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

Metabolomic approach to search for fungal resistant forms of Aegilops tauschii Coss. from the VIR collection

Broadening of the genetic diversity of donors of resistance to biotic environmental factors is a challenging problem concerning Triticum L., which can be solved by using wild relatives of wheat, in particular, Aegilops tauschii Coss., in breeding programs. This species, believed to be the donor of D genome of common wheat (T. aestivum L.), is a source of some traits important for breeding. This greatly facilitates the possibility of crossing Ae. tauschii with common wheat. Aegilops L. species are donors of effective genes for resistance to fungal diseases in wheat. For instance, genes that determine resistance to rust agents in common wheat were successfully introgressed from Ae. tauschii into the genome of T. aestivum L. The aim of our study was to identify differences in metabolomic profiles of Ae. tauschii forms (genotypes), resistant or susceptible to such fungal pathogens as Puccinia triticina f. sp. tritici and Erysiphe graminis f. sp. tritici. These indicators may be used as biochemical markers of resistance. A comparative analysis of groups of Ae. tauschii accessions showed that metabolomic profiles of the forms with or without resistance to fungal pathogens differed significantly in the contents of nonproteinogenic amino acids, polyols, phytosterols, acylglycerols, mono- and oligosaccharides, glycosides, phenolic compounds (hydroquinone, kempferol), etc. This fact was consistent with the previously obtained data on the relationship between Fusarium resistance in oats (Avena sativa L.) and certain components of the metabolomic profile, such as acylglycerols, nonproteinogenic amino acids, galactinol, etc. Thus, our studies once again confirmed the possibility and effectiveness of the use of metabolomic analysis for screening the genetic diversity of accessions in the VIR collection, of Ae. tauschii in particular, in order to identify forms with a set of compounds in their metabolomic profile, which characterize them as resistant. Ae. tauschii accessions with a high content of pipecolic acids, acylglycerols, galactinol, stigmasterol, glycerol, azelaic and pyrogallic acids, campesterol, hydroquinone, etc., can be used for creating wheat and triticale cultivars with high resistance to fungal pathogens causing powdery mildew, brown rust, and yellow rust.

Metabolomic Fingerprinting of Potato Cultivars Differing in Susceptibility to Spongospora subterranea f. sp. subterranea Root Infection

Plants defend themselves from pathogens by producing bioactive defense chemicals. The biochemical mechanisms relating to quantitative resistance of potato to root infection by Spongospora subterranea f. sp. subterranea (Sss) are, however, not understood, and are not efficiently utilized in potato breeding programs. Untargeted metabolomics using ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) was used to elucidate the biochemical mechanisms of susceptibility to Sss root infection. Potato roots and root exudate metabolic profiles of five tolerant cultivars were compared with those of five susceptible cultivars, following Sss inoculation, to identify tolerance-related metabolites. Comparison of the relative metabolite abundance of tolerant versus susceptible cultivars revealed contrasting responses to Sss infection. Metabolites belonging to amino acids, organic acids, fatty acids, phenolics, and sugars, as well as well-known cell wall thickening compounds were putatively identified and were especially abundant in the tolerant cultivars relative to the susceptible cultivars. Metabolites known to activate plant secondary defense metabolism were significantly increased in the tolerant cultivars compared to susceptible cultivars following Sss inoculation. Root-exuded compounds belonging to the chemical class of phenolics were also found in abundance in the tolerant cultivars compared to susceptible cultivars. This study illustrated that Sss infection of potato roots leads to differential expression of metabolites in tolerant and susceptible potato cultivars.

DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN 2 of Arabidopsis thaliana is a negative regulator of local and systemic acquired resistance

To fine tune defense response output, plants recruit both positive and negative regulators. Here we report Arabidopsis DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN 2(DAP2) gene as a negative regulator of basal defense against virulent bacterial pathogens. Expression of DAP2 enhances upon pathogen inoculation. Our experiments show that DAP2 suppressed resistance against virulent strains of bacterial pathogens, pathogen-induced callose deposition, and ROS accumulation; however, it did not influence effector-triggered immunity. In addition, DAP2 negatively regulated systemic acquired resistance (SAR). DAP2 expression was enhanced in the pathogen-free systemic tissues of SAR-induced plants. Previously, Arabidopsis Flowering locus D (FLD) gene has been shown to be essential for SAR but not for local resistance. We show here that FLD function is necessary for SAR-induced expression of DAP2, suggesting DAP2 as a target of FLD for activation of SAR in Arabidopsis.

Arabidopsis Response Regulator 6 (ARR6) Modulates Plant Cell-Wall Composition and Disease Resistance

The cytokinin signaling pathway, which is mediated by Arabidopsis response regulator (ARR) proteins, has been involved in the modulation of some disease-resistance responses. Here, we describe novel functions of ARR6 in the control of plant disease-resistance and cell-wall composition. Plants impaired in ARR6 function (arr6) were more resistant and susceptible, respectively, to the necrotrophic fungus Plectosphaerella cucumerina and to the vascular bacterium Ralstonia solanacearum, whereas Arabidopsis plants that overexpress ARR6 showed the opposite phenotypes, which further support a role of ARR6 in the modulation of disease-resistance responses against these pathogens. Transcriptomics and metabolomics analyses revealed that, in arr6 plants, canonical disease-resistance pathways, like those activated by defensive phytohormones, were not altered, whereas immune responses triggered by microbe-associated molecular patterns were slightly enhanced. Cell-wall composition of arr6 plants was found to be severely altered compared with that of wild-type plants. Remarkably, pectin-enriched cell-wall fractions extracted from arr6 walls triggered more intense immune responses than those activated by similar wall fractions from wild-type plants, suggesting that arr6 pectin fraction is enriched in wall-related damage-associated molecular patterns, which trigger immune responses. This work supports a novel function of ARR6 in the control of cell-wall composition and disease resistance and reinforces the role of the plant cell wall in the modulation of specific immune responses.

The plant cuticle regulates apoplastic transport of salicylic acid during systemic acquired resistance

The plant cuticle is often considered a passive barrier from the environment. We show that the cuticle regulates active transport of the defense hormone salicylic acid (SA). SA, an important regulator of systemic acquired resistance (SAR), is preferentially transported from pathogen-infected to uninfected parts via the apoplast. Apoplastic accumulation of SA, which precedes its accumulation in the cytosol, is driven by the pH gradient and deprotonation of SA. In cuticle-defective mutants, increased transpiration and reduced water potential preferentially routes SA to cuticle wax rather than to the apoplast. This results in defective long-distance transport of SA, which in turn impairs distal accumulation of the SAR-inducer pipecolic acid. High humidity reduces transpiration to restore systemic SA transport and, thereby, SAR in cuticle-defective mutants. Together, our results demonstrate that long-distance mobility of SA is essential for SAR and that partitioning of SA between the symplast and cuticle is regulated by transpiration.

Plant Dynamic Metabolic Response to Bacteriophage Treatment After Xanthomonas campestris pv. campestris Infection

Periodic epidemics of black rot disease occur worldwide causing substantial yield losses. Xanthomonas campestris pv. campestris (Xcc) represents one of the most common bacteria able to cause the above disease in cruciferous plants such as broccoli, cabbage, cauliflower, and Arabidopsis thaliana. In agriculture, several strategies are being developed to contain the Xanthomonas infection. The use of bacteriophages could represent a valid and efficient approach to overcome this widespread phenomenon. Several studies have highlighted the potential usefulness of implementing phage therapy to control plant diseases as well as Xcc infection. In the present study, we characterized the effect of a lytic phage on the plant Brassica oleracea var. gongylodes infected with Xcc and, for the first time, the correlated plant metabolic response. The results highlighted the potential benefits of bacteriophages: reduction of bacterium proliferation, alteration of the biofilm structure and/or modulation of the plant metabolism and defense response.

