Molecular aspects of defence priming

Arabidopsis thaliana FLOWERING LOCUS D is required for systemic acquired resistance

Localized infection in plants often induces systemic acquired resistance (SAR), which provides long-term protection against subsequent infections. A signal originating in the SAR-inducing organ is transported to the distal organs, where it stimulates salicylic acid (SA) accumulation and priming, a mechanism that results in more robust activation of defenses in response to subsequent pathogen infection. In recent years, several metabolites that promote long-distance SAR signaling have been identified. However, the mechanism or mechanisms by which plants perceive and respond to the SAR signals are largely obscure. Here, we show that, in Arabidopsis thaliana, the FLOWERING LOCUS D (FLD) is required for responding to the SAR signals leading to the systemic accumulation of SA and enhancement of disease resistance. Although the fld mutant was competent in accumulating the SAR-inducing signal, it was unable to respond to the SAR signal that accumulates in petiole exudates of wild-type leaves inoculated with a SAR-inducing pathogen. Supporting FLD's role in systemic SAR signaling, we observed that dehydroabietinal and azelaic acid, two metabolites that, in wild-type plants, promote SAR-associated systemic accumulation of SA and priming, respectively, were unable to promote SAR in the fld mutant. FLD also participates in flowering, where it functions to repress expression of the flowering repressor FLOWERING LOCUS C (FLC). However, epistasis analysis indicates that FLD's function in SAR is independent of FLC.

Induced fungal resistance to insect grazing: reciprocal fitness consequences and fungal gene expression in the Drosophila-Aspergillus model system

BACKGROUND

Fungi are key dietary resources for many animals. Fungi, in consequence, have evolved sophisticated physical and chemical defences for repelling and impairing fungivores. Expression of such defences may entail costs, requiring diversion of energy and nutrients away from fungal growth and reproduction. Inducible resistance that is mounted after attack by fungivores may allow fungi to circumvent the potential costs of defence when not needed. However, no information exists on whether fungi display inducible resistance. We combined organism and fungal gene expression approaches to investigate whether fungivory induces resistance in fungi.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that grazing by larval fruit flies, Drosophila melanogaster, induces resistance in the filamentous mould, Aspergillus nidulans, to subsequent feeding by larvae of the same insect. Larval grazing triggered the expression of various putative fungal resistance genes, including the secondary metabolite master regulator gene laeA. Compared to the severe pathological effects of wild type A. nidulans, which led to 100% insect mortality, larval feeding on a laeA loss-of-function mutant resulted in normal insect development. Whereas the wild type fungus recovered from larval grazing, larvae eradicated the chemically deficient mutant. In contrast, mutualistic dietary yeast, Saccharomyces cerevisiae, reached higher population densities when exposed to Drosophila larval feeding.

CONCLUSIONS/SIGNIFICANCE

Our study presents novel evidence that insect grazing is capable of inducing resistance to further grazing in a filamentous fungus. This phenotypic shift in resistance to fungivory is accompanied by changes in the expression of genes involved in signal transduction, epigenetic regulation and secondary metabolite biosynthesis pathways. Depending on reciprocal insect-fungus fitness consequences, fungi may be selected for inducible resistance to maintain high fitness in fungivore-rich habitats. Induced fungal defence responses thus need to be included if we wish to have a complete conception of animal-fungus co-evolution, fungal gene regulation, and multitrophic interactions.

Training innate immunity: the changing concept of immunological memory in innate host defence

The inability of innate immunity to build an immunological memory is considered a main difference with adaptive immunity. This concept has been challenged by studies in plants, invertebrates and mammals. Recently, a paradigm shift in our understanding host defence has been triggered by the mounting evidence for innate immune memory, leading to increased responses to secondary infections. Important differences between the cell populations and the molecular mechanisms exist between the adaptive traits of innate host defence on the one hand and immunological memory of adaptive immunity on the other hand. The lasting state of enhanced innate immunity termed 'trained immunity' is mediated by prototypical innate immune cells such as natural killer cells and monocytes/macrophages. It provides protection against reinfection in a T/B-cell-independent manner, with both specific mechanisms and nonspecific epigenetic reprogramming mediating these effects. This concept represents a paradigm change in immunity, and its putative role in resistance to reinfection may represent the next step in the design of future vaccines.

Systemic signaling during plant defense

Systemic acquired resistance (SAR) is a type of pathogen-induced broad-spectrum resistance in plants. During SAR, primary infection-induced rapid generation and transportation of mobile signal(s) 'prepare' the rest of the plant for subsequent infections. Several, seemingly unrelated, mobile chemical inducers of SAR have been identified, at least two of which function in a feed-back regulatory loop with a lipid transfer-like protein. Signal(s) perception in the systemic tissues relies on the presence of an intact cuticle, the waxy layer covering all aerial parts of the plant. SAR results in chromatin modifications, which prime systemic tissues for enhanced and rapid signaling derived from salicylic acid, which along with its signaling components is key for SAR induction. This review summarizes recent findings related to SAR signal generation, movement, and perception.

Piriformospora indica root colonization triggers local and systemic root responses and inhibits secondary colonization of distal roots

Piriformosporaindica is a basidiomycete fungus colonizing roots of a wide range of higher plants, including crop plants and the model plant Arabidopsis thaliana. Previous studies have shown that P. indica improves growth, and enhances systemic pathogen resistance in leaves of host plants. To investigate systemic effects within the root system, we established a hydroponic split-root cultivation system for Arabidopsis. Using quantitative real-time PCR, we show that initial P. indica colonization triggers a local, transient response of several defense-related transcripts, of which some were also induced in shoots and in distal, non-colonized roots of the same plant. Systemic effects on distal roots included the inhibition of secondary P. indica colonization. Faster and stronger induction of defense-related transcripts during secondary inoculation revealed that a P. indica pretreatment triggers root-wide priming of defense responses, which could cause the observed reduction of secondary colonization levels. Secondary P. indica colonization also induced defense responses in distant, already colonized parts of the root. Endophytic fungi therefore trigger a spatially specific response in directly colonized and in systemic root tissues of host plants.

Hyperosmotic priming of Arabidopsis seedlings establishes a long-term somatic memory accompanied by specific changes of the epigenome

BACKGROUND

In arid and semi-arid environments, drought and soil salinity usually occur at the beginning and end of a plant's life cycle, offering a natural opportunity for the priming of young plants to enhance stress tolerance in mature plants. Chromatin marks, such as histone modifications, provide a potential molecular mechanism for priming plants to environmental stresses, but whether transient exposure of seedlings to hyperosmotic stress leads to chromatin changes that are maintained throughout vegetative growth remains unclear.

