Pipecolic acid, an endogenous mediator of defense amplification and priming, is a critical regulator of inducible plant immunity

ChIP-seq reveals broad roles of SARD1 and CBP60g in regulating plant immunity

Recognition of pathogens by host plants leads to rapid transcriptional reprogramming and activation of defence responses. The expression of many defence regulators is induced in this process, but the mechanisms of how they are controlled transcriptionally are largely unknown. Here we use chromatin immunoprecipitation sequencing to show that the transcription factors SARD1 and CBP60g bind to the promoter regions of a large number of genes encoding key regulators of plant immunity. Among them are positive regulators of systemic immunity and signalling components for effector-triggered immunity and PAMP-triggered immunity, which is consistent with the critical roles of SARD1 and CBP60g in these processes. In addition, SARD1 and CBP60g target a number of genes encoding negative regulators of plant immunity, suggesting that they are also involved in negative feedback regulation of defence responses. Based on these findings we propose that SARD1 and CBP60g function as master regulators of plant immune responses.

SARD1 and CBP60g are two plant transcription factors that regulate salicylic acid biosynthesis in response to pathogens. Here, Sun et al. show that they bind a wide array of loci related to multiple defence signalling pathways suggesting a broader role as regulators of the plant immune response.

The synthetic cationic lipid diC14 activates a sector of the Arabidopsis defence network requiring endogenous signalling components

Natural and synthetic elicitors have contributed significantly to the study of plant immunity. Pathogen-derived proteins and carbohydrates that bind to immune receptors, allow the fine dissection of certain defence pathways. Lipids of a different nature that act as defence elicitors, have also been studied, but their specific effects have been less well characterized, and their receptors have not been identified. In animal cells, nanoliposomes of the synthetic cationic lipid 3-tetradecylamino-tert-butyl-N-tetradecylpropionamidine (diC14) activate the TLR4-dependent immune cascade. Here, we have investigated whether this lipid induces Arabidopsis defence responses. At the local level, diC14 activated early and late defence gene markers (FRK1, WRKY29, ICS1 and PR1), acting in a dose-dependent manner. This lipid induced the salicylic acid (SA)-dependent, but not jasmonic acid (JA)-dependent, pathway and protected plants against Pseudomonas syringae pv. tomato (Pst), but not Botrytis cinerea. diC14 was not toxic to plant or pathogen, and potentiated pathogen-induced callose deposition. At the systemic level, diC14 induced PR1 expression and conferred resistance against Pst. diC14-induced defence responses required the signalling protein EDS1, but not NDR1. Curiously, the lipid-induced defence gene expression was lower in the fls2/efr/cerk1 triple mutant, but still unchanged in the single mutants. The amidine headgroup and chain length were important for its activity. Given the robustness of the responses triggered by diC14, its specific action on a defence pathway and the requirement for well-known defence components, this synthetic lipid is emerging as a useful tool to investigate the initial events involved in plant innate immunity.

Systemic Immunity Requires SnRK2.8-Mediated Nuclear Import of NPR1 in Arabidopsis

In plants, necrotic lesions occur at the site of pathogen infection through the hypersensitive response, which is followed by induction of systemic acquired resistance (SAR) in distal tissues. Salicylic acid (SA) induces SAR by activating NONEXPRESSER OF PATHOGENESIS-RELATED GENES1 (NPR1) through an oligomer-to-monomer reaction. However, SA biosynthesis is elevated only slightly in distal tissues during SAR, implying that SA-mediated induction of SAR requires additional factors. Here, we demonstrated that SA-independent systemic signals induce a gene encoding SNF1-RELATED PROTEIN KINASE 2.8 (SnRK2.8), which phosphorylates NPR1 during SAR. The SnRK2.8-mediated phosphorylation of NPR1 is necessary for its nuclear import. Notably, although SnRK2.8 transcription and SnRK2.8 activation are independent of SA signaling, the SnRK2.8-mediated induction of SAR requires SA. Together with the SA-mediated monomerization of NPR1, these observations indicate that SA signals and SnRK2.8-mediated phosphorylation coordinately function to activate NPR1 via a dual-step process in developing systemic immunity in Arabidopsis thaliana.

Amino Acid Catabolism in Plants

Amino acids have various prominent functions in plants. Besides their usage during protein biosynthesis, they also represent building blocks for several other biosynthesis pathways and play pivotal roles during signaling processes as well as in plant stress response. In general, pool sizes of the 20 amino acids differ strongly and change dynamically depending on the developmental and physiological state of the plant cell. Besides amino acid biosynthesis, which has already been investigated in great detail, the catabolism of amino acids is of central importance for adjusting their pool sizes but so far has drawn much less attention. The degradation of amino acids can also contribute substantially to the energy state of plant cells under certain physiological conditions, e.g. carbon starvation. In this review, we discuss the biological role of amino acid catabolism and summarize current knowledge on amino acid degradation pathways and their regulation in the context of plant cell physiology.

Visualizing the relevance of bacterial blue- and red-light receptors during plant-pathogen interaction

The foliar pathogen Pseudomonas syringae pv. tomato DC3000 (Pst) leads to consistent losses in tomato crops, urging to multiply investigations on the physiological bases for its infectiveness. As other P. syringae pathovars, Pst is equipped with photoreceptors for blue and red light, mimicking the photosensing ability of host plants. In this work we have investigated Pst strains lacking the genes for a blue-light sensing protein (PstLOV), for a bacteriophytochrome (PstBph1) or for heme-oxygenase-1. When grown in culturing medium, all deletion mutants presented a larger growth than wild-type (WT) Pst under all other light conditions, with the exception of blue light which, under our experimental conditions (photon fluence rate = 40 μmol m(-2) s(-1)), completely suppressed the growth of the deletion mutants. Each of the knockout mutants shows stronger virulence towards Arabidopsis thaliana than PstWT, as evidenced by macroscopic damages in the host tissues of infected leaves. Mutated bacteria were also identified in districts distant from the infection site using scanning electron microscopy. These results underscore the importance of Pst photoreceptors in responding to environmental light inputs and the partial protective role that they exert towards host plants during infection, diminishing virulence and invasiveness.

An untargeted global metabolomic analysis reveals the biochemical changes underlying basal resistance and priming in Solanum lycopersicum, and identifies 1-methyltryptophan as a metabolite involved in plant responses to Botrytis cinerea and Pseudomonas syringae

In this study, we have used untargeted global metabolomic analysis to determine and compare the chemical nature of the metabolites altered during the infection of tomato plants (cv. Ailsa Craig) with Botrytis cinerea (Bot) or Pseudomonas syringae pv. tomato DC3000 (Pst), pathogens that have different invasion mechanisms and lifestyles. We also obtained the metabolome of tomato plants primed using the natural resistance inducer hexanoic acid and then infected with these pathogens. By contrasting the metabolomic profiles of infected, primed, and primed + infected plants, we determined not only the processes or components related directly to plant defense responses, but also inferred the metabolic mechanisms by which pathogen resistance is primed. The data show that basal resistance and hexanoic acid-induced resistance to Bot and Pst are associated with a marked metabolic reprogramming. This includes significant changes in amino acids, sugars and free fatty acids, and in primary and secondary metabolism. Comparison of the metabolic profiles of the infections indicated clear differences, reflecting the fact that the plant's chemical responses are highly adapted to specific attackers. The data also indicate involvement of signaling molecules, including pipecolic and azelaic acids, in response to Pst and, interestingly, to Bot. The compound 1-methyltryptophan was shown to be associated with the tomato-Pst and tomato-Bot interactions as well as with hexanoic acid-induced resistance. Root application of this Trp-derived metabolite also demonstrated its ability to protect tomato plants against both pathogens.

