Differential degradation of extraplastidic and plastidic lipids during freezing and post-freezing recovery in Arabidopsis thaliana

Chlorophyll fluorescence characteristics and lipid metabolism in endangered Cycas panzhihuaensis exposed to drought, high temperature and their combination1

Cycas panzhihuaensis, an endangered species distributed in the dry-hot valleys of southwestern China, faces drought (D), heat (H), and their combination (DH) under current and future climatic conditions. To explore the responses of C. panzhihuaensis to D, H, and DH, chlorophyll fluorescence and the lipid and fatty acid profiles were determined. Leaf water loss and leaf damage only occurred following DH treatment. The photochemical activity was least impacted by D stress and most severely impacted by DH stress. D treatment reduced the levels of most lipid categories and total fatty acids. Both the H and DH treatments led to a significant decrease in the levels of saccharolipids, lysophospholipids, sphingolipids, and fatty acyls, while significantly increasing the levels of neutral glycerolipids and fatty acids. Moreover, odd-numbered fatty acids and trans-fatty acids-C18:2ttn-6 accumulated significantly following both H and DH treatments. However, the levels of both total fatty acids and total lipids were significantly lower after DH stress compared to H stress. The proportion of saturated fatty acids increased after D treatment and that of polyunsaturated fatty acids increased after both H and DH treatments. Following various treatments, the degree of unsaturation in total phospholipids decreased, while that in total saccharolipids remained unchanged. Additionally, the unsaturation levels of diacylglycerol and triacylglycerol showed no change after D stress, but increased after H and DH treatments. In conclusion, C. panzhihuaensis exhibited varying levels of tolerance to D, H, and DH treatments, which may be related to the differential adjustments in lipid composition and unsaturation levels.

Remodeling of membrane lipids associated with ABA-induced desiccation tolerance in Physcomitrium patens

Pretreatment of Physcomitrium patens with abscisic acid (ABA) has been shown to induce desiccation tolerance. While previous research suggests that ABA-induced production of proteins and soluble sugars contributes to desiccation stress tolerance, additional mechanisms underlying this tolerance remain unclear. In this study, we found that ABA pretreatment led to increased levels of digalactosyl diacylglycerol (DGDG), phosphatidylcholine (PC), and phosphatidylinositol (PI), along with a decrease in monogalactosyl diacylglycerol (MGDG). These changes elevated the MGDG/DGDG and PC/phosphatidylethanolamine (PE) ratios, potentially stabilizing membranes and enhancing desiccation tolerance. Furthermore, ABA pretreatment effectively prevented membrane lipid degradation during desiccation and subsequent rehydration. These findings highlight ABA's role in desiccation tolerance through membrane lipid modulation, providing new insights into stress tolerance mechanisms in bryophytes.

From sensing to acclimation: The role of membrane lipid remodeling in plant responses to low temperatures

Low temperatures pose a dramatic challenge to plant viability. Chilling and freezing disrupt cellular processes, forcing metabolic adaptations reflected in alterations to membrane compositions. Understanding the mechanisms of plant cold tolerance is increasingly important due to anticipated increases in the frequency, severity, and duration of cold events. This review synthesizes current knowledge on the adaptive changes of membrane glycerolipids, sphingolipids, and phytosterols in response to cold stress. We delve into key mechanisms of low-temperature membrane remodeling, including acyl editing and headgroup exchange, lipase activity, and phytosterol abundance changes, focusing on their impact at the subcellular level. Furthermore, we tabulate and analyze current gycerolipidomic data from cold treatments of Arabidopsis, maize, and sorghum. This analysis highlights congruencies of lipid abundance changes in response to varying degrees of cold stress. Ultimately, this review aids in rationalizing observed lipid fluctuations and pinpoints key gaps in our current capacity to fully understand how plants orchestrate these membrane responses to cold stress.

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.

Deciphering the Enhancing Impact of Exogenous Brassinolide on Physiological Indices of Melon Plants under Downy Mildew-Induced Stress

Melon (Cucumis melo L.) is a valuable horticultural crop of the Cucurbitaceae family. Downy mildew (DM), caused by Pseudoperonospora cubensis, is a significant inhibitor of the production and quality of melon. Brassinolide (BR) is a new type of phytohormone widely used in cultivation for its broad spectrum of resistance- and defense-mechanism-improving activity. In this study, we applied various exogenous treatments (0.5, 1.0, and 2.0 mg·L-1) of BR at four distinct time periods (6 h, 12 h, 24 h, and 48 h) and explored the impact of BR on physiological indices and the genetic regulation of melon seedling leaves infected by downy-mildew-induced stress. It was mainly observed that a 2.0 mg·L-1 BR concentration effectively promoted the enhanced photosynthetic activity of seedling leaves, and quantitative real-time polymerase chain reaction (qRT-PCR) analysis similarly exhibited an upregulated expression of the predicted regulatory genes of photosystem II (PSII) CmHCF136 (MELO3C023596.2) and CmPsbY (MELO3C010708.2), thus indicating the stability of the PSII reaction center. Furthermore, 2.0 mg·L-1 BR resulted in more photosynthetic pigments (nearly three times more than the chlorophyll contents (264.52%)) as compared to the control and other treatment groups and similarly upregulated the expression trend of the predicted key enzyme genes CmLHCP (MELO3C004214.2) and CmCHLP (MELO3C017176.2) involved in chlorophyll biosynthesis. Meanwhile, the maximum contents of soluble sugars and starch (186.95% and 164.28%) were also maintained, which were similarly triggered by the upregulated expression of the predicted genes CmGlgC (MELO3C006552.2), CmSPS (MELO3C020357.2), and CmPEPC (MELO3C018724.2), thereby maintaining osmotic adjustment and efficiency in eliminating reactive oxygen species. Overall, the exogenous 2.0 mg·L-1 BR exhibited maintained antioxidant activities, plastid membranal stability, and malondialdehyde (MDA) content. The chlorophyll fluorescence parameter values of F0 (42.23%) and Fv/Fm (36.67%) were also noticed to be higher; however, nearly three times higher levels of NPQ (375.86%) and Y (NPQ) (287.10%) were observed at 48 h of treatment as compared to all other group treatments. Increased Rubisco activity was also observed (62.89%), which suggested a significant role for elevated carbon fixation and assimilation and the upregulated expression of regulatory genes linked with Rubisco activity and the PSII reaction process. In short, we deduced that the 2.0 mg·L-1 BR application has an enhancing effect on the genetic modulation of physiological indices of melon plants against downy mildew disease stress.

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.

RNA-Seq Identified Putative Genes Conferring Photosynthesis and Root Development of Melon under Salt Stress

Melon is an important fruit crop of the Cucurbitaceae family that is being cultivated over a large area in China. Unfortunately, salt stress has crucial effects on crop plants and damages photosynthesis, membranal lipid components, and hormonal metabolism, which leads to metabolic imbalance and retarded growth. Herein, we performed RNA-seq analysis and a physiological parameter evaluation to assess the salt-induced stress impact on photosynthesis and root development activity in melon. The endogenous quantification analysis showed that the significant oxidative damage in the membranal system resulted in an increased ratio of non-bilayer/bilayer lipid (MGDG/DGDG), suggesting severe irregular stability in the photosynthetic membrane. Meanwhile, root development was slowed down by a superoxidized membrane system, and downregulated genes showed significant contributions to cell wall biosynthesis and IAA metabolism. The comparative transcriptomic analysis also exhibited that major DEGs were more common in the intrinsic membrane component, photosynthesis, and metabolism. These are all processes that are usually involved in negative responses. Further, the WGCN analysis revealed the involvement of two main network modules: the thylakoid membrane and proteins related to photosystem II. The qRT-PCR analysis exhibited that two key genes (MELO3C006053.2 and MELO3C023596.2) had significant variations in expression profiling at different time intervals of salt stress treatments (0, 6, 12, 24, and 48 h), which were also consistent with the RNA-seq results, denoting the significant accuracy of molecular dataset analysis. In summary, we performed an extensive molecular and metabolic investigation to check the salt-stress-induced physiological changes in melon and proposed that the PSII reaction centre may likely be the primary stress target.