RESULTS

We have established an effective protocol for hyperosmotic priming in the model plant Arabidopsis, which includes a transient mild salt treatment of seedlings followed by an extensive period of growth in control conditions. Primed plants are identical to non-primed plants in growth and development, yet they display reduced salt uptake and enhanced drought tolerance after a second stress exposure. ChIP-seq analysis of four histone modifications revealed that the priming treatment altered the epigenomic landscape; the changes were small but they were specific for the treated tissue, varied in number and direction depending on the modification, and preferentially targeted transcription factors. Notably, priming leads to shortening and fractionation of H3K27me3 islands. This effect fades over time, but is still apparent after a ten day growth period in control conditions. Several genes with priming-induced differences in H3K27me3 showed altered transcriptional responsiveness to the second stress treatment.

CONCLUSION

Experience of transient hyperosmotic stress by young plants is stored in a long-term somatic memory comprising differences of chromatin status, transcriptional responsiveness and whole plant physiology.

Priming of antiherbivore defensive responses in plants

Defense priming is defined as increased readiness of defense induction. A growing body of literature indicates that plants (or intact parts of a plant) are primed in anticipation of impending environmental stresses, both biotic and abiotic, and upon the following stimulus, induce defenses more quickly and strongly. For instance, some plants previously exposed to herbivore-inducible plant volatiles (HIPVs) from neighboring plants under herbivore attack show faster or stronger defense activation and enhanced insect resistance when challenged with secondary insect feeding. Research on priming of antiherbivore defense has been limited to the HIPV-mediated mechanism until recently, but significant advances were made in the past three years, including non-HIPV-mediated defense priming, epigenetic modifications as the molecular mechanism of priming, and others. It is timely to consider the advances in research on defense priming in the plant-insect interactions.

Changes of phenolic secondary metabolite profiles in the reaction of narrow leaf lupin (Lupinus angustifolius) plants to infections with Colletotrichum lupini fungus or treatment with its toxin

Plant interactions with environmental factors cause changes in the metabolism and regulation of biochemical and physiological processes. Plant defense against pathogenic microorganisms depends on an innate immunity system that is activated as a result of infection. There are two mechanisms of triggering this system: basal immunity activated as a result of a perception of microbe-associated molecular patterns through pattern recognition receptors situated on the cell surface and effector-triggered immunity (ETI). An induced biosynthesis of bioactive secondary metabolites, in particular phytoalexins, is one of the mechanisms of plant defense to fungal infection. Results of the study on narrow leaf lupin (Lupinus angustifolius L.) plants infected with the anthracnose fungus Colletotrichum lupini and treated with fungal phytotoxic metabolites are described in the paper. The C. lupini phytotoxins were isolated from liquid cultures, purified and partially characterized with physicochemical methods. Accumulation of secondary metabolites on leaf surface and within the tissues of plants either infected, treated with the fungal phytotoxin or submitted to both treatments was studied using GC-MS and LC-MS, respectively. Substantial differences in isoflavone aglycones and glycoconjugate profiles occurred in response to different ways of plant treatment.

A small jab - a big effect: nonspecific immunomodulation by vaccines

Recent epidemiological studies have shown that, in addition to disease-specific effects, vaccines against infectious diseases have nonspecific effects on the ability of the immune system to handle other pathogens. For instance, in randomized trials tuberculosis and measles vaccines are associated with a substantial reduction in overall child mortality, which cannot be explained by prevention of the target disease. New research suggests that the nonspecific effects of vaccines are related to cross-reactivity of the adaptive immune system with unrelated pathogens, and to training of the innate immune system through epigenetic reprogramming. Hence, epidemiological findings are backed by immunological data. This generates a new understanding of the immune system and about how it can be modulated by vaccines to impact the general resistance to disease.

Label-free quantitative proteomic analysis of systemic responses to local wounding and virus infection in Arabidopsis thaliana

Plants are continuously exposed to changing environmental conditions and must, as sessile organisms, possess sophisticated acclimative mechanisms. To gain insight into systemic responses to local virus infection or wounding, we performed comparative LC-MS/MS protein profiling of distal, virus-free leaves four and five days after local inoculation of Arabidopsis thaliana plants with either Oilseed rape mosaic virus (ORMV) or inoculation buffer alone. Our study revealed biomarkers for systemic signaling in response to wounding and compatible virus infection in Arabidopsis, which should prove useful in further addressing the trigger-specific systemic response network and the elusive systemic signals. We observed responses common to ORMV and mock treatment as well as protein profile changes that are specific to local virus infection or mechanical wounding (mock treatment) alone, which provides evidence for the existence of more than one systemic signal to induce these distinct changes. Comparison of the systemic responses between time points indicated that the responses build up over time. Our data indicate stress-specific changes in proteins involved in jasmonic and abscisic acid signaling, intracellular transport, compartmentalization of enzyme activities, protein folding and synthesis, and energy and carbohydrate metabolism. In addition, a virus-triggered systemic signal appears to suppress antiviral host defense.

Hexanoic acid is a resistance inducer that protects tomato plants against Pseudomonas syringae by priming the jasmonic acid and salicylic acid pathways

Hexanoic acid-induced resistance (Hx-IR) is effective against several pathogens in tomato plants. Our study of the mechanisms implicated in Hx-IR against Pseudomonas syringae pv. tomato DC3000 suggests that hexanoic acid (Hx) treatment counteracts the negative effect of coronatine (COR) and jasmonyl-isoleucine (JA-Ile) on the salicylic acid (SA) pathway. In Hx-treated plants, an increase in the expression of jasmonic acid carboxyl methyltransferase (JMT) and the SA marker genes PR1 and PR5 indicates a boost in this signalling pathway at the expense of a decrease in JA-Ile. Moreover, Hx treatment potentiates 12-oxo-phytodienoic acid accumulation, which suggests that this molecule might play a role per se in Hx-IR. These results support a positive relationship between the SA and JA pathways in Hx-primed plants. Furthermore, one of the mechanisms of virulence mediated by COR is stomatal re-opening on infection with P. syringae. In this work, we observed that Hx seems to inhibit stomatal opening in planta in the presence of COR, which suggests that, on infection in tomato, this treatment suppresses effector action to prevent bacterial entry into the mesophyll.

