Chloroplast Membrane Remodeling during Freezing Stress Is Accompanied by Cytoplasmic Acidification Activating SENSITIVE TO FREEZING2

Exploring cotton SFR2's conundrum in response to cold stress

Cotton is an important agricultural crop to many regions across the globe but is sensitive to low-temperature exposure. The activity of the enzyme SENSITIVE TO FREEZING 2 (SFR2) improves cold tolerance of plants and produces trigalactosylsyldiacylglycerol (TGDG), but its role in cold sensitive plants, such as cotton remains unknown. Recently, it was reported that cotton SFR2 produced very little TGDG under normal and cold conditions. Here, we investigate cotton SFR2 activation and TGDG production. Using multiple approaches in the native system and transformation into Arabidopsis thaliana, as well as heterologous yeast expression, we provide evidence that cotton SFR2 activates differently than previously found among other plant species. We conclude with the hypothesis that SFR2 in cotton is not activated in a similar manner regarding acidification or freezing like Arabidopsis and that other regions of SFR2 protein are critical for activation of the enzyme than previously reported.

SENSITIVE TO FREEZING2 is crucial for growth of Marchantia polymorpha under acidic conditions

Land plants have evolved many systems to adapt to a wide range of environmental stresses. In seed plants, oligogalactolipid synthesis is involved in tolerance to freezing and dehydration, but it has not been analyzed in non-vascular plants. Here we analyzed trigalactosyldiacylglycerol (TGDG) synthesis in Marchantia polymorpha. TGDG is synthesized by galactolipid: galactolipid galactosyltransferase [GGGT; SENSITIVE TO FREEZING2 (SFR2) in Arabidopsis]. We analyzed the subcellular localization and GGGT activity of two M. polymorpha SFR2 homologs (MpGGGT1 and MpGGGT2, each as a GFP-fusion protein) using a transient expression system in Nicotiana benthamiana leaves and found that MpGGGT1-GFP localized in the chloroplast envelope membrane. We produced mutants Mpgggt1 and Mpgggt2 and found that TGDG did not accumulate in Mpgggt1 upon treatment of the thallus with acetic acid. Moreover, growth of Mpgggt1 mutants was impaired by acetic acid treatment. Microscopy revealed that the acetic acid treatment of M. polymorpha plants damaged intracellular membranes. The fact that the effect was similar for wild-type and Mpgggt1 plants suggested that MpGGGT has a role in recovery from damage. These results indicate that MpGGGT plays a crucial role in M. polymorpha growth under conditions of acid stress, which may have been encountered during the ancient terrestrial colonization of plants.

Using a thermal gradient table to study plant temperature signalling and response across a temperature spectrum

Plants must cope with ever-changing temperature conditions in their environment. In many plant species, suboptimal high and low temperatures can induce adaptive mechanisms that allow optimal performance. Thermomorphogenesis is the acclimation to high ambient temperature, whereas cold acclimation refers to the acquisition of cold tolerance following a period of low temperatures. The molecular mechanisms underlying thermomorphogenesis and cold acclimation are increasingly well understood but neither signalling components that have an apparent role in acclimation to both cold and warmth, nor factors determining dose-responsiveness, are currently well defined. This can be explained in part by practical limitations, as applying temperature gradients requires the use of multiple growth conditions simultaneously, usually unavailable in research laboratories. Here we demonstrate that commercially available thermal gradient tables can be used to grow and assess plants over a defined and adjustable steep temperature gradient within one experiment. We describe technical and thermodynamic aspects and provide considerations for plant growth and treatment. We show that plants display the expected morphological, physiological, developmental and molecular responses that are typically associated with high temperature and cold acclimation. This includes temperature dose-response effects on seed germination, hypocotyl elongation, leaf development, hyponasty, rosette growth, temperature marker gene expression, stomatal conductance, chlorophyll content, ion leakage and hydrogen peroxide levels. In conclusion, thermal gradient table systems enable standardized and predictable environments to study plant responses to varying temperature regimes and can be swiftly implemented in research on temperature signalling and response.

Lipidomic and Metabolomic Analyses Reveal Changes of Lipid and Metabolite Profiles in Rapeseed during Nitrogen Deficiency

Nitrogen is one of the most essential macronutrients for plant growth and its availability in soil is vital for agricultural sustainability and productivity. However, excessive nitrogen application could reduce the nitrogen use efficiency and produce environmental pollution. Here, we systematically determined the response in lipidome and metabolome in rapeseed during nitrogen starvation. Plant growth was severely retarded during nitrogen deficiency, while the levels of most amino acids were significantly decreased. The level of monogalactosyldiacyglycerol (MGDG) in leaves and roots was significantly decreased, while the level of digalactosyldiacylglycerol (DGDG) was significantly decreased in roots, resulting in a significant reduction of the MGDG/DGDG ratio during nitrogen starvation. Meanwhile, the levels of sulfoquinovosyl diacylglycerol, phosphatidylglycerol and glucuronosyl diacylglycerol were reduced to varying extents. Moreover, the levels of metabolites in the tricarboxylic acid cycle, Calvin cycle and energy metabolism were changed during nitrogen deficiency. These findings show that nitrogen deprivation alters the membrane lipid metabolism and carbon metabolism, and our study provides valuable information to further understand the response of rapeseed to nitrogen deficiency at the metabolism level.

