Role of phospholipid signalling in plant environmental responses

Integrating conceptual and machine learning models to enhance daily-Scale streamflow simulation and assessing climate change impact in the watersheds of the Godavari basin, India

This study examined and addressed climate change's effects on hydrological patterns, particularly in critical places like the Godavari River basin. This study used daily gridded rainfall and temperature datasets from the Indian Meteorological Department (IMD) for model training and testing, 70% and 30%, respectively. To anticipate future hydrological shifts, the study harnessed the EC-Earth3 data, presenting an innovative methodology tailored to the unique hydrological dynamics of the Godavari River basin. The Sacramento model provided initial streamflow estimates for Kanhargaon, Nowrangpur, and Wairagarh. This approach melded traditional hydrological modeling with advanced multi-layer perceptron (MLP) capabilities. When combined with parameters like lagged rainfall, lagged streamflow, potential evapotranspiration (PET), and temperature variations, these initial outputs were further refined using the Sac-MLP model. A comparison with Sacramento revealed the superior performance of the Sac-MLP model. For instance, during training, the Nash Sutcliffe efficiency (NSE) values for the Sac-MLP witnessed an improvement from 0.610 to 0.810 in Kanhargaon, 0.580 to 0.692 in Nowrangpur, and 0.675 to 0.849 in Wairagarh. The results of the testing further corroborated these findings, as evidenced by the increase in the NSE for Kanhargaon from 0.890 to 0.910. Additionally, Nowrangpur and Wairagarh experienced notable improvements, with their NSE values rising from 0.629 to 0.785 and 0.725 to 0.902, respectively. Projections based on EC-Earth3 data across various scenarios highlighted significant shifts in rainfall and temperature patterns, especially in the far future (2071-2100). Regarding the relative change in annual streamflow, Kanhargaon projections under SSP370 and SSP585 for the far future indicate increases of 584.38% and 662.74%. Similarly, Nowrangpur and Wairagarh are projected to see increases of 98.27% and 114.98%, and 81.68% and 108.08%, respectively. This study uses EC-Earth3 estimates to demonstrate the Sac-MLP model's accuracy and importance in climate change water resource planning. The unique method for region-specific hydrological analysis provides vital insights for sustainable water resource management. This research provides a deeper understanding of climate-induced hydrological changes and a robust modeling approach for accurate predictions in changing environmental conditions.

The dehydration-responsive protein PpFAS1.3 in moss Physcomitrium patens plays a regulatory role in lipid metabolism

Moss plants appear in the early stages of land colonization and possess varying degrees of dehydration tolerance. In this study, a protein called PpFAS1.3 was identified, which contains a fasciclin 1-like domain and is essential for the moss Physcomitrium patens' response to short-term rapid dehydration. When the FAS1.3 protein was knocked out, leafyshoots showed a significant decrease in tolerance to rapid dehydration, resulting in accelerated water loss and increased membrane leakage. Phylogenetic analysis suggests that PpFAS1.3 and its homologous proteins may have originated from bacteria and are specifically found in non-vascular plants like mosses and liverworts. As a dehydration-related protein, FAS1.3 plays a significant role in regulating lipid metabolism, particularly in the synthesis of free fatty acids (FFA) and the metabolism of two phospholipids, PC and PA. This discovery highlights the close connection between PpFAS1.3 and lipid metabolism, providing new insights into the molecular mechanisms underlying plant adaptation to stresses.

Phospholipid Signaling in Crop Plants: A Field to Explore

In plant models such as Arabidopsis thaliana, phosphatidic acid (PA), a key molecule of lipid signaling, was shown not only to be involved in stress responses, but also in plant development and nutrition. In this article, we highlight lipid signaling existing in crop species. Based on open access databases, we update the list of sequences encoding phospholipases D, phosphoinositide-dependent phospholipases C, and diacylglycerol-kinases, enzymes that lead to the production of PA. We show that structural features of these enzymes from model plants are conserved in equivalent proteins from selected crop species. We then present an in-depth discussion of the structural characteristics of these proteins before focusing on PA binding proteins. For the purpose of this article, we consider RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), the most documented PA target proteins. Finally, we present pioneering experiments that show, by different approaches such as monitoring of gene expression, use of pharmacological agents, ectopic over-expression of genes, and the creation of silenced mutants, that lipid signaling plays major roles in crop species. Finally, we present major open questions that require attention since we have only a perception of the peak of the iceberg when it comes to the exciting field of phospholipid signaling in plants.

Rice myo-inositol-3-phosphate synthase 2 (RINO2) alleviates heat injury-induced impairment in pollen germination and tube growth by modulating Ca2+ signaling and actin filament cytoskeleton

Low phytic acid (lpa) crop is considered as an effective strategy to improve crop nutritional quality, but a substantial decrease in phytic acid (PA) usually has negative effect on agronomic performance and its response to environment adversities. Myo-inositol-3-phosphate synthase (MIPS) is the rate-limiting enzyme in PA biosynthesis pathway, and regarded as the prime target for engineering lpa crop. In this paper, the rice MIPS gene (RINO2) knockout mutants and its wild type were performed to investigate the genotype-dependent alteration in the heat injury-induced spikelet fertility and its underlying mechanism for rice plants being imposed to heat stress at anthesis. Results indicated that RINO2 knockout significantly enhanced the susceptibility of rice spikelet fertility to heat injury, due to the severely exacerbated obstacles in pollen germination and pollen tube growth in pistil for RINO2 knockout under high temperature (HT) at anthesis. The loss of RINO2 function caused a marked reduction in inositol and phosphatidylinositol derivative concentrations in the HT-stressed pollen grains, which resulted in the strikingly lower content of phosphatidylinositol 4,5-diphosphate (PI (4,5) P2) in germinating pollen grain and pollen tube. The insufficient supply of PI (4,5) P2 in the HT-stressed pollen grains disrupted normal Ca2+ gradient in the apical region of pollen tubes and actin filament cytoskeleton in growing pollen tubes. The severely repressed biosynthesis of PI (4,5) P2 was among the regulatory switch steps leading to the impaired pollen germination and deformed pollen tube growth for the HT-stressed pollens of RINO2 knockout mutants.

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

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

Comparative metabolomic analysis between tomato and halophyte plants under intercropping conditions

Halophyte-based intercropping appears nowadays as a valuable approach in soil remediation and agriculture. In this work, intercropping between the halophyte Arthrocaulon macrostachyum and tomato (Solanum lycopersicum var. Sargento) was studied in both plant species using comparative mass spectrometry-based metabolomics coupled to metabolic pathway predictions. A significant number of changes in metabolites was observed in the halophyte. In terms of alteration of specific metabolic pathways, intercropping conditions stimulated sugar and starch metabolisms in tomato, whereas in the halophyte, intercropping mainly altered amino acid-related pathways. In addition, arginine and proline metabolism were commonly affected in both tomato and halophyte plants. Moreover, metabolomic changes were associated with physiological alterations in tomato. In this sense, mild oxidative stress was induced in intercropped tomato plants, which, in turn, could trigger signaling events leading to plant adjustment to intercropping conditions. This study represents the first approach toward understanding intercropping interactions at the metabolome level and its effect on plant physiology, opening up prospects for further characterization of this crop cultivation strategy.

