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Scooped from: PLoS Pathogens, 2013 (via Suayib Üstün's scoopit) Authors: Suayib Üstün, Verena Bartetzko, Frederik Börnke Summary: The phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) requires type III effector proteins (T3Es) for virulence. After translocation into the host cell, T3Es are thought to interact with components of host immunity to suppress defence responses. XopJ is a T3E protein from Xcv that interferes with plant immune responses; however, its host cellular target is unknown. Here we show that XopJ interacts with the proteasomal subunit RPT6 in yeast andin planta to inhibit proteasome activity. A C235A mutation within the catalytic triad of XopJ as well as a G2A exchange within the N-terminal myristoylation motif abolishes the ability of XopJ to inhibit the proteasome. Xcv ΔxopJ mutants are impaired in growth and display accelerated symptom development including tissue necrosis on susceptible pepper leaves. Application of the proteasome inhibitor MG132 restored the ability of the Xcv ΔxopJ to attenuate the development of leaf necrosis. The XopJ dependent delay of tissue degeneration correlates with reduced levels of salicylic acid (SA) and changes in defence- and senescence-associated gene expression. Necrosis upon infection with Xcv ΔxopJ was greatly reduced in pepper plants with reduced expression of NPR1, a central regulator of SA responses, demonstrating the involvement of SA-signalling in the development of XopJ dependent phenotypes. Our results suggest that XopJ-mediated inhibition of the proteasome interferes with SA-dependent defence response to attenuate onset of necrosis and to alter host transcription. A central role of the proteasome in plant defence is discussed.
Via Suayib Üstün
Scooped from: OMICS, 2013 Authors: Budheswar Dehury, Mousumi Sahu, Kishore Sarma, Jagajjit Sahu, Priyabrata Sen, Mahendra Kumar Modi, Gauri Dutta Sharma, Manabendra Dutta Choudhury and Madhumita Barooah Summary: Rice (Oryza sativa L.), a model plant belonging to the family Poaceae, is a staple food for a majority of the people worldwide. Grown in the tropical and subtropical regions of the world, this important cereal crop is under constant and serious threat from both biotic and abiotic stresses. Among the biotic threats, Xanthomonas oryzae pv. oryzae, causing the damaging bacterial blight disease in rice, is a prominent pathogen. The xa5 gene in the host plant rice confers race-specific resistance to this pathogen. This recessive gene belongs to the Xa gene family of rice and encodes a gamma subunit of transcription factor IIA (TFIIAγ). In view of the importance of this gene in conferring resistance to the devastating disease, we reconstructed the phylogenetic relationship of this gene, developed a three-dimensional protein model, followed by long-term molecular dynamics simulation studies to gain a better understanding of the evolution, structure, and function of xa5. The modeled structure was found to fit well with the small subunit of TFIIA from human, suggesting that it may also act as a small subunit of TFIIA in rice. The model had a stable conformation in response to the atomic flexibility and interaction, when subjected to MD simulation at 20 nano second in aqueous solution. Further structural analysis of xa5 indicated that the protein retained its basic transcription factor function, suggesting that it might govern a novel pathway responsible for bacterial blight resistance. Future molecular docking studies of xa5 underway with its corresponding avirulence gene is expected to shed more direct light into plant–pathogen interactions at the molecular level and thus pave the way for richer agriproteomic insights.
Scooped from: New Phytologist, 2013 Authors: Michael F. Allen and Kuni Kitajima Summary: Understanding the temporal variation of soil and root dynamics is a major step towards determining net carbon in ecosystems. We describe the installation and structure of an in situ soil observatory and sensing network consisting of an automated minirhizotron with associated soil and atmospheric sensors.Ectomycorrhizal hyphae were digitized daily during 2011 in a Mediterranean climate, high-elevation coniferous forest. Hyphal length was high, but stable during winter in moist and cold soil. As soil began to warm and dry, simultaneous mortality and production indicating turnover followed precipitation events. Mortality continued through the dry season, although some hyphae persisted through the extremes. With autumn monsoons, rapid hyphal re-growth occurred following each event.Relative hyphal length is dependent upon soil temperature and moisture. Soil respiration is related to the daily change in hyphal production, but not hyphal mortality.Continuous sensor and observation systems can provide more accurate assessments of soil carbon dynamics.
