Plant-Microbe Interactions: Pathogenesis & Symbiosis
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Four Arabidopsis berberine‐bridge enzyme‐like proteins are specific oxidases that inactivate the elicitor‐active oligogalacturonides

Recognition of endogenous molecules acting as “damage-associated molecular patterns” (DAMPs) is a key feature of immunity in both animals and plants. Oligogalacturonides (OGs), i.e. fragments derived from the hydrolysis of homogalacturonan, a major component of pectin are a well-known class of DAMPs that activate immunity and protect plants against several microbes. However, hyper-accumulation of OGs severely affects growth, eventually leading to cell death and clearly pointing to OGs as players in the growth-defence trade-off. Here we report a mechanism that may control the homeostasis of OGs avoiding their deleterious hyper-accumulation. By combining affinity chromatography on acrylamide-trapped OGs and other procedures, an Arabidopsis thaliana enzyme that specifically oxidizes OGs was purified and identified. The enzyme was named OG OXIDASE 1 (OGOX1) and shown to be encoded by the gene At4g20830. As a typical flavo-protein, OGOX1 is a sulphite-sensitive H2O2–producing enzyme that displays maximal activity on OGs with a degree of polymerization >4. OGOX1 belongs to a large gene family of mainly apoplastic putative FAD-binding proteins [Berberine-Bridge Enzyme-like (BBE-like); 27 members], whose biochemical and biological function is largely unexplored. We have found that at least four BBE-like enzymes in Arabidopsis are OG oxidases (OGOX1-4). Oxidized OGs display a reduced capability of activating the immune responses and are less hydrolysable by fungal polygalacturonases. Plants overexpressing OGOX1 are more resistant to Botrytis cinerea, pointing to a crucial role of OGOX enzymes in plant immunity.
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A glycine-rich protein MoGrp1 functions as a novel splicing factor to regulate fungal virulence and growth in Magnaporthe oryzae | Phytopathology Research | Full Text

Glycine-rich proteins (GRPs) have diverse amino acid sequences and are involved in a variety of biological processes. The role of GRPs in plant pathogenic fungi has not been reported. In this study, we identified and functionally characterized a novel gene named MoGRP1 in Magnaporthe oryzae, which encodes a protein that has an N-terminal RNA recognition motif (RRM) and a C-terminal glycine-rich domain with four Arg-Gly-Gly (RGG) repeats. Deletion of MoGRP1 resulted in dramatic reductions in fungal virulence, mycelial growth, and conidiation. The ΔMogrp1 mutants were also defective in cell wall integrity and in their responses to different stresses. MoGrp1 was localized to the nucleus and was co-immunoprecipitated with several components of the spliceosome, including subunits of the U1 snRNP and U2 snRNP complexes. Moreover, MoGrp1 exhibited binding affinity for poly(U). Importantly, MoGrp1 was responsible for the normal splicing of genes involved in infection-related morphogenesis. Domain deletion assays showed that both the RRM domain and its two adjacent RGG repeats were essential to the full function of MoGrp1. Notably, the nine amino acids between the first and the second RGG repeats were indispensable for nuclear localization and for the biological functions of MoGrp1. Taken together, our data suggest that MoGrp1 functions as a novel splicing factor with poly(U) binding activity to regulate fungal virulence, development, and stress responses in the rice blast fungus.
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TCP transcription factors interact with ZED1‐related kinases as components of the temperature‐regulated immunity - Wang - - Plant, Cell & Environment - Wiley Online Library

The elevation of ambient temperature generally inhibits plant immunity, but the molecular regulations of immunity by ambient temperature in plants are largely elusive. We previously reported that the Arabidopsis HOPZ‐ETI‐DEFICIENT 1 (ZED1)‐related kinases (ZRKs) mediate the temperature‐sensitive immunity by inhibiting the transcription of SUPPRESSOR OF NPR1‐1, CONSTITUTIVE 1 (SNC1). Here, we further demonstrate that the nucleus‐localized ZED1 and ZRKs facilitate such inhibitory role in associating with the TEOSINTE BRANCHED1, CYCLOIDEA AND PROLIFERATING CELL FACTOR (TCP) transcription factors. We show that some of TCP members could physically interact with ZRKs and are induced by elevated temperature. Disruption of TCPs leads to a mild autoimmune phenotype, while overexpression of the TCP15 could suppress the autoimmunity activated by the overexpressed SNC1 in the snc1‐2. These findings demonstrate that the TCP transcription factors associate with nuclear ZRK as components of the temperature‐regulated immunity, which discloses a possible molecular mechanism underlying the regulation of immunity by ambient temperature in plants.
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Exploring the natural microbiome of the model liverwort: fungal endophyte diversity in Marchantia polymorpha L