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.

Insights into the mechanism of phospholipid hydrolysis by plant non-specific phospholipase C

Non-specific phospholipase C (NPC) hydrolyzes major membrane phospholipids to release diacylglycerol (DAG), a potent lipid-derived messenger regulating cell functions. Despite extensive studies on NPCs reveal their fundamental roles in plant growth and development, the mechanistic understanding of phospholipid-hydrolyzing by NPCs, remains largely unknown. Here we report the crystal structure of Arabidopsis NPC4 at a resolution of 2.1 Å. NPC4 is divided into a phosphoesterase domain (PD) and a C-terminal domain (CTD), and is structurally distinct from other characterized phospholipases. The previously uncharacterized CTD is indispensable for the full activity of NPC4. Mechanistically, CTD contributes NPC4 activity mainly via CTD^α1-PD interaction, which ultimately stabilizes the catalytic pocket in PD. Together with a series of structure-guided biochemical studies, our work elucidates the structural basis and provides molecular mechanism of phospholipid hydrolysis by NPC4, and adds new insights into the members of phospholipase family.

The Regulatory Mechanisms and Control Technologies of Chilling Injury and Fungal Diseases of Postharvest Loquat Fruit

Loquat is a popular fruit widely cultivated all over the world. It is rich in minerals and carotenoids and has high commercial value. At room temperature, loquat fruit is impressionable to water and nutritional losses, physical damage, and microbial decay, resulting in a short postharvest life. Low-temperature storage is routinely used to prolong the shelf life of loquat fruit; however, cold storage can also lead to lignification of flesh tissue, which is one of the major symptoms of chilling injury (CI), reducing the quality and economic value of the fruit. In addition, fruit decay caused by microbial infection is another important reason for postharvest losses of loquat. To reduce quality deterioration and optimize the postharvest storage strategies of loquat fruit, considerable progress has been made in the physiological and molecular biological studies of CI, microbial decay, and preservation technologies of loquat fruit during the postharvest phase in recent decades. This review summarizes the current research progress and provides a reference for the improvement of loquat fruit quality.

DIACYLGLYCEROL KINASE 5 participates in flagellin-induced signaling in Arabidopsis

Flagellin perception is a keystone of pattern-triggered immunity in plants. The recognition of this protein by a plasma membrane (PM) receptor complex is the beginning of a signaling cascade that includes protein phosphorylation and the production of reactive oxygen species (ROS). In both Arabidopsis (Arabidopsis thaliana) seedlings and suspension cells, we found that treatment with flg22, a peptide corresponding to the most conserved domain of bacterial flagellin, caused a rapid and transient decrease in the level of phosphatidylinositol (PI) 4,5-bisphosphate along with a parallel increase in phosphatidic acid (PA). In suspension cells, inhibitors of either phosphoinositide-dependent phospholipases C (PLC) or diacylglycerol kinases (DGKs) inhibited flg22-triggered PA production and the oxidative burst. In response to flg22, receptor-like kinase-deficient fls2, bak1, and bik1 mutants (FLAGELLIN SENSITIVE 2, BRASSINOSTEROID INSENSITIVE 1-associated kinase 1, and BOTRYTIS-INDUCED KINASE 1, respectively) produced less PA than wild-type (WT) plants, whereas this response did not differ in NADPH oxidase-deficient rbohD (RESPIRATORY BURST OXIDASE HOMOLOG D) plants. Among the DGK-deficient lines tested, the dgk5.1 mutant produced less PA and less ROS after flg22 treatment compared with WT seedlings. In response to flg22, dgk5.1 plants showed lower callose accumulation and impaired resistance to Pseudomonas syringae pv. tomato DC3000 hrcC-. Transcriptomics revealed that the basal expression of defense-related genes was altered in dgk5.1 seedlings compared with the WT. A GFP-DGK5 fusion protein localized to the PM, where RBOHD and PLC2 (proteins involved in plant immunity) are also located. The role of DGK5 and its enzymatic activity in flagellin signaling and fine-tuning of early immune responses in plant-microbe interactions is discussed.

32Pi Labeled Transgenic Wheat Shows the Accumulation of Phosphatidylinositol 4,5-bisphosphate and Phosphatidic Acid Under Heat and Osmotic Stress

The ensuing heat stress drastically affects wheat plant growth and development, consequently compromising its grain yield. There are many thermoregulatory processes/mechanisms mediated by ion channels, lipids, and lipid-modifying enzymes that occur in the plasma membrane and the chloroplast. With the onset of abiotic or biotic stresses, phosphoinositide-specific phospholipase C (PI-PLC), as a signaling enzyme, hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) to generate inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) which is further phosphorylated into phosphatidic acid (PA) as a secondary messenger and is involved in multiple processes. In the current study, a phospholipase C (PLC) signaling pathway was investigated in spring wheat (Triticum aestivum L.) and evaluated its four AtPLC5 overexpressed (OE)/transgenic lines under heat and osmotic stresses through ³²P_i radioactive labeling. Naturally, the wheat harbors only a small amount of PIP₂. However, with the sudden increase in temperature (40°C), PIP₂ levels start to rise within 7.5 min in a time-dependent manner in wild-type (Wt) wheat. While the Phosphatidic acid (PA) level also elevated up to 1.6-fold upon exposing wild-type wheat to heat stress (40°C). However, at the anthesis stage, a significant increase of ∼4.5-folds in PIP₂ level was observed within 30 min at 40°C in AtPLC5 over-expressed wheat lines. Significant differences in PIP₂ level were observed in Wt and AtPLC5-OE lines when treated with 1200 mM sorbitol solution. It is assumed that the phenomenon might be a result of the activation of PLC/DGK pathways. Together, these results indicate that heat stress and osmotic stress activate several lipid responses in wild-type and transgenic wheat and can explain heat and osmotic stress tolerance in the wheat plant.

The Enigmatic Aliphatic Acetogenins and Their Correlations With Lipids During Seed Germination and Leaf Development of Avocado (Persea americana Mill.)

