Induction of Arabidopsis defense genes by virulent and avirulent Pseudomonas syringae strains and by a cloned avirulence gene

Transcriptomic and Metabolomic Analysis of a Pseudomonas-Resistant versus a Susceptible Arabidopsis Accession

Accessions of one plant species may show significantly different levels of susceptibility to stresses. The Arabidopsis thaliana accessions Col-0 and C24 differ significantly in their resistance to the pathogen Pseudomonas syringae pv. tomato (Pst). To help unravel the underlying mechanisms contributing to this naturally occurring variance in resistance to Pst, we analyzed changes in transcripts and compounds from primary and secondary metabolism of Col-0 and C24 at different time points after infection with Pst. Our results show that the differences in the resistance of Col-0 and C24 mainly involve mechanisms of salicylic-acid-dependent systemic acquired resistance, while responses of jasmonic-acid-dependent mechanisms are shared between the two accessions. In addition, arginine metabolism and differential activity of the biosynthesis pathways of aliphatic glucosinolates and indole glucosinolates may also contribute to the resistance. Thus, this study highlights the difference in the defense response strategies utilized by different genotypes.

A Phakopsora pachyrhizi Effector Suppresses PAMP-Triggered Immunity and Interacts with a Soybean Glucan Endo-1,3-β-Glucosidase to Promote Virulence

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is one of the most important diseases affecting soybean production in tropical areas. During infection, P. pachyrhizi secretes proteins from haustoria that are transferred into plant cells to promote virulence. To date, only one candidate P. pachyrhizi effector protein has been characterized in detail to understand the mechanism by which it suppresses plant defenses to enhance infection. Here, we aimed to extend understanding of the pathogenic mechanisms of P. pachyrhizi based on the discovery of host proteins that interact with the effector candidate Phapa-7431740. We demonstrated that Phapa-7431740 suppresses pathogen-associated molecular pattern-triggered immunity (PTI) and that it interacts with a soybean glucan endo-1,3-β-glucosidase (GmβGLU), a pathogenesis-related (PR) protein belonging to the PR-2 family. Structural and phylogenetic characterization of the PR-2 protein family predicted in the soybean genome and comparison to PR-2 family members in Arabidopsis thaliana and cotton, demonstrated that GmβGLU is a type IV β-1,3-glucanase. Transcriptional profiling during an infection time course showed that the GmβGLU mRNA is highly induced during the initial hours after infection, coinciding with peak of expression of Phapa-7431740. The effector was able to interfere with the activity of GmβGLU in vitro, with a dose-dependent inhibition. Our results suggest that Phapa-7431740 may suppress PTI by interfering with glucan endo-1,3-β-glucosidase activity. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Mechanosensory trichome cells evoke a mechanical stimuli-induced immune response in Arabidopsis thaliana

Perception of pathogen-derived ligands by corresponding host receptors is a pivotal strategy in eukaryotic innate immunity. In plants, this is complemented by circadian anticipation of infection timing, promoting basal resistance even in the absence of pathogen threat. Here, we report that trichomes, hair-like structures on the epidermis, directly sense external mechanical forces, including raindrops, to anticipate pathogen infections in Arabidopsis thaliana. Exposure of leaf surfaces to mechanical stimuli initiates the concentric propagation of intercellular calcium waves away from trichomes to induce defence-related genes. Propagating calcium waves enable effective immunity against pathogenic microbes through the CALMODULIN-BINDING TRANSCRIPTION ACTIVATOR 3 (CAMTA3) and mitogen-activated protein kinases. We propose an early layer of plant immunity in which trichomes function as mechanosensory cells that detect potential risks.

A rapid and non-destructive method for spatial-temporal quantification of colonization by Pseudomonas syringae pv. tomato DC3000 in Arabidopsis and tomato

BACKGROUND

The bacterial leaf pathogen Pseudomonas syringae pv tomato (Pst) is the most popular model pathogen for plant pathology research. Previous methods to study the plant-Pst interactions rely on destructive quantification of Pst colonisation, which can be labour- and time-consuming and does not allow for spatial-temporal monitoring of the bacterial colonisation. Here, we describe a rapid and non-destructive method to quantify and visualise spatial-temporal colonisation by Pst in intact leaves of Arabidopsis and tomato.

RESULTS

The method presented here uses a bioluminescent Pst DC3000 strain that constitutively expresses the luxCDABE operon from Photorhabdus luminescens (Pst::LUX) and requires a common gel documentation (Gel Doc) system with a sensitive CCD/CMOS camera and imaging software (Photoshop or Image J). By capturing bright field and bioluminescence images from Pst::LUX-infected leaves, we imaged the spatiotemporal dynamics of Pst infection. Analysis of bioluminescence from live Pst bacteria over a 5-day time course after spray inoculation of Arabidopsis revealed transition of the bacterial presence from the older leaves to the younger leaves and apical meristem. Colonisation by Pst:LUX bioluminescence was obtained from digital photos by calculating relative bioluminescence values, which is adjusted for bioluminescence intensity and normalised by leaf surface. This method detected statistically significant differences in Pst::LUX colonisation between Arabidopsis genotypes varying in basal resistance, as well as statistically significant reductions in Pst::LUX colonisation by resistance-inducing treatments in both Arabidopsis and tomato. Comparison of relative bioluminescence values to conventional colony counting on selective agar medium revealed a statistically significant correlation, which was reproducible between different Gel Doc systems.

CONCLUSIONS

We present a non-destructive method to quantify colonisation by bioluminescent Pst::LUX in plants. Using a common Gel Doc system and imaging software, our method requires less time and labour than conventional methods that are based on destructive sampling of infected leaf material. Furthermore, in contrast to conventional strategies, our method provides additional information about the spatial-temporal patterns of Pst colonisation.

Differential Regulation of the Ribosomal Association of mRNA Transcripts in an Arabidopsis Mutant Defective in Jasmonate-Dependent Wound Response

Jasmonoyl-L-isoleucine (JA-Ile) is a powerful oxylipin responsible for the genome-wide transcriptional reprogramming in plants that results in major physiological shifts from growth to defense. The double T-DNA insertion Arabidopsis mutant, cyp94b1cyp94b3 (b1b3), defective in cytochrome p450s, CYP94B1 and CYP94B3, which are responsible for oxidizing JA-Ile, accumulates several fold higher levels of JA-Ile yet displays dampened JA-Ile-dependent wound responses-the opposite of what is expected. Transcriptomic and proteomic analyses showed that while the transcriptional response to wounding was largely unchanged in b1b3 compared to wild type (WT), many proteins were found to be significantly reduced in the mutant, which was verified by immunoblot analyses of marker proteins. To understand this protein phenotype and their hypothesized contribution to the b1b3 phenotypes, wounded rosette leaf samples from both WT and b1b3 were subject to a translating ribosome affinity purification RNA sequencing analysis. More than 1,600 genes whose transcripts do not change in abundance by wounding changed their association with the ribosomes after wounding in WT leaves. Consistent with previous observations, the total pool of mRNA transcripts was similar between WT and b1b3; however, the ribosome-associated pool of transcripts was changed significantly. Most notably, fewer transcripts were associated with the ribosome pool in b1b3 than in WT, potentially explaining the reduction of many proteins in the mutant. Among those genes with fewer ribosome-associated transcripts in b1b3 were genes relating to stress response, specialized metabolism, protein metabolism, ribosomal subunits, and transcription factors, consistent with the biochemical phenotypes of the mutant. These results show previously unrecognized regulations at the translational level that are affected by misregulation of JA homeostasis during the wound response in plants.