Scooped from: PLoS Biology, 2013 (via Pamela J. Hines - Science Editor) Authors: Ann Demogines, Jonathan Abraham, Hyeryun Choe, Michael Farzan and Sara L. Sawyer Summary: Transferrin Receptor (TfR1) is the cell-surface receptor that regulates iron uptake into cells, a process that is fundamental to life. However, TfR1 also facilitates the cellular entry of multiple mammalian viruses. We use evolutionary and functional analyses of TfR1 in the rodent clade, where two families of viruses bind this receptor, to mechanistically dissect how essential housekeeping genes like TFR1 successfully balance the opposing selective pressures exerted by host and virus. We find that while the sequence of rodent TfR1 is generally conserved, a small set of TfR1 residue positions has evolved rapidly over the speciation of rodents. Remarkably, all of these residues correspond to the two virus binding surfaces of TfR1. We show that naturally occurring mutations at these positions block virus entry while simultaneously preserving iron-uptake functionalities, both in rodent and human TfR1. Thus, by constantly replacing the amino acids encoded at just a few residue positions, TFR1 divorces adaptation to ever-changing viruses from preservation of key cellular functions. These dynamics have driven genetic divergence at the TFR1 locus that now enforces species-specific barriers to virus transmission, limiting both the cross-species and zoonotic transmission of these viruses. Science Editor Comment: Pamela J. Hines "Viruses gain entry to host cells through binding to specific receptors, which subsequently mutate to block virus binding. This can be problematic, however, because many viral receptors perform essential functions for their hosts. One such example is transferrin receptor 1, which regulates iron uptake by host cells. How are such mutations selected so as to not disrupt the essential function of the receptor? Demogines et al. now show how a small amount of mutations in transferrin receptor 1 can alter viral host specificity. Analysis of the ratio of synonymous to nonsynonymous DNA mutations in the rodent transferrin receptor identified a handful of residues that were under positive selection. These residues corresponded to the region of the receptor that interacts with the virus rather than those residues necessary for iron uptake. A search for human single-nucleotide polymorphisms identified transferrin receptor variants that reduced viral uptake in cultured human cells but maintained iron regulation. These results demonstrate that in the constant arms race between viruses and the cells they infect, positive selection of residues involved in viral entry can be divorced from regions of the receptor that are essential for host function."
Scooped from: PLoS Pathogens, 2013 Authors: Chong Zhang, Qiguang Xie, Ryan G. Anderson, Gina Ng, Nicholas C. Seitz, Thomas Peterson, C. Robertson McClung, John M. McDowell, Dongdong Kong, June M. Kwak and Hua Lu Summary: The circadian clock integrates temporal information with environmental cues in regulating plant development and physiology. Recently, the circadian clock has been shown to affect plant responses to biotic cues. To further examine this role of the circadian clock, we tested disease resistance in mutants disrupted in CCA1 and LHY, which act synergistically to regulate clock activity. We found that cca1 and lhy mutants also synergistically affect basal and resistance gene-mediated defense against Pseudomonas syringae and Hyaloperonospora arabidopsidis. Disrupting the circadian clock caused by overexpression of CCA1 or LHY also resulted in severe susceptibility to P. syringae. We identified a downstream target of CCA1 and LHY, GRP7, a key constituent of a slave oscillator regulated by the circadian clock and previously shown to influence plant defense and stomatal activity. We show that the defense role of CCA1 and LHY against P. syringae is at least partially through circadian control of stomatal aperture but is independent of defense mediated by salicylic acid. Furthermore, we found defense activation by P. syringae infection and treatment with the elicitor flg22 can feedback-regulate clock activity. Together this data strongly supports a direct role of the circadian clock in defense control and reveal for the first time crosstalk between the circadian clock and plant innate immunity.
Scooped from: Molecular Microbiology, 2013 Author: Frank-Jörg Vorhölter Summary: RNA-Seq is opening new doors for the functional understanding of microorganisms. Advances in RNA-Seq technology are allowing investigators to focus their studies on specific functional questions. An interesting example is presented by An et al. (2013) in this issue of Molecular Microbiology. New genes were identified for proteins and ncRNAs when the authors concentrated on the role of the rpf genes, which code for key components of a signal transduction hub in the plant pathogen Xanthomonas campestris pv. campestris. Although rpf gene products were already known to be involved in controlling transcription of many genes, including those encoding several important virulence factors, novel and unexpected properties of this signal transduction system emerged from the RNA-Seq analysis. In addition to identifying new target genes influenced by the rpf genes, the study found that the regulons of RpfC and RpfG, the sensor and response regulator of the master two-component regulatory system, only partially overlapped, indicating that the Rpf signalling system is even more complex than previously appreciated.