Role of Lipids of the Evergreen Shrub Ephedra monosperma in Adaptation to Low Temperature in the Cryolithozone

Lipids are the fundamental components of cell membranes and they play a significant role in their integrity and fluidity. The alteration in lipid composition of membranes has been reported to be a major response to abiotic environmental stresses. Seasonal dynamics of membrane lipids in the shoots of Ephedra monosperma J.G. Gmel. ex C.A. Mey. growing in natural conditions of permafrost ecosystems was studied using HPTLC, GC-MS and ESI-MS. An important role of lipid metabolism was established during the autumn-winter period when the shoots of the evergreen shrub were exposed to low positive (3.6 °C), negative (-8.3 °C) and extremely low temperatures (-38.4 °C). Maximum accumulation of phosphatidic acid (PA), the amount of which is times times greater than the sum of phosphatidylcholine and phosphatidylethanolamine (PC + PE) was noted in shoots of E. monosperma in the summer-autumn period. The autumn hardening period (3.6 °C) is accompanied by active biosynthesis and accumulation of membrane lipids, a decrease of saturated 34:1 PCs, 34:1 PEs and 34:1 PAs, and an increase in unsaturated long-chain 38:5 PEs, 38:6 PEs, indicating that the adaptation of E. monosperma occurs not at the level of lipid classes but at the level of molecular species. At a further decrease of average daily air temperature in October (-8.3 °C) a sharp decline of PA level was registered. At an extreme reduction of environmental temperature (-38.4 °C) the content of non-bilayer PE and PA increases, the level of unsaturated fatty acids (FA) rises due to the increase of C18:2(Δ9,12) and C18:3(Δ9,12,15) acids and the decrease of C16:0 acids. It is concluded that changes in lipid metabolism reflect structural and functional reorganization of cell membranes and are an integral component of the complex process of plant hardening to low temperatures, which contributes to the survival of E. monosperma monocotyledonous plants in the extreme conditions of the Yakutia cryolithozone.

Dendrobium Multi-Omics Reveal Lipid Remodeling in Response to Freezing

Freezing damage is a common phenomenon responsible for reduced yields of economic crops. Regulation of lipid metabolism plays an important role in plant growth and adaptation during freezing. We previously carried out transcriptome and untargeted metabolome analyses to determine the regulation of flavonol and anthocyanin biosynthesis during freezing treatment (FT) and post-freezing recovery (FR) in Dendrobium catenatum. However, changes in lipid levels are hard to confirm by untargeted metabolomics analysis alone. Regulation of lipid metabolism in response to freezing is largely unknown in Dendrobium. In this study, a multi-omics strategy was used to offer a better means of studying metabolic flow during FT and FR. To this end, 6976 proteins were identified by the 4D_label-free proteome, including 5343 quantified proteins. For each of the two conditions, we enriched differentially accumulated proteins (DAPs) into 15 gene ontology (GO) terms, including primary metabolism, lipid metabolism, and photosynthesis processes. We also identified 7 lipid categories and 3672 lipid species using lipidome assays. We found significant remodeling occurring in the phospholipid category during FT and FR. We also found that most sphingolipids were significantly upregulated. An integrated multi-omics analysis revealed significant changes in the expression levels of 141 mRNAs and encoding proteins under both FT and FR conditions. During FT, phospholipase A (PLA) and phospholipase D (PLD) were associated with phospholipid editing and galactolipid remodeling. These results provide valuable new insights into how the freezing tolerance of D. catenatum might be improved by genetic engineering.

Comparative changes in sugars and lipids show evidence of a critical node for regeneration in safflower seeds during aging

During seed aging, there is a critical node (CN) where the population viability drops sharply. Exploring the specific locations of the CN in different species of plants is crucial for understanding the biological storage properties of seeds and refining seed life span management. Safflower, a bulk oil crop that relies on seeds for propagation, has a short seed life. However, at present, its biological characteristics during storage are not clear, especially the changes in metabolic capability and cell structures. Such knowledge is needed to improve the management of safflower seed life span and effective preservation in gene banks. Here, the seed survival curve of oilseed safflower under the controlled deterioration conditions of 60% relative humidity and 50°C was detected. The seed population showed an inverted S shape for the fall in germination. In the first 12 days of aging, germination remained above 86%. Prior to the CN at approximately day 10 (C10), when viability was in the "plateau" interval, seed vigor reduced at the same imbibition time point. Further analysis of the changes in sugar concentration found that the sucrose content decreased slowly with aging and the content of raffinose and two monosaccharides decreased abruptly at C10. Differentially metabolized lipids, namely lysophospholipids [lyso-phosphatidylcholine (LPC) and lyso-phosphatidylethanolamines (LPE)] and PMeOH, increased at day 3 of aging (C3). Fatty acid content increased by C6, and the content of phospholipids [phosphatidylcholines (PC), phosphatidylethanolamines (PE), and phosphatidylinositols (PI) and glycolipids [digalactosyl diacylglycerol, monogalactosyl diacylglycerol, and sulphoquinovosyl diglycerides (SQDG)] decreased significantly from C10. In addition, the activities of raffinose hydrolase alpha-galactosidase and the glyoxylate key enzyme isocitrate lyase decreased with seed aging. Confocal microscopy and transmission electron microscopy revealed shrinkage of the seed plasma membrane at C10 and the later fragmentation. Seedling phenotypic indicators and 2,3,5-triphenyltetrazolium chloride activity assays also verified that there were significant changes in seeds quality at the CN. In summary, the time point C10 is a CN during seed population aging. Before the CN, sugar and lipid metabolism, especially fatty acid metabolism into sugar, can make up for the energy consumed by aging. After this point, the seeds were irreversibly damaged, and their viability was greatly and rapidly reduced as the cell structure became increasingly destroyed.

Genome-wide identification and expression analysis of phospholipase D gene in leaves of sorghum in response to abiotic stresses

Abiotic stress caused by unsuitable environmental changes brings serious impacts on the growth and development of sorghum, resulting in significant loss in yield and quality every year. Phospholipase D is one of the key enzymes that catalyze the hydrolysis of phospholipids, and participates in plants response to abiotic stresses and phytohormones, whereas as the main producers of Phosphatidic acid (PA) signal, the detailed information about Phospholipase D associated (SbPLD) family in sorghum has been rarely reported. This study was performed to identify the PLD family gene in sorghum based on the latest genome annotation and to determine the expression of PLDs under abiotic stresses by qRT-PCR analysis. In this study, 13 PLD genes were identified in sorghum genome and further divided into 7 groups according to the phylogenetic analysis. All sorghum PLD family members harbored two conserved domains (HDK1&2) with catalytic activity, and most members contained a C2 domain. In ζ subfamily, C2 domain was replaced by PX and PH domain. The exon-intron structure of SbPLD genes within the same subfamily was highly conservative. The tissue specific expression analysis revealed different expression of SbPLD genes in various developmental stages. High level expression of SbPLDα3 was observed in almost all tissues, whereas SbPLDα4 was mainly expressed in roots. Under abiotic stress conditions, SbPLD genes responded actively to NaCl, ABA, drought (PEG) and cold (4 °C) treatment at the transcriptional level. The expression of SbPLDβ1 was significantly up-regulated, while the transcription of SbPLDζ was suppressed under various stress conditions. In addition, SbPLDβ1 and SbPLDδ2 were predicted to be the target genes of sbi-miR159 and sbi-miR167, respectively. This study will help to decipher the roles of PLDs in sorghum growth and abiotic stress responses.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01200-9.