Opposing functions of the plant TOPLESS gene family during SNC1-mediated autoimmunity

Regulation of the plant immune system is important for controlling the specificity and amplitude of responses to pathogens and in preventing growth-inhibiting autoimmunity that leads to reductions in plant fitness. In previous work, we reported that SRFR1, a negative regulator of effector-triggered immunity, interacts with SNC1 and EDS1. When SRFR1 is non-functional in the Arabidopsis accession Col-0, SNC1 levels increase, causing a cascade of events that lead to autoimmunity phenotypes. Previous work showed that some members of the transcriptional co-repressor family TOPLESS interact with SNC1 to repress negative regulators of immunity. Therefore, to explore potential connections between SRFR1 and TOPLESS family members, we took a genetic approach that examined the effect of each TOPLESS member in the srfr1 mutant background. The data indicated that an additive genetic interaction exists between SRFR1 and two members of the TOPLESS family, TPR2 and TPR3, as demonstrated by increased stunting and elevated PR2 expression in srfr1 tpr2 and srfr1 tpr2 tpr3 mutants. Furthermore, the tpr2 mutation intensifies autoimmunity in the auto-active snc1-1 mutant, indicating a novel role of these TOPLESS family members in negatively regulating SNC1-dependent phenotypes. This negative regulation can also be reversed by overexpressing TPR2 in the srfr1 tpr2 background. Similar to TPR1 that positively regulates snc1-1 phenotypes by interacting with SNC1, we show here that TPR2 directly binds the N-terminal domain of SNC1. In addition, TPR2 interacts with TPR1 in vivo, suggesting that the opposite functions of TPR2 and TPR1 are based on titration of SNC1-TPR1 complexes by TPR2 or altered functions of a SNC1-TPR1-TPR2 complex. Thus, this work uncovers diverse functions of individual members of the TOPLESS family in Arabidopsis and provides evidence for the additive effect of transcriptional and post-transcriptional regulation of SNC1.

The immune system is a double-edged sword that affords organisms with protection against infectious diseases but can also lead to negative effects if not properly controlled. Plants only possess an innate antimicrobial immune system that relies on rapid upregulation of defenses once immune receptors detect the presence of microbes. Plant immune receptors known as resistance proteins play a key role in rapidly triggering defenses if pathogens breach other defenses. A common model of unregulated immunity in the reference Arabidopsis variety Columbia-0 involves a resistance gene called SNC1. When the SNC1 protein accumulates to unnaturally high levels or possesses auto-activating mutations, the visible manifestations of immune overactivity include stunted growth and low biomass and seedset. Consequently, expression of this gene and accumulation of the encoded protein are tightly regulated on multiple levels. Despite careful study the mechanisms of SNC1 gene regulation are not fully understood. Here we present data on members of the well-known TOPLESS family of transcriptional repressors. While previously characterized members were shown to function in indirect activation of defenses, TPR2 and TPR3 are shown here to function in preventing high defense activity. This study therefore contributes to the understanding of complex regulatory processes in plant immunity.

Carbonic anhydrases CA1 and CA4 function in atmospheric CO2-modulated disease resistance

Carbonic anhydrases CA1 and CA4 attenuate plant immunity and can contribute to altered disease resistance levels in response to changing atmospheric CO₂ conditions. β-Carbonic anhydrases (CAs) play an important role in CO₂ metabolism and plant development, but have also been implicated in plant immunity. Here we show that the bacterial pathogen Pseudomonas syringae and application of the microbe-associated molecular pattern (MAMP) flg22 repress CA1 and CA4 gene expression in Arabidopsis thaliana. Using the CA double-mutant ca1ca4, we provide evidence that CA1 and CA4 play an attenuating role in pathogen- and flg22-triggered immune responses. In line with this, ca1ca4 plants exhibited enhanced resistance against P. syringae, which was accompanied by an increased expression of the defense-related genes FRK1 and ICS1. Under low atmospheric CO₂ conditions (150 ppm), when CA activity is typically low, the levels of CA1 transcription and resistance to P. syringae in wild-type Col-0 were similar to those observed in ca1ca4. However, under ambient (400 ppm) and elevated (800 ppm) atmospheric CO₂ conditions, CA1 transcription was enhanced and resistance to P. syringae reduced. Together, these results suggest that CA1 and CA4 attenuate plant immunity and that differential CA gene expression in response to changing atmospheric CO₂ conditions contribute to altered disease resistance levels.

Isolation of Open Chromatin Identifies Regulators of Systemic Acquired Resistance

Upon local infection, plants activate a systemic immune response called systemic acquired resistance (SAR). During SAR, systemic leaves become primed for the superinduction of defense genes upon reinfection. We used formaldehyde-assisted isolation of regulatory DNA elements coupled to next-generation sequencing to identify SAR regulators. Our bioinformatic analysis produced 10,129 priming-associated open chromatin sites in the 5' region of 3,025 genes in the systemic leaves of Arabidopsis (Arabidopsis thaliana) plants locally infected with Pseudomonas syringae pv. maculicola Whole transcriptome shotgun sequencing analysis of the systemic leaves after challenge enabled the identification of genes with priming-linked open chromatin before (contained in the formaldehyde-assisted isolation of regulatory DNA elements sequencing dataset) and enhanced expression after (included in the whole transcriptome shotgun sequencing dataset) the systemic challenge. Among them, Arabidopsis MILDEW RESISTANCE LOCUS O3 (MLO3) was identified as a previously unidentified positive regulator of SAR. Further in silico analysis disclosed two yet unknown cis-regulatory DNA elements in the 5' region of genes. The P-box was mainly associated with priming-responsive genes, whereas the C-box was mostly linked to challenge. We found that the P- or W-box, the latter recruiting WRKY transcription factors, or combinations of these boxes, characterize the 5' region of most primed genes. Therefore, this study provides a genome-wide record of genes with open and accessible chromatin during SAR and identifies MLO3 and two previously unidentified DNA boxes as likely regulators of this immune response.

Nicotiana species as surrogate host for studying the pathogenicity of Acidovorax citrulli, the causal agent of bacterial fruit blotch of cucurbits

Bacterial fruit blotch (BFB) caused by Acidovorax citrulli is one of the most important bacterial diseases of cucurbits worldwide. However, the mechanisms associated with A. citrulli pathogenicity and genetics of host resistance have not been extensively investigated. We idenitfied Nicotiana benthamiana and Nicotiana tabacum as surrogate hosts for studying A. citrulli pathogenicity and non-host resistance triggered by type III secreted (T3S) effectors. Two A. citrulli strains, M6 and AAC00-1, that represent the two major groups amongst A. citrulli populations, induced disease symptoms on N. benthamiana, but triggered a hypersensitive response (HR) on N. tabacum plants. Transient expression of 19 T3S effectors from A. citrulli in N. benthamiana leaves revealed that three effectors, Aave_1548, Aave_2708, and Aave_2166, trigger water-soaking-like cell death in N. benthamiana. Aave_1548 knockout mutants of M6 and AAC00-1 displayed reduced virulence on N. benthamiana and melon (Cucumis melo L.). Transient expression of Aave_1548 and Aave_2166 effectors triggered a non-host HR in N. tabacum, which was dependent on the functionality of the immune signalling component, NtSGT1. Hence, employing Nicotiana species as surrogate hosts for studying A. citrulli pathogenicity may help characterize the function of A. citrulli T3S effectors and facilitate the development of new strategies for BFB management.