Degradation of FATTY ACID EXPORT PROTEIN1 by RHOMBOID-LIKE PROTEASE11 contributes to cold tolerance in Arabidopsis

Plants need to acclimate to different stresses to optimize growth under unfavorable conditions. In Arabidopsis (Arabidopsis thaliana), the abundance of the chloroplast envelope protein FATTY ACID EXPORT PROTEIN1 (FAX1) decreases after the onset of low temperatures. However, how FAX1 degradation occurs and whether altered FAX1 abundance contributes to cold tolerance in plants remains unclear. The rapid cold-induced increase in RHOMBOID-LIKE PROTEASE11 (RBL11) transcript levels, the physical interaction of RBL11 with FAX1, the specific FAX1 degradation after RBL11 expression, and the absence of cold-induced FAX1 degradation in rbl11 loss-of-function mutants suggest that this enzyme is responsible for FAX1 degradation. Proteomic analyses showed that rbl11 mutants have higher levels of FAX1 and other proteins involved in membrane lipid homeostasis, suggesting that RBL11 is a key element in the remodeling of membrane properties during cold conditions. Consequently, in the cold, rbl11 mutants show a shift in lipid biosynthesis toward the eukaryotic pathway, which coincides with impaired cold tolerance. To test whether cold sensitivity is due to increased FAX1 levels, we analyzed FAX1 overexpressors. The rbl11 mutants and FAX1 overexpressor lines show superimposable phenotypic defects upon exposure to cold temperatures. Our re-sults show that the cold-induced degradation of FAX1 by RBL11 is critical for Arabidop-sis to survive cold and freezing periods.

All members of the Arabidopsis DGAT and PDAT acyltransferase families operate during high and low temperatures

The accumulation of triacylglycerol (TAG) in vegetative tissues is necessary to adapt to changing temperatures. It has been hypothesized that TAG accumulation is required as a storage location for maladaptive membrane lipids. The TAG acyltransferase family has five members (DIACYLGLYCEROL ACYLTRANSFERSE1/2/3 and PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1/2), and their individual roles during temperature challenges have either been described conflictingly or not at all. Therefore, we used Arabidopsis (Arabidopsis thaliana) loss of function mutants in each acyltransferase to investigate the effects of temperature challenge on TAG accumulation, plasma membrane integrity, and temperature tolerance. All mutants were tested under one high- and two low-temperature regimens, during which we quantified lipids, assessed temperature sensitivity, and measured plasma membrane electrolyte leakage. Our findings revealed reduced effectiveness in TAG production during at least one temperature regimen for all acyltransferase mutants compared to the wild type, resolved conflicting roles of pdat1 and dgat1 by demonstrating their distinct temperature-specific actions, and uncovered that plasma membrane integrity and TAG accumulation do not always coincide, suggesting a multifaceted role of TAG beyond its conventional lipid reservoir function during temperature stress.

The metabolism of nonstructural carbohydrates, lipids, and energy in two Cycas species with differential tolerance to unexpected freezing stress

Introduction

With the climate warming, the occurrence of freezing events is projected to increase in late spring and early autumn in the Northern Hemisphere. Observation of morphological traits showed that Cycas panzhihuaensis was more tolerant to unexpected freezing stress than C. bifida. Energy balance is crucial for plant tolerance to stress. Here, we aimed to determine whether the different responses of the two species to the unpredicted freezing stress were associated with the metabolism of energy and related substances.

Methods

The effects of unexpected freezing temperatures on C. panzhihuaensis and C. bifida were studied by measuring chlorophyll fluorescence parameters, energy charge and the profile of nonstructural carbohydrates (NSC) and lipids.

Results

C. panzhihuaensis exhibited higher stability of photosynthetic machinery than C. bifida under unpredicted freezing events. Significant interaction between species and treatments were observed in the energy charge, the level of NSC and its most components and the amount of most lipid categories and lipid classes. The decrease of soluble sugar and the increase of neutral glycerolipids at the early freezing stage, the accumulation of membrane glycerolipids at the late freezing stage and the continuous decrease of energy charge during the freezing period were the characteristics of C. panzhihuaensis responding to unexpected freezing stress. The degradation of membrane glycerolipids and the continuous decrease of soluble sugar during the freezing period and the accumulation of neutral glycerolipids and energy charge at the late freezing stage represented the characteristics of C. bifida responses.

Discussion

The different freezing sensitivity between C. panzhihuaensis and C. bifida might be associated with the differential patterns of the metabolism of energy, NSC and lipids. C. panzhihuaensis possesses the potential to be introduced to the areas of higher latitudes and altitudes.

The Morphological Parameters and Cytosolic pH of Cells of Root Zones in Tobacco Plants (Nicotiana tabacum L.): Nonlinear Effects of NaCl Concentrations

Salinity impacts important processes in plants, reducing their yield. The effect of salinity on the cytosolic pH (pHcyt) has been little studied. In this research, we employed transgenic tobacco plants expressing the pH sensor Pt-GFP to investigate the alterations in pHcyt in cells across various root zones. Furthermore, we examined a wide spectrum of NaCl concentrations (ranging from 0 to 150 mM) and assessed morphological parameters and plant development. Our findings revealed a pattern of cytosolic acidification in cells across all root zones at lower NaCl concentrations (50, 100 mM). Interestingly, at 150 mM NaCl, pHcyt levels either increased or returned to normal, indicating a nonlinear effect of salinity on pHcyt. Most studied parameters related to development and morphology exhibited an inhibitory influence in response to NaCl. Notably, a nonlinear relationship was observed in the cell length within the elongation and differentiation zones. While cell elongation occurred at 50 and 100 mM NaCl, it was not evident at 150 mM NaCl. This suggests a complex interplay between stimulating and inhibitory effects of salinity, contributing to the nonlinear relationship observed between pHcyt, cell length, and NaCl concentration.

Oligogalactolipid production during cold challenge is conserved in early diverging lineages

Severe cold, defined as a damaging cold beyond acclimation temperatures, has unique responses, but the signaling and evolution of these responses are not well understood. Production of oligogalactolipids, which is triggered by cytosolic acidification in Arabidopsis (Arabidopsis thaliana), contributes to survival in severe cold. Here, we investigated oligogalactolipid production in species from bryophytes to angiosperms. Production of oligogalactolipids differed within each clade, suggesting multiple evolutionary origins of severe cold tolerance. We also observed greater oligogalactolipid production in control samples than in temperature-challenged samples of some species. Further examination of representative species revealed a tight association between temperature, damage, and oligogalactolipid production that scaled with the cold tolerance of each species. Based on oligogalactolipid production and transcript changes, multiple angiosperm species share a signal of oligogalactolipid production initially described in Arabidopsis, namely cytosolic acidification. Together, these data suggest that oligogalactolipid production is a severe cold response that originated from an ancestral damage response that remains in many land plant lineages and that cytosolic acidification may be a common signaling mechanism for its activation.

