Arabidopsis phospholipase Dβ1 modulates defense responses to bacterial and fungal pathogens

A GDSL motif-containing lipase modulates Sclerotinia sclerotiorum resistance in Brassica napus

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum (Lib.) De Bary is a devastating disease infecting hundreds of plant species. It also restricts the yield, quality, and safe production of rapeseed (Brassica napus) worldwide. However, the lack of resistance sources and genes to S. sclerotiorum has greatly restricted rapeseed SSR-resistance breeding. In this study, a previously identified GDSL motif-containing lipase gene, B. napus GDSL LIPASE-LIKE 1 (BnaC07.GLIP1), encoding a protein localized to the intercellular space, was characterized as functioning in plant immunity to S. sclerotiorum. The BnaC07.GLIP1 promoter is S. sclerotiorum-inducible and the expression of BnaC07.GLIP1 is substantially enhanced after S. sclerotiorum infection. Arabidopsis (Arabidopsis thaliana) heterologously expressing and rapeseed lines overexpressing BnaC07.GLIP1 showed enhanced resistance to S. sclerotiorum, whereas RNAi suppression and CRISPR/Cas9 knockout B. napus lines were hyper-susceptible to S. sclerotiorum. Moreover, BnaC07.GLIP1 affected the lipid composition and induced the production of phospholipid molecules, such as phosphatidylethanolamine, phosphatidylcholine, and phosphatidic acid, which were correlated with decreased levels of reactive oxygen species (ROS) and enhanced expression of defense-related genes. A B. napus bZIP44 transcription factor specifically binds the CGTCA motif of the BnaC07.GLIP1 promoter to positively regulate its expression. BnbZIP44 responded to S. sclerotiorum infection, and its heterologous expression inhibited ROS accumulation, thereby enhancing S. sclerotiorum resistance in Arabidopsis. Thus, BnaC07.GLIP1 functions downstream of BnbZIP44 and is involved in S. sclerotiorum resistance by modulating the production of phospholipid molecules and ROS homeostasis in B. napus, providing insights into the potential roles and functional mechanisms of BnaC07.GLIP1 in plant immunity and for improving rapeseed SSR disease-resistance breeding.

OsPLDα1 mediates cadmium stress response in rice by regulating reactive oxygen species accumulation and lipid remodeling

Lipid remodeling is crucial for various cellular activities and the stress tolerance of plants; however, little is known about the lipid dynamics induced by the heavy metal cadmium (Cd). In this study, we investigated the phospholipid profiles in rice (Oryza sativa) under Cd exposure. We observed a significant decline in the total amounts of phosphatidylcholine and phosphatidylserine, contrasted with an elevation in phosphatidic acid (PA) due to Cd stress. Additionally, Cd stress prompted the activation of phospholipase D (PLD) and induced the expression of PLDα1. OsPLDα1 knockout mutants (Ospldα1) showed increased sensitivity to Cd, characterized by a heightened accumulation of hydrogen peroxide in roots and diminished PA production following Cd treatment. Conversely, PLDα1-overexpressing (OsPLDα1-OE) lines demonstrated enhanced tolerance to Cd, with suppressed transcription of the respiratory burst oxidase homolog (Rboh) genes. The transcription levels of genes associated with Cd uptake and transport were accordingly modulated in Ospldα1 and OsPLDα1-OE plants relative to the wild-type. Taken together, our findings underscore the pivotal role of OsPLDα1 in conferring tolerance to Cd by modulating reactive oxygen species homeostasis and lipid remodeling in rice.

Genomic identification and expression profiling of DMP genes in oat (Avena sativa) elucidate their responsiveness to seed aging

BACKGROUND

The Domain of unknown function 679 membrane protein (DMP) family, which is unique to plants, plays a crucial role in reproductive development, stress response and aging. A comprehensive study was conducted to identify the DMP gene members of oat (Avena sativa) and to investigate their structural features and tissue-specific expression profiles. Utilizing whole genome and transcriptome data, we analyzed the physicochemical properties, gene structure, cis-acting elements, phylogenetic relationships, conserved structural (CS) domains, CS motifs and expression patterns of the AsDMP family in A. sativa.

RESULTS

The DMP family genes of A. sativa were distributed across 17 chromosomal scaffolds, encompassing a total of 33 members. Based on phylogenetic relationships, the AsDMP genes were classified into five distinct subfamilies. The gene structure also suggests that A. sativa may have undergone an intron loss event during its evolution. Covariance analysis indicates that genome-wide duplication and segmental duplication may be the major contributor to the expansion of the AsDMP gene family. Ka/Ks selective pressure analysis of the AsDMP gene family suggests that DMP gene pairs are generally conserved over evolutionary time. The upstream promoters of these genes contain several cis-acting elements, suggesting a potential role in abiotic stress responses and hormone induction. Transcriptome data revealed that the expression patterns of the DMP genes are involved in tissue and organ development. In this study, the AsDMP genes (AsDMP1, AsDMP19, and AsDMP22) were identified as potential regulators of seed senescence in A. sativa. These genes could serve as candidates for breeding studies focused on seed longevity and anti-aging germplasm in A. sativa. The study provides valuable insights into the regulatory mechanisms of the AsDMP gene family in the aging process of A. sativa germplasm and offers theoretical support for further function investigation into the functions of AsDMP genes and the molecular mechanisms underlying seed anti-aging.

CONCLUSIONS

This study identified the AsDMP genes as being involved in the aging process of A. sativa seeds, marking the first report on the potential role of DMP genes in seed aging for A. sativa.

Molecular responses of agroinfiltrated Nicotiana benthamiana leaves expressing suppressor of silencing P19 and influenza virus-like particles

The production of influenza vaccines in plants is achieved through transient expression of viral hemagglutinins (HAs), a process mediated by the bacterial vector Agrobacterium tumefaciens. HA proteins are then produced and matured through the secretory pathway of plant cells, before being trafficked to the plasma membrane where they induce formation of virus-like particles (VLPs). Production of VLPs unavoidably impacts plant cells, as do viral suppressors of RNA silencing (VSRs) that are co-expressed to increase recombinant protein yields. However, little information is available on host molecular responses to foreign protein expression. This work provides a comprehensive overview of molecular changes occurring in Nicotiana benthamiana leaf cells transiently expressing the VSR P19, or co-expressing P19 and an influenza HA. Our data identifies general responses to Agrobacterium-mediated expression of foreign proteins, including shutdown of chloroplast gene expression, activation of oxidative stress responses and reinforcement of the plant cell wall through lignification. Our results also indicate that P19 expression promotes salicylic acid (SA) signalling, a process dampened by co-expression of the HA protein. While reducing P19 level, HA expression also induces specific signatures, with effects on lipid metabolism, lipid distribution within membranes and oxylipin-related signalling. When producing VLPs, dampening of P19 responses thus likely results from lower expression of the VSR, crosstalk between SA and oxylipin pathways, or a combination of both outcomes. Consistent with the upregulation of oxidative stress responses, we finally show that reduction of oxidative stress damage through exogenous application of ascorbic acid improves plant biomass quality during production of VLPs.