Scooped from: Front. Plant Sci., 2013 via Nicolas Denancé Authors: Nicolas Denancé, Andrea Sánchez-Vallet, Deborah Goffner and Antonio Molina Plant growth and response to environmental cues are largely governed by phytohormones. The plant hormones ethylene, jasmonic acid, and salicylic acid (SA) play a central role in the regulation of plant immune responses. In addition, other plant hormones, such as auxins, abscisic acid (ABA), cytokinins, gibberellins, and brassinosteroids, that have been thoroughly described to regulate plant development and growth, have recently emerged as key regulators of plant immunity. Plant hormones interact in complex networks to balance the response to developmental and environmental cues and thus limiting defense-associated fitness costs. The molecular mechanisms that govern these hormonal networks are largely unknown. Moreover, hormone signaling pathways are targeted by pathogens to disturb and evade plant defense responses. In this review, we address novel insights on the regulatory roles of the ABA, SA, and auxin in plant resistance to pathogens and we describe the complex interactions among their signal transduction pathways. The strategies developed by pathogens to evade hormone-mediated defensive responses are also described. Based on these data we discuss how hormone signaling could be manipulated to improve the resistance of crops to pathogens.
Via Nicolas Denancé
Scooped from: PLoS One, 2013 Authors: Fan Zhang, Zhenglin Du, Liyu Huang, Casiana Vera Cruz, Yongli Zhou and Zhikang Li
Summary: Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of rice bacterial blight, which is a major rice disease in tropical Asian countries. An attempt has been made to investigate gene expression patterns of three Xoo strains on the minimal medium XOM2, PXO99 (P6) and PXO86 (P2) from the Philippines, and GD1358 (C5) from China, which exhibited different virulence in 30 rice varieties, with putative virulence factors using deep sequencing. In total, 4,781 transcripts were identified in this study, and 1,151 and 3,076 genes were differentially expressed when P6 was compared with P2 and with C5, respectively. Our results indicated that Xoo strains from different regions exhibited distinctly different expression patterns of putative virulence-relevant genes. Interestingly, 40 and 44 genes involved in chemotaxis and motility exhibited higher transcript alterations in C5 compared with P6 and P2, respectively. Most other genes associated with virulence, including exopolysaccharide (EPS) synthesis, Hrp genes and type III effectors, including Xanthomonas outer protein (Xop) effectors and transcription activator-like (TAL) effectors, were down-regulated in C5 compared with P6 and P2. The data were confirmed by real-time quantitative RT-PCR, tests of bacterial motility, and enzyme activity analysis of EPS and xylanase. These results highlight the complexity of Xoo and offer new avenues for improving our understanding of Xoo-rice interactions and the evolution of Xoo virulence.
Scooped from: The Journal of the Federation of American Societies for Experimental Biology, 2013 Authors: Takashi Ohgita, Naoki Hayashi, Susumu Hama, Hiroyuki Tsuchiya, Naomasa Gotoh and Kentaro Kogure Summary: The type III secretion apparatus (T3SA) participates in the secretion of bacterial proteins called effectors, although the detailed mechanism of effector secretion remains unclear. T3SA and flagellum were shown to branch from a common ancestor and also show structural similarity. In addition, both T3SA-dependent effector secretion and flagellar rotation were reported to require proton-motive force (PMF) for activity. From these reports, we hypothesized that T3SA, like the flagellum, would rotate via PMF and that this rotation is responsible for effector secretion. To observe T3SA rotation, we constructed a novel observation system by modifying the tip of T3SA on bacterial cell membranes with an observation probe, which allowed documentation of T3SA rotation for the first time. T3SA rotation was stopped by the addition of a protonophore that decreases PMF. Moreover, increased viscosity of the observation medium inhibited both rotation of T3SA associated with beads and effector secretion. These results suggested that effector secretion would follow the PMF-dependent rotation of T3SA and could be inhibited by preventing T3SA rotation. Moreover, the motion-track analysis of bead rotation suggested that the T3SA needle might be flexible. Consequently, we propose a “rotational secretion model” as a novel effector secretion mechanism of T3SA.
