also significantly increased; hence, it was shown to significantly enhance the salt tolerance of transgenic plants. Physiological and biochemical analysis showed that overexpression of PfFAD3a increased the relative chlorophyll content and PSII maximum photochemical efficiency of transgenic soybean plants under salt stress; meanwhile, a decreased accumulation of MDA, H₂O₂, and O2•-, increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbic acid peroxidase (APX), as well as the production of proline and soluble sugar. In summary, the overexpression of PfFAD3a may enhance the salt tolerance in transgenic soybean plants through enhanced membrane fluidity and through the antioxidant capacity induced by C18:3.

Sources of variability in seagrass fatty acid profiles and the need of identifying reliable warming descriptors

Global warming is expected to have inexorable and profound effects on marine ecosystems, particularly in foundation species such as seagrasses. Identifying responses to warming and comparing populations across natural temperature gradients can inform how future warming will impact the structure and function of ecosystems. Here, we investigated how thermal environment, intra-shoot and spatial variability modulate biochemical responses of the Mediterranean seagrass Posidonia oceanica. Through a space-for-time substitution study, Fatty acid (FA) profiles on the second and fifth leaf of the shoots were quantified at eight sites in Sardinia along a natural sea surface temperature (SST) summer gradient (about 4 °C). Higher mean SST were related to a decrease in the leaf total fatty acid content (LTFA), a reduction in polyunsaturated fatty acids (PUFA), omega-3/omega-6 PUFA and PUFA/saturated fatty acids (SFA) ratios and an increase in SFA, monounsaturated fatty acids and carbon elongation index (CEI, C18:2 n-6/C16:2 n-6) ratio. Results also revealed that FA profiles were strongly influenced by leaf age, independently of SST and spatial variability within sites. Overall, this study evidenced that the sensitive response of P. oceanica FA profiles to intra-shoot and spatial variability must not be overlooked when considering their response to temperature changes.

Dissecting the effect of ethylene in the transcriptional regulation of chilling treatment in grapevine leaves

Ethylene (ETH) plays important roles in various development programs and stress responses in plants. In grapevines, ETH increased dramatically under chilling stress and is known to positively regulate cold tolerance. However, the role of ETH in transcriptional regulation during chilling stress of grapevine leaves is still not clear. To address this gap, targeted hormone profiling and transcriptomic analysis were performed on leaves of Vitis amurensis under chilling stress with and without aminoethoxyvinylglycine (AVG, a inhibitor of ETH synthesis) treatment. APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) and WRKY transcription factors (TF) were only the two highly enriched TF families that were consistently up-regulated during chilling stress but inhibited by AVG. The comparison of leaf transcriptomes between chilling treatment and chilling with AVG allowed the identification of potential ETH-regulated genes. Potential genes that are positively regulated by ETH are enriched in solute transport, protein biosynthesis, phytohormone action, antioxidant and carbohydrate metabolism. Conversely, genes related to the synthesis and signaling of ETH, indole-3-acetic acid (IAA), abscisic acid (ABA) were up-regulated by chilling treatment but inhibited by AVG. The contents of ETH, ABA and IAA also paralleled with the transcriptome data, which suggests that the response of ABA and IAA during chilling stress may regulate by ETH signaling, and together may belong to an integrated network of hormonal signaling pathways underpinning chilling stress response in grapevine leaves. Together, these findings provide new clues for further studying the complex regulatory mechanism of ETH under low-temperature stress in plants more generally and new opportunities for breeding cold-resilient grapevines.

Fatty acid-based index development in estuarine organisms to pinpoint environmental contamination

Estuaries have long been preferred areas of human settlement, where multiple anthropogenic activities take place, which have contributed to a significant decrease in environmental quality of these ecosystems. Accordingly, environmental monitoring and management have long relied on the development of tools that summarize and simplify complex information and provide direct interpretation of quality status. Here, the fatty acid profiles of three abundant estuarine species, namely Hediste diversicolor, Carcinus maenas and Pomatoschistus microps, were used to develop and validate a multimetric index, based on the Euclidean dissimilarities of profiles between sites, in response to contamination gradient in a large urban estuary. Spatial differences were generally related to unsaturated fatty acids (mono- and polyunsaturated, of the n-3 and n-6 series) in all species, albeit more pronounced in P. microps. Multivariate models returned high classification accuracies for the three sampled sites, varying from 73.3% in the invertebrate species to 100.0% in the fish species. Results show the applicability of the developed FA-based index, particularly due to the easy of communication, for managers and the public alike, but also highlight the need for prior validation on species suitability or sensitivity to depict environmental contamination.

Ocean Acidification Alleviates Dwarf Eelgrass (Zostera noltii) Lipid Landscape Remodeling under Warming Stress

Coastal seagrass meadows provide a variety of essential ecological and economic services, including nursery grounds, sediment stabilization, nutrient cycling, coastal protection, and blue carbon sequestration. However, these ecosystems are highly threatened by ongoing climatic change. This study was aimed to understand how the dwarf eelgrass Zostera noltii leaf lipid landscapes are altered under predicted ocean warming (+4 °C) and hypercapnic (ΔpH 0.4) conditions. Warming and hypercapnic conditions were found to induce a severe reduction in the leaf total fatty acid, though the combined treatment substantially alleviated this depletion. The lipid discrimination revealed a significant increase in the relative monogalactosyldiacylglycerol (MGDG) content in both hypercapnic and warming conditions, allied to plastidial membrane stabilization mechanisms. Hypercapnia also promoted enhanced phosphatidylglycerol (PG) leaf contents, a mechanism often associated with thylakoid reinvigoration. In addition to changing the proportion of storage, galacto- and phospholipids, the tested treatments also impacted the FA composition of all lipid classes, with warming exposure leading to decreases in polyunsaturated fatty acids (PUFAs); however, the combination of both stress conditions alleviated this effect. The observed galactolipid and phospholipid PUFA decreases are compatible with a homeoviscous adaptation, allowing for the maintenance of membrane stability by counteracting excessive membrane fluidity. Neutral lipid contents were substantially increased under warming conditions, especially in C18 fatty acids (C18), impairing their use as substrates for fatty acylated derivatives essential for maintaining the osmotic balance of cells. An analysis of the phospholipid and galactolipid fatty acid profiles as a whole revealed a higher degree of discrimination, highlighting the higher impact of warming and the proposed stress alleviation effect induced by increased water-dissolved CO2 availability. Still, it is essential to remember that the pace at which the ocean is warming can overcome the ameliorative capacity induced by higher CO2 availability, leaving seagrasses under severe heat stress beyond their lipid remodeling capacity.