Direct Regulation of the EFR-Dependent Immune Response by Arabidopsis TCP Transcription Factors

One layer of the innate immune system allows plants to recognize pathogen-associated molecular patterns (PAMPS), activating a defense response known as PAMP-triggered immunity (PTI). Maintaining an active immune response, however, comes at the cost of plant growth and development; accordingly, optimization of the balance between defense and development is critical to plant fitness. The TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor family consists of well-characterized transcriptional regulators of plant development and morphogenesis. The three closely related class I TCP transcription factors TCP8, TCP14, and TCP15 have also been implicated in the regulation of effector-triggered immunity, but there has been no previous characterization of PTI-related phenotypes. To identify TCP targets involved in PTI, we screened a PAMP-induced gene promoter library in a yeast one-hybrid assay and identified interactions of these three TCPs with the EF-Tu RECEPTOR (EFR) promoter. The direct interactions between TCP8 and EFR were confirmed to require an intact TCP binding site in planta. A tcp8 tcp14 tcp15 triple mutant was impaired in EFR-dependent PTI and exhibited reduced levels of PATHOGENESIS-RELATED PROTEIN 2 and induction of EFR expression after elicitation with elf18 but also increased production of reactive oxygen species relative to Col-0. Our data support an increasingly complex role for TCPs at the nexus of plant development and defense.

Duckweed (Lemna minor) and Alfalfa (Medicago sativa) as Bacterial Infection Model Systems

Alternative animal host models of bacterial infection have been developed which reproduce some of the disease conditions observed in higher animals. Analogously, plants are useful for modeling bacterial pathogenesis, in some cases revealing broadly conserved infection mechanisms. Similar to animals, plants have been shown to possess innate immune systems that respond to invading viruses, bacteria, and fungi. Plant infection models often yield results faster, are more convenient, and less expensive than many animal infection models. Here, we describe the use of two different plant-based infection models for the discovery of virulence genes and factors involved in bacterial pathogenesis.

Proteome of olive non-glandular trichomes reveals protective protein network against (a)biotic challenge

Olive is one of the most important fruit crop trees in the history of Mediterranean because of the high quality oil. Olive oil has a well-balanced fatty acid composition along with biophenols, which make it exceptional in human diet and provide an exceptional value to the olive oil. Leaf non-glandular peltate trichomes are specialized cell types representing a protective barrier against acute environmental conditions. To characterize the proteome of this highly differentiated cell type, we performed a comparative proteomic analysis among isolated trichomes and trichome-less leaves. Proteins were separated and identified using the 2-DE MALDI-TOF/MS method. A number of enzymes involved in abiotic and biotic stress responses are present and may be responsible for the adaptation to prolonged adverse environmental conditions. The results show that this highly differentiated cell type is physiologically active fulfilling the demands of the trichomes in furnishing the leaf with a highly protective mechanism.

Tracing My Roots: How I Became a Plant Biologist

My trajectory to becoming a plant biologist was shaped by a complex mix of scientific, political, sociological, and personal factors. I was trained as a microbiologist and molecular biologist in the late 1960s and early 1970s, a time of political upheaval surrounding the Vietnam War. My political activism taught me to be wary of the potential misuses of scientific knowledge and to promote the positive applications of science for the benefit of society. I chose agricultural science for my postdoctoral work. Because I was not trained as a plant biologist, I devised a postdoctoral project that took advantage of my microbiological training, and I explored using genetic technologies to transfer the ability to fix nitrogen from prokaryotic nitrogen-fixing species to the model plant Arabidopsis thaliana with the ultimate goal of engineering crop plants. The invention of recombinant DNA technology greatly facilitated the cloning and manipulation of bacterial nitrogen-fixation ( nif) genes, but it also forced me to consider how much genetic engineering of organisms, including human beings, is acceptable. My laboratory has additionally studied host–pathogen interactions using Arabidopsis and the nematode Caenorhabditis elegans as model hosts.

MicroRNA396a-5p and -3p induce tomato disease susceptibility by suppressing target genes and upregulating salicylic acid

Plants have evolved a variety of mechanisms to perceive and resist the assault of pathogens. The biotrophs, necrotrophs and hemibiotrophs are types of plant pathogens that activate diverse salicylic acid (SA) and jasmonic acid (JA) signaling pathways. In this study we showed that the expressions of miR396a-5p and -3p in Solanum lycopersicum (S. lycopersicum) were both down-regulated after infection by hemibiotroph Phytophthora infestans (P. infestans) and necrotroph Botrytis cinerea (B. cinerea) infection. Overexpression of miR396a-5p and -3p in transgenic tomato enhanced the susceptibility of S. lycopersicum to P. infestans and B. cinerea infection and the tendency to produce reactive oxygen species (ROS) under pathogen-related biotic stress. Additionally, miR396a regulated growth-regulating factor1 (GRF1), salicylic acid carboxyl methyltransferase (SAMT), glycosyl hydrolases (GH) and nucleotide-binding site-leucine-rich repeat (NBS-LRR) and down-regulated their levels. This ultimately led to inhibition of the expression of pathogenesis-related 1 (PR1), TGA transcription factors1 and 2 (TGA1 and TGA2) and JA-dependent proteinase inhibitors I and II (PI I and II), but enhanced the endogenous SA content and nonexpressor of pathogenesis-related genes 1 (NPR1) expression. Taken together, our results showed that negative regulation of target genes and their downstream genes expressions by miR396a-5p and -3p are critical for tomato abiotic stresses via affecting SA or JA signaling pathways.

Early Arabidopsis root hair growth stimulation by pathogenic strains of Pseudomonas syringae

BACKGROUND AND AIMS

Selected beneficial Pseudomonas spp. strains have the ability to influence root architecture in Arabidopsis thaliana by inhibiting primary root elongation and promoting lateral root and root hair formation. A crucial role for auxin in this long-term (1week), long-distance plant-microbe interaction has been demonstrated.

METHODS

Arabidopsis seedlings were cultivated in vitro on vertical plates and inoculated with pathogenic strains Pseudomonas syringae pv. maculicola (Psm) and P. syringae pv. tomato DC3000 (Pst), as well as Agrobacterium tumefaciens (Atu) and Escherichia coli (Eco). Root hair lengths were measured after 24 and 48h of direct exposure to each bacterial strain. Several Arabidopsis mutants with impaired responses to pathogens, impaired ethylene perception and defects in the exocyst vesicle tethering complex that is involved in secretion were also analysed.

KEY RESULTS

Arabidopsis seedling roots infected with Psm or Pst responded similarly to when infected with plant growth-promoting rhizobacteria; root hair growth was stimulated and primary root growth was inhibited. Other plant- and soil-adapted bacteria induced similar root hair responses. The most compromised root hair growth stimulation response was found for the knockout mutants exo70A1 and ein2. The single immune pathways dependent on salicylic acid, jasmonic acid and PAD4 are not directly involved in root hair growth stimulation; however, in the mutual cross-talk with ethylene, they indirectly modify the extent of the stimulation of root hair growth. The Flg22 peptide does not initiate root hair stimulation as intact bacteria do, but pretreatment with Flg22 prior to Psm inoculation abolished root hair growth stimulation in an FLS2 receptor kinase-dependent manner. These early response phenomena are not associated with changes in auxin levels, as monitored with the pDR5::GUS auxin reporter.

CONCLUSIONS

Early stimulation of root hair growth is an effect of an unidentified component of living plant pathogenic bacteria. The root hair growth response is triggered in the range of hours after bacterial contact with roots and can be modulated by FLS2 signalling. Bacterial stimulation of root hair growth requires functional ethylene signalling and an efficient exocyst-dependent secretory machinery.