Lipids in avocados have been widely studied due to their nutritional value and several reported bioactivities. Aliphatic acetogenins are a relevant component of the avocado lipidome and have been tested for several potential food and pharma industries applications. This work followed the evolution of avocado fatty acids (FAs) and aliphatic acetogenins during seed germination and leaf growth. Oil extracts of embryonic axes, cotyledons, and leaves from seedlings and trees were divided to analyze free acetylated acetogenins (AcO-acetogenins), and free FAs. Embryonic axes from germinating seeds contained the highest amount of AcO-acetogenins and FAs; this tissue also accumulated the most diverse FA profile with up to 22 detected moieties. Leaves presented the highest variations in AcO-acetogenin profiles during development, although leaves from seedlings accumulated the simplest FA profile with only 10 different FAs. Remarkably, AcO-acetogenins represented half of the carbons allocated to lipids in grown leaves, while embryonic axes and cotyledons always contained more carbons within FAs during germination. Thus, we hypothesized the use of the AcO-acetogenin acyl chain for energy production toward β-oxidation. Also, α-linolenic and docosahexaenoic acids (DHAs) were proposed as close AcO-acetogenin intermediaries based on a correlation network generated using all these data. Another part of the oil extract was fractionated into different lipid classes before transesterification to profile FAs and acetogenins bound to lipids. Acetogenin backbones were identified for the first time in triglycerides from cotyledons and mainly in polar lipids (which include phospholipids) in all developing avocado tissues analyzed. Seed tissues accumulated preferentially polar lipids during germination, while triglycerides were consumed in cotyledons. Seedling leaves contained minute amounts of triglycerides, and polar lipids increased as they developed. Results from this work suggest acetogenins might be part of the energy and signaling metabolisms, and possibly of membrane structures, underlining the yet to establish role(s) of these unusual lipids in the avocado plant physiology.

Identification of Arabidopsis Phospholipase A Mutants With Increased Susceptibility to Plasmodiophora brassicae

Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is one of the most devastating diseases of canola (Brassica napus) in Canada. The identification of novel genes that contribute to clubroot resistance is important for the sustainable management of clubroot, as these genes may be used in the development of resistant canola cultivars. Phospholipase As (PLAs) play important roles in plant defense signaling and stress tolerance, and thus are attractive targets for crop breeding. However, since canola is an allopolyploid and has multiple copies of each PLA gene, it is time-consuming to test the functions of PLAs directly in this crop. In contrast, the model plant Arabidopsis thaliana has a simpler genetic background and only one copy of each PLA. Therefore, it would be reasonable and faster to validate the potential utility of PLA genes in Arabidopsis first. In this study, we identified seven homozygous atpla knockout/knockdown mutants of Arabidopsis, and tested their performance following inoculation with P. brassicae. Four mutants (pla₁-iiα, pla₁-iγ3, pla₁-iii, ppla-iiiβ, ppla-iiiδ) developed more severe clubroot than the wild-type, suggesting increased susceptibility to P. brassicae. The homologs of these Arabidopsis PLAs (AtPLAs) in B. napus (BnPLAs) were identified through Blast searches and phylogenic analysis. Expression of the BnPLAs was subsequently examined in transcriptomic datasets generated from canola infected by P. brassicae, and promising candidates for further characterization identified.

Phospholipase Ds in plants: Their role in pathogenic and symbiotic interactions

Phospholipase Ds (PLDs) are a heterogeneous group of enzymes that are widely distributed in organisms. These enzymes hydrolyze the structural phospholipids of the plasma membrane, releasing phosphatidic acid (PA), an important secondary messenger. Plant PLDs play essential roles in several biological processes, including growth and development, abiotic stress responses, and plant-microbe interactions. Although the roles of PLDs in plant-pathogen interactions have been extensively studied, their roles in symbiotic relationships are not well understood. The establishment of the best-studied symbiotic interactions, those between legumes and rhizobia and between most plants and mycorrhizae, requires the regulation of several physiological, cellular, and molecular processes. The roles of PLDs in hormonal signaling, lipid metabolism, and cytoskeletal dynamics during rhizobial symbiosis were recently explored. However, to date, the roles of PLDs in mycorrhizal symbiosis have not been reported. Here, we present a critical review of the participation of PLDs in the interactions of plants with pathogens, nitrogen-fixing bacteria, and arbuscular mycorrhizal fungi. We describe how PLDs regulate rhizobial and mycorrhizal symbiosis by modulating reactive oxygen species levels, hormonal signaling, cytoskeletal rearrangements, and G-protein activity.

The specific glycerolipid composition is responsible for maintaining the membrane stability of Physcomitrella patens under dehydration stress

Land colonization is a major event in plant evolution. Little is known about the evolutionary characteristics of lipids during this process. Here, we proved that Physcomitrella patens, a bryophyte that appeared in the early evolution of terrestrial plants, has short-term desiccation resistance. The maintenance of membrane integrity is related to its specific glycerolipid composition and key genes for lipid metabolism. We analyzed 414 types of lipid molecules, and found that phospholipids accounted for 61.7%, mainly PC and PI; glycolipids accounted for only 26.5%, with a special MGDG molecular map. The most abundant MDGD, that is, MGDG34:6, contained rare 15- and 19-carbon acyl chains; the level of neutral lipids was higher. This was consistent with the results observed by TEM, with fewer lamellae and obvious lipid droplets. Slight dehydration accumulated a large number of TAG molecules, and severe dehydration degraded phospholipids and caused membrane leakage, but PA and MGDG fluctuated less. The key genes of lipid metabolism, DGAT and PAP, were actively transcribed, suggesting that PA was one of the main DAG sources for TAG synthesis. This work proves that Physcomitrella patens adopts high-constitutive PC and PI similar to plant seeds, abundant TAG, and its own specific MGDG to resist extreme dehydration. This result provides a new insight into the lipid evolution of early terrestrial plants against unfavorable terrestrial environments.

Phospholipase Dα1 Acts as a Negative Regulator of High Mg2+-Induced Leaf Senescence in Arabidopsis

Magnesium (Mg2+) is a macronutrient involved in essential cellular processes. Its deficiency or excess is a stress factor for plants, seriously affecting their growth and development and therefore, its accurate regulation is essential. Recently, we discovered that phospholipase Dα1 (PLDα1) activity is vital in the stress response to high-magnesium conditions in Arabidopsis roots. This study shows that PLDα1 acts as a negative regulator of high-Mg2+-induced leaf senescence in Arabidopsis. The level of phosphatidic acid produced by PLDα1 and the amount of PLDα1 in the leaves increase in plants treated with high Mg2+. A knockout mutant of PLDα1 (pldα1-1), exhibits premature leaf senescence under high-Mg2+ conditions. In pldα1-1 plants, higher accumulation of abscisic and jasmonic acid (JA) and impaired magnesium, potassium and phosphate homeostasis were observed under high-Mg2+ conditions. High Mg2+ also led to an increase of starch and proline content in Arabidopsis plants. While the starch content was higher in pldα1-1 plants, proline content was significantly lower in pldα1-1 compared with wild type plants. Our results show that PLDα1 is essential for Arabidopsis plants to cope with the pleiotropic effects of high-Mg2+ stress and delay the leaf senescence.