Fatty acid profiles of estuarine macroalgae are biomarkers of anthropogenic pressures: Development and application of a multivariate pressure index

Transitional ecosystems are among the most degraded ecosystems worldwide, with several groups of organisms investigated for their reliability as biological indicators of human-driven disturbances. Recently non-traditional biochemical biomarkers such as an individual's fatty acids profile have been identified as promising tools for assessing contaminant exposure. In this work, two abundant Atlantic benthic macroalgae (Ulva lactuca and Fucus vesiculosus species) were surveyed in three mudflat areas of the highly urbanized Tejo estuary, with increasing anthropogenic disturbance degrees (Alcochete, Rosário and Seixal mudflats, increasing in contamination by this order) and their fatty acids evaluated as potential biomarkers for exposure to contaminants known to have toxic effects on biota. In terms of contamination the metal pollution index of all the compartments analysed (sediment bioavailable and total metal concentrations and thallus metal concentrations) revealed the same tendencies with lower contamination levels in Alcochete, intermediate in Rosário and high contamination levels in Seixal. In the thallus of U. lactuca thallus could be observed a strong decrease in C18-fatty acids along the contamination gradient, likely due to lipid peroxidation from metal-generated reactive oxygen species. Nevertheless, an increase in stearic and hexadecatrienoic acids in the thallus from the most contaminated site suggested counteractive mechanisms maintain the production of C18-fatty acid pool. A similar response was found in F. vesiculosus but with palmitic acid acting as precursor for the synthesis of stearic acid, allowing the maintenance of oleic and linoleic acids levels in the membranes to counteract oxidative stress. Beyond the physiological interest of these mechanisms, fatty acid profiles were used to develop a novel multivariate pressure index (Multi-PI), that beyond the contaminant concentration would reflect the response of these biomonitor species towards anthropogenic disturbance, through the evaluation of fatty acid profiles, which are also key molecules from a trophic perspective within the estuarine system. The Multi-PI efficiency in responding to different environmental contamination degrees, was substantiated by strong and positive correlations with thallus and sediment contamination. This indicated that fatty acid profiles reflect thallus and benthic habitat contamination and are efficient biomarkers of environmental metal contamination. Therefore, the sessile and abundant nature of benthic macroalgae allied to their fatty acid responses can be leveraged as suitable biomarkers for contaminant monitoring in future impact assessment and ecotoxicology studies.

Membrane dynamics during individual and combined abiotic stresses in plants and tools to study the same

The plasma membrane (PM) is possibly the most diverse biological membrane of plant cells; it separates and guards the cell against its external environment. It has an extremely complex structure comprising a mosaic of lipids and proteins. The PM lipids are responsible for maintaining fluidity, permeability and integrity of the membrane and also influence the functioning of membrane proteins. However, the PM is the primary target of environmental stress, which affects its composition, conformation and properties, thereby disturbing the cellular homeostasis. Maintenance of integrity and fluidity of the PM is a prerequisite for ensuring the survival of plants during adverse environmental conditions. The ability of plants to remodel membrane lipid and protein composition plays a crucial role in adaptation towards varying abiotic environmental cues, including high or low temperature, drought, salinity and heavy metals stress. The dynamic changes in lipid composition affect the functioning of membrane transporters and ultimately regulate the physical properties of the membrane. Plant membrane-transport systems play a significant role in stress adaptation by cooperating with the membrane lipidome to maintain the membrane integrity under stressful conditions. The present review provides a holistic view of stress responses and adaptations in plants, especially the changes in the lipidome and proteome of PM under individual or combined abiotic stresses, which cause alterations in the activity of membrane transporters and modifies the fluidity of the PM. The tools to study the varying lipidome and proteome of the PM are also discussed.

Pigment and Fatty Acid Production under Different Light Qualities in the Diatom Phaeodactylum tricornutum

Diatoms are microscopic biorefineries producing value-added molecules, including unique pigments, triglycerides (TAGs) and long-chain polyunsaturated fatty acids (LC-PUFAs), with potential implications in aquaculture feeding and the food or biofuel industries. These molecules are utilized in vivo for energy harvesting from sunlight to drive photosynthesis and as photosynthetic storage products, respectively. In the present paper, we evaluate the effect of narrow-band spectral illumination on carotenoid, LC-PUFAs and TAG contents in the model diatom Phaeodactylum tricornutum. Shorter wavelengths in the blue spectral range resulted in higher production of total fatty acids, namely saturated TAGs. Longer wavelengths in the red spectral range increased the cell’s content in Hexadecatrienoic acid (HTA) and Eicosapentaenoic acid (EPA). Red wavelengths induced higher production of photoprotective carotenoids, namely fucoxanthin. In combination, the results demonstrate how diatom value-added molecule production can be modulated by spectral light control during the growth. How diatoms could use such mechanisms to regulate efficient light absorption and cell buoyancy in the open ocean is discussed.

An overview of heat stress in tomato (Solanum lycopersicum L.)

Heat stress has been defined as the rise of temperature for a period of time higher than a threshold level, thereby permanently affecting the plant growth and development. Day or night temperature is considered as the major limiting factor for plant growth. Earlier studies reported that night temperature is an important factor in the heat reaction of the plants. Tomato cultivars capable of setting viable fruits under night temperatures above 21 °C are considered as heat-tolerant cultivars. The development of breeding objectives is generally summarized in four points: (a) cultivars with higher yield, (b) disease resistant varieties in the 1970s, (c) long shelf-life in 1980s, and (d) nutritive and taste quality during 1990s. Some unique varieties like the dwarf "Micro-Tom", and the first transgenic tomato (FlavrSavr) were developed through breeding; they were distributed late in the 1980s. High temperature significantly affects seed, pollen viability and root expansion. Researchers have employed different parameters to evaluate the tolerance to heat stress, including membrane thermo stability, floral characteristics (Stigma exertion and antheridia cone splitting), flower number, and fruit yield per plant. Reports on pollen viability and fruit set/plant under heat stress by comparing the pollen growth and tube development in heat-treated and non-heat-stressed conditions are available in literature. The electrical conductivity (EC) have been used to evaluate the tolerance of some tomato cultivars in vitro under heat stress conditions as an indication of cell damage due to electrolyte leakage; they classified the cultivars into three groups: (a) heat tolerant, (b) moderately heat tolerant, and (c) heat sensitive. It is important to determine the range in genetic diversity for heat tolerance in tomatoes. Heat stress experiments under field conditions offer breeders information to identify the potentially heat tolerant germplasm.

Effects of Propranolol on Growth, Lipids and Energy Metabolism and Oxidative Stress Response of Phaeodactylum tricornutum

Simple Summary

In the past two decades, increasing attention has been directed to investigate the incidence and consequences of pharmaceuticals in the aquatic environment. Propranolol is a non-selective β-adrenoceptor blocker used in large quantities worldwide to treat cardiovascular conditions. Diatoms (model organism) exposed to this compound showed evident signs of oxidative stress, a significant reduction of the autotrophic O₂ production and an increase in the heterotrophic mitochondrial respiration. Additionally, diatoms exposed to propranolol showed a consumption of its storage lipids. In ecological terms this will have cascading impacts in the marine trophic webs, where these organisms are key elements, through a reduction of the water column oxygenation and essential fatty acid availability to the heterotrophic organisms that depend on these primary producers. In ecotoxicological terms, diatoms photochemical and fatty acid traits showed to be potential good biomarkers for toxicity assessment of diatoms exposed to this widespread pharmaceutical compound.