Microwounding is a pivotal factor for the induction of actin-dependent penetration resistance against fungal attack

Induced penetration resistance is triggered by failed penetration attempts of nonpathogenic fungi. The resistance mechanism is an important nonhost reaction in plants that can block the invasion of filamentous pathogens such as fungi and oomycetes. However, it remains unclear whether the mechanical stimuli accompanying fungal penetration play a role in induced penetration resistance, whereas the perforation of the cell wall may provide significant stimuli to plant cells. Here, we used microneedles or biolistic bombardment to mimic fungal penetration pegs and a micromanipulation transfer technique of the bio-probe, a germling of Blumeria graminis hordei, to the wounded cells to demonstrate that microwounds derived from fungal penetration attempts may trigger induced penetration resistance in plant cells. When preinoculated with the nonpathogenic fungi Erysiphe pisi and Colletotrichum orbiculare, which were unable to penetrate a barley cell, the penetration of a bio-probe that was transferred by micromanipulation onto the same cell was completely blocked. Fungal penetration was essential to the triggering of induced penetration resistance because a penetration-peg-defective mutant of C. orbiculare completely lacked the ability to trigger resistance. The artificial microwounds significantly, but not completely, blocked the penetration of the bio-probe. Treatment with the actin polymerization inhibitor cytochalasin A or expression of the actin depolymerizing protein HvPro1 caused complete ablation of the induced penetration resistance triggered by either failed fungal penetration or artificial microwounds. These results strongly suggest that microwounding may trigger actin-dependent induced penetration resistance. Manipulation of induced penetration resistance may be a promising target to improve basic disease resistance in plants.

Towards establishing broad-spectrum disease resistance in plants: silicon leads the way

Plants are constantly threatened by a wide array of microbial pathogens. Pathogen invasion can lead to vast yield losses and the demand for sustainable plant-protection strategies has never been greater. Chemical plant activators and selected strains of rhizobacteria can increase resistance against specific types of pathogens but these treatments are often ineffective or even cause susceptibility against others. Silicon application is one of the scarce examples of a treatment that effectively induces broad-spectrum disease resistance. The prophylactic effect of silicon is considered to be the result of both passive and active defences. Although the phenomenon has been known for decades, very little is known about the molecular basis of silicon-afforded disease control. By combining knowledge on how silicon interacts with cell metabolism in diatoms and plants, this review describes silicon-induced regulatory mechanisms that might account for broad-spectrum plant disease resistance. Priming of plant immune responses, alterations in phytohormone homeostasis, regulation of iron homeostasis, silicon-driven photorespiration and interaction with defence signalling components all are potential mechanisms involved in regulating silicon-triggered resistance responses. Further elucidating how silicon exerts its beneficial properties may create new avenues for developing plants that are better able to withstand multiple attackers.

The Arabidopsis elongator complex subunit2 epigenetically regulates plant immune responses

The Arabidopsis thaliana Elongator complex subunit2 (ELP2) genetically interacts with NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), a key transcription coactivator of plant immunity, and regulates the induction kinetics of defense genes. However, the mechanistic relationship between ELP2 and NPR1 and how ELP2 regulates the kinetics of defense gene induction are unclear. Here, we demonstrate that ELP2 is an epigenetic regulator required for pathogen-induced rapid transcriptome reprogramming. We show that ELP2 functions in a transcriptional feed-forward loop regulating both NPR1 and its target genes. An elp2 mutation increases the total methylcytosine number, reduces the average methylation levels of methylcytosines, and alters (increases or decreases) methylation levels of specific methylcytosines. Interestingly, infection of plants with the avirulent bacterial pathogen Pseudomonas syringae pv tomato DC3000/avrRpt2 induces biphasic changes in DNA methylation levels of NPR1 and PHYTOALEXIN DEFICIENT4 (PAD4), which encodes another key regulator of plant immunity. These dynamic changes are blocked by the elp2 mutation, which is correlated with delayed induction of NPR1 and PAD4. The elp2 mutation also reduces basal histone acetylation levels in the coding regions of several defense genes. Together, our data demonstrate a new role for Elongator in somatic DNA demethylation/methylation and suggest a function for Elongator-mediated chromatin regulation in pathogen-induced transcriptome reprogramming.

Organ-specific alterations in tobacco transcriptome caused by the PVX-derived P25 silencing suppressor transgene

BACKGROUND

RNA silencing affects a broad range of regulatory processes in all eukaryotes ranging from chromatin structure maintenance to transcriptional and translational regulation and longevity of the mRNAs. Particularly in plants, it functions as the major defense mechanism against viruses. To counter-act this defense, plant viruses produce suppressors of RNA silencing (Viral suppressors of RNA silencing, VSRSs), which are essential for viruses to invade their specific host plants. Interactions of these VSRSs with the hosts' silencing pathways, and their direct and indirect interference with different cellular regulatory networks constitute one of the main lines of the molecular virus-host interactions. Here we have used a microarray approach to study the effects of the Potato virus X Potexvirus (PVX)-specific P25 VSRS protein on the transcript profile of tobacco plants, when expressed as a transgene in these plants.

RESULTS

The expression of the PVX-specific P25 silencing suppressor in transgenic tobacco plants caused significant up-regulation of 1350 transcripts, but down-regulation of only five transcripts in the leaves, and up- and down-regulation of 51 and 13 transcripts, respectively, in the flowers of these plants, as compared to the wild type control plants. Most of the changes occurred in the transcripts related to biotic and abiotic stresses, transcription regulation, signaling, metabolic pathways and cell wall modifications, and many of them appeared to be induced through up-regulation of the signaling pathways regulated by ethylene, jasmonic acid and salicylic acid. Correlations of these alterations with the protein profile and related biological functions were analyzed. Surprisingly, they did not cause significant alterations in the protein profile, and caused only very mild alteration in the phenotype of the P25-expressing transgenic plants.

CONCLUSION

Expression of the PVX-specific P25 VSRS protein causes major alterations in the transcriptome of the leaves of transgenic tobacco plants, but very little of any effects in the young flowers of the same plants. The fairly stable protein profile in the leaves and lack of any major changes in the plant phenotype indicate that the complicated interplay and interactions between different regulatory levels are able to maintain homeostasis in the plants.

