Single-cell transcript profiling of barley attacked by the powdery mildew fungus

A unique effector secreted by Pseudozyma flocculosa mediates its biocontrol activity

BACKGROUND

Pseudozyma flocculosa is a highly efficient biocontrol agent (BCA) of powdery mildews whose mode of action remains elusive. It is known to secrete unique effectors during its interaction with powdery mildews but effectors have never been shown to be part of the arsenal of a BCA. Here, we characterize the role of the effector Pf2826 released by Pseudozyma flocculosa during its tripartite interaction with barley and the pathogen fungus Blumeria graminis f. sp. hordei.

RESULTS

We utilized CRISPR-Cas9-based genome editing and confirmed that secreted P. flocculosa effector Pf2826 is required for full biocontrol activity. We monitored the localization of the effector Pf2826 with C-terminal mCherry tag and found it localized around the haustoria and on powdery mildew spores. His-tagged Pf2826 recombinant protein was expressed, purified, and used as bait in a pull-down assay from total proteins extracted during the tripartite interaction. Potential interactors were identified by LC-MS/MS analysis after removing unspecific interactions found in the negative controls. A two-way yeast two-hybrid assay validated that Pf2826 interacted with barley pathogenesis-related (PR) proteins HvPR1a and chitinase and with an effector protein from powdery mildew.

CONCLUSIONS

In contrast to the usual modes of action of competition, parasitism, and antibiosis ascribed to BCAs, this study shows that effector pf2826 plays a vital role in the biocontrol activity of P. flocculosa by interacting with plant PR proteins and a powdery mildew effector, altering the host-pathogen interaction.

Network organization of the plant immune system: from pathogen perception to robust defense induction

The plant immune system has been explored essentially through the study of qualitative resistance, a simple form of immunity, and from a reductionist point of view. The recent identification of genes conferring quantitative disease resistance revealed a large array of functions, suggesting more complex mechanisms. In addition, thanks to the advent of high-throughput analyses and system approaches, our view of the immune system has become more integrative, revealing that plant immunity should rather be seen as a distributed and highly connected molecular network including diverse functions to optimize expression of plant defenses to pathogens. Here, we review the recent progress made to understand the network complexity of regulatory pathways leading to plant immunity, from pathogen perception, through signaling pathways and finally to immune responses. We also analyze the topological organization of these networks and their emergent properties, crucial to predict novel immune functions and test them experimentally. Finally, we report how these networks might be regulated by environmental clues. Although system approaches remain extremely scarce in this area of research, a growing body of evidence indicates that the plant response to combined biotic and abiotic stresses cannot be inferred from responses to individual stresses. A view of possible research avenues in this nascent biology domain is finally proposed.

Comparison of leaf transcriptome in response to Rhizoctonia solani infection between resistant and susceptible rice cultivars

Background

Sheath blight (SB), caused by Rhizoctonia solani, is a common rice disease worldwide. Currently, rice cultivars with robust resistance to R. solani are still lacking. To provide theoretic basis for molecular breeding of R. solani-resistant rice cultivars, the changes of transcriptome profiles in response to R. solani infection were compared between a moderate resistant cultivar (Yanhui-888, YH) and a susceptible cultivar (Jingang-30, JG).

Results

In the present study, 3085 differentially express genes (DEGs) were detected between the infected leaves and the control in JG, with 2853 DEGs in YH. A total of 4091 unigenes were significantly upregulated in YH than in JG before infection, while 3192 were significantly upregulated after infection. Further analysis revealed that YH and JG showed similar molecular responses to R. solani infection, but the responses were earlier in JG than in YH. Expression levels of trans-cinnamate 4-monooxygenase (C4H), ethylene-insensitive protein 2 (EIN2), transcriptome factor WRKY33 and the KEGG pathway plant-pathogen interaction were significantly affected by R. solani infection. More importantly, these components were all over-represented in YH cultivar than in JG cultivar before and/or after infection.

Conclusions

These genes possibly contribute to the higher resistance of YH to R. solani than JG and were potential target genes to molecularly breed R. solani-resistant rice cultivar.