2-Hydroxy Acids in Plant Metabolism

Glycolate, malate, lactate, and 2-hydroxyglutarate are important 2-hydroxy acids (2HA) in plant metabolism. Most of them can be found as D- and L-stereoisomers. These 2HA play an integral role in plant primary metabolism, where they are involved in fundamental pathways such as photorespiration, tricarboxylic acid cycle, glyoxylate cycle, methylglyoxal pathway, and lysine catabolism. Recent molecular studies in Arabidopsis thaliana have helped elucidate the participation of these 2HA in in plant metabolism and physiology. In this chapter, we summarize the current knowledge about the metabolic pathways and cellular processes in which they are involved, focusing on the proteins that participate in their metabolism and cellular/intracellular transport in Arabidopsis.

Priming and memory of stress responses in organisms lacking a nervous system

Experience and memory of environmental stimuli that indicate future stress can prepare (prime) organismic stress responses even in species lacking a nervous system. The process through which such organisms prepare their phenotype for an improved response to future stress has been termed 'priming'. However, other terms are also used for this phenomenon, especially when considering priming in different types of organisms and when referring to different stressors. Here we propose a conceptual framework for priming of stress responses in bacteria, fungi and plants which allows comparison of priming with other terms, e.g. adaptation, acclimation, induction, acquired resistance and cross protection. We address spatial and temporal aspects of priming and highlight current knowledge about the mechanisms necessary for information storage which range from epigenetic marks to the accumulation of (dormant) signalling molecules. Furthermore, we outline possible patterns of primed stress responses. Finally, we link the ability of organisms to become primed for stress responses (their 'primability') with evolutionary ecology aspects and discuss which properties of an organism and its environment may favour the evolution of priming of stress responses.

Hybrid mimics and hybrid vigor in Arabidopsis

F1 hybrids can outperform their parents in yield and vegetative biomass, features of hybrid vigor that form the basis of the hybrid seed industry. The yield advantage of the F1 is lost in the F2 and subsequent generations. In Arabidopsis, from F2 plants that have a F1-like phenotype, we have by recurrent selection produced pure breeding F5/F6 lines, hybrid mimics, in which the characteristics of the F1 hybrid are stabilized. These hybrid mimic lines, like the F1 hybrid, have larger leaves than the parent plant, and the leaves have increased photosynthetic cell numbers, and in some lines, increased size of cells, suggesting an increased supply of photosynthate. A comparison of the differentially expressed genes in the F1 hybrid with those of eight hybrid mimic lines identified metabolic pathways altered in both; these pathways include down-regulation of defense response pathways and altered abiotic response pathways. F6 hybrid mimic lines are mostly homozygous at each locus in the genome and yet retain the large F1-like phenotype. Many alleles in the F6 plants, when they are homozygous, have expression levels different to the level in the parent. We consider this altered expression to be a consequence of transregulation of genes from one parent by genes from the other parent. Transregulation could also arise from epigenetic modifications in the F1. The pure breeding hybrid mimics have been valuable in probing the mechanisms of hybrid vigor and may also prove to be useful hybrid vigor equivalents in agriculture.

Shifts in metabolomic profiles of the parasitoid Nasonia vitripennis associated with elevated cold tolerance induced by the parasitoid's diapause, host diapause and host diet augmented with proline

The ectoparasitoid wasp, Nasonia vitripennis can enhance its cold tolerance by exploiting a maternally-induced larval diapause. A simple manipulation of the fly host diapause status and supplementation of the host diet with proline also dramatically increase cold tolerance in the parasitoid. In this study, we used a metabolomics approach to define alterations in metabolite profiles of N. vitripennis caused by diapause in the parasitoid, diapause of the host, and augmentation of the host's diet with proline. Metabolic profiles of diapausing and nondiapausing parasitoid were significantly differentiated, with pronounced distinctions in levels of multiple cryoprotectants, amino acids, and carbohydrates. The dynamic nature of diapause was underscored by a shift in the wasp's metabolomic profile as the duration of diapause increased, a feature especially evident for increased concentrations of a suite of cryoprotectants. Metabolic pathways involved in amino acid and carbohydrate metabolism were distinctly enriched during diapause in the parasitoid. Host diapause status also elicited a pronounced effect on metabolic signatures of the parasitoid, noted by higher cryoprotectants and elevated compounds derived from glycolysis. Proline supplementation of the host diet did not translate directly into elevated proline in the parasitoid but resulted in an alteration in the abundance of many other metabolites, including elevated concentrations of essential amino acids, and reduction in metabolites linked to energy utilization, lipid and amino acid metabolism. Thus, the enhanced cold tolerance of N. vitripennis associated with proline augmentation of the host diet appears to be an indirect effect caused by the metabolic perturbations associated with diet supplementation.

Folic acid induces salicylic acid-dependent immunity in Arabidopsis and enhances susceptibility to Alternaria brassicicola

Folates are essential for one-carbon transfer reactions in all organisms and contribute, for example, to de novo DNA synthesis. Here, we detected the folate precursors 7,8-dihydropteroate (DHP) and 4-amino-4-deoxychorismate (ADC) in extracts from Arabidopsis thaliana plants by Fourier transform ion cyclotron resonance-mass spectrometry. The accumulation of DHP, but not ADC, was induced after infection of plants with Pseudomonas syringae delivering the effector protein AvrRpm1. Application of folic acid or the DHP precursor 7,8-dihydroneopterin (DHN) enhanced resistance in Arabidopsis to P. syringae and elevated the transcript accumulation of the salicylic acid (SA) marker gene pathogenesis-related1 in both the treated and systemic untreated leaves. DHN- and folic acid-induced systemic resistance was dependent on SA biosynthesis and signalling. Similar to SA, folic acid application locally enhanced Arabidopsis susceptibility to the necrotrophic fungus Alternaria brassicicola. Together, the data associate the folic acid pathway with innate immunity in Arabidopsis, simultaneously activating local and systemic SA-dependent resistance to P. syringae and suppressing local resistance to A. brassicicola.

What the transcriptome does not tell - proteomics and metabolomics are closer to the plants' patho-phenotype

The proteome and metabolome of the plant provide a wealth of additional information on plant-microbe interactions since they not only represent additional levels of regulation, but often they harbor the end products of regulatory processes. Proteomics has contributed to our understanding of plant-microbe research by increasing the spatial resolution of the analysis within the infected tissue, because components of the basal immunity were uncovered in the apoplast. Metabolomics has developed into a powerful approach to discover the role of small molecules during plant-microbe interactions in non-model plants since it does not depend on the availability of genome or transcriptome data. Moreover, novel molecules involved in systemic acquired resistance and the precursors for the formation of molecules that provide physical barriers to prevent spreading of pathogens were identified.

