Saccharomyces cerevisiae SSD1 orthologues are essential for host infection by the ascomycete plant pathogens Colletotrichum lagenarium and Magnaporthe grisea

The phosphorylation landscape of infection-related development by the rice blast fungus

Many of the world's most devastating crop diseases are caused by fungal pathogens that elaborate specialized infection structures to invade plant tissue. Here, we present a quantitative mass-spectrometry-based phosphoproteomic analysis of infection-related development by the rice blast fungus Magnaporthe oryzae, which threatens global food security. We mapped 8,005 phosphosites on 2,062 fungal proteins following germination on a hydrophobic surface, revealing major re-wiring of phosphorylation-based signaling cascades during appressorium development. Comparing phosphosite conservation across 41 fungal species reveals phosphorylation signatures specifically associated with biotrophic and hemibiotrophic fungal infection. We then used parallel reaction monitoring (PRM) to identify phosphoproteins regulated by the fungal Pmk1 MAPK that controls plant infection by M. oryzae. We define 32 substrates of Pmk1 and show that Pmk1-dependent phosphorylation of regulator Vts1 is required for rice blast disease. Defining the phosphorylation landscape of infection therefore identifies potential therapeutic interventions for the control of plant diseases.

A reference transcriptome for walnut anthracnose pathogen, Ophiognomonia leptostyla, guides the discovery of candidate virulence genes

Despite the economic losses due to the walnut anthracnose, Ophiognomonia leptostyla is an orphan fungus with respect to genomic resources. In the present study, the transcriptome of O. leptostyla was assembled for the first time. RNA sequencing was conducted for the fungal mycelia grown in a liquid media, and the inoculated leaf samples of walnut with the fungal conidia sampled at 48, 96 and 144 h post inoculation (hpi). The completeness, correctness, and contiguity of the de novo transcriptome assemblies generated with Trinity, Oases, SOAPdenovo-Trans and Bridger were compared to identify a single superior reference assembly. In most of the assessment criteria including N50, Transrate score, number of ORFs with known description in gene bank, the percentage of reads mapped back to the transcript (RMBT), BUSCO score, Swiss-Prot coverage bin and RESM-EVAL score, the Bridger assembly was the superior and thus used as a reference for profiling the O. leptostyla transcriptome in liquid media vs. during walnut infection. The k-means clustering of transcripts resulted in four distinct transcription patterns across the three sampling time points. Most of the detected CAZy transcripts had elevated transcription at 96 hpi that is hypothetically concurrent with the start of intracellular growth. The in-silico analysis revealed 103 candidate effectors of which six were members of Necrosis and Ethylene Inducing Like Protein (NLP) gene family belonging to three distinct k-means clusters. This study provided a complex and temporal pattern of the CAZys and candidate effectors transcription during six days post O. leptostyla inoculation on walnut leaves, introducing a list of candidate virulence genes for validation in future studies.

Tandem metalloenzymes gate plant cell entry by pathogenic fungi

Global food security is endangered by fungal phytopathogens causing devastating crop production losses. Many of these pathogens use specialized appressoria cells to puncture plant cuticles. Here, we unveil a pair of alcohol oxidase-peroxidase enzymes to be essential for pathogenicity. Using Colletotrichum orbiculare, we show that the enzyme pair is cosecreted by the fungus early during plant penetration and that single and double mutants have impaired penetration ability. Molecular modeling, biochemical, and biophysical approaches revealed a fine-tuned interplay between these metalloenzymes, which oxidize plant cuticular long-chain alcohols into aldehydes. We show that the enzyme pair is involved in transcriptional regulation of genes necessary for host penetration. The identification of these infection-specific metalloenzymes opens new avenues on the role of wax-derived compounds and the design of oxidase-specific inhibitors for crop protection.

Post-transcriptional control of fungal cell wall synthesis

Pathogenic fungi hide from their hosts by camouflage, obscuring immunogenic cell wall components such as beta-glucan with innocuous coverings such as mannoproteins and alpha-glucan that are less readily recognised by the host. Attempts to understand how such processes are regulated have met with varying success. Typically studies focus on understanding the transcriptional response of fungi to either their reservoir environment or the host. However, such approaches do not fully address this research question, due to the layers of post-transcriptional and post-translational regulation that occur within a cell. Although in animals the impact of post-transcriptional and post-translational regulation has been well characterised, our knowledge of these processes in the fungal kingdom is more limited. Mutations in RNA-binding proteins, like Ssd1 and Candida albicans Slr1, affect cell wall composition and fungal virulence indicating that post-transcriptional regulation plays a key role in these processes. Here, we review the current state of knowledge of fungal post-transcriptional regulation, and link this to potential mechanisms of immune evasion by drawing on studies from model yeast and plant pathogenic fungi. We highlight several RNA-binding proteins that regulate cell wall synthesis and could be involved in local translation of cell wall components. Expanding our knowledge on post-transcriptional regulation in human fungal pathogens is essential to fully comprehend fungal virulence strategies and for the design of novel antifungal therapies.

Niemann-Pick Type C Proteins Are Required for Sterol Transport and Appressorium-Mediated Plant Penetration of Colletotrichum orbiculare

Many biotrophic and hemibiotrophic fungal pathogens use appressoria to directly penetrate the host plant surface. In the cucumber anthracnose fungus Colletotrichum orbiculare, differentiation of appressoria requires a proper G₁/S cell cycle progression, regulated by the GTPase-activating protein complex CoBub2-CoBfa1 and its downstream GTPase CoTem1. To explore the mechanisms by which the CoTem1 cascade regulates plant infection, we screened for CoTem1 interaction factors and identified a Niemann-Pick type C2 homolog (CoNpc2). Niemann-Pick type C proteins NPC1 and NPC2 are sterol-binding proteins required for sterol export from lysosomes (vacuoles) in humans and yeasts. We showed that CoNpc2 colocalized with CoNpc1 in late endosomes and vacuoles and that disruption of its gene resulted in aberrant sterol accumulation in vacuoles and loss of sterol membrane localization, indicating that NPC proteins are engaged in sterol transport in C. orbiculare. For appressorium infection, sterol transport and proper distribution mediated by CoNpc1 and CoNpc2 are critical for membrane integrity and membrane curvature with actin assembly, leading to penetration peg emergence and appressorial cone formation. Our results revealed a novel mechanism by which NPC proteins regulate appressorium-mediated plant infection.

Genome Analyses of Two Blueberry Pathogens: Diaportheamygdali CAA958 and Diaporthe eres CBS 160.32

The genus Diaporthe includes pathogenic species distributed worldwide and affecting a wide variety of hosts. Diaporthe amygdali and Diaporthe eres have been found to cause cankers, dieback, or twig blights on economically important crops such as soybean, almond, grapevine, and blueberry. Despite their importance as plant pathogens, the strategies of species of Diaporthe to infect host plants are poorly explored. To provide a genomic basis of pathogenicity, the genomes of D. amygdali CAA958 and D. eres CBS 160.32 were sequenced and analyzed. Cellular transporters involved in the transport of toxins, ions, sugars, effectors, and genes implicated in pathogenicity were detected in both genomes. Hydrolases and oxidoreductases were the most prevalent carbohydrate-active enzymes (CAZymes). However, analyses of the secreted proteins revealed that the secretome of D. eres CBS 160.32 is represented by 5.4% of CAZymes, whereas the secreted CAZymes repertoire of D. amygdali CAA958 represents 29.1% of all secretomes. Biosynthetic gene clusters (BGCs) encoding compounds related to phytotoxins and mycotoxins were detected in D. eres and D. amygdali genomes. The core gene clusters of the phytotoxin Fusicoccin A in D. amygdali are reported here through a genome-scale assembly. Comparative analyses of the genomes from 11 Diaporthe species revealed an average of 874 CAZymes, 101 secondary metabolite BGCs, 1640 secreted proteins per species, and genome sizes ranging from 51.5 to 63.6 Mbp. This study offers insights into the overall features and characteristics of Diaporthe genomes. Our findings enrich the knowledge about D. eres and D. amygdali, which will facilitate further research into the pathogenicity mechanisms of these species.