Within their tissues, plants host diverse communities of fungi, termed fungal endophytes. These fungi can affect plant growth, competitiveness, and resistance to stressors, thereby influencing plant community structure. Research characterizing fungal endophyte communities has so far mostly focused on seed plants, but information on the endophytes of other plant lineages is needed to understand how plant microbiomes impact whole ecosystems and how major changes through land plant evolution have affected plant-microbe relationships. In this study, we assess the fungal endophyte community of the model liverwort Marchantia polymorpha L. by both culturing and Illumina amplicon sequencing methods. We detect a very diverse fungal community that is distinct between M. polymorpha patches and only shares a few core fungi between populations across the United States. We also show low overlap in taxa detected by the different methods. This study helps build a foundation for using M. polymorpha and other Marchantia species as models for the ecology and dynamics of bryophyte microbiomes.
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Smut infection of perennial hosts: the genome and the transcriptome of the Brassicaceae smut fungus Thecaphora thlaspeos reveal functionally conserved and novel effectors - Courville - - New Phytol...

Biotrophic fungal plant pathogens can balance their virulence and form intricate relationships with their hosts. Sometimes, this leads to systemic host colonization over long timescales without macroscopic symptoms. However, how plant‐pathogenic endophytes manage to establish their sustained systemic infection remains largely unknown.

Here, we present a genomic and transcriptomic analysis of Thecaphora thlaspeos. This relative of the well‐studied grass smut Ustilago maydis is the only smut fungus adapted to Brassicaceae hosts. Its ability to overwinter with perennial hosts and its systemic plant infection including roots are unique characteristics among smut fungi.

The T. thlaspeos genome was assembled to the chromosome level. It is a typical smut genome in terms of size and genome characteristics. In silico prediction of candidate effector genes revealed common smut effector proteins and unique members. For three candidates, we have functionally demonstrated effector activity. One of them, TtTue1, suggests a potential link to cold acclimation. On the plant side, we found evidence for a typical immune response as it is present in other infection systems, despite the absence of any macroscopic symptoms during infection.

Our findings suggest that T. thlaspeos distinctly balances its virulence during biotrophic growth ultimately allowing for long‐lived infection of its perennial hosts.
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Frontiers | Roots and nodules response differently to P starvation in the Mediterranean-type legume Virgilia divaricata | Plant Science

Virgilia divaricata is a tree legume that grows in the Cape Floristic Region (CFR) in poor soils. A comparison between high and low phosphate growth conditions between roots and nodules was conducted and evaluated for the plants ability to handle low phosphate (LP) stress conditions in Virgilia divaricata. We showed that the plant copes with low phosphate stress through an increased allocation of resources, reliance on biological nitrogen fixation (BNF) and enhanced enzyme activity, especially phosphoenolpyruvate carboxylase . Nodules had a lower percentage decline in P compared to roots to uphold their metabolic functions. These strategies partly explain how V. divaricata can sustain growth despite LP conditions. Although the number of nodules declined with LP (i.e., 34% compared to the 88% decline in roots), their biomass remained unchanged in spite of an overall decline in plant dry weight. We attribute this behaviour to P conservation strategies in LP nodules that imply an increase in a metabolic bypass that operates at the phosphoenolpyruvate branchpoint in glycolysis. The enhanced activities of nodule phosphoenolpyruvate carboxylase, malate dehydrogenase and malic enzime, whilst pyruvate kinase declines, suggests that under LP conditions an adenylate bypass was in operation either to synthesize more organic acids or to mediate pyruvate via a non-adenylate requiring metabolic route. Both possibilities represent a P-stress adaptation route and this is the first report of its kind for legume trees that are indigenous to low P, acid soils. Although BNF declined by a small percentage during LP, this P conservation was evident in the unchanged BNF efficiency per weight, and the increase in BNF efficiency per mol of P. It appears that legumes that are indigenous to acid soils, may be able to continue their reliance on BNF via increased allocation to nodules and also due to increase their efficiency for BNF on a P basis, owing to P-saving mechanisms such as the organic acid routes.
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The Identity of a Single Residue of the RNA-dependent RNA Polymerase of Grapevine Fanleaf Virus Modulates Vein Clearing Symptoms in Nicotiana benthamiana | Molecular Plant-Microbe Interactions