VpSTS29/STS2 enhances fungal tolerance in grapevine through a positive feedback loop

Accumulation of stilbene phytoalexins stimulates resistance mechanisms against the grapevine fungus Uncinula necator. However, the defensive mechanisms triggered by stilbene synthase (STS) genes, remain largely unknown. Here, we report the function and molecular mechanism of the stilbene synthase gene VpSTS29/STS2 from Vitis pseudoreticulata in the regulation of plant responses to powdery mildew. Stilbene synthesis occurred mainly in root tips and mesophyll cells of transgenic grapevines via transport through the vascular bundles. Overexpression of VpSTS29/STS2 in Vitis vinifera increased the abundance of STSs in mesophyll tissue and resulted in the accumulation of biologically active resveratrol derivatives at the invasion site. Similarly, expression of VpSTS29/STS2 in Arabidopsis increased resistance to Golovinomyces cichoracearum. The VpSTS29/STS2-expressing Arabidopsis lines showed increased piceid accumulation together with more local hypersensitive reactions, inhibition of mycelial growth, and a reduced incidence of pathogens. Transcriptome profiling analyses demonstrated that VpSTS29/STS2-induced defences led to reprograming of global gene expression and activation of salicylic acid (SA) signalling, thus increasing expression of WRKY-MYB transcription factors and other defence response genes. We propose a model for resveratrol-mediated coordination of defence responses in which SA participates in a positive feedback loop.

Driving factors of epiphytic bacterial communities: A review

Bacteria establish complex, compositionally consistent communities on healthy leaves. Ecological processes such as dispersal, diversification, ecological drift, and selection as well as leaf surface physicochemistry and topology impact community assembly. Since the leaf surface is an oligotrophic environment, species interactions such as competition and cooperation may be major contributors to shape community structure. Furthermore, the plant immune system impacts on microbial community composition, as plant cells respond to bacterial molecules and shape their responses according to the mixture of molecules present. Such tunability of the plant immune network likely enables the plant host to differentiate between pathogenic and non-pathogenic colonisers, avoiding costly immune responses to non-pathogenic colonisers. Plant immune responses are either systemically distributed or locally confined, which in turn affects the colonisation pattern of the associated microbiota. However, how each of these factors impacts the bacterial community is unclear. To better understand this impact, bacterial communities need to be studied at a micrometre resolution, which is the scale that is relevant to the members of the community. Here, current insights into the driving factors influencing the assembly of leaf surface-colonising bacterial communities are discussed, with a special focus on plant host immunity as an emerging factor contributing to bacterial leaf colonisation.

Plant glutathione transferase-mediated stress tolerance: functions and biotechnological applications

Plant glutathione transferases (EC 2.5.1.18, GSTs) are an ancient, multimember and diverse enzyme class. Plant GSTs have diverse roles in plant development, endogenous metabolism, stress tolerance, and xenobiotic detoxification. Their study embodies both fundamental aspects and agricultural interest, because of their ability to confer tolerance against biotic and abiotic stresses and to detoxify herbicides. Here we review the biotechnological applications of GSTs towards developing plants that are resistant to biotic and abiotic stresses. We integrate recent discoveries, highlight, and critically discuss the underlying biochemical and molecular pathways involved. We elaborate that the functions of GSTs in abiotic and biotic stress adaptation are potentially a result of both catalytic and non-catalytic functions. These include conjugation of reactive electrophile species with glutathione and the modulation of cellular redox status, biosynthesis, binding, and transport of secondary metabolites and hormones. Their major universal functions under stress underline the potential in developing climate-resilient cultivars through a combination of molecular and conventional breeding programs. We propose that future GST engineering efforts through rational and combinatorial approaches, would lead to the design of improved isoenzymes with purpose-designed catalytic activities and novel functional properties. Concurrent GST-GSH metabolic engineering can incrementally increase the effectiveness of GST biotechnological deployment.

Are ineffective defence reactions potential target for induced resistance during the compatible wheat-powdery mildew interaction?