Scooped from: Microbiology - SGM, 2013 Authors: Yong Zhang, Li Chen, Takeshi Yoshimochi, Akinori Kiba, Yasufumi Hikichi and Kouhei Ohnishi Summary: Genes in the hrp regulon encode component proteins of the type III secretion system and are essential for the pathogenicity of Ralstonia solanacearum. The hrp regulon is controlled by HrpB. We isolated several genes regulating hrpB expression from the Japanese strain OE1-1 using minitransposon mutagenesis. Among them, we mainly focused on 2 genes, hrpG and prhG, which are the positive regulators of hrpB. Although the global virulence regulator PhcA negatively regulated hrpG expression via prhIR, it positively regulated prhG expression. We further investigated the contrasting regulation of hrpG and prhG by PhcA and speculated that R. solanacearum may switch from HrpG to PrhG for hrpB activation in a cell density-dependent manner. Although the prhG mutant proliferated similarly to the wild type in leaf intercellular spaces and in xylem vessels of the host plants, it was less virulent than the wild type. The expression of the popA operon, which belongs to the hrp regulon, was significantly reduced in the prhG mutant by more than half in the leaf intercellular spaces and more than two thirds in the xylem vessels when compared with the wild type.
The analyses presented here provides a global view of the responses to multiple stresses, further validates known resistance-associated genes, and highlights new potential target genes, some lineage specific to rice, that play important roles in response to stress, providing a roadmap to develop varieties of rice that are more resistant to multiple biotic and abiotic stresses, as encountered in nature.
Via Elsa Ballini
Scooped from: Cellular Microbiology, 2013. Authors: Yousef Abu Kwaik and Dirk Bumann Summary: Microbial access to host nutrients is a fundamental aspect of infectious diseases. Pathogens face complex dynamic nutritional host microenvironments that change with increasing inflammation and local hypoxia. Since the host can actively limit microbial access to nutrient supply, pathogens have evolved various metabolic adaptations to successfully exploit available host nutrients for proliferation. Recent studies have unraveled an emerging paradigm that we propose to designate as ‘nutritional virulence’. This paradigm is based on specific virulence mechanisms that target major host biosynthetic and degradation pathways (proteasomes, autophagy and lysosomes) or nutrient-rich sources, such as glutathione, to enhance host supply of limiting nutrients, such as cysteine. Although Cys is the most limiting cellular amino acid, it is a metabolically favourable source of carbon and energy for various pathogens that are auxotrophic for Cys but utilize idiosyncratic nutritional virulence strategies to generate a gratuitous supply of host Cys. Therefore, proliferation of some intracellular pathogens is restricted by a host nutritional rheostat regulated by certain limiting amino acids, and pathogens have evolved idiosyncratic strategies to short circuit the host nutritional rheostat. Deciphering mechanisms of microbial ‘nutritional virulence’ and metabolism in vivo will facilitate identification of novel microbialand host targets for treatment and prevention of infectious diseases. Host–pathogen synchronization of amino acid auxotrophy indicates that this nutritional synchronization has been a major driving force in the evolution of many intracellular bacterial pathogens.
Scooped from: PLoS One. 2013 Authors: Michael D. Carruthers, Paul A. Nicholson, Erin N. Tracy, and Robert S. Munson, Jr Summary: Type VI secretion systems (T6SS) are a class of macromolecular secretion machines that are utilized by a number of bacteria for inter-bacterial competition or to elicit responses in eukaryotic cells. Acinetobacter baumannii is an opportunistic pathogen that causes severe infections in humans. These infections, including pneumonia and bacteremia, are important, as they are often associated with hospitals and medical-settings where they disproportionally affect critically ill patients like those residing in intensive care units. While it is known that A. baumannii genomes carry genes whose predicted products have homology with T6SS-associated gene products from other bacteria, and secretion of a major T6SS structural protein Hcp has been demonstrated, no additional work on an A. baumannii T6SS has been reported. Herein, we demonstrated that A. baumannii strain M2 secretes Hcp and this secretion was dependent upon TssB, an ortholog of a bacteriophage contractile sheath protein, confirming that strain M2 produces a functional T6SS. Additionally, we demonstrated that the ability of strain M2 to out-compete Escherichia coli was reliant upon the products of tssB and hcp. Collectively, our data have provided the first evidence demonstrating function in inter-bacterial competition, for a T6SS produced by A. baumannii.