Abstract

Present demographic trends suggest a rise in the contributions of human pharmaceuticals into coastal ecosystems, underpinning an increasing demand to evaluate the ecotoxicological effects and implications of drug residues in marine risk assessments. Propranolol, a non-selective β-adrenoceptor blocker, is used worldwide to treat high blood pressure conditions and other related cardiovascular conditions. Although diatoms lack β-adrenoceptors, this microalgal group presents receptor-like kinases and proteins with a functional analogy to the animal receptors and that can be targeted by propranolol. In the present work, the authors evaluated the effect of this non-selective β-adrenoceptor blocker in diatom cells using P. tricornutum as a model organism, to evaluate the potential effect of this compound in cell physiology (growth, lipids and energy metabolism and oxidative stress) and its potential relevance for marine ecosystems. Propranolol exposure leads to a significant reduction in diatom cell growth, more evident in the highest concentrations tested. This is likely due to the observed impairment of the main primary photochemistry processes and the enhancement of the mitochondrial respiratory activity. More specifically, propranolol decreased the energy transduction from photosystem II (PSII) to the electron transport chain, leading to an increase in oxidative stress levels. Cells exposed to propranolol also exhibited high-dissipated energy flux, indicating that this excessive energy is efficiently diverted, to some extent, from the photosystems, acting to prevent irreversible photoinhibition. As energy production is impaired at the PSII donor side, preventing energy production through the electron transport chain, diatoms appear to be consuming storage lipids as an energy backup system, to maintain essential cellular functions. This consumption will be attained by an increase in respiratory activity. Considering the primary oxygen production and consumption pathways, propranolol showed a significant reduction of the autotrophic O₂ production and an increase in the heterotrophic mitochondrial respiration. Both mechanisms can have negative effects on marine trophic webs, due to a decrease in the energetic input from marine primary producers and a simultaneous oxygen production decrease for heterotrophic species. In ecotoxicological terms, bio-optical and fatty acid data appear as highly efficient tools for ecotoxicity assessment, with an overall high degree of classification when these traits are used to build a toxicological profile, instead of individually assessed.

Lipid landscape remodelling in Sarcocornia fruticosa green and red physiotypes

Under certain abiotic conditions (elevated irradiance, temperature and sediment salinity) observed mostly during the Mediterranean summer, the halophyte Sarcocornia fruticosa suffers a metabolic shift evidenced by a red coloration, evidencing the presence of two physiotypes (green and red). Previous works indicated that this metabolic shift has severe implications in the primary photochemistry of this species, impairing the light and carbon harvesting. Under stress plants have lower light use efficiencies and are more prone to photoinhibition, and thus this metabolic shift is essential for this species to deal with the high light intensities characteristic from this time of the year. Nevertheless, the fatty acid and lipid remodelling in green and red S. fruticosa physiotypes was not previously evaluated nor its relations with this metabolic shift. The evaluation of the lipid landscape suggests several lipid and fatty acid remodelling when comparing both red and green physiotype, as strategies to overcome stress. The galactolipids of the red physiotype suffer several changes aiming to keep chloroplast membrane structural and functional stability during water stress and can also be related to an improvement of the plants response to osmotic stress. At the phospholipid level, a readjustment of its fatty acid profiles was also observable. This remodelling allows the plants to adjust membrane fluidity the imposed osmotic stress, being this action transversal to choroplastidial, extraplastidial, and involves the action of the different phospholipids. Additionally, neutral lipids (NLs) also appear to play a role in osmotic stress adaptation, with an increase content in C18 fatty acids in the red physiotype. The resulting lipid landscape in both physiotypes presents very specific signatures that can be used as biomarkers to track this kind of metabolic shifts, in future studies with similar species.

Photobiological and lipidic responses reveal the drought tolerance of Aster tripolium cultivated under severe and moderate drought: Perspectives for arid agriculture in the mediterranean

In the past Aster tripolium has already proved to be a good candidate for saline agriculture in soils with low water availability. Thus, the aim of the present work was to disentangle the photobiological and biochemical mechanisms underlying the response of A. tripolium to PEG-induced drought stress, by exposing plants to PEG-induced moderate and severe drought conditions. Plant primary productivity was maintained under moderate drought conditions, due to the presence of alternative electron donors fueling the PSII. Additionally, the high anthocyanin production under drought conditions, act as photoprotective shields against photoinhibition. Moreover, the increased quinone turnover rate simultaneously with a net rate of RC closure and density increase, acted as a counteractive measure, allowing high energy fluxes into the photosystems under drought conditions. PSI showed an activity reduction, indicating that under drought conditions the ETC activity acts as an energetic escape route. Furthermore, membrane remodeling could also be observed under drought. The total fatty acid and omega-3 linolenic acid (18:3) contents were maintained, under osmotic stress. Membrane restructuring with lower amounts of polyunsaturated fatty acids (18:3) is considered an adaptation to osmotically stressful environments. Increased 18:1 and 16:1t fatty acids production improve the LHCs and chloroplast membrane stabilization, allowing the LHC to maintain its efficient functioning. The results here presented are very similar to the ones observed in the past regarding A. tripolium feedback to salinity stress, indicating that the mechanisms to overcome osmotic stress, either due to increased salinity or reduced water availability, are the same.

Repair of sub-lethal freezing damage in leaves of Arabidopsis thaliana

BACKGROUND

The detrimental effects of global climate change direct more attention to the survival and productivity of plants during periods of highly fluctuating temperatures. In particular in temperate climates in spring, temperatures can vary between above-zero and freezing temperatures, even during a single day. Freeze-thaw cycles cause cell membrane lesions that can lead to tissue damage and plant death. Whereas the processes of cold acclimation and freeze-thaw injury are well documented, not much is known about the recovery of plants after a freezing event. We therefore addressed the following questions: i. how does the severity of freezing damage influence repair; ii. how are respiration and content of selected metabolites influenced during the repair process; and iii. how do transcript levels of selected genes respond during repair?

RESULTS

We have investigated the recovery from freezing to sub-lethal temperatures in leaves of non-acclimated and cold acclimated Arabidopsis thaliana plants over a period of 6 days. Fast membrane repair and recovery of photosynthesis were observed 1 day after recovery (1D-REC) and continued until 6D-REC. A substantial increase in respiration accompanied the repair process. In parallel, concentrations of sugars and proline, acting as compatible solutes during freezing, remained unchanged or declined, implicating these compounds as carbon and nitrogen sources during recovery. Similarly, cold-responsive genes were mainly down regulated during recovery of cold acclimated leaves. In contrast, genes involved in cell wall remodeling and ROS scavenging were induced during recovery. Interestingly, also the expression of genes encoding regulatory proteins, such as 14-3-3 proteins, was increased suggesting their role as regulators of repair processes.

CONCLUSIONS

Recovery from sub-lethal freezing comprised membrane repair, restored photosynthesis and increased respiration rates. The process was accompanied by transcriptional changes including genes encoding regulatory proteins redirecting the previous cold response to repair processes, e.g. to cell wall remodeling, maintenance of the cellular proteome and to ROS scavenging. Understanding of processes involved in repair of freeze-thaw injury increases our knowledge on plant survival in changing climates with highly fluctuating temperatures.

