Phosphatidic acid, a versatile water-stress signal in roots

Understanding of Plant Salt Tolerance Mechanisms and Application to Molecular Breeding

Soil salinization is a widespread hindrance that endangers agricultural production and ecological security. High salt concentrations in saline soils are primarily caused by osmotic stress, ionic toxicity and oxidative stress, which have a negative impact on plant growth and development. In order to withstand salt stress, plants have developed a series of complicated physiological and molecular mechanisms, encompassing adaptive changes in the structure and function of various plant organs, as well as the intricate signal transduction networks enabling plants to survive in high-salinity environments. This review summarizes the recent advances in salt perception under different tissues, physiological responses and signaling regulations of plant tolerance to salt stress. We also examine the current knowledge of strategies for breeding salt-tolerant plants, including the applications of omics technologies and transgenic approaches, aiming to provide the basis for the cultivation of salt-tolerant crops through molecular breeding. Finally, future research on the application of wild germplasm resources and muti-omics technologies to discover new tolerant genes as well as investigation of crosstalk among plant hormone signaling pathways to uncover plant salt tolerance mechanisms are also discussed in this review.

Ectopic Expression of Distinct PLC Genes Identifies 'Compactness' as a Possible Architectural Shoot Strategy to Cope with Drought Stress

Phospholipase C (PLC) has been implicated in several stress responses, including drought. Overexpression (OE) of PLC has been shown to improve drought tolerance in various plant species. Arabidopsis contains nine PLC genes, which are subdivided into four clades. Earlier, OE of PLC3, PLC5 or PLC7 was found to increase Arabidopsis' drought tolerance. Here, we confirm this for three other PLCs: PLC2, the only constitutively expressed AtPLC; PLC4, reported to have reduced salt tolerance and PLC9, of which the encoded enzyme was presumed to be catalytically inactive. To compare each PLC and to discover any other potential phenotype, two independent OE lines of six AtPLC genes, representing all four clades, were simultaneously monitored with the GROWSCREEN-FLUORO phenotyping platform, under both control- and mild-drought conditions. To investigate which tissues were most relevant to achieving drought survival, we additionally expressed AtPLC5 using 13 different cell- or tissue-specific promoters. While no significant differences in plant size, biomass or photosynthesis were found between PLC lines and wild-type (WT) plants, all PLC-OE lines, as well as those tissue-specific lines that promoted drought survival, exhibited a stronger decrease in 'convex hull perimeter' (= increase in 'compactness') under water deprivation compared to WT. Increased compactness has not been associated with drought or decreased water loss before although a hyponastic decrease in compactness in response to increased temperatures has been associated with water loss. We propose that the increased compactness could lead to decreased water loss and potentially provide a new breeding trait to select for drought tolerance.

PLAT domain protein 1 (PLAT1/PLAFP) binds to the Arabidopsis thaliana plasma membrane and inserts a lipid

Robust agricultural yields depend on the plant's ability to fix carbon amid variable environmental conditions. Over seasonal and diurnal cycles, the plant must constantly adjust its metabolism according to available resources or external stressors. The metabolic changes that a plant undergoes in response to stress are well understood, but the long-distance signaling mechanisms that facilitate communication throughout the plant are less studied. The phloem is considered the predominant conduit for the bidirectional transport of these signals in the form of metabolites, nucleic acids, proteins, and lipids. Lipid trafficking through the phloem in particular attracted our attention due to its reliance on soluble lipid-binding proteins (LBP) that generate and solubilize otherwise membrane-associated lipids. The Phloem Lipid-Associated Family Protein (PLAFP) from Arabidopsis thaliana is generated in response to abiotic stress as is its lipid-ligand phosphatidic acid (PA). PLAFP is proposed to transport PA through the phloem in response to drought stress. To understand the interactions between PLAFP and PA, nearly 100 independent systems comprised of the protein and one PA, or a plasma membrane containing varying amounts of PA, were simulated using atomistic classical molecular dynamics methods. In these simulations, PLAFP is found to bind to plant plasma membrane models independent of the PA concentration. When bound to the membrane, PLAFP adopts a binding pose where W41 and R82 penetrate the membrane surface and anchor PLAFP. This triggers a separation of the two loop regions containing W41 and R82. Subsequent simulations indicate that PA insert into the β-sandwich of PLAFP, driven by interactions with multiple amino acids besides the W41 and R82 identified during the insertion process. Fine-tuning the protein-membrane and protein-PA interface by mutating a selection of these amino acids may facilitate engineering plant signaling processes by modulating the binding response.

Lipid signaling and proline catabolism are activated in barley roots (Hordeum vulgare L.) during recovery from cold stress

Previous findings have shown that phospholipase D (PLD) contributes to the response to long-term chilling stress in barley by regulating the balance of proline (Pro) levels. Although Pro accumulation is one of the most prominent changes in barley roots exposed to this kind of stress, the regulation of its metabolism during recovery from stress remains unclear. Research has mostly focused on the responses to stress per se, and not much is known about the dynamics and mechanisms underlying the subsequent recovery. The present study aimed to evaluate how PLD, its product phosphatidic acid (PA), and diacylglycerol pyrophosphate (DGPP) modulate Pro accumulation in barley during recovery from long-term chilling stress. Pro metabolism involves different pathways and enzymes. The rate-limiting step is mediated by pyrroline-5-carboxylate synthetase (P5CS) in its biosynthesis, and by proline dehydrogenase (ProDH) in its catabolism. We observed that Pro levels decreased in recovering barley roots due to an increase in ProDH activity. The addition of 1-butanol, a PLD inhibitor, reverted this effect and altered the relative gene expression of ProDH. When barley tissues were treated with PA before recovery, the fresh weight of roots increased and ProDH activity was stimulated. These data contribute to our understanding of how acidic membrane phospholipids like PA help to control Pro degradation during recovery from stress.

Phosphatidic acid-regulated SOS2 controls sodium and potassium homeostasis in Arabidopsis under salt stress

The maintenance of sodium/potassium (Na+ /K+ ) homeostasis in plant cells is essential for salt tolerance. Plants export excess Na+ out of cells mainly through the Salt Overly Sensitive (SOS) pathway, activated by a calcium signal; however, it is unknown whether other signals regulate the SOS pathway and how K+ uptake is regulated under salt stress. Phosphatidic acid (PA) is emerging as a lipid signaling molecule that modulates cellular processes in development and the response to stimuli. Here, we show that PA binds to the residue Lys57 in SOS2, a core member of the SOS pathway, under salt stress, promoting the activity and plasma membrane localization of SOS2, which activates the Na+ /H+ antiporter SOS1 to promote the Na+ efflux. In addition, we reveal that PA promotes the phosphorylation of SOS3-like calcium-binding protein 8 (SCaBP8) by SOS2 under salt stress, which attenuates the SCaBP8-mediated inhibition of Arabidopsis K+ transporter 1 (AKT1), an inward-rectifying K+ channel. These findings suggest that PA regulates the SOS pathway and AKT1 activity under salt stress, promoting Na+ efflux and K+ influx to maintain Na+ /K+ homeostasis.

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids - including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids - also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.

The impact of boron nutrient supply in mulberry (Morus alba) response to metabolomics, enzyme activities, and physiological parameters

Boron (B) is essential for normal and healthy plant growth. Therefore, Boron stress is a common abiotic stress that limits plant growth and productivity. However, how mulberry copes with boron stress remains unclear. In this study, seedlings of the Morus alba cultivar, Yu-711, were treated with five different concentrations of boric acid (H₃BO₃), including deficient (0 and 0.02 mM), sufficient (0.1 mM) and toxic (0.5 and 1 mM) levels. Physiological parameters, enzymatic activities and non-targeted liquid chromatography-mass spectrometry (LC-MS) technique were employed to evaluate the effects of boron stress on the net photosynthetic rate (Pn), chlorophyll content, stomatal conductance (Gs), transpiration rate (Tr), intercellular CO₂ concentration (Ci) and metabolome signatures. Physiological analysis revealed that Boron deficiency and toxicity induced a decline in P_n, C_i, G_s, T_r, and chlorophyll content. Also, enzymatic activities, including catalase (CAT) and superoxide dismutase (SOD), decreased, while POD activity increased in response to Boron stress. Osmotic substances such as soluble sugars, soluble proteins, and proline (PRO) presented elevated levels under all Boron concentrations. Metabolome analysis indicated that differential metabolites, including amino acids, secondary metabolites, carbohydrates, and lipids, played a key role in Yu-711's response to Boron stress. These metabolites were mainly involved in amino acid metabolism, biosynthesis of other secondary metabolites, lipid metabolism, metabolism of cofactors and vitamins, and metabolism of other amino acids pathways. Our findings reveal the various metabolites pathways in mulberry response to boron nutrient supply and may serve as fundamental knowledge in breeding resistance mulberry plants, so that it can cope with climate changes.