The development of extracellular vesicle markers for the fungal phytopathogen Colletotrichum higginsianum

Fungal phytopathogens secrete extracellular vesicles (EVs) associated with enzymes and phytotoxic metabolites. While these vesicles are thought to promote infection, defining the true contents and functions of fungal EVs, as well as suitable protein markers, is an ongoing process. To expand our understanding of fungal EVs and their possible roles during infection, we purified EVs from the hemibiotrophic phytopathogen Colletotrichum higginsianum, the causative agent of anthracnose disease in multiple plant species, including Arabidopsis thaliana. EVs were purified in large numbers from the supernatant of protoplasts but not the supernatant of intact mycelial cultures. We purified two separate populations of EVs, each associated with over 700 detected proteins, including proteins involved in vesicle transport, cell wall biogenesis and the synthesis of secondary metabolites. We selected two SNARE proteins (Snc1 and Sso2) and one 14-3-3 protein (Bmh1) as potential EV markers and generated transgenic strains expressing fluorescent fusions. Each marker was confirmed to be protected inside EVs. Fluorescence microscopy was used to examine the localization of each marker during infection on Arabidopsis leaves. These findings further our understanding of EVs in fungal phytopathogens and will help build an experimental system to study EV interkingdom communication between plants and fungi.

Yeast Ssd1 is a non-enzymatic member of the RNase II family with an alternative RNA recognition site

Ssd1, a conserved fungal RNA-binding protein, is important in stress responses, cell division and virulence. Ssd1 is closely related to Dis3L2 of the RNase II family of nucleases, but lacks catalytic activity and likely suppresses translation of bound mRNAs. Previous studies identified RNA motifs enriched in Ssd1-associated transcripts, yet the sequence requirements for Ssd1 binding are not defined. Here, we identify precise binding sites of Ssd1 on RNA using in vivo cross-linking and cDNA analysis. These sites are enriched in 5' untranslated regions of a subset of mRNAs encoding cell wall proteins. We identified a conserved bipartite motif that binds Ssd1 with high affinity in vitro. Active RNase II enzymes have a characteristic, internal RNA binding path; the Ssd1 crystal structure at 1.9 Å resolution shows that remnants of regulatory sequences block this path. Instead, RNA binding activity has relocated to a conserved patch on the surface of the protein. Structure-guided mutations of this surface prevent Ssd1 from binding RNA in vitro and phenocopy Ssd1 deletion in vivo. These studies provide a new framework for understanding the function of a pleiotropic post-transcriptional regulator of gene expression and give insights into the evolution of regulatory and binding elements in the RNase II family.

Repeated Evolution of Inactive Pseudonucleases in a Fungal Branch of the Dis3/RNase II Family of Nucleases

The RNase II family of 3'-5' exoribonucleases is present in all domains of life, and eukaryotic family members Dis3 and Dis3L2 play essential roles in RNA degradation. Ascomycete yeasts contain both Dis3 and inactive RNase II-like "pseudonucleases." The latter function as RNA-binding proteins that affect cell growth, cytokinesis, and fungal pathogenicity. However, the evolutionary origins of these pseudonucleases are unknown: What sequence of events led to their novel function, and when did these events occur? Here, we show how RNase II pseudonuclease homologs, including Saccharomyces cerevisiae Ssd1, are descended from active Dis3L2 enzymes. During fungal evolution, active site mutations in Dis3L2 homologs have arisen at least four times, in some cases following gene duplication. In contrast, N-terminal cold-shock domains and regulatory features are conserved across diverse dikarya and mucoromycota, suggesting that the nonnuclease function requires these regions. In the basidiomycete pathogenic yeast Cryptococcus neoformans, the single Ssd1/Dis3L2 homolog is required for cytokinesis from polyploid "titan" growth stages. This phenotype of C. neoformans Ssd1/Dis3L2 deletion is consistent with those of inactive fungal pseudonucleases, yet the protein retains an active site sequence signature. We propose that a nuclease-independent function for Dis3L2 arose in an ancestral hyphae-forming fungus. This second function has been conserved across hundreds of millions of years, whereas the RNase activity was lost repeatedly in independent lineages.

CoGRIM19 is required for invasive hyphal growth of Colletotrichum orbiculare inside epidermal cells of cucumber cotyledons

Colletotrichum orbiculare, an anthracnose disease fungus of cucurbit plants, extends penetration hyphae inside the epidermal cells of host plants. Unlike vegetative hyphae formed on a nutrient rich medium, this pathogen initially develops biotrophic penetration hyphae, which acquire nutrient resources from living host cells and secret effector proteins to suppress host defense responses. Subsequently, the nature of penetration hyphae changes from biotrophy to necrotrophy in response to the interaction with a host plant. Hence, controlling the extension of penetration hyphae is crucial for C. orbiculare infection. Here, we identified CoGRIM19 encoding Nadh-ubiquinone oxidoreductase subunit as a pathogenicity gene. Pathogenicity assays showed that the cogrim19 mutant caused no visible symptoms on cucumber cotyledons. Microscopic observations revealed that the cogrim19 mutant developed an appressorium and penetration hyphae under artificial conditions such as on coverslips or cellulose membranes, but the penetration hyphae of the mutant were retarded in the cucumber cotyledons. Microscopic observations of biotrophy-specific expression fluorescent signals revealed that the biotrophic stage was maintained in the retarded penetration hyphae of the cogrim19 mutant as the penetration of the wild type. In addition to cytological observations, pathogenicity assays using wounded leaves showed that the cogrim19 mutant had an attenuated pathogenesis. Taking our results together, CoGRIM19 is required for invasive hyphal growth inside the epidermal cells of cucumber cotyledons in C. orbiculare.

Cell Wall Biogenesis Protein Phosphatase CrSsd1 Is Required for Conidiation, Cell Wall Integrity, and Mycoparasitism in Clonostachys rosea

Cell wall biogenesis protein phosphatases play important roles in various cellular processes in fungi. However, their functions in the widely distributed mycoparasitic fungus Clonostachys rosea remain unclear, as do their potential for controlling plant fungal diseases. Herein, the function of cell wall biogenesis protein phosphatase CrSsd1 in C. rosea 67-1 was investigated using gene disruption and complementation approaches. The gene-deficient mutant ΔCrSsd1 exhibited much lower conidiation, hyphal growth, mycoparasitic ability, and biocontrol efficacy than the wild-type (WT) strain, and it was more sensitive to sorbitol and Congo red. The results indicate that CrSsd1 is involved in fungal conidiation, osmotic stress adaptation, cell wall integrity, and mycoparasitism in C. rosea.