Drought and heat stress mediated activation of lipid signaling in plants: a critical review

Lipids are a principal component of plasma membrane, acting as a protective barrier between the cell and its surroundings. Abiotic stresses such as drought and temperature induce various lipid-dependent signaling responses, and the membrane lipids respond differently to environmental challenges. Recent studies have revealed that lipids serve as signal mediators forreducing stress responses in plant cells and activating defense systems. Signaling lipids, such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, and N-acylethanolamines, are generated in response to stress. Membrane lipids are essential for maintaining the lamellar stack of chloroplasts and stabilizing chloroplast membranes under stress. However, the effects of lipid signaling targets in plants are not fully understood. This review focuses on the synthesis of various signaling lipids and their roles in abiotic stress tolerance responses, providing an essential perspective for further investigation into the interactions between plant lipids and abiotic stress.

Low-temperature and circadian signals are integrated by the sigma factor SIG5

Chloroplasts are a common feature of plant cells and aspects of their metabolism, including photosynthesis, are influenced by low-temperature conditions. Chloroplasts contain a small circular genome that encodes essential components of the photosynthetic apparatus and chloroplast transcription/translation machinery. Here, we show that in Arabidopsis, a nuclear-encoded sigma factor that controls chloroplast transcription (SIGMA FACTOR5) contributes to adaptation to low-temperature conditions. This process involves the regulation of SIGMA FACTOR5 expression in response to cold by the bZIP transcription factors ELONGATED HYPOCOTYL5 and ELONGATED HYPOCOTYL5 HOMOLOG. The response of this pathway to cold is gated by the circadian clock, and it enhances photosynthetic efficiency during long-term cold and freezing exposure. We identify a process that integrates low-temperature and circadian signals, and modulates the response of chloroplasts to low-temperature conditions.

LC-ESI-MS/MS Analysis of Sulfolipids and Galactolipids in Green and Red Lettuce (Lactuca sativa L.) as Influenced by Sulfur Nutrition

Sulfur (S) deprivation leads to abiotic stress in plants. This can have a significant impact on membrane lipids, illustrated by a change in either the lipid class and/or the fatty acid distribution. Three different levels of S (deprivation, adequate, and excess) in the form of potassium sulfate were used to identify individual thylakoid membrane lipids, which might act as markers in S nutrition (especially under stress conditions). The thylakoid membrane consists of the three glycolipid classes: monogalactosyl- (MGDG), digalactosyl- (DGDG), and sulfoquinovosyl diacylglycerols (SQDG). All of them have two fatty acids linked, differing in chain length and degree of saturation. LC-ESI-MS/MS served as a powerful method to identify trends in the change in individual lipids and to understand strategies of the plant responding to stress. Being a good model plant, but also one of the most important fresh-cut vegetables in the world, lettuce (Lactuca sativa L.) has already been shown to respond significantly to different states of sulfur supply. The results showed a transformation of the glycolipids in lettuce plants and trends towards a higher degree of saturation of the lipids and an increased level of oxidized SQDG under S-limiting conditions. Changes in individual MGDG, DGDG, and oxidized SQDG were associated to S-related stress for the first time. Promisingly, oxidized SQDG might even serve as markers for further abiotic stress factors.

Cell-Type-Specific Length and Cytosolic pH Response of Superficial Cells of Arabidopsis Root to Chronic Salinity

Soil salinity negatively affects the growth, development and yield of plants. Acidification of the cytosol in cells of glycophytes was reported under salinity, while various types of plant cells can have a specific reaction under the same conditions. Transgenic Arabidopsis plants expressing the pH sensor Pt-GFP in the cytosol were used in this work for determination of morphometric changes and cytosolic pH changes in the superficial cells of Arabidopsis roots under chronic salinity in vitro. We did not find changes in the length of the root cap cells, while there was a decrease in the length of the differentiation zone under 50, 75 mM NaCl and the size of the epidermal cells of the differentiation zone under 75 mM NaCl. The most significant changes of cytosolic pH to chronic salinity was noted in columella (decrease by 1 pH unit at 75 mM NaCl) and epidermal cells of the differentiation zone (decrease by 0.6 and 0.4 pH units at 50 and 75 mM NaCl, respectively). In developed lateral root cap cells, acidification of cytosol by 0.4 units occurred only under 75 mM NaCl in the medium. In poorly differentiated lateral cells of the root cap, there were no changes in pH under chronic salinity.

The Effect of Cold Stress on the Root-Specific Lipidome of Two Wheat Varieties with Contrasting Cold Tolerance

Complex glycerolipidome analysis of wheat upon low temperature stress has been reported for above-ground tissues only. There are no reports on the effects of cold stress on the root lipidome nor on tissue-specific responses of cold stress wheat roots. This study aims to investigate the changes of lipid profiles in the different developmental zones of the seedling roots of two wheat varieties with contrasting cold tolerance exposed to chilling and freezing temperatures. We analyzed 273 lipid species derived from 21 lipid classes using a targeted profiling approach based on MS/MS data acquired from schedule parallel reaction monitoring assays. For both the tolerant Young and sensitive Wyalkatchem species, cold stress increased the phosphatidylcholine and phosphatidylethanolamine compositions, but decreased the monohexosyl ceramide compositions in the root zones. We show that the difference between the two varieties with contrasting cold tolerance could be attributed to the change in the individual lipid species, rather than the fluctuation of the whole lipid classes. The outcomes gained from this study may advance our understanding of the mechanisms of wheat adaptation to cold and contribute to wheat breeding for the improvement of cold-tolerance.