The Role of Triacylglycerol in the Protection of Cells against Lipotoxicity under Drought in Lolium multiflorum/Festucaarundinacea Introgression Forms

Triacylglycerol is a key lipid compound involved in maintaining homeostasis of both membrane lipids and free fatty acids (FFA) in plant cells under adverse environmental conditions. However, its role in the process of lipid remodeling has not been fully recognized, especially in monocots, including grass species. For our study, two closely related introgression forms of Lolium multiflorum (Italian ryegrass) and Festuca arundinacea (tall fescue), distinct in their level of drought tolerance, were selected as plant models to study rearrangements in plant lipidome under water deficit and further re-watering. The low drought tolerant (LDT) form revealed an elevated level of cellular membrane damage accompanied by an increased content of polyunsaturated FFA and triacylglycerol under water deficit, compared with the high drought tolerant (HDT) form. However, the LDT introgression form demonstrated also the ability to regenerate its membranes after stress cessation. The obtained results clearly indicated that accumulation of triacylglycerol under advanced drought in the LDT form could serve as a cellular protective mechanism against overaccumulation of toxic polyunsaturated FFA and other lipid intermediates. Furthermore, accumulation of triacylglycerol under drought conditions could serve also as storage of substrates required for further regeneration of membranes after stress cessation. The rearrangements in triacylglycerol metabolism were supported by the upregulation of several genes, involved in a biosynthesis of triacylglycerol. With respect to this process, diacylglycerol O-acyltransferase DGAT2 seems to play the most important role in the analyzed grasses.

Phosphatidic acid modulates MPK3- and MPK6-mediated hypoxia signaling in Arabidopsis

Phosphatidic acid (PA) is an important lipid essential for several aspects of plant development and biotic and abiotic stress responses. We previously suggested that submergence induces PA accumulation in Arabidopsis thaliana; however, the molecular mechanism underlying PA-mediated regulation of submergence-induced hypoxia signaling remains unknown. Here, we showed that in Arabidopsis, loss of the phospholipase D (PLD) proteins PLDα1 and PLDδ leads to hypersensitivity to hypoxia, but increased tolerance to submergence. This enhanced tolerance is likely due to improvement of PA-mediated membrane integrity. PA bound to the mitogen-activated protein kinase 3 (MPK3) and MPK6 in vitro and contributed to hypoxia-induced phosphorylation of MPK3 and MPK6 in vivo. Moreover, mpk3 and mpk6 mutants were more sensitive to hypoxia and submergence stress compared with wild type, and fully suppressed the submergence-tolerant phenotypes of pldα1 and pldδ mutants. MPK3 and MPK6 interacted with and phosphorylated RELATED TO AP2.12, a master transcription factor in the hypoxia signaling pathway, and modulated its activity. In addition, MPK3 and MPK6 formed a regulatory feedback loop with PLDα1 and/or PLDδ to regulate PLD stability and submergence-induced PA production. Thus, our findings demonstrate that PA modulates plant tolerance to submergence via both membrane integrity and MPK3/6-mediated hypoxia signaling in Arabidopsis.

Arabidopsis PLDζ1 and PLDζ2 localize to post-Golgi membrane compartments in a partially overlapping manner

Arabidopsis PLDζ1 and PLDζ2 localize to the trans-Golgi network and to compartments including the trans-Golgi network, multi-vesicular bodies, and the tonoplast, respectively, depending on their N-terminal regions containing PX-PH domains. Phospholipase D (PLD) is involved in dynamic cellular processes, including membrane trafficking, cytoskeletal reorganization, and signal transduction for gene expression, through the production of phosphatidic acid in membrane compartments specific to each process. Although PLD plays crucial roles in various plant phenomena, the underlying processes involving PLD for each phenomenon remain largely elusive, partly because the subcellular localization of PLD remains obscure. In this study, we performed comparative subcellular localization analyses of the Arabidopsis thaliana PX-PH-PLDs PLDζ1 and PLDζ2. In mature lateral root cap cells, own promoter-driven fluorescence protein fusions of PLDζ1 localized to the entire trans-Golgi network (TGN) while that of PLDζ2 localized to punctate structures including part of the TGN and multi-vesicular bodies as well as the tonoplast. These localization patterns were reproduced using N-terminal partial proteins, which contain PX-PH domains. An inducibly overexpressed fluorescence protein fusion of the PLDζ2 partial protein first localized to punctate structures, and then accumulated predominantly on the tonoplast. Further domain dissection analysis revealed that the N-terminal moiety preceding the PX-PH domain of PLDζ2 was required for the tonoplast-predominant accumulation. These findings suggest that PLDζ1 and PLDζ2 play partially overlapping but nonetheless distinctive roles in post-Golgi compartments along the membrane trafficking pathway from the TGN to the tonoplast.

Salinity tolerance mechanisms and their breeding implications

BACKGROUND

The era of first green revolution brought about by the application of chemical fertilizers surely led to the explosion of food grains, but left behind the notable problem of salinity. Continuous application of these fertilizers coupled with fertilizer-responsive crops make the country self-reliant, but continuous deposition of these led to altered the water potential and thus negatively affecting the proper plant functioning from germination to seed setting.

MAIN BODY

Increased concentration of anion and cations and their accumulation and distribution cause cellular toxicity and ionic imbalance. Plants respond to salinity stress by any one of two mechanisms, viz., escape or tolerate, by either limiting their entry via root system or controlling their distribution and storage. However, the understanding of tolerance mechanism at the physiological, biochemical, and molecular levels will provide an insight for the identification of related genes and their introgression to make the crop more resilient against salinity stress.

SHORT CONCLUSION

Novel emerging approaches of plant breeding and biotechnologies such as genome-wide association studies, mutational breeding, marker-assisted breeding, double haploid production, hyperspectral imaging, and CRISPR/Cas serve as engineering tools for dissecting the in-depth physiological mechanisms. These techniques have well-established implications to understand plants' adaptions to develop more tolerant varieties and lower the energy expenditure in response to stress and, constitutively fulfill the void that would have led to growth resistance and yield penalty.

Transcriptome and Metabolome Analyses Reveal Potential Salt Tolerance Mechanisms Contributing to Maintenance of Water Balance by the Halophytic Grass Puccinellia nuttalliana

Elevated soil salinity exacerbated by human activities and global climate change poses serious threats to plant survival. Although halophytes provide many important clues concerning salt tolerance in plants, some unanswered questions remain to be addressed, including the processes of water and solute transport regulation. We performed high-throughput RNA-sequencing in roots and metabolome characterizations in roots and leaves of Puccinellia nuttalliana halophytic grass subjected to 0 (control) and 150 mM NaCl. In RNAseq, a total of 31 Gb clean bases generated were de novo assembled into 941,894 transcripts. The PIP2;2 and HKT1;5 transcript levels increased in response to the NaCl treatment implying their roles in water and ion homeostasis. Several transcription factors, including WRKY39, DEK3, HY5, and ABF2, were also overexpressed in response to NaCl. The metabolomic analysis revealed that proline and dopamine significantly increased due to the upregulation of the pathway genes under salt stress, likely contributing to salt tolerance mechanisms. Several phosphatidylcholines significantly increased in roots suggesting that the alterations of membrane lipid composition may be an important strategy in P. nuttalliana for maintaining cellular homeostasis and membrane integrity under salt stress. In leaves, the TCA cycle was enriched suggesting enhanced energy metabolism to cope with salt stress. Other features contributing to the ability of P. nuttalliana to survive under high salinity conditions include salt secretion by the salt glands and enhanced cell wall lignification of the root cells. While most of the reported transcriptomic, metabolomics, and structural alterations may have consequences to water balance maintenance by plants under salinity stress, the key processes that need to be further addressed include the role of the changes in the aquaporin gene expression profiles in the earlier reported enhancement of the aquaporin-mediated root water transport.