The mechanisms underlying host plant symptom development upon infection by viruses of the genus Nepovirus in the family Secoviridae, including grapevine fanleaf virus (GFLV), are poorly understood. In the systemic host Nicotiana benthamiana, GFLV strain GHu produces characteristic vein clearing symptoms in apical leaves unlike other GFLV strains, such as F13, which cause an asymptomatic infection. In this study, we expanded on earlier findings and used reverse genetics to identify residue 802 (lysine, K) of the GFLV-GHu RNA1-encoded RNA-dependent RNA polymerase (1EPol) as a modulator of vein clearing symptom development in N. benthamiana. Mutations to this site abolished (K to G, A, or Q) or attenuated (K to N or P) symptom expression. Noteworthy, residue 802 is necessary but not sufficient for vein clearing symptoms as GFLV-F13 RNA1 carrying K802 remained asymptomatic in N. benthamiana. No correlation was found between symptom expression and RNA1 accumulation, as shown by RT-qPCR. Additionally, the involvement of RNA silencing in vein clearing symptoms was ruled out by VIGS experiments and structure predictions for protein 1EPol suggested that residue 802 is flanked by strongly predicted stable secondary structures, including a conserved motif of unknown function (805LLKT/AHLK/RT/ALR814). Together, these results reveal the protein nature of the GFLV-GHu symptom determinant in N. benthamiana and provide a solid basis for probing and determining the virus-host proteome network for vein clearing symptoms.
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Frontiers | Lipid Trafficking at Membrane Contact Sites During Plant Development and Stress Response | Plant Science

Frontiers | Lipid Trafficking at Membrane Contact Sites During Plant Development and Stress Response | Plant Science | Plant-Microbe Interactions: Pathogenesis & Symbiosis | Scoop.it
The biogenesis of cellular membranes involves an important traffic of lipids from their site of synthesis to their final destination. Lipid transfer can be mediated by vesicular or non-vesicular pathways. The non-vesicular pathway requires the close apposition of two membranes to form a functional platform, called membrane contact sites (MCSs), where lipids are exchanged. These last decades, MCSs have been observed between virtually all organelles and a role in lipid transfer has been demonstrated for some of them. In plants, the lipid composition of membranes is highly dynamic and can be drastically modified in response to environmental changes. This highlights the importance of understanding the mechanisms involved in the regulation of membrane lipid homeostasis in plants. This review summarizes our current knowledge about the non-vesicular transport of lipids at MCSs in plants and its regulation during stress.
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Foliar fungi alter reproductive timing and allocation in Arabidopsis under normal and water-stressed conditions

Microbes influence plant phenotypes but most known examples of this are from the study of below-ground microbes and plant disease modification. To examine the potential importance of phyllosphere microbes on non-disease related plant traits, we used sterile Arabidopsis clones to test the effects of foliar fungi on flowering phenology and reproductive allocation under conditions of varying water stress. We inoculated the sterile plants with fully-factorial combinations of four fungal isolates, then measured flowering time and reproductive allocation for each treatment group under normal and water-stressed conditions. All plants inoculated with foliar fungi had significantly later flowering and greater seed mass than the sterile control groups. The magnitude of this effect depended on the specific fungi present, but individual fungal effects diminished as inoculum richness increased. Above-ground microbes likely influence other plant traits as well and should be considered in any study measuring plant phenotypes.
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Selection of reference genes for quantitative real-time PCR normalization in the plant pathogen Puccinia helianthi Schw. | BMC Plant Biology | Full Text

Real-time RT-PCR has become a common and robust technique to detect and quantify low-abundance mRNA expression and is a prefered tool when examining fungal gene expression in infected host tissues. However, correct evaluation of gene expression data requires accurate and reliable normalization against a reference transcript. Thus, the identification of reference genes with stable expression during different conditions is of paramount importance. Here, we present a study where in vitro and in planta experiments were used to validate the expression stability of reference gene candidates of Puccinia helianthi Schw., an obligate pathogen that causes rust in sunflower (Helianthus annuus).

Results
Eleven reference genes of P. helianthi were validated at different growth stages. Excel-based software geNorm, BestKeeper and NormFinder were used to evaluate the reference gene transcript stabilities. Of eleven reference gene candidates tested, three were stably expressed in urediniospores, germinating growth stage and in planta. Two of these genes (UBC, EF2), encoding ubiquitin-conjugating enzyme and elongation factor 2, proved to be the most stable set of reference genes under the experimental conditions used.