Small signaling molecules in plant response to cold stress

Cold stress is one of the harsh environmental stresses that adversely affect plant growth and crop yields in the Qinghai-Tibet Plateau. However, plants have evolved mechanisms to overcome the impact of cold stress. Progress has been made in understanding how plants perceive and transduce low-temperature signals to tolerate cold stress. Small signaling molecules are crucial for cellular signal transduction by initiating the downstream signaling cascade that helps plants to respond to cold stress. These small signaling molecules include calcium, reactive oxygen species, nitric oxide, hydrogen sulfide, cyclic guanosine monophosphate, phosphatidic acid, and sphingolipids. The small signaling molecules are involved in many aspects of cellular and physiological functions, such as inducing gene expression and activating hormone signaling, resulting in upregulation of the antioxidant enzyme activities, osmoprotectant accumulation, malondialdehyde reduction, and photosynthesis improvement. We summarize our current understanding of the roles of the small signaling molecules in cold stress in plants, and highlight their crosstalk in cold signaling transduction. These discoveries help us understand how the plateau plants adapt to the severe alpine environment as well as to develop new crops tolerating cold stress in the Qinghai-Tibet Plateau.

Identification, evolution, expression analysis of phospholipase D (PLD) gene family in tea (Camellia sinensis)

Phospholipase D (PLD) (EC 3.1.4.4) plays important roles in plants growth, development, and response to environmental stresses. Tea plant (Camellia sinensis) is the most important non-alcoholic beverage in the world with health benefits, but tea production decreases in response to environmental stresses such as cold and drought. Therefore, a genome-wide analysis of the C. sinensis PLD gene family (CsPLDs) was carried out. In the current study, identification, evolutionary relationship, duplication, selection pressure, gene structure, promoter analysis, transcript-targeted miRNA, and simple sequence repeat markers prediction, RNA-seq data analysis, and three-dimensional structure of the CsPLDs have been investigated using bioinformatics tools. 15 PLDs were identified from the tea genome which belongs to five groups, including CsPLDα, CsPLDβ, CsPLDδ, CsPLDε, and CsPLDζ. Both segmental and tandem duplications have occurred in the CsPLD gene family. Ka/Ks ratio for the most duplicated pair genes was less than 1 which implies negative selection to conserve their function during the tea evolution. 68 cis-elements have been found in CsPLDs indicating the contribution of these genes in response to environmental stresses. Likewise, 72 SSR loci and 96 miRNA molecules in 14 and 15 CsPLDs have been detected. According to RNA-seq data, the highest expression in all tissues under various abiotic stresses was related to CsPLDα1. Besides, a three-dimensional structure of the CsPLDα1 was evaluated to better understand its biological activity. This research provides comprehensive information that could be useful in future studies to develop stress-tolerant tea.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01007-0.

Fight Hard or Die Trying: Current Status of Lipid Signaling during Plant-Pathogen Interaction

Plant diseases pose a substantial threat to food availability, accessibility, and security as they account for economic losses of nearly $300 billion on a global scale. Although various strategies exist to reduce the impact of diseases, they can introduce harmful chemicals to the food chain and have an impact on the environment. Therefore, it is necessary to understand and exploit the plants' immune systems to control the spread of pathogens and enable sustainable agriculture. Recently, growing pieces of evidence suggest a functional myriad of lipids to be involved in providing structural integrity, intracellular and extracellular signal transduction mediators to substantial cross-kingdom cell signaling at the host-pathogen interface. Furthermore, some pathogens recognize or exchange plant lipid-derived signals to identify an appropriate host or development, whereas others activate defense-related gene expression. Typically, the membrane serves as a reservoir of lipids. The set of lipids involved in plant-pathogen interaction includes fatty acids, oxylipins, phospholipids, glycolipids, glycerolipids, sphingolipids, and sterols. Overall, lipid signals influence plant-pathogen interactions at various levels ranging from the communication of virulence factors to the activation and implementation of host plant immune defenses. The current review aims to summarize the progress made in recent years regarding the involvement of lipids in plant-pathogen interaction and their crucial role in signal transduction.

Plant Acyl-CoA-Binding Proteins-Their Lipid and Protein Interactors in Abiotic and Biotic Stresses

Plants are constantly exposed to environmental stresses during their growth and development. Owing to their immobility, plants possess stress-sensing abilities and adaptive responses to cope with the abiotic and biotic stresses caused by extreme temperatures, drought, flooding, salinity, heavy metals and pathogens. Acyl-CoA-binding proteins (ACBPs), a family of conserved proteins among prokaryotes and eukaryotes, bind to a variety of acyl-CoA esters with different affinities and play a role in the transport and maintenance of subcellular acyl-CoA pools. In plants, studies have revealed ACBP functions in development and stress responses through their interactions with lipids and protein partners. This review summarises the roles of plant ACBPs and their lipid and protein interactors in abiotic and biotic stress responses.

Membrane Lipids' Metabolism and Transcriptional Regulation in Maize Roots Under Cold Stress

Low temperature is one of the major abiotic stresses that restrict the growth and development of maize seedlings. Membrane lipid metabolism and remodeling are key strategies for plants to cope with temperature stresses. In this study, an integrated lipidomic and transcriptomic analysis was performed to explore the metabolic changes of membrane lipids in the roots of maize seedlings under cold stress (5°C). The results revealed that major extraplastidic phospholipids [phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), and phosphatidylinositol (PI)] were dominant membrane lipids in maize root tissues, accounting for more than 70% of the total lipids. In the transcriptome data of maize roots under cold stress, a total of 189 lipid-related differentially expressed genes (DEGs) were annotated and classified into various lipid metabolism pathways, and most of the DEGs were enriched in the "Eukaryotic phospholipid synthesis" (12%), "Fatty acid elongation" (12%), and "Phospholipid signaling" (13%) pathways. Under low temperature stress, the molar percentage of the most abundant phospholipid PC decreased around 10%. The significantly up-regulated expression of genes encoding phospholipase [phospholipase D (PLD)] and phosphatase PAP/LPP genes implied that PC turnover was triggered by cold stress mainly via the PLD pathway. Consequently, as the central product of PC turnover, the level of PA increased drastically (63.2%) compared with the control. The gene-metabolite network and co-expression network were constructed with the prominent lipid-related DEGs to illustrate the modular regulation of metabolic changes of membrane lipids. This study will help to explicate membrane lipid remodeling and the molecular regulation mechanism in field crops encountering low temperature stress.

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.