Integrated transcriptomics and metabolomics analysis to characterize alkali stress responses in canola (Brassica napus L.)

BACKGROUND

Soil salinization is a major constraint limiting agricultural development and affecting crop growth and productivity, especially in arid and semi-arid regions. Understanding the molecular mechanism of the adaptability of canola to salt stress is very important to improve the salt tolerance of canola and promote its cultivation in saline alkali soil.

RESULTS

To identify the metabolomic and transcriptomic mechanisms of canola under alkaline salt stress, we collected roots of control (no salt treatment) and 72 h Na₂CO₃-stressed canola seedlings (hydroponics) for metabolic profiling of metabolites, supplemented with RNA-Seq analysis and real-time quantitative PCR validation. Metabolomic analysis showed that the metabolites of amino acids and fatty acids were higher accumulated under alkaline salt stress, including L-proline, L-glutamate, L-histidine, L-phenylalanine, L-citrulline, L-tyrosine, L-saccharopine, L-tryptophan, linoleic acid, dihomo gamma linolenic acid, alpha linolenic acid, Eric acid, oleic acid and neuronic acid, while the metabolism of carbohydrate (sucrase, alpha, alpha trehalose), polyol (ribitol), UDP-D-galactose, D-mannose, D-fructose and D-glucose 6-phosphate decreased. Transcriptomic and metabolomic pathway analysis indicated that carbohydrate metabolism may not play an important role in the resistance of canola to alkaline salt stress. Organic acid metabolism (fatty acid accumulation) and amino acid metabolism are important metabolic pathways in the root of canola under alkaline salt stress.

CONCLUSIONS

These results suggest that the genes and metabolites involved in fatty acid metabolism and amino acids metabolism in roots of canola may regulate salt tolerance of canola seedlings under alkaline salt stress, which improves our understanding of the molecular mechanisms of salt tolerance in canola.

Link between Lipid Second Messengers and Osmotic Stress in Plants

Plants are subject to different types of stress, which consequently affect their growth and development. They have developed mechanisms for recognizing and processing an extracellular signal. Second messengers are transient molecules that modulate the physiological responses in plant cells under stress conditions. In this sense, it has been shown in various plant models that membrane lipids are substrates for the generation of second lipid messengers such as phosphoinositide, phosphatidic acid, sphingolipids, and lysophospholipids. In recent years, research on lipid second messengers has been moving toward using genetic and molecular approaches to reveal the molecular setting in which these molecules act in response to osmotic stress. In this sense, these studies have established that second messengers can transiently recruit target proteins to the membrane and, therefore, affect protein conformation, activity, and gene expression. This review summarizes recent advances in responses related to the link between lipid second messengers and osmotic stress in plant cells.

Lipids in xylem sap of woody plants across the angiosperm phylogeny

Lipids have been observed attached to lumen-facing surfaces of mature xylem conduits of several plant species, but there has been little research on their functions or effects on water transport, and only one lipidomic study of the xylem apoplast. Therefore, we conducted lipidomic analyses of xylem sap from woody stems of seven plants representing six major angiosperm clades, including basal magnoliids, monocots and eudicots, to characterize and quantify phospholipids, galactolipids and sulfolipids in sap using mass spectrometry. Locations of lipids in vessels of Laurus nobilis were imaged using transmission electron microscopy and confocal microscopy. Xylem sap contained the galactolipids di- and monogalactosyldiacylglycerol, as well as all common plant phospholipids, but only traces of sulfolipids, with total lipid concentrations in extracted sap ranging from 0.18 to 0.63 nmol ml^-1 across all seven species. Contamination of extracted sap from lipids in cut living cells was found to be negligible. Lipid composition of sap was compared with wood in two species and was largely similar, suggesting that sap lipids, including galactolipids, originate from cell content of living vessels. Seasonal changes in lipid composition of sap were observed for one species. Lipid layers coated all lumen-facing vessel surfaces of L. nobilis, and lipids were highly concentrated in inter-vessel pits. The findings suggest that apoplastic, amphiphilic xylem lipids are a universal feature of angiosperms. The findings require a reinterpretation of the cohesion-tension theory of water transport to account for the effects of apoplastic lipids on dynamic surface tension and hydraulic conductance in xylem.

Choline-Mediated Lipid Reprogramming as a Dominant Salt Tolerance Mechanism in Grass Species Lacking Glycine Betaine

Choline, as a precursor of glycine betaine (GB) and phospholipids, is known to play roles in plant tolerance to salt stress, but the downstream metabolic pathways regulated by choline conferring salt tolerance are still unclear for non-GB-accumulating species. The objectives were to examine how choline affects salt tolerance in a non-GB-accumulating grass species and to determine major metabolic pathways of choline regulating salt tolerance involving GB or lipid metabolism. Kentucky bluegrass (Poa pratensis) plants were subjected to salt stress (100 mM NaCl) with or without foliar application of choline chloride (1 mM) in a growth chamber. Choline or GB alone and the combined application increased leaf photochemical efficiency, relative water content and osmotic adjustment and reduced leaf electrolyte leakage. Choline application had no effects on the endogenous GB content and GB synthesis genes did not show responses to choline under nonstress and salt stress conditions. GB was not detected in Kentucky bluegrass leaves. Lipidomic analysis revealed an increase in the content of monogalactosyl diacylglycerol, phosphatidylcholine and phosphatidylethanolamine and a decrease in the phosphatidic acid content by choline application in plants exposed to salt stress. Choline-mediated lipid reprogramming could function as a dominant salt tolerance mechanism in non-GB-accumulating grass species.

Targeting salt stress coping mechanisms for stress tolerance in Brassica: A research perspective

Brassica genus comprises numerous cultivated brassica species with various economic importance. Salt stress is an overwhelming problem causing serious losses in Brassica species (e.g. B. napus, B. rapa, B. oleracea, B. juncea) growth and grain yield production by inducing ionic and ROS toxicity. Given that a significant variation exists in salt tolerance level in Brassica genus, Brassica species exhibited numerous salt tolerance mechanisms which were either overlooked or given less importance to improve and understand innate salt stress tolerance mechanism in Brassica species. In this review, we tried to highlight the importance and recent findings relating to some overlooked and potential mechanisms such as role of neurotransmitters, and role of cytosolic Ca²⁺ and ROS as signaling elements to enhance salt stress tolerance. Studies revealed that salt tolerant brassica species retained more K⁺ in leaf mesophyll which confers overall salinity tolerance in salt tolerance brassica species. Neurotransmitter such as melatonin, dopamiane and eATP regulates K⁺ and Ca²⁺ permeable ion channels and plays a very crucial role in ionic homeostasis under salinity stress in brassica. At the end, the numerous possible salt stress agronomic strategies were also discussed to mitigate the severity of the salt stress in Brassica species.

Silencing of phosphoinositide dependent protein kinase orthologs reduces hypersensitive cell death in Nicotiana benthamiana

Phosphatidic acid plays an important role in plant immune responses against phytopathogenic bacteria in Nicotiana benthamiana. Here we focused on phosphoinositide dependent protein kinases (PDKs) as a candidate required for phosphatidic acid signaling. Based on Arabidopsis PDK sequences, we identified four putative PDK orthologs in N. benthamiana genome. To address the role of PDKs in plant defense responses, we created all four NbPDKs-silenced plants by virus-induced gene silencing. the NbPDKs-silenced plants showed a moderately reduced growth phenotype. Induction of hypersensitive cell death was compromised in the NbPDKs-silenced plants challenged with Ralstonia solanacearum. The hypersensitive cell death induced by bacterial effectors was also reduced in the NbPDKs-silenced plants. the NbPDKs-silenced plants showed decreased production of salicylic acid, jasmonic acid and jasmonoyl-L-isoleucine, as well as hydrogen peroxide after inoculation with R. solanacearum. These results suggest that NbPDKs might have an important role in the regulation of the hypersensitive cell death via plant hormone signaling and oxidative burst.

