ATML1 Regulates the Differentiation of ER Body-containing Large Pavement Cells in Rosette Leaves of Brassicaceae Plants

Endoplasmic reticulum (ER)-derived organelles, ER bodies, participate in the defense against herbivores in Brassicaceae plants. ER bodies accumulate β-glucosidases, which hydrolyse specialized thioglucosides known as glucosinolates to generate bioactive substances. In Arabidopsis thaliana, the leaf ER (LER) bodies are formed in large pavement cells, which are found in the petioles, margins, and blades of rosette leaves. However, the regulatory mechanisms involved in establishing large pavement cells are unknown. Here, we show that the ARABIDOPSIS THALIANA MERISTEM L1 LAYER (ATML1) transcription factor regulates the formation of LER bodies in large pavement cells of rosette leaves. Overexpression of ATML1 enhanced the expression of LER body-related genes and the number of LER body-containing large pavement cells, whereas its knockout resulted in opposite effects. ATML1 enhances endoreduplication and cell size through LOSS OF GIANT CELLS FROM ORGANS (LGO). Although the overexpression and knockout of LGO affected the appearance of large pavement cells in Arabidopsis, the effect on LER body-related gene expression and LER body formation was weak. LER body-containing large pavement cells were also found in Eutrema salsugineum, another Brassicaceae species. Our results demonstrate that ATML1 establishes large pavement cells to induce LER body formation in Brassicaceae plants, and thereby possibly contributes to the defense against herbivores.

ER body-resident myrosinases and tryptophan specialized metabolism modulate root microbiota assembly

Endoplasmic reticulum (ER) bodies are ER-derived structures that contain a large amount of PYK10 myrosinase, which hydrolyzes tryptophan (Trp)-derived indole glucosinolates (IGs). Given the well-described role of IGs in root-microbe interactions, we hypothesized that ER bodies in roots are important for interaction with soil-borne microbes at the root-soil interface. We used mutants impaired in ER bodies (nai1), ER body-resident myrosinases (pyk10bglu21), IG biosynthesis (myb34/51/122), and Trp specialized metabolism (cyp79b2b3) to profile their root microbiota community in natural soil, evaluate the impact of axenically collected root exudates on soil or synthetic microbial communities, and test their response to fungal endophytes in a mono-association setup. Tested mutants exhibited altered bacterial and fungal communities in rhizoplane and endosphere, respectively. Natural soils and bacterial synthetic communities treated with mutant root exudates exhibited distinctive microbial profiles from those treated with wild-type (WT) exudates. Most tested endophytes severely restricted the growth of cyp79b2b3, a part of which also impaired the growth of pyk10bglu21. Our results suggest that root ER bodies and their resident myrosinases modulate the profile of root-secreted metabolites and thereby influence root-microbiota interactions.

Loss of MYB34 Transcription Factor Supports the Backward Evolution of Indole Glucosinolate Biosynthesis in a Subclade of the Camelineae Tribe and Releases the Feedback Loop in This Pathway in Arabidopsis

Glucosinolates are specialized defensive metabolites characteristic of the Brassicales order. Among them, aliphatic and indolic glucosinolates (IGs) are usually highly abundant in species from the Brassicaceae family. The exceptions this trend are species representing a subclade of the Camelineae tribe, including Capsella and Camelina genera, which have reduced capacity to produce and metabolize IGs. Our study addresses the contribution of specific glucosinolate-related myeloblastosis (MYB) transcription factors to this unprecedented backward evolution of IG biosynthesis. To this end, we performed phylogenomic and functional studies of respective MYB proteins. The obtained results revealed weakened conservation of glucosinolate-related MYB transcription factors, including loss of functional MYB34 protein, in the investigated species. We showed that the introduction of functional MYB34 from Arabidopsis thaliana partially restores IG biosynthesis in Capsella rubella, indicating that the loss of this transcription factor contributes to the backward evolution of this metabolic pathway. Finally, we performed an analysis of the impact of particular myb mutations on the feedback loop in IG biosynthesis, which drives auxin overproduction, metabolic dysregulation and strong growth retardation caused by mutations in IG biosynthetic genes. This uncovered the unique function of MYB34 among IG-related MYBs in this feedback regulation and consequently in IG conservation in Brassicaceae plants.

Specialized metabolites as versatile tools in shaping plant-microbe associations

Plants are rich repository of a large number of chemical compounds collectively referred to as specialized metabolites. These compounds are of importance for adaptive processes including responses against changing abiotic conditions and interactions with various co-existing organisms. One of the strikingly affirmed functions of these specialized metabolites is their involvement in plants' life-long interactions with complex multi-kingdom microbiomes including both beneficial and harmful microorganisms. Recent developments in genomic and molecular biology tools not only help to generate well-curated information about regulatory and structural components of biosynthetic pathways of plant specialized metabolites but also to create and screen mutant lines defective in their synthesis. In this review, we have comprehensively surveyed the function of these specialized metabolites and discussed recent research findings demonstrating the responses of various microbes on tested mutant lines having defective biosynthesis of particular metabolites. In addition, we attempt to provide key clues about the impact of these metabolites on the assembly of the plant microbiome by summarizing the major findings of recent comparative metagenomic analyses of available mutant lines under customized and natural microbial niches. Subsequently, we delineate benchmark initiatives that aim to engineer or manipulate the biosynthetic pathways to produce specialized metabolites in heterologous systems but also to diversify their immune function. While denoting the function of these metabolites, we also discuss the critical bottlenecks associated with understanding and exploiting their function in improving plant adaptation to the environment.

Targeted and Untargeted Metabolomic Analyses Reveal Organ Specificity of Specialized Metabolites in the Model Grass Brachypodium distachyon

Brachypodium distachyon, because of its fully sequenced genome, is frequently used as a model grass species. However, its metabolome, which constitutes an indispensable element of complex biological systems, remains poorly characterized. In this study, we conducted comprehensive, liquid chromatography-mass spectrometry (LC-MS)-based metabolomic examination of roots, leaves and spikes of Brachypodium Bd21 and Bd3-1 lines. Our pathway enrichment analysis emphasised the accumulation of specialized metabolites representing the flavonoid biosynthetic pathway in parallel with processes related to nucleotide, sugar and amino acid metabolism. Similarities in metabolite profiles between both lines were relatively high in roots and leaves while spikes showed higher metabolic variance within both accessions. In roots, differences between Bd21 and Bd3-1 lines were manifested primarily in diterpenoid metabolism, while differences within spikes and leaves concerned nucleotide metabolism and nitrogen management. Additionally, sulphate-containing metabolites differentiated Bd21 and Bd3-1 lines in spikes. Structural analysis based on MS fragmentation spectra enabled identification of 93 specialized metabolites. Among them phenylpropanoids and flavonoids derivatives were mainly determined. As compared with closely related barley and wheat species, metabolic profile of Brachypodium is characterized with presence of threonate derivatives of hydroxycinnamic acids.