Effect of pulsed light on postharvest disease control-related metabolomic variation in melon (Cucumis melo) artificially inoculated with Fusarium pallidoroseum

Pulsed light, as a postharvest technology, is an alternative to traditional fungicides, and can be used on a wide variety of fruit and vegetables for sanitization or pathogen control. In addition to these applications, other effects also are detected in vegetal cells, including changes in metabolism and secondary metabolite production, which directly affect disease control response mechanisms. This study aimed to evaluate pulsed ultraviolet light in controlling postharvest rot, caused by Fusarium pallidoroseum in 'Spanish' melon, in natura, and its implications in disease control as a function of metabolomic variation to fungicidal or fungistatic effects. The dose of pulsed light (PL) that inhibited F. pallidoroseum growth in melons (Cucumis melo var. Spanish) was 9 KJ m-2. Ultra-performance liquid chromatography (UPLC) coupled to a quadrupole-time-of-flight (QTOF) mass analyzer identified 12 compounds based on tandem mass spectrometry (MS/MS) fragmentation patterns. Chemometric analysis by Principal Components Analysis (PCA) and Orthogonal Partial Least Squared Discriminant Analysis (OPLS-DA) and corresponding S-Plot were used to evaluate the changes in fruit metabolism. PL technology provided protection against postharvest disease in melons, directly inhibiting the growth of F. pallidoroseum through the upregulation of specific fruit biomarkers such as pipecolic acid (11), saponarin (7), and orientin (3), which acted as major markers for the defense system against pathogens. PL can thus be proposed as a postharvest technology to prevent chemical fungicides and may be applied to reduce the decay of melon quality during its export and storage.

The “Green” FMOs: Diversity, Functionality and Application of Plant Flavoproteins

Flavin-dependent monooxygenases (FMOs) are ancient enzymes present in all kingdoms of life. FMOs typically catalyze the incorporation of an oxygen atom from molecular oxygen into small molecules. To date, the majority of functional characterization studies have been performed on mammalian, fungal and bacterial FMOs, showing that they play fundamental roles in drug and xenobiotic metabolism. By contrast, our understanding of FMOs across the plant kingdom is very limited, despite plants possessing far greater FMO diversity compared to both bacteria and other multicellular organisms. Here, we review the progress of plant FMO research, with a focus on FMO diversity and functionality. Significantly, of the FMOs characterized to date, they all perform oxygenation reactions that are crucial steps within hormone metabolism, pathogen resistance, signaling and chemical defense. This demonstrates the fundamental role FMOs have within plant metabolism, and presents significant opportunities for future research pursuits and downstream applications.

JMJ14 encoded H3K4 demethylase modulates immune responses by regulating defence gene expression and pipecolic acid levels

Epigenetic modifications have emerged as an important mechanism underlying plant defence against pathogens. We examined the role of JMJ14, a Jumonji (JMJ) domain-containing H3K4 demethylase, in local and systemic plant immune responses in Arabidopsis. The function of JMJ14 in local or systemic defence response was investigated by pathogen growth assays and by analysing expression and H3K4me3 enrichments of key defence genes using qPCR and ChIP-qPCR. Salicylic acid (SA) and pipecolic acid (Pip) levels were quantified and function of JMJ14 in SA- and Pip-mediated defences was analysed in Col-0 and jmj14 plants. jmj14 mutants were compromised in both local and systemic defences. JMJ14 positively regulates pathogen-induced H3K4me3 enrichment and expression of defence genes involved in SA- and Pip-mediated defence pathways. Consequently, loss of JMJ14 results in attenuated defence gene expression and reduced Pip accumulation during establishment of systemic acquired resistance (SAR). Exogenous Pip partially restored SAR in jmj14 plants, suggesting that JMJ14 regulated Pip biosynthesis and other downstream factors regulate SAR in jmj14 plants. JMJ14 positively modulates defence gene expressions and Pip levels in Arabidopsis.

β-Aminobutyric Acid Pretreatment Confers Salt Stress Tolerance in Brassica napus L. by Modulating Reactive Oxygen Species Metabolism and Methylglyoxal Detoxification

Salinity is a serious environmental hazard which limits world agricultural production by adversely affecting plant physiology and biochemistry. Hence, increased tolerance against salt stress is very important. In this study, we explored the function of β-aminobutyric acid (BABA) in enhancing salt stress tolerance in rapeseed (Brassica napus L.). After pretreatment with BABA, seedlings were exposed to NaCl (100 and 150 mM) for 2 days. Salt stress increased Na content and decreased K content in shoot and root. It disrupted the antioxidant defense system by producing reactive oxygen species (ROS; H2O2 and O2•-), methylglyoxal (MG) content and causing oxidative stress. It also reduced the growth and photosynthetic pigments of seedlings but increased proline (Pro) content. However, BABA pretreatment in salt-stressed seedlings increased ascorbate (AsA) and glutathione (GSH) contents; GSH/GSSG ratio; and the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II) as well as the growth and photosynthetic pigments of plants. In addition, compared to salt stress alone, BABA increased Pro content, reduced the H2O2, MDA and MG contents, and decreased Na content in root and increased K content in shoot and root of rapeseed seedlings. Our findings suggest that BABA plays a double role in rapeseed seedlings by reducing Na uptake and enhancing stress tolerance through upregulating the antioxidant defense and glyoxalase systems.

N-3-oxo-octanoyl-homoserine lactone-mediated priming of resistance to Pseudomonas syringae requires the salicylic acid signaling pathway in Arabidopsis thaliana

BACKGROUD

Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) to communicate each other and to coordinate their collective behaviors. Recently, accumulating evidence shows that host plants are able to sense and respond to bacterial AHLs. Once primed, plants are in an altered state that enables plant cells to more quickly and/or strongly respond to subsequent pathogen infection or abiotic stress.

RESULTS

In this study, we report that pretreatment with N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) confers resistance against the pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 (PstDC3000) in Arabidopsis. Pretreatment with 3OC8-HSL and subsequent pathogen invasion triggered an augmented burst of hydrogen peroxide, salicylic acid accumulation, and fortified expression of the pathogenesis-related genes PR1 and PR5. Upon PstDC3000 challenge, plants treated with 3OC8-HSL showed increased activities of defense-related enzymes including peroxidase, catalase, phenylalanine ammonialyase, and superoxide dismutase. In addition, the 3OC8-HSL-primed resistance to PstDC3000 in wild-type plants was impaired in plants expressing the bacterial NahG gene and in the npr1 mutant. Moreover, the expression levels of isochorismate synthases (ICS1), a critical salicylic acid biosynthesis enzyme, and two regulators of its expression, SARD1 and CBP60g, were potentiated by 3OC8-HSL pretreatment followed by pathogen inoculation.

CONCLUSIONS

Our data indicate that 3OC8-HSL primes the Arabidopsis defense response upon hemibiotrophic bacterial infection and that 3OC8-HSL-primed resistance is dependent on the SA signaling pathway. These findings may help establish a novel strategy for the control of plant disease.

Dissection of flag leaf metabolic shifts and their relationship with those occurring simultaneously in developing seed by application of non-targeted metabolomics

Rice flag leaves are major source organs providing more than half of the nutrition needed for rice seed development. The dynamic metabolic changes in rice flag leaves and the detailed metabolic relationship between source and sink organs in rice, however, remain largely unknown. In this study, the metabolic changes of flag leaves in two japonica and two indica rice cultivars were investigated using non-targeted metabolomics approach. Principal component analysis (PCA) revealed that flag leaf metabolomes varied significantly depending on both species and developmental stage. Only a few of the metabolites in flag leaves displayed the same change pattern across the four tested cultivars along the process of seed development. Further association analysis found that levels of 45 metabolites in seeds that are associated with human nutrition and health correlated significantly with their levels in flag leaves. Comparison of metabolomics of flag leaves and seeds revealed that some flavonoids were specific or much higher in flag leaves while some lipid metabolites such as phospholipids were much higher in seeds. This reflected not only the function of the tissue specific metabolism but also the different physiological properties and metabolic adaptive features of these two tissues.