Specific Changes in Arabidopsis thaliana Rosette Lipids during Freezing Can Be Associated with Freezing Tolerance

While the roles of a few specific lipids in plant freezing tolerance are understood, the effect of many plant lipids remains to be determined. Acclimation of plants to non-freezing cold before exposure to freezing temperatures improves the outcome of plants, compared to plants exposed to freezing without acclimation. Arabidopsis thaliana plants were subjected to one of three treatments: (1) "control", i.e., growth at 21 °C, (2) "non-acclimated", i.e., 3 days at 21 °C, 2 h at -8 °C, and 24 h recovery at 21 °C, and (3) "acclimated", i.e., 3 days at 4 °C, 2 h at -8 °C, and 24 h recovery at 21 °C. Plants were harvested at seven time points during the treatments, and lipid levels were measured by direct-infusion electrospray ionization tandem mass spectrometry. Ion leakage was measured at the same time points. To examine the function of lipid species in relation to freezing tolerance, the lipid levels in plants immediately following the freezing treatment were correlated with the outcome, i.e., ion leakage 24-h post-freezing. Based on the correlations, hypotheses about the functions of specific lipids were generated. Additionally, analysis of the lipid levels in plants with mutations in genes encoding patatin-like phospholipases, lipoxygenases, and 12-oxophytodienoic acid reductase 3 (opr3), under the same treatments as the wild-type plants, identified only the opr3-2 mutant as having major lipid compositional differences compared to wild-type plants.

Bioprospecting Bioactive Polar Lipids from Olive (Olea europaea cv. Galega vulgar) Fruit Seeds: LC-HR-MS/MS Fingerprinting and Sub-Geographic Comparison

Olive seeds have been considered as a new nutritionally healthy food supplement. They are rich in monounsaturated n-9 and essential polyunsaturated n-6 lipids. However, little is known about their polar lipids, potentially bioactive and chemical identity markers for olive pulp and oil. This work aimed to identify the polar lipidome of olive seeds to find possible bioactive compounds and markers of geographic origin, by studying samples from six Portuguese sub-regions. Polar lipids were obtained by solid/liquid extraction, NH₂-solid-phase extraction, and identified by hydrophilic interaction liquid chromatography (HILIC)-HR-ESI-MS and MS/MS. Ninety-four compounds were identified, including phospholipids, glycolipids, sphingolipids, and acyl sterol glycosides, several of which bear polyunsaturated fatty acids. Multivariate statistical analysis found unique profiles within each sub-region and markers of geographic identity, primarily phosphatidylcholines, phosphatidylethanolamines, and lysophosphatidylethanolamines. Therefore, polar lipid signatures should be further investigated, to assess their bioactivity, nutritional value, and chemical identity for valuing olive seeds and their oil.

Genome-wide in silico identification of phospholipase D (PLD) gene family from Corchorus capsularis and Corchorus olitorius: reveals their responses to plant stress

Background

Plant grows in nature facing various types of abiotic stresses for their normal growth and development. During abiotic stress, plants evolve different types of mechanisms to survive in a hostile environment. Phospholipase D (PLD) plays important role in the regulation of diverse cellular processes including stress responses in plants. Member of PLD genes are well studied in different model plants; however, their functions in the jute are not clear yet.

Result

In the present study, a total of 12 and 11 PLD genes were identified in the genome of C. capsularis and C. olitorius, respectively. The presence of the two conserved HKD motifs in PLD genes except for CoPLDδ-2 in jute suggests their strong lipase activity. Twenty different motifs were found in the identified PLD genes, and PLD-β1, PLD-γ1, and all members of PLD-δ1 of both jute species contained the highest number of motifs. Phylogenetic analysis showed the close evolutionary relationship among the five groups of jute PLD proteins along with the PLD proteins from Arabidopsis. Tissue-specific expression pattern of PLDα1-2, PLD-α2, PLDβ1, PLDγ1, and PLDδ1 of two jute species suggested their involvement in plant growth and development. However, the expression pattern of PLDα1-2, PLDα1-3, PLD-α4, PLDδ1, and PLDδ3 indicated their association during waterlogging stress. In addition, PLD-α2, PLDβ1, and PLDδ2 seemed to be involved in drought stress as well as salinity stress.

Conclusion

This genome-wide identification of jute PLD genes from C. capsularis and C. olitorius will help to further functional characterization of the PLD genes for developing stress-tolerant jute variety.

ICE-CBF-COR Signaling Cascade and Its Regulation in Plants Responding to Cold Stress

Cold stress limits plant geographical distribution and influences plant growth, development, and yields. Plants as sessile organisms have evolved complex biochemical and physiological mechanisms to adapt to cold stress. These mechanisms are regulated by a series of transcription factors and proteins for efficient cold stress acclimation. It has been established that the ICE-CBF-COR signaling pathway in plants regulates how plants acclimatize to cold stress. Cold stress is perceived by receptor proteins, triggering signal transduction, and Inducer of CBF Expression (ICE) genes are activated and regulated, consequently upregulating the transcription and expression of the C-repeat Binding Factor (CBF) genes. The CBF protein binds to the C-repeat/Dehydration Responsive Element (CRT/DRE), a homeopathic element of the Cold Regulated genes (COR gene) promoter, activating their transcription. Transcriptional regulations and post-translational modifications regulate and modify these entities at different response levels by altering their expression or activities in the signaling cascade. These activities then lead to efficient cold stress tolerance. This paper contains a concise summary of the ICE-CBF-COR pathway elucidating on the cross interconnections with other repressors, inhibitors, and activators to induce cold stress acclimation in plants.

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.

Recovery from chilling modulates the acyl-editing of phosphatidic acid molecular species in barley roots (Hordeum vulgare L.)

In plants, lipid metabolism and remodelling are key mechanisms for survival under temperature stress. The present study attempted to compare the lipid profile in barley roots both under chilling stress treatment and in the subsequent recovery to stress. Lipids were obtained through a single-extraction method with a polar solvent mixture, followed by mass spectrometry analysis. The results indicate that lipid metabolism was significantly affected by chilling. Most of the glycerolipids analysed returned to control values during short- and long-term recovery, whereas several representative phosphatidic acid (PA) molecular species were edited during long-term recovery. Most of the PA molecular species that increased in the long-term had the same acyl chains as the phosphatidylcholine (PC) species that decreased. C34:2 and C36:4 underwent the most remarkable changes. Given that the mechanisms underlying the acyl-editing of PC in barley roots remain elusive, we also evaluated the contribution of lysophosphatidylcholine acyltransferases (HvLPCAT) and phospholipase A (HvPLA). In line with the aforementioned results, the expression of the HvLPCAT and HvPLA genes was up-regulated during recovery from chilling. The differential acyl-editing of PA during recovery, which involves the remodelling of PC, might therefore be a regulatory mechanism of cold tolerance in barley.

Identification and Expression Profiling of Nonphosphorus Glycerolipid Synthase Genes in Response to Abiotic Stresses in Dendrobium catenatum

Dendrobium catenatum, a valuable Chinese herb, frequently experiences abiotic stresses, such as cold and drought, under natural conditions. Nonphosphorus glycerolipid synthase (NGLS) genes are closely linked to the homeostasis of membrane lipids under abiotic stress in plants. However, there is limited information on NGLS genes in D. catenatum. In this study, a total of eight DcaNGLS genes were identified from the D. catenatum genome; these included three monogalactosyldiacylglycerol synthase (DcaMGD1, 2, 3) genes, two digalactosyldiacylglycerol synthase (DcaDGD1, 2) genes, and three sulfoquinovosyldiacylglycerol synthase (DcaSQD1, 2.1, 2.2) genes. The gene structures and conserved motifs in the DcaNGLSs showed a high conservation during their evolution. Gene expression profiling showed that the DcaNGLSs were highly expressed in specific tissues and during rapid growth stages. Furthermore, most DcaNGLSs were strongly induced by freezing and post-freezing recovery. DcaMGD1 and DcaSQDs were greatly induced by salt stress in leaves, while DcaDGDs were primarily induced by salt stress in roots. Under drought stress, most DcaNGLSs were regulated by circadian rhythms, and DcaSQD2 was closely associated with drought recovery. Transcriptome analysis also revealed that MYB might be regulated by circadian rhythm and co-expressed with DcaNGLSs under drought stress. These results provide insight for the further functional investigation of NGLS and the regulation of nonphosphorus glycerolipid biosynthesis in Dendrobium.