Spatial and temporal regulation of biosynthesis of the plant immune signal salicylic acid

The plant hormone salicylic acid (SA) is essential for local defense and systemic acquired resistance (SAR). When plants, such as Arabidopsis, are challenged by different pathogens, an increase in SA biosynthesis generally occurs through transcriptional induction of the key synthetic enzyme isochorismate synthase 1 (ICS1). However, the regulatory mechanism for this induction is poorly understood. Using a yeast one-hybrid screen, we identified two transcription factors (TFs), NTM1-like 9 (NTL9) and CCA1 hiking expedition (CHE), as activators of ICS1 during specific immune responses. NTL9 is essential for inducing ICS1 and two other SA synthesis-related genes, phytoalexin-deficient 4 (PAD4) and enhanced disease susceptibility 1 (EDS1), in guard cells that form stomata. Stomata can quickly close upon challenge to block pathogen entry. This stomatal immunity requires ICS1 and the SA signaling pathway. In the ntl9 mutant, this response is defective and can be rescued by exogenous application of SA, indicating that NTL9-mediated SA synthesis is essential for stomatal immunity. CHE, the second identified TF, is a central circadian clock oscillator and is required not only for the daily oscillation in SA levels but also for the pathogen-induced SA synthesis in systemic tissues during SAR. CHE may also regulate ICS1 through the known transcription activators calmodulin binding protein 60g (CBP60g) and systemic acquired resistance deficient 1 (SARD1) because induction of these TF genes is compromised in the che-2 mutant. Our study shows that SA biosynthesis is regulated by multiple TFs in a spatial and temporal manner and therefore fills a gap in the signal transduction pathway between pathogen recognition and SA production.

Arabidopsis AZI1 family proteins mediate signal mobilization for systemic defence priming

Priming is a major mechanism behind the immunological 'memory' observed during two key plant systemic defences: systemic acquired resistance (SAR) and induced systemic resistance (ISR). Lipid-derived azelaic acid (AZA) is a mobile priming signal. Here, we show that the lipid transfer protein (LTP)-like AZI1 and its closest paralog EARLI1 are necessary for SAR, ISR and the systemic movement and uptake of AZA in Arabidopsis. Imaging and fractionation studies indicate that AZI1 and EARLI1 localize to expected places for lipid exchange/movement to occur. These are the ER/plasmodesmata, chloroplast outer envelopes and membrane contact sites between them. Furthermore, these LTP-like proteins form complexes and act at the site of SAR establishment. The plastid targeting of AZI1 and AZI1 paralogs occurs through a mechanism that may enable/facilitate their roles in signal mobilization.

Polycomb and Trithorax group protein-mediated control of stress responses in plants

A plant's experience of abiotic or biotic stress can lead to stress memory in order to react faster and more efficiently to subsequent stresses. Molecularly, the memory of a stress can rely on stable inheritance through mitotic and meiotic cell divisions, thus epigenetic inheritance. The key epigenetic regulators are DNA cytosine methyltransferases and the Polycomb group (PcG) and Trithorax group (TrxG) proteins, which control numerous developmental processes. PcG and TrxG proteins act antagonistically on stable gene repression through mediating trimethylation of histone H3 lysine 27 (H3K27me3) and H3K4me3, respectively, and target thousands of genes in plants, including many genes responsive to stress. The role of PcG/TrxG proteins in regulating stress responses and memory, however, is just emerging. While it is well investigated that stress can induce changes of histone modifications at genes regulated by stress, it is largely unclear whether these changes are mitotically and/or meiotically heritable, hence confer somatic and/or transgenerational stress memory. As the literature on the role of DNA methylation in regulating stress responses has recently been extensively summarized, we focus this review on the current knowledge on the role of PcG and TrxG in stress responses and memory.

Metabolomics Suggests That Soil Inoculation with Arbuscular Mycorrhizal Fungi Decreased Free Amino Acid Content in Roots of Durum Wheat Grown under N-Limited, P-Rich Field Conditions

Arbuscular mycorrhizal fungi (AMF) have a major impact on plant nutrition, defence against pathogens, a plant’s reaction to stressful environments, soil fertility, and a plant’s relationship with other microorganisms. Such effects imply a broad reprogramming of the plant’s metabolic activity. However, little information is available regarding the role of AMF and their relation to other soil plant growth—promoting microorganisms in the plant metabolome, especially under realistic field conditions. In the present experiment, we evaluated the effects of inoculation with AMF, either alone or in combination with plant growth–promoting rhizobacteria (PGPR), on the metabolome and changes in metabolic pathways in the roots of durum wheat (Triticum durum Desf.) grown under N-limited agronomic conditions in a P-rich environment. These two treatments were compared to infection by the natural AMF population (NAT). Soil inoculation with AMF almost doubled wheat root colonization by AMF and decreased the root concentrations of most compounds in all metabolic pathways, especially amino acids (AA) and saturated fatty acids, whereas inoculation with AMF+PGPR increased the concentrations of such compounds compared to inoculation with AMF alone. Enrichment metabolomics analyses showed that AA metabolic pathways were mostly changed by the treatments, with reduced amination activity in roots most likely due to a shift from the biosynthesis of common AA to γ-amino butyric acid. The root metabolome differed between AMF and NAT but not AMF+PGPR and AMF or NAT. Because the PGPR used were potent mineralisers, and AMF can retain most nitrogen (N) taken as organic compounds for their own growth, it is likely that this result was due to an increased concentration of mineral N in soil inoculated with AMF+PGPR compared to AMF alone.

Single cell-type comparative metabolomics of epidermal bladder cells from the halophyte Mesembryanthemum crystallinum

One of the remarkable adaptive features of the halophyte Mesembryanthemum crystallinum are the specialized modified trichomes called epidermal bladder cells (EBC) which cover the leaves, stems, and peduncle of the plant. They are present from an early developmental stage but upon salt stress rapidly expand due to the accumulation of water and sodium. This particular plant feature makes it an attractive system for single cell type studies, with recent proteomics and transcriptomics studies of the EBC establishing that these cells are metabolically active and have roles other than sodium sequestration. To continue our investigation into the function of these unusual cells we carried out a comprehensive global analysis of the metabolites present in the EBC extract by gas chromatography Time-of-Flight mass spectrometry (GC-TOF) and identified 194 known and 722 total molecular features. Statistical analysis of the metabolic changes between control and salt-treated samples identified 352 significantly differing metabolites (268 after correction for FDR). Principal components analysis provided an unbiased evaluation of the data variance structure. Biochemical pathway enrichment analysis suggested significant perturbations in 13 biochemical pathways as defined in KEGG. More than 50% of the metabolites that show significant changes in the EBC, can be classified as compatible solutes and include sugars, sugar alcohols, protein and non-protein amino acids, and organic acids, highlighting the need to maintain osmotic homeostasis to balance the accumulation of Na(+) and Cl(-) ions. Overall, the comparison of metabolic changes in salt treated relative to control samples suggests large alterations in M. crystallinum epidermal bladder cells.

Investigation of the Chemical Interface in the Soybean-Aphid and Rice-Bacteria Interactions Using MALDI-Mass Spectrometry Imaging

Mass spectrometry imaging (MSI) is an emerging technology for high-resolution plant biology. It has been utilized to study plant-pest interactions, but limited to the surface interfaces. Here we expand the technology to explore the chemical interactions occurring inside the plant tissues. Two sample preparation methods, imprinting and fracturing, were developed and applied, for the first time, to visualize internal metabolites of leaves in matrix-assisted laser desorption ionization (MALDI)-MSI. This is also the first time nanoparticle-based ionization was implemented to ionize diterpenoid phytochemicals that were difficult to analyze with traditional organic matrices. The interactions between rice-bacterium and soybean-aphid were investigated as two model systems to demonstrate the capability of high-resolution MSI based on MALDI. Localized molecular information on various plant- or pest-derived chemicals provided valuable insight for the molecular processes occurring during the plant-pest interactions. Specifically, salicylic acid and isoflavone based resistance was visualized in the soybean-aphid system and antibiotic diterpenoids in rice-bacterium interactions.