Genome wide transcriptome analysis reveals vital role of heat responsive genes in regulatory mechanisms of lentil (Lens culinaris Medikus)

The present study reports the role of morphological, physiological and reproductive attributes viz. membrane stability index (MSI), osmolytes accumulations, antioxidants activities and pollen germination for heat stress tolerance in contrasting genotypes. Heat stress increased proline and glycine betaine (GPX) contents, induced superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione peroxidase (GPX) activities and resulted in higher MSI in PDL-2 (tolerant) compared to JL-3 (sensitive). In vitro pollen germination of tolerant genotype was higher than sensitive one under heat stress. In vivo stressed pollens of tolerant genotype germinated well on stressed stigma of sensitive genotype, while stressed pollens of sensitive genotype did not germinate on stressed stigma of tolerant genotype. De novo transcriptome analysis of both the genotypes showed that number of contigs ranged from 90,267 to 104,424 for all the samples with N50 ranging from 1,755 to 1,844 bp under heat stress and control conditions. Based on assembled unigenes, 194,178 high-quality Single Nucleotide Polymorphisms (SNPs), 141,050 microsatellites and 7,388 Insertion-deletions (Indels) were detected. Expression of 10 genes was evaluated using quantitative Real Time Polymerase Chain Reaction (RT-qPCR). Comparison of differentially expressed genes (DEGs) under different combinations of heat stress has led to the identification of candidate DEGs and pathways. Changes in expression of physiological and pollen phenotyping related genes were also reaffirmed through transcriptome data. Cell wall and secondary metabolite pathways are found to be majorly affected under heat stress. The findings need further analysis to determine genetic mechanism involved in heat tolerance of lentil.

Involvement of OpsLTP1 from Opuntia streptacantha in abiotic stress adaptation and lipid metabolism

Plant lipid transfer proteins (LTPs) exhibit the ability to transfer lipids between membranes in vitro, and have been implicated in diverse physiological processes associated to plant growth, reproduction, development, biotic and abiotic stress responses. However, their mode of action is not yet fully understood. To explore the functions of the OpsLTP1 gene encoding a LTP from cactus pear Opuntia streptacantha Lem., we generated transgenic Arabidopsis thaliana (L.) Heynh. plants to overexpress OpsLTP1 and contrasted our results with the loss-of-function mutant ltp3 from A. thaliana under abiotic stress conditions. The ltp3 mutant seeds showed impaired germination under salt and osmotic treatments, in contrast to OpsLTP1 overexpressing lines that displayed significant increases in germination rate. Moreover, stress recovery assays showed that ltp3 mutant seedlings were more sensitive to salt and osmotic treatments than wild-type plants suggesting that AtLTP3 is required for stress-induced responses, while the OpsLTP1 overexpressing line showed no significant differences. In addition, OpsLTP1 overexpressing and ltp3 mutant seeds stored lower amount of total lipids compared with wild-type seeds, showing changes primarily on 16C and 18C fatty acids. However, ltp3 mutant also lead changes in lipid profile and no over concrete lipids which may suggest a compensatory activation of other LTPs. Interestingly, linoleic acid (18:2ω6) was consistently increased in neutral, galactoglycerolipids and phosphoglycerolipids of OpsLTP1 overexpressing line indicating a role of OpsLTP1 in the modulation of lipid composition in A. thaliana.

Identification of alternative oxidase encoding genes in Caulerpa cylindracea by de novo RNA-Seq assembly analysis

Alternative oxidases (AOX) are defined in plants, fungi and algae. The main function of AOX proteins has been described for electron flow through electron transport chain and regulation of mitochondrial retrograde signaling pathway. The roles of AOX proteins have been characterized in reproduction and resistance against oxidative stress, cold stress, starvation, and biotic attacks. Caulerpa cylindracea is an invasive marine green alga. Although the natural habitats of the species are Australia coasts, the impact of the invasion has been monitored through the Mediterranean Sea and the Aegean Sea. C. cylindracea species have advantages against others by showing higher resistance to stress conditions such as cold, starvation, pathogen attacks and by their capability of sexual and vegetative reproduction. Comparing the advantages of C. cylindracea over the niche and defined functional roles of mitochondrial AOX proteins, it is evident that AOX proteins are likely involved in developing those advantageous skills in C. cylindracea. However, there is limited data about biochemical and molecular mechanisms that take part in stress resistance and invasion characteristics. We aimed to identify mitochondrial alternative oxidase encoding genes in C. cylindracea while annotating whole transcriptome data for the species. Samples were collected from Seferihisar/İzmir. Transcriptome analysis from pooled RNA samples revealed 47,400 assembled contigs represented by 33,340 unigenes. Using standalone Blast analysis, we were able to identify two alternative oxidase encoding genes.

Investigating the physiological mechanisms underlying Salicornia ramosissima response to atmospheric CO2 enrichment under coexistence of prolonged soil flooding and saline excess

A 45-days long climatic chamber experiment was design to evaluate the effect of 400 and 700 ppm atmospheric CO₂ treatments with and without soil water logging in combination with 171 and 510 mM NaCl in the halophyte Salicornia ramosissima. In order to ascertain the possible synergetic impact of these factors associate to climatic change in this plant species physiological and growth responses. Our results indicated that elevated atmospheric CO₂ concentration improved plant physiological performance under suboptimal root-flooding and saline conditions plants. Thus, this positive impact was mainly ascribed to an enhancement of energy transport efficiency, as indicated the greater P_G, N and Sm values, and the maintaining of carbon assimilation capacity due to the higher net photosynthetic rate (A_N) and water use efficiency (_iWUE). This could contribute to reduce the risk of oxidative stress owing to the accumulation of reactive oxygen species (ROS). Moreover, plants grown at 700 ppm had a greater capacity to cope with flooding and salinity synergistic impact by a greater efficiency in the modulation in enzyme antioxidant machinery and by the accumulation of osmoprotective compounds and saturated fatty acids in its tissues. These responses indicate that atmospheric CO₂ enrichment would contribute to preserve the development of Salicornia ramosissima against the ongoing process of increment of soil stressful conditions linked with climatic change.

Lipid remodelling: Unravelling the response to cold stress in Arabidopsis and its extremophile relative Eutrema salsugineum

Environmental constraints limit the geographic distribution of many economically important crops. Cold stress is an important abiotic stress that affects plant growth and development, resulting in loss of vigour and surface lesions. These symptoms are caused by, among other metabolic processes, the altered physical and chemical composition of cell membranes. As a major component of cell membranes lipids have been recognized as having a significant role in cold stress, both as a mechanical defence through leaf surface protection and plasma membrane remodelling, and as signal transduction molecules. We present an overview integrating gene expression and lipidomic data published so far in Arabidopsis and its relative the extremophile Eutrema salsugineum. This data enables a better understanding of the contribution of the lipidome in determining the ability to tolerate suboptimal temperature conditions. Collectively this information will allow us to identify the key lipids and pathways responsible for resilience, enabling the development of new approaches for crop tolerance to stress.