Phospholipid Signaling Is a Component of the Salicylic Acid Response in Plant Cell Suspension Cultures

Salicylic acid (SA) is an important signaling molecule involved in plant defense. While many proteins play essential roles in SA signaling, increasing evidence shows that responses to SA appear to involve and require lipid signals. The phospholipid-generated signal transduction involves a family of enzymes that catalyze the hydrolysis or phosphorylation of phospholipids in membranes to generate signaling molecules, which are important in the plant cellular response. In this review, we focus first, the role of SA as a mitigator in biotic/abiotic stress. Later, we describe the experimental evidence supporting the phospholipid–SA connection in plant cells, emphasizing the roles of the secondary lipid messengers (phosphatidylinositol 4,5-bisphosphate (PIP₂) and phosphatidic acid (PA)) and related enzymes (phospholipase D (PLD) and phospholipase C (PLC)). By placing these recent finding in context of phospholipids and SA in plant cells, we highlight the role of phospholipids as modulators in the early steps of SA triggered transduction in plant cells.

Regulation of ABA-Non-Activated SNF1-Related Protein Kinase 2 Signaling Pathways by Phosphatidic Acid

Phosphatidic acid (PA) is involved in the regulation of plant growth and development, as well as responses to various environmental stimuli. Several PA targets in plant cells were identified, including two SNF1-related protein kinases 2 (SnRK2s), SnRK2.10 and SnRK2.4, which are not activated by abscisic acid (ABA). Here, we investigated the effects of PA on various elements of ABA-non-activated SnRK2 signaling. PA 16:0/18:1 was found to modulate the SnRK2 structure and the phosphorylation of some SnRK2 targets. Conversely, phosphorylation by the ABA-non-activated SnRK2s, of one of such targets, dehydrin Early Responsive to Dehydration 14 (ERD14), affects its interaction with PA and subcellular localization. Moreover, PA 16:0/18:1 modulates the activity and/or localization of negative regulators of the ABA-non-activated SnRK2s, not only of the ABA insensitive 1 (ABI1) phosphatase, which was identified earlier, but also of another protein phosphatase 2C, PP2CA. The activity of both phosphatases was inhibited by about 50% in the presence of 50 μM PA. PA 16:0/18:1 also impacts the phosphorylation and subcellular localization of SnRK2-interacting calcium sensor, known to inhibit SnRK2 activity in a calcium-dependent manner. Thus, PA was found to regulate ABA-non-activated SnRK2 signaling at several levels: the activity, phosphorylation status and/or localization of SnRK2 cellular partners.

Phosphatidic acid: an emerging versatile class of cellular mediators

Lipids function not only as the major structural components of cell membranes, but also as molecular messengers that transduce signals to trigger downstream signaling events in the cell. Phosphatidic acid (PA), the simplest and a minor class of glycerophospholipids, is a key intermediate for the synthesis of membrane and storage lipids, and also plays important roles in mediating diverse cellular and physiological processes in eukaryotes ranging from microbes to mammals and higher plants. PA comprises different molecular species that can act differently, and is found in virtually all organisms, tissues, and organellar membranes, with variations in total content and molecular species composition. The cellular levels of PA are highly dynamic in response to stimuli and multiple enzymatic reactions can mediate its production and degradation. Moreover, its unique physicochemical properties compared with other glycerophospholipids allow PA to influence membrane structure and dynamics, and interact with various proteins. PA has emerged as a class of new lipid mediators modulating various signaling and cellular processes via its versatile effects, such as membrane tethering, conformational changes, and enzymatic activities of target proteins, and vesicular trafficking.

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.

Impact of phenol on the glycerophospholipid turnover and potential role of circadian clock in the plant response against this pollutant in tobacco hairy roots

Glycerophospholipids (GPLs) from cell membranes (CM) are a proper source for the synthesis of lipid messengers able to activate signal pathways that will define the plant survival under changing and stressful environmental conditions. Little is known about how GPLs metabolism (GPLsM) is regulated and the effects of phenol treatment on GPLs composition. In this work, we studied the effects of phenol both on GPLs turnover and on the expression of GPLsM-related genes potentially regulated by the circadian clock, using tobacco hairy root cultures (HRC). Phenol decreased the total PC levels and increased PE, PG and CL levels in the dark phase. Different molecular species of PC and PE showed the same trend than the total PC and PE upon phenol treatment. Besides, significant differences in the expression of all studied genes related to GPLsM were found. NtCCT2 expression was affected at all analyzed times while NtPECT1 and NtAAPT1 showed similar expression patterns. NtCDS1, NtPGPS2 and NtCLS genes showed significant and differential expression profiles both in untreated and treated HRC. PECT1 and NtPGPS2 genes seem to conserve a circadian expression profile mainly in untreated HRC. However, phenol was able to modify the GPLs composition and the expression of genes related to GPLs synthesis. The GPLs modification could be explained by the up-regulation of NtPECT1, NtAAPT1 and NtCLS genes during the dark phase, suggesting for being a crucial moment for HRC to trigger an adaptive response against this organic pollutant.

Heat Sensing and Lipid Reprograming as a Signaling Switch for Heat Stress Responses in Wheat

Temperature is an essential physical factor that affects the plant life cycle. Almost all plant species have evolved a robust signal transduction system that enables them to sense changes in the surrounding temperature, relay this message and accordingly adjust their metabolism and cellular functions to avoid heat stress-related damage. Wheat (Triticum aestivum), being a cool-season crop, is very sensitive to heat stress. Any increase in the ambient temperature, especially at the reproductive and grain-filling stages, can cause a drastic loss in wheat yield. Heat stress causes lipid peroxidation due to oxidative stress, resulting in the damage of thylakoid membranes and the disruption of their function, which ultimately decreases photosynthesis and crop yield. The cell membrane/plasma membrane plays prominent roles as an interface system that perceives and translates the changes in environmental signals into intracellular responses. Thus, membrane lipid composition is a critical factor in heat stress tolerance or susceptibility in wheat. In this review, we elucidate the possible involvement of calcium influx as an early heat stress-responsive mechanism in wheat plants. In addition, the physiological implications underlying the changes in lipid metabolism under high-temperature stress in wheat and other plant species will be discussed. In-depth knowledge about wheat lipid reprograming can help develop heat-tolerant wheat varieties and provide approaches to solve the impact of global climate change.

Salt Tolerance Mechanisms of Plants

Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context in order to link them to observed phases in growth rate that occur in response to stress. Recent advances in phenotyping can now functionally or genetically link cellular signaling responses, ion transport, water management, and gene expression to growth, development, and survival. Halophytes, which are naturally salt-tolerant plants, are highlighted as success stories to learn from. We emphasize that (a) filling the major knowledge gaps in salt-induced signaling pathways, (b) increasing the spatial and temporal resolution of our knowledge of salt stress responses, (c) discovering and considering crop-specific responses, and (d) including halophytes in our comparative studies are all essential in order to take our approaches to increasing crop yields in saline soils to the next level.

Biochemical characterization of phospholipases C from Coffea arabica in response to aluminium stress

Signal transduction in plants determines their successful adaptation to diverse stress factors. Our group employed suspension cells to study the phosphoinositide pathway, which is triggered by aluminium stress. We investigated about members of the PI-specific phospholipase C (PLC) family and evaluated their transcription profiles in Coffea arabica (Ca) suspension cells after 14days of culture when treated or not with 100μM AlCl₃. The four CaPLC1-4 members showed changes in their transcript abundance upon AlCl₃ treatment. The expression profiles of CaPLC1/2 exhibited a rapid and transitory increase in abundance. In contrast, CaPLC3 and CaPLC4 showed that transcript levels were up-regulated in short times (at 30s), while only CaPLC4 kept high levels and CaPLC3 was reduced to basal after 3h of treatment. CaPLC proteins were heterologously expressed, and CaPLC2 and CaPLC4 were tested for in vitro activity in the presence or absence of AlCl₃ and compared to Arabidopsis PLC2 (AtPLC2). A crude extract was isolated from coffee cells. CaPLC2 showed a similar inhibition (30%) as in AtPLC2 and in the crude extract, while in CaPLC4, the activity was enhanced by AlCl₃. Additionally, we visualized the yellow fluorescent protein PH domain of human PLCδ1 (YFP-PH_PLCδ1) subcellular localization in cells that were treated or not with AlCl₃. In non-treated cells, we observed a polar fluorescence signal towards the fused membrane. However, when cells were treated with AlCl₃, these signals were disrupted. Finally, this is the first time that PLC activity has been shown to be stimulated in vitro by AlCl₃.