The possible involvement of salicylic acid and hydrogen peroxide in the systemic promotion of phenolic biosynthesis in clover roots colonized by arbuscular mycorrhizal fungus

Arbuscular mycorrhizal fungal (AMF) colonization can induce both the local and the systemic increase in phenolic accumulation in hosts. However, the signaling molecules responsible for the systemic induction is still unclear. In this study, a split-root rhizobox system was designed to explore these molecules, with one half of clover (Trifolium repense) roots colonized by AMF, Funneliformis mosseae (formerly known as Glomus mosseae), and the other not (NM/M). Plants with two halves both (M/M) or neither (NM/NM) inoculated were also established for comparison. The contents of phenols and the accumulation of salicylic acid (SA), hydrogen peroxide (H2O2) and nitric oxide (NO) in roots were monitored, the activities of L-phenylalanine ammonia-lyase (PAL) and nitric oxide synthase (NOS) in roots were assayed, and the expressions of pal and chs (gene encoding chalcone synthase) genes in roots were also quantified using qRT-PCR. Results indicated that when phenolic content in NM/NM plants was lower than that in M/M plants, AMF colonization systemically induced the increase in phenolic content in NM/M plants. Similarly, the accumulations of SA and H2O2 were increased by AMF both locally and systemically, while that of NO was only increased locally. Moreover, enzyme assay and qRT-PCR were in accordance with these results. These data suggest that AMF colonization can systemically increase the phenolic biosynthesis, and SA and H2O2 are possibly the signaling molecules involved. The role of MeSA, a signaling molecule capable of long distance transport in this process, is also discussed.

ALD1 Regulates Basal Immune Components and Early Inducible Defense Responses in Arabidopsis

Robust immunity requires basal defense machinery to mediate timely responses and feedback cycles to amplify defenses against potentially spreading infections. AGD2-LIKE DEFENSE RESPONSE PROTEIN 1 (ALD1) is needed for the accumulation of the plant defense signal salicylic acid (SA) during the first hours after infection with the pathogen Pseudomonas syringae and is also upregulated by infection and SA. ALD1 is an aminotransferase with multiple substrates and products in vitro. Pipecolic acid (Pip) is an ALD1-dependent bioactive product induced by P. syringae. Here, we addressed roles of ALD1 in mediating defense amplification as well as the levels and responses of basal defense machinery. ALD1 needs immune components PAD4 and ICS1 (an SA synthesis enzyme) to confer disease resistance, possibly through a transcriptional amplification loop between them. Furthermore, ALD1 affects basal defense by controlling microbial-associated molecular pattern (MAMP) receptor levels and responsiveness. Vascular exudates from uninfected ALD1-overexpressing plants confer local immunity to the wild type and ald1 mutants yet are not enriched for Pip. We infer that, in addition to affecting Pip accumulation, ALD1 produces non-Pip metabolites that play roles in immunity. Thus, distinct metabolite signals controlled by the same enzyme affect basal and early defenses versus later defense responses, respectively.

The metabolomics of oxidative stress

Oxidative stress resulting from increased availability of reactive oxygen species (ROS) is a key component of many responses of plants to challenging environmental conditions. The consequences for plant metabolism are complex and manifold. We review data on small compounds involved in oxidative stress, including ROS themselves and antioxidants and redox buffers in the membrane and soluble phases, and we discuss the wider consequences for plant primary and secondary metabolism. While metabolomics has been exploited in many studies on stress, there have been relatively few non-targeted studies focused on how metabolite signatures respond specifically to oxidative stress. As part of the discussion, we present results and reanalyze published datasets on metabolite profiles in catalase-deficient plants, which can be considered to be model oxidative stress systems. We emphasize the roles of ROS-triggered changes in metabolites as potential oxidative signals, and discuss responses that might be useful as markers for oxidative stress. Particular attention is paid to lipid-derived compounds, the status of antioxidants and antioxidant breakdown products, altered metabolism of amino acids, and the roles of phytohormone pathways.

Phospholipase D and phosphatidic acid in plant defence response: from protein-protein and lipid-protein interactions to hormone signalling

Phospholipase Ds (PLDs) and PLD-derived phosphatidic acids (PAs) play vital roles in plant hormonal and environmental responses and various cellular dynamics. Recent studies have further expanded the functions of PLDs and PAs into plant-microbe interaction. The molecular diversities and redundant functions make PLD-PA an important signalling complex regulating lipid metabolism, cytoskeleton dynamics, vesicle trafficking, and hormonal signalling in plant defence through protein-protein and protein-lipid interactions or hormone signalling. Different PLD-PA signalling complexes and their targets have emerged as fast-growing research topics for understanding their numerous but not yet established roles in modifying pathogen perception, signal transduction, and downstream defence responses. Meanwhile, advanced lipidomics tools have allowed researchers to reveal further the mechanisms of PLD-PA signalling complexes in regulating lipid metabolism and signalling, and their impacts on jasmonic acid/oxylipins, salicylic acid, and other hormone signalling pathways that essentially mediate plant defence responses. This review attempts to summarize the progress made in spatial and temporal PLD/PA signalling as well as PLD/PA-mediated modification of plant defence. It presents an in-depth discussion on the functions and potential mechanisms of PLD-PA complexes in regulating actin filament/microtubule cytoskeleton, vesicle trafficking, and hormonal signalling, and in influencing lipid metabolism-derived metabolites as critical signalling components in plant defence responses. The discussion puts PLD-PA in a broader context in order to guide future research.

Plasmodesmata-located protein overexpression negatively impacts the manifestation of systemic acquired resistance and the long-distance movement of Defective in Induced Resistance1 in Arabidopsis

Systemic acquired resistance (SAR) is a plant defence response that provides immunity to distant uninfected leaves after an initial localised infection. The lipid transfer protein (LTP) Defective in Induced Resistance1 (DIR1) is an essential component of SAR that moves from induced to distant leaves following a SAR-inducing local infection. To understand how DIR1 is transported to distant leaves during SAR, we analysed DIR1 movement in transgenic Arabidopsis lines with reduced cell-to-cell movement caused by the overexpression of Plasmodesmata-Located Proteins PDLP1 and PDLP5. These PDLP-overexpressing lines were defective for SAR, and DIR1 antibody signals were not observed in phloem sap-enriched petiole exudates collected from distant leaves. Our data support the idea that cell-to-cell movement of DIR1 through plasmodesmata is important during long-distance SAR signalling in Arabidopsis.