Silencing of AtRAP, a target gene of a bacteria-induced small RNA, triggers antibacterial defense responses through activation of LSU2 and down-regulation of GLK1

Plants fine-tune their sophisticated immunity systems in response to pathogen infections. We previously showed that AtlsiRNA-1, a bacteria-induced plant endogenous small interfering RNA, silences the AtRAP gene, which encodes a putative RNA binding protein. In this study, we demonstrate that AtRAP functions as a negative regulator in plant immunity by characterizing molecular and biological responses of the knockout mutant and overexpression lines of AtRAP upon bacterial infection. AtRAP is localized in chloroplasts and physically interacts with Low Sulfur Upregulated 2 (LSU2), which positively regulates plant defense. Our results suggest that AtRAP negatively regulates defense responses by suppressing LSU2 through physical interaction. We also detected downregulation of the transcription factor GOLDEN2-LIKE 1 (GLK1) in atrap-1 using microarray analysis. The glk1 glk2 double mutant showed enhanced resistance to Pseudomonas syringae pv. tomato, which is consistent with a previous study showing enhanced resistance of a glk1 glk2 double mutant to Hyaloperonospora arabidopsidis. Taken together, our data suggest that silencing of AtRAP by AtlsiRNA-1 upon bacterial infection triggers defense responses through regulation of LSU2 and GLK1.

Identification of a Vitis vinifera endo-β-1,3-glucanase with antimicrobial activity against Plasmopara viticola

Inducible plant defences against pathogens are stimulated by infections and comprise several classes of pathogenesis-related (PR) proteins. Endo-β-1,3-glucanases (EGases) belong to the PR-2 class and their expression is induced by many pathogenic fungi and oomycetes, suggesting that EGases play a role in the hydrolysis of pathogen cell walls. However, reports of a direct effect of EGases on cell walls of plant pathogens are scarce. Here, we characterized three EGases from Vitis vinifera whose expression is induced during infection by Plasmopara viticola, the causal agent of downy mildew. Recombinant proteins were expressed in Escherichia coli. The enzymatic characteristics of these three enzymes were measured in vitro and in planta. A functional assay performed in vitro on germinated P. viticola spores revealed a strong anti-P. viticola activity for EGase3, which strikingly was that with the lowest in vitro catalytic efficiency. To our knowledge, this work shows, for the first time, the direct effect against downy mildew of EGases of the PR-2 family from Vitis.

ELF18-INDUCED LONG-NONCODING RNA Associates with Mediator to Enhance Expression of Innate Immune Response Genes in Arabidopsis

The plant immune response is a complex process involving transcriptional and posttranscriptional regulation of gene expression. Responses to plant immunity are initiated upon the perception of pathogen-associated molecular patterns, including peptide fragment of bacterial flagellin (flg22) or translation elongation factor Tu (elf18). Here, we identify an Arabidopsis thaliana long-noncoding RNA, designated ELF18-INDUCED LONG-NONCODING RNA1 (ELENA1), as a factor enhancing resistance against Pseudomonas syringe pv tomato DC3000. ELENA1 knockdown plants show decreased expression of PATHOGENESIS-RELATED GENE1 (PR1) and the plants are susceptible to pathogens. By contrast, plants overexpressing ELENA1 show elevated PR1 expression after elf18 treatment and display a pathogen resistance phenotype. RNA-sequencing analysis of ELENA1-overexpressing plants after elf18 treatment confirms increased expression of defense-related genes compared with the wild type. ELENA1 directly interacts with Mediator subunit 19a (MED19a) and affects enrichment of MED19a on the PR1 promoter. These results show that MED19a regulates PR1 expression through ELENA1. Our findings uncover an additional layer of complexity, implicating long-noncoding RNAs in the transcriptional regulation of plant innate immunity.

Protein phosphatase AP2C1 negatively regulates basal resistance and defense responses to Pseudomonas syringae

Mitogen-activated protein kinases (MAPKs) mediate plant immune responses to pathogenic bacteria. However, less is known about the cell autonomous negative regulatory mechanism controlling basal plant immunity. We report the biological role of Arabidopsis thaliana MAPK phosphatase AP2C1 as a negative regulator of plant basal resistance and defense responses to Pseudomonas syringae. AP2C2, a closely related MAPK phosphatase, also negatively controls plant resistance. Loss of AP2C1 leads to enhanced pathogen-induced MAPK activities, increased callose deposition in response to pathogen-associated molecular patterns or to P. syringae pv. tomato (Pto) DC3000, and enhanced resistance to bacterial infection with Pto. We also reveal the impact of AP2C1 on the global transcriptional reprogramming of transcription factors during Pto infection. Importantly, ap2c1 plants show salicylic acid-independent transcriptional reprogramming of several defense genes and enhanced ethylene production in response to Pto. This study pinpoints the specificity of MAPK regulation by the different MAPK phosphatases AP2C1 and MKP1, which control the same MAPK substrates, nevertheless leading to different downstream events. We suggest that precise and specific control of defined MAPKs by MAPK phosphatases during plant challenge with pathogenic bacteria can strongly influence plant resistance.

Characterization of VvPAL-like promoter from grapevine using transgenic tobacco plants

A 2000-bp 5'-flanking region of VvPAL-like was isolated from 'Summer Black' grapevine by PCR amplification, named pVvPAL-like. To gain a better understanding of the expression and regulatory mechanism of VvPAL-like, a chimeric expression unit consisting of the β-glucuronidase (GUS) reporter gene under the control of a 2000-bp fragment of the VvPAL-like promoter was transformed into tobacco via Agrobacterium tumefaciens. Histochemical staining showed that the full-length promoter directs efficient expression of the reporter gene in cotyledons and hypocotyls, stigma, style, anthers, pollen, ovary, trichomes, and vascular bundles of transgenic plants. A series of 5' progressive deletions of the promoter revealed the presence of a negative regulatory region (-424 to -292) in the VvPAL-like promoter. Exposure of the transgenic tobacco plants to various abiotic stresses demonstrated that the full-length construct could be induced by light, copper (Cu), abscisic acid (ABA), indole-3-acetic (IAA), methyl jasmonate (MeJA) (N-1-naphthylphthalamic acid), ethylene, and drought. Furthermore, the ethylene-responsive region was found to be located in the -1461/-930 fragment, while the element(s) for the MeJA-responsive expression may be present in the -424/-292 region in the VvPAL-like promoter. These findings will help us to better understand the molecular mechanisms by which VvPAL-like participates in biosynthesis of flavonoids and stress responses.

A De Novo-Assembly Based Data Analysis Pipeline for Plant Obligate Parasite Metatranscriptomic Studies

Current and emerging plant diseases caused by obligate parasitic microbes such as rusts, downy mildews, and powdery mildews threaten worldwide crop production and food safety. These obligate parasites are typically unculturable in the laboratory, posing technical challenges to characterize them at the genetic and genomic level. Here we have developed a data analysis pipeline integrating several bioinformatic software programs. This pipeline facilitates rapid gene discovery and expression analysis of a plant host and its obligate parasite simultaneously by next generation sequencing of mixed host and pathogen RNA (i.e., metatranscriptomics). We applied this pipeline to metatranscriptomic sequencing data of sweet basil (Ocimum basilicum) and its obligate downy mildew parasite Peronospora belbahrii, both lacking a sequenced genome. Even with a single data point, we were able to identify both candidate host defense genes and pathogen virulence genes that are highly expressed during infection. This demonstrates the power of this pipeline for identifying genes important in host-pathogen interactions without prior genomic information for either the plant host or the obligate biotrophic pathogen. The simplicity of this pipeline makes it accessible to researchers with limited computational skills and applicable to metatranscriptomic data analysis in a wide range of plant-obligate-parasite systems.