Conclusion
We found that UBC and EF2 are suitable candidates for for the standardization of gene expression studies in the plant pathogen P. helianthi and potentially other related pathogens.
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A Plant Immune Receptor Adopts a Two-Step Recognition Mechanism to Enhance Viral Effector Perception

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors play critical roles in pathogen surveillance. Most plant NLRs characterized so far were found to use single domain/sensor to recognize pathogen effectors. Here we report that Sw-5b NLR immune receptor uses two distinct domains to detect the viral movement protein NSm encoded by tospovirus. In addition to the previously reported LRR domain, we found that the N-terminal Solanaceae domain (SD) of Sw-5b also interacts with NSm and a conserved 21-amino acids region of NSm (NSm21). The specific interaction between Sw-5b SD and NSm is required for releasing the CC inhibition on NB-ARC-LRR region. Our results showed that the binding of NSm affects the nucleotide-binding activity of the NB-ARC-LRR in vitro. However, Sw-5b NB-ARC-LRR is activated only when NSm and NSm21 levels were higher. Strikingly, Sw-5b SD could significantly enhance the ability of NB-ARC-LRR to detect low levels of NSm effector and facilitate its activation to induce defense response. The Sw-5b SD mutant that is disrupted in NSm recognition failed to enhance the ability of NB-ARC-LRR in sensing low levels of NSm and NSm21. Taken together, our results suggest that Sw-5b SD functions as an extra sensor and the NB-ARC-LRR functions as an activator and Sw-5b NLR adopts a two-step recognition mechanism to enhance viral effector perception.
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Exploring the role of ectomycorrhizal fungi in soil carbon dynamics - Zak - - New Phytologist - Wiley Online Library

The extent to which ectomycorrhizal fungi (ECM) enable plants to access organic nitrogen (N) bound in soil organic matter (SOM) and transfer this growth‐limiting nutrient to their plant host, has important implications for our understanding of plant–fungal interactions and the cycling and storage of carbon (C) and N in terrestrial ecosystems. Empirical evidence currently supports a range of perspectives, suggesting that ECM vary in their ability to provide their host with N bound in SOM, and that this capacity can both positively and negatively influence soil C storage. To help resolve the multiplicity of observations, we gathered a group of researchers to explore the role of ECM fungi in soil C dynamics, and propose new directions that hold promise to resolve competing hypotheses and contrasting observations. In this Viewpoint, we summarize these deliberations and identify areas of inquiry that hold promise for increasing our understanding of these fundamental and widespread plant symbionts and their role in ecosystem level biogeochemistry.
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Osa‐miR398b boosts H2O2 production and rice blast disease‐resistance via multiple superoxide dismutases - Li - - New Phytologist - Wiley Online Library

miRNAs contribute to plant resistance against pathogens. Previously, we found that the function of miR398b in immunity in rice differs from that in Arabidopsis. However, the underlying mechanisms are unclear.

In this study, we characterized the mutants of miR398b target genes and demonstrated that multiple superoxide dismutase genes contribute to miR398b‐regulated rice immunity against the blast fungus Magnaporthe oryzae.

Out of the four target genes of miR398b, mutations in Cu/Zn‐Superoxidase Dismutase1 (CSD1), CSD2 and Os11g09780 (SODX) led to enhanced resistance to M. oryzae and increased H2O2 accumulation. By contrast, mutations in Copper Chaperone for Superoxide Dismutase (CCSD) resulted in enhanced susceptibility. Biochemical studies revealed that csd1, csd2 and sodx displayed altered expression of CSDs and other SOD family members, leading to increased total SOD enzyme activity that positively contributed to higher H2O2 production. In contrast, the ccsd mutant showed CSD protein deletion, resulting in decreased CSD and total SOD enzyme activity.

Our results demonstrate the roles of different SODs in miR398b‐regulated resistance to rice blast disease, and uncover an integrative regulatory network in which miR398b boosts total SOD activity to up‐regulate H2O2 level and thereby improve disease resistance.
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A novel fungal effector from Puccinia graminis suppressing RNA silencing and plant defense responses - Yin - - New Phytologist - Wiley Online Library

Fungal plant pathogens like rust‐causing biotrophic fungi secrete hundreds of effectors into plant cells to subvert host immunity and promote pathogenicity on their host plants by manipulating specific physiological processes or signal pathways, but the actual function has been demonstrated for very few of these proteins.