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Scooped from: Current Biology, 2013 Authors: Yang Zhang, Eugenio Butelli, Rosalba De Stefano, Henk-jan Schoonbeek, Andreas Magusin, Chiara Pagliarani, Nikolaus Wellner, Lionel Hill, Diego Orzaez, Antonio Granell, Jonathan D.G. Jones and Cathie Martin Summary: Shelf life is an important quality trait for many fruit, including tomatoes. We report that enrichment of anthocyanin, a natural pigment, in tomatoes can significantly extend shelf life. Processes late in ripening are suppressed by anthocyanin accumulation, and susceptibility to Botrytis cinerea, one of the most important postharvest pathogens, is reduced in purple tomato fruit. We show that reduced susceptibility to B. cinerea is dependent specifically on the accumulation of anthocyanins, which alter the spreading of the ROS burst during infection. The increased antioxidant capacity of purple fruit likely slows the processes of overripening. Enhancing the levels of natural antioxidants in tomato provides a novel strategy for extending shelf life by genetic engineering or conventional breeding.
Scooped from: Environmental Microbiology Reports, 2012 Authors: Maarten G. K. Ghequire, Wen Li, Paul Proost, Remy Loris and René De Mot Summary: The genomes of Pseudomonas syringae pv. syringae 642 and Xanthomonas citri pv. malvacearum LMG 761 each carry a putative homologue of the plant lectin-like bacteriocin (llpA) genes previously identified in the rhizosphere isolate Pseudomonas putida BW11M1 and the biocontrol strain Pseudomonas fluorescens Pf-5. The respective purified recombinant proteins, LlpAPss642 and LlpAXcm761, display genus-specific antibacterial activity across species boundaries. The inhibitory spectrum of the P. syringae bacteriocin overlaps partially with those of the P. putida and P. fluorescens LlpAs. Notably, Xanthomonas axonopodis pv. citri str. 306 secretes a protein identical to LlpAXcm761. The functional characterization of LlpA proteins from two different phytopathogenic γ-proteobacterial species expands the lectin-like bacteriocin family beyond the Pseudomonas genus and suggests its involvement in competition among closely related plant-associated bacteria with different lifestyles.
Scooped from: Journal of Experimental Botany, 2013 Authors: Kapuganti J. Gupta, Yariv Brotman, Shruthi Segu, Tatiana Zeier, Jürgen Zeier, Stefan T. Persijn, Simona M. Cristescu, Frans J. M. Harren, Hermann Bauwe, Alisdair R. Fernie, Werner M. Kaiser and Luis A. J. Mur Summary: Different forms of nitrogen (N) fertilizer affect disease development; however, this study investigated the effects of N forms on the hypersensitivity response (HR)-a pathogen-elicited cell death linked to resistance. HR-eliciting Pseudomonas syringae pv. phaseolicola was infiltrated into leaves of tobacco fed with either NO₃⁻ or NH₄⁺. The speed of cell death was faster in NO₃⁻-fed compared with NH₄⁺-fed plants, which correlated, respectively, with increased and decreased resistance. Nitric oxide (NO) can be generated by nitrate reductase (NR) to influence the formation of the HR. NO generation was reduced in NH₄⁺-fed plants where N assimilation bypassed the NR step. This was similar to that elicited by the disease-forming P. syringae pv. tabaci strain, further suggesting that resistance was compromised with NH₄⁺ feeding. PR1a is a biomarker for the defence signal salicylic acid (SA), and expression was reduced in NH₄⁺-fed compared with NO₃⁻ fed plants at 24h after inoculation. This pattern correlated with actual SA measurements. Conversely, total amino acid, cytosolic and apoplastic glucose/fructose and sucrose were elevated in - treated plants. Gas chromatography/mass spectroscopy was used to characterize metabolic events following different N treatments. Following NO₃⁻ nutrition, polyamine biosynthesis was predominant, whilst after NH₄⁺ nutrition, flux appeared to be shifted towards the production of 4-aminobutyric acid. The mechanisms whereby feeding enhances SA, NO, and polyamine-mediated HR-linked defence whilst these are compromised with NH₄⁺, which also increases the availability of nutrients to pathogens, are discussed.