Phospholipase Dα1 mediates the high-Mg2+ stress response partially through regulation of K+ homeostasis

Intracellular levels of Mg²⁺ are tightly regulated, as Mg²⁺ deficiency or excess affects normal plant growth and development. In Arabidopsis, we determined that phospholipase Dα1 (PLDα1) is involved in the stress response to high-magnesium conditions. The T-DNA insertion mutant pldα1 is hypersensitive to increased concentrations of magnesium, exhibiting reduced primary root length and fresh weight. PLDα1 activity increases rapidly after high-Mg²⁺ treatment, and this increase was found to be dose dependent. Two lines harbouring mutations in the HKD motif, which is essential for PLDα1 activity, displayed the same high-Mg²⁺ hypersensitivity of pldα1 plants. Moreover, we show that high concentrations of Mg²⁺ disrupt K⁺ homeostasis, and that transcription of K⁺ homeostasis-related genes CIPK9 and HAK5 is impaired in pldα1. Additionally, we found that the akt1, hak5 double mutant is hypersensitive to high-Mg²⁺ . We conclude that in Arabidopsis, the enzyme activity of PLDα1 is vital in the response to high-Mg²⁺ conditions, and that PLDα1 mediates this response partially through regulation of K⁺ homeostasis.

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.

Comparative spatial lipidomics analysis reveals cellular lipid remodelling in different developmental zones of barley roots in response to salinity

Salinity-induced metabolic, ionic, and transcript modifications in plants have routinely been studied using whole plant tissues, which do not provide information on spatial tissue responses. The aim of this study was to assess the changes in the lipid profiles in a spatial manner and to quantify the changes in the elemental composition in roots of seedlings of four barley cultivars before and after a short-term salt stress. We used a combination of liquid chromatography-tandem mass spectrometry, inductively coupled plasma mass spectrometry, matrix-assisted laser desorption/ionization mass spectrometry imaging, and reverse transcription - quantitative real time polymerase chain reaction platforms to examine the molecular signatures of lipids, ions, and transcripts in three anatomically different seminal root tissues before and after salt stress. We found significant changes to the levels of major lipid classes including a decrease in the levels of lysoglycerophospholipids, ceramides, and hexosylceramides and an increase in the levels of glycerophospholipids, hydroxylated ceramides, and hexosylceramides. Our results revealed that modifications to lipid and transcript profiles in plant roots in response to a short-term salt stress may involve recycling of major lipid species, such as phosphatidylcholine, via resynthesis from glycerophosphocholine.

An efficient and comprehensive plant glycerolipids analysis approach based on high-performance liquid chromatography-quadrupole time-of-flight mass spectrometer

In past two decades, numerous lipidomics approaches based on mass spectrometry with or without liquid chromatography separation have been established for identification and quantification of lipids in plants. In this study, we developed an efficient and comprehensive lipidomics approach based on UPLC with an Acquity UPLCTM BEH C18 column coupled to TripleTOF using ESI in positive ion mode and MS/MSALL scan for data collection. Lipid extract was prepared to 2 mg/ml solution according to dry tissue weight and mixed with 13 kinds of internal standards including PA, PC, PE, and PG. Each analysis required single injection of 5-10 μl lipid solvent and completed in 32 min. A target method dataset was generated using the LipidView software for prediction of the accurate mass of target lipid species. The dataset was uploaded into the PeakView to create processing datasets to search target lipid species, which achieved batch data processing of multiple samples for lipid species-specific identification and quantification. As proof of concept, we profiled the lipids of different tissues of rapeseed. Thirteen lipid classes including 218 glycerolipids were identified including 46 TAGs, 15 DAGs, 20 PCs, 24 PEs, 13 PGs, 14 PIs, 26 PSs, 12 PAs, 16 MGDGs, 16 DGDGs, 6 LysoPCs, 5 LysoPEs, and 5 LysoPGs. Together, our approach permits the analysis of glycerolipids in plant tissues with simplicity in sample analysis and data processing using UPLC-TripleTOF.

Phosphatidic acid promotes the activation and plasma membrane localization of MKK7 and MKK9 in response to salt stress

Phosphatidic acid (PA) is a lipid secondary messenger involved in intracellular signaling in eukaryotes. It has been confirmed that PA mediates salt stress signaling by promoting activation of Mitogen-activated Protein Kinase 6 (MPK6) which phosphorylates Na⁺/H⁺ antiporter SOS1. However, the MPK6-upstream kinases and their relationship to PA remain unclear. Here, we found that, among the six tested Arabidopsis Mitogen-activated Protein Kinase Kinases (MKKs), PA specifically bound to MKK7 and MKK9 which phosphorylate MPK6, and promoted the activation of MKK7/MKK9. Based on phenotypic and physiological analyses, we found that MKK7 and MKK9 positively regulate Arabidopsis salt tolerance and are functionally redundant. NaCl treatment can induce significant increase in MKK7/MKK9 activities, and this depends, in part, on the Phospholipase Dα1 (PLDα1). MKK7 and MKK9 also mediate the NaCl-induced activation of MPK6. Furthermore, PA or NaCl treatment could induce translocation of MKK7/MKK9 to the plasma membrane, whereas this translocation disappeared in pldα1. These results indicate that PA binds to MKK7 and MKK9, increases their kinase activity and plasma membrane localization during Arabidopsis response to salt stress. Together with the PA-MPK6-SOS1 pathway identified previously, this mechanism may maximize the signal transduction efficiency, providing novel insights into the link between lipid signaling and MAPK cascade.

Membrane Rigidity and Phosphatidic Acid (PtdOH) Signal: Two Important Events in Acinetobacter guillouiae SFC 500-1A Exposed to Chromium(VI) and Phenol

The remodeling of membrane lipids is a mechanism that allows microorganisms to survive in unfavorable environments such as industrial effluents, which often contain inorganic and organic pollutants, like chromium and phenol. In the present work, we evaluated the effect of Cr(VI) and phenol on the membrane of Acinetobacter guillouiae SFC 500-1A, a bacterial strain isolated from tannery sediments where such pollutants can be found. The presence of lipid kinases and phospholipases and the changes in their activities under exposure to these pollutants were determined. Cr(VI) and Cr(VI) + phenol caused the membrane to become more rigid for up to 16 h after exposure. This could be due to an increase in cardiolipin (Ptd₂ Gro) and a decrease in phosphatidylethanolamine (PtdEtn), which are indicative of more order and rigidity in the membrane. Increased phospholipase A activity (PLA, EC 3.1.1.4) could be responsible for the decrease in PtdEtn levels. Moreover, our results indicate that Cr(VI) and Cr(VI) + phenol trigger the phosphatidic acid (PtdOH) signal. The finding of significantly increased phosphatidylinositol-4-phosphate (PtdIns-4-P) levels means this is likely achieved via PtdIns-PLC/DGK. This report provides the first evidence that A. guillouiae SFC 500-1A is able to sense Cr(VI) and phenol, transduce this signal through changes in the physical state of the membrane, and trigger lipid-signaling events.