Here, we show that the PgtSR1 effector proteins, encoded by two allelic genes (PgtSR1‐a and PgtSR1‐b), from the wheat stem rust pathogen Puccinia graminis f. sp. tritici (Pgt), suppress RNA silencing in plants and impede plant defenses by altering the abundance of small RNAs that serve as defense regulators.

Expression of the PgtSR1s in plants revealed that the PgtSR1s promote susceptibility to multiple pathogens and partially suppress cell death triggered by multiple R proteins.

Overall, our study provides the first evidence that the filamentous fungus Puccinia graminis has evolved to produce fungal suppressors of RNA silencing (FSR) and indicates that PgtSR1s suppress both basal defenses and effector triggered immunity.
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NAC transcription factors in plant immunity | Phytopathology Research | Full Text

The NAC (NAM, ATAF and CUC) family is one of the largest plant-specific transcription factor (TF) families. Members of this family are implicated in plant growth, development and stress responses. Recent functional studies demonstrate that a number of NAC TFs function as positive or negative regulators of plant immunity to biotrophic, hemibiotrophic or necotrophic pathogens, as modulators of the hypersensitive responses and stomatal immunity or as virulence targets of pathogen effectors. They affect plant immunity through their regulatory impact on signaling of plant hormones, which in turn are key players in plant immune responses. This review summarizes current knowledge and recent progress in our understanding of the biological functions of NAC TFs in plant immunity and discusses perspectives and directions for further study to elucidate the molecular mechanisms of NAC TF functions in immunity and potential application in improvement of crop disease resistance.
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The Agrobacterium VirD5 protein hyper‐activates the mitotic Aurora kinase in host cells - Zhang - - New Phytologist - Wiley Online Library

Aided by translocated virulence proteins Agrobacterium tumefaciens transforms plant cells with oncogenic T‐DNA. In the host cells the virulence protein VirD5 moves to the nucleus, where it becomes localized at the kinetochores, and disturbs faithful chromosome segregation, but the molecular mechanism underlying this is still unknown.

In order to gain more insight in this, we screened amongst the kinetochore proteins for VirD5 interactors using BIFC assays, and tested chromosome segregation in yeast cells.

We discovered that VirD5 interacts with the conserved mitotic Aurora kinase Ipl1 in yeast and likewise with plant Aurora kinases. In vitro VirD5 was found to stimulate the activity of the Ipl1. Phosphorylation of substrates by Ipl1 in vivo is known to result in the detachment between kinetochore and spindle microtubule. This is necessary for error correction, but increased Ipl1/Aurora kinase activity is known to cause spindle instability, explaining enhanced chromosome mis‐segregation seen in the presence of VirD5. That activation of the Ipl1/Aurora kinase at least partially underlies the toxicity of VirD5, became apparent by artificial boosting of the activity of the specific counteracting phosphatase Glc7 in vivo, which relieved the toxicity.

These findings reveal a novel mechanism by which a pathogenic bacterium manipulates host cells.
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An Arabidopsis berberine‐bridge enzyme‐like protein specifically oxidizes cellulose oligomers and plays a role in immunity - Locci - - The Plant Journal - Wiley Online Library

The plant cell wall is the barrier that pathogens must overcome to cause a disease and to this purpose they secrete degrading enzymes of the various cell wall components. Due to the complexity of these components, several types of oligosaccharide fragments may be released during pathogenesis and some of these can act as Damage‐Associated Molecular Pattern (DAMPs). Well‐known DAMPs are the oligogalacturonides (OGs) released upon degradation of homogalacturonan and the products of the cellulose breakdown, i.e. the cellodextrins (CDs). We have previously reported that four Arabidopsis Berberine Bridge Enzyme‐like (BBE‐like) proteins (OGOX1‐4) oxidize OGs and impair their elicitor activity. We show here that another Arabidopsis BBE‐like protein, which is expressed coordinately with OGOX1 during immunity, specifically oxidizes CDs with a preference for cellotriose (CD3) and longer fragments (CD4‐6). Oxidized CDs show a negligible elicitor activity and are less utilizable by the fungus Botrytis cinerea as a carbon source. The enzyme, named CELLOX (CELLODEXTRIN OXIDASE), is encoded by the gene At4g20860. Plants overexpressing CELLOX display an enhanced resistance to B. cinerea likely because oxidized CDs are a less valuable carbon source. Thus, the capacity of oxidizing and impairing the biological activity of the cell wall‐derived oligosaccharides seems to be a general trait of the family of BBE‐like proteins, which may serve for the homeostatic control of the level of DAMPs to prevent their hyper‐accumulation.
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Frontiers | BnaMPK3 is a key regulator of defense responses to the devastating plant pathogen Sclerotinia sclerotiorum in oilseed rape | Plant Science