Long distance movement of DIR1 and investigation of the role of DIR1-like during systemic acquired resistance in Arabidopsis

DIR1 is a lipid transfer protein (LTP) postulated to complex with and/or chaperone a signal(s) to distant leaves during Systemic Acquired Resistance (SAR) in Arabidopsis. DIR1 was detected in phloem sap-enriched petiole exudates collected from wild-type leaves induced for SAR, suggesting that DIR1 gains access to the phloem for movement from the induced leaf. Occasionally the defective in induced resistance1 (dir1-1) mutant displayed a partially SAR-competent phenotype and a DIR1-sized band in protein gel blots was detected in dir1-1 exudates suggesting that a highly similar protein, DIR1-like (At5g48490), may contribute to SAR. Recombinant protein studies demonstrated that DIR1 polyclonal antibodies recognize DIR1 and DIR1-like. Homology modeling of DIR1-like using the DIR1-phospholipid crystal structure as template, provides clues as to why the dir1-1 mutant is rarely SAR-competent. The contribution of DIR1 and DIR1-like during SAR was examined using an Agrobacterium-mediated transient expression-SAR assay and an estrogen-inducible DIR1-EGFP/dir1-1 line. We provide evidence that upon SAR induction, DIR1 moves down the leaf petiole to distant leaves. Our data also suggests that DIR1-like displays a reduced capacity to move to distant leaves during SAR and this may explain why dir1-1 is occasionally SAR-competent.

Long-distance communication and signal amplification in systemic acquired resistance

Systemic acquired resistance (SAR) is an inducible defense mechanism in plants that confers enhanced resistance against a variety of pathogens. SAR is activated in the uninfected systemic (distal) organs in response to a prior (primary) infection elsewhere in the plant. SAR is associated with the activation of salicylic acid (SA) signaling and the priming of defense responses for robust activation in response to subsequent infections. The activation of SAR requires communication by the primary infected tissues with the distal organs. The vasculature functions as a conduit for the translocation of factors that facilitate long-distance intra-plant communication. In recent years, several metabolites putatively involved in long-distance signaling have been identified. These include the methyl ester of SA (MeSA), the abietane diterpenoid dehydroabietinal (DA), the dicarboxylic acid azelaic acid (AzA), and a glycerol-3-phosphate (G3P)-dependent factor. Long-distance signaling by some of these metabolites also requires the lipid-transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE 1). The relative contribution of these factors in long-distance signaling is likely influenced by environmental conditions, for example light. In the systemic leaves, the AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1)-dependent production of the lysine catabolite pipecolic acid (Pip), FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) signaling, as well as SA synthesis and downstream signaling are required for the activation of SAR. This review summarizes the involvement and interaction between long-distance SAR signals and details the recently discovered role of Pip in defense amplification and priming that allows plants to acquire immunity at the systemic level. Recent advances in SA signaling and perception are also highlighted.

Long-distance communication and signal amplification in systemic acquired resistance

Systemic acquired resistance (SAR) is an inducible defense mechanism in plants that confers enhanced resistance against a variety of pathogens. SAR is activated in the uninfected systemic (distal) organs in response to a prior (primary) infection elsewhere in the plant. SAR is associated with the activation of salicylic acid (SA) signaling and the priming of defense responses for robust activation in response to subsequent infections. The activation of SAR requires communication by the primary infected tissues with the distal organs. The vasculature functions as a conduit for the translocation of factors that facilitate long-distance intra-plant communication. In recent years, several metabolites putatively involved in long-distance signaling have been identified. These include the methyl ester of SA (MeSA), the abietane diterpenoid dehydroabietinal (DA), the dicarboxylic acid azelaic acid (AzA), and a glycerol-3-phosphate (G3P)-dependent factor. Long-distance signaling by some of these metabolites also requires the lipid-transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE 1). The relative contribution of these factors in long-distance signaling is likely influenced by environmental conditions, for example light. In the systemic leaves, the AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1)-dependent production of the lysine catabolite pipecolic acid (Pip), FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) signaling, as well as SA synthesis and downstream signaling are required for the activation of SAR. This review summarizes the involvement and interaction between long-distance SAR signals and details the recently discovered role of Pip in defense amplification and priming that allows plants to acquire immunity at the systemic level. Recent advances in SA signaling and perception are also highlighted.

Priming of the Arabidopsis pattern-triggered immunity response upon infection by necrotrophic Pectobacterium carotovorum bacteria

Boosted responsiveness of plant cells to stress at the onset of pathogen- or chemically induced resistance is called priming. The chemical β-aminobutyric acid (BABA) enhances Arabidopsis thaliana resistance to hemibiotrophic bacteria through the priming of the salicylic acid (SA) defence response. Whether BABA increases Arabidopsis resistance to the necrotrophic bacterium Pectobacterium carotovorum ssp. carotovorum (Pcc) is not clear. In this work, we show that treatment with BABA protects Arabidopsis against the soft-rot pathogen Pcc. BABA did not prime the expression of the jasmonate/ethylene-responsive gene PLANT DEFENSIN 1.2 (PDF1.2), the up-regulation of which is usually associated with resistance to necrotrophic pathogens. Expression of the SA marker gene PATHOGENESIS RELATED 1 (PR1) on Pcc infection was primed by BABA treatment, but SA-defective mutants demonstrated a wild-type level of BABA-induced resistance against Pcc. BABA primed the expression of the pattern-triggered immunity (PTI)-responsive genes FLG22-INDUCED RECEPTOR-LIKE KINASE 1 (FRK1), ARABIDOPSIS NON-RACE SPECIFIC DISEASE RESISTANCE GENE (NDR1)/HAIRPIN-INDUCED GENE (HIN1)-LIKE 10 (NHL10) and CYTOCHROME P450, FAMILY 81 (CYP81F2) after inoculation with Pcc or after treatment with purified bacterial microbe-associated molecular patterns, such as flg22 or elf26. PTI-mediated callose deposition was also potentiated in BABA-treated Arabidopsis, and BABA boosted Arabidopsis stomatal immunity to Pcc. BABA treatment primed the PTI response in the SA-defective mutants SA induction deficient 2-1 (sid2-1) and phytoalexin deficient 4-1 (pad4-1). In addition, BABA priming was associated with open chromatin configurations in the promoter region of PTI marker genes. Our data indicate that BABA primes the PTI response upon necrotrophic bacterial infection and suggest a role for the PTI response in BABA-induced resistance.