Practices towards SARS-CoV-2 transmission prevention among Polish high school students

Background

Adolescents play a significant role in SARS-CoV-2 spread. The study objective was to assess practices towards COVID-19 among final year Polish high school students.

Methods

Between December 2020-March 2021 an online cross-sectional survey was conducted. Multistage random sampling selected 24 high schools in the Lubuskie region then 51 classes. Students' practices were assessed by an anonymous questionnaire. Significance was tested by Chi-square test.

Results

Response rate was 72.3%; 638 students (360 females) participated (mean age 18.4±0.6 years); 68.7% attended schools in the cities with <100,000 inhabitants; 31.5% were in the science program. Most (81.3%) declared they always used masks in public spaces in the last month (74.3% males vs 86.1% females, p < 0.001; 77.2% of those living in smaller vs 90.5% in larger cities, p < 0.001; 89.6% in the science program vs 77.6% - in the other programs, p < 0.001); 35% always avoided crowded areas (26.9% males vs 39.4% females, p = 0.001; 29.5% living in smaller vs 47% in larger cities, p < 0.001; 43.8% in the science program vs 30.9% - in the other programs, p = 0.002); 24.3% declared always keeping social distance (19.4% males vs 27.2% females, p = 0.02), 29.9% stated they did not touch their faces (25% males vs 33.3% females, p = 0.02; 27.2% living in smaller vs 36% in larger cities, p = 0.02; 37.8% in the science program vs 26.3% - in the other programs, p = 0.003). Among those wearing masks - uncovered noses (64.6%) and masks around the neck (42.3%; 48.1% males vs 38.9% females, p = 0.02) were the most frequent incorrect practices.

Conclusions

Most high school students, in particular males, living in small cities and attending non-science program have inappropriate preventive practices towards COVID-19. Health education programs are urgently needed to better equip students with COVID-19 prevention skills.

Key messages

The study shows data of final year high-school students' practices towards COVID-19 and maybe used by public health experts to better address educational campaigns oriented to Polish adolescents. Health education aimed at high school students would be crucial to improve compliance with SARS-CoV-2 infection control procedures.

Gene expression evolution in pattern-triggered immunity within Arabidopsis thaliana and across Brassicaceae species

Plants recognize surrounding microbes by sensing microbe-associated molecular patterns (MAMPs) to activate pattern-triggered immunity (PTI). Despite their significance for microbial control, the evolution of PTI responses remains largely uncharacterized. Here, by employing comparative transcriptomics of six Arabidopsis thaliana accessions and three additional Brassicaceae species to investigate PTI responses, we identified a set of genes that commonly respond to the MAMP flg22 and genes that exhibit species-specific expression signatures. Variation in flg22-triggered transcriptome responses across Brassicaceae species was incongruent with their phylogeny, while expression changes were strongly conserved within A. thaliana. We found the enrichment of WRKY transcription factor binding sites in the 5'-regulatory regions of conserved and species-specific responsive genes, linking the emergence of WRKY-binding sites with the evolution of gene expression patterns during PTI. Our findings advance our understanding of the evolution of the transcriptome during biotic stress.

A Network of Phosphate Starvation and Immune-Related Signaling and Metabolic Pathways Controls the Interaction between Arabidopsis thaliana and the Beneficial Fungus Colletotrichum tofieldiae

The beneficial root-colonizing fungus Colletotrichum tofieldiae mediates plant growth promotion (PGP) upon phosphate (Pi) starvation in Arabidopsis thaliana. This activity is dependent on the Trp metabolism of the host, including indole glucosinolate (IG) hydrolysis. Here, we show that C. tofieldiae resolves several Pi starvation-induced molecular processes in the host, one of which is the downregulation of auxin signaling in germ-free plants, which is restored in the presence of the fungus. Using CRISPR/Cas9 genome editing, we generated an Arabidopsis triple mutant lacking three homologous nitrilases (NIT1 to NIT3) that are thought to link IG-hydrolysis products with auxin biosynthesis. Retained C. tofieldiae-induced PGP in nit1/2/3 mutant plants demonstrated that this metabolic connection is dispensable for the beneficial activity of the fungus. This suggests that either there is an alternative metabolic link between IG-hydrolysis products and auxin biosynthesis, or C. tofieldiae restores auxin signaling independently of IG metabolism. We show that C. tofieldiae, similar to pathogenic microorganisms, triggers Arabidopsis immune pathways that rely on IG metabolism as well as salicylic acid and ethylene signaling. Analysis of IG-deficient myb mutants revealed that these metabolites are, indeed, important for control of in planta C. tofieldiae growth: however, enhanced C. tofieldiae biomass does not necessarily negatively correlate with PGP. We show that Pi deficiency enables more efficient colonization of Arabidopsis by C. tofieldiae, possibly due to the MYC2-mediated repression of ethylene signaling and changes in the constitutive IG composition in roots.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Evolutionary changes in the glucosinolate biosynthetic capacity in species representing Capsella, Camelina and Neslia genera

Glucosinolates are unique thioglucosides that evolved in the order Brassicales. These compounds function in plant adaptation to the environment, including combating plant pathogens, herbivore deterrence and abiotic stress tolerance. In line with their defensive functions glucosinolates usually accumulate constitutively in relatively high amounts in all tissues of Brassicaceae plants. Here we performed glucosinolate analysis in different organs of selected species representing Capsella, Camelina and Neslia genera, which similarly as the model plant Arabidopsis thaliana belong to the Camelineae tribe. We also identified orthologs of A. thaliana glucosinolate biosynthetic genes in the published genomes of some of the investigated species. Subsequent gene expression and phylogenetic analyses enabled us an insight into the evolutionary changes in the transcription of these genes and in the sequences of respective proteins that occurred within the Camelineae tribe. Our results indicated that glucosinolates are highly abundant in siliques and roots of the investigated species but hardly, if at all, produced in leaves. In addition to this unusual tissular distribution we revealed reduced structural diversity of methionine-derived aliphatic glucosinolates (AGs) with elevated accumulation of rare long chain AGs. This preference seems to correlate with evolutionary changes in genes encoding methylthioalkylmalate synthases that are responsible for the elongation of AG side chains. Finally, our results indicate that the biosynthetic pathway for tryptophan-derived indolic glucosinolates likely lost its main functions in immunity and resistance towards sucking insects and is on its evolutionary route to be shut off in the investigated species.