The disease caused by Sclerotinia sclerotiorum has traditionally been difficult to control, resulting in tremendous economic losses in oilseed rape (Brassica napus). Identification of important genes in the defense responses is critical for molecular breeding, an important strategy for controlling the disease. Here, we report that a B. napus mitogen-activated protein kinase gene, BnaMPK3, plays an important role in the defense against S. sclerotiorum in oilseed rape. BnaMPK3 is highly expressed in the stems, flowers and leaves, and its product is localized in the nucleus. Furthermore, BnaMPK3 is highly responsive to infection by S. sclerotiorum and treatment with jasmonic acid (JA) or the biosynthesis precursor of ethylene (ET), but not to treatment with salicylic acid (SA) or abscisic acid. Moreover, overexpression (OE) of BnaMPK3 in B. napus and Nicotiana benthamiana results in significantly enhanced resistance to S. sclerotiorum, whereas resistance is diminished in RNAi transgenic plants. After S. sclerotiorum infection, defense responses associated with ET, JA and SA signaling are intensified in the BnaMPK3-OE plants but weakened in the BnaMPK3-RNAi plants when compared to those in the wild type plants; by contrast the level of both H2O2 accumulation and cell death exhibits a reverse pattern. The candidate gene association analyses show that the BnaMPK3-encoding BnaA06g18440D locus is a cause of variation in the resistance to S. sclerotiorum in natural B. napus population. These results suggest that BnaMPK3 is a key regulator of multiple defense responses to S. sclerotiorum, which may guide the resistance improvement of oilseed rape and related economic crops.
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1-aminocyclopropane-1-carboxylate: a novel and strong chemoattractant for the plant beneficial rhizobacterium Pseudomonas putida UW4 | Molecular Plant-Microbe Interactions

PGPR and fungi-bacterial biofilms are both important biofertilizer inoculants for sustainable agriculture. However, the strongest chemoattractant for bacteria to colonize the rhizosphere and mycelia is not clear. Coincidentally, almost all of the PGPRs possess 1-aminocyclopropane-1-carboxylate (ACC) deaminase (AcdS) and can utilize ACC as the sole nitrogen source. Here, we found that ACC was a novel, metabolic dependent and methyl-accepting chemoreceptor (MCP)-involved chemoattractant for Pseudomonas putida UW4. The chemotactic response of UW4 to ACC is significantly greater than that to the amino acids and organic acids identified in the plant root and fungal hyphal exudates. The colonization accounts of the acdS or cheR deletion mutants of UW4 in the wheat rhizosphere and on Agaricus bisporus mycelia were reduced one magnitude compared to those of UW4. The colonization accounts of UW4 on A. bisporus antisense ACO mycelia with a high ACC production significantly increased compared to A. bisporus, following by UW4 cheR complementary strain and ethylene chemoreceptor gene deletion mutant. The colonization accounts of the UW4 strains on A. bisporus acdS+ mycelia with a low ACC production decreased significantly compared to A. bisporus wt. The results suggested that ACC, not ethylene, should be the strongest chemoattractant for the PGPR that contain AcdS.
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Biocontrol of soft-rot: confocal microscopy highlights virulent pectobacterial communication and its jamming by rhodococcal quorum-quenching | Molecular Plant-Microbe Interactions

Confocal laser scanning microscopy was chosen to observe the colonization and damage caused by the soft-rot Pectobacterium atrosepticum and the protection mediated by the biocontrol agent Rhodococcus erythropolis. We developed dual-color reporter strains suited for monitoring quorum-sensing and quorum-quenching activities leading to maceration or biocontrol, respectively. A constitutively expressed cyan or red fluorescent protein served as a cell tag for plant colonization, while an inducible expression reporter system based on the green fluorescent protein gene enabled the simultaneous recording of signaling molecule production, detection and/or degradation. The dual-colored pathogen and biocontrol strains were used to co-inoculate potato tubers. At cellular quorum, images revealed a strong pectobacterial quorum-sensing activity, especially at the plant cell walls as well as a concomitant rhodococcal quorum-quenching response, both at the single-cell and microcolony levels. The generated biosensors appear to be promising and complementary tools useful for molecular and cellular studies of bacterial communication and interference.
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Comparative transcriptomic analysis reveals different responses of Arabidopsis thaliana roots and shoots to infection by Agrobacterium tumefaciens in a hydroponic co-cultivation system - ScienceDirect