HSP70-3 Interacts with Phospholipase Dδ and Participates in Heat Stress Defense

Heat shock proteins (HSPs) function as molecular chaperones and are key components responsible for protein folding, assembly, translocation, and degradation under stress conditions. However, little is known about how HSPs stabilize proteins and membranes in response to different hormonal or environmental cues in plants. Here, we combined molecular, biochemical, and genetic approaches to elucidate the involvement of cytosolic HSP70-3 in plant stress responses and the interplay between HSP70-3 and plasma membrane (PM)-localized phospholipase Dδ (PLDδ) in Arabidopsis (Arabidopsis thaliana). Analysis using pull-down, coimmunoprecipitation, and bimolecular fluorescence complementation revealed that HSP70-3 specifically interacted with PLDδ. HSP70-3 bound to microtubules, such that it stabilized cortical microtubules upon heat stress. We also showed that heat shock induced recruitment of HSP70-3 to the PM, where HSP70-3 inhibited PLDδ activity to mediate microtubule reorganization, phospholipid metabolism, and plant thermotolerance, and this process depended on the HSP70-3-PLDδ interaction. Our results suggest a model whereby the interplay between HSP70-3 and PLDδ facilitates the re-establishment of cellular homeostasis during plant responses to external stresses and reveal a regulatory mechanism in regulating membrane lipid metabolism.

The toc132toc120 heterozygote mutant of Arabidopsis thaliana accumulates decreased levels of the major chloroplast lipids

We used ESI-MS/MS to profile glycerolipids in a mutant of Arabidopsis thaliana that is null and heterozygous for the TOC132 and TOC120 genes, and is referred to as the toc132toc120± mutant. The goal was to assess the impact of a defective atToc132/120 receptor on the accumulation of chloroplast lipids. The mutant accumulated decreased amounts of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and phosphatidylglycerol (PG). In the cold-acclimated mutant, PG accumulated at the control levels. However, 34:4-PG (18:3/16:1Δ^3trans) was significantly decreased, which indicates that the mutant was impaired in synthesis of the chloroplast-derived PG. Major molecular species of MGDG and DGDG were significantly decreased, which was indicative of the decreased levels of triunsaturated fatty acids in galactolipids. The cold-acclimated mutant accumulated increased levels of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS), which indicate that defect in the atToc132/120 receptor did not impair the ER pathway of lipid synthesis. Both cold-acclimated wildtype and mutant plants accumulated increased levels of phosphatidic acid (PA). The increased levels of major molecular species of PA suggest that some pool of PA was derived from degradation of both the chloroplast and extra-chloroplast lipids. The cold-acclimated mutant had decreased double bond index (DBI) and increased acyl chain length (ACL), which was indicative of decreased membrane fluidity. However, a decrease in the ratio of MGDG to DGDG indicate that the mutant was capable of remodeling membrane lipids in response to low temperatures. We conclude that the defective Toc132/120 receptor resulted in decreased synthesis of chloroplast lipids and decreased membrane fluidity.

Lipid Remodeling Confers Osmotic Stress Tolerance to Embryogenic Cells during Cryopreservation

Plant species conservation through cryopreservation using plant vitrification solutions (PVS) is based in empiricism and the mechanisms that confer cell integrity are not well understood. Using ESI-MS/MS analysis and quantification, we generated 12 comparative lipidomics datasets for membranes of embryogenic cells (ECs) of Magnolia officinalis during cryogenic treatments. Each step of the complex PVS-based cryoprotocol had a profoundly different impact on membrane lipid composition. Loading treatment (osmoprotection) remodeled the cell membrane by lipid turnover, between increased phosphatidic acid (PA) and phosphatidylglycerol (PG) and decreased phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The PA increase likely serves as an intermediate for adjustments in lipid metabolism to desiccation stress. Following PVS treatment, lipid levels increased, including PC and PE, and this effectively counteracted the potential for massive loss of lipid species when cryopreservation was implemented in the absence of cryoprotection. The present detailed cryobiotechnology findings suggest that the remodeling of membrane lipids and attenuation of lipid degradation are critical for the successful use of PVS. As lipid metabolism and composition varies with species, these new insights provide a framework for technology development for the preservation of other species at increasing risk of extinction.

The TOC159 null mutant of Arabidopsis thaliana is impaired in the accumulation of plastid lipids and phosphatidylcholine

We used electrospray ionization tandem mass spectrometry to profile glycerolipids in the TOC159 null mutant of Arabidopsis, which is referred to as plastid protein import 2, or ppi2. The goal was to evaluate the impact of a defective atToc159 receptor in the accumulation of plastid lipids. The ppi2 mutant is severely impaired in the accumulation of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and phosphatidylglycerol (PG), which are major components of the thylakoid membranes. Major molecular species of MGDG and DGDG are drastically decreased, which is consistent with our previous findings of decreased levels of hexadecatrienoic and linolenic acids. Under normal growth conditions, the ppi2 mutant accumulated significantly lower levels of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). In the cold-acclimated mutant, the amounts of PE and PI were similar to the wildtype level, which indicates that the ER pathway of lipid synthesis was functional in the mutant. The cold-acclimated ppi2 mutant accumulated increased amounts of phosphatidic acid (PA), which was mirrored by an increase in phospholipase Dα (PLDα) transcript levels. These data suggest that PLDα activity contributed to the accumulation of cold-induced PA in the ppi2 mutant. The accumulation of major molecular species in PA indicates that cold-induced PA originated from the degradation of both plastidial and extraplastidial lipids. Compared with the wildtype, the ppi2 mutant had a low double bond index and high acyl chain length, which is indicative of decreased membrane fluidity. Taken together, these data indicate that a defective atToc159 receptor severely impaired the plastid pathway of lipid synthesis, which negatively affected the synthesis and/or accumulation of PC.

Ozone-induced lipid changes in the wildtype and toc132toc120 heterozygote mutant of Arabidopsis thaliana

We measured the fatty acids and lipid content in the wildtype and toc132toc120 heterozygote mutant of Arabidopsis thaliana that were exposed to elevated levels of ozone. The goal was to assess whether a defective atToc132/120 receptor would alter the mutant's response to ozone-induced stress. Increased malondialdehyde (MDA) levels were measured in wildtype plants that were exposed to ozone for 3 h and left in an ozone-free environment for 21 h. The increased levels of MDA were not positively correlated with changes in the levels of triunsaturated fatty acids from which MDA is derived. In both the wildtype and mutant plants, absolute amounts of the glycerolipids were not altered by ozone treatment. The untreated mutant, however, accumulated decreased levels of chloroplast lipids and triunsaturated fatty acids. In ozone-treated wildtype, the levels of 16:3 were significantly decreased and this was mirrored by decreased levels of TOC132 and FAD5 transcripts, and increased levels of SP1 E3 ligase transcripts. These data suggest a possible increase in protein ubiquitination under ozone-induced stress. In contrast, in ozone-treated mutant, the FAD5 transcripts accumulated at the control level. The untreated mutant, however, accumulated significantly increased levels of CAT1 and FAD7 transcripts, which indicates that a defective chloroplast receptor induced cellular stress. In ozone-treated wildtype, there was a small increase in 34:6-phosphatidic acid, which indicates that a small amount of the chloroplast-derived MGDG was degraded in response to ozone-induced stress. Overall, these data indicate that the wildtype and mutant responded differently in lipid composition and oxidation to ozone-induced stress.