Moonlighting Function of Phytochelatin Synthase1 in Extracellular Defense against Fungal Pathogens

Phytochelatin synthase (PCS) is a key component of heavy metal detoxification in plants. PCS catalyzes both the synthesis of the peptide phytochelatin from glutathione and the degradation of glutathione conjugates via peptidase activity. Here, we describe a role for PCS in disease resistance against plant pathogenic fungi. The pen4 mutant, which is allelic to cadmium insensitive1 (cad1/pcs1) mutants, was recovered from a screen for Arabidopsis mutants with reduced resistance to the nonadapted barley fungal pathogen Blumeria graminis f. sp. hordei PCS1, which is found in the cytoplasm of cells of healthy plants, translocates upon pathogen attack and colocalizes with the PEN2 myrosinase on the surface of immobilized mitochondria. pcs1 and pen2 mutant plants exhibit similar metabolic defects in the accumulation of pathogen-inducible indole glucosinolate-derived compounds, suggesting that PEN2 and PCS1 act in the same metabolic pathway. The function of PCS1 in this pathway is independent of phytochelatin synthesis and deglycination of glutathione conjugates, as catalytic-site mutants of PCS1 are still functional in indole glucosinolate metabolism. In uncovering a peptidase-independent function for PCS1, we reveal this enzyme to be a moonlighting protein important for plant responses to both biotic and abiotic stresses.

The role of CYP71A12 monooxygenase in pathogen-triggered tryptophan metabolism and Arabidopsis immunity

Effective defense of Arabidopsis against filamentous pathogens requires two mechanisms, both of which involve biosynthesis of tryptophan (Trp)-derived metabolites. Extracellular resistance involves products of PEN2-dependent metabolism of indole glucosinolates (IGs). Restriction of further fungal growth requires PAD3-dependent camalexin and other, as yet uncharacterized, indolics. This study focuses on the function of CYP71A12 monooxygenase in pathogen-triggered Trp metabolism, including the biosynthesis of indole-3-carboxylic acid (ICA). Moreover, to investigate the contribution of CYP71A12 and its products to Arabidopsis immunity, we analyzed infection phenotypes of multiple mutant lines combining pen2 with pad3, cyp71A12, cyp71A13 or cyp82C2. Metabolite profiling of cyp71A12 lines revealed a reduction in ICA accumulation. Additionally, analysis of mutant plants showed that low amounts of ICA can form during an immune response by CYP71B6/AAO1-dependent metabolism of indole acetonitrile, but not via IG hydrolysis. Infection assays with Plectosphaerella cucumerina and Colletotrichum tropicale, two pathogens with different lifestyles, revealed cyp71A12-, cyp71A13- and cyp82C2-associated defects associated with Arabidopsis immunity. Our results indicate that CYP71A12, but not CYP71A13, is the major enzyme responsible for the accumulation of ICA in Arabidopsis in response to pathogen ingression. We also show that both enzymes are key players in the resistance of Arabidopsis against selected filamentous pathogens after they invade.

Identification of drought responsive proteins and related proteomic QTLs in barley

Drought is a major abiotic stress that negatively influences crop yield. Breeding strategies for improved drought resistance require an improved knowledge of plant drought responses. We therefore applied drought to barley recombinant inbred lines and their parental genotypes shortly before tillering. A large-scale proteomic analysis of leaf and root tissue revealed proteins that respond to drought in a genotype-specific manner. Of these, Rubisco activase in chloroplast, luminal binding protein in endoplasmic reticulum, phosphoglycerate mutase, glutathione S-transferase, heat shock proteins and enzymes involved in phenylpropanoid biosynthesis showed strong genotype×environment interactions. These data were subjected to genetic linkage analysis and the identification of proteomic QTLs that have potential value in marker-assisted breeding programs.

Early first trimester uteroplacental flow and the progressive disintegration of spiral artery plugs: new insights from contrast-enhanced ultrasound and tissue histopathology

STUDY QUESTION

Does the use of a vascular contrast agent facilitate earlier detection of maternal flow to the placental intervillous space (IVS) in the first trimester of pregnancy?

SUMMARY ANSWER

Microvascular filling of the IVS was demonstrated by contrast-enhanced ultrasound from 6 weeks of gestation onwards, earlier than previously believed.

WHAT IS KNOWN ALREADY

During placental establishment and remodeling of maternal spiral arteries, endovascular trophoblast cells invade and accumulate in the lumen of these vessels to form 'trophoblast plugs'. Prior evidence from morphological and Doppler ultrasound studies has been conflicting as to whether the spiral arteries are completely plugged, preventing maternal blood flow to the IVS until late in the first trimester.

STUDY DESIGN, SIZE, DURATION

Uteroplacental flow was examined across the first trimester in human subjects given an intravenous infusion of lipid-shelled octofluoropropane microbubbles with ultrasound measurement of destruction and replenishment kinetics. We also performed a comprehensive histopathological correlation using two separately archived uteroplacental tissue collections to evaluate the degree of spiral artery plugging and evaluate remodeling of the upstream myometrial radial and arcurate arteries.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Pregnant women (n = 34) were recruited in the first trimester (range: 6+3 to 13+6 weeks gestation) for contrast-enhanced ultrasound studies with destruction-replenishment analysis of signal intensity for assessment of microvascular flux rate. Histological samples from archived in situ (Boyd Collection, n = 11) and fresh first, second, and third trimester decidual and post-hysterectomy uterine specimens (n = 16) were evaluated by immunohistochemistry (using markers of epithelial, endothelial and T-cells, as well as cell adhesion and proliferation) and ultrastructural analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

Contrast agent entry into the IVS was visualized as early as 6+3 weeks of gestation with some variability in microvascular flux rate noted in the 6-7+6 week samples. Spiral artery plug canalization was observed from 7 weeks with progressive disintegration thereafter. Of note, microvascular flux rate did not progressively increase until 13 weeks, which suggests that resistance to maternal flow in the early placenta may be mediated more proximally by myometrial radial arteries that begin remodeling at the end of the first trimester.

LIMITATIONS REASONS FOR CAUTION

Gestational age was determined by crown-rump length measurements obtained by transvaginal ultrasound on the day of contrast-enhanced imaging studies, which may explain the variability in the earliest gestational age samples due to the margin of error in this type of measurement.

WIDER IMPLICATIONS OF THE FINDINGS

Our comprehensive in situ histological analysis, in combination with the use of an in vivo imaging modality that has the sensitivity to permit visualization of microvascular filling, has allowed us to reveal new evidence in support of increasing blood flow to the IVS from 6 weeks of gestation. Histologic review suggested the mechanism may be blood flow through capillary-sized channels that form through the loosely cohesive 'plugs' by 7 weeks gestation. However, spiral artery remodeling on its own did not appear to explain why there is significantly more blood flow at 13 weeks gestation. Histologic studies suggest it may be related to radial artery remodeling, which begins at the end of the first trimester.

STUDY FUNDING/COMPETING INTEREST(S)

This project was supported by the Oregon Health and Science University Knight Cardiovascular Institute, Center for Developmental Health and the Struble Foundation. There are no competing interests.