The proteome response of potato leaves to priming agents and S-nitrosoglutathione

The primed mobilization for more potent defense responses to subsequent stress has been shown for many plant species, but there is a growing need to identify reliable molecular markers for this unique phenomenon. In the present study a proteomic approach was used to screen similarities in protein abundance in leaves of primed potato (Solanum tuberosum L.) treated with four well-known inducers of plant resistance, i.e. β-aminobutyric acid (BABA), γ-aminobutyric acid (GABA), Laminarin and 2,6-dichloroisonicotinic acid (INA), respectively. Moreover, to gain insight into the importance of nitric oxide (NO) in primed protein accumulation the potato leaves were supplied by S-nitrosoglutathione (GSNO), as an NO donor. The comparative analysis, using two-dimensional electrophoresis and mass spectrometry, revealed that among 25 proteins accumulated specifically after BABA, GABA, INA and Laminarin treatments, 13 proteins were accumulated also in response to GSNO. Additionally, overlapping proteomic changes between BABA-primed and GSNO-treated leaves showed 5 protein spots absent in the proteome maps obtained in response to the other priming agents. The identified 18 proteins belonged, in most cases, to functional categories of primary metabolism. The selected proteins including three redox-regulated enzymes, i.e. glyceraldehyde 3-phosphate dehydrogenase, carbonic anhydrase, and fructose-bisphosphate aldolase, were discussed in relation to the plant defence responses. Taken together, the overlapping effects in the protein profiles obtained between priming agents, GSNO and cPTIO treatments provide insight indicating that the primed potato exhibits unique changes in the primary metabolism, associated with selective protein modification via NO.

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

Metabolic signals orchestrate plant defenses against microbial pathogen invasion. Here, we report the identification of the non-protein amino acid pipecolic acid (Pip), a common Lys catabolite in plants and animals, as a critical regulator of inducible plant immunity. Following pathogen recognition, Pip accumulates in inoculated Arabidopsis thaliana leaves, in leaves distal from the site of inoculation, and, most specifically, in petiole exudates from inoculated leaves. Defects of mutants in AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) in systemic acquired resistance (SAR) and in basal, specific, and β-aminobutyric acid-induced resistance to bacterial infection are associated with a lack of Pip production. Exogenous Pip complements these resistance defects and increases pathogen resistance of wild-type plants. We conclude that Pip accumulation is critical for SAR and local resistance to bacterial pathogens. Our data indicate that biologically induced SAR conditions plants to more effectively synthesize the phytoalexin camalexin, Pip, and salicylic acid and primes plants for early defense gene expression. Biological priming is absent in the pipecolate-deficient ald1 mutants. Exogenous pipecolate induces SAR-related defense priming and partly restores priming responses in ald1. We conclude that Pip orchestrates defense amplification, positive regulation of salicylic acid biosynthesis, and priming to guarantee effective local resistance induction and the establishment of SAR.

Epigenetics of host-pathogen interactions: the road ahead and the road behind

A growing body of evidence points towards epigenetic mechanisms being responsible for a wide range of biological phenomena, from the plasticity of plant growth and development to the nutritional control of caste determination in honeybees and the etiology of human disease (e.g., cancer). With the (partial) elucidation of the molecular basis of epigenetic variation and the heritability of certain of these changes, the field of evolutionary epigenetics is flourishing. Despite this, the role of epigenetics in shaping host-pathogen interactions has received comparatively little attention. Yet there is plenty of evidence supporting the implication of epigenetic mechanisms in the modulation of the biological interaction between hosts and pathogens. The phenotypic plasticity of many key parasite life-history traits appears to be under epigenetic control. Moreover, pathogen-induced effects in host phenotype may have transgenerational consequences, and the bases of these changes and their heritability probably have an epigenetic component. The significance of epigenetic modifications may, however, go beyond providing a mechanistic basis for host and pathogen plasticity. Epigenetic epidemiology has recently emerged as a promising area for future research on infectious diseases. In addition, the incorporation of epigenetic inheritance and epigenetic plasticity mechanisms to evolutionary models and empirical studies of host-pathogen interactions will provide new insights into the evolution and coevolution of these associations. Here, we review the evidence available for the role epigenetics on host-pathogen interactions, and the utility and versatility of the epigenetic technologies available that can be cross-applied to host-pathogen studies. We conclude with recommendations and directions for future research on the burgeoning field of epigenetics as applied to host-pathogen interactions.

Downy mildew resistance induced by Trichoderma harzianum T39 in susceptible grapevines partially mimics transcriptional changes of resistant genotypes

BACKGROUND

Downy mildew, caused by Plasmopara viticola, is one of the most severe diseases of grapevine and is commonly controlled by fungicide treatments. The beneficial microorganism Trichoderma harzianum T39 (T39) can induce resistance to downy mildew, although the molecular events associated with this process have not yet been elucidated in grapevine. A next generation RNA sequencing (RNA-Seq) approach was used to study global transcriptional changes associated with resistance induced by T39 in Vitis vinifera Pinot Noir leaves. The long-term aim was to develop strategies to optimize the use of this agent for downy mildew control.

RESULTS

More than 14.8 million paired-end reads were obtained for each biological replicate of T39-treated and control leaf samples collected before and 24 h after P. viticola inoculation. RNA-Seq analysis resulted in the identification of 7,024 differentially expressed genes, highlighting the complex transcriptional reprogramming of grapevine leaves during resistance induction and in response to pathogen inoculation. Our data show that T39 has a dual effect: it directly modulates genes related to the microbial recognition machinery, and it enhances the expression of defence-related processes after pathogen inoculation. Whereas several genes were commonly affected by P. viticola in control and T39-treated plants, opposing modulation of genes related to responses to stress and protein metabolism was found. T39-induced resistance partially inhibited some disease-related processes and specifically activated defence responses after P. viticola inoculation, causing a significant reduction of downy mildew symptoms.

CONCLUSIONS

The global transcriptional analysis revealed that defence processes known to be implicated in the reaction of resistant genotypes to downy mildew were partially activated by T39-induced resistance in susceptible grapevines. Genes identified in this work are an important source of markers for selecting novel resistance inducers and for the analysis of environmental conditions that might affect induced resistance mechanisms.