Analysis of cell wall synthesis and metabolism during early germination of Blumeria graminis f. sp. hordei conidial cells induced in vitro

As an obligate biotroph, Blumeria graminis f. sp. hordei (Bgh) cannot be grown in an axenic culture, and instead must be cultivated on its host species, Hordeum vulgare (barley). In this study an in vitro system utilizing n-hexacosanal, a constituent of the barley cuticle and known inducer of Bgh germination, was used to cultivate Bgh and differentiate conidia up to the appressorial germ tube stage for analysis. Transcriptomic and proteomic profiling of the appressorial germ tube stage revealed that there was a significant shift towards energy and protein production during the pre-penetrative phase of development, with an up-regulation of enzymes associated with cellular respiration and protein synthesis, modification and transport. Glycosidic linkage analysis of the cell wall polysaccharides demonstrated that during appressorial development an increase in 1,3- and 1,4-linked glucosyl residues and xylosyl residues was detected along with a significant decrease in galactosyl residues. The use of this in vitro cultivation method demonstrates that it is possible to analyse the pre-penetrative processes of Bgh development in the absence of a plant host.

An Ssd1 Homolog Impacts Trehalose and Chitin Biosynthesis and Contributes to Virulence in Aspergillus fumigatus

Regulation of fungal cell wall biosynthesis is critical to maintain cell wall integrity in dynamic fungal infection microenvironments. Genes involved in this response that impact fungal fitness and host immune responses remain to be fully defined. In this study, we observed that a yeast ssd1 homolog, ssdA, in the filamentous fungus Aspergillus fumigatus is involved in trehalose and cell wall homeostasis. An ssdA null mutant strain exhibited an increase in trehalose levels and a reduction in fungal colony growth rate. In contrast, overexpression of ssdA perturbed trehalose biosynthesis and reduced germination of conidia. The ssdA null mutant strain was more resistant to cell wall-perturbing agents, while overexpression of ssdA increased sensitivity. Overexpression of ssdA significantly increased chitin levels, and both loss and overexpression of ssdA altered subcellular localization of the class V chitin synthase CsmA. Strikingly, overexpression of ssdA abolished adherence to abiotic surfaces and severely attenuated the virulence of A. fumigatus in a murine model of invasive pulmonary aspergillosis. Despite the severe in vitro fitness defects observed upon loss of ssdA, neither surface adherence nor murine survival was impacted. In conclusion, A. fumigatus SsdA plays a critical role in cell wall homeostasis impacting A. fumigatus-host interactions.IMPORTANCE The incidence of life-threatening infections caused by the filamentous fungus Aspergillus fumigatus is increasing along with an increase in the number of fungal strains resistant to contemporary antifungal therapies. The fungal cell wall and the associated carbohydrates required for its synthesis and maintenance are attractive drug targets given that many genes encoding proteins involved in cell wall biosynthesis and integrity are absent in humans. Importantly, genes and associated cell wall biosynthesis and homeostasis regulatory pathways remain to be fully defined in A. fumigatus In this report, we identify SsdA as an important component of trehalose and fungal cell wall biosynthesis in A. fumigatus that consequently impacts the host immune response and fungal virulence in animal models of infection.

Colletotrichum orbiculare MTF4 Is a Key Transcription Factor Downstream of MOR Essential for Plant Signal-Dependent Appressorium Development and Pathogenesis

The cucumber anthracnose fungus Colletotrichum orbiculare forms a specialized infection structure, called an appressorium. Appressorium differentiation relies on fungal perception of physical and biochemical signals at the plant surface. Our previous report showed that the morphogenesis-related NDR (nuclear Dbf2-related) kinase pathway (MOR) is crucial for translating plant-derived signals for appressorium development. Here, we focused on identifying transcriptional regulators downstream of MOR that are involved in plant signal sensing and transduction for appressorium development. Based on whole-genome transcript profiling, we identified a Zn(II)₂Cys₆ transcription factor, CoMTF4, as a potential downstream factor of MOR. CoMTF4 was expressed in planta rather than in vitro under the control of the NDR kinase CoCbk1. Phenotypes of comtf4 mutants, strains with constitutively active CoCbk1 and strains with constitutive overexpression of CoMTF4 suggested that CoMtf4 acts downstream of MOR. Furthermore, nuclear localization of CoMtf4 was dependent on the MOR and responsive to plant-derived signals that lead to appressorium morphogenesis. Thus, we conclude that CoMtf4 is a transcription factor downstream of MOR that is essential for appressorium morphogenesis and pathogenesis and is regulated in response to plant-derived signals. This study provides insights into fungal sensing of plant signals and subsequent responses critical for appressorium formation.

Transcriptional profiling and localization of GUL-1, a COT-1 pathway component, in Neurospora crassa

Impairment of theNeurospora crassaCOT-1 kinase results in defects in hyphal polarity. Some of these effects are partially suppressed by inactivation of gul-1 (encoding an mRNA-binding protein involved in translational regulation). Here, we report on the transcriptional profiling of cot-1 inactivation and demonstrate that gul-1 affects transcript abundance of multiple genes in the COT-1 pathway, including processes such as cell wall remodeling, nitrogen and amino acid metabolism. The GUL-1 protein itself was found to be distributed within the entire hyphal cell, along with a clear presence of aggregates that traffic within the cytoplasm. Live imaging of GUL-1-GFP demonstrated that GUL-1 transport is microtubule-dependent. Cellular stress, as imposed by the presence of the cell wall biosynthesis inhibitor Nikkomycin Z or by nitrogen limitation, resulted in a 2-3-fold increase of GUL-1 aggregate association with nuclei. Taken together, this study demonstrates that GUL-1 affects multiple processes, its function is stress-related and linked with cellular traffic and nuclear association.

Disruption of gul-1 decreased the culture viscosity and improved protein secretion in the filamentous fungus Neurospora crassa

BACKGROUND

The cellulolytic fungus Neurospora crassa is considered a potential host for enzyme and bioethanol production. However, large scale applications are hindered by its filamentous growth. Although previous investigations have shown that mycelial morphology in submerged culture can be controlled by altering physical factors, there is little knowledge available about the potential for morphology control by genetic modification.

RESULTS

In this study, we screened morphological mutants in the filamentous fungus N. crassa. Of the 90 morphological mutants screened, 14 mutants exhibited considerably higher viscosity compared with that of the wild type strain, and only two mutants showed low-viscosity morphologies in submerged culture. We observed that disruption of gul-1 (NCU01197), which encodes an mRNA binding protein involved in cell wall remodeling, caused pellet formation as the fermentation progressed, and resulted in the most significant decrease in viscosity of culture broth. Moreover, over-expression of gul-1 caused dramatically increased viscosity, suggesting that the gul-1 had an important function in mycelial morphology during submerged cultivation. Transcriptional profiling showed that expression of genes encoding eight GPI-anchored cell wall proteins was lowered in Δgul-1 while expression of genes associated with two non-anchored cell wall proteins was elevated. Meanwhile, the expression levels of two hydrophobin genes were also significantly altered. These results suggested that GUL-1 affected the transcription of cell wall-related genes, thereby influencing cell wall structure and mycelial morphology. Additionally, the deletion of gul-1 caused increased protein secretion, probably due to a defect in cell wall integrity, suggesting this as an alternative strategy of strain improvement for enzyme production. To confirm practical applications, deletion of gul-1 in the hyper-cellulase producing strain (∆ncw-1∆Ncap3m) significantly reduced the viscosity of culture broth.