DGK5β-derived phosphatidic acid regulates ROS production in plant immunity by stabilizing NADPH oxidase

In plant immunity, phosphatidic acid (PA) regulates reactive oxygen species (ROS) by binding to respiratory burst oxidase homolog D (RBOHD), an NADPH oxidase responsible for ROS production. Here, we analyze the influence of PA binding on RBOHD activity and the mechanism of RBOHD-bound PA generation. PA binding enhances RBOHD protein stability by inhibiting vacuolar degradation, thereby increasing chitin-induced ROS production. Mutations in diacylglycerol kinase 5 (DGK5), which phosphorylates diacylglycerol to produce PA, impair chitin-induced PA and ROS production. The DGK5 transcript DGK5β (but not DGK5α) complements reduced PA and ROS production in dgk5-1 mutants, as well as resistance to Botrytis cinerea. Phosphorylation of S506 residue in the C-terminal calmodulin-binding domain of DGK5β contributes to the activation of DGK5β to produce PA. These findings suggest that DGK5β-derived PA regulates ROS production by inhibiting RBOHD protein degradation, elucidating the role of PA-ROS interplay in immune response regulation.

Arabidopsis thaliana phosphoinositide-specific phospholipase C 2 is required for Botrytis cinerea proliferation

Phospholipase C (PLC) plays a key role in lipid signaling during plant development and stress responses. PLC activation is one of the earliest responses during pathogen perception. Arabidopsis thaliana contains seven PLC encoding genes (AtPLC1 to AtPLC7) and two pseudogenes (AtPLC8 and AtPLC9), being AtPLC2 the most abundant isoform with constitutive expression in all plant organs. PLC has been linked to plant defense signaling, in particular to the production of reactive oxygen species (ROS). Previously, we demonstrated that AtPLC2 is involved in ROS production via the NADPH oxidase isoforms RBOHD activation during stomata plant immunity. Here we studied the role of AtPLC2 on plant resistance against the necrotrophic fungus Botrytis cinerea, a broad host-range and serious agricultural pathogen. We show that the AtPLC2-silenced (amiR PLC2) or null mutant (plc2-1) plants developed smaller B. cinerea lesions. Moreover, plc2-1 showed less ROS production and an intensified SA-dependent signaling upon infection, indicating that B. cinerea uses AtPLC2-triggered responses for a successful proliferation. Therefore, AtPLC2 is a susceptibility (S) gene that facilitates B. cinerea infection and proliferation.

CsPM5.2, a phosphate transporter protein-like gene, promotes powdery mildew resistance in cucumber

Powdery mildew (PM) is one of the most serious fungal diseases affecting cucumbers (Cucumis sativus L.). The mechanism of PM resistance in cucumber is intricate and remains fragmentary as it is controlled by several genes. In this study, we detected the major-effect Quantitative Trait Locus (QTL), PM5.2, involved in PM resistance by QTL mapping. Through fine mapping, the dominant PM resistance gene, CsPM5.2, was cloned and its function was confirmed by transgenic complementation and natural variation identification. In cultivar 9930, a dysfunctional CsPM5.2 mutant resulted from a single nucleotide polymorphism in the coding region and endowed susceptibility to PM. CsPM5.2 encodes a phosphate transporter-like protein PHO1; H3. The expression of CsPM5.2 is ubiquitous and induced by the PM pathogen. In cucumber, both CsPM5.2 and Cspm5.1 (Csmlo1) are required for PM resistance. Transcriptome analysis suggested that the salicylic acid (SA) pathway may play an important role in CsPM5.2-mediated PM resistance. Our findings help parse the mechanisms of PM resistance and provide strategies for breeding PM-resistant cucumber cultivars.

Activation of MAPK-mediated immunity by phosphatidic acid in response to positive-strand RNA viruses

Increasing evidence suggests that mitogen-activated protein kinase (MAPK) cascades play a crucial role in plant defense against viruses. However, the mechanisms that underlie the activation of MAPK cascades in response to viral infection remain unclear. In this study, we discovered that phosphatidic acid (PA) represents a major class of lipids that respond to Potato virus Y (PVY) at an early stage of infection. We identified NbPLDα1 (Nicotiana benthamiana phospholipase Dα1) as the key enzyme responsible for increased PA levels during PVY infection and found that it plays an antiviral role. 6K2 of PVY interacts with NbPLDα1, leading to elevated PA levels. In addition, NbPLDα1 and PA are recruited by 6K2 to membrane-bound viral replication complexes. On the other hand, 6K2 also induces activation of the MAPK pathway, dependent on its interaction with NbPLDα1 and the derived PA. PA binds to WIPK/SIPK/NTF4, prompting their phosphorylation of WRKY8. Notably, spraying with exogenous PA is sufficient to activate the MAPK pathway. Knockdown of the MEK2-WIPK/SIPK-WRKY8 cascade resulted in enhanced accumulation of PVY genomic RNA. 6K2 of Turnip mosaic virus and p33 of Tomato bushy stunt virus also interacted with NbPLDα1 and induced the activation of MAPK-mediated immunity. Loss of function of NbPLDα1 inhibited virus-induced activation of MAPK cascades and promoted viral RNA accumulation. Thus, activation of MAPK-mediated immunity by NbPLDα1-derived PA is a common strategy employed by hosts to counteract positive-strand RNA virus infection.

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Phosphatidic acid signaling and function in nuclei

Membrane lipidomes are dynamic and their changes generate lipid mediators affecting various biological processes. Phosphatidic acid (PA) has emerged as an important class of lipid mediators involved in a wide range of cellular and physiological responses in plants, animals, and microbes. The regulatory functions of PA have been studied primarily outside the nuclei, but an increasing number of recent studies indicates that some of the PA effects result from its action in nuclei. PA levels in nuclei are dynamic in response to stimuli. Changes in nuclear PA levels can result from activities of enzymes associated with nuclei and/or from movements of PA generated extranuclearly. PA has also been found to interact with proteins involved in nuclear functions, such as transcription factors and proteins undergoing nuclear translocation in response to stimuli. The nuclear action of PA affects various aspects of plant growth, development, and response to stress and environmental changes.

Gene co-expression network analysis for the selection of candidate early warning indicators of heat and nutrient stress in Posidonia oceanica

The continuous worldwide seagrasses decline calls for immediate actions in order to preserve this precious marine ecosystem. The main stressors that have been linked with decline in seagrasses are 1) the increasing ocean temperature due to climate change and 2) the continuous inputs of nutrients (eutrophication) associated with coastal human activities. To avoid the loss of seagrass populations, an "early warning" system is needed. We used Weighed Gene Co-expression Network Analysis (WGCNA), a systems biology approach, to identify potential candidate genes that can provide an early warning signal of stress in the Mediterranean iconic seagrass Posidonia oceanica, anticipating plant mortality. Plants were collected from both eutrophic (EU) and oligotrophic (OL) environments and were exposed to thermal and nutrient stress in a dedicated mesocosm. By correlating the whole-genome gene expression after 2-weeks exposure with the shoot survival percentage after 5-weeks exposure to stressors, we were able to identify several transcripts that indicated an early activation of several biological processes (BP) including: protein metabolic process, RNA metabolic process, organonitrogen compound biosynthetic process, catabolic process and response to stimulus, which were shared among OL and EU plants and among leaf and shoot apical meristem (SAM), in response to excessive heat and nutrients. Our results suggest a more dynamic and specific response of the SAM compared to the leaf, especially the SAM from plants coming from a stressful environment appeared more dynamic than the SAM from a pristine environment. A vast list of potential molecular markers is also provided that can be used as targets to assess field samples.