Powdery mildew caused by Blumeria graminis f.sp. tritici, an obligate aerial biotrophic fungus, would be one of the most damaging wheat (Triticum aestivum) diseases without the extensive use of conventional fungicides. In our study, the expression levels of some basal defence-related genes were investigated during a compatible interaction in order to evaluate wheat reactions to infection, along with the different stages of the infectious process in planta. As fungal conidia initiated their germination and developed appressorial germ tube (AGT), early defence reactions involved the expression of a lipoxygenase (LOX)- and an oxalate oxidase (OXO)-encoding genes, followed by activations of corresponding LOX (EC 1.13.11.12) and OXO (EC 1.2.3.4) activities, respectively. When penetration of AGT took place, up-regulation of chitinases (CHI) and PR1-encoding genes expression occurred along with an increase of CHI (EC 3.2.1.14) activity. Meanwhile, expression of a phenylalanine ammonia-lyase-encoding gene also took place. Up-regulation of a phospholipase C- and lipid transfer proteins-encoding genes expression occurred during the latest stages of infection. Neither the phi glutathione S-transferase (GST)-encoding gene expression nor the GST (EC 2.5.1.13) activity was modified upon wheat infection by powdery mildew. Whether these defence reactions during such a compatible interaction are markers of immunity or susceptibility, and whether they have the ability to contribute to protection upon modulation of their timing and their intensity by resistance inducers are discussed.

Accumulation of Transcripts Abundance after Barley Inoculation with Cochliobolus sativus

Spot blotch caused by the hemibiotrophic pathogen Cochliobolus sativus has been the major yield-reducing factor for barley production during the last decade. Monitoring transcriptional reorganization triggered in response to this fungus is an essential first step for the functional analysis of genes involved in the process. To characterize the defense responses initiated by barley resistant and susceptible cultivars, a survey of transcript abundance at early time points of C. sativus inoculation was conducted. A notable number of transcripts exhibiting significant differential accumulations in the resistant and susceptible cultivars were detected compared to the non-inoculated controls. At the p-value of 0.0001, transcripts were divided into three general categories; defense, regulatory and unknown function, and the resistant cultivar had the greatest number of common transcripts at different time points. Quantities of differentially accumulated gene transcripts in both cultivars were identified at 24 h post infection, the approximate time when the pathogen changes trophic lifestyles. The unique and common accumulated transcripts might be of considerable interest for enhancing effective resistance to C. sativus.

Interaction of barley powdery mildew effector candidate CSEP0055 with the defence protein PR17c

A large number of effector candidates have been identified recently in powdery mildew fungi. However, their roles and how they perform their functions remain unresolved. In this study, we made use of host-induced gene silencing and confirmed that the secreted barley powdery mildew effector candidate, CSEP0055, contributes to the aggressiveness of the fungus. This result suggests that CSEP0055 is involved in the suppression of plant defence. A yeast two-hybrid screen indicated that CSEP0055 interacts with members of the barley pathogenesis-related protein families, PR1 and PR17. Interaction with PR17c was confirmed by bimolecular fluorescence complementation analyses. Down-regulation and over-expression of PR17c in epidermal cells of barley confirmed that this protein is important for penetration resistance against the powdery mildew fungus. In line with this, PR17c was found to be apoplastic, localizing to the papillae formed in response to this fungus. The CSEP0055 transcript did not start to accumulate until 24 h after inoculation. This suggests that this gene is expressed too late to influence primary penetration events, but rather sustains the fungus at sites of secondary penetration, where PR17c appears to be able to accumulate.

Differential accumulation of host mRNAs on polyribosomes during obligate pathogen-plant interactions