Unravelling the Roles of Nitrogen Nutrition in Plant Disease Defences

Nitrogen (N) is one of the most important elements that has a central impact on plant growth and yield. N is also widely involved in plant stress responses, but its roles in host-pathogen interactions are complex as each affects the other. In this review, we summarize the relationship between N nutrition and plant disease and stress its importance for both host and pathogen. From the perspective of the pathogen, we describe how N can affect the pathogen’s infection strategy, whether necrotrophic or biotrophic. N can influence the deployment of virulence factors such as type III secretion systems in bacterial pathogen or contribute nutrients such as gamma-aminobutyric acid to the invader. Considering the host, the association between N nutrition and plant defence is considered in terms of physical, biochemical and genetic mechanisms. Generally, N has negative effects on physical defences and the production of anti-microbial phytoalexins but positive effects on defence-related enzymes and proteins to affect local defence as well as systemic resistance. N nutrition can also influence defence via amino acid metabolism and hormone production to affect downstream defence-related gene expression via transcriptional regulation and nitric oxide (NO) production, which represents a direct link with N. Although the critical role of N nutrition in plant defences is stressed in this review, further work is urgently needed to provide a comprehensive understanding of how opposing virulence and defence mechanisms are influenced by interacting networks.

Biosynthesis and Regulation of Salicylic Acid and N-Hydroxypipecolic Acid in Plant Immunity

Salicylic acid (SA) has long been known to be essential for basal defense and systemic acquired resistance (SAR). N-Hydroxypipecolic acid (NHP), a recently discovered plant metabolite, also plays a key role in SAR and to a lesser extent in basal resistance. Following pathogen infection, levels of both compounds are dramatically increased. Analysis of SA- or SAR-deficient mutants has uncovered how SA and NHP are biosynthesized. The completion of the SA and NHP biosynthetic pathways in Arabidopsis allowed better understanding of how they are regulated. In this review, we discuss recent progress on SA and NHP biosynthesis and their regulation in plant immunity.

Redundant CAMTA Transcription Factors Negatively Regulate the Biosynthesis of Salicylic Acid and N-Hydroxypipecolic Acid by Modulating the Expression of SARD1 and CBP60g

Two signal molecules, salicylic acid (SA) and N-hydroxypipecolic acid (NHP), play critical roles in plant immunity. The biosynthetic genes of both compounds are positively regulated by master immune-regulating transcription factors SARD1 and CBP60g. However, the relationship between the SA and NHP pathways is unclear. CALMODULIN-BINDING TRANSCRIPTION FACTOR 1 (CAMTA1), CAMTA2, and CAMTA3 are known redundant negative regulators of plant immunity, but the underlying mechanism also remains largely unknown. In this study, through chromatin immunoprecipitation and electrophoretic mobility shift assays, we uncovered that CBP60g is a direct target of CAMTA3, which also negatively regulates the expression of SARD1, presumably via an indirect effect. The autoimmunity of camta3-1 is suppressed by sard1 cbp60g double mutant as well as ald1 and fmo1, two single mutants defective in NHP biosynthesis. Interestingly, a suppressor screen conducted in the camta1/2/3 triple mutant background yielded various mutants blocking biosynthesis or signaling of either SA or NHP, leading to nearly complete suppression of the extreme autoimmunity of camta1/2/3, suggesting that the SA and NHP pathways can mutually amplify each other. Together, these results reveal that CAMTAs repress the biosynthesis of SA and NHP by modulating the expression of SARD1 and CBP60g, and that the SA and NHP pathways are coordinated to optimize plant immune response.

Arabidopsis CAMTA Transcription Factors Regulate Pipecolic Acid Biosynthesis and Priming of Immunity Genes

The Arabidopsis thaliana Calmodulin-binding Transcription Activator (CAMTA) transcription factors CAMTA1, CAMTA2, and CAMTA3 (CAMTA123) serve as master regulators of salicylic acid (SA)-mediated immunity, repressing the biosynthesis of SA in healthy plants. Here, we show that CAMTA123 also repress the biosynthesis of pipecolic acid (Pip) in healthy plants. Loss of CAMTA123 function resulted in the induction of AGD2-like defense response protein 1 (ALD1), which encodes an enzyme involved in Pip biosynthesis. Induction of ALD1 resulted in the accumulation of high levels of Pip, which brought about increased levels of the SA receptor protein NPR1 without induction of NPR1 expression or requirement for an increase in SA levels. Pip-mediated induction of ALD1 and genes regulating the biosynthesis of SA-CBP60g, SARD1, PAD4, and EDS1-was largely dependent on NPR1. Furthermore, Pip-mediated increase in NPR1 protein levels was associated with priming of Pip and SA biosynthesis genes to induction by low levels of SA. Taken together, our findings expand the role for CAMTA123 in regulating key immunity genes and suggest a working model whereby loss of CAMTA123 repression leads to the induction of plant defense genes and initiation of SAR.

Translational Regulation of Metabolic Dynamics during Effector-Triggered Immunity

Recent studies have shown that global translational reprogramming is an early activation event in pattern-triggered immunity, when plants recognize microbe-associated molecular patterns. However, it is not fully known whether translational regulation also occurs in subsequent immune responses, such as effector-triggered immunity (ETI). In this study, we performed genome-wide ribosome profiling in Arabidopsis upon RPS2-mediated ETI activation and discovered that specific groups of genes were translationally regulated, mostly in coordination with transcription. These genes encode enzymes involved in aromatic amino acid, phenylpropanoid, camalexin, and sphingolipid metabolism. The functional significance of these components in ETI was confirmed by genetic and biochemical analyses. Our findings provide new insights into diverse translational regulation of plant immune responses and demonstrate that translational coordination of metabolic gene expression is an important strategy for ETI.

The Lifecycle of the Plant Immune System

Throughout their life span, plants confront an endless barrage of pathogens and pests. To successfully defend against biotic threats, plants have evolved a complex immune system responsible for surveillance, perception, and the activation of defense. Plant immunity requires multiple signaling processes, the outcome of which vary according to the lifestyle of the invading pathogen(s). In short, these processes require the activation of host perception, the regulation of numerous signaling cascades, and transcriptome reprograming, all of which are highly dynamic in terms of temporal and spatial scales. At the same time, the development of a single immune event is subjective to the development of plant immune system, which is co-regulated by numerous processes, including plant ontogenesis and the host microbiome. In total, insight into each of these processes provides a fuller understanding of the mechanisms that govern plant-pathogen interactions. In this review, we will discuss the "lifecycle" of plant immunity: the development of individual events of defense, including both local and distal processes, as well as the development and regulation of the overall immune system by ontogenesis regulatory genes and environmental microbiota. In total, we will integrate the output of recent discoveries and theories, together with several hypothetical models, to present a dynamic portrait of plant immunity.

Construction and applications of a B vitamin genetic resource for investigation of vitamin-dependent metabolism in maize

The B vitamins provide essential co-factors for central metabolism in all organisms. In plants, B vitamins have surprising emerging roles in development, stress tolerance and pathogen resistance. Hence, there is a paramount interest in understanding the regulation of vitamin biosynthesis as well as the consequences of vitamin deficiency in crop species. To facilitate genetic analysis of B vitamin biosynthesis and functions in maize, we have mined the UniformMu transposon resource to identify insertional mutations in vitamin pathway genes. A screen of 190 insertion lines for seed and seedling phenotypes identified mutations in biotin, pyridoxine and niacin biosynthetic pathways. Importantly, isolation of independent insertion alleles enabled genetic confirmation of genotype-to-phenotype associations. Because B vitamins are essential for survival, null mutations often have embryo lethal phenotypes that prevent elucidation of subtle, but physiologically important, metabolic consequences of sub-optimal (functional) vitamin status. To circumvent this barrier, we demonstrate a strategy for refined genetic manipulation of vitamin status based on construction of heterozygotes that combine strong and hypomorphic mutant alleles. Dosage analysis of pdx2 alleles in endosperm revealed that endosperm supplies pyridoxine to the developing embryo. Similarly, a hypomorphic bio1 allele enabled analysis of transcriptome and metabolome responses to incipient biotin deficiency in seedling leaves. We show that systemic pipecolic acid accumulation is an early metabolic response to sub-optimal biotin status highlighting an intriguing connection between biotin, lysine metabolism and systemic disease resistance signaling. Seed-stocks carrying insertions for vitamin pathway genes are available for free, public distribution via the Maize Genetics Cooperation Stock Center.