Glutathione Transferase U13 Functions in Pathogen-Triggered Glucosinolate Metabolism

Glutathione (GSH) and indole glucosinolates (IGs) exert key functions in the immune system of the model plant Arabidopsis (Arabidopsis thaliana). Appropriate GSH levels are important for execution of both pre- and postinvasive disease resistance mechanisms to invasive pathogens, whereas an intact PENETRATION2 (PEN2)-pathway for IG metabolism is essential for preinvasive resistance in this species. Earlier indirect evidence suggested that the latter pathway involves conjugation of GSH with unstable products of IG metabolism and further processing of the resulting adducts to biologically active molecules. Here we describe the identification of Glutathione-S-Transferase class-tau member 13 (GSTU13) as an indispensable component of the PEN2 immune pathway for IG metabolism. gstu13 mutant plants are defective in the pathogen-triggered biosynthesis of end products of the PEN2 pathway, including 4-O-β-d-glucosyl-indol-3-yl formamide, indole-3-ylmethyl amine, and raphanusamic acid. In line with this metabolic defect, lack of functional GSTU13 results in enhanced disease susceptibility toward several fungal pathogens including Erysiphe pisi, Colletotrichum gloeosporioides, and Plectosphaerella cucumerina Seedlings of gstu13 plants fail also to deposit the (1,3)-β-glucan cell wall polymer, callose, after recognition of the bacterial flg22 epitope. We show that GSTU13 mediates specifically the role of GSH in IG metabolism without noticeable impact on other immune functions of this tripeptide. We postulate that GSTU13 connects GSH with the pathogen-triggered PEN2 pathway for IG metabolism to deliver metabolites that may have numerous functions in the innate immune system of Arabidopsis.

Glutathione S-Transferases in the Biosynthesis of Sulfur-Containing Secondary Metabolites in Brassicaceae Plants

Plants in the Brassicaceae family have evolved the capacity to produce numerous unique and structurally diverse sulfur-containing secondary metabolites, including constitutively present thio-glucosides, also known as glucosinolates, and indole-type phytoalexins, which are induced upon pathogen recognition. Studies on the glucosinolate and phytoalexin biosynthetic pathways in the model plant Arabidopsis thaliana have shown that glutathione donates the sulfur atoms that are present in these compounds, and this further suggests that specialized glutathione S-transferases (GSTs) are involved in the biosynthesis of glucosinolates and sulfur-containing phytoalexins. In addition, experimental evidence has shown that GSTs also participate in glucosinolate catabolism. Several candidate GSTs have been suggested based on co-expression analysis, however, the function of only a few of these enzymes have been validated by enzymatic assays or with phenotypes of respective mutant plants. Thus, it remains to be determined whether biosynthesis of sulfur-containing metabolites in Brassicaceae plants requires specific or nonspecific GSTs.

Chemical suppressors of mlo-mediated powdery mildew resistance

Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2',3'-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.

Key Components of Different Plant Defense Pathways Are Dispensable for Powdery Mildew Resistance of the Arabidopsis mlo2 mlo6 mlo12 Triple Mutant

Loss of function mutations of particular plant MILDEW RESISTANCE LOCUS O (MLO) genes confer durable and broad-spectrum penetration resistance against powdery mildew fungi. Here, we combined genetic, transcriptomic and metabolomic analyses to explore the defense mechanisms in the fully resistant Arabidopsis thaliana mlo2 mlo6 mlo12 triple mutant. We found that this genotype unexpectedly overcomes the requirement for indolic antimicrobials and defense-related secretion, which are critical for incomplete resistance of mlo2 single mutants. Comparative microarray-based transcriptome analysis of mlo2 mlo6 mlo12 mutants and wild type plants upon Golovinomyces orontii inoculation revealed an increased and accelerated accumulation of many defense-related transcripts. Despite the biotrophic nature of the interaction, this included the non-canonical activation of a jasmonic acid/ethylene-dependent transcriptional program. In contrast to a non-adapted powdery mildew pathogen, the adapted powdery mildew fungus is able to defeat the accumulation of defense-relevant indolic metabolites in a MLO protein-dependent manner. We suggest that a broad and fast activation of immune responses in mlo2 mlo6 mlo12 plants can compensate for the lack of single or few defense pathways. In addition, our results point to a role of Arabidopsis MLO2, MLO6, and MLO12 in enabling defense suppression during invasion by adapted powdery mildew fungi.

PYK10 myrosinase reveals a functional coordination between endoplasmic reticulum bodies and glucosinolates in Arabidopsis thaliana

The endoplasmic reticulum body (ER body) is an organelle derived from the ER that occurs in only three families of the order Brassicales and is suggested to be involved in plant defense. ER bodies in Arabidopsis thaliana contain large amounts of β-glucosidases, but the physiological functions of ER bodies and these enzymes remain largely unclear. Here we show that PYK10, the most abundant β-glucosidase in A. thaliana root ER bodies, hydrolyzes indole glucosinolates (IGs) in addition to the previously reported in vitro substrate scopolin. We found a striking co-expression between ER body-related genes (including PYK10), glucosinolate biosynthetic genes and the genes for so-called specifier proteins affecting the terminal products of myrosinase-mediated glucosinolate metabolism, indicating that these systems have been integrated into a common transcriptional network. Consistent with this, comparative metabolite profiling utilizing a number of A. thaliana relatives within Brassicaceae identified a clear phylogenetic co-occurrence between ER bodies and IGs, but not between ER bodies and scopolin. Collectively, our findings suggest a functional link between ER bodies and glucosinolate metabolism in planta. In addition, in silico three-dimensional modeling, combined with phylogenomic analysis, suggests that PYK10 represents a clade of 16 myrosinases that arose independently from the other well-documented class of six thioglucoside glucohydrolases. These findings provide deeper insights into how glucosinolates are metabolized in cruciferous plants and reveal variation of the myrosinase-glucosinolate system within individual plants.

Dysfunction of Arabidopsis MACPF domain protein activates programmed cell death via tryptophan metabolism in MAMP-triggered immunity

Plant immune responses triggered upon recognition of microbe-associated molecular patterns (MAMPs) typically restrict pathogen growth without a host cell death response. We isolated two Arabidopsis mutants, derived from accession Col-0, that activated cell death upon inoculation with nonadapted fungal pathogens. Notably, the mutants triggered cell death also when treated with bacterial MAMPs such as flg22. Positional cloning identified NSL1 (Necrotic Spotted Lesion 1) as a responsible gene for the phenotype of the two mutants, whereas nsl1 mutations of the accession No-0 resulted in necrotic lesion formation without pathogen inoculation. NSL1 encodes a protein of unknown function containing a putative membrane-attack complex/perforin (MACPF) domain. The application of flg22 increased salicylic acid (SA) accumulation in the nsl1 plants derived from Col-0, while depletion of isochorismate synthase 1 repressed flg22-inducible lesion formation, indicating that elevated SA is needed for the cell death response. nsl1 plants of Col-0 responded to flg22 treatment with an RBOHD-dependent oxidative burst, but this response was dispensable for the nsl1-dependent cell death. Surprisingly, loss-of-function mutations in PEN2, involved in the metabolism of tryptophan (Trp)-derived indole glucosinolates, suppressed the flg22-induced and nsl1-dependent cell death. Moreover, the increased accumulation of SA in the nsl1 plants was abrogated by blocking Trp-derived secondary metabolite biosynthesis, whereas the nsl1-dependent hyperaccumulation of PEN2-dependent compounds was unaffected when the SA biosynthesis pathway was blocked. Collectively, these findings suggest that MAMP-triggered immunity activates a genetically programmed cell death in the absence of the functional MACPF domain protein NSL1 via Trp-derived secondary metabolite-mediated activation of the SA pathway.