Scooped from:Plant Cell, 2013 (via @KamounLab and @PlantTeaching) Authors: Kranthi K. Mandadi and Karen-Beth G. Scholthof
Summary: Plants respond to pathogens using elaborate networks of genetic interactions. Recently, significant progress has been made in understanding RNA silencing and how viruses counter this apparently ubiquitous antiviral defense. In addition, plants also induce hypersensitive and systemic acquired resistance responses, which together limit the virus to infected cells and impart resistance to the noninfected tissues. Molecular processes such as the ubiquitin proteasome system and DNA methylation are also critical to antiviral defenses. Here, we provide a summary and update of advances in plant antiviral immune responses, beyond RNA silencing mechanisms—advances that went relatively unnoticed in the realm of RNA silencing and nonviral immune responses. We also document the rise of Brachypodium and Setaria species as model grasses to study antiviral responses in Poaceae, aspects that have been relatively understudied, despite grasses being the primary source of our calories, as well as animal feed, forage, recreation, and biofuel needs in the 21st century. Finally, we outline critical gaps, future prospects, and considerations central to studying plant antiviral immunity. To promote an integrated model of plant immunity, we discuss analogous viral and nonviral immune concepts and propose working definitions of viral effectors, effector-triggered immunity, and viral pathogen-triggered immunity.
Via Mary Williams, Kamoun Lab @ TSL
Scooped from: Molecular Microbiology, 2013 Authors: Shi-Qi An, Melanie Febrer, Yvonne McCarthy, Dong-Jie Tang, Leah Clissold, Gemy Kaithakottil, David Swarbreck, Ji-Liang Tang, Jane Rogers, J. Maxwell Dow and Robert P. Ryan. Summary: The bacterium Xanthomonas campestris is an economically important pathogen of many crop species and a model for the study of bacterial phytopathogenesis. In X. campestris, a regulatory system mediated by the signal molecule DSF controls virulence to plants. The synthesis and recognition of the DSF signal depends upon different Rpf proteins. DSF signal generation requires RpfF whereas signal perception and transduction depends upon a system comprising the sensor RpfC and regulator RpfG. Here we have addressed the action and role of Rpf/DSF signalling in phytopathogenesis by high-resolution transcriptional analysis coupled to functional genomics. We detected transcripts for many genes that were unidentified by previous computational analysis of the genome sequence. Novel transcribed regions included intergenic transcripts predicted as coding or non-coding as well as those that were antisense to coding sequences. In total, mutation of rpfF, rpfG and rpfC led to alteration in transcript levels (more than fourfold) of approximately 480 genes. The regulatory influence of RpfF and RpfC demonstrated considerable overlap. Contrary to expectation, the regulatory influence of RpfC and RpfG had limited overlap, indicating complexities of the Rpf signalling system. Importantly, functional analysis revealed over 160 new virulence factors within the group of Rpf-regulated genes.
Scooped from: PLoS ONE, 2011 Authors: Vaibhav Bhardwaj, Stuart Meier, Lindsay N. Petersen, Robert A. Ingle, Laura C. Roden Summary: The circadian clock allows plants to anticipate predictable daily changes in abiotic stimuli, such as light; however, whether the clock similarly allows plants to anticipate interactions with other organisms is unknown. Here we show that Arabidopsis thaliana (Arabidopsis) has circadian clock-mediated variation in resistance to the virulent bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), with plants being least susceptible to infection in the subjective morning. We suggest that the increased resistance to Pst DC3000 observed in the morning in Col-0 plants results from clock-mediated modulation of pathogen associated molecular pattern (PAMP)-triggered immunity. Analysis of publicly available microarray data revealed that a large number of Arabidopsis defence-related genes showed both diurnal- and circadian-regulation, including genes involved in the perception of the PAMP flagellin which exhibit a peak in expression in the morning. Accordingly, we observed that PAMP-triggered callose deposition was significantly higher in wild-type plants inoculated with Pst DC3000 hrpA in the subjective morning than in the evening, while no such temporal difference was evident in arrhythmic plants. Our results suggest that PAMP-triggered immune responses are modulated by the circadian clock and that temporal regulation allows plants to anticipate and respond more effectively to pathogen challenges in the daytime.