Genetic loci associated with freezing tolerance in a European rapeseed (Brassica napus L.) diversity panel identified by genome-wide association mapping

Winter biotypes of rapeseed (Brassica napus L.) require a vernalization treatment to enter the reproductive phase and generally produce greater yields than spring rapeseed. To find genetic loci associated with freezing tolerance in rapeseed, we first performed genotyping-by-sequencing (GBS) on a diversity panel consisting of 222 rapeseed accessions originating primarily from Europe, which identified 69,554 high-quality single-nucleotide polymorphisms (SNPs). Model-based cluster analysis suggested that there were eight subgroups. The diversity panel was then phenotyped for freezing survival (visual damage and Fv/Fo and Fv/Fm) after 2 months of cold acclimation (5°C) and a freezing treatment (-15°C for 4 h). The genotypic and phenotypic data for each accession in the rapeseed diversity panel was then used to conduct a genome-wide association study (GWAS). GWAS results showed that 14 significant markers were mapped to seven chromosomes for the phenotypes scored. Twenty-four candidate genes located within the mapped loci were identified as previously associated with lipid, photosynthesis, flowering, ubiquitination, and cytochrome P450 in rapeseed or other plant species.

Chloroplast membrane lipid remodeling protects against dehydration by limiting membrane fusion and distortion

Dehydration damages the structural integrity of the chloroplast membrane and, consequently, the normal photosynthetic function of this organelle. Remodeling of galactolipids by converting monogalactosyl-diacylglycerol (MGDG) to digalactosyl-diacylglycerol (DGDG) and oligo-galactolipids is an effective adaptation strategy for protecting against dehydration damage to the chloroplast membrane. However, detailed molecular mechanisms are missing. In this study, by performing molecular-level simulations of bi-lamellar membranes under various dehydration conditions, we find that MGDG-to-DGDG remodeling protects the chloroplast membrane in a unique manner by simultaneously dictating both the extent and the pattern of fusion stalks formed with the apposed membrane. Specifically, MGDG-rich membranes form elongated stalks at a moderate dehydration level, whereas DGDG-rich membranes form smaller, rounded stalks. Simulations of wild-type and mutant Arabidopsis (Arabidopsis thaliana) outer chloroplast membranes further confirm that the mutant membrane without galactolipid remodeling is more susceptible to membrane fusion due to its higher MGDG content. Our work reveals the underlying physical mechanisms that govern the pattern and extent of membrane fusion structures, paving the way for rational genetic engineering of crops with improved dehydration tolerance.

Protein kinase and phosphatase control of plant temperature responses

Plants must cope with ever-changing temperature conditions in their environment. Suboptimal high and low temperatures, and stressful extreme temperatures, induce adaptive mechanisms that allow optimal performance and survival, respectively. These processes have been extensively studied at the physiological, transcriptional and (epi)genetic level. Cellular temperature signalling cascades and tolerance mechanisms also involve post-translational modifications (PTMs), particularly protein phosphorylation. Many protein kinases are known to be involved in cold acclimation and heat stress responsiveness but research on the role and importance of kinases and phosphatases in triggering responses to mild changes in temperature such as thermomorphogenesis is inadequately understood. In this review, we summarize the current knowledge on the roles of kinases and phosphatases in plant temperature responses. We discuss how kinases can function over a range of temperatures in different signalling pathways and provide an outlook to the application of PTM-modifying factors for the development of thermotolerant crops.

Mechanisms and functions of membrane lipid remodeling in plants

Lipid remodeling, defined herein as post-synthetic structural modifications of membrane lipids, play crucial roles in regulating the physicochemical properties of cellular membranes and hence their many functions. Processes affected by lipid remodeling include lipid metabolism, membrane repair, cellular homeostasis, fatty acid trafficking, cellular signaling and stress tolerance. Glycerolipids are the major structural components of cellular membranes and their composition can be adjusted by modifying their head groups, their acyl chain lengths and the number and position of double bonds. This review summarizes recent advances in our understanding of mechanisms of membrane lipid remodeling with emphasis on the lipases and acyltransferases involved in the modification of phosphatidylcholine and monogalactosyldiacylglycerol, the major membrane lipids of extraplastidic and photosynthetic membranes, respectively. We also discuss the role of triacylglycerol metabolism in membrane acyl chain remodeling. Finally, we discuss emerging data concerning the functional roles of glycerolipid remodeling in plant stress responses. Illustrating the molecular basis of lipid remodeling may lead to novel strategies for crop improvement and other biotechnological applications such as bioenergy production.

Sweet Modifications Modulate Plant Development

Plant development represents a continuous process in which the plant undergoes morphological, (epi)genetic and metabolic changes. Starting from pollination, seed maturation and germination, the plant continues to grow and develops specialized organs to survive, thrive and generate offspring. The development of plants and the interplay with its environment are highly linked to glycosylation of proteins and lipids as well as metabolism and signaling of sugars. Although the involvement of these protein modifications and sugars is well-studied, there is still a long road ahead to profoundly comprehend their nature, significance, importance for plant development and the interplay with stress responses. This review, approached from the plants' perspective, aims to focus on some key findings highlighting the importance of glycosylation and sugar signaling for plant development.

Advances in Understanding of Desiccation Tolerance of Lichens and Lichen-Forming Algae

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.

The Role of Chloroplast Membrane Lipid Metabolism in Plant Environmental Responses

Plants are nonmotile life forms that are constantly exposed to changing environmental conditions during the course of their life cycle. Fluctuations in environmental conditions can be drastic during both day–night and seasonal cycles, as well as in the long term as the climate changes. Plants are naturally adapted to face these environmental challenges, and it has become increasingly apparent that membranes and their lipid composition are an important component of this adaptive response. Plants can remodel their membranes to change the abundance of different lipid classes, and they can release fatty acids that give rise to signaling compounds in response to environmental cues. Chloroplasts harbor the photosynthetic apparatus of plants embedded into one of the most extensive membrane systems found in nature. In part one of this review, we focus on changes in chloroplast membrane lipid class composition in response to environmental changes, and in part two, we will detail chloroplast lipid-derived signals.

