Maintenance or collapse: responses of extraplastidic membrane lipid composition to desiccation in the resurrection plant Paraisometrum mileense

Lipids signaling and unsaturation of fatty acids participate in ramie response to submergence stress and hypoxia-responsive gene regulation

Ramie is a valuable crop that produces high-quality fibers and holds promise in ecological management and potential therapeutic properties. The damage of submergence during the fertile period seriously affects the growth of ramie. This study used transcriptomics and UPLC-QTOF/MS-based lipidomics analysis to reveal the lipids remodeling and stress adaptation mechanism in ramie response to submergence. The results of subcellular distribution showed that lipids in ramie leaf cells mostly aggregate in the inter-chloroplast cytoplasm to form lipid droplets under submergence stress. High-performance thin-layer chromatography (HPTLC) and lipidomics analysis showed that the composition and content of lipids in ramie leaves significantly changed under submergence stress, and the content of fatty acids (FAs) gradually accumulated with the extension of the submergence treatment time. Further analysis revealed that the content of 18:3 (n3) Coenzyme A (C18:3-CoA) increased significantly with the prolongation of submergence stress, and the exogenous addition of C18:3-CoA activated the expression of hypoxia-responsive marker genes such as BnADH1, BnPCO2, BnADH1, and BnPDC1. These results suggest that the ramie lipid metabolism pathways were significantly affected under submergence, and the C18:3-CoA may act directly or indirectly on the hypoxia-responsive genes to activate their transcriptional activities, thereby enhancing the tolerance of ramie to submergence stress.

Specific metabolic and cellular mechanisms of the vegetative desiccation tolerance in resurrection plants for adaptation to extreme dryness

MAIN CONCLUSION

Substantial advancements have been made in our comprehension of vegetative desiccation tolerance in resurrection plants, and further research is still warranted to elucidate the mechanisms governing distinct cellular adaptations. Resurrection plants are commonly referred to as a small group of extremophile vascular plants that exhibit vegetative desiccation tolerance (VDT), meaning that their vegetative tissues can survive extreme drought stress (> 90% water loss) and subsequently recover rapidly upon rehydration. In contrast to most vascular plants, which typically employ water-saving strategies to resist partial water loss and optimize water absorption and utilization to a limited extent under moderate drought stress, ultimately succumbing to cell death when confronted with severe and extreme drought conditions, resurrection plants have evolved unique mechanisms of VDT, enabling them to maintain viability even in the absence of water for extended periods, permitting them to rejuvenate without harm upon water contact. Understanding the mechanisms associated with VDT in resurrection plants holds the promise of expanding our understanding of how plants adapt to exceedingly arid environments, a phenomenon increasingly prevalent due to global warming. This review offers an updated and comprehensive overview of recent advances in VDT within resurrection plants, with particular emphasis on elucidating the metabolic and cellular adaptations during desiccation, including the intricate processes of cell wall folding and the prevention of cell death. Furthermore, this review highlights existing unanswered questions in the field, suggests potential avenues for further research to gain deeper insights into the remarkable VDT adaptations observed in resurrection plants, and highlights the potential application of VDT-derived techniques in crop breeding to enhance tolerance to extreme drought stress.

Wound Healing Activity of the Flavonoid-Enriched Fraction of Selaginella bryopteris Linn. against Streptozocin-Induced Diabetes in Rats

Diabetes and its complications, such as delayed wound healing, are increasing at an alarming rate in India, putting an enormous strain on the country’s limited healthcare resources. Hence, the present study proposes to screen/identify the possible mechanisms and to study the effect of the flavonoid-enriched fraction of Selaginella bryopteris extract against human keratinocyte cell lines (HaCaT) and streptozocin (STZ)-induced diabetic wounds in a male Wistar rat model. Chemical profiling was performed by an MTT assay. The obtained GC–MS analysis results showed the presence of amentoflavone, gallic acid, imidazole, palmitic acid, catechine, L-fucitol, lupeol, and myo-inositol as the major bioactive phytoconstituents. S. bryopteris induces the generation of ROS, the condensation of chromatin in the nucleus, and changes in the membrane potential of mitochondria in HaCaT cell lines. An S. bryopteris-dependent induction of apoptosis-mediated cell death in HaCaT cell lines was confirmed by an AO/PI analysis. Mitochondrial depolarization was reflected in JC-1 staining of cells. The wound size was reduced and epithelialization was enhanced. Keratinocyte migration decreased interleukins, TNF-α, IL-2, and IL-6 and the expression of pro-apoptotic (p53, caspase-3, caspase-9, and Bax) and anti-apoptotic (Bcl-2) genes in a dose-dependent manner. Keratinocyte migration increased antioxidant enzyme levels (CAT, SOD, MDA, and GSH). Wound healing is facilitated through the mitochondria-mediated apoptosis pathway, revealing a new area of diabetic wound therapy.