Proteomic Insights of Cowpea Response to Combined Biotic and Abiotic Stresses

The co-occurrence of biotic and abiotic stresses in agricultural areas severely affects crop performance and productivity. Drought is one of the most adverse environmental stresses, and its association with root-knot nematodes further limits the development of several economically important crops, such as cowpea. Plant responses to combined stresses are complex and require novel adaptive mechanisms through the induction of specific biotic and abiotic signaling pathways. Therefore, the present work aimed to identify proteins involved in the resistance of cowpea to nematode and drought stresses individually and combined. We used the genotype CE 31, which is resistant to the root-knot nematode Meloidogyne spp. And tolerant to drought. Three biological replicates of roots and shoots were submitted to protein extraction, and the peptides were evaluated by LC-MS/MS. Shotgun proteomics revealed 2345 proteins, of which 1040 were differentially abundant. Proteins involved in essential biological processes, such as transcriptional regulation, cell signaling, oxidative processes, and photosynthesis, were identified. However, the main defense strategies in cowpea against cross-stress are focused on the regulation of hormonal signaling, the intense production of pathogenesis-related proteins, and the downregulation of photosynthetic activity. These are key processes that can culminate in the adaptation of cowpea challenged by multiple stresses. Furthermore, the candidate proteins identified in this study will strongly contribute to cowpea genetic improvement programs.

Jasmonic Acid as a Mediator in Plant Response to Necrotrophic Fungi

Jasmonic acid (JA) and its derivatives, all named jasmonates, are the simplest phytohormones which regulate multifarious plant physiological processes including development, growth and defense responses to various abiotic and biotic stress factors. Moreover, jasmonate plays an important mediator's role during plant interactions with necrotrophic oomycetes and fungi. Over the last 20 years of research on physiology and genetics of plant JA-dependent responses to pathogens and herbivorous insects, beginning from the discovery of the JA co-receptor CORONATINE INSENSITIVE1 (COI1), research has speeded up in gathering new knowledge on the complexity of plant innate immunity signaling. It has been observed that biosynthesis and accumulation of jasmonates are induced specifically in plants resistant to necrotrophic fungi (and also hemibiotrophs) such as mostly investigated model ones, i.e., Botrytis cinerea, Alternaria brassicicola or Sclerotinia sclerotiorum. However, it has to be emphasized that the activation of JA-dependent responses takes place also during susceptible interactions of plants with necrotrophic fungi. Nevertheless, many steps of JA function and signaling in plant resistance and susceptibility to necrotrophs still remain obscure. The purpose of this review is to highlight and summarize the main findings on selected steps of JA biosynthesis, perception and regulation in the context of plant defense responses to necrotrophic fungal pathogens.

Grapevine-Associated Lipid Signalling Is Specifically Activated in an Rpv3 Background in Response to an Aggressive P. viticola Pathovar

Vitis vinifera L. is highly susceptible to the biotrophic pathogen Plasmopara viticola. To control the downy mildew disease, several phytochemicals are applied every season. Recent European Union requirements to reduce the use of chemicals in viticulture have made it crucial to use alternative and more sustainable approaches to control this disease. Our previous studies pinpoint the role of fatty acids and lipid signalling in the establishment of an incompatible interaction between grapevine and P. viticola. To further understand the mechanisms behind lipid involvement in an effective defence response we have analysed the expression of several genes related to lipid metabolism in three grapevine genotypes: Chardonnay (susceptible); Regent (tolerant), harbouring an Rpv3-1 resistance loci; and Sauvignac (resistant) that harbours a pyramid of Rpv12 and Rpv3-1 resistance loci. A highly aggressive P. viticola isolate was used (NW-10/16). Moreover, we have characterised the grapevine phospholipases C and D gene families and monitored fatty acid modulation during infection. Our results indicate that both susceptible and resistant grapevine hosts did not present wide fatty acid or gene expression modulation. The modulation of genes associated with lipid signalling and fatty acids seems to be specific to Regent, which raises the hypothesis of being specifically linked to the Rpv3 loci. In Sauvignac, the Rpv12 may be dominant concerning the defence response, and, thus, this genotype may present the activation of other pathways rather than lipid signalling.

Plant N-acylethanolamines play a crucial role in defense and its variation in response to elevated CO2 and temperature in tomato

The ubiquitous lipid-derived molecules N-acylethanolamines (NAEs) have multiple immune functions in mammals, but their roles and mechanisms in plant defense response during changing environment remain largely unclear. Here, we found that exogenous NAE18:0 and NAE18:2 promoted defense against the necrotrophic pathogen Botrytis cinerea but suppressed defense to the hemi-biotrophic pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 in tomato. The knocking-down and overexpression function analysis of the pathogen-responsive NAE synthetic gene PHOSPHOLIPASE Dγ (PLDγ) and hydrolytic gene FATTY ACID AMID HYDROLASE 1 (FAAH1) revealed that the NAE pathway is crucial for plant defense response. Using exogenous applications and SA-abolished NahG plants, we unveiled the antagonistic relationship between NAE and SA in plant defense response. Elevated CO₂ and temperature significantly changed the NAE pathway in response to pathogens, while inhibition of the NAE pathway led to the alternation of environment-mediated defense variations against Pst DC3000 in tomato, indicating that NAE pathway is associated with plant defense variations in response to elevated CO₂ and temperature. The results herein reveal a new function of NAE in plant defense, and its involvement in environment-mediated defense variation in tomato. These findings shed light on the NAE-based plant defense, which may have relevance to crop disease management in future changing climate.

The phospholipase D gene GhPLDδ confers resistance to Verticillium dahliae and improves tolerance to salt stress

Plant phospholipase D (PLD) and its product phosphatidic acid (PA) function in both abiotic and biotic stress signaling. However, to date, a PLD gene conferring the desired resistance to both biotic and abiotic stresses has not been found in cotton. Here, we isolated and identified a PLD gene GhPLDδ from cotton (Gossypium hirsutum), which functions in Verticillium wilt resistance and salt tolerance. GhPLDδ was highly induced by salicylic acid (SA), methyl jasmonate (MeJA), abscisic acid (ABA), hydrogen peroxide, PEG 6000, NaCl, and Verticillium dahliae in cotton plants. The positive role of GhPLDδ in regulating plant resistance to V. dahliae was confirmed by loss- and gain-of-function analyses. Upon chitin treatment, accumulation of PA, hydrogen peroxide, JA, SA, and the expression of genes involved in MAPK cascades, JA- and SA-related defense responses were positively related to the level of GhPLDδ in plants. The treatment by exogenous PA could activate the expression of genes related to MAPK, SA, and JA signaling pathways. Moreover, GhPLDδ overexpression enhanced salt tolerance in Arabidopsis as demonstrated by the increased germination rate, longer seedling root, higher chlorophyll content, larger fresh weight, lower malondialdehyde content, and fully expand rosette leaves. Additionally, the PA content and the expression of the genes of the MAPK cascades regulated by PA were increased in GhPLDδ-overexpressed Arabidopsis under salt stress. Taken together, these findings suggest that GhPLDδ and PA are involved in regulating plant defense against both V. dahliae infection and salt stress.