Molecular cloning, characterization and expression analysis of CbPLDδ gene from Chorispora bungeana in low temperature

Chorispora bungeana (C. bungeana) is a rare alpine subnival species that is highly tolerant to low temperature stress. Phospholipase D (PLD) is a key enzyme involved in membrane phospholipid catabolism during plant growth and the stress response. In this study, one member of CbPLD gene family, CbPLDδ, was cloned from C. bungeana and was introduced into tobacco. This gene encodes an 864-amino acid protein with two catalytic HxKxxxxD motifs which are essential for phospholipase D activity. After the CbPLDδ gene is fused with the vector containing the GFP tag, subcellular localization showed that CbPLDδ was predominately located in the cell membrane. RT-qPCR and histochemical GUS assays showed that CbPLDδ gene was induced by low temperature and expressed predominantly in leaf and root. Compared with wild-type tobacco, CbPLDδ transgenic tobacco showed higher activities of antioxidant enzymes, and lower levels of malonidiadehyde and electrolyte leakage under low temperature stress. These results reflected that CbPLDδ is involved in the response to low temperature stress, and has the potential to improve the low temperature tolerance of plants.

Comprehensive evaluation of physiological traits under nitrogen stress and participation of linolenic acid in nitrogen-deficiency response in wheat seedlings

BACKGROUND

Nitrogen (N) deficiency is a major constraint for plant production in many areas. Developing the new crop genotypes with high productivity under N deficiency is an important approach to maintain agricultural production. Therefore, understanding how plant response to N deficiency and the mechanism of N-deficiency tolerance are very important for sustainable development of modern crop production.

RESULTS

In this study, the physiological responses and fatty acid composition were investigated in 24 wheat cultivars under N-deficient stress. Through Pearson's correlation analysis and principal component analysis, the responses of 24 wheat cultivars were evaluated. The results showed that the plant growth and carbohydrate metabolism were all differently affected by N deficiency in all tested wheat cultivars. The seedlings that had high shoot biomass also maintained high level of chlorophyll content under N deficiency. Moreover, the changes in fatty acid composition, especially the linolenic acid (18:3) and the double bond index (DBI), showed close positive correlations with the shoot dry weight and chlorophyll content alterations in response to N-deficient condition. These results indicated that beside the chlorophyll content, the linolenic acid content and DBI may also contribute to N-deficiency adaptation, thus could be considered as efficient indicators for evaluation of different response in wheat seedlings under N-deficient condition.

CONCLUSIONS

The alteration in fatty acid composition can potentially contribute to N-deficiency tolerance in plants, and the regulation of fatty acid compositions maybe an effective strategy for plants to adapt to N-deficient stress.

Membrane lipids are involved in plant response to oxygen deprivation

Membrane lipids change drastically in plants when they suffered from hypoxia (oxygen deficiency) stress. Overall, hypoxia stress lowers the contents of total lipids, inhabits lipid biosynthesis, and stimulates lipid degradation, leading to the accumulation of free fatty acids. Lipid alterations include changes in the contents of lipid classes, the extent of saturation, and the length of acyl chains. But the detail and systematic studies about lipid changes, as well as the function mechanism in hypoxia stress are poorly understood. Here, the major unanswered questions and suggestions on the study of the function of lipid in hypoxia stress were provided.

Quantitative Profiling of Arabidopsis Polar Glycerolipids under Two Types of Heat Stress

At the cellular level, the remodelling of membrane lipids and production of heat shock proteins are the two main strategies whereby plants survive heat stress. Although many studies related to glycerolipids and HSPs under heat stress have been reported separately, detailed alterations of glycerolipids and the role of HSPs in the alterations of glycerolipids still need to be revealed. In this study, we profiled the glycerolipids of wild-type Arabidopsis and its HSP101-deficient mutant hot-1 under two types of heat stress. Our results demonstrated that the alterations of glycerolipids were very similar in wild-type Arabidopsis and hot-1 during heat stress. Although heat acclimation led to a slight decrease of glycerolipids, the decrease of glycerolipids in plants without heat acclimation is more severe under heat shock. The contents of 36:x monogalactosyl diacylglycerol (MGDG) were slightly increased, whereas that of 34:6 MGDG and 34:4 phosphatidylglycerol (PG) were severely decreased during moderate heat stress. Our findings suggested that heat acclimation could reduce the degradation of glycerolipids under heat shock. Synthesis of glycerolipids through the prokaryotic pathway was severely suppressed, whereas that through the eukaryotic pathway was slightly enhanced during moderate heat stress. In addition, HSP101 has a minor effect on the alterations of glycerolipids under heat stress.

Analysis of changes in the Panax notoginseng glycerolipidome in response to long-term chilling and heat

Long-term moderately high or low temperatures can damage economically important plants. In the present study, we treated Panax notoginseng, an important traditional Chinese medicine, with temperatures of 10, 20, and 30 °C for 30 days. We then investigated P. notoginseng glycerolipidome responses to these moderate temperature stresses using an ESI/MS-MS-based lipidomic approach. Both long-term chilling (LTC, 10 °C) and long-term heat (LTH, 30 °C) decreased photo pigment levels and photosynthetic rate. LTH-induced degradation of photo pigments and glycerolipids may further cause the decline of photosynthesis and thereafter the senescence of leaves. LTC-induced photosynthesis decline is attributed to the degradation of photosynthetic pigments rather than the degradation of chloroplastidic lipids. P. notoginseng has an especially high level of lysophosphatidylglycerol, which may indicate that either P. notoginseng phospholipase A acts in a special manner on phosphatidylglycerol (PG), or that phospholipase B acts. The ratio of sulfoquinovosyldiacylglycerol (SQDG) to PG increased significantly after LTC treatment, which may indicate that SQDG partially substitutes for PG. After LTC treatment, the increase in the degree of unsaturation of plastidic lipids was less than that of extraplastidic lipids, and the increase in the unsaturation of PG was the largest among the ten lipid classes tested. These results indicate that increasing the level of unsaturated PG may play a special role in maintaining the function and stability of P. notoginseng photosystems after LTC treatment.

Dynamic changes in membrane lipid composition of leaves of winter wheat seedlings in response to PEG-induced water stress

BACKGROUND

Membrane lipid composition associates closely with membrane stability and fluidity under water stress. In this study, lipidomic analyses based on electrospray ionization mass spectrometry (ESI-MS/MS) were carried out to explore dynamic changes of membrane lipids in term of molecular species caused by PEG (Polyethylene glycol-6000)-induced water stress in wheat seedlings.

RESULTS

Among the main phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) are primary degradation targets, and PC was degraded in the largest degree. Membrane ion leakage dramatically increased later than the significant reduction of these phospholipids, indicating that the loss of membrane integrity lagged behind severe phospholipid degradation. Monogalactosyldiacylglycerol (MGDG) increased firstly and decreased later, while digalactosyldiacylglycerol (DGDG) ratcheted up with stress. DGDG/MGDG increased after stress for 3 days, and unsaturation of DGDG was promoted with stress. Variation trends of galactolipids differed among molecular species. The time when MGDG (34:3), DGDG (34:3) began to decline approached to the time when non-stomatal limitation impaired photosynthesis. While the two predominant molecular species MGDG (36:6) and DGDG (36:6) began to decline later. So we speculated that MGDG (34:3), DGDG (34:3) might be key components in photosynthesis apparatus and participate in photosynthesis directly. While the two predominant molecular species, MGDG (36:6) and DGDG (36:6) might locate in thylakoid lipid bilayer matrix and play roles in stabilizing the membrane. The research provides new insights into the dynamic response of lipid metabolism to PEG-induced water stress.

CONCLUSION

In wheat plants under water stress, the major molecular species of PC, PE and PG were degraded, MGDG and DGDG molecular species had differing degradation time courses.