Bio-based resistance inducers for sustainable plant protection against pathogens

An increasing demand for environmentally acceptable alternative for traditional pesticides provides an impetus to conceive new bio-based strategies in crop protection. Employing induced resistance is one such strategy, consisting of boosting the natural plant immunity. Upon infections, plants defend themselves by activating their immune mechanisms. These are initiated after the recognition of an invading pathogen via the microbe-associated molecular patterns (MAMPs) or other microbe-derived molecules. Triggered responses inhibit pathogen spread from the infected site. Systemic signal transport even enables to prepare, i.e. prime, distal uninfected tissues for more rapid and enhanced response upon the consequent pathogen attack. Similar defense mechanisms can be triggered by purified MAMPs, pathogen-derived molecules, signal molecules involved in plant resistance to pathogens, such as salicylic and jasmonic acid, or a wide range of other chemical compounds. Induced resistance can be also conferred by plant-associated microorganisms, including beneficial bacteria or fungi. Treatment with resistance inducers or beneficial microorganisms provides long-lasting resistance for plants to a wide range of pathogens. This study surveys current knowledge on resistance and its mechanisms provided by microbe-, algae- and plant-derived elicitors in different crops. The main scope deals with bacterial substances and fungus-derived molecules chitin and chitosan and algae elicitors, including naturally sulphated polysaccharides such as ulvans, fucans or carageenans. Recent advances in the utilization of this strategy in practical crop protection are also discussed.

Priming for enhanced defense

When plants recognize potential opponents, invading pathogens, wound signals, or abiotic stress, they often switch to a primed state of enhanced defense. However, defense priming can also be induced by some natural or synthetic chemicals. In the primed state, plants respond to biotic and abiotic stress with faster and stronger activation of defense, and this is often linked to immunity and abiotic stress tolerance. This review covers recent advances in disclosing molecular mechanisms of priming. These include elevated levels of pattern-recognition receptors and dormant signaling enzymes, transcription factor HsfB1 activity, and alterations in chromatin state. They also comprise the identification of aspartyl-tRNA synthetase as a receptor of the priming activator β-aminobutyric acid. The article also illustrates the inheritance of priming, exemplifies the role of recently identified priming activators azelaic and pipecolic acid, elaborates on the similarity to defense priming in mammals, and discusses the potential of defense priming in agriculture.

Signal regulators of systemic acquired resistance

Salicylic acid (SA) is an important phytohormone that plays a vital role in a number of physiological responses, including plant defense. The last two decades have witnessed a number of breakthroughs related to biosynthesis, transport, perception and signaling mediated by SA. These findings demonstrate that SA plays a crictical role in both local and systemic defense responses. Systemic acquired resistance (SAR) is one such SA-dependent response. SAR is a long distance signaling mechanism that provides broad spectrum and long-lasting resistance to secondary infections throughout the plant. This unique feature makes SAR a highly desirable trait in crop production. This review summarizes the recent advances in the role of SA in SAR and discusses its relationship to other SAR inducers.

Interconnection between flowering time control and activation of systemic acquired resistance

The ability to avoid or neutralize pathogens is inherent to all higher organisms including plants. Plants recognize pathogens through receptors, and mount resistance against the intruders, with the help of well-elaborated defense arsenal. In response to some localinfections, plants develop systemic acquired resistance (SAR), which provides heightened resistance during subsequent infections. Infected tissues generate mobile signaling molecules that travel to the systemic tissues, where they epigenetically modify expression o a set of genes to initiate the manifestation of SAR in distant tissues. Immune responses are largely regulated at transcriptional level. Flowering is a developmental transition that occurs as a result of the coordinated action of large numbers of transcription factors that respond to intrinsic signals and environmental conditions. The plant hormone salicylic acid (SA) which is required for SAR activation positively regulates flowering. Certain components of chromatin remodeling complexes that are recruited for suppression of precocious flowering are also involved in suppression of SAR in healthy plants. FLOWERING LOCUS D, a putative histone demethylase positively regulates SAR manifestation and flowering transition in Arabidopsis. Similarly, incorporation of histone variant H2A.Z in nucleosomes mediated by PHOTOPERIOD-INDEPENDENT EARLY FLOWERING 1, an ortholog of yeast chromatin remodeling complex SWR1, concomitantly influences SAR and flowering time. SUMO conjugation and deconjugation mechanisms also similarly affect SAR and flowering in an SA-dependent manner. The evidences suggest a common underlying regulatory mechanism for activation of SAR and flowering in plants.

Insect eggs induce a systemic acquired resistance in Arabidopsis

Although they constitute an inert stage of the insect's life, eggs trigger plant defences that lead to egg mortality or attraction of egg parasitoids. We recently found that salicylic acid (SA) accumulates in response to oviposition by the Large White butterfly Pieris brassicae, both in local and systemic leaves, and that plants activate a response that is similar to the recognition of pathogen-associated molecular patterns (PAMPs), which are involved in PAMP-triggered immunity (PTI). Here we discovered that natural oviposition by P. brassicae or treatment with egg extract inhibit growth of different Pseudomonas syringae strains in Arabidopsis through the activation of a systemic acquired resistance (SAR). This egg-induced SAR involves the metabolic SAR signal pipecolic acid, depends on ALD1 and FMO1, and is accompanied by a stronger induction of defence genes upon secondary infection. Although P. brassicae larvae showed a reduced performance when feeding on Pseudomonas syringae-infected plants, this effect was less pronounced when infected plants had been previously oviposited. Altogether, our results indicate that egg-induced SAR might have evolved as a strategy to prevent the detrimental effect of bacterial pathogens on feeding larvae.

Bacteria-triggered systemic immunity in barley is associated with WRKY and ETHYLENE RESPONSIVE FACTORs but not with salicylic acid

Leaf-to-leaf systemic immune signaling known as systemic acquired resistance is poorly understood in monocotyledonous plants. Here, we characterize systemic immunity in barley (Hordeum vulgare) triggered after primary leaf infection with either Pseudomonas syringae pathovar japonica (Psj) or Xanthomonas translucens pathovar cerealis (Xtc). Both pathogens induced resistance in systemic, uninfected leaves against a subsequent challenge infection with Xtc. In contrast to systemic acquired resistance in Arabidopsis (Arabidopsis thaliana), systemic immunity in barley was not associated with NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 or the local or systemic accumulation of salicylic acid. Instead, we documented a moderate local but not systemic induction of abscisic acid after infection of leaves with Psj. In contrast to salicylic acid or its functional analog benzothiadiazole, local applications of the jasmonic acid methyl ester or abscisic acid triggered systemic immunity to Xtc. RNA sequencing analysis of local and systemic transcript accumulation revealed unique gene expression changes in response to both Psj and Xtc and a clear separation of local from systemic responses. The systemic response appeared relatively modest, and quantitative reverse transcription-polymerase chain reaction associated systemic immunity with the local and systemic induction of two WRKY and two ETHYLENE RESPONSIVE FACTOR (ERF)-like transcription factors. Systemic immunity against Xtc was further associated with transcriptional changes after a secondary/systemic Xtc challenge infection; these changes were dependent on the primary treatment. Taken together, bacteria-induced systemic immunity in barley may be mediated in part by WRKY and ERF-like transcription factors, possibly facilitating transcriptional reprogramming to potentiate immunity.