Gallic acid improves the antioxidant ability against cadmium toxicity: Impact on leaf lipid composition of sunflower (Helianthus annuus) seedlings

In the present work, the effect of seed pre-soaking with gallic acid (GA; 3,4,5-triphydroxyl-benzoic acid) in conferring subsequent tolerance to Cd stress in sunflower (Helianthus annuus) seedlings was investigated. Exposing sunflower seedlings to increasing Cd concentrations (5, 10 and 20 μM) caused a gradual decrease in root and shoot biomass and increased the metal accumulation in both organs. Seed pretreatment with 75 µM GA significantly restricted Cd uptake, markedly alleviated Cd-induced plant growth inhibition, and mitigated the oxidative damages caused by this metal, as compared to plants directly exposed to Cd. GA pre-soaking prior to Cd stress also enhanced catalase, ascorbate peroxidase and glutathione reductase activities, while inhibiting that of superoxide dismutase. This was associated with increased levels of total thiols and glutathione along with a decreased level of oxidized glutathione in leaves. Moreover, GA pre-soaking led to changes in leaf fatty acid composition of seedlings challenged with Cd, as evidenced by the higher total lipid content and lipid unsaturation degree. As a whole, this study provides strong arguments highlighting the potential role of GA as a growth promoter for sunflower seedlings submitted to Cd stress, notably by boosting the antioxidant defense system and improving leaf membrane stability.

Molecular cloning and functional characterization of GmAAPTs from soybean (Glycine max)

Aminoalcoholphosphotransferase (AAPT) utilizes diacylglycerols and cytidine diphosphate-choline/ethanolamine as substrates for the synthesis of phosphatidylcholine (PC)/phosphatidylethanolamine (PE). Plant AAPTs involved in phospholipid metabolism mediate diverse physiological processes; however, little is known about their functions in triacylglycerol (TAG) metabolism and seed germination. In the present study, we isolated and characterized two AAPTs, GmAAPT1 and GmAAPT2, from soybean (Glycine max). GmAAPT1 and GmAAPT2 exhibited strong similarity in their amino acid contents and expression patterns, and both were found to localize to the endoplasmic reticulum and Golgi apparatus. In vitro enzymatic analyses showed that GmAAPT1 and GmAAPT2 contributed to PC and PE synthesis and exhibited choline/ethanolamine phosphotransferase-like enzymatic properties. The overexpression of GmAAPT1 and GmAAPT2 in Arabidopsis led to reduced levels of seed TAG and polyunsaturated fatty acids and decreased seed germination under freezing stress. Together, these findings suggest that GmAAPTs mediate TAG metabolism and negatively regulate seed freezing tolerance.

The Role of Triacylglycerol in Plant Stress Response

Vegetable oil is mainly composed of triacylglycerol (TAG), a storage lipid that serves as a major commodity for food and industrial purposes, as well as an alternative biofuel source. While TAG is typically not produced at significant levels in vegetative tissues, emerging evidence suggests that its accumulation in such tissues may provide one mechanism by which plants cope with abiotic stress. Different types of abiotic stress induce lipid remodeling through the action of specific lipases, which results in various alterations in membrane lipid composition. This response induces the formation of toxic lipid intermediates that cause membrane damage or cell death. However, increased levels of TAG under stress conditions are believed to function, at least in part, as a means of sequestering these toxic lipid intermediates. Moreover, the lipid droplets (LDs) in which TAG is enclosed also function as a subcellular factory to provide binding sites and substrates for the biosynthesis of bioactive compounds that protect against insects and fungi. Though our knowledge concerning the role of TAG in stress tolerance is expanding, many gaps in our understanding of the mechanisms driving these processes are still evident. In this review, we highlight progress that has been made to decipher the role of TAG in plant stress response, and we discuss possible ways in which this information could be utilized to improve crops in the future.

Cold tolerance in the genus Arabidopsis

PREMISE

Cold tolerance is an important factor limiting the geographic distribution and growing season for many plant species, yet few studies have examined variation in cold tolerance extensively within and among closely related species and compared that to their geographic distribution.

METHODS

This study examines cold tolerance within and among species in the genus Arabidopsis. We assessed cold tolerance by measuring electrolyte leakage from detached leaves in multiple populations of five Arabidopsis taxa. The temperature at which 50% of cells were lysed was considered the lethal temperature (LT₅₀ ).

RESULTS

We found variability within and among taxa in cold tolerance. There was no significant within-species relationship between latitude and cold tolerance. However, the northern taxa, A. kamchatica, A. lyrata subsp. petraea, and A. lyrata subsp. lyrata, were more cold tolerant than A. thaliana and A. halleri subsp. gemmifera both before and after cold acclimation. Cold tolerance increased after cold acclimation (exposure to low, but nonfreezing temperatures) for all taxa, although the difference was not significant for A. halleri subsp. gemmifera. For all taxa except A. lyrata subsp. lyrata, the LT₅₀ values for cold-acclimated plants were higher than the January mean daily minimum temperature (T_min ), indicating that if plants were not insulated by snow cover, they would not likely survive winter at the northern edge of their range.

CONCLUSIONS

Arabidopsis lyrata and A. kamchatica were far more cold tolerant than A. thaliana. These extremely cold-tolerant taxa are excellent candidates for studying both the molecular and ecological aspects of cold tolerance.

A Lipidomic Approach to Identify Cold-Induced Changes in Arabidopsis Membrane Lipid Composition

Lipid changes that occur in leaves of plants (e.g., Arabidopsis thaliana), during cold and freezing stress can be analyzed with electrospray ionization triple quadrupole mass spectrometry, using high-throughput multiple reaction monitoring (MRM). An online tool, LipidomeDB Data Calculation Environment, is employed for mass spectral data processing.