Genes underlying cold acclimation in the tea plant (<i>Camellia sinensis</i> (L.) Kuntze)

The article reviews the latest studies showing the diversity of genetic mechanisms and gene families underlying the increased cold and frost tolerance of tea and other plant species. It has been shown that cell responses to chilling (0…+15°C) and freezing (< 0°C) are not the same and gene expression under cold stress is genotype-specific. In recent decades, progress has been made in understanding the genetic mechanisms underlying the cold response of plants – ICE1 (inducer of CBF expression 1), CBF (C-repeat-binding factor), COR (cold-regulated genes) pathways and signaling have been discovered. The ICE, CBF and DHN gene groups play a key role in the cold acclimation of the tea plant. The accumulation of CBF transcripts occurs after 15 min of chilling induction, and longer cold stress leads to accumulation of CBF transcripts. It is shown that the transcripts of the CsDHN1, CsDHN2 and CsDHN3 genes accumulate at a higher level in resistant genotypes of tea in comparison with susceptible cultivars during freezing. CBF-independent pathways include genes involved in metabolism and transcription factors such as HSFC1, ZAT12, CZF1, PLD (phospholipase D), WRKY, HD-Zip, CsLEA, LOX, NAC, HSP, which are widely distributed in plants and are involved in the basic mechanisms of tea resistance to cold and frost. The most recent studies show an important role of miRNA in the mechanisms of response to chilling and freezing in tea. The data obtained on different plant species may correlate with the mechanisms of frost tolerance of tea and are the basis for future studies of the signaling pathways of response to cold in the tea plant. The results of the research emphasize the need to further explore the ways in which various genes regulate the tolerance of tea to cold stress to find the molecular markers of frost tolerance.

Metabolomic analyses reveal substances that contribute to the increased freezing tolerance of alfalfa (Medicago sativa L.) after continuous water deficit

BACKGROUND

Alfalfa is a high-quality forage cultivated widely in northern China. Recently, the failure of alfalfa plants to survive the winter has caused substantial economic losses. Water management has attracted considerable attention as a method for the potential improvement of winter survival. The aim of this study was to determine whether and how changes in the water regime affect the freezing tolerance of alfalfa.

RESULTS

The alfalfa variety WL353LH was cultivated under water regimes of 80 and 25% of water-holding capacity, and all the plants were subjected to low temperatures at 4/0 °C (light/dark) and then - 2/- 6 °C (light/dark). The semi-lethal temperatures were lower for water-stressed than well-watered alfalfa. The pool sizes of total soluble sugars, total amino acids, and proline changed substantially under water-deficit and low-temperature conditions. Metabolomics analyses revealed 72 subclasses of differential metabolites, among which lipid and lipid-like molecules (e.g., fatty acids, unsaturated fatty acids, and glycerophospholipids) and amino acids, peptides, and analogues (e.g., proline betaine) were upregulated under water-deficit conditions. Some carbohydrates (e.g., D-maltose and raffinose) and flavonoids were also upregulated at low temperatures. Finally, Kyoto Encyclopedia of Genes and Genomes analyses revealed 18 significantly enriched pathways involved in the biosynthesis and metabolism of carbohydrates, unsaturated fatty acids, amino acids, and glycerophospholipids.

CONCLUSIONS

Water deficit significantly enhanced the alfalfa' freezing tolerance, and this was correlated with increased soluble sugar, amino acid, and lipid and lipid-like molecule contents. These substances are involved in osmotic regulation, cryoprotection, and the synthesis, fluidity, and stability of the cellular membrane. Our study provides a reference for improving alfalfa' winter survival through water management.

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.

An Advanced Lipid Metabolism System Revealed by Transcriptomic and Lipidomic Analyses Plays a Central Role in Peanut Cold Tolerance

Cold stress restricts peanut (Arachis hypogaea L.) growth, development, and yield. However, the specific mechanism of cold tolerance in peanut remains unknown. Here, the comparative physiological, transcriptomic, and lipidomic analyses of cold tolerant variety NH5 and cold sensitive variety FH18 at different time points of cold stress were conducted to fill this gap. Transcriptomic analysis revealed lipid metabolism including membrane lipid and fatty acid metabolism may be a significant contributor in peanut cold tolerance, and 59 cold-tolerant genes involved in lipid metabolism were identified. Lipidomic data corroborated the importance of membrane lipid remodeling and fatty acid unsaturation. It indicated that photosynthetic damage, resulted from the alteration in fluidity and integrity of photosynthetic membranes under cold stress, were mainly caused by markedly decreased monogalactosyldiacylglycerol (MGDG) levels and could be relieved by increased digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) levels. The upregulation of phosphatidate phosphatase (PAP1) and phosphatidate cytidylyltransferase (CDS1) inhibited the excessive accumulation of PA, thus may prevent the peroxidation of membrane lipids. In addition, fatty acid elongation and fatty acid β-oxidation were also worth further studied in peanut cold tolerance. Finally, we constructed a metabolic model for the regulatory mechanism of peanut cold tolerance, in which the advanced lipid metabolism system plays a central role. This study lays the foundation for deeply analyzing the molecular mechanism and realizing the genetic improvement of peanut cold tolerance.

Metabolomic analysis of the occurrence of bitter fruits on grafted oriental melon plants

Grafting has been widely applied to melon (Cucumis melo L.) production to alleviate obstacles of continuous cropping and control soil-borne diseases. However, grafting often leads to a decline of fruit quality. For example, sometimes bitter fruits are produced on grafted plants. However, the underlying physiological mechanism still remains unclear. This study investigated the effects of different rootstocks on the taste of fruits of the Balengcui, an oriental melon cultivar, during summer production. The results showed that all grafted plants with Cucurbita maxima Duch. rootstocks produced bitter fruits, while non-grafted plants and plants grafted onto muskmelon rootstocks produced no bitter fruits. Liquid chromatography-mass spectrometry and metabonomic analysis were performed to investigate the mechanism underlying the occurrence of bitter fruits. Metabolite comparisons of fruits from plants grafted onto Ribenxuesong rootstocks both with non-grafted plants and plants grafted onto muskmelon rootstocks showed that 17 metabolites including phospholipids, cucurbitacins and flavonoids, exhibited changes. The three Cucurbitacins, Cucurbitacin O, Cucurbitacin C, and Cucurbitacin S, increased dramatically. The 10 phospholipids PS(18:1(9Z)/18:2(9Z,12Z)), PS(P-18:0/15:0), PA(18:1(11Z)/18:1(11Z)), PE(16:0/18:0), PS(O-16:0/17:2(9Z,12Z)), PI(16:0/18:2(9Z,12Z)), PA(15:0/22:6(4Z,7Z,10Z,13Z,16Z,19Z)), PS(P-16:0/17:2(9Z,12Z)), PS(22:0/22:1(11Z)), and PA(17:1(9Z)/0:0)) were significantly decreased, while two PA (16:0/18:2 (9Z, 12Z) and 16:0/18:1 (11Z)), two flavonoids (pelargonidin 3-(6''-malonylglucoside)-5-glucoside and malvidin 3-rutinoside) significantly increased in fruits of plants grafted onto Cucurbita maxima Duch. rootstocks. These metabolites were involved in the glycerophospholipid metabolic pathway, the mevalonate pathway, and the phenylpropanoid pathway. In summary, these results showed that the bitter fruits of grafted Balengcui were caused by Cucurbita maxima Duch. rootstocks. Phospholipids, cucurbitacins, and flavonoids were the key contributors for the occurrence of bitter fruits in Balengcui melon after grafting onto Cucurbita maxima Duch. rootstocks.

Comparative transcriptomic analysis reveals gene expression associated with cold adaptation in the tea plant Camellia sinensis

BACKGROUND

Low temperature restricts the planting range of all crops, but cold acclimation induces adaption to cold stress in many plants. Camellia sinensis, a perennial evergreen tree that is the source of tea, is mainly grown in warm areas. Camellia sinensis var. sinensis (CSS) has greater cold tolerance than Camellia sinensis var. assamica (CSA). To gain deep insight into the molecular mechanisms underlying cold adaptation, we investigated the physiological responses and transcriptome profiles by RNA-Seq in two tea varieties, cold resistant SCZ (classified as CSS) and cold susceptible YH9 (classified as CSA), during cold acclimation.