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

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

Supplementary Information

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

Characterization and Expression of Phospholipase D Putatively Involved in Colletotrichummusae Disease Development of Postharvest Banana Fruit

Phospholipase D (PLD) in plants plays an important role in growth, development, and stress response. The effect of hexanal on PLD in banana fruit responding to Colletotrichum musae infection remains poorly understood. In this study, four putative PLD genes, named as MaPLD1, MaPLD2, MaPLD3, and MaPLD4 were identified from banana fruit. The four MaPLDs can be classified into three of the seven known PLD families according to sequence characterization. Their deduced amino acid sequences displayed homology of PLDs from other plant species. Furthermore, the specific expression analysis of PLD genes in banana fruit in response to infection in C. musae was studied and the response relationship between PLD family members and banana fruit under anthracnose stress was clarified. Changes in both the activity of PLD and PLC, and the connection between hexanal and phospholipases in the banana fruit C. musae infection were compared. The results showed that the incidence of disease in banana inoculated with C. musae was dramatically increased after 6 days of storage, the activation of PLD and PLC in infected anthracnose fruit before disease development, and that this activation was inhibited by hexanal treatment, which suggested that both enzymes play a protective role in banana fruit to cope with C. musae infection and the participation of hexanal in their regulation. Of the four MaPLD genes, the anthracnose had a stronger effect on MaPLD1 and MaPLD4. These data demonstrated that hexanal treatment could enhance fruit disease resistance to C. musae, and that PLD could take part in the disease defensive system of harvested banana fruit to C. musae by modulating the metabolism of cell membrane lipids, and thus suppress disease development in C. musae -inoculated banana during storage.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

Genome-wide investigation of the PLD gene family in alfalfa (Medicago sativa L.): identification, analysis and expression

Background

External environmental factors, such as salt, alkali and drought, severely limit the acreage and yield of alfalfa. The mining of tolerance-related genes in alfalfa and improving the stress resistance of this plant are essential for increasing alfalfa yield. PLD is the main phospholipid hydrolase in plants and plays an important role in plant growth, development, signaling, and resistance to adverse stress. With the availability of whole genome sequences, the annotation and expression of PLDs in alfalfa can now be achieved. At present, few studies have investigated PLDs in alfalfa. Here, we conducted a study of PLDs in alfalfa and identified and analyzed the expression pattern of PLDs under different treatments.

Results

Fifty-nine MsPLDs were identified in alfalfa and classified into six subtypes: MsPLDα, β, γ, δ and ε belong to the C2-PLD subfamily, and MsPLDζ belongs to the PXPH-PLD subfamily. Members of the same PLD subtype have similar physicochemical properties, sequence structure and domains, but their cis-acting elements are different. A qRT-PCR analysis revealed that MsPLDs are expressed in multiple tissues. MsPLDs can respond to alkali, drought, ABA, IAA, and GA3 treatments and particularly to salt stress. Different expression patterns were found for the same gene under different treatments and different genes under the same treatment. Expression of MsPLD05 improved salt tolerance in yeast.

Conclusion

This study represents the first genome-wide characterization of MsPLDs in alfalfa. Most MsPLDs are expressed mainly in mature leaves and respond positively to abiotic stresses and hormonal treatments. This study further expands the resistance gene pool in legume forage grasses and provides a reference for further in-depth study of MsPLDs in alfalfa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08424-9.

Lipid Signaling Requires ROS Production to Elicit Actin Cytoskeleton Remodeling during Plant Innate Immunity

In terrestrial plants a basal innate immune system, pattern-triggered immunity (PTI), has evolved to limit infection by diverse microbes. The remodeling of actin cytoskeletal arrays is now recognized as a key hallmark event during the rapid host cellular responses to pathogen attack. Several actin binding proteins have been demonstrated to fine tune the dynamics of actin filaments during this process. However, the upstream signals that stimulate actin remodeling during PTI signaling remain poorly characterized. Two second messengers, reactive oxygen species (ROS) and phosphatidic acid (PA), are elevated following pathogen perception or microbe-associated molecular pattern (MAMP) treatment, and the timing of signaling fluxes roughly correlates with actin cytoskeletal rearrangements. Here, we combined genetic analysis, chemical complementation experiments, and quantitative live-cell imaging experiments to test the role of these second messengers in actin remodeling and to order the signaling events during plant immunity. We demonstrated that PHOSPHOLIPASE Dβ (PLDβ) isoforms are necessary to elicit actin accumulation in response to flg22-associated PTI. Further, bacterial growth experiments and MAMP-induced apoplastic ROS production measurements revealed that PLDβ-generated PA acts upstream of ROS signaling to trigger actin remodeling through inhibition of CAPPING PROTEIN (CP) activity. Collectively, our results provide compelling evidence that PLDβ/PA functions upstream of RBOHD-mediated ROS production to elicit actin rearrangements during the innate immune response in Arabidopsis.

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.

Defence-related pathways, phytohormones and primary metabolism are key players in kiwifruit plant tolerance to Pseudomonas syringae pv. actinidiae

The reasons underlying the differential tolerance of Actinidia spp. to the pandemic pathogen Pseudomonas syringae pv. actinidiae (Psa) have not yet been elucidated. We hypothesized that differential plant-defence strategies linked to transcriptome regulation, phytohormones and primary metabolism might be key and that Actinidia chinensis susceptibility results from an inefficient activation of defensive mechanisms and metabolic impairments shortly following infection. Here, 48 h postinoculation bacterial density was 10-fold higher in A. chinensis var. deliciosa than in Actinidia arguta, accompanied by significant increases in glutamine, ornithine, jasmonic acid (JA) and salicylic acid (SA) (up to 3.2-fold). Actinidia arguta showed decreased abscisic acid (ABA) (0.7-fold), no changes in primary metabolites, and 20 defence-related genes that were only differentially expressed in this species. These include GLOX1, FOX1, SN2 and RBOHA, which may contribute to its higher tolerance. Results suggest that A. chinensis' higher susceptibility to Psa is due to an inefficient activation of plant defences, with the involvement of ABA, JA and SA, leading to impairments in primary metabolism, particularly the ammonia assimilation cycle. A schematic overview on the interaction between Psa and genotypes with distinct tolerance is provided, highlighting the key transcriptomic and metabolomic aspects contributing to the different plant phenotypes after infection.

Deciphering the Genome-Wide Transcriptomic Changes during Interactions of Resistant and Susceptible Genotypes of American Elm with Ophiostoma novo-ulmi

Dutch elm disease (DED), caused by Ophiostoma novo-ulmi (Onu), is a destructive disease of American elm (Ulmus americana L.). The molecular mechanisms of resistance and susceptibility against DED in American elm are still largely uncharacterized. In the present study, we performed a de novo transcriptome (RNA-sequencing; RNA-Seq) assembly of U. americana and compared the gene expression in a resistant genotype, 'Valley Forge', and a susceptible (S) elm genotype at 0 and 96 h post-inoculation of Onu. A total of 85,863 non-redundant unigenes were identified. Compared to the previously characterized U. minor transcriptome, U. americana has 35,290 similar and 55,499 unique genes. The transcriptomic variations between 'Valley Forge' and 'S' were found primarily in the photosynthesis and primary metabolism, which were highly upregulated in the susceptible genotype irrespective of the Onu inoculation. The resistance to DED was associated with the activation of RPM1-mediated effector-triggered immunity that was demonstrated by the upregulation of genes involved in the phenylpropanoids biosynthesis and PR genes. The most significantly enriched gene ontology (GO) terms in response to Onu were response to stimulus (GO:0006950), response to stress (GO:0050896), and secondary metabolic process (GO:0008152) in both genotypes. However, only in the resistant genotype, the defense response (GO:0006952) was among the topmost significantly enriched GO terms. Our findings revealed the molecular regulations of DED resistance and susceptibility and provide a platform for marker-assisted breeding of resistant American elm genotypes.