Plant pathogens elicit dramatic changes in the expression of host genes during both compatible and incompatible interactions. Gene expression profiling studies of plant-pathogen interactions have only considered messenger RNAs (mRNAs) present in total RNA, which contains subpopulations of actively translated mRNAs associated with polyribosomes (polysomes) and non-translated mRNAs that are not associated with polysomes. The goal of this study was to enhance previous gene expression analyses by identifying host mRNAs that become differentially associated with polysomes following pathogen inoculation. Total and polysomal RNA were extracted from barley (Hordeum vulgare) plants at 32 h after inoculation with Blumeria graminis f. sp. hordei, and Arabidopsis thaliana plants at 10 days after inoculation with Turnip mosaic virus. Gene expression profiles were obtained for each pathosystem, which represent diverse plant host-obligate pathogen interactions. Using this approach, host mRNAs were identified that were differentially associated with polysomes in response to pathogen treatment. Approximately 18% and 26% of mRNAs represented by probe sets on the Affymetrix Barley1 and Arabidopsis ATH1 GeneChips, respectively, differentially accumulated in the two populations in one or more combinations of treatment and genotype. Gene ontology analysis of mRNAs sharing the same pattern of accumulation in total and polysomal RNA identified gene sets that contained a significant number of functionally related annotations, suggesting both transcript accumulation and recruitment to polyribosomes are coordinately regulated in these systems.

Plant-pathogen interactions: what microarray tells about it?

Plant defense responses are mediated by elementary regulatory proteins that affect expression of thousands of genes. Over the last decade, microarray technology has played a key role in deciphering the underlying networks of gene regulation in plants that lead to a wide variety of defence responses. Microarray is an important tool to quantify and profile the expression of thousands of genes simultaneously, with two main aims: (1) gene discovery and (2) global expression profiling. Several microarray technologies are currently in use; most include a glass slide platform with spotted cDNA or oligonucleotides. Till date, microarray technology has been used in the identification of regulatory genes, end-point defence genes, to understand the signal transduction processes underlying disease resistance and its intimate links to other physiological pathways. Microarray technology can be used for in-depth, simultaneous profiling of host/pathogen genes as the disease progresses from infection to resistance/susceptibility at different developmental stages of the host, which can be done in different environments, for clearer understanding of the processes involved. A thorough knowledge of plant disease resistance using successful combination of microarray and other high throughput techniques, as well as biochemical, genetic, and cell biological experiments is needed for practical application to secure and stabilize yield of many crop plants. This review starts with a brief introduction to microarray technology, followed by the basics of plant-pathogen interaction, the use of DNA microarrays over the last decade to unravel the mysteries of plant-pathogen interaction, and ends with the future prospects of this technology.

Barley leaf transcriptome and metabolite analysis reveals new aspects of compatibility and Piriformospora indica-mediated systemic induced resistance to powdery mildew

Colonization of barley roots with the basidiomycete fungus Piriformospora indica (Sebacinales) induces systemic resistance against the biotrophic leaf pathogen Blumeria graminis f. sp. hordei (B. graminis). To identify genes involved in this mycorrhiza-induced systemic resistance, we compared the leaf transcriptome of P. indica-colonized and noncolonized barley plants 12, 24, and 96 h after challenge with a virulent race of B. graminis. The leaf pathogen induced specific gene sets (e.g., LRR receptor kinases and WRKY transcription factors) at 12 h postinoculation (hpi) (prepenetration phase) and vesicle-localized gene products 24 hpi (haustorium establishment). Metabolic analysis revealed a progressing shift of steady state contents of the intermediates glucose-1-phosphate, uridinediphosphate-glucose, and phosphoenolpyruvate 24 and 96 hpi, indicating that B. graminis shifts central carbohydrate metabolism in favor of sucrose biosynthesis. Both B. graminis and P. indica increased glutamine and alanine contents, whereas substrates for starch and nitrogen assimilation (adenosinediphosphate- glucose and oxoglutarate) decreased. In plants that were more B. graminis resistant due to P. indica root colonization, 22 transcripts, including those of pathogenesis-related genes and genes encoding heat-shock proteins, were differentially expressed ?twofold in leaves after B. graminis inoculation compared with non-mycorrhized plants. Detailed expression analysis revealed a faster induction after B. graminis inoculation between 8 and 16 hpi, suggesting that priming of these genes is an important mechanism of P. indica-induced systemic disease resistance.