Uncovering early response to drought by proteomic, physiological and biochemical changes in the almond × peach rootstock 'Garnem'

Drought affects growth and metabolism in plants. To investigate the changes in root protein function involved in the early response to drought stress, a proteomic analysis in combination to a physiological and biochemical analysis was performed in plants of 'Garnem', an almond × peach hybrid rootstock, subjected to short-term drought stress. Abscisic acid (ABA) accumulation levels increased during the drought exposure, which induced stomatal closure, and thus, minimised water losses. These effects were reflected in stomatal conductance and leaf water potential levels. However, 'Garnem' was able to balance water content and maintain an osmotic adjustment in cell membranes, suggesting a dehydration avoidance strategy. The proteomic analysis revealed significant abundance changes in 29 and 24 spots after 2 and 24 h of drought stress respectively. Out of these, 15 proteins were identified by LC-ESI-MS/MS. The abundance changes of these proteins suggest the influence in drought-responsive mechanisms present in 'Garnem', allowing its adaptation to drought conditions. Overall, our study improves existing knowledge on the root proteomic changes in the early response to drought. This will lead to a better understanding of dehydration avoidance and tolerance strategies, and finally, help in new drought-tolerance breeding approaches.

Phosphatidic acids mediate transport of Ca2+ and H+ through plant cell membranes

Phosphatidic acids (PAs) are a key intermediate in phospholipid biosynthesis, and a central element in numerous signalling pathways. Functions of PAs are related to their fundamental role in molecular interactions within cell membranes modifying membrane bending, budding, fission and fusion. Here we tested the hypothesis that PAs are capable of direct transport of ions across bio-membranes. We have demonstrated that PAs added to the maize plasma membrane vesicles induced ionophore-like transmembrane transport of Ca2+, H+ and Mg2+. PA-induced Ca2+ fluxes increased with an increasing PAs acyl chain unsaturation. For all the PAs analysed, the effect on Ca2+ permeability increased with increasing pH (pH 8.0>pH 7.2>pH 6.0). The PA-induced Ca2+, Mg2+ and H+ permeability was also more pronounced in the endomembrane vesicles as compared with the plasma membrane vesicles. Addition of PA to protoplasts from Arabidopsis thaliana (L.) Heynh. roots constitutively expressing aequorin triggered elevation of the cytosolic Ca2+ activity, indicating that the observed PA-dependent Ca2+ transport occurs in intact plants.

Lipid remodelling plays an important role in wheat (Triticum aestivum) hypoxia stress

Membrane lipid remodelling is one of the strategies that plants have developed to combat abiotic stress. In this study, physiological, lipidomic and proteome analyses were conducted to investigate the changes in glycerolipid and phospholipid concentrations in the wheat (Triticum aestivum L.) cultivars CIGM90.863 and Seri M82 under hypoxia treatment. The growth of CIGM90.863 remained unaffected, whereas Seri M82 was significantly stunted after 8 days of hypoxia treatment. The concentrations of all lipids except lysophosphatidylglycerol were significantly higher in the leaves of Seri M82 than in CIGM90.863 under normal growth conditions. The lipid profile changed significantly under hypoxia stress and varied between genotypes for some of the lipids. Phosphatidic acids remained unchanged in Seri M82 but they were gradually induced in CIGM90.863 in response to hypoxia stress because of the higher phospholipase D expression and lower expression of diglycerol kinase and phosphatidate phosphatases. In contrast, digalactosyldiacylglycerol content was highly stable in CIGM90.863 following hypoxia treatment, although it decreased significantly in Seri M82. Phosphatidylglycerol and lipoxygenase showed a stronger and faster response in CIGM90.863 than in Seri M82 under hypoxia stress. Different membrane lipid adjustments in wheat under oxygen deficiency conditions could be partly responsible for the differing tolerance of Seri M82 and CIGM90.863. This study will help us to better understand how wheat tolerates hypoxia stress by regulating lipid remodelling.

Role for Arabidopsis PLC7 in Stomatal Movement, Seed Mucilage Attachment, and Leaf Serration

Phospholipase C (PLC) has been suggested to play important roles in plant stress and development. To increase our understanding of PLC signaling in plants, we have started to analyze knock-out (KO), knock-down (KD) and overexpression mutants of Arabidopsis thaliana, which contains nine PLCs. Earlier, we characterized PLC2, PLC3 and PLC5. Here, the role of PLC7 is functionally addressed. Promoter-GUS analyses revealed that PLC7 is specifically expressed in the phloem of roots, leaves and flowers, and is also present in trichomes and hydathodes. Two T-DNA insertion mutants were obtained, i.e., plc7-3 being a KO- and plc7-4 a KD line. In contrast to earlier characterized phloem-expressed PLC mutants, i.e., plc3 and plc5, no defects in primary- or lateral root development were found for plc7 mutants. Like plc3 mutants, they were less sensitive to ABA during stomatal closure. Double-knockout plc3 plc7 lines were lethal, but plc5 plc7 (plc5/7) double mutants were viable, and revealed several new phenotypes, not observed earlier in the single mutants. These include a defect in seed mucilage, enhanced leaf serration, and an increased tolerance to drought. Overexpression of PLC7 enhanced drought tolerance too, similar to what was earlier found for PLC3-and PLC5 overexpression. In vivo 32Pi-labeling of seedlings and treatment with sorbitol to mimic drought stress, revealed stronger PIP2 responses in both drought-tolerant plc5/7 and PLC7-OE mutants. Together, these results show novel functions for PLC in plant stress and development. Potential molecular mechanisms are discussed.

Phosphatidic acid plays key roles regulating plant development and stress responses

Phospholipids, including phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylserine (PS) and phosphoinositides, have emerged as an important class of cellular messenger molecules in various cellular and physiological processes, of which PA attracts much attention of researchers. In addition to its effect on stimulating vesicle trafficking, many studies have demonstrated that PA plays a crucial role in various signaling pathways by binding target proteins and regulating their activity and subcellular localization. Here, we summarize the functional mechanisms and target proteins underlying PA-mediated regulation of cellular signaling, development, hormonal responses, and stress responses in plants.

Bioorthogonal click chemistry for fluorescence imaging of choline phospholipids in plants

BACKGROUND

Phospholipids are important structural and signaling molecules in plant membranes. Some fluorescent dyes can stain general lipids of membranes, but labeling and visualization of specific lipid classes have yet to be developed for most components of the membrane. New techniques for visualizing membrane lipids are needed to further delineate their dynamic structural and signaling roles in plant cells. In this study we examined whether propargylcholine, a bioortholog of choline, can be used to label the major membrane lipid, phosphatidylcholine, and other choline phospholipids in plants. We established that propargylcholine is readily taken up by roots, and that its incorporation is not detrimental to plant growth. After plant tissue is harvested and fixed, a click-chemistry reaction covalently links the alkyne group of propargylcholine to a fluorescently-tagged azide, resulting in specific labeling of choline phospholipids.

RESULTS

Uptake of propargylcholine, followed by click chemistry with fluorescein or Alexa Fluor 594 azide was used to visualize choline phospholipids in cells of root, leaf, stem, silique and seed tissues from Arabidopsis thaliana. Co-localization with various subcellular markers indicated coinciding fluorescent signals in cell membranes, such as the tonoplast and the ER. Among different cell types in the leaf epidermis, guard cells displayed strong labeling. Mass spectrometry-based lipidomic analysis of the various plant tissues revealed that incorporation of propargylcholine was strongest in roots with approximately 50% of total choline phospholipids being labeled, but it was also incorporated in the other tissues including seeds. Phospholipid profiling confirmed that, in each tissue analyzed, incorporation of the bioortholog had little impact on the pool of choline plus choline-like phospholipids or other lipid species.