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.

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.

Plant phospholipase D: novel structure, regulatory mechanism, and multifaceted functions with biotechnological application

Phospholipases D (PLDs) are important membrane lipid-modifying enzymes in eukaryotes. Phosphatidic acid, the product of PLD activity, is a vital signaling molecule. PLD-mediated lipid signaling has been the subject of extensive research leading to discovery of its crystal structure. PLDs are involved in the pathophysiology of several human diseases, therefore, viewed as promising targets for drug design. The availability of a eukaryotic PLD crystal structure will encourage PLD targeted drug designing. PLDs have been implicated in plants response to biotic and abiotic stresses. However, the molecular mechanism of response is not clear. Recently, several novel findings have shown that PLD mediated modulation of structural and developmental processes, such as: stomata movement, root growth and microtubule organization are crucial for plants adaptation to environmental stresses. Involvement of PLDs in regulating membrane remodeling, auxin mediated alteration of root system architecture and nutrient uptake to combat nitrogen and phosphorus deficiencies and magnesium toxicity is established. PLDs via vesicle trafficking modulate cytoskeleton and exocytosis to regulate self-incompatibility (SI) signaling in flowering plants, thereby contributes to plants hybrid vigor and diversity. In addition, the important role of PLDs has been recognized in biotechnologically important functions, including oil/TAG synthesis and maintenance of seed quality. In this review, we describe the crystal structure of a plant PLD and discuss the molecular mechanism of catalysis and activity regulation. Further, the role of PLDs in regulating plant development under biotic and abiotic stresses, nitrogen and phosphorus deficiency, magnesium ion toxicity, SI signaling and pollen tube growth and in important biotechnological applications has been discussed.

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.

Phospholipases C and D and Their Role in Biotic and Abiotic Stresses

Plants, as sessile organisms, have adapted a fine sensing system to monitor environmental changes, therefore allowing the regulation of their responses. As the interaction between plants and environmental changes begins at the surface, these changes are detected by components in the plasma membrane, where a molecule receptor generates a lipid signaling cascade via enzymes, such as phospholipases (PLs). Phospholipids are the key structural components of plasma membranes and signaling cascades. They exist in a wide range of species and in different proportions, with conversion processes that involve hydrophilic enzymes, such as phospholipase-C (PLC), phospholipase-D (PLD), and phospholipase-A (PLA). Hence, it is suggested that PLC and PLD are highly conserved, compared to their homologous genes, and have formed clusters during their adaptive history. Additionally, they generate responses to different functions in accordance with their protein structure, which should be reflected in specific signal transduction responses to environmental stress conditions, including innate immune responses. This review summarizes the phospholipid systems associated with signaling pathways and the innate immune response.

Speaking the language of lipids: the cross-talk between plants and pathogens in defence and disease

Lipids and fatty acids play crucial roles in plant immunity, which have been highlighted over the past few decades. An increasing number of studies have shown that these molecules are pivotal in the interactions between plants and their diverse pathogens. The roles played by plant lipids fit in a wide spectrum ranging from the first physical barrier encountered by the pathogens, the cuticle, to the signalling pathways that trigger different immune responses and expression of defence-related genes, mediated by several lipid molecules. Moreover, lipids have been arising as candidate biomarkers of resistance or susceptibility to different pathogens. Studies on the apoplast and extracellular vesicles have been highlighting the possible role of lipids in the intercellular communication and the establishment of systemic acquired resistance during plant-pathogen interactions. From the pathogen perspective, lipid metabolism and specific lipid molecules play pivotal roles in the pathogen's life cycle completion, being crucial during recognition by the plant and evasion from the host immune system, therefore potentiating infection. Studies conducted in the last years have contributed to a better understanding of the language of lipids during the cross-talk between plants and pathogens. However, it is essential to continue exploring the knowledge brought up to light by transcriptomics and proteomics studies towards the elucidation of lipid signalling processes during defence and disease. In this review, we present an updated overview on lipids associated to plant-pathogen interactions, exploiting their roles from the two sides of this battle.

Phospholipase D- and phosphatidic acid-mediated phospholipid metabolism and signaling modulate symbiotic interaction and nodulation in soybean (Glycine max)

Symbiotic rhizobium-legume interactions, such as root hair curling, rhizobial invasion, infection thread expansion, cell division and proliferation of nitrogen-fixing bacteroids, and nodule formation, involve extensive membrane synthesis, lipid remodeling and cytoskeleton dynamics. However, little is known about these membrane-cytoskeleton interfaces and related genes. Here, we report the roles of a major root phospholipase D (PLD), PLDα1, and its enzymatic product, phosphatidic acid (PA), in rhizobium-root interaction and nodulation. PLDα1 was activated and the PA content transiently increased in roots after rhizobial infection. Levels of PLDα1 transcript and PA, as well as actin and tubulin cytoskeleton-related gene expression, changed markedly during root-rhizobium interactions and nodule development. Pre-treatment of the roots of soybean seedlings with n-butanol suppressed the generation of PLD-derived PA, the expression of early nodulation genes and nodule numbers. Overexpression or knockdown of GmPLDα1 resulted in changes in PA levels, glycerolipid profiles, nodule numbers, actin cytoskeleton dynamics, early nodulation gene expression and hormone levels upon rhizobial infection compared with GUS roots. The transcript levels of cytoskeleton-related genes, such as GmACTIN, GmTUBULIN, actin capping protein 1 (GmCP1) and microtubule-associating protein (GmMAP1), were modified in GmPLDα1-altered hairy roots compared with those of GUS roots. Phosphatidic acid physically bound to GmCP1 and GmMAP1, which could be related to cytoskeletal changes in rhizobium-infected GmPLDα1 mutant roots. These data suggest that PLDα1 and PA play important roles in soybean-rhizobium interaction and nodulation. The possible underlying mechanisms, including PLDα1- and PA-mediated lipid signaling, membrane remodeling, cytoskeleton dynamics and related hormone signaling, are discussed herein.

Integrative Proteomic and Phosphoproteomic Analyses of Pattern- and Effector-Triggered Immunity in Tomato

Plants have evolved a two-layered immune system consisting of pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI and ETI are functionally linked, but also have distinct characteristics. Unraveling how these immune systems coordinate plant responses against pathogens is crucial for understanding the regulatory mechanisms underlying plant defense. Here we report integrative proteomic and phosphoproteomic analyses of the tomato-Pseudomonas syringae (Pst) pathosystem with different Pst mutants that allow the dissection of PTI and ETI. A total of 225 proteins and 79 phosphopeptides differentially accumulated in tomato leaves during Pst infection. The abundances of many proteins and phosphoproteins changed during PTI or ETI, and some responses were triggered by both PTI and ETI. For most proteins, the ETI response was more robust than the PTI response. The patterns of protein abundance and phosphorylation changes revealed key regulators involved in Ca²⁺ signaling, mitogen-activated protein kinase cascades, reversible protein phosphorylation, reactive oxygen species (ROS) and redox homeostasis, transcription and protein turnover, transport and trafficking, cell wall remodeling, hormone biosynthesis and signaling, suggesting their common or specific roles in PTI and/or ETI. A NAC (NAM, ATAF, and CUC family) domain protein and lipid particle serine esterase, two PTI-specific genes identified from previous transcriptomic work, were not detected as differentially regulated at the protein level and were not induced by PTI. Based on integrative transcriptomics and proteomics data, as well as qRT-PCR analysis, several potential PTI and ETI-specific markers are proposed. These results provide insights into the regulatory mechanisms underlying PTI and ETI in the tomato-Pst pathosystem, and will promote future validation and application of the disease biomarkers in plant defense.