Quantitative and temporal definition of the Mla transcriptional regulon during barley-powdery mildew interactions

Barley Mildew resistance locus a (Mla) is a major determinant of immunity to the powdery mildew pathogen, Blumeria graminis f. sp. hordei. Alleles of Mla encode cytoplasmic- and membrane-localized coiled-coil, nucleotide binding site, leucine-rich repeat proteins that mediate resistance when complementary avirulence effectors (AVR(a)) are present in the pathogen. Presence of an appropriate AVR(a) protein triggers nuclear relocalization of MLA, in which MLA binds repressing host transcription factors. Timecourse expression profiles of plants harboring Mla1, Mla6, and Mla12 wild-type alleles versus paired loss-of-function mutants were compared to discover conserved transcriptional targets of MLA and downstream signaling cascades. Pathogen-dependent gene expression was equivalent or stronger in susceptible plants at 20 h after inoculation (HAI) and was attenuated at later timepoints, whereas resistant plants exhibited a time-dependent strengthening of the transcriptional response, increasing in both fold change and the number of genes differentially expressed. Deregulation at 20 HAI implicated 16 HAI as a crucial point in determining the future trajectory of this interaction and was interrogated by quantitative analysis. In total, 28 potential transcriptional targets of the MLA regulon were identified. These candidate targets possess a diverse set of predicted functions, suggesting that multiple pathways are required to mediate the hypersensitive reaction.

Pathogenesis-associated transcriptional patterns in Triticeae

The Triticeae tribe of the plant Poaceae family contains some of the most important cereal crop plants for nutrition of humans and livestock such as wheat and barley. Despite the agronomical relevance of plant immunity, knowledge on mechanisms of disease or resistance in Triticeae is limited. It is hardly understood what actually stops a microbial invader when restricted by the plant and in how far a susceptible host plant contributes to pathogenesis. Transcriptional reprogramming of the host plant may be involved in both immunity and disease. This paper gives an overview about recent analyses of global pathogenesis-related transcriptional patterns in response of Triticeae to biotrophic or non-biotrophic fungal pathogens and their toxins. It highlights enriched biological functions in association with successful plant defence or disease as well as experiments that successfully translated gene expression data into analysis of gene functions.

Tissue-adapted invasion strategies of the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae causes rice blast, the most serious foliar fungal disease of cultivated rice (Oryza sativa). During hemibiotrophic leaf infection, the pathogen simultaneously combines biotrophic and necrotrophic growth. Here, we provide cytological and molecular evidence that, in contrast to leaf tissue infection, the fungus adopts a uniquely biotrophic infection strategy in roots for a prolonged period and spreads without causing a loss of host cell viability. Consistent with a biotrophic lifestyle, intracellularly growing hyphae of M. oryzae are surrounded by a plant-derived membrane. Global, temporal gene expression analysis used to monitor rice responses to progressive root infection revealed a rapid but transient induction of basal defense-related gene transcripts, indicating perception of the pathogen by the rice root. Early defense gene induction was followed by suppression at the onset of intracellular fungal growth, consistent with the biotrophic nature of root invasion. By contrast, during foliar infection, the vast majority of these transcripts continued to accumulate or increased in abundance. Furthermore, induction of necrotrophy-associated genes during early tissue penetration, previously observed in infected leaves, was not seen in roots. Collectively, our results not only report a global characterization of transcriptional root responses to a biotrophic fungal pathogen but also provide initial evidence for tissue-adapted fungal infection strategies.

Nonhost resistance of barley to different fungal pathogens is associated with largely distinct, quantitative transcriptional responses

Nonhost resistance protects plants against attack by the vast majority of potential pathogens, including phytopathogenic fungi. Despite its high biological importance, the molecular architecture of nonhost resistance has remained largely unexplored. Here, we describe the transcriptional responses of one particular genotype of barley (Hordeum vulgare subsp. vulgare 'Ingrid') to three different pairs of adapted (host) and nonadapted (nonhost) isolates of fungal pathogens, which belong to the genera Blumeria (powdery mildew), Puccinia (rust), and Magnaporthe (blast). Nonhost resistance against each of these pathogens was associated with changes in transcript abundance of distinct sets of nonhost-specific genes, although general (not nonhost-associated) transcriptional responses to the different pathogens overlapped considerably. The powdery mildew- and blast-induced differences in transcript abundance between host and nonhost interactions were significantly correlated with differences between a near-isogenic pair of barley lines that carry either the Mlo wild-type allele or the mutated mlo5 allele, which mediates basal resistance to powdery mildew. Moreover, during the interactions of barley with the different host or nonhost pathogens, similar patterns of overrepresented and underrepresented functional categories of genes were found. The results suggest that nonhost resistance and basal host defense of barley are functionally related and that nonhost resistance to different fungal pathogens is associated with more robust regulation of complex but largely nonoverlapping sets of pathogen-responsive genes involved in similar metabolic or signaling pathways.