Lipid remodeling under acidic conditions and its interplay with low Pi stress in Arabidopsis

Here we show that accumulation of galactose-containing lipids in plastid membranes in shoots and the other membranes in roots maintains Arabidopsis growth under acidic stress and acidic phosphate deficiency. Soil acidification and phosphate deficiency are closely related to each other in natural environments. In addition to the toxicity of high proton concentrations, acid soil can lead to imbalances of ion availability and nutritional deficiencies, including inorganic phosphate (Pi). Among plants, activation of non-phosphorus-containing galactolipid, digalactosyldiacylglycerol (DGDG), synthesis concomitant with phospholipid degradation, namely membrane lipid remodeling, is crucial for coping with Pi starvation. However, regulation mechanisms of membrane lipid composition during acidic stress have not been clarified. Here, we investigated lipid metabolism in Arabidopsis thaliana grown under acidic stress with or without Pi. Under Pi-sufficient acidic conditions, DGDG was increased in shoot membranes, and some Pi starvation-responsive genes that are involved in lipid remodeling were upregulated without reducing Pi content in leaves. In contrast, under acidic Pi deficiency, membrane lipid remodeling in roots was partially repressed at a lower external pH. Nevertheless, phenotypic comparison between wild type and the double mutant of MGD2/3, which are responsible for DGDG accumulation during Pi starvation, indicated that the complete absence of lipid remodeling in roots resulted in a loss of tolerance to Pi deficiency rather specifically under acidic conditions. This result suggested important physiological roles of galactolipid-enriched membranes under acidic Pi deficiency.

Transcriptomic profiling revealed genes involved in response to cold stress in maize

Maize is an important food crop. Chilling stress can decrease maize production by affecting seed germination and seedling growth, especially in early spring. We analysed chlorophyll fluorescence, membrane lipids, secondary metabolites and the transcriptome of two maize inbred lines (chilling-tolerant M54 and chilling-sensitive 753F) after 0, 4 and 24 h cold stress. M54 showed better ability to protect PSII and accumulate secondary metabolites. From RNA sequencing data, we determined that the majority of cold-affected genes were involved in photosynthesis, secondary metabolism, and signal transduction. Genes important for maintaining photosystem structure and for regulating electron transport were less affected by cold stress in M54 than in 753F. Expression of genes related to secondary metabolism and unsaturated fatty acid synthesis were upregulated more strongly in M54 than in 753F and M54 accumulated more unsaturated fatty acids and secondary metabolites. As a result, M54 achieved relatively high cold tolerance by protecting the photosystems and maintaining the stability of cell membranes.

An Arabidopsis protoplast isolation method reduces cytosolic acidification and activation of the chloroplast stress sensor SENSITIVE TO FREEZING 2

Chloroplasts adapt to freezing and other abiotic stresses in part by modifying their membranes. One key-remodeling enzyme is SENSITIVE TO FREEZING2 (SFR2). SFR2 is unusual because it does not respond to initial cold stress or cold acclimation, instead it responds during freezing conditions in Arabidopsis. This response has been shown to be sensitive to cytosolic acidification. The unique lipid products of SFR2 have also been detected in response to non-freezing stresses, but what causes SFR2 to respond in these stresses is unknown. Here, we investigate protoplast isolation as a representative of wounding stress. We show that SFR2 oligogalactolipid products accumulate during protoplast isolation. Notably, we show that protoplast cytosol is acidified during isolation. Modification of the buffers reduces oligogalactolipid accumulation, while prolonged incubation in the isolated state increases it. We conclude that SFR2 activation during protoplast isolation correlates with cytosolic acidification, implying that all SFR2 activation may be dependent on cytosolic acidification. We also conclude that protoplasts can be more gently isolated, reducing their stress.

Cellular Organization and Regulation of Plant Glycerolipid Metabolism

Great strides have been made in understanding how membranes and lipid droplets are formed and maintained in land plants, yet much more is to be learned given the complexity of plant lipid metabolism. A complicating factor is the multi-organellar presence of biosynthetic enzymes and unique compositional requirements of different membrane systems. This necessitates a rich network of transporters and transport mechanisms that supply fatty acids, membrane lipids and storage lipids to their final cellular destination. Though we know a large number of the biosynthetic enzymes involved in lipid biosynthesis and a few transport proteins, the regulatory mechanisms, in particular, coordinating expression and/or activity of the majority remain yet to be described. Plants undergoing stress alter their membranes' compositions, and lipids such as phosphatidic acid have been implicated in stress signaling. Additionally, lipid metabolism in chloroplasts supplies precursors for jasmonic acid (JA) biosynthesis, and perturbations in lipid homeostasis has consequences on JA signaling. In this review, several aspects of plant lipid metabolism are discussed that are currently under investigation: cellular transport of lipids, regulation of lipid biosynthesis, roles of lipids in stress signaling, and lastly the structural and oligomeric states of lipid enzymes.

LcFIN2, a novel chloroplast protein gene from sheepgrass, enhances tolerance to low temperature in Arabidopsis and rice

Adverse environmental stresses affect plant growth and crop yields. Sheepgrass (Leymus chinensis (Trin.) Tzvel), an important forage grass that is widely distributed in the east of Eurasia steppe, has high tolerance to extreme low temperature. Many genes that respond to cold stress were identified in sheepgrass by RNA-sequencing, but more detailed studies are needed to dissect the function of those genes. Here, we found that LcFIN2, a sheepgrass freezing-induced protein 2, encoded a chloroplast-targeted protein. Expression of LcFIN2 was upregulated by freezing, chilling, NaCl and abscisic acid (ABA) treatments. Overexpression of LcFIN2 enhanced the survival rate of transgenic Arabidopsis after freezing stress. Importantly, heterologous expression of LcFIN2 in rice exhibited not only higher survival rate but also accumulated various soluble substances and reduced membrane damage in rice under chilling stress. Furthermore, the chlorophyll content, the quantum photochemistry efficiency of photosystem II (ΦPSII), the non-photochemical quenching (NPQ), the net photosynthesis rate (Pn) and the expression of some chloroplast ribosomal-related and photosynthesis-related genes were higher in the transgenic rice under chilling stress. These findings suggested that the LcFIN2 gene could potentially be used to improve low-temperature tolerance in crops.

Research Progress in Membrane Lipid Metabolism and Molecular Mechanism in Peanut Cold Tolerance

Early sowing has been extensively used in high-latitude areas to avoid drought stress during sowing; however, cold damage has become the key limiting factor of early sowing. To relieve cold stress, plants develop a series of physiological and biochemical changes and sophisticated molecular regulatory mechanisms. The biomembrane is the barrier that protects cells from injury as well as the primary place for sensing cold signals. Chilling tolerance is closely related to the composition, structure, and metabolic process of membrane lipids. This review focuses on membrane lipid metabolism and its molecular mechanism, as well as lipid signal transduction in peanut (Arachis hypogaea L.) under cold stress to build a foundation for explicating lipid metabolism regulation patterns and physiological and molecular response mechanisms during cold stress and to promote the genetic improvement of peanut cold tolerance.