Disentangling the photochemical salinity tolerance in Aster tripolium L.: connecting biophysical traits with changes in fatty acid composition

A profound analysis of A. tripolium photochemical traits under salinity exposure is lacking in the literature, with very few references focusing on its fatty acid profile role in photophysiology. To address this, the deep photochemical processes were evaluated by Pulse Amplitude Modulated (PAM) Fluorometry coupled with a discrimination of its leaf fatty acid profile. Plants exposed to 125-250 mm NaCl showed higher photochemical light harvesting efficiencies and lower energy dissipation rates. under higher NaCl exposure, there is evident damage of the oxygen evolving complexes (OECs). On the other hand, Reaction Centre (RC) closure net rate and density increased, improving the energy fluxes entering the PS II, in spite of the high amounts of energy dissipated and the loss of PS II antennae connectivity. Energy dissipation was mainly achieved through the auroxanthin pathway. Total fatty acid content displayed a similar trend, being also higher under 125-250 mm NaCl with high levels of omega-3 and omega-6 fatty acids. The increase in oleic acid and palmitic acid allows the maintenance of the good functioning of the PS II. Also relevant was the high concentration of chloroplastic C16:1t in the individuals subjected to 125-250 mm NaCl, related with a higher electron transport activity and with the organization of the Light Harvesting Complexes (LHC) and thus reducing the activation of energy dissipation mechanisms. All these new insights shed some light not only on the photophysiology of this potential cash-crop, but also highlight its important saline agriculture applications of this species as forage and potential source of essential fatty acids.

RNA-seq based transcriptomic analysis uncovers α-linolenic acid and jasmonic acid biosynthesis pathways respond to cold acclimation in Camellia japonica

Camellia is a well-known ornamental flower native to Southeast of Asia, including regions such as Japan, Korea and South China. However, most species in the genus Camellia are cold sensitive. To elucidate the cold stress responses in camellia plants, we carried out deep transcriptome sequencing of 'Jiangxue', a cold-tolerant cultivar of Camellia japonica, and approximately 1,006 million clean reads were generated using Illumina sequencing technology. The assembly of the clean reads produced 367,620 transcripts, including 207,592 unigenes. Overall, 28,038 differentially expressed genes were identified during cold acclimation. Detailed elucidation of responses of transcription factors, protein kinases and plant hormone signalling-related genes described the interplay of signal that allowed the plant to fine-tune cold stress responses. On the basis of global gene regulation of unsaturated fatty acid biosynthesis- and jasmonic acid biosynthesis-related genes, unsaturated fatty acid biosynthesis and jasmonic acid biosynthesis pathways were deduced to be involved in the low temperature responses in C. japonica. These results were supported by the determination of the fatty acid composition and jasmonic acid content. Our results provide insights into the genetic and molecular basis of the responses to cold acclimation in camellia plants.

Effects of high temperature on the ultrastructure and microtubule organization of interphase and dividing cells of the seagrass Cymodocea nodosa

Short-time temperature effects (34-40 °C) on microtubule (MT) organization and on cell structure of young epidermal leaf cells of the seagrass Cymodocea nodosa were investigated under laboratory conditions using transmission electron microscopy (TEM) and tubulin immunofluorescence. The interphase MT network was affected by the increased temperature, the effect being time dependent and expressed in both the form and the orientation of the MT bundles. After 1 h at 38 °C, there was also a severe disturbance in dividing cells with thick and short MTs in the mitotic spindle and atypically organized phragmoplasts, while after 2 h at 38 °C the mitotic index was tenfold reduced compared with the control material. After 6 h at 38 °C, a large number of telophase cells were observed, meaning that cytokinesis was blocked. TEM observation revealed cells with uncompleted cell plates consisting of swollen vesicles and branched cisternae, with no phragmoplast MTs. These cells bear a nucleolus with segregated fibrillar and granular zones, an increased number of swollen mitochondria, and numerous parallel arrays of endoplasmic reticulum cisternae in the cortical cytoplasm. The possible relationship of these changes in C. nodosa with a response mechanism in order to face elevated temperature effects of climate change is discussed.

Alternative Oxidase Pathway Optimizes Photosynthesis During Osmotic and Temperature Stress by Regulating Cellular ROS, Malate Valve and Antioxidative Systems

The present study reveals the importance of alternative oxidase (AOX) pathway in optimizing photosynthesis under osmotic and temperature stress conditions in the mesophyll protoplasts of Pisum sativum. The responses of photosynthesis and respiration were monitored at saturating light intensity of 1000 μmoles m(-2) s(-1) at 25°C under a range of sorbitol concentrations from 0.4 to 1.0 M to induce hyper-osmotic stress and by varying the temperature of the thermo-jacketed pre-incubation chamber from 25 to 10°C to impose sub-optimal temperature stress. Compared to controls (0.4 M sorbitol and 25°C), the mesophyll protoplasts showed remarkable decrease in NaHCO3-dependent O2 evolution (indicator of photosynthetic carbon assimilation), under both hyper-osmotic (1.0 M sorbitol) and sub-optimal temperature stress conditions (10°C), while the decrease in rates of respiratory O2 uptake were marginal. The capacity of AOX pathway increased significantly in parallel to increase in intracellular pyruvate and reactive oxygen species (ROS) levels under both hyper-osmotic stress and sub-optimal temperature stress under the background of saturating light. The ratio of redox couple (Malate/OAA) related to malate valve increased in contrast to the ratio of redox couple (GSH/GSSG) related to antioxidative system during hyper-osmotic stress. Further, the ratio of GSH/GSSG decreased in the presence of sub-optimal temperature, while the ratio of Malate/OAA showed no visible changes. Also, the redox ratios of pyridine nucleotides increased under hyper-osmotic (NADH/NAD) and sub-optimal temperature (NADPH/NADP) stresses, respectively. However, upon restriction of AOX pathway by using salicylhydroxamic acid (SHAM), the observed changes in NaHCO3-dependent O2 evolution, cellular ROS, redox ratios of Malate/OAA, NAD(P)H/NAD(P) and GSH/GSSG were further aggravated under stress conditions with concomitant modulations in NADP-MDH and antioxidant enzymes. Taken together, the results indicated the importance of AOX pathway in optimizing photosynthesis under both hyper-osmotic stress and sub-optimal temperatures. Regulation of ROS through redox couples related to malate valve and antioxidant system by AOX pathway to optimize photosynthesis under these stresses are discussed.

Mitochondrial energy-dissipating systems (alternative oxidase, uncoupling proteins, and external NADH dehydrogenase) are involved in development of frost-resistance of winter wheat seedlings

Gene expression, protein synthesis, and activities of alternative oxidase (AOX), uncoupling proteins (UCP), adenine nucleotide translocator (ANT), and non-coupled NAD(P)H dehydrogenases (NDex, NDPex, and NDin) were studied in shoots of etiolated winter wheat (Triticum aestivum L.) seedlings after exposure to hardening low positive (2°C for 7 days) and freezing (-2°C for 2 days) temperatures. The cold hardening efficiently increased frost-resistance of the seedlings and decreased the generation of reactive oxygen species (ROS) during further cold shock. Functioning of mitochondrial energy-dissipating systems can represent a mechanism responsible for the decrease in ROS under these conditions. These systems are different in their response to the action of the hardening low positive and freezing temperatures. The functioning of the first system causes induction of AOX and UCP synthesis associated with an increase in electron transfer via AOX in the mitochondrial respiratory chain and also with an increase in the sensitivity of mitochondrial non-phosphorylating respiration to linoleic and palmitic acids. The increase in electron transfer via AOX upon exposure of seedlings to hardening freezing temperature is associated with retention of a high activity of NDex. It seems that NDex but not the NDPex and NDin can play an important role in maintaining the functional state of mitochondria in heterotrophic tissues of plants under the influence of freezing temperatures. The involvement of the mitochondrial energy-dissipating systems and their possible physiological role in the adaptation of winter crops to cold and frost are discussed.