Ectopic expression of a grape aspartic protease gene, AP13, in Arabidopsis thaliana improves resistance to powdery mildew but increases susceptibility to Botrytis cinerea

The grape aspartic protease gene, AP13 was previously reported to be responsive, in Chinese wild Vitis quinquangularis cv. 'Shang-24', to infection by Erysiphe necator, the causal agent of powdery mildew disease, as well as to treatment with salicylic acid in V. labrusca×V. vinifera cv. 'Kyoho'. In the current study, we evaluated the expression levels of AP13 in 'Shang-24' in response to salicylic acid (SA), methyl jasmonate (MeJA) and ethylene (ET) treatments, as well as to infection by the necrotrophic fungus, Botrytis cinerea, and the transcript levels of VqAP13 decreased after B. cinerea infection and MeJA treatment, but increased following ET and SA treatments. Transgenic Arabidopsis thaliana lines over-expressing VqAP13 under the control of a constitutive promoter showed enhanced resistance to powdery mildew and to the bacterium Pseudomonas syringae pv. tomato DC3000, and accumulated more callose than wild type plants, while the resistance of transgenic A. thaliana lines to B. cinerea inoculation was reduced. In addition, the expression profiles of various disease resistance- related genes in the transgenic A. thaliana lines following infection by different pathogens were compared to the equivalent profiles in the wild type plants. The results suggest that VqAP13 action promotes the SA dependent signal transduction pathway, but suppresses the JA signal transduction pathway.

Genes from pUM505 plasmid contribute to Pseudomonas aeruginosa virulence

The pUM505 plasmid was isolated from a clinical strain of Pseudomonas aeruginosa. This plasmid contains a genomic island with sequence similar to islands found in chromosomes of virulent P. aeruginosa clinical isolates. The objective of this work was to determine whether pUM505 increases the virulence of P. aeruginosa and to identify the genes responsible for this property. First, using the lettuce-leaf model, we found that pUM505 significantly increases the virulence of P. aeruginosa reference strain PAO1. pUM505 also increased the PAO1 virulence in a murine model and increased cytotoxicity of this strain toward HeLa cells. Thus, we generated a pUM505 gene library of 103 clones in the pUCP20 binary vector. The library was transferred to Escherichia coli TOP10 and P. aeruginosa PAO1 to identify genes. The lettuce-leaf model allowed us to identify three recombinant plasmids that increased the virulence of both E. coli and P. aeruginosa strains. These recombinant plasmids also increased the virulence of the PAO1 strain in mice and induced a cytotoxic effect in HeLa cells. Eleven genes were identified in the virulent transformants. Of these genes, only the pUM505 ORF 2 has homology with a gene previously implicated in virulence. These results indicate that pUM505 contains several genes that encode virulence factors, suggesting that the plasmid may contribute directly to bacterial virulence.

Danger peptide receptor signaling in plants ensures basal immunity upon pathogen-induced depletion of BAK1

Pathogens infect a host by suppressing defense responses induced upon recognition of microbe-associated molecular patterns (MAMPs). Despite this suppression, MAMP receptors mediate basal resistance to limit host susceptibility, via a process that is poorly understood. The Arabidopsis leucine-rich repeat (LRR) receptor kinase BAK1 associates and functions with different cell surface LRR receptors for a wide range of ligands, including MAMPs. We report that BAK1 depletion is linked to defense activation through the endogenous PROPEP peptides (Pep epitopes) and their LRR receptor kinases PEPR1/PEPR2, despite critical defects in MAMP signaling. In bak1-knockout plants, PEPR elicitation results in extensive cell death and the prioritization of salicylate-based defenses over jasmonate-based defenses, in addition to elevated proligand and receptor accumulation. BAK1 disruption stimulates the release of PROPEP3, produced in response to Pep application and during pathogen challenge, and renders PEPRs necessary for basal resistance. These findings are biologically relevant, since specific BAK1 depletion coincides with PEPR-dependent resistance to the fungal pathogen Colletotrichum higginsianum. Thus, the PEPR pathway ensures basal resistance when MAMP-triggered defenses are compromised by BAK1 depletion.

Decreased Polysaccharide Feruloylation Compromises Plant Cell Wall Integrity and Increases Susceptibility to Necrotrophic Fungal Pathogens

The complexity of cell wall composition and structure determines the strength, flexibility, and function of the primary cell wall in plants. However, the contribution of the various components to cell wall integrity (CWI) and function remains unclear. Modifications of cell wall composition can induce plant responses known as CWI control. In this study, we used transgenic expression of the fungal feruloyl esterase AnFAE to examine the effect of post-synthetic modification of Arabidopsis and Brachypodium cell walls. Transgenic Arabidopsis plants expressing AnFAE showed a significant reduction of monomeric ferulic acid, decreased amounts of wall-associated extensins, and increased susceptibility to Botrytis cinerea, compared with wild type. Transgenic Brachypodium showed reductions in monomeric and dimeric ferulic acids and increased susceptibility to Bipolaris sorokiniana. Upon infection, transgenic Arabidopsis and Brachypodium plants also showed increased expression of several defense-related genes compared with wild type. These results demonstrate a role, in both monocot and dicot plants, of polysaccharide feruloylation in plant CWI, which contributes to plant resistance to necrotrophic pathogens.

Host target modification as a strategy to counter pathogen hijacking of the jasmonate hormone receptor

In the past decade, characterization of the host targets of pathogen virulence factors took a center stage in the study of pathogenesis and disease susceptibility in plants and humans. However, the impressive knowledge of host targets has not been broadly exploited to inhibit pathogen infection. Here, we show that host target modification could be a promising new approach to "protect" the disease-vulnerable components of plants. In particular, recent studies have identified the plant hormone jasmonate (JA) receptor as one of the common targets of virulence factors from highly evolved biotrophic/hemibiotrophic pathogens. Strains of the bacterial pathogen Pseudomonas syringae, for example, produce proteinaceous effectors, as well as a JA-mimicking toxin, coronatine (COR), to activate JA signaling as a mechanism to promote disease susceptibility. Guided by the crystal structure of the JA receptor and evolutionary clues, we succeeded in modifying the JA receptor to allow for sufficient endogenous JA signaling but greatly reduced sensitivity to COR. Transgenic Arabidopsis expressing this modified receptor not only are fertile and maintain a high level of insect defense, but also gain the ability to resist COR-producing pathogens Pseudomonas syringae pv. tomato and P. syringae pv. maculicola. Our results provide a proof-of-concept demonstration that host target modification can be a promising new approach to prevent the virulence action of highly evolved pathogens.

Bacterial Leaf Infiltration Assay for Fine Characterization of Plant Defense Responses using the Arabidopsis thaliana-Pseudomonas syringae Pathosystem

In the absence of specialized mobile immune cells, plants utilize their localized programmed cell death and Systemic Acquired Resistance to defend themselves against pathogen attack. The contribution of a specific Arabidopsis gene to the overall plant immune response can be specifically and quantitatively assessed by assaying the pathogen growth within the infected tissue. For over three decades, the hemibiotrophic bacterium Pseudomonas syringae pv. maculicola ES4326 (Psm ES4326) has been widely applied as the model pathogen to investigate the molecular mechanisms underlying the Arabidopsis immune response. To deliver pathogens into the leaf tissue, multiple inoculation methods have been established, e.g., syringe infiltration, dip inoculation, spray, vacuum infiltration, and flood inoculation. The following protocol describes an optimized syringe infiltration method to deliver virulent Psm ES4326 into leaves of adult soil-grown Arabidopsis plants and accurately screen for enhanced disease susceptibility (EDS) towards this pathogen. In addition, this protocol can be supplemented with multiple pre-treatments to further dissect specific immune defects within different layers of plant defense, including Salicylic Acid (SA)-Triggered Immunity (STI) and MAMP-Triggered Immunity (MTI).