Gesneriads, a Source of Resurrection and Double-Tolerant Species: Proposal of New Desiccation- and Freezing-Tolerant Plants and Their Physiological Adaptations

Gesneriaceae is a pantropical family of plants that, thanks to their lithophytic and epiphytic growth forms, have developed different strategies for overcoming water scarcity. Desiccation tolerance or "resurrection" ability is one of them: a rare phenomenon among angiosperms that involves surviving with very little relative water content in their tissues until water is again available. Physiological responses of desiccation tolerance are also activated during freezing temperatures, a stress that many of the resurrection gesneriads suffer due to their mountainous habitat. Therefore, research on desiccation- and freezing-tolerant gesneriads is a great opportunity for crop improvement, and some of them have become reference resurrection angiosperms (Dorcoceras hygrometrica, Haberlea rhodopensis and Ramonda myconi). However, their difficult indoor cultivation and outdoor accessibility are major obstacles for their study. Therefore, this review aims to identify phylogenetic, geoclimatic, habitat, and morphological features in order to propose new tentative resurrection gesneriads as a way of making them more reachable to the scientific community. Additionally, shared and species-specific physiological responses to desiccation and freezing stress have been gathered as a stress response metabolic basis of the family.

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.

Redox imbalance impedes photosynthetic activity in rice by disrupting cellular membrane integrity and induces programmed cell death under submergence

Climate change negatively impacts the global hydrological resources leading to detrimental flood events. Submergence impedes the cellular membrane integrity, consequently affecting the membrane fluidity. Different abiotic stresses influence membrane lipid composition. Therefore, the remodeling of membrane lipids plays a major role in stress adaptation. Submergence-induced membrane lipid peroxidation is well established in plants. However, dynamic changes in lipid composition for regulating submergence tolerance in rice remain so far unexplored. The present study explored the effect of submergence on the lipidomic profile of the Sub1 near-isogenic lines (NILs) of rice, viz. Swarna, and Swarna Sub1 with contrasting submergence tolerance. The study also examined the association of lipidomic alteration with the membrane integrity and submergence tolerance. Submergence caused increased accumulation of reactive oxygen species (ROS), which was significantly higher in Swarna than Swarna Sub1. The lipid profile was also considerably altered under submergence. Following submergence, Swarna exhibited a significant decrease in phospholipid content accompanied by increased lipid peroxidation and electrolyte leakage. Furthermore, the disintegration of the thylakoid membrane resulted in a significant decrease in the chlorophyll content and photosynthesis rate under submergence. Submergence-induced hypoxic condition also promoted starch depletion to fulfill the energy requirement. In contrast, submergence acclimation in Swarna Sub1 was associated with the shift to anaerobic respiration mediated by increased alcohol dehydrogenase (ADH) activity. Effective ROS detoxification in Swarna Sub1 facilitated by increased antioxidant enzyme activities contributed to the submergence tolerance by maintaining membrane integrity and photosynthetic activity. The present study established the direct association of lipid remodeling with membrane integrity, cell viability, and photosynthesis and also devised a crop model to reveal the molecular background of submergence tolerance in plants.

Rehydration Process in Rustyback Fern (Asplenium ceterach L.): Profiling of Volatile Organic Compounds

Simple Summary

Severe environmental changes, such as drought, can delay growth, the development of plants, and induce injury to their tissues. However, a group of land plant species, called resurrection or desiccation-tolerant plants, is able to lose 95% of their cellular water and still remain viable for long periods, resuming full metabolic activity upon rehydration. Recovery from near-complete water loss is complex and requires the coordination of physical and chemical processes in the resurrection plants. Under stress conditions plants also synthesize and release a wide variety of volatile organic compounds with diverse biological and ecological functions. The rehydration process in resurrection rustyback fern (Asplenium ceterach) resulted in complete plant recovery within 72 h, accompanied by high emission of volatiles, mainly belonging to the group of fatty acid derivatives. These findings could have significant implications from biotechnological and ecological perspectives since the rustyback fern has been recently recognized as a valuable source of bioactive compounds.