Regulation of Pathogen-Triggered Tryptophan Metabolism in Arabidopsis thaliana by MYB Transcription Factors and Indole Glucosinolate Conversion Products

MYB34, MYB51, and MYB122 transcription factors are known as decisive regulators of indolic glucosinolate (IG) biosynthesis with a strong impact on expression of genes encoding CYP79B2 and CYP79B3 enzymes that redundantly convert tryptophan to indole-3-acetaldoxime (IAOx). This intermediate represents a branching point for IG biosynthesis, and pathways leading to camalexin and indole-carboxylic acids (ICA). Here we investigate how these MYBs affect the pathogen-triggered Trp metabolism. Our experiments indicated that these three MYBs affect not only IG production but also constitutive biosynthesis of other IAOx-derived metabolites. Strikingly, the PENETRATION 2 (PEN2)-dependent IG-metabolism products, which are absent in myb34/51/122 and pen2 mutants, were indispensable for full flg22-mediated induction of other IAOx-derived compounds. However, gene induction and accumulation of ICAs and camalexin upon pathogen infection was not compromised in myb34/51/122 plants, despite strongly reduced IG levels. Hence, in comparison with cyp79B2/B3, which lacks all IAOx-derived metabolites, we found myb34/51/122 an ideal tool to analyze IG contribution to resistance against the necrotrophic fungal pathogen Plectosphaerella cucumerina. The susceptibility of myb34/51/122 was similar to that of pen2, but much lower than susceptibility of cyp79B2/B3, indicating that MYB34/51/122 contribute to resistance toward P. cucumerina exclusively through IG biosynthesis, and that PEN2 is the main leaf myrosinase activating IGs in response to microbial pathogens.

Analysis of Drought-Induced Proteomic and Metabolomic Changes in Barley (Hordeum vulgare L.) Leaves and Roots Unravels Some Aspects of Biochemical Mechanisms Involved in Drought Tolerance

In this study, proteomic and metabolomic changes in leaves and roots of two barley (Hordeum vulgare L.) genotypes, with contrasting drought tolerance, subjected to water deficit were investigated. Our two-dimensional electrophoresis (2D-PAGE) combined with matrix-assisted laser desorption time of flight mass spectrometry (MALDI-TOF and MALDI-TOF/TOF) analyses revealed 121 drought-responsive proteins in leaves and 182 in roots of both genotypes. Many of the identified drought-responsive proteins were associated with processes that are typically severely affected during water deficit, including photosynthesis and carbon metabolism. However, the highest number of identified leaf and root proteins represented general defense mechanisms. In addition, changes in the accumulation of proteins that represent processes formerly unassociated with drought response, e.g., phenylpropanoid metabolism, were also identified. Our tandem gas chromatography - time of flight mass spectrometry (GC/MS TOF) analyses revealed approximately 100 drought-affected low molecular weight compounds representing various metabolite types with amino acids being the most affected metabolite class. We compared the results from proteomic and metabolomic analyses to search for existing relationship between these two levels of molecular organization. We also uncovered organ specificity of the observed changes and revealed differences in the response to water deficit of drought susceptible and tolerant barley lines. Particularly, our results indicated that several of identified proteins and metabolites whose accumulation levels were increased with drought in the analyzed susceptible barley variety revealed elevated constitutive accumulation levels in the drought-resistant line. This may suggest that constitutive biochemical predisposition represents a better drought tolerance mechanism than inducible responses.

Mutant Allele-Specific Uncoupling of PENETRATION3 Functions Reveals Engagement of the ATP-Binding Cassette Transporter in Distinct Tryptophan Metabolic Pathways

Arabidopsis (Arabidopsis thaliana) penetration (PEN) genes quantitatively contribute to the execution of different forms of plant immunity upon challenge with diverse leaf pathogens. PEN3 encodes a plasma membrane-resident pleiotropic drug resistance-type ATP-binding cassette transporter and is thought to act in a pathogen-inducible and PEN2 myrosinase-dependent metabolic pathway in extracellular defense. This metabolic pathway directs the intracellular biosynthesis and activation of tryptophan-derived indole glucosinolates for subsequent PEN3-mediated efflux across the plasma membrane at pathogen contact sites. However, PEN3 also functions in abiotic stress responses to cadmium and indole-3-butyric acid (IBA)-mediated auxin homeostasis in roots, raising the possibility that PEN3 exports multiple functionally unrelated substrates. Here, we describe the isolation of a pen3 allele, designated pen3-5, that encodes a dysfunctional protein that accumulates in planta like wild-type PEN3. The specific mutation in pen3-5 uncouples PEN3 functions in IBA-stimulated root growth modulation, callose deposition induced with a conserved peptide epitope of bacterial flagellin (flg22), and pathogen-inducible salicylic acid accumulation from PEN3 activity in extracellular defense, indicating the engagement of multiple PEN3 substrates in different PEN3-dependent biological processes. We identified 4-O-β-D-glucosyl-indol-3-yl formamide (4OGlcI3F) as a pathogen-inducible, tryptophan-derived compound that overaccumulates in pen3 leaf tissue and has biosynthesis that is dependent on an intact PEN2 metabolic pathway. We propose that a precursor of 4OGlcI3F is the PEN3 substrate in extracellular pathogen defense. These precursors, the shared indole core present in IBA and 4OGlcI3F, and allele-specific uncoupling of a subset of PEN3 functions suggest that PEN3 transports distinct indole-type metabolites in distinct biological processes.

Secondary metabolites in plant innate immunity: conserved function of divergent chemicals

Plant secondary metabolites carry out numerous functions in interactions between plants and a broad range of other organisms. Experimental evidence strongly supports the indispensable contribution of many constitutive and pathogen-inducible phytochemicals to plant innate immunity. Extensive studies on model plant species, particularly Arabidopsis thaliana, have brought significant advances in our understanding of the molecular mechanisms underpinning pathogen-triggered biosynthesis and activation of defensive secondary metabolites. However, despite the proven significance of secondary metabolites in plant response to pathogenic microorganisms, little is known about the precise mechanisms underlying their contribution to plant immunity. This insufficiency concerns information on the dynamics of cellular and subcellular localization of defensive phytochemicals during the encounters with microbial pathogens and precise knowledge on their mode of action. As many secondary metabolites are characterized by their in vitro antimicrobial activity, these compounds were commonly considered to function in plant defense as in planta antibiotics. Strikingly, recent experimental evidence suggests that at least some of these compounds alternatively may be involved in controlling several immune responses that are evolutionarily conserved in the plant kingdom, including callose deposition and programmed cell death.