Calcium-dependent protein kinase 5 links calcium signaling with N-hydroxy-l-pipecolic acid- and SARD1-dependent immune memory in systemic acquired resistance

Systemic acquired resistance (SAR) prepares infected plants for faster and stronger defense activation upon subsequent attacks. SAR requires an information relay from primary infection to distal tissue and the initiation and maintenance of a self-maintaining phytohormone salicylic acid (SA)-defense loop. In spatial and temporal resolution, we show that calcium-dependent protein kinase CPK5 contributes to immunity and SAR. In local basal resistance, CPK5 functions upstream of SA synthesis, perception, and signaling. In systemic tissue, CPK5 signaling leads to accumulation of SAR-inducing metabolite N-hydroxy-L-pipecolic acid (NHP) and SAR marker genes, including Systemic Acquired Resistance Deficient 1 (SARD1) Plants of increased CPK5, but not CPK6, signaling display an 'enhanced SAR' phenotype towards a secondary bacterial infection. In the sard1-1 background, CPK5-mediated basal resistance is still mounted, but NHP concentration is reduced and enhanced SAR is lost. The biochemical analysis estimated CPK5 half maximal kinase activity for calcium, K50 [Ca²⁺ ], to be c. 100 nM, close to the cytoplasmic resting level. This low threshold uniquely qualifies CPK5 to decode subtle changes in calcium, a prerequisite to signal relay and onset and maintenance of priming at later time points in distal tissue. Our data explain why CPK5 functions as a hub in basal and systemic plant immunity.

Primary Metabolite Responses to Oxidative Stress in Early-Senescing and Paraquat Resistant Arabidopsis thaliana rcd1 (Radical-Induced Cell Death1)

Rcd1 (radical-induced cell death1) is an Arabidopsis thaliana mutant, which exhibits high tolerance to paraquat [methyl viologen (MV)], herbicide that interrupts photosynthetic electron transport chain causing the formation of superoxide and inhibiting NADPH production in the chloroplast. To understand the biochemical mechanisms of MV-resistance and the role of RCD1 in oxidative stress responses, we performed metabolite profiling of wild type (Col-0) and rcd1 plants in light, after MV exposure and after prolonged darkness. The function of RCD1 has been extensively studied at transcriptomic and biochemical level, but comprehensive metabolite profiling of rcd1 mutant has not been conducted until now. The mutant plants exhibited very different metabolic features from the wild type under light conditions implying enhanced glycolytic activity, altered nitrogen and nucleotide metabolism. In light conditions, superoxide production was elevated in rcd1, but no metabolic markers of oxidative stress were detected. Elevated senescence-associated metabolite marker levels in rcd1 at early developmental stage were in line with its early-senescing phenotype and possible mitochondrial dysfunction. After MV exposure, a marked decline in the levels of glycolytic and TCA cycle intermediates in Col-0 suggested severe plastidic oxidative stress and inhibition of photosynthesis and respiration, whereas in rcd1 the results indicated sustained photosynthesis and respiration and induction of energy salvaging pathways. The accumulation of oxidative stress markers in both plant lines indicated that MV-resistance in rcd1 derived from the altered regulation of cellular metabolism and not from the restricted delivery of MV into the cells or chloroplasts. Considering the evidence from metabolomic, transcriptomic and biochemical studies, we propose that RCD1 has a negative effect on reductive metabolism and rerouting of the energy production pathways. Thus, the altered, highly active reductive metabolism, energy salvaging pathways and redox transfer between cellular compartments in rcd1 could be sufficient to avoid the negative effects of MV-induced toxicity.

A Specific and Sensitive Enzymatic Assay for the Quantitation of L-Proline

Because proline accumulates rapidly in response to several stress conditions such as drought and excess salt, increased intracellular levels of free proline are considered a hallmark of adaptive reactions in plants, particularly in response to water stress. Proline quantitation is easily achievable by reaction with ninhydrin, since under acidic conditions peculiar red or yellow reaction products form with this unique cyclic amino acid. However, little attention has been paid to date to cross-reaction of ninhydrin with other amino acids at high levels, or with structurally related compounds that may also be present at significant concentrations in plant tissues, possibly leading to proline overestimation. In vitro at high pH values, δ¹-pyrroline-5-carboxylate reductase, the enzyme catalyzing the second and last step in proline synthesis from glutamate, was early found to catalyze the reverse oxidation of proline with the concomitant reduction of NAD(P)⁺ to NAD(P)H. Here we characterized this reverse reaction using recombinant enzymes from Arabidopsis thaliana and Oryza sativa, and demonstrated its utility for the specific quantification of L-proline. By optimizing the reaction conditions, fast, easy, and reproducible measurement of L-proline concentration was achieved, with similar sensitivity but higher specificity than the commonly used ninhydrin methods.

Stressed Out About Hormones: How Plants Orchestrate Immunity

Plants are under relentless challenge by pathogenic bacteria, fungi, and oomycetes, for whom they provide a resource of living space and nutrients. Upon detection of pathogens, plants carry out multiple layers of defense response, orchestrated by a tightly organized network of hormones. In this review, we provide an overview of the phytohormones involved in immunity and the ways pathogens manipulate their biosynthesis and signaling pathways. We highlight recent developments, including the discovery of a defense signaling molecule, new insights into hormone biosynthesis, and the increasing importance of signaling hubs at which hormone pathways intersect.

Studies on the constituents of Helleborus purpurascens: use of derivatives from calix[6]arene, homooxacalix[3]arene and homoazacalix[3]arene as extractant agents for amino acids from the aqueous extract

The task of this work was to investigate the extraction capacity of various calixarenes for free and esterified amino acids from aqueous acid phases. Furthermore, this method was applied to aqueous extracts of Helleborus purpurascens. Generally, it is known that calixarenes can be used as extractants for ammonium compounds due to π-cation and lone pair cation interactions. As first, tert-Butyl-calix[6]arene and derivatives thereof were used. They had already proven their worth in previous investigations. In addition, tert-Butyl-hexahomooxa-calix[3]arene was used also, which can also enter into lone pair cation interactions. In addition to these well-known calixarenes, new calixarenes were produced and tested. Based on the tert-Butyl-hexahomooxa-calix[3]arene, a phosphor(III)bridged derivative was prepared, combining the three aromatic hydroxyl groups to a phosphite. As a seldom-described class of calixarenes, tert-Butyl-hexahomoaza-calix[3]arene derivatives were used. The nitrogen analogues of tert-Butyl-hexahomooxa-calix[3]arene could be produced as N-benzyl derivatives. The structure of the esterified carboxymethylated derivative of N,N',N″-Tribenzyl-tert-Butyl-hexahomoaza-calix[3]arene could be verified by X-ray structure analysis. It crystallized as a partial cone. The extraction capacity of the described calixarenes was investigated for amino acids from aqueous acidic solutions into an organic phase. For the testing were chosen asparagine, aspartic acid, tyrosine, tryptophane, phenylalanine and pipecolinic acid and their methyl esters. The amino acids and their methyl esters were dissolved in water at different pH values. The calixarenes were dissolved in dichloromethane (DCM) or chloroform. After this preparation, the aqueous acidic amino acid solutions were mixed with the solutions and shaken intensively. In addition, blank values were determined by extracting the aqueous stock solutions of the amino acids and their methyl esters with pure solvents. To determine the extraction rate, the phases were separated and each analysed using GC-FID, partially GC-MS(EI). The evaluation is performed in two ways. On the one hand the depletion in the aqueous phase and on the other hand the content in the organic phase was determined.