Scooped from Molecular Plant-Microbe Interactions, 2013 (via @KamounLab) Authors: Jorge LuisBadel, Sophie J. M.Piquerez, DavidGreenshields, GhanasyamRallapalli, GeorginaFabro, NaveedIshaque and Jonathan D. G.Jones Summary: The genome of the pathogenic oomycete Hyaloperonospora arabidopsidisis predicted to encode at least 134 high-confidence effectors (HaRxL) carrying the RxLR motif implicated in their translocation into plant cells. However, only four avirulence genes (ATR1, ATR13, ATR5, and ATR39) have been isolated. This indicates that identification of HaRxL effectors based on avirulence is low throughput. We aimed at rapidly identifying H. arabidopsidis effectors that contribute to virulence by developing methods to detect and quantify multiple candidates in bacterial mixed infections using either Illumina sequencing or capillary electrophoresis. In these assays, referred to here as in planta effector competition assays, we estimate the contribution to virulence of individual effectors by calculating the abundance of each HaRxL in the bacterial population recovered from leaves 3 days after inoculation relative to abundance in the initial mixed inoculum. We identified HaRxL that enhancePseudomonas syringae pv. tomato DC3000 growth in some but not allArabidopsis accessions. Further analysis showed that HaRxLL464, HaRxL75, HaRxL22, HaRxLL441, and HaRxL89 suppress pathogen-associated molecular pattern-triggered immunity (PTI) and localize to different subcellular compartments in Nicotiana benthamiana, providing evidence for a multilayered suppression of PTI by pathogenic oomycetes and molecular probes for the dissection of PTI.
Via Kamoun Lab @ TSL
Scooped from: Biochemistry, 2013 Authors: Srirupa Chatterjee, Sukanya Chaudhury, Andrew C. McShan, Kawaljit Kaur and Roberto N. De Guzman Summary: Many plant and animal bacterial pathogens assemble a needle-like nanomachine, the type III secretion system (T3SS), to inject virulence proteins directly into eukaryotic cells to initiate infection. The ability of bacteria to inject effectors into host cells is essential for infection, survival, and pathogenesis for many Gram-negative bacteria, including Salmonella, Escherichia, Shigella, Yersinia, Pseudomonas, and Chlamydia spp. These pathogens are responsible for a wide variety of diseases, such as typhoid fever, large-scale food-borne illnesses, dysentery, bubonic plague, secondary hospital infections, and sexually transmitted diseases. The T3SS consists of structural and nonstructural proteins. The structural proteins assemble the needle apparatus, which consists of a membrane-embedded basal structure, an external needle that protrudes from the bacterial surface, and a tip complex that caps the needle. Upon host cell contact, a translocon is assembled between the needle tip complex and the host cell, serving as a gateway for translocation of effector proteins by creating a pore in the host cell membrane. Following delivery into the host cytoplasm, effectors initiate and maintain infection by manipulating host cell biology, such as cell signaling, secretory trafficking, cytoskeletal dynamics, and the inflammatory response. Finally, chaperones serve as regulators of secretion by sequestering effectors and some structural proteins within the bacterial cytoplasm. This review will focus on the latest developments and future challenges concerning the structure and biophysics of the needle apparatus.
Scooped from: Annual Review of Phytopathology, 2013
Authors: Xiu-Fang Xin and Sheng Yang He
Summary: Since the early 1980s, various strains of the gram-negative bacterial pathogen Pseudomonas syringae have been used as models for understanding plant-bacterial interactions. In 1991, a P. syringae pathovar tomato (Pst) strain, DC3000, was reported to infect not only its natural host tomato but also Arabidopsis in the laboratory, a finding that spurred intensive efforts in the subsequent two decades to characterize the molecular mechanisms by which this strain causes disease in plants. Genomic analysis shows that Pst DC3000 carries a large repertoire of potential virulence factors, including proteinaceous effectors that are secreted through the type III secretion system and a polyketide phytotoxin called coronatine, which structurally mimics the plant hormone jasmonate ( JA). Study of Pst DC3000 pathogenesis has not only provided several conceptual advances in understanding how a bacterial pathogen employs type III effectors to suppress plant immune responses and promote disease susceptibility but has also facilitated the discovery of the immune function of stomata and key components of JA signaling in plants. The concepts derived from the study of Pst DC3000 pathogenesis may prove useful in understanding pathogenesis mechanisms of other plant pathogens.