RESULTS

Under freezing stress, lower relative electrical conductivity and higher chlorophyll fluorescence (Fv/Fm) values were detected in SCZ than in YH9 when subjected to freezing acclimation. During cold treatment, 6072 and 7749 DEGs were observed for SCZ and YH9, respectively. A total of 978 DEGs were common for both SCZ and YH9 during the entire cold acclimation process. DEGs were enriched in pathways of photosynthesis, hormone signal transduction, and transcriptional regulation of plant-pathogen interactions. Further analyses indicated that decreased expression of Lhca2 and higher expression of SnRK2.8 are correlated with cold tolerance in SCZ.

CONCLUSIONS

Compared with CSA, CSS was significantly more resistant to freezing after cold acclimation, and this increased resistance was associated with an earlier expression of cold-induced genes. Because the greater transcriptional differentiation during cold acclimation in SCZ may contribute to its greater cold tolerance, our studies identify specific genes involved in photoinhibition, ABA signal conduction, and plant immunity that should be studied for understanding the processes involved in cold tolerance. Marker-assisted breeding focused on the allelic variation at these loci provides an avenue for the possible generation of CSA cultivars that have CSS-level cold tolerance.

Genome-Wide Analysis of Phospholipase D Gene Family and Profiling of Phospholipids under Abiotic Stresses in Brassica napus

Oil crop Brassica napus is subjected to environmental stresses such as drought, cold and salt. Phospholipase Ds (PLDs) have vital roles in regulation of plant growth, development and stress tolerance. In this study, 32 BnaPLD genes were identified and classified into six subgroups depending on the conserved protein structures. High similarity in gene and protein structures exists between BnaPLDs and AtPLDs. Gene expression analysis showed that BnaPLDα1s and BnaPLDδs had higher expression than other PLDs. BnaPLDα1 and BnaPLDδ were significantly induced by abiotic stresses including dehydration, NaCl, abscisic acid (ABA) and 4�C. Lipidomic analysis showed that the content of main membrane phospholipids decreased gradually under stresses, except phosphatidylglycerol increased under the treatment of ABA and phosphatidylethanolamine increased under 4�C. Correspondingly, their product of phosphatidic acid increased often with a transient peak at 8 h. The plant height of mutants of PLDα1 was significantly reduced. Agronomic traits such as yield, seed number, silique number and branches were significantly impaired in PLDα1 mutants. These results indicate that there is a large family of PLD genes in B. napus, especially BnaPLDα1s and BnaPLDδs may play important roles in membrane lipids remodeling and maintaining of the growth and stress tolerance of B. napus.

Changes in gene expression during germination reveal pea genotypes with either "quiescence" or "escape" mechanisms of waterlogging tolerance

Waterlogging causes germination failure in pea (Pisum sativum L.). Three genotypes (BARI Motorshuti-3, Natore local-2 [NL-2], and Kaspa) contrasting in ability to germinate in waterlogged soil were exposed to different durations of waterlogging. Whole genome RNAseq was employed to capture differentially expressing genes. The ability to germinate in waterlogged soil was associated with testa colour and testa membrane integrity as confirmed by electrical conductivity measurements. Genotypes Kaspa and NL-2 displayed different mechanisms of tolerance. In Kaspa, an energy conserving strategy was indicated by a strong upregulation of tyrosine protein kinsase and down regulation of linoleate 9S-lipoxygenase 5, a fat metabolism gene. In contrast, a faster energy utilization strategy was suggested in NL-2 by the marked upregulation of a subtilase family protein and peroxisomal adenine nucleotide carrier 2, a fat metabolizing gene. Waterlogging susceptibility in germinating seeds of genotype BARI Motorshuti-3 was linked to upregulation of a kunitz-type trypsin/protease inhibitor that blocks protein metabolism and may lead to excessive lipid metabolism and the membrane leakage associated with waterlogging damage. Pathway analyses based on gene ontologies showed seed storage protein metabolism as upregulated in tolerant genotypes and downregulated in the sensitive genotype. Understanding the tolerance mechanism provides a platform to breed for adaptation to waterlogging stress at germination in pea.

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.

Differential phosphatidic acid metabolism in barley leaves and roots induced by chilling temperature

Phosphatidic acid (PA) is an important bioactive lipid that mediates chilling responses in barley. Modifications in the lipid composition of cellular membranes during chilling are essential to maintain their integrity and fluidity. First, we investigated the molecular species of PA present in leaves and roots by ESI-MS/MS, to evaluate the modifications that occur in response to chilling. We demonstrated that PA pools in leaves differ from PA fatty acid composition in roots. Compared with plants grown at 25 °C, the short-term and long-term chilling for 3 h and 36 h at 4 °C not produced significant changes in PA molecular species. The endogenous DAG and PA phosphorylation by in vitro DAG and PA kinase activities showed higher activity in leaves compared with that in root, and they showed contrasting responses to chilling. Similarly, PA removal by phosphatidate phosphohydrolase was tested, showing that this activity was specifically increased in response to chilling in roots. The findings presented here may be helpful to understand how the PA signal is modulated between tissues, and may serve to highlight the importance of knowing the basal PA pools in different plant organs.

Chloroplast Galactolipids: The Link Between Photosynthesis, Chloroplast Shape, Jasmonates, Phosphate Starvation and Freezing Tolerance

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) together constitute approximately 80% of chloroplast lipids. Apart from facilitating the photosynthesis light reaction in the thylakoid membrane, these two lipids are important for maintaining chloroplast morphology and for plant survival under abiotic stresses such as phosphate starvation and freezing. Recently it was shown that severe growth retardation phenotypes of the DGDG-deficient mutant dgd1 were due to jasmonate overproduction, linking MGDG and DGDG homeostasis with phytohormone production and suggesting MGDG as a major substrate for jasmonate biosynthesis. Induction of jasmonate synthesis and jasmonic acid (JA) signaling was also observed under conditions of phosphate starvation. We hypothesize that when DGDG is recruited to substitute for phospholipids in extraplastidic membranes during phosphate deficiency, the altered MGDG to DGDG ratio in the chloroplast envelope triggers the conversion of galactolipids into jasmonates. The conversion may contribute to rebalancing the MGDG to DGDG ratio rapidly to maintain chloroplast shape, and jasmonate production can reduce the growth rate and enhance predator deterrence. We also hypothesize that other conditions, such as suppression of dgd1 phenotypes by trigalactosyldiacylglycerol (tgd) mutations, may all be linked to altered jasmonate production, indicating that caution should be exercised when interpreting phenotypes caused by conditions that may alter the MGDG to DGDG ratio at the chloroplast envelope.

Turnover of Glycerolipid Metabolite Pool and Seed Viability

Hydration–dehydration cycles can frequently cause stress to seeds, but can also be used to improve germination. However, the molecular basis of the stress caused is poorly understood. Herein, we examine the effects of hydration–dehydration cycles on seed viability and profile the membrane glycerolipid molecular species. We find that seed viability was not affected during the first two cycles, but significantly decreased as further cycles were applied, until all viability was lost. The abundances of seven glycerolipid classes increased and decreased through hydration and dehydration, respectively, but the phosphatidic acid and diacylglycerol abundances changed in the opposite sense, while total glycerolipid contents remained constant. This suggests that during hydration–dehydration cycles, turnover of glycerolipid metabolite pools take place, while no significant lipid synthesis or degradation is involved. As further hydration–dehydration cycles occurred, lipid unsaturation increased, plastidic lipids decreased, and phosphatidylserine acyl chains lengthened. The latter two could be lethal for seeds. Our findings reveal a novel model of membrane lipid changes, and provide new insights into the responses of seeds to hydration–dehydration cycles.