Glycerol-3-phosphate mediates rhizobia-induced systemic signaling in soybean

Glycerol-3-phosphate (G3P) is a well-known mobile regulator of systemic acquired resistance (SAR), which provides broad spectrum systemic immunity in response to localized foliar pathogenic infections. We show that G3P-derived foliar immunity is also activated in response to genetically-regulated incompatible interactions with nitrogen-fixing bacteria. Using gene knock-down we show that G3P is essential for strain-specific exclusion of non-desirable root-nodulating bacteria and the associated foliar pathogen immunity in soybean. Grafting studies show that while recognition of rhizobium incompatibility is root driven, bacterial exclusion requires G3P biosynthesis in the shoot. Biochemical analyses support shoot-to-root transport of G3P during incompatible rhizobia interaction. We describe a root-shoot-root signaling mechanism which simultaneously enables the plant to exclude non-desirable nitrogen-fixing rhizobia in the root and pathogenic microbes in the shoot.

Oxidative post-translational modifications controlling plant-pathogen interaction

Pathogen recognition is linked to the perception of microbe/pathogen-associated molecular patterns triggering a specific and transient accumulation of reactive oxygen species (ROS) at the pathogen attack site. The apoplastic oxidative "burst" generated at the pathogen attack site depends on the ROS-generator systems including enzymes such as plasma membrane NADP (H) oxidases, cell wall peroxidases and lipoxygenase. ROS are cytotoxic molecules that inhibit invading pathogens or signalling molecules that control the local and systemic induction of defence genes. Post-translational modifications induced by ROS are considered as a potential signalling mechanism that can modify protein structure and/or function, localisation and cellular stability. Thus, this review focuses on how ROS are essential molecules regulating the function of proteins involved in the plant response to a pathogen attack through post-translational modifications.

A Role for Tocopherol Biosynthesis in Arabidopsis Basal Immunity to Bacterial Infection

Tocopherols are lipid-soluble antioxidants synthesized in plastids of plants and other photosynthetic organisms. The four known tocopherols, α-, β-, γ-, and δ-tocopherol, differ in number and position of methyl groups on their chromanol head group. In unstressed Arabidopsis (Arabidopsis thaliana) leaves, α-tocopherol constitutes the main tocopherol form, whereas seeds predominantly contain γ-tocopherol. Here, we show that inoculation of Arabidopsis leaves with the bacterial pathogen Pseudomonas syringae induces the expression of genes involved in early steps of tocopherol biosynthesis and triggers strong accumulation of γ-tocopherol, moderate production of δ-tocopherol, and generation of the benzoquinol precursors of tocopherols. The pathogen-inducible biosynthesis of tocopherols is promoted by the immune regulators ENHANCED DISEASE SUSCEPTIBILITY1 and PHYTOALEXIN-DEFICIENT4. In addition, tocopherols accumulate in response to bacterial flagellin and reactive oxygen species. By quantifying tocopherol forms in inoculated wild-type plants and biosynthetic pathway mutants, we provide biochemical insights into the pathogen-inducible tocopherol pathway. Notably, vitamin E deficient2 (vte2) mutant plants, which are compromised in both tocopherol and benzoquinol precursor accumulation, exhibit increased susceptibility toward compatible P. syringae and possess heightened levels of markers of lipid peroxidation after bacterial infection. The deficiency of triunsaturated fatty acids in vte2-1 fatty acid desaturase3-2 (fad3-2) fad7-2 fad8 quadruple mutants prevents increased lipid peroxidation in the vte2 background and restores pathogen resistance to wild-type levels. Therefore, the tocopherol biosynthetic pathway positively influences salicylic acid accumulation and guarantees effective basal resistance of Arabidopsis against compatible P. syringae, possibly by protecting leaves from the pathogen-induced oxidation of trienoic fatty acid-containing lipids.

Gene expression and evidence of coregulation of the production of some metabolites of chilli pepper inoculated with Pectobacterium carotovorum ssp. carotovorum

Chilli pepper (Capsicum annuum L.) is susceptible to Pectobacterium carotovorum subsp. carotovorum (Pcc), the causal agent of soft rot disease in crops. Understanding the molecular principles of systemic acquired resistance, which is poorly understood in chilli pepper, represents an important step towards understanding inducible defence responses and can assist in designing appropriate intervention strategies for crop disease management. Accordingly, we investigated (via real-time PCR and metabolomics profiling) the molecular response of chilli pepper to Pcc by characterisation of the crucial metabolic regulators involved in the establishment of defence response. We profiled 13 key inducible defence response genes, which included MYB transcriptor factor, ethylene response element-binding protein, suppressor of the G2 allele of Skp1, cytochrome P450, small Sar1 (GTPase), hydroxycinnamoyl-CoA:quinate hydroxycinnamoyl transferase, pathogenesis-related protein 1a, endo-1,3-β-glucanase, chitinase, proteinase inhibitor, defensin, coiled-coil-nucleotide-binding site-leucine-rich repeat (CC-NBS-LRR) resistance and phenylalanine ammonia lyase. In addition, we determined metabolomic shifts induced by Pcc in pepper. The PCR results revealed a significant induction of the selected plant defence-related genes in response to Pcc inoculation; the metabolomic profiling showed that of 99 primary metabolites profiled the quantities of acetylcarnitine, adenosine, adenosine 3',5' cyclic monophosphate, guanosine 3',5' cyclic monophosphate and inosine decreased in pepper leaves inoculated with Pcc.

Extracellular pyridine nucleotides trigger plant systemic immunity through a lectin receptor kinase/BAK1 complex

Systemic acquired resistance (SAR) is a long-lasting broad-spectrum plant immunity induced by mobile signals produced in the local leaves where the initial infection occurs. Although multiple structurally unrelated signals have been proposed, the mechanisms responsible for perception of these signals in the systemic leaves are unknown. Here, we show that exogenously applied nicotinamide adenine dinucleotide (NAD⁺) moves systemically and induces systemic immunity. We demonstrate that the lectin receptor kinase (LecRK), LecRK-VI.2, is a potential receptor for extracellular NAD⁺ (eNAD⁺) and NAD⁺ phosphate (eNADP⁺) and plays a central role in biological induction of SAR. LecRK-VI.2 constitutively associates with BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) in vivo. Furthermore, BAK1 and its homolog BAK1-LIKE1 are required for eNAD(P)⁺ signaling and SAR, and the kinase activities of LecR-VI.2 and BAK1 are indispensable to their function in SAR. Our results indicate that eNAD⁺ is a putative mobile signal, which triggers SAR through its receptor complex LecRK-VI.2/BAK1 in Arabidopsis thaliana.

Systemic signals allows plants to mount immune responses in sites that are distal from the local infection site. Here, the authors provide evidence that nicotinamide adenine dinucleotide (NAD ⁺ ) is a potential systemic signal that induces immunity via the lectin receptor kinase LecRK-VI.2 and BAK1.

An engineered pathway for N-hydroxy-pipecolic acid synthesis enhances systemic acquired resistance in tomato

Systemic acquired resistance (SAR) is a powerful immune response that triggers broad-spectrum disease resistance throughout a plant. In the model plant Arabidopsis thaliana, long-distance signaling and SAR activation in uninfected tissues occur without circulating immune cells and instead rely on the metabolite N-hydroxy-pipecolic acid (NHP). Engineering SAR in crop plants would enable external control of a plant's ability to mount a global defense response upon sudden changes in the environment. Such a metabolite-engineering approach would require the molecular machinery for producing and responding to NHP in the crop plant. Here, we used heterologous expression in Nicotiana benthamiana leaves to identify a minimal set of Arabidopsis genes necessary for the biosynthesis of NHP. Local expression of these genes in tomato leaves triggered SAR in distal tissues in the absence of a pathogen, suggesting that the SAR trait can be engineered to enhance a plant's endogenous ability to respond to pathogens. We also showed tomato produces endogenous NHP in response to a bacterial pathogen and that NHP is present across the plant kingdom, raising the possibility that an engineering strategy to enhance NHP-induced defenses could be possible in many crop plants.