CONCLUSIONS

Using the model filamentous fungus N. crassa, genes that affect mycelial morphology in submerged culture were explored through systematic screening of morphological mutants. Disrupting several candidate genes altered viscosities in submerged culture. This work provides an example for controlling fungal morphology in submerged fermentation by genetic engineering, and will be beneficial for industrial fungal strain improvement.

Inappropriate Expression of an NLP Effector in Colletotrichum orbiculare Impairs Infection on Cucurbitaceae Cultivars via Plant Recognition of the C-Terminal Region

The hemibiotrophic pathogen Colletotrichum orbiculare preferentially expresses a necrosis and ethylene-inducing peptide 1 (Nep1)-like protein named NLP1 during the switch to necrotrophy. Here, we report that the constitutive expression of NLP1 in C. orbiculare blocks pathogen infection in multiple Cucurbitaceae cultivars via their enhanced defense responses. NLP1 has a cytotoxic activity that induces cell death in Nicotiana benthamiana. However, C. orbiculare transgenic lines constitutively expressing a mutant NLP1 lacking the cytotoxic activity still failed to infect cucumber, indicating no clear relationship between cytotoxic activity and the NLP1-dependent enhanced defense. NLP1 also possesses the microbe-associated molecular pattern (MAMP) sequence called nlp24, recognized by Arabidopsis thaliana at its central region, similar to NLPs of other pathogens. Surprisingly, inappropriate expression of a mutant NLP1 lacking the MAMP signature is also effective for blocking pathogen infection, uncoupling the infection block from the corresponding MAMP. Notably, the deletion analyses of NLP1 suggested that the C-terminal region of NLP1 is critical to enhance defense in cucumber. The expression of mCherry fused with the C-terminal 32 amino acids of NLP1 was enough to trigger the defense of cucurbits, revealing that the C-terminal region of the NLP1 protein is recognized by cucurbits and, then, terminates C. orbiculare infection.

ChMob2 binds to ChCbk1 and promotes virulence and conidiation of the fungal pathogen Colletotrichum higginsianum

Background

Mob family proteins are conserved between animals, plants and fungi and are essential for the activation of NDR kinases that control crucial cellular processes like cytokinesis, proliferation and morphology.

Results

We identified a hypomorphic allele of ChMOB2 in a random insertional mutant (vir-88) of the hemibiotrophic ascomycete fungus Colletotrichum higginsianum. The mutant is impaired in conidiation, host penetration and virulence on Arabidopsis thaliana. ChMob2 binds to and co-localizes with the NDR/LATS kinase homolog ChCbk1. Mutants in the two potential ChCbk1 downstream targets ChSSD1 and ChACE2 show defects in pathogenicity. The genome of C. higginsianum encodes two more Mob proteins. While we could not detect any effect on pathogenicity in ΔChmob3 mutants, ChMob1 is involved in conidiation, septae formation and virulence.

Conclusion

This study shows that ChMob2 binds to the conserved NDR/LATS Kinase ChCbk1 and plays an important role in pathogenicity of Colletotrichum higginsianum on Arabidopsis thaliana.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0932-7) contains supplementary material, which is available to authorized users.

Regulation of Neurospora crassa cell wall remodeling via the cot-1 pathway is mediated by gul-1

Impairment of the Neurospora crassa Nuclear DBF2-related kinase-encoding gene cot-1 results in pleiotropic effects, including abnormally thick hyphal cell walls and septa. An increase in the transcript abundance of genes encoding chitin and glucan synthases and the chitinase gh18-5, but not the cell wall integrity pathway transcription factor rlm-1, accompany the phenotypic changes observed. Deletion of chs-5 or chs-7 in a cot-1 background results in a reduction of hyperbranching frequency characteristic of the cot-1 parent. gul-1 (a homologue of the yeast SSD1 gene) encodes a translational regulator and has been shown to partially suppress cot-1. We demonstrate that the high expression levels of the cell wall remodeling genes analyzed is curbed, and reaches near wild type levels, when gul-1 is inactivated. This is accompanied by morphological changes that include reduced cell wall thickness and restoration of normal chitin levels. We conclude that gul-1 is a mediator of cell wall remodeling within the cot-1 pathway.

Colletotrichum orbiculare WHI2, a Yeast Stress-Response Regulator Homolog, Controls the Biotrophic Stage of Hemibiotrophic Infection Through TOR Signaling

The hemibiotrophic fungus Colletotrichum orbiculare first establishes a biotrophic infection stage in cucumber (Cucumber sativus) epidermal cells and subsequently transitions to a necrotrophic stage. Here, we found that C. orbiculare established hemibiotrophic infection via C. orbiculare WHI2, a yeast stress regulator homolog, and TOR (target of rapamycin) signaling. Plant defense responses such as callose deposition, H2O2, and antimicrobial proteins were strongly induced by the C. orbiculare whi2Δ mutant, resulting in defective pathogenesis. Expression analysis of biotrophy-specific genes evaluated by the promoter VENUS fusion gene indicated weaker VENUS signal intensity in the whi2Δ mutant, thereby suggesting that C. orbiculare WHI2 plays a key role in regulating biotrophic infection of C. orbiculare. The involvement of CoWHI2 in biotrophic infection was further explored with a DNA microarray. In the Cowhi2Δ mutant, TOR-dependent ribosomal protein-related genes were strikingly upregulated compared with the wild type. Moreover, callose deposition in the host plant after inoculation with the Cowhi2Δ mutant treated with rapamycin, which inhibits TOR activity, was reduced, and the mutant remained biotrophic in contrast to the untreated mutant. Thus, regulation of TOR by Whi2 is apparently crucial to the biotrophic stage of hemibiotrophic infection in C. orbiculare.

A Genetic Screen for Saccharomyces cerevisiae Mutants That Fail to Enter Quiescence

Budding yeast begin the transition to quiescence by prolonging G1 and accumulating limited nutrients. They undergo asymmetric cell divisions, slow cellular expansion, acquire significant stress tolerance and construct elaborate cell walls. These morphologic changes give rise to quiescent (Q) cells, which can be distinguished from three other cell types in a stationary phase culture by flow cytometry. We have used flow cytometry to screen for genes that are required to obtain the quiescent cell fraction. We find that cell wall integrity is critical and these genes may help define quiescence-specific features of the cell wall. Genes required to evade the host innate immune response are common. These may be new targets for antifungal drugs. Acquired thermotolerance is also a common property, and we show that the stress-response transcription factors Msn2 and Msn4 promote quiescence. Many other pathways also contribute, including a subset of genes involved in autophagy, ubiquitin-mediated proteolysis, DNA replication, bud site selection, and cytokinesis.