Determination of glycerophospholipids in vegetable edible oils: Proof of concept to discriminate olive oil categories

Glycerophospholipids (GPLs) constitute a chemical family within the saponifiable fraction of vegetable oils. GPLs have been scarcely studied in edible oils owing to the lack of sensitive and selective analytical methods. We have developed a method for identification, confirmation and relative quantitation of GPLs in vegetable oils. The method is based on solid-phase extraction (SPE) for isolation of GPLs and determination by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). As proof of concept, the approach has been applied to characterize GPLs in different olive oil categories, thus revealing compositional changes, which could be explained by factors such as the quality of fruits and the extraction process. Families such as glycerophosphatidic acids and phosphatidylglycerides are remarkable because of their capability to discriminate virgin olive oils from the rest of categories. These results open a door to additional studies targeted at the identification of olive oil quality by monitoring these lipids.

Seed germination, respiratory processes and phosphatidic acid accumulation in Arabidopsis diacylglycerol kinase knockouts - The effect of brassinosteroid, brassinazole and salinity

Using Arabidopsis thaliana wild type (WT) plants and diacylglycerol kinase knockouts (single mutants - dgk3, dgk1, dgk6; double mutants - dgk3dgk7, dgk5dgk6, dgk1dgk2) we observed that the inhibitor of brassinosteroid (BR) biosynthesis, brassinazole (BRZ), drastically decreased germination of dgk mutants under salt stress, while BRZ co-administration with 24-epibrassinolide (EBL) partially improved germination rates. We also observed a statistically significant decrease in alternative and cytochrome respiratory pathways in response to BRZ treatment under salinity conditions. We showed that production of the lipid second messenger phosphatidic acid (PA) is impaired in dgk mutants in response to EBL treatment and inhibitor of diacylglycerol kinase (DGK) - R59022. This study demonstrates that dgk mutants possess lower germination rates, lower total respiration rates, an alternative respiratory pathway and PA content under optimal and high salinity conditions in response to EBL treatment comparing to WT plants.

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

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

Involvement of phosphatidylinositol metabolism in aluminum-induced malate secretion in Arabidopsis

To identify the upstream signaling of aluminum-induced malate secretion through aluminum-activated malate transporter 1 (AtALMT1), a pharmacological assay using inhibitors of human signal transduction pathways was performed. Early aluminum-induced transcription of AtALMT1 and other aluminum-responsive genes was significantly suppressed by phosphatidylinositol 4-kinase (PI4K) and phospholipase C (PLC) inhibitors, indicating that the PI4K-PLC metabolic pathway activates early aluminum signaling. Inhibitors of phosphatidylinositol 3-kinase (PI3K) and PI4K reduced aluminum-activated malate transport by AtALMT1, suggesting that both the PI3K and PI4K metabolic pathways regulate this process. These results were validated using T-DNA insertion mutants of PI4K and PI3K-RNAi lines. A human protein kinase inhibitor, putatively inhibiting homologous calcineurin B-like protein-interacting protein kinase and/or Ca-dependent protein kinase in Arabidopsis, suppressed late-phase aluminum-induced expression of AtALMT1, which was concomitant with the induction of an AtALMT1 repressor, WRKY46, and suppression of an AtALMT1 activator, Calmodulin-binding transcription activator 2 (CAMTA2). In addition, a human deubiquitinase inhibitor suppressed aluminum-activated malate transport, suggesting that deubiquitinases can regulate this process. We also found a reduction of aluminum-induced citrate secretion in tobacco by applying inhibitors of PI3K and PI4K. Taken together, our results indicated that phosphatidylinositol metabolism regulates organic acid secretion in plants under aluminum stress.

Cellular Organization and Regulation of Plant Glycerolipid Metabolism

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

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.

Lipidomic reprogramming associated with drought stress priming-enhanced heat tolerance in tall fescue (Festuca arundinacea)

Stress priming by exposing plants to a mild or moderate drought could enhance plant tolerance to subsequent heat stress. Lipids play vital roles in stress adaptation, but how lipidomic profiles change, affecting the cross-stress tolerance, is largely unknown. The objectives of this study were to perform lipidomics, to analyse the content, composition, and saturation levels of lipids in leaves of tall fescue (Festuca arundinacea) following drought priming and subsequent heat stress, and to identify major lipids and molecular species associated with priming-enhanced heat tolerance. Plants were initially exposed to drought for 8 days by withholding irrigation and subsequently subjected to 25 days of heat stress (38/33°C day/night) in growth chambers. Drought-primed plants maintained significantly higher leaf relative water content, chlorophyll content, photochemical efficiency, and lower electrolyte leakage than nonprimed plants under heat stress. Drought priming enhanced the accumulation of phospholipids and glycolipids involved in membrane stabilization and stress signalling (phosphatidic acid, phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol, and digalactosyl diacylglycerol) during subsequent exposure to heat stress. The reprogramming of lipid metabolism for membrane stabilization and signalling in response to drought priming and subsequent exposure to heat stress could contribute to drought priming-enhanced heat tolerance in cool-season grass species.

Accumulation of phosphoinositides in distinct regions of the periarbuscular membrane

Phosphoinositides and phosphatidic acid are small anionic lipids that comprise a minor proportion of total membrane lipids in eukaryotic cells but influence a broad range of cellular processes including endomembrane trafficking, signaling, exocytosis and endocytosis. To investigate the spatial distribution of phosphoinositides during arbuscular mycorrhizal symbiosis, we generated fluorescent reporters of PI(4,5)P₂ and PI4P, as well as phosphatidic acid and diacylglycerol and used them to monitor lipid distribution on the cytoplasmic side of membrane bilayers in colonized cortical cells. The PI4P reporter accumulated strongly on the periarbuscular membrane (PAM) and transiently labeled Golgi bodies, while the PA reporter showed differential labeling of endomembranes and the PAM. Surprisingly, the PI(4,5)P₂ reporter accumulated in small, discrete regions of the PAM on the arbuscule trunks, frequently in two regions on opposing sides of the hypha. A mutant reporter with reduced PI(4,5)P₂ binding capacity did not show these accumulations. The PI(4,5)P₂ -rich regions were detected at all phases of arbuscule development following branching, co-localized with membrane marker proteins potentially indicating high membrane bilayer content, and were associated with an alteration in morphology of the hypha. A possible analogy to the biotrophic interfacial membrane complex formed in rice infected with Magnaporthe orzyae is discussed.