Phloem cytochemical modification and gene expression following the recovery of apple plants from apple proliferation disease

Recovery of apple trees from apple proliferation was studied by combining ultrastructural, cytochemical, and gene expression analyses to possibly reveal changes linked to recovery-associated resistance. When compared with either healthy or visibly diseased plants, recovered apple trees showed abnormal callose and phloem-protein accumulation in their leaf phloem. Although cytochemical localization detected Ca(2+) ions in the phloem of all the three plant groups, Ca(2+) concentration was remarkably higher in the phloem cytosol of recovered trees. The expression patterns of five genes encoding callose synthase and of four genes encoding phloem proteins were analyzed by quantitative real-time reverse transcription-polymerase chain reaction. In comparison to both healthy and diseased plants, four of the above nine genes were remarkably up-regulated in recovered trees. As in infected apple trees, phytoplasma disappear from the crown during winter, but persist in the roots, and it is suggested that callose synthesis/deposition and phloem-protein plugging of the sieve tubes would form physical barriers preventing the recolonization of the crown during the following spring. Since callose deposition and phloem-protein aggregation are both Ca(2+)-dependent processes, the present results suggest that an inward flux of Ca(2+) across the phloem plasma membrane could act as a signal for activating defense reactions leading to recovery in phytoplasma-infected apple trees.

Primary Metabolism and Plant Defense—Fuel for the Fire

Plants have the ability to recognize and respond to a multitude of microorganisms. Recognition of pathogens results in a massive reprogramming of the plant cell to activate and deploy defense responses to halt pathogen growth. Such responses are associated with increased demands for energy, reducing equivalents, and carbon skeletons that are provided by primary metabolic pathways. Although pathogen recognition and downstream resistance responses have been the focus of major study, an intriguing and comparatively understudied phenomenon is how plants are able to recruit energy for the defense response. To that end, this review will summarize current research on energy-producing primary metabolism pathways and their role in fueling the resistance response.

Lr34-mediated leaf rust resistance in wheat: transcript profiling reveals a high energetic demand supported by transient recruitment of multiple metabolic pathways

The wheat gene Lr34 confers partial resistance to all races of Puccinia triticina, the causal agent of wheat leaf rust. However, the biological basis for the exceptional durability of Lr34 is unclear. We used the Affymetrix GeneChip Wheat Genome Array to compare transcriptional changes of near-isogenic lines of Thatcher wheat in a compatible interaction, an incompatible interaction conferred by the resistance gene Lr1, and the race-nonspecific response conditioned by Lr34 3 and 7 days postinoculation (dpi) with P. triticina. No differentially expressed genes were detected in Lr1 plants at either timepoint whereas, in the compatible Thatcher interaction, differentially expressed genes were detected only at 7 dpi. In contrast, differentially expressed genes were identified at both timepoints in P. triticina-inoculated Lr34 plants. At 3 dpi, upregulated genes associated with Lr34-mediated resistance encoded various defense and stress-related proteins, secondary metabolism enzymes, and transcriptional regulation and cellular-signaling proteins. Further, coordinated upregulation of key genes in several metabolic pathways that can contribute to increased carbon flux through the tricarboxylic cycle was detected. This indicates that Lr34-mediated resistance imposes a high energetic demand that leads to the induction of multiple metabolic responses to support cellular energy requirements. These metabolic responses were not sustained through 7 dpi, and may explain why Lr34 fails to inhibit the pathogen fully but does increase the latent period.