Comparative proteomics reveals differential induction of both biotic and abiotic stress response associated proteins in rice during Xanthomonas oryzae pv. oryzae infection

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight disease in rice and brutally affects the yield up to 50 % of total production. Here, we report a comparative proteomics analysis of total foliar protein isolated from infected rice leaves of susceptible Pusa Basmati 1 (PB1) and resistant Oryza longistaminata genotypes. Two-dimensional gel electrophoresis (2-DE) coupled with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) approaches identified 29 protein spots encoding unique proteins from both the genotypes. Identified proteins belonged to a large number of biological and molecular functions related to biotic and abiotic stress proteins which are potentially involved during Xoo infection. Biotic and abiotic stress-related proteins were induced during Xoo infection, indicating the activation of common stress pathway during bacterial blight infection. Candidate genes conferring tolerance against bacterial blight, which include germin-like protein, putative r40c1, cyclin-dependent kinase C, Ent-isokaur-15-ene synthase and glutathione-dependent dehydroascorbate reductase 1 (GSH-DHAR1), were also induced, with germin-like proteins induced only in the resistant rice genotype O. longistaminata. Energy, metabolism and hypothetical proteins were common among both the genotypes. Further, host defence/stress-related proteins were mostly expressed in resistant genotype O. longistaminata, indicating possible co-evolution of the pathogen and the wild rice, O. longistaminata.

Colletotrichum orbiculare Secretes Virulence Effectors to a Biotrophic Interface at the Primary Hyphal Neck via Exocytosis Coupled with SEC22-Mediated Traffic

The hemibiotrophic pathogen Colletotrichum orbiculare develops biotrophic hyphae inside cucumber (Cucumis sativus) cells via appressorial penetration; later, the pathogen switches to necrotrophy. C. orbiculare also expresses specific effectors at different stages. Here, we found that virulence-related effectors of C. orbiculare accumulate in a pathogen-host biotrophic interface. Fluorescence-tagged effectors accumulated in a ring-like region around the neck of the biotrophic primary hyphae. Fluorescence imaging of cellular components and transmission electron microscopy showed that the ring-like signals of the effectors localized at the pathogen-plant interface. Effector accumulation at the interface required induction of its expression during the early biotrophic phase, suggesting that transcriptional regulation may link to effector localization. We also investigated the route of effector secretion to the interface. An exocytosis-related component, the Rab GTPase SEC4, localized to the necks of biotrophic primary hyphae adjacent to the interface, thereby suggesting focal effector secretion. Disruption of SEC4 in C. orbiculare reduced virulence and impaired effector delivery to the ring signal interface. Disruption of the v-SNARE SEC22 also reduced effector delivery. These findings suggest that biotrophy-expressed effectors are secreted, via the endoplasmic reticulum-to-Golgi route and subsequent exocytosis, toward the interface generated between C. orbiculare and the host cell.

Mycosphaerella graminicola LysM effector-mediated stealth pathogenesis subverts recognition through both CERK1 and CEBiP homologues in wheat

Fungal cell-wall chitin is a well-recognized pathogen-associated molecular pattern. Recognition of chitin in plants by pattern recognition receptors activates pathogen-triggered immunity (PTI). In Arabidopsis, this process is mediated by a plasma membrane receptor kinase, CERK1, whereas in rice, a receptor-like protein, CEBiP, in addition to CERK1 is required. Secreted chitin-binding lysin motif (LysM) containing fungal effector proteins, such as Ecp6 from the biotrophic fungus Cladosporium fulvum, have been reported to interfere with PTI. Here, we identified wheat homologues of CERK1 and CEBiP and investigated their role in the interaction with the nonbiotrophic pathogen of wheat Mycosphaerella graminicola (synonym Zymoseptoria tritici). We show that silencing of either CERK1 or CEBiP in wheat, using Barley stripe mosaic virus-mediated virus-induced gene silencing, is sufficient in allowing leaf colonization by the normally nonpathogenic M. graminicola Mg3LysM (homologue of Ecp6) deletion mutant, while the Mg1LysM deletion mutant was fully pathogenic toward both silenced and wild-type wheat leaves. These data indicate that Mg3LysM is important for fungal evasion of PTI in wheat leaf tissue and that both CERK1 and CEBiP are required for activation of chitin-induced defenses, a feature conserved between rice and wheat, and perhaps, also in other cereal species.

Identification of virulence genes in the crucifer anthracnose fungus Colletotrichum higginsianum by insertional mutagenesis

To investigate the molecular and genetic mechanisms underlying virulence of Colletotrichum higginsianum on Arabidopsis thaliana, a T-DNA insertion mutant library of C. higginsianum, the causal agent of crucifer anthracnose, was established using Agrobacterium tumefaciens-mediated transformation. Among 875 transformants tested for virulence on Arabidopsis, six mutants with altered virulence, including an appressorial melanin-deficient mutant T734, two mutants defective in penetration, T45 and B30, and three mutants, T679, T732 and T801, that cause hypersensitive reactions on host Arabidopsis, were obtained. Southern blot analysis indicated that the mutants T732 and T734 harbored single-site T-DNA integrations, while B30 harbored two T-DNA insertions. Border flanking sequences of T-DNAs from these mutants were recovered by inverse polymerase chain reaction (PCR) and thermal asymmetric interlaced PCR. Sequence analyses revealed that single T-DNA insertions in mutant T734 targeted the coding region of a gene with unknown function, and in mutant T732 targeted a gene encoding a copper amine oxidase. The two T-DNA insertion sites in mutant B30 were found in the coding region of a gene encoding an exosome component and in the upstream region of a DUF221-domain gene. None of these genes have previously been implicated in virulence of the phytopathogenic fungi. Among these avirulent mutants, T734 showed altered color in colony growth and produced melanin-deficient, albino appressoria. The T-DNA insert in T734 was detected in the coding region of a gene named C. higginsianum melanin-deficiency gene (Ch-MEL1), which is highly similar to a gene encoding a hypothetical protein in Colletotrichum gloeosporioides (GenBank ELA33048). To validate whether the Ch-MEL1 gene was associated with virulence of the mutant T734, a targeted gene disruption and complementation approach was used. The appressoria of ▵Ch-mel1 null mutants were defective in melanization and failed to penetrate the host epidermal cells. When inoculated onto the wounded leaf tissues, the ▵Ch-mel1 mutants grew on host tissues but failed to cause lesions beyond the wound site. In contrast, both the complement C▵Ch-mel1-2 and the wild type produced melanized appressoria and caused necrosis on leaves of Arabidopsis. Ch-MEL1 is required for both appressorial melanin production in C. higginsianum and post-invasive growth in host tissues. Together with identification of other avirulent mutants and their associated genes, this study provides novel insights into molecular mechanisms underlying virulence of the hemibiotroph, C. higginsianum.

Vascular plant one-zinc-finger protein 1/2 transcription factors regulate abiotic and biotic stress responses in Arabidopsis

Plants adapt to abiotic and biotic stresses by activating abscisic acid-mediated (ABA) abiotic stress-responsive and salicylic acid-(SA) or jasmonic acid-mediated (JA) biotic stress-responsive pathways, respectively. Although the abiotic stress-responsive pathway interacts antagonistically with the biotic stress-responsive pathways, the mechanisms that regulate these pathways remain largely unknown. In this study, we provide insight into the function of vascular plant one-zinc-finger proteins (VOZs) that modulate various stress responses in Arabidopsis. The expression of many stress-responsive genes was changed in the voz1voz2 double mutant under normal growth conditions. Consistent with altered stress-responsive gene expression, freezing- and drought-stress tolerances were increased in the voz1voz2 double mutant. In contrast, resistance to a fungal pathogen, Colletotrichum higginsianum, and to a bacterial pathogen, Pseudomonas syringae, was severely impaired. Thus, impairing VOZ function simultaneously conferred increased abiotic tolerance and biotic stress susceptibility. In a chilling stress condition, both the VOZ1 and VOZ2 mRNA expression levels and the VOZ2 protein level gradually decreased. VOZ2 degradation during cold exposure was completely inhibited by the addition of the 26S proteasome inhibitor, MG132, a finding that suggested that VOZ2 degradation is dependent on the ubiquitin/26S proteasome system. In voz1voz2, ABA-inducible transcription factor CBF4 expression was enhanced significantly even under normal growth conditions, despite an unchanged endogenous ABA content. A finding that suggested that VOZs negatively affect CBF4 expression in an ABA-independent manner. These results suggest that VOZs function as both negative and positive regulators of the abiotic and biotic stress-responsive pathways, and control Arabidopsis adaptation to various stress conditions.