Molecular and physiological stages of priming: how plants prepare for environmental challenges

Being sessile organisms, plants must respond to various challenges in the environment. The priming process consists of three clear stages. The first stage includes all the cellular changes in the absence of the challenge so-called pre-challenge priming stage. These changes are expected to be rather subtle, affecting the preparation of the plant to properly manage subsequent responses to pathogens with no major fitness costs. Most of the research that has been conducted at this stage has been dedicated to the study of changes in gene expression and protein phosphorylation. However, the metabolic changes that occur during the pre-challenge priming stage are poorly understood. The second stage affects the early to late stages of the defence response, which occurs after the interaction with a pathogen has been established. Most studies involving priming are dedicated to the molecular events that take place during this stage. Most studies have shown that defence priming is strongly hormonally regulated; however, there is also evidence of the involvement of phenolic derivative compounds and many other secondary metabolites, leading to stronger and faster plant responses. The third priming phase ranges from long lasting defence priming to trans-generational acquired resistance. Long-term metabolic transitions, that occur in the offspring of primed plants, remain to be elucidated. Here we review existing information in the literature that relates to the metabolic changes that occur during all three defence priming stages and highlight the metabolic transitions that are associated with the stimulation of priming and the characteristics of the pathogens whenever possible.

Cotton cytochrome P450 CYP82D regulates systemic cell death by modulating the octadecanoid pathway

Plant oxylipins are derived from unsaturated fatty acids and play roles in plant growth and development as well as defence. Although recent studies have revealed that fatty acid metabolism is involved in systemic acquired resistance, the precise function of oxylipins in plant defence remains unknown. Here we report a cotton P450 gene SILENCE-INDUCED STEM NECROSIS (SSN), RNAi suppression of which causes a lesion mimic phenotype. SSN is also involved in jasmonate metabolism and the response to wounding. Fatty acid and oxylipin metabolite analysis showed that SSN overexpression causes hyperaccumulation of hydroxide and ketodiene fatty acids and reduced levels of 18:2 fatty acids, whereas silencing causes an imbalance in LOX (lipoxygenase) expression and excessive hydroperoxide fatty acid accumulation. We also show that an unknown oxylipin-derived factor is a putative mobile signal required for systemic cell death and hypothesize that SSN acts as a valve to regulate HR on pathogen infection.

Oxylipin signalling is known to play important roles in plant growth, development and defence against pathogens. Here Sun et al. identify a novel cytochrome P450 in cotton and show that its suppression leads to activation of plant defence responses and alteration of oxylipin metabolism.

A metabolic profiling strategy for the dissection of plant defense against fungal pathogens

Here we present a metabolic profiling strategy employing direct infusion Orbitrap mass spectrometry (MS) and gas chromatography-mass spectrometry (GC/MS) for the monitoring of soybean's (Glycine max L.) global metabolism regulation in response to Rhizoctonia solani infection in a time-course. Key elements in the approach are the construction of a comprehensive metabolite library for soybean, which accelerates the steps of metabolite identification and biological interpretation of results, and bioinformatics tools for the visualization and analysis of its metabolome. The study of metabolic networks revealed that infection results in the mobilization of carbohydrates, disturbance of the amino acid pool, and activation of isoflavonoid, α-linolenate, and phenylpropanoid biosynthetic pathways of the plant. Components of these pathways include phytoalexins, coumarins, flavonoids, signaling molecules, and hormones, many of which exhibit antioxidant properties and bioactivity helping the plant to counterattack the pathogen's invasion. Unraveling the biochemical mechanism operating during soybean-Rhizoctonia interaction, in addition to its significance towards the understanding of the plant's metabolism regulation under biotic stress, provides valuable insights with potential for applications in biotechnology, crop breeding, and agrochemical and food industries.

β-Aminobutyric acid (BABA)-induced resistance in Arabidopsis thaliana: link with iron homeostasis

β-Aminobutyric acid (BABA) is a nonprotein amino acid inducing resistance in many different plant species against a wide range of abiotic and biotic stresses. Nevertheless, how BABA primes plant natural defense reactions remains poorly understood. Based on its structure, we hypothesized and confirmed that BABA is able to chelate iron (Fe) in vitro. In vivo, we showed that it led to a transient Fe deficiency response in Arabidopsis thaliana plants exemplified by a reduction of ferritin accumulation and disturbances in the expression of genes related to Fe homeostasis. This response was not correlated to changes in Fe concentrations, suggesting that BABA affects the availability or the distribution of Fe rather than its assimilation. The phenotype of BABA-treated plants was similar to those of plants cultivated in Fe-deficient conditions. A metabolomic analysis indicated that both BABA and Fe deficiency induced the accumulation of common metabolites, including p-coumaroylagmatine, a metabolite previously shown to be synthesized in several plant species facing pathogen attack. Finally, we showed that the protective effect induced by BABA against Botrytis cinerea was mimicked by Fe deficiency. In conclusion, the Fe deficiency response caused by BABA could bring the plant to a defense-ready state, participating in the plant resistance against the pathogens.

Arabidopsis ENHANCED DISEASE SUSCEPTIBILITY1 promotes systemic acquired resistance via azelaic acid and its precursor 9-oxo nonanoic acid

Systemic acquired resistance (SAR) is a form of inducible disease resistance that depends on salicylic acid and its upstream regulator ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1). Although local Arabidopsis thaliana defence responses activated by the Pseudomonas syringae effector protein AvrRpm1 are intact in eds1 mutant plants, SAR signal generation is abolished. Here, the SAR-specific phenotype of the eds1 mutant is utilized to identify metabolites that contribute to SAR. To this end, SAR bioassay-assisted fractionation of extracts from the wild type compared with eds1 mutant plants that conditionally express AvrRpm1 was performed. Using high-performance liquid chromatography followed by mass spectrometry, systemic immunity was associated with the accumulation of 60 metabolites, including the putative SAR signal azelaic acid (AzA) and its precursors 9-hydroperoxy octadecadienoic acid (9-HPOD) and 9-oxo nonanoic acid (ONA). Exogenous ONA induced SAR in systemic untreated leaves when applied at a 4-fold lower concentration than AzA. The data suggest that in planta oxidation of ONA to AzA might be partially responsible for this response and provide further evidence that AzA mobilizes Arabidopsis immunity in a concentration-dependent manner. The AzA fragmentation product pimelic acid did not induce SAR. The results link the C9 lipid peroxidation products ONA and AzA with systemic rather than local resistance and suggest that EDS1 directly or indirectly promotes the accumulation of ONA, AzA, or one or more of their common precursors possibly by activating one or more pathways that either result in the release of these compounds from galactolipids or promote lipid peroxidation.

Knockdown of LjALD1, AGD2-like defense response protein 1, influences plant growth and nodulation in Lotus japonicus

The discovery of the enzyme L,L-diaminopimelate aminotransferase (LL-DAP-AT, EC 2.6.1.83) uncovered a unique step in the L-lysine biosynthesis pathway in plants. In Arabidopsis thaliana, LL-DAP-AT has been shown to play a key role in plant-pathogen interactions by regulation of the salicylic acid (SA) signaling pathway. Here, a full-length cDNA of LL-DAP-AT named as LjALD1 from Lotus japonicus (Regel) Larsen was isolated. The deduced amino acid sequence shares 67% identity with the Arabidopsis aminotransferase AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (AtALD1) and is predicted to contain the same key elements: a conserved aminotransferase domain and a pyridoxal-5'-phosphate cofactor binding site. Quantitative real-time PCR analysis showed that LjALD1 was expressed in all L. japonicus tissues tested, being strongest in nodules. Expression was induced in roots that had been infected with the symbiotic rhizobium Mesorhizobium loti or treated with SA agonist benzo-(1, 2, 3)-thiadiazole-7-carbothioic acid. LjALD1 Knockdown exhibited a lower SA content, an increased number of infection threads and nodules, and a slight reduction in nodule size. In addition, compared with wild-type, root growth was increased and shoot growth was suppressed in LjALD1 RNAi plant lines. These results indicate that LjALD1 may play important roles in plant development and nodulation via SA signaling in L. japonicus.