Using fluorescent lipids contributes to the active learning of principles underlying lipid signaling

The concepts of phospholipase activity is often taught in undergraduate biology and biochemistry classes and reinforced in laboratory exercises. However, very rarely does the design of these exercises allow students to directly gain experience in the use of modern instruments such as digital imaging systems and fluorescence spectrophotometers. The laboratory exercise described here involves the use of fluorescent lipids to evaluate phospholipase activity. Students use thin layer chromatography (TLC) to understand how lipids change under different conditions (i.e. abiotic and biotic stress). They explore strategies to separate, visualize and quantify lipids by TLC, digital imaging, and fluorometry. They also have increased opportunities for hands-on practise with experimental design, liposome sample preparation, and implementation of instrumentation commonly used by experienced researchers; all while learning and applying fundamental concepts about lipids. © 2018 International Union of Biochemistry and Molecular Biology, 47(1):100-105, 2018.

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.

Photosynthetic response to increased irradiance correlates to variation in transcriptional response of lipid-remodeling and heat-shock genes

Plants have evolved several mechanisms for sensing increased irradiance, involving signal perception by photoreceptors (cryptochromes), and subsequent biochemical (reactive oxygen species, ROS) and metabolic clues to transmit the signals. This results in the increased expression of heat-shock response genes and of the transcription factor LONG HYPOCOTYL 5 (HY5, mediated by the cryptochrome photoreceptor 1, CRY1). Here, we show the existence of another response pathway in Arabidopsis. This pathway evokes the SPX1-mediated expression activation of the transcription factor PHR1 and leads to the expression of several galactolipid biosynthesis genes. Gene expression analysis of accessions Col-0, Ga-0, and Ts-1, showed activated expression of the SPX1/PHR1-mediated gene expression activation pathway acting on galactolipids biosynthesis genes in both Ga-0 and Col-0, but not in Ts-1. The activation of the SPX1/PHR1-mediated response pathway can be associated with lower photosynthesis efficiency in Ts-1, compared to Col-0 and Ga-0. Besides the accession-associated activation of the SPX1/PHR1-mediated response pathway, comparing gene expression in the accessions showed stronger activation of several heat responsive genes in Ga-0, and the opposite in Ts-1, when compared to Col-0, in line with the differences in their efficiency of photosynthesis. We conclude that natural variation in activation of both heat responsive genes and of galactolipids biosynthesis genes contribute to the variation in photosynthesis efficiency in response to irradiance increase.

Production of Plant Secondary Metabolites: Examples, Tips and Suggestions for Biotechnologists

Plants are sessile organisms and, in order to defend themselves against exogenous (a)biotic constraints, they synthesize an array of secondary metabolites which have important physiological and ecological effects. Plant secondary metabolites can be classified into four major classes: terpenoids, phenolic compounds, alkaloids and sulphur-containing compounds. These phytochemicals can be antimicrobial, act as attractants/repellents, or as deterrents against herbivores. The synthesis of such a rich variety of phytochemicals is also observed in undifferentiated plant cells under laboratory conditions and can be further induced with elicitors or by feeding precursors. In this review, we discuss the recent literature on the production of representatives of three plant secondary metabolite classes: artemisinin (a sesquiterpene), lignans (phenolic compounds) and caffeine (an alkaloid). Their respective production in well-known plants, i.e., Artemisia, Coffea arabica L., as well as neglected species, like the fibre-producing plant Urtica dioica L., will be surveyed. The production of artemisinin and caffeine in heterologous hosts will also be discussed. Additionally, metabolic engineering strategies to increase the bioactivity and stability of plant secondary metabolites will be surveyed, by focusing on glycosyltransferases (GTs). We end our review by proposing strategies to enhance the production of plant secondary metabolites in cell cultures by inducing cell wall modifications with chemicals/drugs, or with altered concentrations of the micronutrient boron and the quasi-essential element silicon.

The phosphatidic acid paradox: Too many actions for one molecule class? Lessons from plants

Phosphatidic acid (PA) is a simple phospholipid observed in most organisms. PA acts as a key metabolic intermediate and a second messenger that regulates many cell activities. In plants, PA is involved in numerous cell responses induced by hormones, stress inputs and developmental processes. Interestingly, PA production can be triggered by opposite stressors, such as cold and heat, or by hormones that are considered to be antagonistic, such as abscisic acid and salicylic acid. This property questions the specificity of the responses controlled by PA. Are there generic responses to PA, meaning that cell regulation triggered by PA would be always the same, even in opposite physiological situations? Alternatively, do the responses to PA differ according to the physiological context within the cells? If so, the mechanisms that regulate the divergence of PA-controlled reactions are poorly defined. This review summarizes the latest opinions on how PA signalling is directed in plant cells and examines the intrinsic properties of PA that enable its regulatory diversity. We propose a concept whereby PA regulatory messages are perceived as complex "signatures" that take into account their production site, the availability of target proteins and the relevant cellular environments.

Genomewide identification of genes involved in the potato response to drought indicates functional evolutionary conservation with Arabidopsis plants

Potato is one of the four most important food crop plants worldwide and is strongly affected by drought. The following two pairs of potato cultivars, which are related in ancestry but show different drought tolerances, were chosen for comparative gene expression studies: Gwiazda/Oberon and Tajfun/Owacja. Comparative RNA-seq analyses of gene expression differences in the transcriptomes obtained from drought-tolerant versus drought-sensitive plants during water shortage conditions were performed. The 23 top-ranking genes were selected, 22 of which are described here as novel potato drought-responsive genes. Moreover, all but one of the potato genes selected have homologues in the Arabidopsis genome. Of the seven tested A. thaliana mutants with altered expression of the selected homologous genes, compared to the wild-type Arabidopsis plants, six showed an improved tolerance to drought. These genes encode carbohydrate transporter, mitogen-activated protein kinase kinase kinase 15 (MAPKKK15), serine carboxypeptidase-like 19 protein (SCPL19), armadillo/beta-catenin-like repeat-containing protein, high-affinity nitrate transporter 2.7 and nonspecific lipid transfer protein type 2 (nsLPT). The evolutionary conservation of the functions of the selected genes in the plant response to drought confirms the importance of these identified potato genes in the ability of plants to cope with water shortage conditions. Knowledge regarding these gene functions can be used to generate potato cultivars that are resistant to unfavourable conditions. The approach used in this work and the obtained results allowed for the identification of new players in the plant response to drought.