Rice WRKY45 plays important roles in fungal and bacterial disease resistance

Plant 'activators', such as benzothiadiazole (BTH), protect plants from various diseases by priming the plant salicylic acid (SA) signalling pathway. We have reported previously that a transcription factor identified in rice, WRKY45 (OsWRKY45), plays a pivotal role in BTH-induced disease resistance by mediating SA signalling. Here, we report further functional characterization of WRKY45. Different plant activators vary in their action points, either downstream (BTH and tiadinil) or upstream (probenazole) of SA. Rice resistance to Magnaporthe grisea, induced by both types of plant activator, was markedly reduced in WRKY45-knockdown (WRKY45-kd) rice, indicating a universal role for WRKY45 in chemical-induced resistance. Fungal invasion into rice cells was blocked at most attempted invasion sites (pre-invasive defence) in WRKY45-overexpressing (WRKY45-ox) rice. Hydrogen peroxide accumulated within the cell wall underneath invading fungus appressoria or between the cell wall and the cytoplasm, implying a possible role for H(2)O(2) in pre-invasive defence. Moreover, a hypersensitive reaction-like reaction was observed in rice cells, in which fungal growth was inhibited after invasion (post-invasive defence). The two levels of defence mechanism appear to correspond to Type I and II nonhost resistances. The leaf blast resistance of WRKY45-ox rice plants was much higher than that of other known blast-resistant varieties. WRKY45-ox plants also showed strong panicle blast resistance. BTH-induced resistance to Xanthomonas oryzae pv. oryzae was compromised in WRKY45-kd rice, whereas WRKY45-ox plants were highly resistant to this pathogen. However, WRKY45-ox plants were susceptible to Rhizoctonia solani. These results indicate the versatility and limitations of the application of this gene.

Nucleocytoplasmic shuttling of Ssd1 defines the destiny of its bound mRNAs

Mechanisms that control mRNA metabolism are critical for cell function, development and stress response. The Saccharomyces cerevisiae mRNA-binding protein Ssd1 has been implicated in mRNA processing, ageing, stress response and maintenance of cell integrity. Ssd1 is a substrate of the LATS/NDR tumour suppressor orthologue Cbk1 kinase. Previous data indicate that Ssd1 localizes to the cytoplasm; however, biochemical interactions suggest that Ssd1 at least transiently localizes to the nucleus. We therefore explored whether nuclear localization is important for Ssd1 cytoplasmic functions. We identified a functional NLS in the N-terminal domain of Ssd1. An Ssd1-derived NLS-GFP fusion protein and several C-terminally truncated Ssd1 proteins, which presumably lack nuclear export sequences, accumulate in the nucleus. Alanine substitution of the Ssd1 NLS prevents Ssd1 nuclear entry, mRNA binding and disrupts Srl1 mRNA localization. Moreover, Ssd1-NLS mutations abolish Ssd1 toxicity in the absence of Cbk1 phosphorylation and cause Ssd1 to localize prominently to cytoplasmic puncta. These data indicate that nuclear shuttling is critical for Ssd1 mRNA binding and Ssd1-mRNA localization in the cytoplasm. Collectively these data support the model that Ssd1 functions analogously to hnRNPs, which bind mRNA co-transcriptionally, are exported to the cytoplasm and target mRNAs to sites of localized translation and P-bodies.

HvCEBiP, a gene homologous to rice chitin receptor CEBiP, contributes to basal resistance of barley to Magnaporthe oryzae

BACKGROUND

Rice CEBiP recognizes chitin oligosaccharides on the fungal cell surface or released into the plant apoplast, leading to the expression of plant disease resistance against fungal infection. However, it has not yet been reported whether CEBiP is actually required for restricting the growth of fungal pathogens. Here we evaluated the involvement of a putative chitin receptor gene in the basal resistance of barley to the ssd1 mutant of Magnaporthe oryzae, which induces multiple host defense responses.

RESULTS

The mossd1 mutant showed attenuated pathogenicity on barley and appressorial penetration was restricted by the formation of callose papillae at attempted entry sites. When conidial suspensions of mossd1 mutant were spotted onto the leaves of HvCEBiP-silenced plants, small brown necrotic flecks or blast lesions were produced but these lesions did not expand beyond the inoculation site. Wild-type M. oryzae also produced slightly more severe symptoms on the leaves of HvCEBiP-silenced plants. Cytological observation revealed that these lesions resulted from appressorium-mediated penetration into plant epidermal cells.

CONCLUSIONS

These results suggest that HvCEBiP is involved in basal resistance against appressorium-mediated infection and that basal resistance might be triggered by the recognition of chitin oligosaccharides derived from M. oryzae.

A yeast STE11 homologue CoMEKK1 is essential for pathogenesis-related morphogenesis in Colletotrichum orbiculare

Several signal transduction pathways, including mitogen-activated protein kinase (MAPK) pathways, are involved in appressorium development in Colletotrichum orbiculare, the causal agent of cucumber anthracnose disease. In this study, CoMEKK1, a yeast MAPK kinases (MAPKK) kinase STE11 homolog, was identified as a disrupted gene in an Agrobacterium tumefaciens-mediated transformation mutant. The phenotype of comekk1 disruptant was similar to that of cmk1, a Saccharomyces cerevisiae Fus3/Kss1 MAPK homolog mutant. Moreover, comekk1 and cmk1 mutants were sensitive to high osmotic and salinity stresses, indicating that Comekk1p/Cmk1p signal transduction is involved in stress tolerance. The transformants of the wild type and the comekk1 mutant expressing a constitutively active form of the CoMEKK1 showed slower hyphal growth and abnormal appressorium formation, whereas those of the cmk1 disruptant did not. A Cmk1p-green fluorescent protein (GFP) intracellular localization experiment indicated that nuclear localization of the Cmk1p-GFP fusion protein induced by salt stress was diminished in comekk1 mutants. These results indicate that Comekk1p functions upstream of Cmk1p.

Development of an efficient gene targeting system in Colletotrichum higginsianum using a non-homologous end-joining mutant and Agrobacterium tumefaciens-mediated gene transfer

The hemibiotrophic ascomycete Colletotrichum higginsianum is the casual agent of anthracnose disease of cruciferous plants. High efficiency transformation by Agrobacterium tumefaciens-mediated gene transfer has been established for this fungus. However, targeted gene mutagenesis through homologous recombination rarely occurs in C. higginsianum. We have identified and disrupted the C. higginsianum homologue of the human Ku70 gene, ChKU70, which encodes a protein that plays a role in non-homologous end-joining for repair of DNA breaks. chku70 mutants showed a dramatic increase in the frequency of integration of introduced exogenous DNA fragments by homologous recombination without any detectable phenotypic defects. This result demonstrates that the chku70 mutant is an efficient recipient for targeted gene mutagenesis in C. higginsianum. We have also developed a novel approach [named direct repeat recombination-mediated gene targeting (DRGT)] for targeted gene disruption through Agrobacterium tumefaciens-mediated gene transfer. DRGT utilizes homologous recombination between repeated sequences on the T-DNA flanking a partial fragment of the target gene. Our results suggest that DRGT in the chku70 mutant background could be a useful tool for rapid isolation of targeted gene disruptants in C. higginsianum.