A tomato endoplasmic reticulum (ER)-type omega-3 fatty acid desaturase (LeFAD3) functions in early seedling tolerance to salinity stress

Transgenic tomato plants overexpressing LeFAD3 sense and antisense sequences were generated. Salt stress suppressed the growth of WT and antisense plants to a higher extent than that in sense plants. In this study, we investigated the role of the LeFAD3-encoding ER-type omega-3 fatty acid desaturase in salt tolerance in tomato plants. We created transgenic tomato plants by overexpressing its sense and antisense sequences under the control of the cauliflower mosaic virus 35S promoter. Based on the results of northern and western blotting as well as quantitative reverse transcription-polymerase chain reaction, sense plants expressed more desaturase than wild-type (WT) plants, whereas antisense plants expressed less desaturase than WT. Salt stress suppressed the growth of both WT and antisense plants to a higher extent than that in sense plants, which can be attributed to the fact that sense plants performed better in maintaining the integrity of the membrane system, as revealed by electron microscopy. The concomitant increase in superoxide dismutase (EC 1.15.1.1) and ascorbate peroxidase (EC 1.11.1.7) may have alleviated the photoinhibition caused by the increased level of ROS in sense plants. Our results suggest that LeFAD3 overexpression can enhance the tolerance of early seedlings to salinity stress.

Expression and signal regulation of the alternative oxidase genes under abiotic stresses

Plants in their natural environment frequently face various abiotic stresses, such as drought, high salinity, and chilling. Plant mitochondria contain an alternative oxidase (AOX), which is encoded by a small family of nuclear genes. AOX genes have been shown to be highly responsive to abiotic stresses. Using transgenic plants with varying levels of AOX expression, it has been confirmed that AOX genes are important for abiotic stress tolerance. Although the roles of AOX under abiotic stresses have been extensively studied and there are several excellent reviews on this topic, the differential expression patterns of the AOX gene family members and the signal regulation of AOX gene(s) under abiotic stresses have not been extensively summarized. Here, we review and discuss the current progress of these two important issues.

Evidence for extensive heterotrophic metabolism, antioxidant action, and associated regulatory events during winter hardening in Sitka spruce

BACKGROUND

Cold acclimation in woody perennials is a metabolically intensive process, but coincides with environmental conditions that are not conducive to the generation of energy through photosynthesis. While the negative effects of low temperatures on the photosynthetic apparatus during winter have been well studied, less is known about how this is reflected at the level of gene and metabolite expression, nor how the plant generates primary metabolites needed for adaptive processes during autumn.

RESULTS

The MapMan tool revealed enrichment of the expression of genes related to mitochondrial function, antioxidant and associated regulatory activity, while changes in metabolite levels over the time course were consistent with the gene expression patterns observed. Genes related to thylakoid function were down-regulated as expected, with the exception of plastid targeted specific antioxidant gene products such as thylakoid-bound ascorbate peroxidase, components of the reactive oxygen species scavenging cycle, and the plastid terminal oxidase. In contrast, the conventional and alternative mitochondrial electron transport chains, the tricarboxylic acid cycle, and redox-associated proteins providing reactive oxygen species scavenging generated by electron transport chains functioning at low temperatures were all active.

CONCLUSIONS

A regulatory mechanism linking thylakoid-bound ascorbate peroxidase action with "chloroplast dormancy" is proposed. Most importantly, the energy and substrates required for the substantial metabolic remodeling that is a hallmark of freezing acclimation could be provided by heterotrophic metabolism.

Correlations between the temperature dependence of chlorophyll fluorescence and the fluidity of thylakoid membranes

To monitor changes in membrane fluidity in Arabidopsis leaves and thylakoid membranes, we investigated the temperature dependence of a chlorophyll fluorescence parameter, minimum fluorescence (Fo), and calculated the threshold temperature [T(Fo)] at which the rise of the fluorescence level Fo was considered to be started. For the modification of membrane fluidity we took three different approaches: (1) an examination of wild-type leaves initially cultured at room temperature (22°C), then exposed to either a lower (4°C) or higher (35°C) temperature for 5 days; (2) measurements of the shift in T(Fo) by two mutants deficient in fatty acid desaturase genes - fad7 and fad7fad8 and (3) an evaluation of the performance of wild-type plants when leaves were infiltrated with chemicals that modify fluidity. When wild-type plants were grown at 22°C, the T(Fo) was 48.3 ± 0.3°C. Plants that were then transferred to a chamber set at 4 or 35°C showed a shift in their T(Fo) to 42.7 ± 0.9°C or 48.9 ± 0.1°C, respectively. Under low-temperature acclimation, the decline in this putative transition temperature was significantly less in fad7 and fad7fad8 mutants compared with the wild-type. In both leaf and thylakoid samples, values for T(Fo) were reduced in samples treated with benzyl alcohol, a membrane fluidizer, whereas T(Fo) rose in samples treated with dimethylsulfoxide, a membrane rigidifier. These results indicate that the heat-induced rise of chlorophyll fluorescence is strongly correlated with the fluidity of thylakoid membranes.

Flexible change and cooperation between mitochondrial electron transport and cytosolic glycolysis as the basis for chilling tolerance in tomato plants

To find if cytosolic glycolysis dynamical metabolism plays a role in mediating respiration homeostasis and its relationship with mitochondrial electron transport chain (miETC) flexibility, we selected two tomato genotypes that differ in chilling tolerance and compared the responses of miETC, cytosolic glycolysis and respiratory homeostasis at 7 °C. Our results showed that the transcripts of both classical and bypass component genes for miETC and glycolysis were comparable for both genotypes when grown at 25 °C. However, there was a rapid global increase in the expression of most respiratory genes in response to chilling at 7 °C for both genotypes. When normally grown plant was set as the control for each genotype, the transcripts of most COX family members, ATP synthase, AOX1b, and UCP are highly up-regulated in chilling-tolerant Zhefen No. 208 plants in contrast to the sensitive Zhefen No. 212 plants. Both genotypes mobilized the energy-saving sucrose synthase pathway for sucrose degradation by cytosolic glycolysis, but this mechanism is evidently more effective in tolerant Zhefen No. 208 plants. Furthermore, only Zhefen No. 208 plants were able to partially switch from low-energy efficiency pathways to ATP conserving pathways to carry out fructose-6-phosphate conversion and pyruvate production. This metabolic flexibility in miETC and cytosolic glycolysis were coupled to higher ATP synthesis and lower ROS accumulation, which may be essential for sustaining the higher leaf respiration and homeostasis of chilling-tolerant plants.