Abstract

When exposed to stressful conditions, plants produce numerous volatile organic compounds (VOCs) that have different biological and environmental functions. VOCs emitted during the rehydration process by the fronds of desiccation tolerant fern Asplenium ceterach L. were investigated. Headspace GC–MS analysis revealed that the volatiles profile of rustyback fern is mainly composed of fatty acid derivatives: isomeric heptadienals (over 25%) and decadienals (over 20%), other linear aldehydes, alcohols, and related compounds. Aerial parts of the rustyback fern do not contain monoterpene-type, sesquiterpene-type, and diterpene-type hydrocarbons or corresponding terpenoids. Online detection of VOCs using proton-transfer reaction mass spectrometry (PTR–MS) showed a significant increase in emission intensity of dominant volatiles during the first hours of the rehydration process. Twelve hours after re-watering, emission of detected volatiles had returned to the basal levels that corresponded to hydrated plants. During the early phase of rehydration malondialdehyde (MDA) content in fronds, as an indicator of membrane damage, decreased rapidly which implies that lipoxygenase activity is not stimulated during the recovery process of rustyback fern.

Physiological and biochemical responses involved in vegetative desiccation tolerance of resurrection plant Selaginella brachystachya

The vegetative desiccation tolerance of Selaginella brachystachya has been evaluated for its ability to revive from a desiccation (air dry) state and start normal functioning when rehydrated. In this study, S. brachystachya was identified by DNA barcoding. Experiments were conducted using the detached hydrated, desiccated and rehydrated fronds under laboratory conditions to understand the mechanism of revival upon the water availability. Scanning Electron Microscope images during desiccation showed closed stomata and inside curled leaves. Chlorophyll concentration decreased by 1.1 fold in desiccated state and recovered completely upon rehydration. However, the total carotenoid content decreased 4.5 fold while the anthocyanin concentration increased 5.98 fold and the CO2 exchange rate became negative during desiccation. Lipid peroxidation and superoxide radical production were enhanced during desiccation by 68.32 and 73.4%, respectively. Relative electrolyte leakage was found to be minimal during desiccation. Activities of antioxidant enzymes, namely peroxidase (158.33%), glutathione reductase (107.70%), catalase (92.95%) and superoxide dismutase (184.70%) were found to be higher in the desiccated state. The proline concentration increased by 1.4 fold, starch concentration decreased 3.9 fold and sucrose content increased 2.8 fold during desiccation. Upon rehydration, S. brachystachya recovered its original morphology, physiological and biochemical functions. Our results demonstrate that S. brachystachya minimizes desiccation stress through a range of morphological, physiological and biochemical mechanisms. These results provide useful insights into desiccation tolerance mechanisms for potential utilization in enhancing stress tolerance in crop plants.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02667-1.

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.

New insights into the role of lipids in plant hypoxia responses

In plants, hypoxia (low-oxygen stress) is induced by soil waterlogging or submergence and this major abiotic stress has detrimental effects on plant growth, development, distribution, and productivity. To survive low-oxygen stress, plants have evolved a set of morphological, physiological, and biochemical adaptations. These adaptations integrate metabolic acclimation and signaling networks allowing plants to endure or escape from low-oxygen environments by altering their metabolism and growth. Lipids are ubiquitously involved in regulating plant responses to hypoxia and post-hypoxic reoxygenation. In particular, the polyunsaturation of long-chain acyl-CoAs regulates hypoxia sensing in plants by modulating acyl-CoA-binding protein-Group VII ethylene response factor dynamics. Moreover, unsaturated very-long-chain ceramide species protect plants from hypoxia-induced cellular damage by regulating the kinase activity of CONSTITUTIVE TRIPLE RESPONSE1 in the ethylene signaling pathway. Finally, the oxylipin jasmonate specifically regulates plant responses to reoxygenation stress by transcriptionally modulating antioxidant biosynthesis. Here we provide an overview of the roles of lipid remodeling and signaling in plant responses to hypoxia/reoxygenation and their effects on the downstream events affecting plant survival. In addition, we highlight the key remaining challenges in this important field.