ER bodies in plants of the Brassicales order: biogenesis and association with innate immunity

The endoplasmic reticulum (ER) forms highly organized network structures composed of tubules and cisternae. Many plant species develop additional ER-derived structures, most of which are specific for certain groups of species. In particular, a rod-shaped structure designated as the ER body is produced by plants of the Brassicales order, which includes Arabidopsis thaliana. Genetic analyses and characterization of A. thaliana mutants possessing a disorganized ER morphology or lacking ER bodies have provided insights into the highly organized mechanisms responsible for the formation of these unique ER structures. The accumulation of proteins specific for the ER body within the ER plays an important role in the formation of ER bodies. However, a mutant that exhibits morphological defects of both the ER and ER bodies has not been identified. This suggests that plants in the Brassicales order have evolved novel mechanisms for the development of this unique organelle, which are distinct from those used to maintain generic ER structures. In A. thaliana, ER bodies are ubiquitous in seedlings and roots, but rare in rosette leaves. Wounding of rosette leaves induces de novo formation of ER bodies, suggesting that these structures are associated with resistance against pathogens and/or herbivores. ER bodies accumulate a large amount of β-glucosidases, which can produce substances that potentially protect against invading pests. Biochemical studies have determined that the enzymatic activities of these β-glucosidases are enhanced during cell collapse. These results suggest that ER bodies are involved in plant immunity, although there is no direct evidence of this. In this review, we provide recent perspectives of ER and ER body formation in A. thaliana, and discuss clues for the functions of ER bodies. We highlight defense strategies against biotic stress that are unique for the Brassicales order, and discuss how ER structures could contribute to these strategies.

Glutathione and tryptophan metabolites are key players in Arabidopsis nonhost resistance against Colletotrichum gloeosporioides

The nonhost resistance of Arabidopsis against hemibiotrophic fungi in the genus Colletotrichum consists of pre- and post-invasive immune responses. Previously, we reported EDR1 and PEN2 as important components of Arabidopsis pre-invasive resistance toward non-adapted Colletotrichum gloeosporioides (Cg). However, despite their defect in entry control pen2 and edr1 mutants terminated further growth of this pathogen by activating the post-invasive hypersensitive response (HR) accompanied by plant cell death. In the present study, we showed that γ-glutamylcysteine synthetase (GSH1), which is required for glutathione biosynthesis, and tryptophan (Trp) metabolism contribute to pre- and post-invasive non-host resistance against Cg. We found GSH1 to be involved in the PEN2-dependent entry control of Cg. Opposite to pen2 and edr1, gsh1 mutants failed to restrict the invasive growth of the pathogen, which demonstrated the requirement for GSH1 during post-invasive non-host resistance. Based on the infection and metabolic phenotypes of Arabidopsis mutants defective in Trp metabolism, we showed that the biosynthesis of Trp-derived phytochemicals is also essential for resistance to Cg during the post-invasive HR. By contrast, GSH1 and these metabolites are dispensable for the induction of HR cell death, which is triggered in the non-invaded mesophyll cells adjacent to the Cg-invaded epidermal cells.

The Arabidopsis Rho of plants GTPase AtROP6 functions in developmental and pathogen response pathways

How plants coordinate developmental processes and environmental stress responses is a pressing question. Here, we show that Arabidopsis (Arabidopsis thaliana) Rho of Plants6 (AtROP6) integrates developmental and pathogen response signaling. AtROP6 expression is induced by auxin and detected in the root meristem, lateral root initials, and leaf hydathodes. Plants expressing a dominant negative AtROP6 (rop6(DN)) under the regulation of its endogenous promoter are small and have multiple inflorescence stems, twisted leaves, deformed leaf epidermis pavement cells, and differentially organized cytoskeleton. Microarray analyses of rop6(DN) plants revealed that major changes in gene expression are associated with constitutive salicylic acid (SA)-mediated defense responses. In agreement, their free and total SA levels resembled those of wild-type plants inoculated with a virulent powdery mildew pathogen. The constitutive SA-associated response in rop6(DN) was suppressed in mutant backgrounds defective in SA signaling (nonexpresser of PR genes1 [npr1]) or biosynthesis (salicylic acid induction deficient2 [sid2]). However, the rop6(DN) npr1 and rop6(DN) sid2 double mutants retained the aberrant developmental phenotypes, indicating that the constitutive SA response can be uncoupled from ROP function(s) in development. rop6(DN) plants exhibited enhanced preinvasive defense responses to a host-adapted virulent powdery mildew fungus but were impaired in preinvasive defenses upon inoculation with a nonadapted powdery mildew. The host-adapted powdery mildew had a reduced reproductive fitness on rop6(DN) plants, which was retained in mutant backgrounds defective in SA biosynthesis or signaling. Our findings indicate that both the morphological aberrations and altered sensitivity to powdery mildews of rop6(DN) plants result from perturbations that are independent from the SA-associated response. These perturbations uncouple SA-dependent defense signaling from disease resistance execution.

Chemical warfare or modulators of defence responses - the function of secondary metabolites in plant immunity

In plants, a host's responses to an attempted infection include activation of various secondary metabolite pathways, some of which are specific for particular plant phylogenetic clades. Phytochemicals that represent respective end products in plant immunity have been stereotypically linked to antimicrobial properties. However, in many cases, owing to the lack of unequivocal evidence for direct antibiotic action in planta, alternative functions of secondary metabolites should be considered. Correspondingly, recent findings have identified novel, and rather unexpected, functions of phytochemicals in plant immunity that mediate regulatory pathways for conserved defence responses. It also seems likely that these conserved responses can be regulated by clade-specific phytochemicals.

Sulfur-containing secondary metabolites from Arabidopsis thaliana and other Brassicaceae with function in plant immunity

Biosynthesis of antimicrobial secondary metabolites in response to microbial infection is one of the features of the plant immune system. Particular classes of plant secondary metabolites involved in plant defence are often produced only by species belonging to certain phylogenetic clades. Brassicaceae plants have evolved the ability to synthesise a wide range of sulfur-containing secondary metabolites, including glucosinolates and indole-type phytoalexins. A subset of these compounds is produced by the model plant Arabidopsis thaliana. Genetic tools available for this species enabled verification of immune functions of glucosinolates and camalexin (A. thaliana phytoalexin), as well as characterisation of their respective biosynthetic pathways. Current knowledge of the biosynthesis of Brassicaceae sulfur-containing metabolites suggests that the key event in the evolution of these compounds is the acquisition of biochemical mechanisms originating from detoxification pathways into secondary metabolite biosynthesis. Moreover, it is likely that glucosinolates and Brassicaceae phytoalexins, traditionally considered as separate groups of compounds, have a common evolutionary origin and are interconnected on the biosynthetic level. This suggests that the diversity of Brassicaceae sulfur-containing phytochemicals reflect phylogenetic clade-specific branches of an ancient biosynthetic pathway.