Nitric oxide-mediated stress imprint in potato as an effect of exposure to a priming agent

We investigated how potato exposed to a chemical agent could activate nitric oxide (NO)-dependent events facilitating more potent defense responses to a subsequent pathogen attack. Obtained data revealed that all applied inducers, i.e., β-aminobutyric acid (BABA), γ-aminobutyric acid (GABA), laminarin, or 2,6-dichloroisonicotinic acid (INA), were active stimuli in potentiating NO synthesis in the primed potato. It is assumed, for the mechanism proposed in this paper, that priming involves reversible S-nitrosylated protein (S-nitrosothiols [SNO]) storage as one of the short-term stress imprint components, apart from epigenetic changes sensitized by NO. Based on BABA- and GABA-induced events, it should be stated that a rise in NO generation and coding the NO message in SNO storage at a relatively low threshold together with histone H2B upregulation might create short-term imprint activation, facilitating acquisition of a competence to react faster after challenge inoculation. Laminarin elicited strong NO upregulation with an enhanced SNO pool-altered biochemical imprint in the form of less effective local recall, nevertheless being fully protective in distal responses against P. infestans. In turn, INA showed the most intensified NO generation and abundant formation of SNO, both after the inducer treatment and challenge inoculation abolishing potato resistance against the pathogen. Our results indicate, for the first time, that a precise control of synthesized NO in cooperation with reversible SNO storage and epigenetic modifications might play an important role in integrating and coordinating defense potato responses in the priming phenomenon.

Oxidative and nitrosative-based signaling and associated post-translational modifications orchestrate the acclimation of citrus plants to salinity stress

Reactive oxygen and nitrogen species are involved in a plethora of cellular responses in plants; however, our knowledge on the outcomes of oxidative and nitrosative signaling is still unclear. To better understand how oxidative and nitrosative signals are integrated to regulate cellular adjustments to external conditions, local and systemic responses were investigated in the roots and leaves of sour orange plants (Citrus aurantium L.) after root treatment with hydrogen peroxide (H(2) O(2) ) or sodium nitroprusside (a nitric oxide donor), followed by NaCl stress for 8 days. Phenotypic and physiological data showed that pre-exposure to these treatments induced an acclimation to subsequent salinity stress that was accompanied by both local and systemic H(2) O(2) and nitric oxide (NO) accumulation. Combined histochemical and fluorescent probe approaches showed the existence of a vascular-driven long-distance reactive oxygen species and NO signaling pathway. Transcriptional analysis of genes diagnostic for H(2) O(2) and NO signaling just after treatments or after 8 days of salt stress revealed tissue- and time-specific mechanisms controlling internal H(2) O(2) and NO homeostasis. Furthermore, evidence is presented showing that protein carbonylation, nitration and S-nitrosylation are involved in acclimation to salinity stress. In addition, this work enabled characterization of potential carbonylated, nitrated and nitrosylated proteins with distinct or overlapping signatures. This work provides a framework to better understand the oxidative and nitrosative priming network in citrus plants subjected to salinity conditions.

Epigenetic responses to stress: triple defense?

Stressful conditions for plants can originate from numerous physical, chemical and biological factors, and plants have developed a plethora of survival strategies including developmental and morphological adaptations, specific signaling and defense pathways as well as innate and acquired immunity. While it has become clear in recent years that many stress responses involve epigenetic components, we are far from understanding the mechanisms and molecular interactions. Extending our knowledge is fundamental, not least for plant breeding and conservation biology. This review will highlight recent insights into epigenetic stress responses at the level of signaling, chromatin modification, and potentially heritable consequences.

The epigenetic machinery controlling transgenerational systemic acquired resistance

Progeny from diseased Arabidopsis shows enhanced resistance, which is associated with priming of defense genes. This transgenerational systemic acquired resistance (SAR) is effective against biotrophic pathogens, such as the downy mildew pathogen Hyaloperonospora arabidopsidis. In this study, we have examined mutants in RNA-directed DNA methylation (RdDM) for transgenerational SAR. Our analysis suggests that transgenerational SAR is regulated by the RdDM pathway and transmitted by hypomethylation at CpNpG sites.

Sugars and plant innate immunity

Sugars are involved in many metabolic and signalling pathways in plants. Sugar signals may also contribute to immune responses against pathogens and probably function as priming molecules leading to pathogen-associated molecular patterns (PAMP)-triggered immunity and effector-triggered immunity in plants. These putative roles also depend greatly on coordinated relationships with hormones and the light status in an intricate network. Although evidence in favour of sugar-mediated plant immunity is accumulating, more in-depth fundamental research is required to unravel the sugar signalling pathways involved. This might pave the way for the use of biodegradable sugar-(like) compounds to counteract plant diseases as cheaper and safer alternatives for toxic agrochemicals.

Genome-Wide Characterization of ISR Induced in Arabidopsis thaliana by Trichoderma hamatum T382 Against Botrytis cinerea Infection

In this study, the molecular basis of the induced systemic resistance (ISR) in Arabidopsis thaliana by the biocontrol fungus Trichoderma hamatum T382 against the phytopathogen Botrytis cinerea B05-10 was unraveled by microarray analysis both before (ISR-prime) and after (ISR-boost) additional pathogen inoculation. The observed high numbers of differentially expressed genes allowed us to classify them according to the biological pathways in which they are involved. By focusing on pathways instead of genes, a holistic picture of the mechanisms underlying ISR emerged. In general, a close resemblance is observed between ISR-prime and systemic acquired resistance, the systemic defense response that is triggered in plants upon pathogen infection leading to increased resistance toward secondary infections. Treatment with T. hamatum T382 primes the plant (ISR-prime), resulting in an accelerated activation of the defense response against B. cinerea during ISR-boost and a subsequent moderation of the B. cinerea induced defense response. Microarray results were validated for representative genes by qRT-PCR. The involvement of various defense-related pathways was confirmed by phenotypic analysis of mutants affected in these pathways, thereby proving the validity of our approach. Combined with additional anthocyanin analysis data these results all point to the involvement of the phenylpropanoid pathway in T. hamatum T382-induced ISR.