Exploratory Study of Fatty Acid Profile in Two Filmy Ferns with Contrasting Desiccation Tolerance Reveal the Production of Very Long Chain Polyunsaturated Omega-3 Fatty Acids

Lipids are fundamental components of cell membranes and play a significant role in their integrity and fluidity. Alteration in lipid composition of membranes has been reported to be a major response to abiotic environmental stresses. This work was focused on the characterization of frond lipid composition and membrane integrity during a desiccation–rehydration cycle of two filmy fern species with contrasting desiccation tolerance: Hymenophyllum caudiculatum (less tolerant) and Hymenophyllum plicatum (more tolerant). The relative water content decreased without differences between species when both filmy ferns were subjected to desiccation. However, H. plicatum reached a higher relative water content than H. caudiculatum after rehydration. Fatty acids profiles showed the presence of a very long chain polyunsaturated fatty acid during the desiccation–rehydration cycle, with eicosatrienoic acid being the most abundant. Additionally, propidium iodide permeation staining and confocal microscopy demonstrated that, following the desiccation–rehydration cycle, H. plicatum exhibited a greater membrane integrity than H. caudiculatum. The lack of some very long chain fatty acids such as C22:1n9 and C24:1n9 in this species contrasting with H. plicatum may be associated with its lower membrane stability during the desiccation–rehydration cycle. This report provides the first insight into the fatty acid composition and dynamics of the membrane integrity of filmy ferns during a desiccation–rehydration cycle. This could potentially play a role in determining the different levels of desiccation tolerance and microhabitat preferences exhibited by Hymenophyllaceae species.

Vegetative desiccation tolerance in Eragrostiella brachyphylla: biochemical and physiological responses

Eragrostiella brachyphylla is an angiosperm desiccation-tolerant resurrection plant, which can survive during desiccation in the air-dry state and recover completely on availability of water. The present study was conducted to understand the vegetative desiccation tolerance of Eragrostiella brachyphylla by evaluating its ability to recover the physiological, biochemical and morphological functions post desiccation. In order to understand the responses of Eragrostiella brachyphylla to desiccation and subsequent rehydration experiments were conducted in the hydrated state (HS), desiccated state (DS) and rehydrated state (RS). Scanning electron microscopy revealed significant changes between the three stages in the internal ultra-structures of leaves and stems. Compared to the other states, photosynthetic parameters such as chlorophyll a, chlorophyll b, total chlorophylland total carotenoid contents decreased significantly in the desiccated state. Superoxide radical (O2•-) content also increased, resulting in an oxidative burst during desiccation. Consequently, antioxidant enzymes such as catalase (CAT) superoxide dismutase (SOD) peroxidase (APX) and glutathione reductase (GR) activities were found to be significantly elevated in the desiccated state to avoid oxidative damage. Increased malondialdehyde (MDA) content and relative electrolyte leakage (REL) during desiccation provide evidence for membrane damage and loss of cell-wall integrity. During desiccation, the contents of osmolytes represented by sucrose and proline were found to increase to maintain cell structure integrity. After rehydration, all physiological, biochemical and morphological properties remain unchanged or slightly changed when compared to the hydrated state. Hence, we believe that these unique adaptations contribute to the remarkable desiccation-tolerance property of this plant.

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.

Mesophyll conductance: the leaf corridors for photosynthesis

Besides stomata, the photosynthetic CO2 pathway also involves the transport of CO2 from the sub-stomatal air spaces inside to the carboxylation sites in the chloroplast stroma, where Rubisco is located. This pathway is far to be a simple and direct way, formed by series of consecutive barriers that the CO2 should cross to be finally assimilated in photosynthesis, known as the mesophyll conductance (gm). Therefore, the gm reflects the pathway through different air, water and biophysical barriers within the leaf tissues and cell structures. Currently, it is known that gm can impose the same level of limitation (or even higher depending of the conditions) to photosynthesis than the wider known stomata or biochemistry. In this mini-review, we are focused on each of the gm determinants to summarize the current knowledge on the mechanisms driving gm from anatomical to metabolic and biochemical perspectives. Special attention deserve the latest studies demonstrating the importance of the molecular mechanisms driving anatomical traits as cell wall and the chloroplast surface exposed to the mesophyll airspaces (Sc/S) that significantly constrain gm. However, even considering these recent discoveries, still is poorly understood the mechanisms about signaling pathways linking the environment a/biotic stressors with gm responses. Thus, considering the main role of gm as a major driver of the CO2 availability at the carboxylation sites, future studies into these aspects will help us to understand photosynthesis responses in a global change framework.