Protein profiling of single epidermal cell types from Arabidopsis thaliana using surface-enhanced laser desorption and ionization technology

Here, we describe a novel approach for investigating differential protein expression within three epidermal cell types. In particular, 3000 single pavement, basal, and trichome cells from leaves of Arabidopsis thaliana were harvested by glass micro-capillaries. Subsequently, these single cell samples were joined to form pools of 100 individual cells and analyzed using the ProteinChip technology; SELDI: surface-enhanced laser desorption and ionization. As a result, numerous protein signals that were differentially expressed in the three epidermal cell types could be detected. One of these proteins was characterized by tryptical digestion and subsequent identification via tandem quadrupole-time of flight (Q-TOF) mass spectrometry. Down regulation of this sequenced small subunit precursor of ribulose-1,5 bisphosphate carboxylase(C) oxygenase(O) (RuBisCo) in trichome and basal cells indicates the sink status of these cell types that are located on the surface of A. thaliana source leaves. Based on the obtained protein profiles, we suggest a close functional relationship between basal and trichome cells at the protein level.

A lesion-mimic syntaxin double mutant in Arabidopsis reveals novel complexity of pathogen defense signaling

The lesion-mimic Arabidopsis mutant, syp121 syp122, constitutively expresses the salicylic acid (SA) signaling pathway and has low penetration resistance to powdery mildew fungi. Genetic analyses of the lesion-mimic phenotype have expanded our understanding of programmed cell death (PCD) in plants. Inactivation of SA signaling genes in syp121 syp122 only partially rescues the lesion-mimic phenotype, indicating that additional defenses contribute to the PCD. Whole genome transcriptome analysis confirmed that SA-induced transcripts, as well as numerous other known pathogen-response transcripts, are up-regulated after inactivation of the syntaxin genes. A suppressor mutant analysis of syp121 syp122 revealed that FMO1, ALD1, and PAD4 are important for lesion development. Mutant alleles of EDS1, NDR1, RAR1, and SGT1b also partially rescued the lesion-mimic phenotype, suggesting that mutating syntaxin genes stimulates TIR-NB-LRR and CC-NB-LRR-type resistances. The syntaxin double knockout potentiated a powdery mildew-induced HR-like response. This required functional PAD4 but not functional SA signaling. However, SA signaling potentiated the PAD4-dependent HR-like response. Analyses of quadruple mutants suggest that EDS5 and SID2 confer separate SA-independent signaling functions, and that FMO1 and ALD1 mediate SA-independent signals that are NPR1-dependent. These studies highlight the contribution of multiple pathways to defense and point to the complexity of their interactions.

Systemic and local modulation of plant responses by Piriformospora indica and related Sebacinales species

Piriformospora indica is a fungus of the order Sebacinales (Basidiomycota) infesting roots of mono- and dicotyledonous plants. Endophytic fungal colonization leads to enhanced plant growth while host cell death is required for proliferation in differentiated root tissue to form a mutualistic interaction. Colonization of barley roots by P. indica and related Sebacina vermifera strains also leads to systemic resistance against the leaf pathogenic fungus Blumeria graminis f.sp. hordei due to a yet unknown mechanism of induced resistance. In order to elucidate plant response pathways governed by these root endophytes, we analyzed gene expression in barley plants exhibiting an established symbiosis with P. indica 3 weeks after inoculation. P. indica-colonized roots showed no induction of defence-related genes, while other genes showed a differential regulation pattern indicating a faster P. indica-dependent root development. Gene expression analysis of leaves detected only few systemically induced mRNAs. Among differentially regulated transcripts, we characterized the pathogenesis-related gene HvPr17b and the molecular chaperone HvHsp70 in more detail. HvPr17b shows similarity with TaWCI5, a wheat gene inducible by chemical resistance inducers and salicylate, and was previously proven to exhibit antifungal activity against B. graminis. HvHsp70 is the first gene found to systemically indicate root colonization with endophytic fungi of the order Sebacinales. Both genes are discussed as markers for endophytic colonization and resulting systemic responses.