A Microbial Fermentation Mixture Primes for Resistance Against Powdery Mildew in Wheat

Since many fungal pathogens develop resistance to fungicides, novel and low-cost alternative methods to improve plant health and fitness need to be developed. An approach to improve productivity in crops is to stimulate the plant's own defence mechanisms via priming. Therefore, we investigated if a fermentation-based elicitor could prime plant defences against powdery mildew in wheat by inducing the expression of endogenous defence-related genes. Wheat seedlings were spray-treated with a fermentation-based elicitor 8 days prior to inoculation with powdery mildew. Disease assays showed a significantly reduced number of powdery mildew pustules were formed on wheat treated with the mixed elicitor. In vitro sensitivity assays tested the ability of powdery mildew conidia to germinate on agar amended with the fermentation-based product and concluded that fungal germination and differentiation were also inhibited. Tissue samples were taken at time points pertaining to different developmental stages of powdery mildew infection. Significantly higher expression of PR genes (PR1, PR4, PR5, and PR9) was observed in the microbial fermentation mixture-treated plants compared with untreated plants. These genes are often associated with the elicitation of plant defence responses to specific biotrophic pathogens, such as powdery mildew, suggesting an elicitor-mediated response in the wheat plants tested. The product components were assessed, and the components were found to act synergistically in the microbial fermentation mixture. Therefore, this fermentation-based elicitor provides an effective method for powdery mildew control.

A new strategy to control Cucumber mosaic virus using fabricated NiO-nanostructures

This study was carried out to fabricate nickel oxide nanostructures (NONS) and to evaluate its ability to control Cucumber mosaic virus (CMV) by direct antiviral activity as well as induction of systemic resistance in treated cucumber plants. The efficacy of nickel oxide nanostructures for control CMV in cucumber plants was biologically evaluated by a reduction in disease severity, reduction in CMV accumulation and expression of regulatory and defense-related genes. Cucumber plants treated with nickel oxide nanostructures showed incredible suppression of CMV infection compared with non-treated plants. The enzyme-linked immunosorbent assay (ELISA) showed a marked reduction in CMV accumulation in cucumber plants treated with nickel oxide nanostructures compared to untreated plants. Based on real-time polymerase chain reaction (RT-PCR) test, cucumber plants treated with nickel oxide nanostructures showed increased expression of regulatory and defense-related genes concerned in salicylic acid (SA) and jasmonic acid (JA)/ethylene (ET) signaling pathways. NONS nanostructures showed direct antiviral activity against CMV resulted in significant reduction in CMV severity and titer relative to untreated plants. Treatment with nickel oxide nanostructures significantly improved cucumber fresh and dry weights as well as number of leaves. The induction of systemic resistance towards CMV by NONS nanostructures considered a novel strategy and first report.

Pipecolic Acid Is Induced in Barley upon Infection and Triggers Immune Responses Associated with Elevated Nitric Oxide Accumulation

Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi)biotrophic pathogens. Here, we studied the potential role of Pip in bacteria-induced systemic immunity in barley. Exudates of barley leaves infected with the systemic immunity-inducing pathogen Pseudomonas syringae pv. japonica induced immune responses in A. thaliana. The same leaf exudates contained elevated Pip levels compared with those of mock-treated barley leaves. Exogenous application of Pip induced resistance in barley against the hemibiotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity-inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus Pyrenophora teres. Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, X. translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in Pip-pretreated compared with mock-pretreated plants. Thus, the data suggest that Pip induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.

Folate Metabolism Interferes with Plant Immunity through 1C Methionine Synthase-Directed Genome-wide DNA Methylation Enhancement

Plants rely on primary metabolism for flexible adaptation to environmental changes. Here, through a combination of chemical genetics and forward genetic studies in Arabidopsis plants, we identified that the essential folate metabolic pathway exerts a salicylic acid-independent negative control on plant immunity. Disruption of the folate pathway promotes enhanced resistance to Pseudomonas syringae DC3000 via activation of a primed immune state in plants, whereas its implementation results in enhanced susceptibility. Comparative proteomics analysis using immune-defective mutants identified a methionine synthase (METS1), in charge of the synthesis of Met through the folate-dependent 1C metabolism, acting as a nexus between the folate pathway and plant immunity. Overexpression of METS1 represses plant immunity and is accompanied by genome-wide global increase in DNA methylation, revealing that imposing a methylation pressure at the genomic level compromises plant immunity. Take together, these results indicate that the folate pathway represents a new layer of complexity in the regulation of plant defense responses.

Arabidopsis mlo3 mutant plants exhibit spontaneous callose deposition and signs of early leaf senescence

Arabidopsis thaliana mlo3 mutant plants are not affected in pathogen infection phenotypes but-reminiscent of mlo2 mutant plants-exhibit spontaneous callose deposition and signs of early leaf senescence. The family of Mildew resistance Locus O (MLO) proteins is best known for its profound effect on the outcome of powdery mildew infections: when the appropriate MLO protein is absent, the plant is fully resistant to otherwise virulent powdery mildew fungi. However, most members of the MLO protein family remain functionally unexplored. Here, we investigate Arabidopsis thaliana MLO3, the closest relative of AtMLO2, AtMLO6 and AtMLO12, which are the Arabidopsis MLO genes implicated in the powdery mildew interaction. The co-expression network of AtMLO3 suggests association of the gene with plant defense-related processes such as salicylic acid homeostasis. Our extensive analysis shows that mlo3 mutants are unaffected regarding their infection phenotype upon challenge with the powdery mildew fungi Golovinomyces orontii and Erysiphe pisi, the oomycete Hyaloperonospora arabidopsidis, and the bacterial pathogen Pseudomonas syringae (the latter both in terms of basal and systemic acquired resistance), indicating that the protein does not play a major role in the response to any of these pathogens. However, mlo3 genotypes display spontaneous callose deposition as well as signs of early senescence in 6- or 7-week-old rosette leaves in the absence of any pathogen challenge, a phenotype that is reminiscent of mlo2 mutant plants. We hypothesize that de-regulated callose deposition in mlo3 genotypes might be the result of a subtle transient aberration of salicylic acid-jasmonic acid homeostasis during development.

Nodulation Induces Systemic Resistance of Medicago truncatula and Pisum sativum Against Erysiphe pisi and Primes for Powdery Mildew-Triggered Salicylic Acid Accumulation

Plants encounter beneficial and detrimental microorganisms both above- and belowground and the health status of the plant depends on the composition of this pan-microbiome. Beneficial microorganisms contribute to plant nutrition or systemically or locally protect plants against pathogens, thus facilitating adaptation to a variety of environments. Induced systemic resistance, caused by root-associated microbes, manifests as aboveground resistance against necrotrophic pathogens and is mediated by jasmonic acid/ethylene-dependent signaling. By contrast, systemic acquired resistance relies on salicylic acid (SA) signaling and confers resistance against secondary infection by (hemi)biotrophic pathogens. To investigate whether symbiotic rhizobia that are ubiquitously found in natural ecosystems are able to modulate resistance against biotrophs, we tested the impact of preestablished nodulation of Medicago truncatula and pea (Pisum sativum) plants against infection by the powdery mildew fungus Erysiphe pisi. We found that root symbiosis interfered with fungal penetration of M. truncatula and reduced asexual spore formation on pea leaves independently of symbiotic nitrogen fixation. Improved resistance of nodulated plants correlated with elevated levels of free SA and SA-dependent marker gene expression upon powdery mildew infection. Our results suggest that nodulation primes the plants systemically for E. pisi-triggered SA accumulation and defense gene expression, resulting in increased resistance.