CONCLUSION

We developed and validated a click-chemistry based method for fluorescence imaging of choline phospholipids using a bioortholog of choline, propargylcholine, in various cell-types and tissues from Arabidopsis. This click-chemistry method provides a direct way to metabolically tag and visualize specific lipid molecules in plant cells. This work paves the way for future studies addressing in situ localization of specific lipids in plants.

Transcriptome sequencing and comparative analysis of differentially-expressed isoforms in the roots of Halogeton glomeratus under salt stress

Although Halogeton glomeratus (H. glomeratus) has been confirmed to have a unique mechanism to regulate Na⁺ efflux from the cytoplasm and compartmentalize Na⁺ into leaf vacuoles, little is known about the salt tolerance mechanisms of roots under salinity stress. In the present study, transcripts were sequenced using the BGISEQ-500 sequencing platform (BGI, Wuhan, China). After quality control, approximately 24.08 million clean reads were obtained and the average mapping ratio to the reference gene was 70.00%. When comparing salt-treated samples with the control, a total of 550, 590, 1411 and 2063 DEIs were identified at 2, 6, 24 and 72h, respectively. Numerous differentially-expressed isoforms that play important roles in response and adaptation to salt condition are related to metabolic processes, cellular processes, single-organism processes, localization, biological regulation, responses to stimulus, binding, catalytic activity and transporter activity. Fifty-eight salt-induced isoforms were common to different stages of salt stress; most of these DEIs were related to signal transduction and transporters, which maybe the core isoforms regulating Na⁺ uptake and transport in the roots of H. glomeratus. The expression patterns of 18 DEIs that were detected by quantitative real-time polymerase chain reaction were consistent with their respective changes in transcript abundance as identified by RNA-Seq technology. The present study thoroughly explored potential isoforms involved in salt tolerance on H. glomeratus roots at five time points. Our results may serve as an important resource for the H. glomeratus research community, improving our understanding of salt tolerance in halophyte survival under high salinity stress.

Phospholipases Dζ1 and Dζ2 have distinct roles in growth and antioxidant systems in Arabidopsis thaliana responding to salt stress

Phospholipases Dζ play different roles in Arabidopsis salt tolerance affecting the regulation of ion transport and antioxidant responses. Lipid signalling mediated by phospholipase D (PLD) plays essential roles in plant growth including stress and hormonal responses. Here we show that PLDζ1 and PLDζ2 have distinct effects on Arabidopsis responses to salinity. A transcriptome analysis of a double pldζ1pldζ2 mutant revealed a cluster of genes involved in abiotic and biotic stresses, such as the high salt-stress responsive genes DDF1 and RD29A. Another cluster of genes with a common expression pattern included ROS detoxification genes involved in electron transport and biotic and abiotic stress responses. Total superoxide dismutase (SOD) activity was induced early in the shoots and roots of all pldζ mutants exposed to mild or severe salinity with the highest SOD activity measured in pldζ2 at 14 days. Lipid peroxidation in shoots and roots was higher in the pldζ1 mutant upon salt treatment and pldζ1 accumulated H₂O₂ earlier than other genotypes in response to salt. Salinity caused less deleterious effects on K⁺ accumulation in shoots and roots of the pldζ2 mutant than of wild type, causing only a slight variation in Na⁺/K⁺ ratio. Relative growth rates of wild-type plants, pldζ1, pldζ2 and pldζ1pldζ2 mutants were similar in control conditions, but strongly affected by salt in WT and pldζ1. The efficiency of photosystem II, estimated by measuring the ratio of chlorophyll fluorescence (F _v/F _m ratio), was strongly decreased in pldζ1 under salt stress. In conclusion, PLDζ2 plays a key role in determining Arabidopsis sensitivity to salt stress allowing ion transport and antioxidant responses to be finely regulated.

Phospholipase Dδ negatively regulates plant thermotolerance by destabilizing cortical microtubules in Arabidopsis

The pattern of cortical microtubule arrays plays an important role in plant growth and adaptation in response to hormonal and environmental changes. Cortical microtubules are connected with the plasma membrane (PM); however, how the membrane affects cortical microtubule organization is not well understood. Here, we showed that phospholipase Dδ (PLDδ) was associated with the PM and co-localized with microtubules in cells. In vitro analysis revealed that PLDδ bound to microtubules, resulting in microtubule disorganization. Site-specific mutations that decreased PLDδ enzymatic activity impaired its effects on destabilizing microtubule organization. Heat shock transiently activated PLDδ, without any change of its PM localization, triggering microtubule dissociation from PM and depolymerization and seedling death in Arabidopsis, but these effects were alleviated in pldδ knockout mutants. Complementation of pldδ with wild-type PLDδ, but not mutated PLDδ, restored the phenotypes of microtubules and seedling survival to those of wild-type Arabidopsis. Thus, we conclude that the PM-associated PLDδ negatively regulates plant thermotolerance via destabilizing cortical microtubules, in an activity-dependent manner, rather than its subcellular translocation.

Arabidopsis phosphoinositide-specific phospholipase C 4 negatively regulates seedling salt tolerance

Previous physiological and pharmacological studies have suggested that the activity of phosphoinositide-specific phospholipase C (PI-PLC) plays an important role in regulating plant salt stress responses by altering the intracellular Ca²⁺ concentration. However, the individual members of plant PLCs involved in this process need to be identified. Here, the function of AtPLC4 in the salt stress response of Arabidopsis seedlings was analysed. plc4 mutant seedlings showed hyposensitivity to salt stress compared with Col-0 wild-type seedlings, and the salt hyposensitive phenotype could be complemented by the expression of native promoter-controlled AtPLC4. Transgenic seedlings with AtPLC4 overexpression (AtPLC4 OE) exhibited a salt-hypersensitive phenotype, while transgenic seedlings with its inactive mutant expression (AtPLC4m OE) did not exhibit this phenotype. Using aequorin as a Ca²⁺ indicator in plc4 mutant and AtPLC4 OE seedlings, AtPLC4 was shown to positively regulate the salt-induced Ca²⁺ increase. The salt-hypersensitive phenotype of AtPLC4 OE seedlings was partially rescued by EGTA. An analysis of salt-responsive genes revealed that the transcription of RD29B, MYB15 and ZAT10 was inversely regulated in plc4 mutant and AtPLC4 OE seedlings. Our findings suggest that AtPLC4 negatively regulates the salt tolerance of Arabidopsis seedlings, and Ca²⁺ may be involved in regulating this process.

Acclimation to salt modifies the activation of several osmotic stress-activated lipid signalling pathways in Chlamydomonas

Osmotic stress rapidly activates several phospholipid signalling pathways in the unicellular alga Chlamydomonas. In this report, we have studied the effects of salt-acclimation on growth and phospholipid signalling. Growing cells on media containing 100 mM NaCl increased their salt-tolerance but did not affect the overall phospholipid content, except that levels of phosphatidylinositol phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] were reduced by one-third. When these NaCl-acclimated cells were treated with increasing concentrations of salt, the same lipid signalling pathways as in non-acclimated cells were activated. This was witnessed as increases in phosphatidic acid (PA), lyso-phosphatidic acid (L-PA), diacylglycerol pyrophosphate (DGPP), PI(4,5)P₂ and its isomer PI(3,5)P₂. However, all dose-dependent responses were shifted to higher osmotic-stress levels, and the responses were lower than in non-acclimated cells. When NaCl-acclimated cells were treated with other osmotica, such as KCl and sucrose, the same effects were found, illustrating that they were due to hyperosmotic rather than hyperionic acclimation. The results indicate that acclimation to moderate salt stress modifies stress perception and the activation of several downstream pathways.