Comparative transcriptomic analysis reveals different responses of Arabidopsis thaliana roots and shoots to infection by Agrobacterium tumefaciens in a hydroponic co-cultivation system - ScienceDirect | Plant-Microbe Interactions: Pathogenesis & Symbiosis | Scoop.it
Agrobacterium tumefaciens-Arabidopsis thaliana provides an excellent model system to study bacterial pathogenesis and plant interactions. It is well established that, upon recognizing plant-derived signals, A. tumefaciens activates its virulence programing, culminated by the transfer and integration of T-DNA into the plant genome. However, the initial signaling and response of the plant host during A. tumefaciens -plant interactions is less understood. In this study, A. thaliana were co-cultivated (infected) with A. tumefaciens in a sterile hydroponic system for 8 h, and the transcriptome profiles of root and shoot were determined by microarray analysis. Overall, hundreds of genes involved in plant responses to stress, regulation of transcription, signal transduction or plant metabolism are differentially expressed in roots and shoots upon A. tumefaciens infection. In total, 323 genes were up-regulated and 226 were down-regulated by a log2 transformed fold change of at least 2 in roots upon A. tumefaciens infection, as compared with mock-inoculated plants. In the shoot, 249 genes were up-regulated and 152 were down-regulated in response to A. tumefaciens infection in the roots. Functional classification and comparative analysis revealed that, in response to pathogen attack, the host plant demonstrated significantly differential signaling loops between locally (roots) and distally (shoots) affected sites, despite of little overlap between transcriptome responses. Upon A. tumefaciens infection, roots displayed a strong induction of defense and hormone signaling pathways involving salicylic acid, abscisic acid, ethylene, and auxins, which play essential roles in plant acclimation responses in the root. On the contrary, jasmonic acid and gibberellic acid signaling processes were most activated in the shoots. The identification of genes and signaling pathways that are differentially regulated in roots and shoots may serve as possible targets in molecular breeding for disease tolerant plants or for improving Agrobacterium-mediated plant transformation.
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feedback loop between CaWRKY41 and H2O2 coordinates pepper’s response to Ralstonia solanacearum and excess cadmium | Journal of Experimental Botany | Oxford Academic

WRKY transcription factors have been implicated in both plant immunity and plant responses to cadmium (Cd); however, the mechanism underlying the crosstalk between these processes is unclear. Here, we characterized the roles of CaWRKY41, a groupⅢ WRKY TF, in immunity against the pathogenic bacterium Ralstonia solanacearum and Cd stress responses in pepper (Capsicum annuum). CaWRKY41 was transcriptionally upregulated in response to Cd exposure, R. solanacearum inoculation, and H2O2 treatment. Virus-induced silencing of CaWRKY41 increased Cd tolerance and R. solanacearum susceptibility, while heterologous overexpression of CaWRKY41 in Arabidopsis impaired Cd tolerance, and enhanced Cd and Zn uptake and H2O2 accumulation. Genes encoding reactive oxygen species (ROS)-scavenging enzymes were downregulated in overexpressing CaWRKY41Arabidopsis plants, whereas those encoding Zn transporters and enzymes involved in H2O2 production were upregulated. Consistent with these findings, the ocp3 (overexpressor of cationic peroxidase 3) mutant, which has elevated H2O2 levels, displayed enhanced sensitivity to Cd stress. These results suggest that a positive feedback loop between H2O2 accumulation and CaWRKY41 upregulation coordinates the responses of pepper to R. solanacearum inoculation and Cd exposure. This mechanism might reduce Cd tolerance by increasing Cd uptake via Zn transporters, while enhancing resistance to R. solanacearum.
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Rice black‐streaked dwarf virus P10 acts as either a synergistic or an antagonistic determinant during superinfection with related or unrelated virus - Zhang - - Molecular Plant Pathology - Wiley O...