PuMYB21/PuMYB54 coordinate to activate PuPLDβ1 transcription during peel browning of cold-stored "Nanguo" pears

Refrigeration is commonly used to extend the storage life of "Nanguo" pears, but fruit in long-term refrigeration is prone to peel browning, which is related to membrane lipid degradation. To determine the mechanism of membrane lipid degradation, we identified two R2R3-MYB transcription factors (TFs), PuMYB21 and PuMYB54, from "Nanguo" pears, which were notably expressed in response to cold stress and during the peel-browning process. The results from yeast one-hybrid, electrophoretic mobility shift, and transient expression assays indicated that both PuMYB21 and PuMYB54 directly bind to the promoter of PuPLDβ1 (a key enzyme catalyzing the hydrolysis of membrane phospholipids) and activate its expression, which probably enhances the degradation of membrane phospholipids and eventually results in peel browning. Moreover, the overexpression of PuMYB21 and PuMYB54 can greatly activate the transcription of endogenous PuPLDβ1 in both "Nanguo" pear fruits and calli, and their silencing can inhibit its transcription. Furthermore, yeast two-hybrid, bimolecular fluorescence complementation, and pull-down assays verified that PuMYB21 interacts with PuMYB54 to enhance the expression of PuPLDβ1. In summary, we demonstrate that PuMYB21 and PuMYB54 may have roles in membrane lipid metabolism by directly binding to the downstream structural gene PuPLDβ1 during the low temperature-induced peel browning of "Nanguo" pears.

Tolerance of Plants to Pathogens: A Unifying View

Increasing evidence indicates that tolerance is a host defense strategy against pathogens as widespread and successful as resistance. Since the concept of tolerance was proposed more than a century ago, it has been in continuous evolution. In parallel, our understanding of its mechanistic bases and its consequences for host and pathogen interactions, ecology, and evolution has grown. This review aims at summarizing the conceptual changes in the meaning of tolerance inside and outside the field of phytopathology, emphasizing difficulties in demonstrating and quantifying this trait. We also discuss evidence of tolerance and current knowledge on its genetic regulation, mechanisms, and role in host-pathogen coevolution, highlighting common patterns across hosts and pathogens. We hope that this comprehensive review attracts more plant pathologists to the study of this key plant defense response.

The BIR2/BIR3-Associated Phospholipase Dγ1 Negatively Regulates Plant Immunity

Plants have evolved effective strategies to defend themselves against pathogen invasion. Starting from the plasma membrane with the recognition of microbe-associated molecular patterns (MAMPs) via pattern recognition receptors, internal cellular signaling pathways are induced to ultimately fend off the attack. Phospholipase D (PLD) hydrolyzes membrane phospholipids to produce phosphatidic acid (PA), which has been proposed to play a second messenger role in immunity. The Arabidopsis (Arabidopsis thaliana) PLD family consists of 12 members, and for some of these, a specific function in resistance toward a subset of pathogens has been shown. We demonstrate here that Arabidopsis PLDγ1, but not its close homologs PLDγ2 and PLDγ3, is specifically involved in plant immunity. Genetic inactivation of PLDγ1 resulted in increased resistance toward the virulent bacterium Pseudomonas syringae pv. tomato DC3000 and the necrotrophic fungus Botrytis cinerea As pldγ1 mutant plants responded with elevated levels of reactive oxygen species to MAMP treatment, a negative regulatory function for this PLD isoform is proposed. Importantly, PA levels in pldγ1 mutants were not affected compared to stressed wild-type plants, suggesting that alterations in PA levels are not likely the cause for the enhanced immunity in the pldγ1 line. Instead, the plasma-membrane-attached PLDγ1 protein colocalized and associated with the BAK1-INTERACTING RECEPTOR-LIKE KINASES BIR2 and BIR3, which are known negative regulators of pattern-triggered immunity. Moreover, complex formation of PLDγ1 and BIR2 was further promoted upon MAMP treatment. Hence, we propose that PLDγ1 acts as a negative regulator of plant immune responses in complex with immunity-related proteins BIR2 and BIR3.

Arabidopsis PLDs with C2-domain function distinctively in hypoxia

Hypoxia (oxygen deprivation) causes metabolic disturbances at physiological, biochemical and genetic levels and results in decreased plant growth and development. Phospholipase D (PLD)-mediated signaling was reported for abiotic and biotic stress signaling events in plants. To investigate the participatory role of PLDs also in hypoxia signaling, we used wild type of Arabidopsis thaliana and 10 pld isoform mutants containing C2-domain. Hypoxia-induced changes in three major signaling players, namely, cytosolic free calcium (Ca²⁺ _cyt ), reactive oxygen species (ROS) and phosphatidic acid (PA), were determined in mesophyll protoplasts. The Ca²⁺ _cyt and ROS levels were monitored by fluorescence microscopy and confocal imaging, while PA levels were quantified by an enzymatic method. Our findings reveal that the elevations of cytosolic calcium and PA are reduced in all the 10 mutants dysfunctional in PLD isoforms. The hypoxia-related changes in both calcium and ROS show different kinetic patterns depending on the type of PLD studied. Pharmacological experiments confirm that both external and internal sources contribute to calcium and ROS accumulation under hypoxia. PLDα1-3, PLDβ1 and PLDγ1-3 are likely involved in calcium signaling under hypoxia as well as in PA production, while all investigated PLDs, except for PLDγ3, take part in ROS elevation.

Proteomic analysis by iTRAQ-PRM provides integrated insight into mechanisms of resistance in pepper to Bemisia tabaci (Gennadius)

BACKGROUND

The Bemisia tabaci is a major leaf feeding insect pest to pepper (Capsicum annuum), causing serious damage to pepper growth and yield. It is particularly important to study the mechanism of pepper resistance to B. tabaci, and to breed and promote the varieties of pepper resistant to B. tabaci. However, very limited molecular mechanism is available about how plants perceive and defend themselves from the destructive pest. Proteome technologies have provided an idea method for studying plant physiological processes in response to B. tabaci.

RESULTS

Here, a highly resistant genotype and a highly susceptible genotype were exposed to B. tabaci feeding for 48 h to explore the defense mechanisms of pepper resistance to B. tabaci. The proteomic differences between both genotypes were compared using isobaric tag for relative and absolute quantification (iTRAQ). The quantitative data were validated by parallel reaction monitoring (PRM). The results showed that 37 differential abundance proteins (DAPs) were identified in the RG (resistant genotype), while 17 DAPs were identified in the SG (susceptible genotype) at 48 h after B. tabaci feeding. 77 DAPs were identified when comparing RG with SG without feeding. The DAP functions were determined for the classification of the pathways, mainly involved in redox regulation, stress response, protein metabolism, lipid metabolism and carbon metabolism. Some candidate DAPs are closely related to B. tabaci resistance such as annexin D4-like (ANN4), calreticulin-3 (CRT3), heme-binding protein 2-like (HBP1), acidic endochitinase pcht28-like (PR3) and lipoxygenase 2 (LOX2).