Metabolomic profiling of the halophyte Prosopis strombulifera shows sodium salt- specific response

Primary and secondary metabolite profiles were analyzed in roots and leaves of the halophytic shrub Prosopis strombulifera in response to control plants (no salt added in the growing media) and to lowering the osmotic potential to -1.0, -1.9, and -2.6 MPa generated by NaCl, Na₂SO₄, and the iso-osmotic combination of them at 24 h after reaching such potential. A rapid production of metabolites in response to sodium salt was found, which was correlated with modifications in growth parameters. Analysis of polar metabolite profiles by GC-MS rendered a total of 108 significantly altered compounds including 18 amino acids, 19 secondary metabolites, 23 carbohydrates, 13 organic acids, 4 indole acids, among others. Primary metabolites showed a differential response under the salt treatments, which was dependent on salt type and concentration, organ and age of plants. Most of identified compounds showed the strongest accumulation at the highest salt concentration assayed for Na₂SO₄-treated plants, which was correlated with damaging effects of sulfate anion on plant growth. Roots of NaCl-treated plants showed a higher number of altered metabolites (analyzed by UPLC-ESI-QqTOF-MS) compared to other treatments, while leaves of Na₂SO₄-treated plants showed the highest number of altered signals. A low degree of overlapping between secondary metabolites altered in roots and leaves of NaCl and Na₂SO₄-treated plants was found. However, when both NaCl and Na₂SO₄ salts were present plants always showed a lower number of altered metabolites. Three compounds were tentatively identified: tryptophan, lysophosphatidylcoline and 13-hydroxyoctadecadienoic acid. Increasing knowledge on P. strombulifera metabolism will contribute to unravel the underlying biochemical mechanism of salt tolerance.

Overexpression of Grain Amaranth (Amaranthus hypochondriacus) AhERF or AhDOF Transcription Factors in Arabidopsis thaliana Increases Water Deficit- and Salt-Stress Tolerance, Respectively, via Contrasting Stress-Amelioration Mechanisms

Two grain amaranth transcription factor (TF) genes were overexpressed in Arabidopsis plants. The first, coding for a group VII ethylene response factor TF (i.e., AhERF-VII) conferred tolerance to water-deficit stress (WS) in transgenic Arabidopsis without affecting vegetative or reproductive growth. A significantly lower water-loss rate in detached leaves coupled to a reduced stomatal opening in leaves of plants subjected to WS was associated with this trait. WS tolerance was also associated with an increased antioxidant enzyme activity and the accumulation of putative stress-related secondary metabolites. However, microarray and GO data did not indicate an obvious correlation between WS tolerance, stomatal closure, and abscisic acid (ABA)-related signaling. This scenario suggested that stomatal closure during WS in these plants involved ABA-independent mechanisms, possibly involving reactive oxygen species (ROS). WS tolerance may have also involved other protective processes, such as those employed for methyl glyoxal detoxification. The second, coding for a class A and cluster I DNA binding with one finger TF (i.e., AhDof-AI) provided salt-stress (SS) tolerance with no evident fitness penalties. The lack of an obvious development-related phenotype contrasted with microarray and GO data showing an enrichment of categories and genes related to developmental processes, particularly flowering. SS tolerance also correlated with increased superoxide dismutase activity but not with augmented stomatal closure. Additionally, microarray and GO data indicated that, contrary to AhERF-VII, SS tolerance conferred by AhDof-AI in Arabidopsis involved ABA-dependent and ABA-independent stress amelioration mechanisms.

Regulation of mRNA decay in plant responses to salt and osmotic stress

Plant acclimation to environmental stresses requires fast signaling to initiate changes in developmental and metabolic responses. Regulation of gene expression by transcription factors and protein kinases acting upstream are important elements of responses to salt and drought. Gene expression can be also controlled at the post-transcriptional level. Recent analyses on mutants in mRNA metabolism factors suggest their contribution to stress signaling. Here we highlight the components of mRNA decay pathways that contribute to responses to osmotic and salt stress. We hypothesize that phosphorylation state of proteins involved in mRNA decapping affect their substrate specificity.

Lipid signalling in plant responses to abiotic stress

Lipids are one of the major components of biological membranes including the plasma membrane, which is the interface between the cell and the environment. It has become clear that membrane lipids also serve as substrates for the generation of numerous signalling lipids such as phosphatidic acid, phosphoinositides, sphingolipids, lysophospholipids, oxylipins, N-acylethanolamines, free fatty acids and others. The enzymatic production and metabolism of these signalling molecules are tightly regulated and can rapidly be activated upon abiotic stress signals. Abiotic stress like water deficit and temperature stress triggers lipid-dependent signalling cascades, which control the expression of gene clusters and activate plant adaptation processes. Signalling lipids are able to recruit protein targets transiently to the membrane and thus affect conformation and activity of intracellular proteins and metabolites. In plants, knowledge is still scarce of lipid signalling targets and their physiological consequences. This review focuses on the generation of signalling lipids and their involvement in response to abiotic stress. We describe lipid-binding proteins in the context of changing environmental conditions and compare different approaches to determine lipid-protein interactions, crucial for deciphering the signalling cascades.

Phloem Proteomics Reveals New Lipid-Binding Proteins with a Putative Role in Lipid-Mediated Signaling

Global climate changes inversely affect our ability to grow the food required for an increasing world population. To combat future crop loss due to abiotic stress, we need to understand the signals responsible for changes in plant development and the resulting adaptations, especially the signaling molecules traveling long-distance through the plant phloem. Using a proteomics approach, we had identified several putative lipid-binding proteins in the phloem exudates. Simultaneously, we identified several complex lipids as well as jasmonates. These findings prompted us to propose that phloem (phospho-) lipids could act as long-distance developmental signals in response to abiotic stress, and that they are released, sensed, and moved by phloem lipid-binding proteins (Benning et al., 2012). Indeed, the proteins we identified include lipases that could release a signaling lipid into the phloem, putative receptor components, and proteins that could mediate lipid-movement. To test this possible protein-based lipid-signaling pathway, three of the proteins, which could potentially act in a relay, are characterized here: (I) a putative GDSL-motif lipase (II) a PIG-P-like protein, with a possible receptor-like function; (III) and PLAFP (phloem lipid-associated family protein), a predicted lipid-binding protein of unknown function. Here we show that all three proteins bind lipids, in particular phosphatidic acid (PtdOH), which is known to participate in intracellular stress signaling. Genes encoding these proteins are expressed in the vasculature, a prerequisite for phloem transport. Cellular localization studies show that the proteins are not retained in the endoplasmic reticulum but surround the cell in a spotted pattern that has been previously observed with receptors and plasmodesmatal proteins. Abiotic signals that induce the production of PtdOH also regulate the expression of GDSL-lipase and PLAFP, albeit in opposite patterns. Our findings suggest that while all three proteins are indeed lipid-binding and act in the vasculature possibly in a function related to long-distance signaling, the three proteins do not act in the same but rather in distinct pathways. It also points toward PLAFP as a prime candidate to investigate long-distance lipid signaling in the plant drought response.

Detection of Mycobacterium tuberculosis and Mycobacterium bovis in Sahiwal cattle from an organized farm using ante-mortem techniques

Aim:

The aim of this study was to investigate the prevalence of bovine tuberculosis (TB) and detection of Mycobacterium bovis in cattle from an organized dairy farm.

Materials and Methods:

A total of 121 animals (93 females and 28 males) of 1 year and above were studied for the prevalence of bovine TB using single intradermal comparative cervical tuberculin (SICCT) test, bovine gamma-interferon (γ-IFN) enzyme immunoassay, and polymerase chain reactions (PCRs).

Results:

Out of total 121 animals, 17 (14.04%) animals were positive reactors to SICCT test while only one (0.82%) animal for γ-IFN assay. By PCR, Mycobacterium TB complex was detected in 19 (15.70%) animals out of which 4 (3.30%) animal were also positive for M. bovis.

Conclusions:

Diagnosis of bovine TB can be done in early stage in live animals with multiple approaches like skin test followed by a molecular technique like PCR which showed promising results.