Dissecting phosphite-induced priming in Arabidopsis infected with Hyaloperonospora arabidopsidis

Phosphite (Phi), a phloem-mobile oxyanion of phosphorous acid (H(3)PO(3)), protects plants against diseases caused by oomycetes. Its mode of action is unclear, as evidence indicates both direct antibiotic effects on pathogens as well as inhibition through enhanced plant defense responses, and its target(s) in the plants is unknown. Here, we demonstrate that the biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) exhibits an unusual biphasic dose-dependent response to Phi after inoculation of Arabidopsis (Arabidopsis thaliana), with characteristics of indirect activity at low doses (10 mm or less) and direct inhibition at high doses (50 mm or greater). The effect of low doses of Phi on Hpa infection was nullified in salicylic acid (SA)-defective plants (sid2-1, NahG) and in a mutant impaired in SA signaling (npr1-1). Compromised jasmonate (jar1-1) and ethylene (ein2-1) signaling or abscisic acid (aba1-5) biosynthesis, reactive oxygen generation (atrbohD), or accumulation of the phytoalexins camalexin (pad3-1) and scopoletin (f6'h1-1) did not affect Phi activity. Low doses of Phi primed the accumulation of SA and Pathogenesis-Related protein1 transcripts and mobilized two essential components of basal resistance, Enhanced Disease Susceptibility1 and Phytoalexin Deficient4, following pathogen challenge. Compared with inoculated, Phi-untreated plants, the gene expression, accumulation, and phosphorylation of the mitogen-activated protein kinase MPK4, a negative regulator of SA-dependent defenses, were reduced in plants treated with low doses of Phi. We propose that Phi negatively regulates MPK4, thus priming SA-dependent defense responses following Hpa infection.

Broad-spectrum acquired resistance in barley induced by the Pseudomonas pathosystem shares transcriptional components with Arabidopsis systemic acquired resistance

Inducible resistance responses play a central role in the defense of plants against pathogen attack. Acquired resistance (AR) is induced alongside defense toward primary attack, providing broad-spectrum protection against subsequent pathogen challenge. The localization and molecular basis of AR in cereals is poorly understood, in contrast with the well-characterized systemic acquired resistance (SAR) response in Arabidopsis. Here, we use Pseudomonas syringae as a biological inducer of AR in barley, providing a clear frame of reference to the Arabidopsis-P. syringae pathosystem. Inoculation of barley leaf tissue with the nonadapted P. syringae pv. tomato avrRpm1 (PstavrRpm1) induced an active local defense response. Furthermore, inoculation of barley with PstavrRpm1 resulted in the induction of broad-spectrum AR at a distance from the local lesion, "adjacent" AR, effective against compatible isolates of P. syringae and Magnaporthe oryzae. Global transcriptional profiling of this adjacent AR revealed similarities with the transcriptional profile of SAR in Arabidopsis, as well as transcripts previously associated with chemically induced AR in cereals, suggesting that AR in barley and SAR in Arabidopsis may be mediated by analogous pathways.

Stress-induced chromatin changes: a critical view on their heritability

The investigation of stress responses has been a focus of plant research, breeding and biotechnology for a long time. Insight into stress perception, signaling and genetic determinants of resistance has recently been complemented by growing evidence for substantial stress-induced changes at the chromatin level. These affect specific sequences or occur genome-wide and are often correlated with transcriptional regulation. The majority of these changes only occur during stress exposure, and both expression and chromatin states typically revert to the pre-stress state shortly thereafter. Other changes result in the maintenance of new chromatin states and modified gene expression for a longer time after stress exposure, preparing an individual for developmental decisions or more effective defence. Beyond this, there are claims for stress-induced heritable chromatin modifications that are transmitted to progeny, thereby improving their characteristics. These effects resemble the concept of Lamarckian inheritance of acquired characters and represent a challenge to the uniqueness of DNA sequence-based inheritance. However, with the growing insight into epigenetic regulation and transmission of chromatin states, it is worth investigating these phenomena carefully. While genetic changes (mainly transposon mobility) in response to stress-induced interference with chromatin are well documented and heritable, in our view there is no unambiguous evidence for transmission of exclusively chromatin-controlled stress effects to progeny. We propose a set of criteria that should be applied to substantiate the data for stress-induced, chromatin-encoded new traits. Well-controlled stress treatments, thorough phenotyping and application of refined genome-wide epigenetic analysis tools should be helpful in moving from interesting observations towards robust evidence.

Plasticity as a plastic response: how submergence-induced leaf elongation in Rumex palustris depends on light and nutrient availability in its early life stage

Plants may experience different environmental cues throughout their development which interact in determining their phenotype. This paper tests the hypothesis that environmental conditions experienced early during ontogeny affect the phenotypic response to subsequent environmental cues. This hypothesis was tested by exposing different accessions of Rumex palustris to different light and nutrient conditions, followed by subsequent complete submergence. Final leaf length and submergence-induced plasticity were affected by the environmental conditions experienced at early developmental stages. In developmentally older leaves, submergence-induced elongation was lower in plants previously subjected to high-light conditions. Submergence-induced elongation of developmentally younger leaves, however, was larger when pregrown in high light. High-light and low-nutrient conditions led to an increase of nonstructural carbohydrates in the plants. There was a positive correlation between submergence-induced leaf elongation and carbohydrate concentration and content in roots and shoots, but not with root and shoot biomass before submergence. These results show that conditions experienced by young plants modulate the responses to subsequent environmental conditions, in both magnitude and direction. Internal resource status interacts with cues perceived at different developmental stages in determining plastic responses to the environment.

A fruitful chromatin harvest: meeting summary of the Second European Workshop on Plant Chromatin 2011 in Versailles, France

In September 2011, the Second European Workshop on Plant Chromatin took place in Versailles, France. The workshop covered a range of topics related to plant chromatin biology, including regulation of gene expression by Polycomb group proteins, chromatin dynamics, reconfiguration of epigenetic marks in response to various cues and chromatin assembly. Here, we summarize some of the highlights discussed during the meeting.

Multiple exposures to drought 'train' transcriptional responses in Arabidopsis

Pre-exposure to stress may alter plants' subsequent responses by producing faster and/or stronger reactions implying that plants exercise a form of 'stress memory'. The mechanisms of plants' stress memory responses are poorly understood leaving this fundamental biological question unanswered. Here we show that during recurring dehydration stresses Arabidopsis plants display transcriptional stress memory demonstrated by an increase in the rate of transcription and elevated transcript levels of a subset of the stress-response genes (trainable genes). During recovery (watered) states, trainable genes produce transcripts at basal (preinduced) levels, but remain associated with atypically high H3K4me3 and Ser5P polymerase II levels, indicating that RNA polymerase II is stalled. This is the first example of a stalled RNA polymerase II and its involvement in transcriptional memory in plants. These newly discovered phenomena might be a general feature of plant stress-response systems and could lead to novel approaches for increasing the flexibility of a plant's ability to respond to the environment.