Rice black‐streaked dwarf virus (RBSDV), a member of the genus Fijivirus, is a devastating pathogen of crop plants. RBSDV S10 encodes a capsid protein (P10) that is an important component of the double‐layered particle. However, little information is available on the roles of RBSDV P10 in viral infection or in interactions with other viruses. Here we demonstrate that expressing P10 in plants alleviated the symptoms of both RBSDV and the closely‐related southern rice black‐streaked dwarf virus (SRBSDV), and reduced the disease incidence, but made the plants more susceptible to the unrelated rice stripe virus (RSV). Further experiments suggested that P10‐mediated resistance to RBSDV and SRBSDV operated at the protein level rather than the RNA level and was not due to post transcriptional gene silencing. Transcriptomic data revealed that expressing P10 in plants significantly suppressed the expression of rice defense‐related genes, which might play important roles in resistance to RSV infection. After infection with RBSDV, plants were more resistant to subsequent challenge by SRBSDV but more susceptible to RSV. Overall, these results indicate that P10 acts as an important effector in virus interactions.
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Inside out: root cortex‐localized LHK1 cytokinin receptor limits epidermal infection of Lotus japonicus roots by Mesorhizobium loti - Miri - - New Phytologist - Wiley Online Library

During Lotus japonicas–Mesorhizobium loti symbiosis, the LOTUS HISTIDINE KINASE1 (LHK1) cytokinin receptor regulates both the initiation of nodule formation and the scope of root infection. However, the exact spatio‐temporal mechanism by which this receptor exerts its symbiotic functions has remained elusive.

•In this study, we performed cell type‐specific complementation experiments in the hyperinfected lhk1‐1 mutant background, targeting LHK1 to either the root epidermis or the root cortex. We also utilized various genetic backgrounds to characterize expression of several genes regulating symbiotic infection.

•We show here that expression of LHK1 in the root cortex is required and sufficient to regulate both nodule formation and epidermal infections. The LHK1‐dependent signalling that restricts subsequent infection events is triggered before initial cell divisions for nodule primordium formation. We demonstrate also that AHK4, the Arabidopsis orthologue of LHK1, is able to regulate M. loti infection in L. japonicus, suggesting that an endogenous cytokinin receptor could be sufficient for engineering nitrogen‐fixing root nodule symbiosis in nonlegumes.

•Our data provide experimental evidence for the existence of an LHK1‐dependent root cortex‐to‐epidermis feedback mechanism regulating rhizobial infection. This root‐localized regulatory module functionally links with the systemic autoregulation of nodulation (AON) to maintain the homeostasis of symbiotic infection.
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Bacterially produced spermidine induces plant systemic susceptibility to pathogens

Plant root-associated microbes promote plant growth, in part by the induction of systemic resistance (ISR) to foliar pathogens. In an attempt to find novel growth-promoting and ISR inducing strains, we previously identified strains of root-associated Pseudomonas spp. that promote plant growth but unexpectedly induced systemic susceptibility (ISS) to foliar pathogens. Here we demonstrate that the ISS-inducing phenotype is common among root-associated Pseudomonas spp. and we identified the underlying genetic and molecular basis of ISS. Using comparative genomics we identified a single P. fluorescens locus containing a novel periplasmic spermidine biosynthesis gene speE2 that is unique to ISS strains. We generated a clean deletion of the speE2 gene in two ISS strains and found that speE2 is necessary for the ISS phenotype. Spermidine but not spermine is sufficient to phenocopy ISS strains. The ISS locus is present in diverse bacteria and has previously been implicated in pathogenesis in animals. Collectively these data show that a single bacterially derived molecule can modulate systemic plant immunity.
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Chloroplasts navigate towards the pathogen interface to counteract infection by the Irish potato famine pathogen

Chloroplasts are light harvesting organelles that arose from ancient endosymbiotic cyanobacteria. Upon immune activation, chloroplasts switch off photosynthesis, produce anti-microbial compounds, and develop tubular extensions called stromules. We report that chloroplasts navigate to the pathogen interface to counteract infection by the Irish potato famine pathogen Phytophthora infestans, physically associating with the specialised membrane that engulfs pathogen haustoria. Outer envelope protein, chloroplast unusual positioning1 (CHUP1), anchors chloroplasts to the host-pathogen interface. Stromules are induced during infection in a CHUP1-dependent manner, embracing haustoria and interconnecting chloroplasts, to form dynamic organelle clusters. Infection-triggered reprogramming of chloroplasts relies on surface immune signalling, whereas pathogen effectors subvert these immune pulses. Chloroplast are deployed focally, and coordinate to restrict pathogen entry into plant cells, a process actively countered by parasite effectors.
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