Cold stress activates disease resistance in Arabidopsis thaliana through a salicylic acid dependent pathway

Exposure to short-term cold stress influences disease resistance by mechanisms that remain poorly characterized. The molecular basis of cold-activated immunity was therefore investigated in Arabidopsis thaliana inoculated with the bacterial pathogen Pst DC3000, using a transcriptomic analysis. Exposure to cold stress for 10 hr was sufficient to activate immunity, as well as H₂ O₂ accumulation and callose deposition. Transcriptome changes induced by the 10-hr cold treatment were similar to those caused by pathogen infection, including increased expression of the salicylic acid (SA) pathway marker genes, PR2 and PR5, and genes playing positive roles in defence against (hemi)-biotrophs. In contrast, transcripts encoding jasmonic acid (JA) pathway markers such as PR4 and MYC2 and transcripts with positive roles in defence against necrotrophs were less abundant following the 10-hr cold treatment. Cold-activated immunity was dependent on SA, being partially dependent on NPR1 and ICS1/SID2. In addition, transcripts encoding SA biosynthesis enzymes such as ICS2, PAL1, PAL2, and PAL4 (but not ICS1/SID2) and MES9 were more abundant, whereas GH3.5/WES1 and SOT12 transcripts that encode components involved in SA modification were less abundant following cold stress treatment. These findings show that cold stress cross-activates innate immune responses via a SA-dependent pathway.

Systemic acquired resistance networks amplify airborne defense cues

Salicylic acid (SA)-mediated innate immune responses are activated in plants perceiving volatile monoterpenes. Here, we show that monoterpene-associated responses are propagated in feed-forward loops involving the systemic acquired resistance (SAR) signaling components pipecolic acid, glycerol-3-phosphate, and LEGUME LECTIN-LIKE PROTEIN1 (LLP1). In this cascade, LLP1 forms a key regulatory unit in both within-plant and between-plant propagation of immunity. The data integrate molecular components of SAR into systemic signaling networks that are separate from conventional, SA-associated innate immune mechanisms. These networks are central to plant-to-plant propagation of immunity, potentially raising SAR to the population level. In this process, monoterpenes act as microbe-inducible plant volatiles, which as part of plant-derived volatile blends have the potential to promote the generation of a wave of innate immune signaling within canopies or plant stands. Hence, plant-to-plant propagation of SAR holds significant potential to fortify future durable crop protection strategies following a single volatile trigger.

Plants immune responses are triggered upon perception of volatile monoterpenes. Here, Wenig et al. show that a feed-forward loop featuring LEGUME LECTIN-LIKE PROTEIN1 propagates monoterpene-associated cues both within and between plants, illustrating how systemic immunity could act at a population level.

Bacterial infection systemically suppresses stomatal density

Many plant pathogens gain entry to their host via stomata. On sensing attack, plants close these pores to restrict pathogen entry. Here, we show that plants exhibit a second longer term stomatal response to pathogens. Following infection, the subsequent development of leaves is altered via a systemic signal. This reduces the density of stomata formed, thus providing fewer entry points for pathogens on new leaves. Arabidopsis thaliana leaves produced after infection by a bacterial pathogen that infects through the stomata (Pseudomonas syringae) developed larger epidermal pavement cells and stomata and consequently had up to 20% reductions in stomatal density. The bacterial peptide flg22 or the phytohormone salicylic acid induced similar systemic reductions in stomatal density suggesting that they might mediate this effect. In addition, flagellin receptors, salicylic acid accumulation, and the lipid transfer protein AZI1 were all required for this developmental response. Furthermore, manipulation of stomatal density affected the level of bacterial colonization, and plants with reduced stomatal density showed slower disease progression. We propose that following infection, development of new leaves is altered by a signalling pathway with some commonalities to systemic acquired resistance. This acts to reduce the potential for future infection by providing fewer stomatal openings.

N-hydroxypipecolic acid and salicylic acid: a metabolic duo for systemic acquired resistance

Recent research has established that the pipecolate pathway, a three-step biochemical sequence from l-lysine to N-hydroxypipecolic acid (NHP), is central for plant systemic acquired resistance (SAR). NHP orchestrates SAR establishment in concert with the immune signal salicylic acid (SA). Here, we outline the biochemistry of NHP formation from l-Lys and address novel progress on SA biosynthesis in Arabidopsis and other plant species. In Arabidopsis, the pathogen-inducible pipecolate and salicylate pathways are activated by common and distinct regulatory elements and mutual interactions between both metabolic branches exist. The mode of action of NHP in SAR involves direct induction of SAR gene expression, signal amplification, priming for enhanced defense activation and positive interplay with SA signaling to ensure elevated plant immunity.

Salicylic acid: biosynthesis, perception, and contributions to plant immunity

Salicylic acid (SA) has emerged as a key plant defense hormone with critical roles in different aspects of plant immunity. Analysis of Arabidopsis mutants revealed complex regulation of pathogen-induced SA biosynthesis. Studies on SA-insensitive mutants led to the identification of the SA receptors and how SA regulates defense gene expression. Consistent with its critical roles in plant immunity, SA is required for the assembly of a normal root microbiome and various pathogen effectors have evolved to target components of SA biosynthesis or signaling. This review discusses recent advances in SA biology, focusing in particular on the regulation of SA biosynthesis and SA perception.

Ammonium mediated changes in carbon and nitrogen metabolisms induce resistance against Pseudomonas syringae in tomato plants

Predominant NH₄⁺ nutrition causes an "ammonium syndrome" that induces metabolic changes and thereby provides resistance against Pseudomonas syringae infection through the activation of systemic acquired acclimation (SAA). Hence, to elucidate the mechanisms underlying NH₄⁺-mediated SAA, the changes in nutrient balance and C and N skeletons were studied in NH₄⁺-treated plants upon infection by P. syringae. A general decrease in cation and an increase in anion levels was observed in roots and leaves of NH₄⁺-treated plants. Upon NH₄⁺-based nutrition and infection, tomato leaves showed an accumulation of S, P, Zn, and of Mn. Mn accumulation might be required for ROS detoxification since it acts as a cofactor of superoxide dismutase (SOD). Primary metabolism was modified in both tissues of NH₄⁺-fed plants to counteract NH₄⁺ toxicity by decreasing TCA intermediates. A significant increase in Arg, Gln, Asn, Lys, Tyr, His and Leu was observed in leaves of NH₄⁺-treated plants. The high level of the putrescine precursor Arg hints towards the importance of the Glu pathway as a key metabolic check-point in NH₄⁺-treated and infected plants. Taken together, NH₄⁺-fed plants displayed a high level of basal responses allowing them to activate SAA and to trigger defense responses against P. syringae through nutrient imbalances and changes in primary metabolism.

Chloroplasts as mediators of plant biotic interactions over short and long distances

In nature, plants interact with numerous other organisms. Some interactions benefit both the plant and the other organism(s), while others lead to disease or even death of the plant hosts. The traditional focus of research into plant biotic interactions has been on the negative effects on plants and the strategies plants use to mitigate or prevent these. Over the last several years there has been increasing appreciation for the diversity and importance of plant biotic interactions in plant success as well as the evolution and stabilization of ecosystems. With this new perspective, it is also becoming clear that the metabolic output of chloroplasts in plants is critical to establishing and maintaining these interactions. Here we highlight the roles of chloroplasts in diverse biotic interactions. Photosynthetic chloroplasts are the source of hormones, small molecules and a prodigious number of secondary metabolites, a significant portion of which influence plant biotic interactions. Importantly, the effects of chloroplasts on these interactions are not limited to sites of direct association or contact but also act at a distance in systemic leaves and roots, in the rhizosphere, in the air surrounding a plant and in neighboring plants, and they can persist over several years.