Use of the plant defense protein osmotin to identify Fusarium oxysporum genes that control cell wall properties

Fusarium oxysporum is the causative agent of fungal wilt disease in a variety of crops. The capacity of a fungal pathogen such as F. oxysporum f. sp. nicotianae to establish infection on its tobacco (Nicotiana tabacum) host depends in part on its capacity to evade the toxicity of tobacco defense proteins, such as osmotin. Fusarium genes that control resistance to osmotin would therefore reflect coevolutionary pressures and include genes that control mutual recognition, avoidance, and detoxification. We identified FOR (Fusarium Osmotin Resistance) genes on the basis of their ability to confer osmotin resistance to an osmotin-sensitive strain of Saccharomyces cerevisiae. FOR1 encodes a putative cell wall glycoprotein. FOR2 encodes the structural gene for glutamine:fructose-6-phosphate amidotransferase, the first and rate-limiting step in the biosynthesis of hexosamine and cell wall chitin. FOR3 encodes a homolog of SSD1, which controls cell wall composition, longevity, and virulence in S. cerevisiae. A for3 null mutation increased osmotin sensitivity of conidia and hyphae of F. oxysporum f. sp. nicotianae and also reduced cell wall beta-1,3-glucan content. Together our findings show that conserved fungal genes that determine cell wall properties play a crucial role in regulating fungal susceptibility to the plant defense protein osmotin.

Peroxisome biogenesis factor PEX13 is required for appressorium-mediated plant infection by the anthracnose fungus Colletotrichum orbiculare

Peroxisomes are ubiquitous organelles of eukaryotic cells that fulfill a variety of biochemical functions, including beta-oxidation of fatty acids. Here, we report that an ortholog of the Saccharomyces cerevisiae peroxisome biogenesis gene PEX13 is required for pathogenicity of Colletotrichum orbiculare. CoPEX13 was identified by screening random insertional mutants for deficiency in fatty acid utilization. Targeted knockout mutants of CoPEX13 were unable to utilize fatty acids as a carbon source. Expression analysis using green fluorescent protein fused to the peroxisomal targeting signals PTS1 and PTS2 revealed that the import machinery for peroxisomal matrix proteins was impaired in copex13 mutants. Appressoria formed by the copex13 mutants were defective in both melanization and penetration ability on host plants, had thin cell walls, and lacked peroxisomes. Moreover, the concentration of intracellular glycerol was lower in copex13 appressoria than those of the wild type. These findings indicate that fatty acid oxidation in peroxisomes is required not only for appressorium melanization but also for cell wall biogenesis and metabolic processes involved in turgor generation, all of which are essential for appressorium penetration ability.

Contribution of peroxisomes to secondary metabolism and pathogenicity in the fungal plant pathogen Alternaria alternata

The filamentous fungus Alternaria alternata includes seven pathogenic variants (pathotypes) which produce different host-selective toxins and cause diseases on different plants. The Japanese pear pathotype produces the host-selective AK-toxin, an epoxy-decatrienoic acid ester, and causes black spot of Japanese pear. Previously, we identified four genes, AKT1, AKT2, AKT3, and AKTR, involved in AK toxin biosynthesis. AKT1, AKT2, and AKT3 encode enzyme proteins with peroxisomal targeting signal type 1 (PTS1)-like tripeptides, SKI, SKL, and PKL, respectively, at the C-terminal ends. In this study, we verified the peroxisome localization of Akt1, Akt2, and Akt3 by using strains expressing N-terminal green fluorescent protein (GFP)-tagged versions of the proteins. To assess the role of peroxisome function in AK-toxin production, we isolated AaPEX6, which encodes a peroxin protein essential for peroxisome biogenesis, from the Japanese pear pathotype and made AaPEX6 disruption-containing transformants from a GFP-Akt1-expressing strain. The DeltaAaPEX6 mutant strains did not grow on fatty acid media because of a defect in fatty acid beta oxidation. The import of GFP-Akt1 into peroxisomes was impaired in the DeltaAaPEX6 mutant strains. These strains completely lost AK toxin production and pathogenicity on susceptible pear leaves. These data show that peroxisomes are essential for AK-toxin biosynthesis. The DeltaAaPEX6 mutant strains showed a marked reduction in the ability to cause lesions on leaves of a resistant pear cultivar with defense responses compromised by heat shock. This result suggests that peroxisome function is also required for plant invasion and tissue colonization in A. alternata. We also observed that mutation of AaPEX6 caused a marked reduction of conidiation.

A Kelch repeat protein, Cokel1p, associates with microtubules and is involved in appressorium development in Colletotrichum orbiculare

Kelch repeat proteins are conserved in diverse organisms and some are known to mediate fundamental cellular functions. We isolated the gene CoKEL1, encoding a novel kelch repeat protein, from Colletotrichum orbiculare. Analysis of a cokel1 mutant indicated that CoKEL1 is involved in proper appressorium development and cell wall synthesis. Appressoria produced by cokel1 disruption mutants showed irregular shape and impairment of turgor generation and the mutant appressoria rarely penetrated to form infection hyphae in host epidermal cells. Accordingly, cokel1 mutants had reduced pathogenicity on host leaves compared with the wild type. Furthermore, the cokel1 mutant was more sensitive to cell-wall-degrading enzymes and showed altered labeling with the cell wall stain Calcofluor white. Cokel1p was localized on cortical and spindle microtubules in vegetative hyphae. These results suggest that Cokel1p is a microtubule-associated protein involved in infection-related morphogenesis and pathogenicity. This is the first report that a kelch repeat protein is required for the pathogenicity of a fungal plant pathogen.

The Colletotrichum orbiculare SSD1 mutant enhances Nicotiana benthamiana basal resistance by activating a mitogen-activated protein kinase pathway

Plant basal resistance is activated by virulent pathogens in susceptible host plants. A Colletotrichum orbiculare fungal mutant defective in the SSD1 gene, which regulates cell wall composition, is restricted by host basal resistance responses. Here, we identified the Nicotiana benthamiana signaling pathway involved in basal resistance by silencing the defense-related genes required for restricting the growth of the C. orbiculare mutant. Only silencing of MAP Kinase Kinase2 or of both Salicylic Acid Induced Protein Kinase (SIPK) and Wound Induced Protein Kinase (WIPK), two mitogen-activated protein (MAP) kinases, allowed the mutant to infect and produce necrotic lesions similar to those of the wild type on inoculated leaves. The fungal mutant penetrated host cells to produce infection hyphae at a higher frequency in SIPK WIPK-silenced plants than in nonsilenced plants, without inducing host cellular defense responses. Immunocomplex kinase assays revealed that SIPK and WIPK were more active in leaves inoculated with mutant fungus than with the wild type, suggesting that induced resistance correlates with MAP kinase activity. Infiltration of heat-inactivated mutant conidia induced both SIPK and WIPK more strongly than did those of the wild type, while conidial exudates of the wild type did not suppress MAP kinase induction by mutant conidia. Therefore, activation of a specific MAP kinase pathway by fungal cell surface components determines the effective level of basal plant resistance.