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

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

Transgenic Arabidopsis thaliana plants expressing a β-1,3-glucanase from sweet sorghum (Sorghum bicolor L.) show reduced callose deposition and increased tolerance to aluminium toxicity

Seventy-one cultivars of sweet sorghum (Sorghum bicolor L.) were screened for aluminium (Al) tolerance by measuring relative root growth (RRG). Two contrasting cultivars, ROMA (Al tolerant) and POTCHETSTRM (Al sensitive), were selected to study shorter term responses to Al stress. POTCHETSTRM had higher callose synthase activity, lower β-1,3-glucanase activity and more callose deposition in the root apices during Al treatment compared with ROMA. We monitored the expression of 12 genes involved in callose synthesis and degradation and found that one of these, SbGlu1 (Sb03g045630.1), which encodes a β-1,3-glucanase enzyme, best explained the contrasting deposition of callose in ROMA and POTCHETSTRM during Al treatment. Full-length cDNAs of SbGlu1 was prepared from ROMA and POTCHETSTRM and expressed in Arabidopsis thaliana using the constitutive cauliflower mosaic virus (CaMV) 35S promoter. Independent transgenic lines displayed significantly greater Al tolerance than wild-type plants and vector-only controls. This phenotype was associated with greater total β-1,3-glucanase activity, less Al accumulation and reduced callose deposition in the roots. These results suggest that callose production is not just an early indicator of Al stress in plants but likely to be part of the toxicity pathway that leads to the inhibition of root growth.

Changes in the proteome of pad2-1, a glutathione depleted Arabidopsis mutant, during Pseudomonas syringae infection

The involvement of glutathione (GSH) in plant defense against pathogen invasion is an established fact. However, the molecular mechanism conferring this tolerance remains to be explored. Here, proteomic analysis of pad2-1, an Arabidopsis thaliana GSH-depleted mutant, in response to Pseudomonas syringae infection has been performed to explore the intricate position of GSH in defense against biotrophic pathogens. The pad2-1 mutant displayed severe susceptibility to P. syringae infection compared to the wild-type (Col-0) thus re-establishing a fundamental role of GSH in defense. Apart from general up-accumulation of energy metabolism-related protein-species in both infected Col-0 and pad2-1, several crucial defense-related protein-species were identified to be differentially accumulated. Leucine-rich repeat-receptor kinase (LRR-RK) and nucleotide-binding site-leucine-rich repeat resistance protein (NBS-LRR), known to play a pioneering role against pathogen attack, were only weakly up-accumulated in pad2-1 after infection. Transcriptional and post-transcriptional regulators like MYB-P1 and glycine-rich repeat RNA-binding protein (GRP) and several other stress-related protein-species like heat shock protein 17 (HSP17) and glutathione-S-transferase (GST) were also identified to be differentially regulated in pad2-1 and Col-0 in response to infection. Together, the present investigation reveals that the optimum GSH-level is essential for the efficient activation of plant defense signaling cascades thus conferring resistance to pathogen invasion.

Apoplastic peroxidases are required for salicylic acid-mediated defense against Pseudomonas syringae

Reactive oxygen species (ROS) generated by NADPH oxidases or apoplastic peroxidases play an important role in the plant defense response. Diminished expression of at least two Arabidopsis thaliana peroxidase encoding genes, PRX33 (At3g49110) and PRX34 (At3g49120), as a consequence of anti-sense expression of a heterologous French bean peroxidase gene (asFBP1.1), were previously shown to result in reduced levels of ROS following pathogen attack, enhanced susceptibility to a variety of bacterial and fungal pathogens, and reduced levels of callose production and defense-related gene expression in response to the microbe associated molecular pattern (MAMP) molecules flg22 and elf26. These data demonstrated that the peroxidase-dependent oxidative burst plays an important role in the elicitation of pattern-triggered immunity (PTI). Further work reported in this paper, however, shows that asFBP1.1 antisense plants are not impaired in all PTI-associated responses. For example, some but not all flg22-elicited genes are induced to lower levels by flg22 in asFPB1.1, and callose deposition in asFPB1.1 is similar to wild-type following infiltration with a Pseudomonas syringae hrcC mutant or with non-host P. syringae pathovars. Moreover, asFPB1.1 plants did not exhibit any apparent defect in their ability to mount a hypersensitive response (HR). On the other hand, salicylic acid (SA)-mediated activation of PR1 was dramatically impaired in asFPB1.1 plants. In addition, P. syringae-elicited expression of many genes known to be SA-dependent was significantly reduced in asFBP1.1 plants. Consistent with this latter result, in asFBP1.1 plants the key regulator of SA-mediated responses, NPR1, showed both dramatically decreased total protein abundance and a failure to monomerize, which is required for its translocation into the nucleus.

A high-sensitivity optical device for the early monitoring of plant pathogen attack via the in vivo detection of ROS bursts

Biotic stressors, especially pathogenic microorganisms, are rather difficult to detect. In plants, one of the earliest cellular responses following pathogen infection is the production of reactive oxygen species (ROS). In this study, a novel optical device for the early monitoring of Pseudomonas attack was developed; this device measures the ROS level via oxidation-sensitive 2', 7'-dichlorodihydrofluorescein diacetate (H2DCFDA)-mediated fluorescence, which could provide early monitoring of attacks by a range of plant pathogen; ROS bursts were detected in vivo in Arabidopsis thaliana with higher sensitivity and accuracy than those of a commercial luminescence spectrophotometer. Additionally, the DCF fluorescence truly reflected early changes in the ROS level, as indicated by an evaluation of the H2O2 content and the tight association between the ROS and Pseudomonas concentration. Moreover, compared with traditional methods for detecting plant pathogen attacks based on physiological and biochemical measurements, our proposed technique also offers significant advantages, such as low cost, simplicity, convenient operation and quick turnaround. These results therefore suggest that the proposed optical device could be useful for the rapid monitoring of attacks by plant pathogen and yield results considerably earlier than the appearance of visual changes in plant morphology or growth.

The Arabidopsis immune adaptor SRFR1 interacts with TCP transcription factors that redundantly contribute to effector-triggered immunity

The plant immune system must be tightly controlled both positively and negatively to maintain normal plant growth and health. We previously identified SUPPRESSOR OF rps4-RLD1 (SRFR1) as a negative regulator specifically of effector-triggered immunity. SRFR1 is localized in both a cytoplasmic microsomal compartment and in the nucleus. Its TPR domain has sequence similarity to TPR domains of transcriptional repressors in other organisms, suggesting that SRFR1 may negatively regulate effector-triggered immunity via transcriptional control. We show here that excluding SRFR1 from the nucleus prevented complementation of the srfr1 phenotype. To identify transcription factors that interact with SRFR1, we screened an Arabidopsis transcription factor prey library by yeast two-hybrid assay and isolated six class I members of the TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factor family. Specific interactions were verified in planta. Although single or double T-DNA mutant tcp8, tcp14 or tcp15 lines were not more susceptible to bacteria expressing AvrRps4, the triple tcp8 tcp14 tcp15 mutant displayed decreased effector-triggered immunity mediated by the resistance genes RPS2, RPS4, RPS6 and RPM1. In addition, expression of PATHOGENESIS-RELATED PROTEIN2 was attenuated in srfr1-4 tcp8-1 tcp14-5 tcp15-3 plants compared to srfr1-4 plants. To date, TCP transcription factors have been implicated mostly in developmental processes. Our data indicate that one function of a subset of TCP proteins is to regulate defense gene expression in antagonism to SRFR1, and suggest a mechanism for an intimate connection between plant development and immunity.