Methyl Salicylate Glucosylation Regulates Plant Defense Signaling and Systemic Acquired Resistance

Plant systemic acquired resistance (SAR) provides an efficient broad-spectrum immune response to pathogens. SAR involves mobile signal molecules that are generated by infected tissues and transported to systemic tissues. Methyl salicylate (MeSA), a molecule that can be converted to salicylic acid (SA), is an essential signal for establishing SAR, particularly under a short period of exposure to light after pathogen infection. Thus, the control of MeSA homeostasis is important for an optimal SAR response. Here, we characterized a uridine diphosphate-glycosyltransferase, UGT71C3, in Arabidopsis (Arabidopsis thaliana), which was induced mainly in leaf tissue by pathogens including Pst DC3000/avrRpt2 (Pseudomonas syringae pv tomato strain DC3000 expressing avrRpt2). Biochemical analysis indicated that UGT71C3 exhibited strong enzymatic activity toward MeSA to form MeSA glucosides in vitro and in vivo. After primary pathogen infection by Pst DC3000/avrRpt2, ugt71c3 knockout mutants exhibited more powerful systemic resistance to secondary pathogen infection than that of wild-type plants, whereas systemic resistance in UGT71C3 overexpression lines was compromised. In agreement, after primary infection of local leaves, ugt71c3 knockout mutants accumulated significantly more systemic MeSA and SA than that in wild-type plants. whereas UGT71C3 overexpression lines accumulated less. Our results suggest that MeSA glucosylation by UGT71C3 facilitates negative regulation of the SAR response by modulating homeostasis of MeSA and SA. This study unveils further SAR regulation mechanisms and highlights the role of glucosylation of MeSA and potentially other systemic signals in negatively modulating plant systemic defense.

Cell Death Triggered by the YUCCA-like Bs3 Protein Coincides with Accumulation of Salicylic Acid and Pipecolic Acid But Not of Indole-3-Acetic Acid

The pepper (Capsicum annuum) resistance gene bacterial spot3 (Bs3) is transcriptionally activated by the matching Xanthomonas euvesicatoria transcription-activator-like effector (TALE) AvrBs3. AvrBs3-induced Bs3 expression triggers a rapid and local cell death reaction, the hypersensitive response (HR). Bs3 is most closely related to plant flavin monooxygenases of the YUCCA (YUC) family, which catalyze the final step in auxin biosynthesis. Targeted mutagenesis of predicted NADPH- and FAD-cofactor sites resulted in Bs3 derivatives that no longer trigger HR, thereby suggesting that the enzymatic activity of Bs3 is crucial to Bs3-triggered HR. Domain swap experiments between pepper Bs3 and Arabidopsis (Arabidopsis thaliana) YUC8 uncovered functionally exchangeable and functionally distinct regions in both proteins, which is in agreement with a model whereby Bs3 evolved from an ancestral YUC gene. Mass spectrometric measurements revealed that expression of YUCs, but not expression of Bs3, coincides with an increase in auxin levels, suggesting that Bs3 and YUCs, despite their sequence similarity, catalyze distinct enzymatic reactions. Finally, we found that expression of Bs3 coincides with increased levels of the salicylic acid and pipecolic acid, two compounds that are involved in systemic acquired resistance.

NbALD1 mediates resistance to turnip mosaic virus by regulating the accumulation of salicylic acid and the ethylene pathway in Nicotiana benthamiana

AGD2-LIKE DEFENCE RESPONSE PROTEIN 1 (ALD1) triggers plant defence against bacterial and fungal pathogens by regulating the salicylic acid (SA) pathway and an unknown SA-independent pathway. We now show that Nicotiana benthamiana ALD1 is involved in defence against a virus and that the ethylene pathway also participates in ALD1-mediated resistance. NbALD1 was up-regulated in plants infected with turnip mosaic virus (TuMV). Silencing of NbALD1 facilitated TuMV infection, while overexpression of NbALD1 or exogenous application of pipecolic acid (Pip), the downstream product of ALD1, enhanced resistance to TuMV. The SA content was lower in NbALD1-silenced plants and higher where NbALD1 was overexpressed or following Pip treatments. SA mediated resistance to TuMV and was required for NbALD1-mediated resistance. However, on NahG plants (in which SA cannot accumulate), Pip treatment still alleviated susceptibility to TuMV, further demonstrating the presence of an SA-independent resistance pathway. The ethylene precursor, 1-aminocyclopropanecarboxylic acid (ACC), accumulated in NbALD1-silenced plants but was reduced in plants overexpressing NbALD1 or treated with Pip. Silencing of ACS1, a key gene in the ethylene pathway, alleviated the susceptibility of NbALD1-silenced plants to TuMV, while exogenous application of ACC compromised the resistance of Pip-treated or NbALD1 transgenic plants. The results indicate that NbALD1 mediates resistance to TuMV by positively regulating the resistant SA pathway and negatively regulating the susceptible ethylene pathway.

Genome-wide identification and characterization of Chitinase gene family in Brassica juncea and Camelina sativa in response to Alternaria brassicae

Chitinases belong to the group of Pathogenesis-related (PR) proteins that provides protection against fungal pathogens. This study presents the, genome-wide identification and characterization of chitinase gene family in two important oilseed crops B. juncea and C. sativa belonging to family Brassicaceae. We have identified 47 and 79 chitinase genes in the genomes of B. juncea and C. sativa, respectively. Phylogenetic analysis of chitinases in both the species revealed four distinct sub-groups, representing different classes of chitinases (I-V). Microscopic and biochemical study reveals the role of reactive oxygen species (ROS) scavenging enzymes in disease resistance of B. juncea and C. sativa. Furthermore, qRT-PCR analysis showed that expression of chitinases in both B. juncea and C. sativa was significantly induced after Alternaria brassicae infection. However, the fold change in chitinase gene expression was considerably higher in C. sativa compared to B. juncea, which further proves their role in C. sativa disease resistance to A. brassicae. This study provides comprehensive analysis on chitinase gene family in B. juncea and C. sativa and in future may serve as a potential candidate for improving disease resistance in B. juncea through transgenic approach.

Suppression of tryptophan synthase activates cotton immunity by triggering cell death via promoting SA synthesis

Primary metabolism plays an important role in plant growth and development, however the relationship between primary metabolism and the adaptive immune response is largely unknown. Here, we employed RNA interference (RNAi), virus-induced gene silencing (VIGS) technology, phytohormone profiling, genetic studies, and transcriptome and metabolome analysis to investigate the function of the tryptophan synthesis pathway in the resistance of cotton to V. dahliae. We found that knock-down of GbTSA1 (Tryptophan Synthase α) and GbTSB1 (tryptophan synthase β) induced a spontaneous cell death phenotype in a salicylic acid (SA)-dependent manner and enhanced resistance to V. dahliae in cotton plants. Metabolome analysis showed that indole and indolic metabolites were highly accumulated in GbTSA1- or GbTSB1-silenced plants. Transcriptomic analysis showed that exogenous indole promotes the expression levels of genes involved in SA synthesis and the defense response. Similarly, indole application strongly enhanced cotton resistance to V. dahliae. These results suggested that metabolic intermediates in the Trp synthesis pathway may be a signal to activate SA synthesis. These results also provided a strategy to elicit plant defense responses by the application of indole.

Fluctuating Light Interacts with Time of Day and Leaf Development Stage to Reprogram Gene Expression

Natural light environments are highly variable. Flexible adjustment between light energy utilization and photoprotection is therefore of vital importance for plant performance and fitness in the field. Short-term reactions to changing light intensity are triggered inside chloroplasts and leaves within seconds to minutes, whereas long-term adjustments proceed over hours and days, integrating multiple signals. While the mechanisms of long-term acclimation to light intensity have been studied by changing constant growth light intensity during the day, responses to fluctuating growth light intensity have rarely been inspected in detail. We performed transcriptome profiling in Arabidopsis (Arabidopsis thaliana) leaves to investigate long-term gene expression responses to fluctuating light (FL). In particular, we examined whether responses differ between young and mature leaves or between morning and the end of the day. Our results highlight global reprogramming of gene expression under FL, including that of genes related to photoprotection, photosynthesis, and photorespiration and to pigment, prenylquinone, and vitamin metabolism. The FL-induced changes in gene expression varied between young and mature leaves at the same time point and between the same leaves in the morning and at the end of the day, indicating interactions of FL acclimation with leaf development stage and time of day. Only 46 genes were up- or down-regulated in both young and mature leaves at both time points. Combined analyses of gene coexpression and cis-elements pointed to a role of the circadian clock and light in coordinating the acclimatory responses of functionally related genes. Our results also suggest a possible cross talk between FL acclimation and systemic acquired resistance-like gene expression in young leaves.