Mobility of Transgenic Nucleic Acids and Proteins within Grafted Rootstocks for Agricultural Improvement

Grafting has been used in agriculture for over 2000 years. Disease resistance and environmental tolerance are highly beneficial traits that can be provided through use of grafting, although the mechanisms, in particular for resistance, have frequently been unknown. As information emerges that describes plant disease resistance mechanisms, the proteins, and nucleic acids that play a critical role in disease management can be expressed in genetically engineered (GE) plant lines. Utilizing transgrafting, the combination of a GE rootstock with a wild-type (WT) scion, or the reverse, has the potential to provide pest and pathogen resistance, impart biotic and abiotic stress tolerance, or increase plant vigor and productivity. Of central importance to these potential benefits is the question of to what extent nucleic acids and proteins are transmitted across a graft junction and whether the movement of these molecules will affect the efficacy of the transgrafting approach. Using a variety of specific examples, this review will report on the movement of organellar DNA, RNAs, and proteins across graft unions. Attention will be specifically drawn to the use of small RNAs and gene silencing within transgrafted plants, with a particular focus on pathogen resistance. The use of GE rootstocks or scions has the potential to extend the horticultural utility of grafting by combining this ancient technique with the molecular strategies of the modern era.

Next-generation systemic acquired resistance

Systemic acquired resistance (SAR) is a plant immune response to pathogen attack. Recent evidence suggests that plant immunity involves regulation by chromatin remodeling and DNA methylation. We investigated whether SAR can be inherited epigenetically following disease pressure by Pseudomonas syringae pv tomato DC3000 (PstDC3000). Compared to progeny from control-treated Arabidopsis (Arabidopsis thaliana; C(1)), progeny from PstDC3000-inoculated Arabidopsis (P(1)) were primed to activate salicylic acid (SA)-inducible defense genes and were more resistant to the (hemi)biotrophic pathogens Hyaloperonospora arabidopsidis and PstDC3000. This transgenerational SAR was sustained over one stress-free generation, indicating an epigenetic basis of the phenomenon. Furthermore, P(1) progeny displayed reduced responsiveness of jasmonic acid (JA)-inducible genes and enhanced susceptibility to the necrotrophic fungus Alternaria brassicicola. This shift in SA- and JA-dependent gene responsiveness was not associated with changes in corresponding hormone levels. Instead, chromatin immunoprecipitation analyses revealed that SA-inducible promoters of PATHOGENESIS-RELATED GENE1, WRKY6, and WRKY53 in P(1) plants are enriched with acetylated histone H3 at lysine 9, a chromatin mark associated with a permissive state of transcription. Conversely, the JA-inducible promoter of PLANT DEFENSIN1.2 showed increased H3 triple methylation at lysine 27, a mark related to repressed gene transcription. P(1) progeny from the defense regulatory mutant non expressor of PR1 (npr1)-1 failed to develop transgenerational defense phenotypes, demonstrating a critical role for NPR1 in expression of transgenerational SAR. Furthermore, the drm1drm2cmt3 mutant that is affected in non-CpG DNA methylation mimicked the transgenerational SAR phenotype. Since PstDC3000 induces DNA hypomethylation in Arabidopsis, our results suggest that transgenerational SAR is transmitted by hypomethylated genes that direct priming of SA-dependent defenses in the following generations.

A deletion in NRT2.1 attenuates Pseudomonas syringae-induced hormonal perturbation, resulting in primed plant defenses

For an efficient defense response against pathogens, plants must coordinate rapid genetic reprogramming to produce an incompatible interaction. Nitrate Trasnporter2 (NRT2) gene family members are sentinels of nitrate availability. In this study, we present an additional role for NRT2.1 linked to plant resistance against pathogens. This gene antagonizes the priming of plant defenses against the bacterial pathogen Pseudomonas syringae pv tomato DC3000 (Pst). The nrt2 mutant (which is deficient in two genes, NRT2.1 and NRT2.2) displays reduced susceptibility to this bacterium. We demonstrate that modifying environmental conditions that stimulate the derepression of the NRT2.1 gene influences resistance to Pst independently of the total level of endogenous nitrogen. Additionally, hormonal homeostasis seemed to be affected in nrt2, which displays priming of salicylic acid signaling and concomitant irregular functioning of the jasmonic acid and abscisic acid pathways upon infection. Effector-triggered susceptibility and hormonal perturbation by the bacterium seem to be altered in nrt2, probably due to reduced sensitivity to the bacterial phytotoxin coronatine. The main genetic and metabolic targets of coronatine in Arabidopsis (Arabidopsis thaliana) remain largely unstimulated in nrt2 mutants. In addition, a P. syringae strain defective in coronatine synthesis showed the same virulence toward nrt2 as the coronatine-producing strain. Taken together, the reduced susceptibility of nrt2 mutants seems to be a combination of priming of salicylic acid-dependent defenses and reduced sensitivity to the bacterial effector coronatine. These results suggest additional functions for NRT2.1 that may influence plant disease resistance by down-regulating biotic stress defense mechanisms and favoring abiotic stress responses.

Descendants of primed Arabidopsis plants exhibit resistance to biotic stress

An attack of plants by pathogens or treatment with certain resistance-inducing compounds can lead to the establishment of a unique primed state of defense. Primed plants show enhanced defense reactions upon further challenge with biotic or abiotic stress. Here, we report that the primed state in Arabidopsis (Arabidopsis thaliana) is still functional in the next generation without additional treatment. We compared the reactions of Arabidopsis plants that had been either primed with β-amino-butyric acid (BABA) or with an avirulent isolate of the bacteria Pseudomonas syringae pv tomato (PstavrRpt2). The descendants of primed plants showed a faster and higher accumulation of transcripts of defense-related genes in the salicylic acid signaling pathway and enhanced disease resistance upon challenge inoculation with a virulent isolate of P. syringae. In addition, the progeny of primed plants was also more resistant against the oomycete pathogen Hyaloperonospora arabidopsidis. When transgenerationally primed plants were subjected to an additional priming treatment, their descendants displayed an even stronger primed phenotype, suggesting that plants can inherit a sensitization for the priming phenomenon. Interestingly, this primed to be primed phenotype was much reduced in the Arabidopsis β-amino-butyric acid priming mutant ibs1 (induced BABA sterility1). Our results demonstrate that the primed state of plants is transferred to their progeny and confers improved protection from pathogen attack as compared to the descendants of unprimed plants.