Lipid and protein fingerprinting for Fusarium oxysporum f. sp. cubense strain-level classification

Banana is one of the most popular fruits in the world but has been substantially impaired by Panama disease in the last years. Fusarium oxysporum f. sp. cubense (Foc) is the causal agent and colonizes banana cultivars from many subgroups with different aggressiveness levels, often leading to plant death while compromising new crops in infested areas. This study has evaluated the ability of MALDI-MS protein and lipid fingerprinting to provide intraspecies classification of Foc isolates and to screen biomolecules related to host-pathogen relationship. The MS data, when inspected via partial least square discriminant analysis (PLS-DA), distinguished the isolates by aggressiveness as well as by specific location and host. Although both lipids and proteins show discriminating tendencies, these differences were more clearly perceived via the protein profiles. Considering that Cavendish cultivar is the more resistant option to endure Foc presence in the field, the lipids and proteins related to this subgroup might have an important role in pathogen adaptation. This study reports a new application of MALDI-MS for the analysis of a banana pathogen with intraspecies classification ability. Graphical abstract MALDI-MS classified Foc isolates by aggressiveness level on banana revealing the additional influence of location and host cultivar on the expression of lipids and proteins.

Circadian and Light Regulated Expression of CBFs and their Upstream Signalling Genes in Barley

CBF (C-repeat binding factor) transcription factors show high expression levels in response to cold; moreover, they play a key regulatory role in cold acclimation processes. Recently, however, more and more information has led to the conclusion that, apart from cold, light-including its spectra-also has a crucial role in regulating CBF expression. Earlier, studies established that the expression patterns of some of these regulatory genes follow circadian rhythms. To understand more of this complex acclimation process, we studied the expression patterns of the signal transducing pathways, including signal perception, the circadian clock and phospholipid signalling pathways, upstream of the CBF gene regulatory hub. To exclude the confounding effect of cold, experiments were carried out at 22 °C. Our results show that the expression of genes implicated in the phospholipid signalling pathway follow a circadian rhythm. We demonstrated that, from among the tested CBF genes expressed in Hordeumvulgare (Hv) under our conditions, only the members of the HvCBF4-phylogenetic subgroup showed a circadian pattern. We found that the HvCBF4-subgroup genes were expressed late in the afternoon or early in the night. We also determined the expression changes under supplemental far-red illumination and established that the transcript accumulation had appeared four hours earlier and more intensely in several cases. Based on our results, we propose a model to illustrate the effect of the circadian clock and the quality of the light on the elements of signalling pathways upstream of the HvCBFs, thus integrating the complex regulation of the early cellular responses, which finally lead to an elevated abiotic stress tolerance.

Fatty Acid- and Lipid-Mediated Signaling in Plant Defense

Fatty acids and lipids, which are major and essential constituents of all plant cells, not only provide structural integrity and energy for various metabolic processes but can also function as signal transduction mediators. Lipids and fatty acids can act as both intracellular and extracellular signals. In addition, cyclic and acyclic products generated during fatty acid metabolism can also function as important chemical signals. This review summarizes the biosynthesis of fatty acids and lipids and their involvement in pathogen defense.

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.

Relationship between aluminum stress and caffeine biosynthesis in suspension cells of Coffea arabica L

Toxicity by aluminum is a growth-limiting factor in plants cultivated in acidic soils. This metal also promotes signal transduction pathways leading to the biosynthesis of defense compounds, including secondary metabolites. In this study, we observed that Coffea arabica L. cells that were kept in the dark did not produce detectable levels of caffeine. However, irradiation with light and supplementation of the culture medium with theobromine were the best conditions for cell maintenance to investigate the role of aluminum in caffeine biosynthesis. The addition of theobromine to the cells did not cause any changes to cell growth and was useful for the bioconversion of theobromine to caffeine. During a short-term AlCl₃-treatment (500μM) of C. arabica cells kept under light irradiation, increases in the caffeine levels in samples that were recovered from both the cells and culture media were evident. This augmentation coincided with increases in the enzyme activity of caffeine synthase (CS) and the transcript level of the gene encoding this enzyme (CS). Together, these results suggest that actions by Al and theobromine on the same pathway lead to the induction of caffeine biosynthesis.

Diacylglycerol kinases activate tobacco NADPH oxidase-dependent oxidative burst in response to cryptogein

Cryptogein is a 10 kDa protein secreted by the oomycete Phytophthora cryptogea that activates defence mechanisms in tobacco plants. Among early signalling events triggered by this microbial-associated molecular pattern is a transient apoplastic oxidative burst which is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity of the RESPIRATORY BURST OXIDASE HOMOLOG isoform D (RBOHD). Using radioactive [33 P]-orthophosphate labelling of tobacco Bright Yellow-2 suspension cells, we here provide in vivo evidence for a rapid accumulation of phosphatidic acid (PA) in response to cryptogein because of the coordinated onset of phosphoinositide-dependent phospholipase C and diacylglycerol kinase (DGK) activities. Both enzyme specific inhibitors and silencing of the phylogenetic cluster III of the tobacco DGK family were found to reduce PA production upon elicitation and to strongly decrease the RBOHD-mediated oxidative burst. Therefore, it appears that PA originating from DGK controls NADPH-oxidase activity. Amongst cluster III DGKs, the expression of DGK5-like was up-regulated in response to cryptogein. Besides DGK5-like is likely to be the main cluster III DGK isoform silenced in one of our mutant lines, making it a strong candidate for the observed response to cryptogein. The relevance of these results is discussed with regard to early signalling lipid-mediated events in plant immunity.

Diacylglycerol Kinases Are Widespread in Higher Plants and Display Inducible Gene Expression in Response to Beneficial Elements, Metal, and Metalloid Ions

Diacylglycerol kinases (DGKs) are pivotal signaling enzymes that phosphorylate diacylglycerol (DAG) to yield phosphatidic acid (PA). The biosynthesis of PA from phospholipase D (PLD) and the coupled phospholipase C (PLC)/DGK route is a crucial signaling process in eukaryotic cells. Next to PLD, the PLC/DGK pathway is the second most important generator of PA in response to biotic and abiotic stresses. In eukaryotic cells, DGK, DAG, and PA are implicated in vital processes such as growth, development, and responses to environmental cues. A plethora of DGK isoforms have been identified so far, making this a rather large family of enzymes in plants. Herein we performed a comprehensive phylogenetic analysis of DGK isoforms in model and crop plants in order to gain insight into the evolution of higher plant DGKs. Furthermore, we explored the expression profiling data available in public data bases concerning the regulation of plant DGK genes in response to beneficial elements and other metal and metalloid ions, including silver (Ag), aluminum (Al), arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), and sodium (Na). In all plant genomes explored, we were able to find DGK representatives, though in different numbers. The phylogenetic analysis revealed that these enzymes fall into three major clusters, whose distribution depends on the composition of structural domains. The catalytic domain conserves the consensus sequence GXGXXG/A where ATP binds. The expression profiling data demonstrated that DGK genes are rapidly but transiently regulated in response to certain concentrations and time exposures of beneficial elements and other ions in different plant tissues analyzed, suggesting that DGKs may mediate signals triggered by these elements. Though this evidence is conclusive, further signaling cascades that such elements may stimulate during hormesis, involving the phosphoinositide signaling pathway and DGK genes and enzymes, remain to be elucidated.