Involvement of hydrogen peroxide, calcium, and ethylene in the induction of the alternative pathway in chilling-stressed Arabidopsis callus

The roles of ethylene, hydrogen peroxide (H(2)O(2)), and calcium in inducing the capacity of the alternative respiratory pathway (AP) under chilling temperature in Arabidopsis thaliana calli were investigated. Exposure of wild-type (WT) calli, but not the calli of ethylene-insensitive mutants, etr1-3 and ein2-1, to chilling led to a marked increase of the AP capacity and triggered a rapid ethylene emission and H(2)O(2) generation. Increasing ethylene emission by applying 1-aminocyclopropane-1-carboxylic (an ethylene precursor) markedly enhanced the AP capacity in WT calli, but not in etr1-3 and ein2-1 calli, whereas suppressing ethylene emission by applying aminooxyacetic acid (an ethylene biosynthesis inhibitor) abolished the chilling-induced AP capacity in WT calli. Furthermore, exogenous H(2)O(2) treatment increased the AP capacity in WT calli, but not in etr1-3 and ein2-1 calli, while both catalase (H(2)O(2) scavenger) and diphenylene iodonium (DPI, an inhibitor of NADPH oxidase) completely inhibited the chilling-induced H(2)O(2) generation and largely inhibited the chilling-induced AP capacity. Interestingly, the chilling-induced AP capacity was completely inhibited by DPI and EGTA (calcium chelator). Further investigation demonstrated that H(2)O(2) and calcium induced ethylene emission under chilling stress. Ethylene modulated the chilling-induced increase of pyruvate content and the expression of alternative oxidase genes (AOX1a and AOX1c). Taken together, these results indicate that H(2)O(2)-, calcium- and ethylene-dependent pathways are required for chilling-induced increase in AP capacity. However, only ethylene is indispensable for the activation of the AP capacity.

Differential expression of cysteine desulfurases in soybean

BACKGROUND

Iron-sulfur [Fe-S] clusters are prosthetic groups required to sustain fundamental life processes including electron transfer, metabolic reactions, sensing, signaling, gene regulation and stabilization of protein structures. In plants, the biogenesis of Fe-S protein is compartmentalized and adapted to specific needs of the cell. Many environmental factors affect plant development and limit productivity and geographical distribution. The impact of these limiting factors is particularly relevant for major crops, such as soybean, which has worldwide economic importance.

RESULTS

Here we analyze the transcriptional profile of the soybean cysteine desulfurases NFS1, NFS2 and ISD11 genes, involved in the biogenesis of [Fe-S] clusters, by quantitative RT-PCR. NFS1, ISD11 and NFS2 encoding two mitochondrial and one plastid located proteins, respectively, are duplicated and showed distinct transcript levels considering tissue and stress response. NFS1 and ISD11 are highly expressed in roots, whereas NFS2 showed no differential expression in tissues. Cold-treated plants showed a decrease in NFS2 and ISD11 transcript levels in roots, and an increased expression of NFS1 and ISD11 genes in leaves. Plants treated with salicylic acid exhibited increased NFS1 transcript levels in roots but lower levels in leaves. In silico analysis of promoter regions indicated the presence of different cis-elements in cysteine desulfurase genes, in good agreement with differential expression of each locus. Our data also showed that increasing of transcript levels of mitochondrial genes, NFS1/ISD11, are associated with higher activities of aldehyde oxidase and xanthine dehydrogenase, two cytosolic Fe-S proteins.

CONCLUSIONS

Our results suggest a relationship between gene expression pattern, biochemical effects, and transcription factor binding sites in promoter regions of cysteine desulfurase genes. Moreover, data show proportionality between NFS1 and ISD11 genes expression.

Analysis of short-term changes in the Arabidopsis thaliana glycerolipidome in response to temperature and light

Although the influence of temperature, particularly cold, on lipid metabolism is well established, previous studies have focused on long-term responses and have largely ignored the influence of other interacting environmental factors. Here, we present a time-resolved analysis of the early responses of the glycerolipidome of Arabidopsis thaliana plants exposed to various temperatures (4, 21 and 32°C) and light intensities (darkness, 75, 150 and 400 μmol m(-2) s(-1)), including selected combinations. Using a UPLC/MS-based lipidomic platform, we reproducibly measured most glycerolipid species reported for Arabidopsis leaves, including the classes phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) phosphatidylglycerol (PG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG). In addition to known lipids, we have identified previously unobserved compounds, such as 36-C PGs and eukaryotic phospholipids containing 16:3 acyl chains. Occurrence of these lipid species implies the action of new biochemical mechanisms. Exposition of Arabidopsis plants to various light and temperature regimes results in two major effects. The first is the dependence of the saturation level of PC and MGDG pools on light intensity, likely arising from light regulation of de novo fatty acid synthesis. The second concerns an immediate decrease in unsaturated species of PG at high-temperature conditions (32°C), which could mark the first stages of adaptation to heat-stress conditions. Observed changes are discussed in the context of current knowledge, and new hypotheses have been formulated concerning the early stages of the plant response to changing light and temperature conditions.

Heat perception and signalling in plants: a tortuous path to thermotolerance

An accurate assessment of the rising ambient temperature by plant cells is crucial for the timely activation of various molecular defences before the appearance of heat damage. Recent findings have allowed a better understanding of the early cellular events that take place at the beginning of mild temperature rise, to timely express heat-shock proteins (HSPs), which will, in turn, confer thermotolerance to the plant. Here, we discuss the key components of the heat signalling pathway and suggest a model in which a primary sensory role is carried out by the plasma membrane and various secondary messengers, such as Ca(2+) ions, nitric oxide (NO) and hydrogen peroxide (H(2)O(2)). We also describe the role of downstream components, such as calmodulins, mitogen-activated protein kinases and Hsp90, in the activation of heat-shock transcription factors (HSFs). The data gathered for land plants suggest that, following temperature elevation, the heat signal is probably transduced by several pathways that will, however, coalesce into the final activation of HSFs, the expression of HSPs and the onset of cellular thermotolerance.

An analytical model of non-photorespiratory CO₂release in the light and dark in leaves of C₃species based on stoichiometric flux balance

Leaf respiration continues in the light but at a reduced rate. This inhibition is highly variable, and the mechanisms are poorly known, partly due to the lack of a formal model that can generate testable hypotheses. We derived an analytical model for non-photorespiratory CO₂ release by solving steady-state supply/demand equations for ATP, NADH and NADPH, coupled to a widely used photosynthesis model. We used this model to evaluate causes for suppression of respiration by light. The model agrees with many observations, including highly variable suppression at saturating light, greater suppression in mature leaves, reduced assimilatory quotient (ratio of net CO₂ and O₂ exchange) concurrent with nitrate reduction and a Kok effect (discrete change in quantum yield at low light). The model predicts engagement of non-phosphorylating pathways at moderate to high light, or concurrent with processes that yield ATP and NADH, such as fatty acid or terpenoid synthesis. Suppression of respiration is governed largely by photosynthetic adenylate balance, although photorespiratory NADH may contribute at sub-saturating light. Key questions include the precise diel variation of anabolism and the ATP : 2e⁻ ratio for photophosphorylation. Our model can focus experimental research and is a step towards a fully process-based model of CO₂ exchange.