Conservation and clade-specific diversification of pathogen-inducible tryptophan and indole glucosinolate metabolism in Arabidopsis thaliana relatives

• A hallmark of the innate immune system of plants is the biosynthesis of low-molecular-weight compounds referred to as secondary metabolites. Tryptophan-derived branch pathways contribute to the capacity for chemical defense against microbes in Arabidopsis thaliana. • Here, we investigated phylogenetic patterns of this metabolic pathway in relatives of A. thaliana following inoculation with filamentous fungal pathogens that employ contrasting infection strategies. • The study revealed unexpected phylogenetic conservation of the pathogen-induced indole glucosinolate (IG) metabolic pathway, including a metabolic shift of IG biosynthesis to 4-methoxyindol-3-ylmethylglucosinolate and IG metabolization. By contrast, indole-3-carboxylic acid and camalexin biosyntheses are clade-specific innovations within this metabolic framework. A Capsella rubella accession was found to be devoid of any IG metabolites and to lack orthologs of two A. thaliana genes needed for 4-methoxyindol-3-ylmethylglucosinolate biosynthesis or hydrolysis. However, C. rubella was found to retain the capacity to deposit callose after treatment with the bacterial flagellin-derived epitope flg22 and pre-invasive resistance against a nonadapted powdery mildew fungus. • We conclude that pathogen-inducible IG metabolism in the Brassicaceae is evolutionarily ancient, while other tryptophan-derived branch pathways represent relatively recent manifestations of a plant-pathogen arms race. Moreover, at least one Brassicaceae lineage appears to have evolved IG-independent defense signaling and/or output pathway(s).

Arabidopsis ENHANCED DISEASE RESISTANCE 1 is required for pathogen-induced expression of plant defensins in nonhost resistance, and acts through interference of MYC2-mediated repressor function

Arabidopsis thaliana exhibits durable resistance, called nonhost resistance, against non-adapted fungal pathogens that typically terminates pathogen entry. The PEN2-dependent indole glucosinolate metabolism pathway is involved in preventing the entry of a range of non-adapted fungi. Here, we report that ENHANCED DISEASE RESISTANCE 1 (EDR1) functions in pre-invasive nonhost resistance. Plants lacking EDR1 exhibit impaired entry resistance to the non-adapted hemibiotrophic Colletotrichum gloeosporioides, in contrast to the enhanced resistance of edr1 against biotrophic infection of a host-adapted powdery mildew fungus. Analysis of the edr1 pen2 double mutant indicates that EDR1 acts in a defense pathway independent from the PEN2 indole glucosinolate pathway. The edr1 mutant also exhibited enhanced susceptibility to host-adapted pathogens, including Colletotrichum higginsianum and necrotrophic Alternaria brassicicola. Comparative transcript profiling revealed that upon C. gloeosporioides inoculation, the expression of four plant defensin genes was severely impaired in edr1, indicating that EDR1 is required for the induced expression of these antifungal proteins. Inactivation of the MYC2-encoded transcription factor fully restored defensin expression in edr1, implying that EDR1 interferes with MYC2 function to abrogate repression of defensin expression. Furthermore, constitutive expression of plant defensin PDF1.2b largely rescued pre-invasive resistance responses in edr1 plants. These results indicate that EDR1 exerts a positive and critical role in resistance responses to hemibiotrophic/necrotrophic fungi, in part by inducing antifungal protein expression through derepression of MYC2 function.

Perturbation of Arabidopsis amino acid metabolism causes incompatibility with the adapted biotrophic pathogen Hyaloperonospora arabidopsidis

Reliance of biotrophic pathogens on living plant tissues to propagate implies strong interdependence between host metabolism and nutrient uptake by the pathogen. However, factors determining host suitability and establishment of infection are largely unknown. We describe a loss-of-inhibition allele of ASPARTATE KINASE2 and a loss-of-function allele of DIHYDRODIPICOLINATE SYNTHASE2 identified in a screen for Arabidopsis thaliana mutants with increased resistance to the obligate biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa). Through different molecular mechanisms, these mutations perturb amino acid homeostasis leading to overaccumulation of the Asp-derived amino acids Met, Thr, and Ile. Although detrimental for the plant, the mutations do not cause defense activation, and both mutants retain full susceptibility to the adapted obligate biotrophic fungus Golovinomyces orontii (Go). Chemical treatments mimicking the mutants' metabolic state identified Thr as the amino acid suppressing Hpa but not Go colonization. We conclude that perturbations in amino acid homeostasis render the mutant plants unsuitable as an infection substrate for Hpa. This may be explained by deployment of the same amino acid biosynthetic pathways by oomycetes and plants. Our data show that the plant host metabolic state can, in specific ways, influence the ability of adapted biotrophic strains to cause disease.

Metabolic engineering in Nicotiana benthamiana reveals key enzyme functions in Arabidopsis indole glucosinolate modification

Indole glucosinolates, derived from the amino acid Trp, are plant secondary metabolites that mediate numerous biological interactions between cruciferous plants and their natural enemies, such as herbivorous insects, pathogens, and other pests. While the genes and enzymes involved in the Arabidopsis thaliana core biosynthetic pathway, leading to indol-3-yl-methyl glucosinolate (I3M), have been identified and characterized, the genes and gene products responsible for modification reactions of the indole ring are largely unknown. Here, we combine the analysis of Arabidopsis mutant lines with a bioengineering approach to clarify which genes are involved in the remaining biosynthetic steps in indole glucosinolate modification. We engineered the indole glucosinolate biosynthesis pathway into Nicotiana benthamiana, showing that it is possible to produce indole glucosinolates in a noncruciferous plant. Building upon this setup, we demonstrate that all members of a small gene subfamily of cytochrome P450 monooxygenases, CYP81Fs, are capable of carrying out hydroxylation reactions of the glucosinolate indole ring, leading from I3M to 4-hydroxy-indol-3-yl-methyl and/or 1-hydroxy-indol-3-yl-methyl glucosinolate intermediates, and that these hydroxy intermediates are converted to 4-methoxy-indol-3-yl-methyl and 1-methoxy-indol-3-yl-methyl glucosinolates by either of two family 2 O-methyltransferases, termed indole glucosinolate methyltransferase 1 (IGMT1) and IGMT2.

Not a peripheral issue: secretion in plant-microbe interactions

The sessile nature of flowering plants and their capacity to thrive in soil habitats likely resulted in specific adaptation mechanisms enabling co-existence with a vast diversity of air-borne and soil-borne microorganisms. Only a small fraction of these microbial encounters result in pathogenic or symbiotic interactions, whilst the majority appear to give rise to commensalistic or mutualistic associations. Considering the abundance of the latter associations the question arises whether plants evolved, besides the plant immune system, other dedicated mechanisms to communicate with and to host microbial communities in the phyllosphere and rhizosphere. We hypothesize that the constitutive and microbe-induced secretion of specialized plant-derived biomolecules creates a critical interface for all types of plant-microbe associations. Thus, the plant secretory machinery might serve an important role in establishing an extended phenotype with microbial life.