To accomplish successful infection, pathogens deploy complex strategies to interfere with host defense systems and subterfuge host physiology to favor pathogen survival and multiplication. Modulation of plant auxin physiology and signaling is emerging as a common virulence strategy for phytobacteria to cause diseases. However, the underlying mechanisms remain largely elusive. We have previously shown that the Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis auxin physiology. Here we report that AvrRpt2 promotes auxin response by stimulating the turnover of Aux/IAA proteins, the key negative regulators in auxin signaling. AvrRpt2 acts additively with auxin to stimulate Aux/IAA turnover, suggesting distinct, yet proteasome-dependent mechanisms operated by AvrRpt2 and auxin to control Aux/IAA stability. The cysteine protease activity is required for AvrRpt2-stimulated auxin signaling and Aux/IAA degradation. Importantly, the transgenic plants expressing the dominant axr2-1 mutation recalcitrant to AvrRpt2-mediated degradation ameliorated virulence functions of AvrRpt2, but did not alter the avirulent function mediated by the corresponding RPS2 resistance protein. Thus, promoting auxin response via modulating the stability of key transcription repressors Aux/IAA is a mechanism used by bacterial type III effector AvrRpt2 to promote pathogenicity.
Via Kamoun Lab @ TSL
Scooped from: Plant Physiology and Biochemistry, 2013 Authors: Woojong Yima, Sundaram Seshadrib, Kiyoon Kima, Gillseung Leec, Tongmin Sa Summary: Bacteria of genus Methylobacterium have been found to promote plant growth and regulate the level of ethylene in crop plants. This work is aimed to test the induction of defense responses in tomato against bacterial wilt by stress ethylene level reduction mediated by the ACC deaminase activity of Methylobacterium strains. Under greenhouse conditions, the disease index value in Methylobacterium sp. inoculated tomato plants was lower than control plants. Plants treated with Methylobacterium sp. challenge inoculated with Ralstonia solanacearum (RS) showed significantly reduced disease symptoms and lowered ethylene emission under greenhouse condition. The ACC and ACO (1-aminocyclopropane-1-carboxylate oxidase) accumulation in tomato leaves were significantly reduced with Methylobacterium strains inoculation. While ACC oxidase gene expression was found higher in plants treated with R. solanacearum than Methylobacterium sp. treatment, PR proteins related to induced systemic resistance like β-1,3-glucanase, PAL, PO and PPO were increased in Methylobacterium sp. inoculated plants. A significant increase in β-1,3-glucanase and PAL gene expression was found in all the Methylobacterium spp. treatments compared to the R. solanacearum treatment. This study confirms the activity of Methylobacterium sp. in increasing the defense enzymes by modulating the ethylene biosynthesis pathway and suggests the use of methylotrophic bacteria as potential biocontrol agents in tomato cultivation.
Scooped from: Journal of Integrative Plant Biology, 2013; via Mary Williams (@PlantTeaching) Authors: William J. Lucas, Andrew Groover, Raffael Lichtenberger, Kaori Furuta, Shri-Ram Yadav, Ykä Helariutta, Xin-Qiang He, Hiroo Fukuda, Julie Kang, Siobhan M. Brady, John W. Patrick, John Sperry, Akiko Yoshida, Ana-Flor López-Millán, Michael A. Grusak, Pradeep Kachroo Summary: The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry. Contents: I. Introduction 295 II. Evolution of the Plant Vascular System 295 III. Phloem Development & Differentiation 300 IV. Molecular Mechanisms Underlying Xylem Cell Differentiation 307 V. Spatial & Temporal Regulation of Vascular Patterning 311 VI. Secondary Vascular Development 318 VII. Physical and Physiological Constraints on Phloem Transport Function 321 VIII. Physical & Physiological Constraints on Xylem Function 328 IX. Long-distance Signaling Through the Phloem 339 X. Root-to-shoot Signaling 347 XI. Vascular Transport of Microelement Minerals 351 XII. Systemic Signaling: Pathogen Resistance 356 XIII. Future Perspectives 361 XIV. Acknowledgements 362 XV. References 362
Via Mary Williams
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