Turnip crinkle virus coat protein inhibits the basal immune response to virus invasion in Arabidopsis by binding to the NAC transcription factor TIP

Turnip crinkle virus (TCV) has been shown to interact with a NAC transcription factor, TIP, of Arabidopsis thaliana, via its coat protein (CP). This interaction correlates with the resistance response manifested in TCV-resistant Arabidopsis ecotype Di-17. We report that failure of a mutated CP to interact with TIP triggered the corresponding TCV mutant (R6A) to cause more severe symptoms in the TCV-susceptible ecotype Col-0. We hypothesized that TCV regulates antiviral basal immunity through TIP-CP interaction. Consistent with this hypothesis, we found that the rate of accumulation of R6A was measurably slower than wild-type TCV over the course of an infection. Notably, R6A was able to accumulate at similar rates as wild-type TCV in mutant plants with defects in salicylic acid (SA) signaling. Finally, plants with altered TIP expression provided evidence R6A's inability to evade the basal resistance response was likely associated with loss of ability for CP to bind TIP.

The CALMODULIN-BINDING PROTEIN60 family includes both negative and positive regulators of plant immunity

Two members of the eight-member CALMODULIN-BINDING PROTEIN60 (CBP60) gene family, CBP60g and SYSTEMIC ACQUIRED RESISTANCE DEFICIENT1 (SARD1), encode positive regulators of plant immunity that promote the production of salicylic acid (SA) and affect the expression of SA-dependent and SA-independent defense genes. Here, we investigated the other six family members in Arabidopsis (Arabidopsis thaliana). Only cbp60a mutations affected growth of the bacterial pathogen Pseudomonas syringae pv maculicola ES4326. In contrast to cbp60g and sard1 mutations, cbp60a mutations reduced pathogen growth, indicating that CBP60a is a negative regulator of immunity. Bacterial growth was increased by cbp60g only in the presence of CBP60a, while the increase in growth due to sard1 was independent of CBP60a, suggesting that the primary function of CBP60g may be to counter the repressive effect of CBP60a. In the absence of pathogen, levels of SA as well as of several SA-dependent and SA-independent pathogen-inducible genes were higher in cbp60a plants than in the wild type, suggesting that the enhanced resistance of cbp60a plants may result from the activation of immune responses prior to pathogen attack. CBP60a bound calmodulin, and the calmodulin-binding domain was defined at the C-terminal end of the protein. Transgenes encoding mutant versions of CBP60a lacking the ability to bind calmodulin failed to complement null cbp60a mutations, indicating that calmodulin-binding ability is required for the immunity-repressing function of CBP60a. Regulation at the CBP60 node involves negative regulation by CBP60a as well as positive regulation by CBP60g and SARD1, providing multiple levels of control over the activation of immune responses.

Allelic variation in two distinct Pseudomonas syringae flagellin epitopes modulates the strength of plant immune responses but not bacterial motility

The bacterial flagellin (FliC) epitopes flg22 and flgII-28 are microbe-associated molecular patterns (MAMPs). Although flg22 is recognized by many plant species via the pattern recognition receptor FLS2, neither the flgII-28 receptor nor the extent of flgII-28 recognition by different plant families is known. Here, we tested the significance of flgII-28 as a MAMP and the importance of allelic diversity in flg22 and flgII-28 in plant-pathogen interactions using purified peptides and a Pseudomonas syringae ∆fliC mutant complemented with different fliC alleles. The plant genotype and allelic diversity in flg22 and flgII-28 were found to significantly affect the plant immune response, but not bacterial motility. The recognition of flgII-28 is restricted to a number of solanaceous species. Although the flgII-28 peptide does not trigger any immune response in Arabidopsis, mutations in both flg22 and flgII-28 have FLS2-dependent effects on virulence. However, the expression of a tomato allele of FLS2 does not confer to Nicotiana benthamiana the ability to detect flgII-28, and tomato plants silenced for FLS2 are not altered in flgII-28 recognition. Therefore, MAMP diversification is an effective pathogen virulence strategy, and flgII-28 appears to be perceived by an as yet unidentified receptor in the Solanaceae, although it has an FLS2-dependent virulence effect in Arabidopsis.

Quantitative trait loci for partial resistance to Pseudomonas syringae pv. maculicola in Arabidopsis thaliana

Segregation of partial resistance to Pseudomonas syringae pv. maculicola (Psm) ES4326 was studied in the recombinant inbred population created from accessions (ecotypes) Columbia (Col-4), the more susceptible parent, and Landsberg (Ler-0). Plants were spray inoculated with lux-transformed bacteria in experiments to measure susceptibility. The amount of disease produced on a range of Col × Ler lines by spray inoculation was highly correlated with that produced by pressure infiltration of bacteria into the apoplast. Quantitative trait locus (QTL) analysis identified four loci that contributed to partial resistance: QRpsJIC-1.1, QRpsJIC-2.1, QRpsJIC-3.1 and QRpsJIC-5.1 on chromosomes 1, 2, 3 and 5, respectively. QRpsJIC-3.1, located 8.45 cM from the top of the consensus genetic map of chromosome 3, had a large, approximately additive effect on partial resistance, explaining 50% of the genetic variation in this population. Fine mapping narrowed the region within which this QTL was located to 62 genes. A list of candidate genes included several major classes of resistance gene.

Subcellular dynamics and role of Arabidopsis β-1,3-glucanases in cell-to-cell movement of tobamoviruses

β-1,3-Glucanases (BG) have been implicated in enhancing virus spread by degrading callose at plasmodesmata (Pd). Here, we investigate the role of Arabidopsis BG in tobamovirus spread. During Turnip vein clearing virus infection, the transcription of two pathogenesis-related (PR)-BG AtBG2 and AtBG3 increased but that of Pd-associated BG AtBG_pap did not change. In transgenic plants, AtBG2 was retained in the endoplasmic reticulum (ER) network and was not secreted. As a stress response mediated by salicylic acid, AtBG2 was secreted and appeared as a free extracellular protein localized in the entire apoplast but did not accumulate at Pd sites. At the leading edge of Tobacco mosaic virus spread, AtBG2 co-localized with the viral movement protein in the ER-derived bodies, similarly to other ER proteins, but was not secreted to the cell wall. In atbg2 mutants, callose levels at Pd and virus spread were unaffected. Likewise, AtBG2 overexpression had no effect on virus spread. However, in atbg_pap mutants, callose at Pd was increased and virus spread was reduced. Our results demonstrate that the constitutive Pd-associated BG but not the stress-regulated extracellular PR-BG are directly involved in regulation of callose at Pd and cell-to-cell transport in Arabidopsis, including the spread of viruses.

JAV1 controls jasmonate-regulated plant defense

Plants evolve effective mechanisms to protect themselves from environmental stresses and employ jasmonates as vital defense signals to defend against insect attack and pathogen infection. Jasmonates are also recognized as an essential growth regulator by which diverse developmental processes are mediated. Despite substantial research, there are no key signaling components reported yet to control jasmonate-regulated plant defense independent of developmental responses. We identify JAV1, a key gene in the jasmonate pathway, which functions as a negative regulator to control plant defense but does not play a detectable role in plant development. Our results suggest that when encountering insect attack and pathogen infection, plants accumulate jasmonates that trigger JAV1 degradation via the 26S proteasome to activate defensive gene expression and elevate resistances against both insects and pathogens. These findings have provided insight into the molecular mechanism by which plants integrate jasmonate signals to protect themselves from insect attack and pathogen infection.