Metabolomic insights into the bioconversion of isonitrosoacetophenone in Arabidopsis thaliana and its effects on defense-related pathways

Plants are constantly exposed to numerous biotic or abiotic stress factors throughout their life-cycle. Pathogens and pathogen-derived molecules are the best studied inducers of plant defense responses, but synthetic and naturally occurring molecules have also been used to induce various types of resistance in plants. Here, an oxime molecule, 2-isonitrosoacetophenone (INAP), related to the stress metabolite citaldoxime, was used to trigger metabolic changes in the metabolome of treated Arabidopsis thaliana plants as monitored by UHPLC-MS in conjunction with principal component analysis (PCA) and orthogonal projection to latent structures discriminant analysis (OPLS-DA). The chemometric methods revealed metabolites found to be significantly present in response to the treatment. These include bioconversion products (2-keto-2-phenylacetaldoxime-glycoside and l-mandelonitrile-glycoside) as well as those of which the levels are affected by the treatment (benzoic acid and derivatives, other phenylpropanoid-derived compounds and glucosinolates). Using in planta bacterial growth evaluations, INAP treatment was furthermore found to induce an anti-microbial environment in vivo.

Multitasking antimicrobial peptides in plant development and host defense against biotic/abiotic stress

Crop losses due to pathogens are a major threat to global food security. Plants employ a multilayer defense against a pathogen including the use of physical barriers (cell wall), induction of hypersensitive defense response (HR), resistance (R) proteins, and synthesis of antimicrobial peptides (AMPs). Unlike a complex R gene-mediated immunity, AMPs directly target diverse microbial pathogens. Many a times, R-mediated immunity breaks down and plant defense is compromised. Although R-gene dependent pathogen resistance has been well studied, comparatively little is known about the interactions of AMPs with host defense and physiology. AMPs are ubiquitous, low molecular weight peptides that display broad spectrum resistance against bacteria, fungi and viruses. In plants, AMPs are mainly classified into cyclotides, defensins, thionins, lipid transfer proteins, snakins, and hevein-like vicilin-like and knottins. Genetic distance lineages suggest their conservation with minimal effect of speciation events during evolution. AMPs provide durable resistance in plants through a combination of membrane lysis and cellular toxicity of the pathogen. Plant hormones - gibberellins, ethylene, jasmonates, and salicylic acid, are among the physiological regulators that regulate the expression of AMPs. Transgenically produced AMP-plants have become a means showing that AMPs are able to mitigate host defense responses while providing durable resistance against pathogens.

Arabidopsis triphosphate tunnel metalloenzyme2 is a negative regulator of the salicylic acid-mediated feedback amplification loop for defense responses

The triphosphate tunnel metalloenzyme (TTM) superfamily represents a group of enzymes that is characterized by their ability to hydrolyze a range of tripolyphosphate substrates. Arabidopsis (Arabidopsis thaliana) encodes three TTM genes, AtTTM1, AtTTM2, and AtTTM3. Although AtTTM3 has previously been reported to have tripolyphosphatase activity, recombinantly expressed AtTTM2 unexpectedly exhibited pyrophosphatase activity. AtTTM2 knockout mutant plants exhibit an enhanced hypersensitive response, elevated pathogen resistance against both virulent and avirulent pathogens, and elevated accumulation of salicylic acid (SA) upon infection. In addition, stronger systemic acquired resistance compared with wild-type plants was observed. These enhanced defense responses are dependent on SA, PHYTOALEXIN-DEFICIENT4, and NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1. Despite their enhanced pathogen resistance, ttm2 plants did not display constitutively active defense responses, suggesting that AtTTM2 is not a conventional negative regulator but a negative regulator of the amplification of defense responses. The transcriptional suppression of AtTTM2 by pathogen infection or treatment with SA or the systemic acquired resistance activator benzothiadiazole further supports this notion. Such transcriptional regulation is conserved among TTM2 orthologs in the crop plants soybean (Glycine max) and canola (Brassica napus), suggesting that TTM2 is involved in immunity in a wide variety of plant species. This indicates the possible usage of TTM2 knockout mutants for agricultural applications to generate pathogen-resistant crop plants.

Nitrogen metabolism meets phytopathology

Nitrogen (N) is essential for life and is a major limiting factor of plant growth. Because soils frequently lack sufficient N, large quantities of inorganic N fertilizers are added to soils for crop production. However, nitrate, urea, and ammonium are a major source of global pollution, because much of the N that is not taken up by plants enters streams, groundwater, and lakes, where it affects algal production and causes an imbalance in aquatic food webs. Many agronomical data indicate that the higher use of N fertilizers during the green revolution had an impact on the incidence of crop diseases. In contrast, examples in which a decrease in N fertilization increases disease severity are also reported, indicating that there is a complex relationship linking N uptake and metabolism and the disease infection processes. Thus, although it is clear that N availability affects disease, the underlying mechanisms remain unclear. The aim of this review is to describe current knowledge of the mechanisms that link plant N status to the plant's response to pathogen infection and to the virulence and nutritional status of phytopathogens.

Arabidopsis flowering locus D influences systemic-acquired-resistance- induced expression and histone modifications of WRKY genes

A plant that is in part infected by a pathogen is more resistant throughout its whole body to subsequent infections--a phenomenon known as systemic acquired resistance (SAR). Mobile signals are synthesized at the site of infection and distributed throughout the plant through vascular tissues. Mechanism of SAR development subsequent to reaching the mobile signal in the distal tissue is largely unknown. Recently we showed that flowering locus D (FLD) gene of Arabidopsis thaliana is required in the distal tissue to activate SAR. FLD codes for a homologue of human-lysine-specific histone demethylase. Here we show that FLD function is required for priming (SAR induced elevated expression during challenge inoculation) of WRKY29 and WRKY6 genes. FLD also differentially influences basal and SAR-induced expression of WRKY38, WRKY65 and WRKY53 genes. In addition, we also show that FLD partly localizes in nucleus and influences histone modifications at the promoters of WRKY29 and WRKY6 genes. The results altogether indicate to the possibility of FLD's involvement in epigenetic regulation of SAR.

Free radical-mediated systemic immunity in plants

Systemic acquired resistance (SAR) is a form of defense that protects plants against a broad-spectrum of secondary infections by related or unrelated pathogens. SAR related research has witnessed considerable progress in recent years and a number of chemical signals and proteins contributing to SAR have been identified. All of these diverse constituents share their requirement for the phytohormone salicylic acid, an essential downstream component of the SAR pathway. However, recent work demonstrating the essential parallel functioning of nitric oxide (NO)-derived and reactive oxygen species (ROS)-derived signaling together with SA provides important new insights in the overlapping pathways leading to SAR. This review discusses the potential significance of branched pathways and the relative contributions of NO/ROS-derived and SA-derived pathways in SAR.