Common and Specific Mechanisms of Desiccation Tolerance in Two Gesneriaceae Resurrection Plants. Multiomics Evidences

Environmental stress, especially water deficiency, seriously limits plant distribution and crop production worldwide. A small group of vascular angiosperm plants termed "resurrection plants," possess desiccation tolerance (DT) to withstand dehydration and to recover fully upon rehydration. In recent years, with the rapid development of life science in plants different omics technologies have been widely applied in resurrection plants to study DT. Boea hygrometrica is native in East and Southeast Asia, and Haberlea rhodopensis is endemic to the Balkans in Europe. They are both resurrection pants from Gesneriaceae family. This paper reviews recent advances in transcriptome and metabolome, and discusses the differences and similarities of DT features between both species. Finally, we believe we provide novel insights into understanding the mechanisms underlying the acquisition and evolution of desiccation tolerance of the resurrection plants that could substantially contribute to develop new approaches for agriculture to overcome water deficiency in future.

Molecular Basis of the Glucose Transport Mechanism in Plants

The SWEET family belongs to a class of transporters in plants that undergoes large conformational changes to facilitate transport of sugar molecules across the cell membrane (SWEET, Sugars Will Eventually Be Exported Transporter). However, the structures of their functionally relevant conformational states in the transport cycle have not been reported. In this study, we have characterized the conformational dynamics and complete transport cycle of glucose in the OsSWEET2b transporter using extensive molecular dynamics simulations. Using Markov state models, we estimated the free energy barrier associated with different states as well as for the glucose transport mechanism. SWEETs undergo a structural transition to outward-facing (OF), occluded (OC), and inward-facing (IF) and strongly support an alternate access transport mechanism. The glucose diffuses freely from outside to inside the cell without causing major conformational changes which means that the conformations of glucose unbound and bound snapshots are exactly the same for OF, OC, and IF states. We identified a network of hydrophobic core residues at the center of the transporter that restricts the glucose entry to the cytoplasmic side and acts as an intracellular hydrophobic gate. The mechanistic predictions from molecular dynamics simulations are validated using site-directed mutagenesis experiments. Our simulation also revealed hourglass-like intermediate states making the pore radius narrower at the center. This work provides new fundamental insights into how substrate-transporter interactions actively change the free energy landscape of the transport cycle to facilitate enhanced transport activity.

Transcriptome reprogramming during severe dehydration contributes to physiological and metabolic changes in the resurrection plant Haberlea rhodopensis

BACKGROUND

Water shortage is a major factor that harms agriculture and ecosystems worldwide. Plants display various levels of tolerance to water deficit, but only resurrection plants can survive full desiccation of their vegetative tissues. Haberlea rhodopensis, an endemic plant of the Balkans, is one of the few resurrection plants found in Europe. We performed transcriptomic analyses of this species under slight, severe and full dehydration and recovery to investigate the dynamics of gene expression and associate them with existing physiological and metabolomics data.

RESULTS

De novo assembly yielded a total of 142,479 unigenes with an average sequence length of 1034 nt. Among them, 18,110 unigenes were differentially expressed. Hierarchical clustering of all differentially expressed genes resulted in seven clusters of dynamic expression patterns. The most significant expression changes, involving more than 15,000 genes, started at severe dehydration (~ 20% relative water content) and were partially maintained at full desiccation (< 10% relative water content). More than a hundred pathways were enriched and functionally organized in a GO/pathway network at the severe dehydration stage. Transcriptomic changes in key pathways were analyzed and discussed in relation to metabolic processes, signal transduction, quality control of protein and DNA repair in this plant during dehydration and rehydration.

CONCLUSION

Reprograming of the transcriptome occurs during severe dehydration, resulting in a profound alteration of metabolism toward alternative energy supply, hormone signal transduction, and prevention of DNA/protein damage under very low cellular water content, underlying the observed physiological and metabolic responses and the resurrection behavior of H. rhodopensis.

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.

Plant Desiccation Tolerance and its Regulation in the Foliage of Resurrection “Flowering-Plant” Species

The majority of flowering-plant species can survive complete air-dryness in their seed and/or pollen. Relatively few species (‘resurrection plants’) express this desiccation tolerance in their foliage. Knowledge of the regulation of desiccation tolerance in resurrection plant foliage is reviewed. Elucidation of the regulatory mechanism in resurrection grasses may lead to identification of genes that can improve stress tolerance and yield of major crop species. Well-hydrated leaves of resurrection plants are desiccation-sensitive and the leaves become desiccation tolerant as they are drying. Such drought-induction of desiccation tolerance involves changes in gene-expression causing extensive changes in the complement of proteins and the transition to a highly-stable quiescent state lasting months to years. These changes in gene-expression are regulated by several interacting phytohormones, of which drought-induced abscisic acid (ABA) is particularly important in some species. Treatment with only ABA induces desiccation tolerance in vegetative tissue of Borya constricta Churchill. and Craterostigma plantagineum Hochstetter. but not in the resurrection grass Sporobolus stapfianus Gandoger. Suppression of drought-induced senescence is also important for survival of drying. Further research is needed on the triggering of the induction of desiccation tolerance, on the transition between phases of protein synthesis and on the role of the phytohormone, strigolactone and other potential xylem-messengers during drying and rehydration.