An extracellular subtilase switch for immune priming in Arabidopsis

In higher eukaryotes, induced resistance associates with acquisition of a priming state of the cells for a more effective activation of innate immunity; however, the nature of the components for mounting this type of immunological memory is not well known. We identified an extracellular subtilase from Arabidopsis, SBT3.3, the overexpression of which enhances innate immune responses while the loss of function compromises them. SBT3.3 expression initiates a durable autoinduction mechanism that promotes chromatin remodeling and activates a salicylic acid(SA)-dependent mechanism of priming of defense genes for amplified response. Moreover, SBT3.3 expression-sensitized plants for enhanced expression of the OXI1 kinase gene and activation of MAP kinases following pathogen attack, providing additional clues for the regulation of immune priming by SBT3.3. Conversely, in sbt3.3 mutant plants pathogen-mediated induction of SA-related defense gene expression is drastically reduced and activation of MAP kinases inhibited. Moreover, chromatin remodeling of defense-related genes normally associated with activation of an immune priming response appear inhibited in sbt3.3 plants, further indicating the importance of the extracellular SBT3.3 subtilase in the establishment of immune priming. Our results also point to an epigenetic control in the regulation of plant immunity, since SBT3.3 is up-regulated and priming activated when epigenetic control is impeded. SBT3.3 represents a new regulator of primed immunity.

Molecular adaptation during a rapid adaptive radiation

"Explosive" adaptive radiations on islands remain one of the most puzzling evolutionary phenomena and the evolutionary genetic processes behind such radiations remain unclear. Rapid morphological and ecological evolution during island radiations suggests that many genes may be under fairly strong selection, although this remains untested. Here, we report that during a rapid recent diversification in the Hawaiian endemic plant genus Schiedea (Caryophyllaceae), 5 in 36 studied genes evolved under positive selection. Positively selected genes are involved in defence mechanisms, photosynthesis, and reproduction. Comparison with eight mainland plant groups demonstrates both the relaxation of purifying selection and more widespread positive selection in Hawaiian Schiedea. This provides compelling evidence that adaptive evolution of protein-coding genes may play a significant role during island adaptive radiations.

Arabidopsis and Brachypodium distachyon transgenic plants expressing Aspergillus nidulans acetylesterases have decreased degree of polysaccharide acetylation and increased resistance to pathogens

The plant cell wall has many significant structural and physiological roles, but the contributions of the various components to these roles remain unclear. Modification of cell wall properties can affect key agronomic traits such as disease resistance and plant growth. The plant cell wall is composed of diverse polysaccharides often decorated with methyl, acetyl, and feruloyl groups linked to the sugar subunits. In this study, we examined the effect of perturbing cell wall acetylation by making transgenic Arabidopsis (Arabidopsis thaliana) and Brachypodium (Brachypodium distachyon) plants expressing hemicellulose- and pectin-specific fungal acetylesterases. All transgenic plants carried highly expressed active Aspergillus nidulans acetylesterases localized to the apoplast and had significant reduction of cell wall acetylation compared with wild-type plants. Partial deacetylation of polysaccharides caused compensatory up-regulation of three known acetyltransferases and increased polysaccharide accessibility to glycosyl hydrolases. Transgenic plants showed increased resistance to the fungal pathogens Botrytis cinerea and Bipolaris sorokiniana but not to the bacterial pathogens Pseudomonas syringae and Xanthomonas oryzae. These results demonstrate a role, in both monocot and dicot plants, of hemicellulose and pectin acetylation in plant defense against fungal pathogens.

Systemic acquired resistance: turning local infection into global defense

Systemic acquired resistance (SAR) is an induced immune mechanism in plants. Unlike vertebrate adaptive immunity, SAR is broad spectrum, with no specificity to the initial infection. An avirulent pathogen causing local programmed cell death can induce SAR through generation of mobile signals, accumulation of the defense hormone salicylic acid, and secretion of the antimicrobial PR (pathogenesis-related) proteins. Consequently, the rest of the plant is protected from secondary infection for a period of weeks to months. SAR can even be passed on to progeny through epigenetic regulation. The Arabidopsis NPR1 (nonexpresser of PR genes 1) protein is a master regulator of SAR. Recent study has shown that salicylic acid directly binds to the NPR1 adaptor proteins NPR3 and NPR4, regulates their interactions with NPR1, and controls NPR1 protein stability. However, how NPR1 interacts with TGA transcription factors to activate defense gene expression is still not well understood. In addition, redox regulators, the mediator complex, WRKY transcription factors, endoplasmic reticulum-resident proteins, and DNA repair proteins play critical roles in SAR.

Bacterial pathogen phytosensing in transgenic tobacco and Arabidopsis plants

Plants are subject to attack by a wide range of phytopathogens. Current pathogen detection methods and technologies are largely constrained to those occurring post-symptomatically. Recent efforts were made to generate plant sentinels (phytosensors) that can be used for sensing and reporting pathogen contamination in crops. Engineered phytosensors indicating the presence of plant pathogens as early-warning sentinels potentially have tremendous utility as wide-area detectors. We previously showed that synthetic promoters containing pathogen and/or defence signalling inducible cis-acting regulatory elements (RE) fused to a fluorescent protein (FP) reporter could detect phytopathogenic bacteria in a transient phytosensing system. Here, we further advanced this phytosensing system by developing stable transgenic tobacco and Arabidopsis plants containing candidate constructs. The inducibility of each synthetic promoter was examined in response to biotic (bacterial pathogens) or chemical (plant signal molecules salicylic acid, ethylene and methyl jasmonate) treatments using stably transgenic plants. The treated plants were visualized using epifluorescence microscopy and quantified using spectrofluorometry for FP synthesis upon induction. Time-course analyses of FP synthesis showed that both transgenic tobacco and Arabidopsis plants were capable to respond in predictable ways to pathogen and chemical treatments. These results provide insights into the potential applications of transgenic plants as phytosensors and the implementation of emerging technologies for monitoring plant disease outbreaks in agricultural fields.

Analysis of the small RNA P16/RgsA in the plant pathogen Pseudomonas syringae pv. tomato strain DC3000

Bacteria contain small non-coding RNAs (ncRNAs) that are responsible for altering transcription, translation or mRNA stability. ncRNAs are important because they regulate virulence factors and susceptibility to various stresses. Here, the regulation of a recently described ncRNA of Pseudomonas syringae pv. tomato DC3000, P16, was investigated. We determined that RpoS regulates the expression of P16. We found that deletion of P16 results in increased sensitivity to hydrogen peroxide compared to the wild-type strain, suggesting that P16 plays a role in the bacteria’s susceptibility to oxidative stress. Additionally the P16 mutant displayed enhanced resistance to heat stress. Our findings provide new information on the regulation and role of this ncRNA in P. syringae.

Coronatine promotes Pseudomonas syringae virulence in plants by activating a signaling cascade that inhibits salicylic acid accumulation

Phytopathogens can manipulate plant hormone signaling to access nutrients and counteract defense responses. Pseudomonas syringae produces coronatine, a toxin that mimics the plant hormone jasmonic acid isoleucine and promotes opening of stomata for bacterial entry, bacterial growth in the apoplast, systemic susceptibility, and disease symptoms. We examined the mechanisms underlying coronatine-mediated virulence and show that coronatine activates three homologous NAC transcription factor (TF) genes, ANAC019, ANAC055, and ANAC072, through direct activity of the TF, MYC2. Genetic characterization of NAC TF mutants demonstrates that these TFs mediate coronatine-induced stomatal reopening and bacterial propagation in both local and systemic tissues by inhibiting the accumulation of the key plant immune signal salicylic acid (SA). These NAC TFs exert this inhibitory effect by repressing ICS1 and activating BSMT1, genes involved in SA biosynthesis and metabolism, respectively. Thus, a signaling cascade by which coronatine confers its multiple virulence activities has been elucidated.