Identification and Functional Characterization of a Cold-Related Protein, BcHHP5, in Pak-Choi (Brassica rapa ssp. chinensis)

In plants, heptahelical proteins (HHPs) have been shown to respond to a variety of abiotic stresses, including cold stress. Up to the present, the regulation mechanism of HHP5 under low temperature stress remains unclear. In this study, BcHHP5 was isolated from Pak-choi (Brassica rapa ssp. chinensis cv. Suzhouqing). Sequence analysis and phylogenetic analysis indicated that BcHHP5 in Pak-choi is similar to AtHHP5 in Arabidopsis thaliana. Structure analysis showed that the structure of the BcHHP5 protein is relatively stable and highly conservative. Subcellular localization indicated that BcHHP5 was localized on the cell membrane and nuclear membrane. Furthermore, real-time quantitative polymerase chain reaction (RT-qPCR) analysis showed that BcHHP5 was induced to express by cold and other abiotic stresses. In Pak-choi, BcHHP5-silenced assay, inhibiting the action of endogenous BcHHP5, indicated that BcHHP5-silenced might have a negative effect on cold tolerance, which was further confirmed. All of these results indicate that BcHHP5 might play a role in abiotic response. This work can serve as a reference for the functional analysis of other cold-related proteins from Pak-choi in the future.

Lipid transport required to make lipids of photosynthetic membranes

Photosynthetic membranes provide much of the usable energy for life on earth. To produce photosynthetic membrane lipids, multiple transport steps are required, including fatty acid export from the chloroplast stroma to the endoplasmic reticulum, and lipid transport from the endoplasmic reticulum to the chloroplast envelope membranes. Transport of hydrophobic molecules through aqueous space is energetically unfavorable and must be catalyzed by dedicated enzymes, frequently on specialized membrane structures. Here, we review photosynthetic membrane lipid transport to the chloroplast in the context of photosynthetic membrane lipid synthesis. We independently consider the identity of transported lipids, the proteinaceous transport components, and membrane structures which may allow efficient transport. Recent advances in lipid transport of chloroplasts, bacteria, and other systems strongly suggest that lipid transport is achieved by multiple mechanisms which include membrane contact sites with specialized protein machinery. This machinery is likely to include the TGD1, 2, 3 complex with the TGD5 and TGD4/LPTD1 systems, and may also include a number of proteins with domains similar to other membrane contact site lipid-binding proteins. Importantly, the likelihood of membrane contact sites does not preclude lipid transport by other mechanisms including vectorial acylation and vesicle transport. Substantial progress is needed to fully understand all photosynthetic membrane lipid transport processes and how they are integrated.

Low-temperature tolerance in land plants: Are transcript and membrane responses conserved?

Plants' tolerance of low temperatures is an economically and ecologically important limitation on geographic distributions and growing seasons. Tolerance for low temperatures varies significantly across different plant species, and different mechanisms likely act in different species. In order to survive low-temperature stress, plant membranes must maintain their fluidity in increasingly cold and oxidative cellular environments. The responses of different species to low-temperature stress include changes to the types and desaturation levels of membrane lipids, though the precise lipids affected tend to vary by species. Regulation of membrane dynamics and other low-temperature tolerance factors are controlled by both transcriptional and post-transcriptional mechanisms. Here, we review low-temperature induced changes in both membrane lipid composition and gene transcription across multiple related plant species with differing degrees of low-temperature tolerance. We attempt to define a core set of changes for transcripts and lipids across species and treatment variations. Some responses appear to be consistent across all species for which data are available, while many others appear likely to be species or family-specific. Potential rationales are presented, including variance in testing, reporting and the importance of considering the level of stress perceived by the plant.

DIACYLGLYCEROL ACYLTRANSFERASE1 Contributes to Freezing Tolerance

Freezing limits plant growth and crop productivity, and plant species in temperate zones have the capacity to develop freezing tolerance through complex modulation of gene expression affecting various aspects of metabolism and physiology. While many components of freezing tolerance have been identified in model species under controlled laboratory conditions, little is known about the mechanisms that impart freezing tolerance in natural populations of wild species. Here, we performed a quantitative trait locus (QTL) study of acclimated freezing tolerance in seedlings of Boechera stricta, a highly adapted relative of Arabidopsis (Arabidopsis thaliana) native to the Rocky Mountains. A single QTL was identified that contained the gene encoding ACYL-COENZYME A:DIACYLGLYCEROL ACYLTRANSFERASE1 (BstDGAT1), whose expression is highly cold responsive. The primary metabolic enzyme DGAT1 catalyzes the final step in assembly of triacylglycerol (TAG) by acyl transfer from acyl-CoA to diacylglycerol. Freezing tolerant plants showed higher DGAT1 expression during cold acclimation than more sensitive plants, and this resulted in increased accumulation of TAG in response to subsequent freezing. Levels of oligogalactolipids that are produced by SFR2 (SENSITIVE TO FREEZING2), an indispensable element of freezing tolerance in Arabidopsis, were also higher in freezing-tolerant plants. Furthermore, overexpression of AtDGAT1 led to increased freezing tolerance. We propose that DGAT1 confers freezing tolerance in plants by supporting SFR2-mediated remodeling of chloroplast membranes.