Phospholipase Dα1-mediated phosphatidic acid change is a key determinant of desiccation-induced viability loss in seeds

High sensitivity of seeds to water loss is a widespread phenomenon in the world's plant species. The molecular basis of this trait is poorly understood but thought to be associated with critical changes in membrane function. We profiled membrane lipids of seeds in eight species with varying levels of desiccation tolerance and found a close association between reducing seed viability and increasing phosphatidic acid (PA). We applied hydration-dehydration cycles to Arabidopsis seeds, which are normally desiccation tolerant, to mimic the onset of desiccation sensitivity with progression towards germination and examined the role of phospholipase D (PLD) in desiccation stress-induced production of PA. We found that PLDα1 became more abundant and migrated from the cytosol to the membrane during desiccation, whereas PLDδ did not change, and that all desiccation-induced PA was derived from PLDα1 hydrolysis. When PLDα1 was suppressed, the germination level after each hydration-dehydration cycle improved significantly. We further demonstrated that PLDα1-mediated PA formation modulates desiccation sensitivity as applying its inhibitor improved seed desiccation tolerance and its suppression in protoplasts enhanced survival under dehydration. The insights provided by comparative lipidomics enable us to propose a new membrane-based model for seed desiccation stress and survival.

Comparative analysis of the response and gene regulation in cold resistant and susceptible tea plants

Cold environment is the main constraint for tea plants (Camellia sinensis) distribution and tea farming. We identified two tea cultivars, called var. sinensis cv. Shuchazao (SCZ) with a high cold-tolerance and var. assamica cv. Yinghong9 (YH9) with low cold-tolerance. To better understand the response mechanism of tea plants under cold stress for improving breeding, we compared physiological and biochemical responses, and associated genes expression in response to 7-day and 14-day cold acclimation, followed by 7-day de-acclimation in these two tea cultivars. We found that the low EL50, low Fv/Fm, and high sucrose and raffinose accumulation are responsible for higher cold tolerance in SCZ comparing with YH9. We then measured the expression of 14 key homologous genes, known as involved in these responses in other plants, for each stages of treatment in both cultivars using RT-qPCR. Our results suggested that the increased expression of CsCBF1 and CsDHNs coupling with the accumulation of sucrose play key roles in conferring higher cold resistance in SCZ. Our findings have revealed key genes regulation responsible for cold resistance, which help to understand the cold-resistant mechanisms and guide breeding in tea plants.

Profiling membrane glycerolipids during γ-ray-induced membrane injury

BACKGROUND

γ-rays are high-energy radiation that cause a range of random injuries to plant cells. Most studies on this issue have focused on γ-ray-induced nucleotide damage and the production of reactive oxygen species in cells, so little is known about the glycerolipid metabolism during γ-rays induced membrane injury. Using an ESI-MS/MS-based lipidomic method, we analysed the lipidome changes in wild-type and phospholipase D (PLD)δ- and α1-deficient Arabidopsis after γ-ray treatment. The aim of this study was to investigate the role of PLD-mediated glycerolipid metabolism in γ-ray-induced membrane injury.

RESULTS

The ion leakage of Arabidopsis leaves after 2885-Gy γ-ray treatment was less than 10%. High does γ-ray treatment could induce the accumulation of intracellular reactive oxygen species (ROS). Inhibition of PLDα1 caused severe lipid degradation under γ-ray treatment. γ-ray-induced glycerolipid degradation mostly happened in chloroplastidic lipids, rather than extraplastidic ones. The levels of lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) were maintained in the WS ecotypes during γ-ray treatments, while increased significantly in the Col ecotype treated with 1100 Gy. After 210- and 1100-Gy γ-ray treatments, the level of lysophosphatidylglycerol (lysoPG) decreased significantly in the four genotypes of Arabidopsis.

CONCLUSIONS

γ-ray-induced membrane injury may occur via an indirect mechanism. The degradation of distinct lipids is not synchronous, and that interconversions among lipids can occur. During γ-ray-induced membrane injury, the degradation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) may be mediated by PLDζ1 or phospholipase A1. The degradation of phosphatidylglycerol was not mediated by PLA, PLDδ or PLDα1, but by phospholipase C or other PLDs. γ-rays can decrease the double-bond index and increase the acyl chain length in membrane lipids, which may make membranes more rigid and further cause injury in membranes.

Does triacylglycerol (TAG) serve a photoprotective function in plant leaves? An examination of leaf lipids under shading and drought

Plant survival in many ecosystems requires tolerance of large radiation loads, unreliable water supply and suboptimal soil fertility. We hypothesized that increased production of neutral lipids (triacylglycerols, TAGs) in plant leaves is a mechanism for dissipating excess radiation energy. In a greenhouse experiment, we combined drought and shade treatments and examined responses among four species differing in life form, habitat, and drought- and shade-tolerance. We also present a lipid extraction protocol suitable for sclerophyllous leaves of native Australian trees (e.g. Acacia, Eucalyptus). Fluorescence measurements indicated that plants exposed to full sunlight experienced mild photoinhibition during our experiment. Accumulation of TAGs did not follow photosynthetic capacity, but instead, TAG concentration increased with non-photochemical quenching. This suggests that plants under oxidative stress may increase biosynthesis of TAGs. Moderate drought stress resulted in a 60% reduction in TAG concentration in wheat (Triticum aestivum). Shading had no effect on TAGs, but increased concentrations of polar lipids in leaves; for example, acclimation to shade in Austrodanthonia spp., a native Australian grass, resulted in a 60% increase in associated polar lipids and higher foliar chlorophyll concentrations. Shading also reduced the digalactosyldiacylglycerol:monogalactosyldiacylglycerol (DGDG:MGDG) ratio in leaves, with a corresponding increase in the degree of unsaturation and thus fluidity of thylakoid membranes of chloroplasts. Our results suggest that prevention of photodamage may be coordinated with accumulation of TAGs, although further research is required to determine if TAGs serve a photoprotective function in plant leaves.

Transcriptomic and metabolic analyses provide new insights into chilling injury in peach fruit

Low temperature conditioning (LTC) alleviates peach fruit chilling injury but the underlying molecular basis is poorly understood. Here, changes in transcriptome, ethylene production, flesh softening, internal browning and membrane lipids were compared in fruit maintained in constant 0 °C and LTC (pre-storage at 8 °C for 5 d before storage at 0 °C). Low temperature conditioning resulted in a higher rate of ethylene production and a more rapid flesh softening as a result of higher expression of ethylene biosynthetic genes and a series of cell wall hydrolases. Reduced internal browning of fruit was observed in LTC, with lower transcript levels of polyphenol oxidase and peroxidase, but higher lipoxygenase. Low temperature conditioning fruit also showed enhanced fatty acid content, increased desaturation, higher levels of phospholipids and a preferential biosynthesis of glucosylceramide. Genes encoding cell wall hydrolases and lipid metabolism enzymes were coexpressed with differentially expressed ethylene response factors (ERFs) and contained ERF binding elements in their promoters. In conclusion, LTC is a special case of cold acclimation which increases ethylene production and, operating through ERFs, promotes both softening and changes in lipid composition and desaturation, which may modulate membrane stability, reducing browning and contributing to alleviation of peach fruit chilling injury.

Metabolomics, a Powerful Tool for Agricultural Research

Metabolomics, which is based mainly on nuclear magnetic resonance (NMR), gas-chromatography (GC) or liquid-chromatography (LC) coupled to mass spectrometry (MS) analytical technologies to systematically acquire the qualitative and quantitative information of low-molecular-mass endogenous metabolites, provides a direct snapshot of the physiological condition in biological samples. As complements to transcriptomics and proteomics, it has played pivotal roles in agricultural and food science research. In this review, we discuss the capacities of NMR, GC/LC-MS in the acquisition of plant metabolome, and address the potential promise and diverse applications of metabolomics, particularly lipidomics, to investigate the responses of Arabidopsis thaliana, a primary plant model for agricultural research, to environmental stressors including heat, freezing, drought, and salinity.