CONCLUSIONS

Taken together, this study indicates complex resistance-related events in B. tabaci interaction, provides novel insights into the molecular mechanism underlying the response of plant to B. tabaci, and identifies some candidate proteins against B. tabaci attack.

A role for diacylglycerol kinase 4 in signalling crosstalk during Arabidopsis pollen tube growth

Diacylglycerol kinases (DGKs) play a major role in the production of phosphatidic acid (PtdOH) and were implicated in endomembrane trafficking and signalling cascades. In plants, the role of DGKs is less clear, as PtdOH seems to arise mostly from phospholipase D activity. Here, we investigated the function of the Arabidopsis gene encoding DGK4, which is highly expressed in pollen. In vitro, pollen tubes from homozygous dgk4 plants showed normal morphology, but reduced growth rate and altered stiffness and adhesion properties (revealed by atomic force microscopy). In vivo, dgk4 pollen was able to fertilize wild-type ovules, but self-pollination in dgk4 plants led to fewer seeds and shorter siliques. Phenotypic analysis revealed that the dgk4 mutation affects not only the male germ line but also the vegetative tissue. DGK4-green fluorescent protein fusion imaging revealed a cytosolic localization with a slightly higher signal in the subapical or apical region. dgk4 pollen tubes were found to exhibit perturbations in membrane recycling, and lipid analysis revealed a minor increase of PtdOH concomitant with decreased phosphatidylcholine, compared with wild-type. In vitro, DGK4 was found to exhibit kinase and guanylyl cyclase activity. Quantitative PCR data revealed downregulation of genes related to actin dynamics and phosphoinositide metabolism in mutant pollen, but upregulation of the DGK6 isoform. Altogether, these results are discussed considering a role of DGK4 in signalling cross-talk.

Phospholipase D and phosphatidic acid in plant immunity

Phospholipase D (PLD) hydrolyzes membrane phospholipids to generate phosphatidic acid (PA). Both PLD and its lipid product PA are involved in various physiological processes, including plant response to pathogens. The PLD family is comprised of multiple members in higher plants, and PLDs have been reported to play positive and/or negative roles in plant immunity, depending on the types of pathogens and specific PLDs involved. Individual PLDs have distinguishable biochemical properties, such as Ca²⁺ and phosphatidylinositide requirements. In addition, PLDs and PA are found to interact with various proteins in hormone and stress signaling. The different biochemical and regulatory properties of PLDs and PA shed light on the mechanisms for the functional diversity of PLDs in plant defense signaling and response.

Fungal attack and host defence pathways unveiled in near-avirulent interactions of Penicillium expansum creA mutants on apples

Amongst the universal diseases affecting apples, blue mould caused by Penicillium expansum is a major concern, resulting in yield and quality losses as a result of the production of the mycotoxin patulin. Despite the characterization of the patulin biosynthetic gene cluster at both the molecular and chemical levels, the underlying regulation of patulin biosynthesis in P. expansum and the mechanisms of apple colonization remain largely obscure. Recent work has indicated that sucrose, a carbon catabolite repressive metabolite, is a critical factor in the regulation of patulin synthesis. Here, CreA, the global carbon catabolite regulator, was assessed for virulence both in vitro and in vivo. We showed that loss-of-function creA strains were nearly avirulent and did not produce patulin in apples. On the basis of RNA-sequencing (RNA-seq) analysis and physiological experimentation, these mutants were unable to successfully colonize apples for a multitude of potential mechanisms including, on the pathogen side, a decreased ability to produce proteolytic enzymes and to acidify the environment and impaired carbon/nitrogen metabolism and, on the host side, an increase in the oxidative defence pathways. Our study defines CreA and its downstream signalling pathways as promising targets for the development of strategies to fight against the development and virulence of this post-harvest pathogen.

Understanding Cytoskeletal Dynamics During the Plant Immune Response

The plant cytoskeleton is a dynamic framework of cytoplasmic filaments that rearranges as the needs of the cell change during growth and development. Incessant turnover mechanisms allow these networks to be rapidly redeployed in defense of host cytoplasm against microbial invaders. Both chemical and mechanical stimuli are recognized as danger signals to the plant, and these are perceived and transduced into cytoskeletal dynamics and architecture changes through a collection of well-recognized, previously characterized players. Recent advances in quantitative cell biology approaches, along with the powerful molecular genetics techniques associated with Arabidopsis, have uncovered two actin-binding proteins as key intermediaries in the immune response to phytopathogens and defense signaling. Certain bacterial phytopathogens have adapted to the cytoskeletal-based defense mechanism during the basal immune response and have evolved effector proteins that target actin filaments and microtubules to subvert transcriptional reprogramming, secretion of defense-related proteins, and cell wall-based defenses. In this review, we describe current knowledge about host cytoskeletal dynamics operating at the crossroads of the molecular and cellular arms race between microbes and plants.

Arabidopsis phospholipase Dα1 and Dδ oppositely modulate EDS1- and SA-independent basal resistance against adapted powdery mildew

Plants use a tightly regulated immune system to fight off various pathogens. Phospholipase D (PLD) and its product, phosphatidic acid, have been shown to influence plant immunity; however, the underlying mechanisms remain unclear. Here, we show that the Arabidopsis mutants pldα1 and pldδ, respectively, exhibited enhanced resistance and enhanced susceptibility to both well-adapted and poorly adapted powdery mildew pathogens, and a virulent oomycete pathogen, indicating that PLDα1 negatively while PLDδ positively modulates post-penetration resistance. The pldα1δ double mutant showed a similar infection phenotype to pldα1, genetically placing PLDα1 downstream of PLDδ. Detailed genetic analyses of pldδ with mutations in genes for salicylic acid (SA) synthesis (SID2) and/or signaling (EDS1 and PAD4), measurement of SA and jasmonic acid (JA) levels, and expression of their respective reporter genes indicate that PLDδ contributes to basal resistance independent of EDS1/PAD4, SA, and JAsignaling. Interestingly, while PLDα1-enhanced green fluorescent protein (eGFP) was mainly found in the tonoplast before and after haustorium invasion, PLDδ-eGFP's focal accumulation to the plasma membrane around the fungal penetration site appeared to be suppressed by adapted powdery mildew. Together, our results demonstrate that PLDα1 and PLDδ oppositely modulate basal, post-penetration resistance against powdery mildew through a non-canonical mechanism that is independent of EDS1/PAD4, SA, and JA.

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.

Leaf blade structure of Verbesina macrophylla (Cass.) F. S. Blake (Asteraceae): ontogeny, duct secretion mechanism and essential oil composition

Secretory structures are common in Asteraceae, where they exhibit a high degree of morphological diversity. The species Verbesina macrophylla, popularly known as assa-peixe, is native to Brazil where it is widely used for medicinal purposes. Despite its potential medical importance, there have been no studies of the anatomy of this species, especially its secretory structures and secreted compounds. This study examined leaves of V. macrophylla with emphasis on secretory structures and secreted secondary metabolites. Development of secretory ducts and the mechanism of secretion production are described for V. macrophylla using ultrastructure, yield and chemical composition of its essential oils. Verbesina macrophylla has a hypostomatic leaf blade with dorsiventral mesophyll and secretory ducts associated with vascular bundles of schizogenous origin. Histochemistry identified the presence of lipids, terpenes, alkaloids and mucopolysaccharides. Ultrastructure suggests that the secretion released into the duct lumen is produced in plastids of transfer cells, parenchymal sheath cells and stored in vacuoles in these cells and duct epithelial cells. The essential oil content was 0.8%, and its major components were germacrene D, germacrene D-4-ol, β-caryophyllene, bicyclogermacrene and α-cadinol. Secretory ducts of V. macrophylla are squizogenous. Substances identified in tissues suggest that both secretions stored in the ducts and in adjacent parenchyma cells are involved in chemical defence. The essential oil is rich in sesquiterpenes, with germacrene D and its derivatives being notable components.