Transcriptional Responses in Root and Leaf of Prunus persica under Drought Stress Using RNA Sequencing

Prunus persica L. Batsch, or peach, is one of the most important crops and it is widely established in irrigated arid and semi-arid regions. However, due to variations in the climate and the increased aridity, drought has become a major constraint, causing crop losses worldwide. The use of drought-tolerant rootstocks in modern fruit production appears to be a useful method of alleviating water deficit problems. However, the transcriptomic variation and the major molecular mechanisms that underlie the adaptation of drought-tolerant rootstocks to water shortage remain unclear. Hence, in this study, high-throughput sequencing (RNA-seq) was performed to assess the transcriptomic changes and the key genes involved in the response to drought in root tissues (GF677 rootstock) and leaf tissues (graft, var. Catherina) subjected to 16 days of drought stress. In total, 12 RNA libraries were constructed and sequenced. This generated a total of 315 M raw reads from both tissues, which allowed the assembly of 22,079 and 17,854 genes associated with the root and leaf tissues, respectively. Subsets of 500 differentially expressed genes (DEGs) in roots and 236 in leaves were identified and functionally annotated with 56 gene ontology (GO) terms and 99 metabolic pathways, which were mostly associated with aminobenzoate degradation and phenylpropanoid biosynthesis. The GO analysis highlighted the biological functions that were exclusive to the root tissue, such as "locomotion," "hormone metabolic process," and "detection of stimulus," indicating the stress-buffering role of the GF677 rootstock. Furthermore, the complex regulatory network involved in the drought response was revealed, involving proteins that are associated with signaling transduction, transcription and hormone regulation, redox homeostasis, and frontline barriers. We identified two poorly characterized genes in P. persica: growth-regulating factor 5 (GRF5), which may be involved in cellular expansion, and AtHB12, which may be involved in root elongation. The reliability of the RNA-seq experiment was validated by analyzing the expression patterns of 34 DEGs potentially involved in drought tolerance using quantitative reverse transcription polymerase chain reaction. The transcriptomic resources generated in this study provide a broad characterization of the acclimation of P. persica to drought, shedding light on the major molecular responses to the most important environmental stressor.

Long-Distance Lipid Signaling and its Role in Plant Development and Stress Response

Lipids are important signaling compounds in plants. They can range from small lipophilic molecules like the dicarboxylic acid Azelaic acid to complex phosphoglycerolipids and regulate plant development as well as the response to biotic and abiotic stress. While their intracellular function is well described, several lipophilic signals are known to be found in the plant phloem and can, thus, also play a role in long-distance signaling. Mostly, they play a role in the pathogen response and systemic acquired resistance. This is particularly true for oxylipins, dehydroabietinal, and azelaic acid. However, several phospholipids have now been described in phloem exudates. Their intracellular function as well as implications and a model for long-distance signaling are discussed in this chapter.

Genome-Wide Analysis and Expression Profiling of the Phospholipase C Gene Family in Soybean (Glycine max)

Phosphatidylinositol-specific phospholipase C (PI-PLC) hydrolyses phosphatidylinositol-4,5-bisphosphate to produce diacylglycerol and inositol 1,4,5-trisphosphate. It plays an important role in plant development and abiotic stress responses. However, systematic analysis and expression profiling of the phospholipase C (PLC) gene family in soybean have not been reported. In this study, 12 putative PLC genes were identified in the soybean genome. Soybean PLCs were found on chromosomes 2, 11, 14 and 18 and encoded 58.8-70.06 kD proteins. Expression pattern analysis by RT-PCR demonstrated that expression of the GmPLCs was induced by PEG, NaCl and saline-alkali treatments in roots and leaves. GmPLC transcripts accumulated specifically in roots after ABA treatment. Furthermore, GmPLC transcripts were analyzed in various tissues. The results showed that GmPLC7 was highly expressed in most tissues, whereas GmPLC12 was expressed in early pods specifically. In addition, subcellular localization analysis was carried out and confirmed that GmPLC10 was localized in the plasma membrane in Nicotiana benthamiana. Our genomic analysis of the soybean PLC family provides an insight into the regulation of abiotic stress responses and development. It also provides a solid foundation for the functional characterization of the soybean PLC gene family.

Tuning plant signaling and growth to survive salt

Salinity is one of the major abiotic factors threatening food security worldwide. Recently, our understanding of early processes underlying salinity tolerance has expanded. In this review, early signaling events, such as phospholipid signaling, calcium ion (Ca(2+)) responses, and reactive oxygen species (ROS) production, together with salt stress-induced abscisic acid (ABA) accumulation, are brought into the context of long-term salt stress-specific responses and alteration of plant growth. Salt-induced quiescent and recovery growth phases rely on modification of cell cycle activity, cell expansion, and cell wall extensibility. The period of initial growth arrest varies among different organs, leading to altered plant morphology. Studying stress-induced changes in growth dynamics can be used for screening to discover novel genes contributing to salt stress tolerance in model species and crops.

Knocking Down of Isoprene Emission Modifies the Lipid Matrix of Thylakoid Membranes and Influences the Chloroplast Ultrastructure in Poplar

Isoprene is a small lipophilic molecule with important functions in plant protection against abiotic stresses. Here, we studied the lipid composition of thylakoid membranes and chloroplast ultrastructure in isoprene-emitting (IE) and nonisoprene-emitting (NE) poplar (Populus × canescens). We demonstrated that the total amount of monogalactosyldiacylglycerols, digalactosyldiacylglycerols, phospholipids, and fatty acids is reduced in chloroplasts when isoprene biosynthesis is blocked. A significantly lower amount of unsaturated fatty acids, particularly linolenic acid in NE chloroplasts, was associated with the reduced fluidity of thylakoid membranes, which in turn negatively affects photosystem II photochemical efficiency. The low photosystem II photochemical efficiency in NE plants was negatively correlated with nonphotochemical quenching and the energy-dependent component of nonphotochemical quenching. Transmission electron microscopy revealed alterations in the chloroplast ultrastructure in NE compared with IE plants. NE chloroplasts were more rounded and contained fewer grana stacks and longer stroma thylakoids, more plastoglobules, and larger associative zones between chloroplasts and mitochondria. These results strongly support the idea that in IE species, the function of this molecule is closely associated with the structural organization and functioning of plastidic membranes.

Quantitative trait loci and candidate genes underlying genotype by environment interaction in the response of Arabidopsis thaliana to drought

Drought stress was imposed on two sets of Arabidopsis thaliana genotypes grown in sand under short-day conditions and analysed for several shoot and root growth traits. The response to drought was assessed for quantitative trait locus (QTL) mapping in a genetically diverse set of Arabidopsis accessions using genome-wide association (GWA) mapping, and conventional linkage analysis of a recombinant inbred line (RIL) population. Results showed significant genotype by environment interaction (G×E) for all traits in response to different watering regimes. For the RIL population, the observed G×E was reflected in 17 QTL by environment interactions (Q×E), while 17 additional QTLs were mapped not showing Q×E. GWA mapping identified 58 single nucleotide polymorphism (SNPs) associated with loci displaying Q×E and an additional 16 SNPs associated with loci not showing Q×E. Many candidate genes potentially underlying these loci were suggested. The genes for RPS3C and YLS7 were found to contain conserved amino acid differences when comparing Arabidopsis accessions with strongly contrasting drought response phenotypes, further supporting their candidacy. One of these candidate genes co-located with a QTL mapped in the RIL population.

Identification and functional characterization of the Arabidopsis Snf1-related protein kinase SnRK2.4 phosphatidic acid-binding domain

Phosphatidic acid (PA) is an important signalling lipid involved in various stress-induced signalling cascades. Two SnRK2 protein kinases (SnRK2.4 and SnRK2.10), previously identified as PA-binding proteins, are shown here to prefer binding to PA over other anionic phospholipids and to associate with cellular membranes in response to salt stress in Arabidopsis roots. A 42 amino acid sequence was identified as the primary PA-binding domain (PABD) of SnRK2.4. Unlike the full-length SnRK2.4, neither the PABD-YFP fusion protein nor the SnRK2.10 re-localized into punctate structures upon salt stress treatment, showing that additional domains of the SnRK2.4 protein are required for its re-localization during salt stress. Within the PABD, five basic amino acids, conserved in class 1 SnRK2s, were found to be necessary for PA binding. Remarkably, plants overexpressing the PABD, but not a non-PA-binding mutant version, showed a severe reduction in root growth. Together, this study biochemically characterizes the PA-SnRK2.4 interaction and shows that functionality of the SnRK2.4 PABD affects root development.