The Powdery Mildew Disease of Arabidopsis: A Paradigm for the Interaction between Plants and Biotrophic Fungi

The powdery mildew diseases, caused by fungal species of the Erysiphales, have an important economic impact on a variety of plant species and have driven basic and applied research efforts in the field of phytopathology for many years. Although the first taxonomic reports on the Erysiphales date back to the 1850's, advances into the molecular biology of these fungal species have been hampered by their obligate biotrophic nature and difficulties associated with their cultivation and genetic manipulation in the laboratory. The discovery in the 1990's of a few species of powdery mildew fungi that cause disease on Arabidopsis has opened a new chapter in this research field. The great advantages of working with a model plant species have translated into remarkable progress in our understanding of these complex pathogens and their interaction with the plant host. Herein we summarize advances in the study of Arabidopsis-powdery mildew interactions and discuss their implications for the general field of plant pathology. We provide an overview of the life cycle of the pathogens on Arabidopsis and describe the structural and functional changes that occur during infection in the host and fungus in compatible and incompatible interactions, with special emphasis on defense signaling, resistance pathways, and compatibility factors. Finally, we discuss the future of powdery mildew research in anticipation of the sequencing of multiple powdery mildew genomes. The cumulative body of knowledge on powdery mildews of Arabidopsis provides a valuable tool for the study and understanding of disease associated with many other obligate biotrophic pathogen species.

Accommodation of powdery mildew fungi in intact plant cells

Parasitic powdery mildew fungi have to overcome basic resistance and manipulate host cells to establish a haustorium as a functional feeding organ in a host epidermal cell. Currently, it is of central interest how plant factors negatively regulate basal defense or whether they even support fungal development in compatible interactions. Additionally, creation of a metabolic sink in infected cells may involve host activity. Here, we review the current progress in understanding potential fungal targets for host reprogramming and nutrient acquisition.

Distinct biphasic mRNA changes in response to Asian soybean rust infection

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is now established in all major soybean-producing countries. Currently, there is little information about the molecular basis of ASR-soybean interactions, which will be needed to assist future efforts to develop effective resistance. Toward this end, abundance changes of soybean mRNAs were measured over a 7-day ASR infection time course in mock-inoculated and infected leaves of a soybean accession (PI230970) carrying the Rpp2 resistance gene and a susceptible genotype (Embrapa-48). The expression profiles of differentially expressed genes (ASR-infected compared with the mock-inoculated control) revealed a biphasic response to ASR in each genotype. Within the first 12 h after inoculation (hai), which corresponds to fungal germination and penetration of the epidermal cells, differential gene expression changes were evident in both genotypes. mRNA expression of these genes mostly returned to levels found in mock-inoculated plants by 24 hai. In the susceptible genotype, gene expression remained unaffected by rust infection until 96 hai, a time period when rapid fungal growth began. In contrast, gene expression in the resistant genotype diverged from the mock-inoculated control earlier, at 72 h, demonstrating that Rpp2-mediated defenses were initiated prior to this time. These data suggest that ASR initially induces a nonspecific response that is transient or is suppressed when early steps in colonization are completed in both soybean genotypes. The race-specific resistance phenotype of Rpp2 is manifested in massive gene expression changes after the initial response prior to the onset of rapid fungal growth that occurs in the susceptible genotype.

Transcript profiling in host-pathogen interactions

Using genomic technologies, it is now possible to address research hypotheses in the context of entire developmental or biochemical pathways, gene networks, and chromosomal location of relevant genes and their inferred evolutionary history. Through a range of platforms, researchers can survey an entire transcriptome under a variety of experimental and field conditions. Interpretation of such data has led to new insights and revealed previously undescribed phenomena. In the area of plant-pathogen interactions, transcript profiling has provided unparalleled perception into the mechanisms underlying gene-for-gene resistance and basal defense, host vs nonhost resistance, biotrophy vs necrotrophy, and pathogenicity of vascular vs nonvascular pathogens, among many others. In this way, genomic technologies have facilitated a system-wide approach to unifying themes and unique features in the interactions of hosts and pathogens.