Antisense-mediated depletion of tomato endoplasmic reticulum omega-3 fatty acid desaturase enhances thermal tolerance

An endoplasmic reticulum-localized tomato omega-3 fatty acid desaturase gene (LeFAD3) was isolated. The antisense tomato plants were obtained under the control of the cauliflower mosaic virus 35S promoter (35S-CaMV). Northern blot analysis confirmed that the expression of LeFAD3 was inhibited in the tomato genome. Levels of 18:3 decreased and correspondingly levels of 18:2 increased in total lipids of leaves and roots. After heat stress, the fresh weight of the aerial parts of antisense transgenic plants was higher than that of the wild type (WT) plants. The membrane system ultrastructure of chloroplasts in leaf cells and all of the subcellular organelles in the root tips of transgenic plants remained more intact than those of WT. Relative electric conductivity increased less in transgenic plants than in WT. Under heat stress, the maximal photochemical efficiency of photosystem II (Fv/Fm) and the O(2) evolution rate decreased more in WT than in transgenic plants. These results suggested that the depletion of LeFAD3 increased the saturation of fatty acids and alleviated high temperature stress.

Proteolytic processing of an Arabidopsis membrane-bound NAC transcription factor is triggered by cold-induced changes in membrane fluidity

Changes in membrane fluidity are the earliest cellular events that occur in plant cells upon exposure to cold. This subsequently triggers physiological processes, such as calcium influx and reorganization of actin cytoskeletons, and induces expression of cold-responsive genes. The plasma-membrane-anchored NAC (NAM/ATAF/CUC) transcription factor NTL6 is of particular interest. Cold triggers proteolytic activation of the dormant NTL6 protein, which in turn elicits pathogen-resistance responses by inducing a small group of cold-inducible PR (pathogenesis-related) genes in Arabidopsis. In the present study, we show that proteolytic processing of NTL6 is regulated by cold-induced remodelling of membrane fluidity. NTL6 processing was stimulated rapidly by cold. The protein stability of NTL6 was also enhanced by cold. The effects of cold on NTL6 processing and protein stability were significantly reduced in cold-acclimatized plants, supporting the regulation of NTL6 processing by membrane fluidity. Consistent with this, although NTL6 processing was stimulated by pharmacological agents that reduce membrane fluidity and thus mimic cold, it was inhibited when plants were treated with a 18:3 unsaturated fatty acid, linolenic acid. In addition, the pattern of NTL6 processing was changed in Arabidopsis mutants with altered membrane lipid compositions. Assays employing chemicals that inhibit activities of the proteasome and proteases showed that NTL6 processing occurs via the regulated intramembrane proteolysis mechanism. Interestingly, a metalloprotease inhibitor blocked the NTL6 processing. These observations indicate that a metalloprotease activity is responsible for NTL6 processing in response to cold-induced changes in membrane fluidity.

Genetically modified Arabidopsis thaliana cells reveal the involvement of the mitochondrial fatty acid composition in membrane basal and uncoupling protein-mediated proton leaks

We investigated the role of membrane fatty acids in basal proton leaks and uncoupling protein (UCP)-dependent proton conductance in Arabidopsis mitochondria. Using wild-type cells, cold-sensitive fad2 mutant cells, deficient in omega-6-oleate desaturase, and cold-tolerant FAD3(+) transformant cells, overexpressing omega-3-linoleate desaturase, we showed that basal proton leak in the non-phosphorylating state was dependent on lipid composition. The extent of membrane proton leak was drastically reduced in the fad2 mutant, containing low amounts of polyunsaturated fatty acids. Conversely, this proton leak was higher in FAD3(+) mitochondria that exhibit a higher polyunsaturated fatty acid content and high protein to lipid ratio. The dependency of membrane leaks upon membrane potential was higher in FAD3(+) and lower in fad2. UCP content was higher in both the fad2 mutant and FAD3(+) transgenic lines compared with wild-type cells and so was the UCP activity, assayed by the reduction of phosphorylation yield (ADP/O) triggered by palmitate as UCP activator. This UCP assay was validated by measurements of UCP-proton leak in the non-phosphorylating state (flux-force relationships between proton flux and membrane potential). The potential uncoupling capacity of the UCP was high enough to allow the loss of respiratory control in the three genotypes. Taken together, the data reported here suggest that the cold tolerance of FAD3(+) cells and the cold sensitivity of fad2 cells are associated with changes in their mitochondrial membrane basal proton leaks, whereas differences in functional expression of UCP are not simply related to cold adaptation in Arabidopsis cells.

Study of the effects of salicylic acid on soybean mitochondrial lipids and respiratory properties using the alternative oxidase as a stress-reporter protein

Biotic and abiotic stresses can lead to modifications in the lipid composition of cell membranes. Although mitochondria appear to be implicated in stress responses, little is known about the membrane lipid changes that occur in these organelles in plants. Besides cytochrome c oxidase, plant mitochondria have an alternative oxidase (AOX) that accepts electrons directly from ubiquinol, dissipating energy as heat. AOX upregulation occurs under a variety of stresses and its induction by salicylic acid (SA) has been observed in different plant species. AOX was also suggested to be used as a functional marker for cell reprogramming under stress. In the present study, we have used etiolated soybean (Glycine max (L.) Merr. cv Cresir) seedlings to study the effects of SA treatment on the lipid composition and the respiratory properties of hypocotyl mitochondria. AOX expression was studied in detail, as a reporter protein, to evaluate whether modifications in mitochondrial energy metabolism were occurring. In mitochondria extracted from SA-treated seedlings, AOX capacity and protein contents increased. Both AOX1 and AOX2b transcripts accumulated in response to SA, but with different kinetics. A reduction in external NADH oxidation capacity was observed, whereas succinate respiration remained unchanged. The phospholipid composition of mitochondria remained similar in control and SA-treated plants, but a reduction in the relative amount of linolenic acid was observed in phosphatidylcholine, phosphatidylethanolamine and cardiolipin. The possible causes of the fatty acid modifications observed, and the implications for mitochondrial metabolism are discussed.

Overexpression of endoplasmic reticulum omega-3 fatty acid desaturase gene improves chilling tolerance in tomato

An endoplasmic reticulum-localized tomato omega-3 fatty acid desaturase gene (LeFAD3) was isolated and characterized with regard to its sequence, response to various temperatures and function in transgenic tomato plants. Northern blot analysis showed that LeFAD3 was expressed in all organs tested and was markedly abundant in roots. Meanwhile, the expression of LeFAD3 was induced by chilling stress (4 degrees C), but inhibited by high temperature (40 degrees C). The transgenic plants were obtained under the control of the cauliflower mosaic virus 35S promoter (35S-CaMV). Northern and western blot analyses confirmed that sense LeFAD3 was transferred into tomato genome and overexpressed. Level of linolenic acids (18:3) increased and correspondingly level of linoleic acid (18:2) decreased in leaves and roots. After chilling stress, the fresh weight of the aerial parts of transgenic plants was higher than that of the wild type (WT) plants, and the membrane system ultrastructure of chloroplast in leaf cell and all the subcellular organelles in root tips of transgenic plants kept more intact than those of WT. Relative electric conductivity increased less in transgenic plants than that in WT, and the respiration rate of the transgenic plants was notably higher than that of WT. The maximal photochemical efficiency of PSII (F(v)/F(m)) and the O(2) evolution rate in WT decreased more than those in transgenic plants under chilling stress. Together with other data, results showed that the overexpression of LeFAD3 led to increased level of 18:3 and alleviated the injuries under chilling stress.