The sesquiterpene botrydial from Botrytis cinerea induces phosphatidic acid production in tomato cell suspensions

The phytotoxin botrydial triggers PA production in tomato cell suspensions via PLD and PLC/DGK activation. PLC/DGK-derived PA is partially required for botrydial-induced ROS generation. Phosphatidic acid (PA) is a phospholipid second messenger involved in the induction of plant defense responses. It is generated via two distinct enzymatic pathways, either via phospholipase D (PLD) or by the sequential action of phospholipase C and diacylglycerol kinase (PLC/DGK). Botrydial is a phytotoxic sesquiterpene generated by the necrotrophic fungus Botrytis cinerea that induces diverse plant defense responses, such as the production of reactive oxygen species (ROS). Here, we analyzed PA and ROS production and their interplay upon botrydial treatments, employing tomato (Solanum lycopersicum) cell suspensions as a model system. Botrydial induces PA production within minutes via PLD and PLC/DGK. Either inhibition of PLC or DGK diminishes ROS generation triggered by botrydial. This indicates that PLC/DGK is upstream of ROS production. In tomato, PLC is encoded by a multigene family constituted by SlPLC1-SlPLC6 and the pseudogene SlPLC7. We have shown that SlPLC2-silenced plants have reduced susceptibility to B. cinerea. In this work, we studied the role of SlPLC2 on botrydial-induced PA production by silencing the expression of SlPLC2 via a specific artificial microRNA. Upon botrydial treatments, SlPLC2-silenced-cell suspensions produce PA levels similar to wild-type cells. It can be concluded that PA is a novel component of the plant responses triggered by botrydial.

Tolerance to Plant Pathogens: Theory and Experimental Evidence

The two major mechanisms of plant defense against pathogens are resistance (the host's ability to limit pathogen multiplication) and tolerance (the host's ability to reduce the effect of infection on its fitness regardless of the level of pathogen multiplication). There is abundant literature on virtually every aspect of plant resistance to pathogens. Although tolerance to plant pathogens is comparatively less understood, studies on this plant defense strategy have led to major insights into its evolution, mechanistic basis and genetic determinants. This review aims at summarizing current theories and experimental evidence on the evolutionary causes and consequences of plant tolerance to pathogens, as well as the existing knowledge on the genetic determinants and mechanisms of tolerance. Our review reveals that (i) in plant-pathogen systems, resistance and tolerance generally coexist, i.e., are not mutually exclusive; (ii) evidence of tolerance polymorphisms is abundant regardless of the pathogen considered; (iii) tolerance is an efficient strategy to reduce the damage on the infected host; and (iv) there is no evidence that tolerance results in increased pathogen multiplication. Taken together, the work discussed in this review indicates that tolerance may be as important as resistance in determining the dynamics of plant-pathogen interactions. Several aspects of plant tolerance to pathogens that still remain unclear and which should be explored in the future, are also outlined.

Overexpression of AtGolS3 and CsRFS in poplar enhances ROS tolerance and represses defense response to leaf rust disease

Plants respond to pathogens through an orchestration of signaling events that coordinate modifications to transcriptional profiles and physiological processes. Resistance to necrotrophic pathogens often requires jasmonic acid, which antagonizes the salicylic acid dependent biotrophic defense response. Recently, myo-inositol has been shown to negatively impact salicylic acid (SA) levels and signaling, while galactinol enhances jasmonic acid (JA)-dependent induced systemic resistance to necrotrophic pathogens. To investigate the function of these compounds in biotrophic pathogen defense, we characterized the defense response of Populus alba × grandidentata overexpressing Arabidopsis GALACTINOL SYNTHASE3 (AtGolS) and Cucumber sativus RAFFINOSE SYNTHASE (CsRFS) challenged with Melampsora aecidiodes, a causative agent of poplar leaf rust disease. Relative to wild-type leaves, the overexpression of AtGolS3 and CsRFS increased accumulation of galactinol and raffinose and led to increased leaf rust infection. During the resistance response, inoculated wild-type leaves displayed reduced levels of galactinol and repressed transcript abundance of two endogenous GolS genes compared to un-inoculated wild-type leaves prior to the up-regulation of NON-EXPRESSOR OF PR1 and PATHOGENESIS-RELATED1. Transcriptome analysis and qRT-PCR validation also revealed the repression of genes participating in calcium influx, phosphatidic acid biosynthesis and signaling, and salicylic acid signaling in the transgenic lines. In contrast, enhanced tolerance to H2O2 and up-regulation of antioxidant biosynthesis genes were exhibited in the overexpression lines. Thus, we conclude that overexpression of AtGolS and CsRFS antagonizes the defense response to poplar leaf rust disease through repressing reactive oxygen species and attenuating calcium and phosphatidic acid signaling events that lead to SA defense.

The Arabidopsis thaliana non-specific phospholipase C2 is involved in the response to Pseudomonas syringae attack

Background and Aims

The non-specific phospholipase C (NPC) is a new member of the plant phospholipase family that reacts to abiotic environmental stresses, such as phosphate deficiency, high salinity, heat and aluminium toxicity, and is involved in root development, silicon distribution and brassinolide signalling. Six NPC genes (NPC1-NPC6) are found in the Arabidopsis genome. The NPC2 isoform has not been experimentally characterized so far.

Methods

The Arabidopsis NPC2 isoform was cloned and heterologously expressed in Escherichia coli. NPC2 enzyme activity was determined using fluorescent phosphatidylcholine as a substrate. Tissue expression and subcellular localization were analysed using GUS- and GFP-tagged NPC2. The expression patterns of NPC2 were analysed via quantitative real-time PCR. Independent homozygous transgenic plant lines overexpressing NPC2 under the control of a 35S promoter were generated, and reactive oxygen species were measured using a luminol-based assay.

Key Results

The heterologously expressed protein possessed phospholipase C activity, being able to hydrolyse phosphatidylcholine to diacylglycerol. NPC2 tagged with GFP was predominantly localized to the Golgi apparatus in Arabidopsis roots. The level of NPC2 transcript is rapidly altered during plant immune responses and correlates with the activation of multiple layers of the plant defence system. Transcription of NPC2 decreased substantially after plant infiltration with Pseudomonas syringae, flagellin peptide flg22 and salicylic acid treatments and expression of the effector molecule AvrRpm1. The decrease in NPC2 transcript levels correlated with a decrease in NPC2 enzyme activity. NPC2-overexpressing mutants showed higher reactive oxygen species production triggered by flg22.

Conclusions

This first experimental characterization of NPC2 provides new insights into the role of the non-specific phospholipase C protein family. The results suggest that NPC2 is involved in the response of Arabidopsis to P. syringae attack.