Tryptophan-derived secondary metabolites in Arabidopsis thaliana confer non-host resistance to necrotrophic Plectosphaerella cucumerina fungi

A defence pathway contributing to non-host resistance to biotrophic fungi in Arabidopsis involves the synthesis and targeted delivery of the tryptophan (trp)-derived metabolites indol glucosinolates (IGs) and camalexin at pathogen contact sites. We have examined whether these metabolites are also rate-limiting for colonization by necrotrophic fungi. Inoculation of Arabidopsis with adapted or non-adapted isolates of the ascomycete Plectosphaerella cucumerina triggers the accumulation of trp-derived metabolites. We found that their depletion in cyp79B2 cyp79B3 mutants renders Arabidopsis fully susceptible to each of three tested non-adapted P. cucumerina isolates, and super-susceptible to an adapted P. cucumerina isolate. This assigns a key role to trp-derived secondary metabolites in limiting the growth of both non-adapted and adapted necrotrophic fungi. However, 4-methoxy-indol-3-ylmethylglucosinolate, which is generated by the P450 monooxygenase CYP81F2, and hydrolyzed by PEN2 myrosinase, together with the antimicrobial camalexin play a minor role in restricting the growth of the non-adapted necrotrophs. This contrasts with a major role of these two trp-derived phytochemicals in limiting invasive growth of non-adapted biotrophic powdery mildew fungi, thereby implying the existence of other unknown trp-derived metabolites in resistance responses to non-adapted necrotrophic P. cucumerina. Impaired defence to non-adapted P. cucumerina, but not to the non-adapted biotrophic fungus Erysiphe pisi, on cyp79B2 cyp79B3 plants is largely restored in the irx1 background, which shows a constitutive accumulation of antimicrobial peptides. Our findings imply differential contributions of antimicrobials in non-host resistance to necrotrophic and biotrophic pathogens.

Entry mode-dependent function of an indole glucosinolate pathway in Arabidopsis for nonhost resistance against anthracnose pathogens

When faced with nonadapted fungal pathogens, Arabidopsis thaliana mounts nonhost resistance responses, which typically result in the termination of early pathogenesis steps. We report that nonadapted anthracnose fungi engage two alternative entry modes during pathogenesis on leaves: turgor-mediated invasion beneath melanized appressoria, and a previously undiscovered hyphal tip-based entry (HTE) that is independent of appressorium formation. The frequency of HTE is positively regulated by carbohydrate nutrients and appears to be subject to constitutive inhibition by the fungal mitogen-activated protein kinase (MAPK) cascade of MAPK ESSENTIAL FOR APPRESSORIUM FORMATION1. The same MAPK cascade is essential for appressorium formation. Unexpectedly, the Arabidopsis indole glucosinolate pathway restricts entry of the nonadapted anthracnose fungi only when these pathogens employ HTE. Arabidopsis mutants defective in indole glucosinolate biosynthesis or metabolism support the initiation of postinvasion growth of nonadapted Colletotrichum gloeosporioides and Colletotrichum orbiculare. However, genetic disruption of Colletotrichum appressorium formation does not permit HTE on host plants. Thus, Colletotrichum appressoria play a critical role in the suppression of preinvasion plant defenses, in addition to their previously described role in turgor-mediated plant cell invasion. We also show that HTE is the predominant morphogenetic response of Colletotrichum at wound sites. This implies the existence of a fungal sensing system to trigger appropriate morphogenetic responses during pathogenesis at wound sites and on intact leaf tissue.

Accumulation of isochorismate-derived 2,3-dihydroxybenzoic 3-O-beta-D-xyloside in arabidopsis resistance to pathogens and ageing of leaves

An intricate network of hormone signals regulates plant development and responses to biotic and abiotic stress. Salicylic acid (SA), derived from the shikimate/isochorismate pathway, is a key hormone in resistance to biotrophic pathogens. Several SA derivatives and associated modifying enzymes have been identified and implicated in the storage and channeling of benzoic acid intermediates or as bioactive molecules. However, the range and modes of action of SA-related metabolites remain elusive. In Arabidopsis, Enhanced Disease Susceptibility 1 (EDS1) promotes SA-dependent and SA-independent responses in resistance against pathogens. Here, we used metabolite profiling of Arabidopsis wild type and eds1 mutant leaf extracts to identify molecules, other than SA, whose accumulation requires EDS1 signaling. Nuclear magnetic resonance and mass spectrometry of isolated and purified compounds revealed 2,3-dihydroxybenzoic acid (2,3-DHBA) as an isochorismate-derived secondary metabolite whose accumulation depends on EDS1 in resistance responses and during ageing of plants. 2,3-DHBA exists predominantly as a xylose-conjugated form (2-hydroxy-3-beta-O-D-xylopyranosyloxy benzoic acid) that is structurally distinct from known SA-glucose conjugates. Analysis of DHBA accumulation profiles in various Arabidopsis mutants suggests an enzymatic route to 2,3-DHBA synthesis that is under the control of EDS1. We propose that components of the EDS1 pathway direct the generation or stabilization of 2,3-DHBA, which as a potentially bioactive molecule is sequestered as a xylose conjugate.

Tryptophan-derived metabolites are required for antifungal defense in the Arabidopsis mlo2 mutant

Arabidopsis (Arabidopsis thaliana) genes MILDEW RESISTANCE LOCUS O2 (MLO2), MLO6, and MLO12 exhibit unequal genetic redundancy with respect to the modulation of defense responses against powdery mildew fungi and the control of developmental phenotypes such as premature leaf decay. We show that early chlorosis and necrosis of rosette leaves in mlo2 mlo6 mlo12 mutants reflects an authentic but untimely leaf senescence program. Comparative transcriptional profiling revealed that transcripts of several genes encoding tryptophan biosynthetic and metabolic enzymes hyperaccumulate during vegetative development in the mlo2 mlo6 mlo12 mutant. Elevated expression levels of these genes correlate with altered steady-state levels of several indolic metabolites, including the phytoalexin camalexin and indolic glucosinolates, during development in the mlo2 single mutant and the mlo2 mlo6 mlo12 triple mutant. Results of genetic epistasis analysis suggest a decisive role for indolic metabolites in mlo2-conditioned antifungal defense against both biotrophic powdery mildews and a camalexin-sensitive strain of the necrotrophic fungus Botrytis cinerea. The wound- and pathogen-responsive callose synthase POWDERY MILDEW RESISTANCE4/GLUCAN SYNTHASE-LIKE5 was found to be responsible for the spontaneous callose deposits in mlo2 mutant plants but dispensable for mlo2-conditioned penetration resistance. Our data strengthen the notion that powdery mildew resistance of mlo2 genotypes is based on the same defense execution machinery as innate antifungal immune responses that restrict the invasion of nonadapted fungal pathogens.