Epigenomic plasticity of Arabidopsis msh1 mutants under prolonged cold stress

Dynamic transcriptional and epigenetic changes enable rapid adaptive benefit to environmental fluctuations. However, the underlying mechanisms and the extent to which this occurs are not well known. MutS Homolog 1 (MSH1) mutants cause heritable developmental phenotypes accompanied by modulation of defense, phytohormone, stress-response, and circadian rhythm genes, as well as heritable changes in DNA methylation patterns. Consistent with gene expression changes, msh1 mutants display enhanced tolerance for abiotic stress including drought and salt stress, while showing increased susceptibility to freezing temperatures. Despite changes in defense and biotic stress-response genes, msh1 mutants showed increasing susceptibility to the bacterial pathogen Pseudomonas syringae. Our results suggest that chronic cold and low light stress (10°C, 150 μmol m-2 s-1) influences non-CG methylation to a greater degree in msh1 mutants compared to wild-type Col-0. Furthermore, CHG changes are more closely pericentromeric, whereas CHH changes are generally more dispersed. This increased variation in non-CG methylation pattern does not significantly affect the msh1-derived enhanced growth behavior after mutants are crossed with isogenic wild type, reiterating the importance of CG methylation changes in msh1-derived enhanced vigor. These results indicate that msh1methylome is hyper-responsive to environmental stress in a manner distinct from the wild-type response, but CG methylation changes are potentially responsible for growth vigor changes in the crossed progeny.

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.

Phospholipid:Diacylglycerol Acyltransferase-Mediated Triacylglyerol Synthesis Augments Basal Thermotolerance

High temperatures rapidly induce a genetically programmed heat-shock response (HSR) that is essential to establish short-term acquired thermotolerance. In addition, an immediate HSR-independent metabolic response is triggered, resulting in an accumulation of unsaturated triacylglycerols (TAGs) in the cytosol. The metabolic processes involved in heat-induced TAG formation in plants and their physiological significance remain to be clarified. Lipidomic analyses of Arabidopsis (Arabidopsis thaliana) seedlings indicated that during heat stress, polyunsaturated fatty acids from thylakoid galactolipids are incorporated into cytosolic TAGs. In addition, rapid conversion of plastidic monogalactosyl diacylglycerols (MGDGs) into oligogalactolipids, acylated MGDGs, and diacylglycerols (DAGs), the direct precursor of TAGs, was observed. For TAG synthesis, DAG requires a fatty acid from the acyl-CoA pool or phosphatidylcholine. Since seedlings deficient in PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1 (PDAT1) were unable to accumulate TAGs after heat stress, phosphatidylcholine appears to be the major fatty acid donor. Results suggest that rapid plastid lipid metabolism drives TAG accumulation during heat stress. PDAT1-mediated TAG accumulation was found to increase heat resistance, since nonacclimated pdat1 mutant seedlings were more sensitive to severe heat stress, as indicated by a more dramatic decline of the maximum efficiency of PSII and lower seedling survival compared to wild-type seedlings. In contrast, nonacclimated trigalactosyldiacylglycerol1 (tgd1) mutants overaccumulating TAGs and oligogalactolipids were more resistant to heat stress. Hence, thylakoid lipid metabolism and TAG formation increases thermotolerance in addition to the genetically encoded HSR.

Coevolution of Domain Interactions in the Chloroplast TGD1, 2, 3 Lipid Transfer Complex Specific to Brassicaceae and Poaceae Plants

The import of lipids into the chloroplast is essential for photosynthetic membrane biogenesis. This process requires an ABC transporter in the inner envelope membrane with three subunits, TRIGALACTOSYLDIACYLGLYCEROL (TGD) 1, 2, and 3, named after the oligogalactolipids that accumulate in the respective Arabidopsis thaliana mutants. Unlike Arabidopsis, in the model grass Brachypodium distachyon, chloroplast lipid biosynthesis is largely dependent on imported precursors, resulting in a characteristic difference in chloroplast lipid acyl composition between the two plants. Accordingly, Arabidopsis is designated as a 16:3 (acyl carbons:double bounds) plant and Brachypodium as an 18:3 plant. Repression of TGD1 (BdTGD1) in Brachypodium affected growth without triggering oligogalactolipid biosynthesis. Moreover, expressing BdTGD1 in the Arabidopsis tgd1-1 mutant restored some phenotypes but did not reverse oligogalactolipid biosynthesis. A 27-amino acid loop (L45) is solely responsible for the incomplete functioning of BdTGD1 in Arabidopsis tgd1-1 Coevolutionary analysis and coimmunoprecipitation assays showed that the TGD1 L45 loop interacts with the mycobacterial cell entry domain of TGD2. To explain the observed differences in oligogalactolipid biosynthesis between the two species, we suggest that excess monogalactosyldiacylglycerol derived from chloroplast-derived precursors in Arabidopsis tgd1-1 is converted into oligogalactolipids, a process absent from Brachypodium with reduced TGD1 levels, which assembles monogalactosyldiacylglycerol exclusively from imported precursors.

SENSITIVE TO FREEZING2 Aides in Resilience to Salt and Drought in Freezing-Sensitive Tomato

SENSITIVE TO FREEZING2 (SFR2) is crucial for protecting chloroplast membranes following freezing in Arabidopsis (Arabidopsis thaliana). It has been shown that SFR2 homologs are present in all land plants, including freezing-sensitive species, raising the question of SFR2 function beyond freezing tolerance. Similar to freezing, salt and drought can cause dehydration. Thus, it is hypothesized that in freezing-sensitive plants SFR2 may play roles in their resilience to salt or drought. To test this hypothesis, SlSFR2 RNAi lines were generated in the cold/freezing-sensitive species tomato (Solanum lycopersicum [M82 cv]). Hypersensitivity to salt and drought of SlSFR2-RNAi lines was observed. Higher tolerance of wild-type tomatoes was correlated with the production of trigalactosyldiacylglycerol, a product of SFR2 activity. Tomato SFR2 in vitro activity is Mg2+-dependent and its optimal pH is 7.5, similar to that of Arabidopsis SFR2, but the specific activity of tomato SFR2 in vitro is almost double that of Arabidopsis SFR2. When salt and drought stress were applied to Arabidopsis, no conditions could be identified at which SFR2 was induced prior to irreversibly impacting plant growth, suggesting that SFR2 protects Arabidopsis primarily against freezing. Discovery of tomato SFR2 function in drought and salt resilience provides further insights into general membrane lipid remodeling-based stress tolerance mechanisms and together with protection against freezing in freezing-resistant plants such as Arabidopsis, it adds lipid remodeling as a possible target for the engineering of abiotic stress-resilient crops.