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.

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.

Submergence induced changes of molecular species in membrane lipids in Arabidopsis thaliana

The composition of membrane lipids is sensitive to environmental stresses. Submergence is a type of stress often encountered by plants. However, how the molecular species of membrane lipids respond to submergence has not yet been characterised. In this study, we used a lipidomic approach to profile the molecular species of membrane lipids in whole plants of Arabidopsis thaliana that were completely submerged for three days. The plants survived one day of submergence, after which, we found that the total membrane lipids were only subtly decreased, showing significant decreases of monogalactosyldiacylglycerol (MGDG) and phosphatidylcholine (PC) and an increase of phosphatidic acid (PA); however, the basic lipid composition was retained. In contrast, three days of submergence caused plants to die, and the membranes deteriorated via the rapid loss of 96% of lipid content together with a 229% increase in PA. The turnover of molecular species from PG and MGDG to PA indicated that submergence-induced lipid changes occurred through PA-mediated degradation. In addition, molecular species of extraplastidic PG degraded sooner than plastidic ones, lyso-phospholipids exhibited various patterns of change, and the double-bond index (DBI) remained unchanged until membrane deterioration. Our results revealed the unique changes of membrane lipids upon submergence and suggested that the major cause of the massive lipid degradation could be anoxia.

Methanol-Promoted Lipid Remodelling during Cooling Sustains Cryopreservation Survival of Chlamydomonas reinhardtii

Cryogenic treatments and cryoprotective agents (CPAs) determine the survival rate of organisms that undergo cryopreservation, but their mechanisms of operation have not yet been characterised adequately. In particular, the way in which membrane lipids respond to cryogenic treatments and CPAs is unknown. We developed comparative profiles of the changes in membrane lipids among cryogenic treatments and between the CPAs dimethyl sulfoxide (DMSO) and methanol (MeOH) for the green alga Chlamydomonas reinhardtii. We found that freezing in liquid nitrogen led to a dramatic degradation of lipids, and that thawing at warm temperature (35°C) induced lipid remodelling. DMSO did not protect membranes, but MeOH significantly attenuated lipid degradation. The presence of MeOH during cooling (from 25°C to -55°C at a rate of 1°C/min) sustained the lipid composition to the extent that membrane integrity was maintained; this phenomenon accounts for successful cryopreservation. An increase in monogalactosyldiacylglycerol and a decrease in diacylglycerol were the major changes in lipid composition associated with survival rate, but there was no transformation between these lipid classes. Phospholipase D-mediated phosphatidic acid was not involved in freezing-induced lipid metabolism in C. reinhardtii. Lipid unsaturation changed, and the patterns of change depended on the cryogenic treatment. Our results provide new insights into the cryopreservation of, and the lipid metabolism in, algae.

How membranes organize during seed germination: three patterns of dynamic lipid remodelling define chilling resistance and affect plastid biogenesis

Imbibitional chilling injury during germination causes agricultural losses, but this can be overcome by osmopriming. It remains unknown how membranes reorganize during germination. Herein, we comparatively profiled changes of membrane lipids during imbibition under normal and chilling temperatures in chilling-tolerant and -sensitive soybean seeds. We found three patterns of dynamic lipid remodelling during the three phases of germination. Pattern 1 involved a gradual increase in plastidic lipids during phases I and II, with an abrupt increase during phase III. This abrupt increase was associated with initiation of photosynthesis. Pattern 3 involved phosphatidic acid (PA) first decreasing, then increasing, and finally decreasing to a low level. Patterns 1 and 3 were interrupted in chilling-sensitive seeds under low temperature, which lead a block in plastid biogenesis and accumulation of harmful PA, respectively. However, they were rescued and returned to their status under normal temperature after polyethylene glycol osmopriming. We specifically inhibited phospholipase D (PLD)-mediated PA formation in chilling-sensitive seeds of soybean, cucumber, and pea, and found their germination under low temperature was significantly improved. These results indicate that membranes undergo specific and functional reorganization of lipid composition during germination and demonstrate that PLD-mediated PA causes imibibitional chilling injury.