Signaling by small metabolites in systemic acquired resistance

Plants can retain the memory of a prior encounter with a pest. This memory confers upon a plant the ability to subsequently activate defenses more robustly when challenged by a pest. In plants that have retained the memory of a prior, localized, foliar infection by a pathogen, the pathogen-free distal organs develop immunity against subsequent infections by a broad-spectrum of pathogens. The long-term immunity conferred by this mechanism, which is termed systemic acquired resistance (SAR), is inheritable over a few generations. Signaling mediated by the phenolic metabolite salicylic acid (SA) is critical for the manifestation of SAR. Recent studies have described the involvement of additional small metabolites in SAR signaling, including methyl salicylate, the abietane diterpenoid dehydroabietinal, the lysine catabolite pipecolic acid, a glycerol-3-phosphate-dependent factor and the dicarboxylic acid azelaic acid. Many of these metabolites can be systemically transported through the plant and probably facilitate communication by the primary infected tissue with the distal tissues, which is essential for the activation of SAR. Some of these metabolites have been implicated in the SAR-associated rapid activation of defenses in response to subsequent exposure to the pathogen, a mechanism termed priming. Here, we summarize the role of these signaling metabolites in SAR, and the relationship between them and SA signaling in SAR.

The Arabidopsis malectin-like leucine-rich repeat receptor-like kinase IOS1 associates with the pattern recognition receptors FLS2 and EFR and is critical for priming of pattern-triggered immunity

Plasma membrane-localized pattern recognition receptors such as FLAGELLIN SENSING2 (FLS2) and EF-TU RECEPTOR (EFR) recognize microbe-associated molecular patterns (MAMPs) to activate the first layer of plant immunity termed pattern-triggered immunity (PTI). A reverse genetics approach with genes responsive to the priming agent β-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hypersusceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants demonstrated defective PTI responses, notably delayed upregulation of PTI marker genes, lower callose deposition, and mitogen-activated protein kinase activities upon bacterial infection or MAMP treatment. Moreover, Arabidopsis lines overexpressing IOS1 were more resistant to P. syringae and demonstrated a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, coimmunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and FLS2 and EFR. IOS1 also associated with BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) in a ligand-independent manner and positively regulated FLS2/BAK1 complex formation upon MAMP treatment. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a regulatory protein of FLS2- and EFR-mediated signaling that primes PTI activation upon bacterial elicitation.

Spatial H2O2 signaling specificity: H2O2 from chloroplasts and peroxisomes modulates the plant transcriptome differentially

Hydrogen peroxide (H2O2) operates as a signaling molecule in eukaryotes, but the specificity of its signaling capacities remains largely unrevealed. Here, we analyzed whether a moderate production of H2O2 from two different plant cellular compartments has divergent effects on the plant transcriptome. Arabidopsis thaliana overexpressing glycolate oxidase in the chloroplast (Fahnenstich et al., 2008; Balazadeh et al., 2012) and plants deficient in peroxisomal catalase (Queval et al., 2007; Inzé et al., 2012) were grown under non-photorespiratory conditions and then transferred to photorespiratory conditions to foster the production of H2O2 in both organelles. We show that H2O2 originating in a specific organelle induces two types of responses: one that integrates signals independently from the subcellular site of H2O2 production and another that is dependent on the H2O2 production site. H2O2 produced in peroxisomes induces transcripts involved in protein repair responses, while H2O2 produced in chloroplasts induces early signaling responses, including transcription factors and biosynthetic genes involved in production of secondary signaling messengers. There is a significant bias towards the induction of genes involved in responses to wounding and pathogen attack by chloroplastic-produced H2O2, including indolic glucosinolates-, camalexin-, and stigmasterol-biosynthetic genes. These transcriptional responses were accompanied by the accumulation of 4-methoxy-indol-3-ylmethyl glucosinolate and stigmasterol.

Preparing to fight back: generation and storage of priming compounds

Immune-stimulated plants are able to respond more rapidly and adequately to various biotic stresses allowing them to efficiently combat an infection. During the priming phase, plant are stimulated in absence of a challenge, and can accumulate and store conjugates or precursors of molecules as well as other compounds that play a role in defense. These molecules can be released during the defensive phase following stress. These metabolites can also participate in the first stages of the stress perception. Here, we report the metabolic changes occuring in primed plants during the priming phase. β-aminobutyric acid (BABA) causes a boost of the primary metabolism through the tricarboxylic acids (TCA) such as citrate, fumarate, (S)-malate and 2-oxoglutarate, and the potentiation of phenylpropanoid biosynthesis and the octodecanoic pathway. On the contrary, Pseudomonas syringae pv tomato (PstAvrRpt2) represses the same pathways. Both systems used to prime plants share some common signals like the changes in the synthesis of amino acids and the production of SA and its glycosides, as well as IAA. Interestingly, a product of the purine catabolism, xanthosine, was found to accumulate following both BABA- and PstAvrRpt2-treatement. The compounds that are strongly affected in this stage are called priming compounds, since their effect on the metabolism of the plant is to induce the production of primed compounds that will help to combat the stress. At the same time, additional identified metabolites suggest the possible defense pathways that plants are using to get ready for the battle.

Salicylic acid signaling inhibits apoplastic reactive oxygen species signaling

BACKGROUND

Reactive oxygen species (ROS) are used by plants as signaling molecules during stress and development. Given the amount of possible challenges a plant face from their environment, plants need to activate and prioritize between potentially conflicting defense signaling pathways. Until recently, most studies on signal interactions have focused on phytohormone interaction, such as the antagonistic relationship between salicylic acid (SA)-jasmonic acid and cytokinin-auxin.

RESULTS

In this study, we report an antagonistic interaction between SA signaling and apoplastic ROS signaling. Treatment with ozone (O3) leads to a ROS burst in the apoplast and induces extensive changes in gene expression and elevation of defense hormones. However, Arabidopsis thaliana dnd1 (defense no death1) exhibited an attenuated response to O3. In addition, the dnd1 mutant displayed constitutive expression of defense genes and spontaneous cell death. To determine the exact process which blocks the apoplastic ROS signaling, double and triple mutants involved in various signaling pathway were generated in dnd1 background. Simultaneous elimination of SA-dependent and SA-independent signaling components from dnd1 restored its responsiveness to O3. Conversely, pre-treatment of plants with SA or using mutants that constitutively activate SA signaling led to an attenuation of changes in gene expression elicited by O3.

CONCLUSIONS

Based upon these findings, we conclude that plants are able to prioritize the response between ROS and SA via an antagonistic action of SA and SA signaling on apoplastic ROS signaling.

Plant perception of β-aminobutyric acid is mediated by an aspartyl-tRNA synthetase

Specific chemicals can prime the plant immune system for augmented defense. β-aminobutyric acid (BABA) is a priming agent that provides broad-spectrum disease protection. However, BABA also suppresses plant growth when applied in high doses, which has hampered its application as a crop defense activator. Here we describe a mutant of Arabidopsis thaliana that is impaired in BABA-induced disease immunity (ibi1) but is hypersensitive to BABA-induced growth repression. IBI1 encodes an aspartyl-tRNA synthetase. Enantiomer-specific binding of the R enantiomer of BABA to IBI1 primed the protein for noncanonical defense signaling in the cytoplasm after pathogen attack. This priming was associated with aspartic acid accumulation and tRNA-induced phosphorylation of translation initiation factor eIF2α. However, mutation of eIF2α-phosphorylating GCN2 kinase did not affect BABA-induced immunity but relieved BABA-induced growth repression. Hence, BABA-activated IBI1 controls plant immunity and growth via separate pathways. Our results open new opportunities to separate broad-spectrum disease resistance from the associated costs on plant growth.