Budding yeast SSD1-V regulates transcript levels of many longevity genes and extends chronological life span in purified quiescent cells

Ssd1 is an RNA-binding protein that affects literally hundreds of different processes and is polymorphic in both wild and lab yeast strains. We have used transcript microarrays to compare mRNA levels in an isogenic pair of mutant (ssd1-d) and wild-type (SSD1-V) cells across the cell cycle. We find that 15% of transcripts are differentially expressed, but there is no correlation with those mRNAs bound by Ssd1. About 20% of cell cycle regulated transcripts are affected, and most show sharper amplitudes of oscillation in SSD1-V cells. Many transcripts whose gene products influence longevity are also affected, the largest class of which is involved in translation. Ribosomal protein mRNAs are globally down-regulated by SSD1-V. SSD1-V has been shown to increase replicative life span currency and we show that SSD1-V also dramatically increases chronological life span (CLS). Using a new assay of CLS in pure populations of quiescent prototrophs, we find that the CLS for SSD1-V cells is twice that of ssd1-d cells.

A novel pathogenicity gene is required in the rice blast fungus to suppress the basal defenses of the host

For successful colonization and further reproduction in host plants, pathogens need to overcome the innate defenses of the plant. We demonstrate that a novel pathogenicity gene, DES1, in Magnaporthe oryzae regulates counter-defenses against host basal resistance. The DES1 gene was identified by screening for pathogenicity-defective mutants in a T-DNA insertional mutant library. Bioinformatic analysis revealed that this gene encodes a serine-rich protein that has unknown biochemical properties, and its homologs are strictly conserved in filamentous Ascomycetes. Targeted gene deletion of DES1 had no apparent effect on developmental morphogenesis, including vegetative growth, conidial germination, appressorium formation, and appressorium-mediated penetration. Conidial size of the mutant became smaller than that of the wild type, but the mutant displayed no defects on cell wall integrity. The Δdes1 mutant was hypersensitive to exogenous oxidative stress and the activity and transcription level of extracellular enzymes including peroxidases and laccases were severely decreased in the mutant. In addition, ferrous ion leakage was observed in the Δdes1 mutant. In the interaction with a susceptible rice cultivar, rice cells inoculated with the Δdes1 mutant exhibited strong defense responses accompanied by brown granules in primary infected cells, the accumulation of reactive oxygen species (ROS), the generation of autofluorescent materials, and PR gene induction in neighboring tissues. The Δdes1 mutant displayed a significant reduction in infectious hyphal extension, which caused a decrease in pathogenicity. Notably, the suppression of ROS generation by treatment with diphenyleneiodonium (DPI), an inhibitor of NADPH oxidases, resulted in a significant reduction in the defense responses in plant tissues challenged with the Δdes1 mutant. Furthermore, the Δdes1 mutant recovered its normal infectious growth in DPI-treated plant tissues. These results suggest that DES1 functions as a novel pathogenicity gene that regulates the activity of fungal proteins, compromising ROS-mediated plant defense.

Coevolution of plants and microbial pathogens leads to interactions that resemble a molecular war. Pathogens generate effector molecules to infect their hosts, and plants produce defense molecules against pathogen attacks. Interactions between these molecules results in plant immunity or disease. Plant disease could be likened to a complex and delicate matter of balance, where a number of molecules are involved in the battlefield. Discovering and understanding the tipping points in the battle are vital for developing disease-free crops. In the interaction of rice and rice blast fungus, a microbe sensor on rice stimulates the generation of reactive oxygen species (ROS) at the site of infection. ROS is known as an antimicrobial material and a stimulator for defense signaling that is important for preparing reinforcement in neighboring tissues. This paper presents the counter-defense mechanism of the fungus against plant-driven ROS. We found that a pathogenicity factor from rice blast fungus, DES1 (Defense Suppressor 1), is involved in overcoming oxidative stress for the counter-defense mechanism, suggesting that this gene is required for fungal pathogenicity.

Atg26-mediated pexophagy is required for host invasion by the plant pathogenic fungus Colletotrichum orbiculare

The number of peroxisomes in a cell can change rapidly in response to changing environmental and physiological conditions. Pexophagy, a type of selective autophagy, is involved in peroxisome degradation, but its physiological role remains to be clarified. Here, we report that cells of the cucumber anthracnose fungus Colletotrichum orbiculare undergo peroxisome degradation as they infect host plants. We performed a random insertional mutagenesis screen to identify genes involved in cucumber pathogenesis by C. orbiculare. In this screen, we isolated a homolog of Pichia pastoris ATG26, which encodes a sterol glucosyltransferase that enhances pexophagy in this methylotrophic yeast. The C. orbiculare atg26 mutant developed appressoria but exhibited a specific defect in the subsequent host invasion step, implying a relationship between pexophagy and fungal phytopathogenicity. Consistent with this, its peroxisomes are degraded inside vacuoles, accompanied by the formation of autophagosomes during infection-related morphogenesis. The autophagic degradation of peroxisomes was significantly delayed in the appressoria of the atg26 mutant. Functional domain analysis of Atg26 suggested that both the phosphoinositide binding domain and the catalytic domain are required for pexophagy and pathogenicity. In contrast with the atg26 mutant, which is able to form appressoria, the atg8 mutant, which is defective in the entire autophagic pathway, cannot form normal appressoria in the earlier steps of morphogenesis. These results indicate a specific function for Atg26-enhanced pexophagy during host invasion by C. orbiculare.

Extracellular transport and integration of plant secretory proteins into pathogen-induced cell wall compartments

Many fungal parasites enter plant cells by penetrating the host cell wall and, thereafter, differentiate specialized intracellular feeding structures, called haustoria, by invagination of the plant's plasma membrane. Arabidopsis PEN gene products are known to act at the cell periphery and function in the execution of apoplastic immune responses to limit fungal entry. This response underneath fungal contact sites is tightly linked with the deposition of plant cell wall polymers, including PMR4/GSL5-dependent callose, in the paramural space, thereby producing localized wall thickenings called papillae. We show that powdery mildew fungi specifically induce the extracellular transport and entrapment of the fusion protein GFP-PEN1 syntaxin and its interacting partner monomeric yellow fluorescent protein (mYFP)-SNAP33 within the papillary matrix. Remarkably, PMR4/GSL5 callose, GFP-PEN1, mYFP-SNAP33, and the ABC transporter GFP-PEN3 are selectively incorporated into extracellular encasements surrounding haustoria of the powdery mildew Golovinomyces orontii, suggesting that the same secretory defense responses become activated during the formation of papillae and haustorial encasements. This is consistent with a time-course analysis of the encasement process, indicating that these extracellular structures are generated through the extension of papillae. We show that PMR4/GSL5 callose accumulation in papillae and haustorial encasements occurs independently of PEN1 syntaxin. We propose a model in which exosome biogenesis/release serves as a common transport mechanism by which the proteins PEN1 and PEN3, otherwise resident in the plasma membrane, together with membrane lipids, become stably incorporated into both pathogen-induced cell wall compartments.

Discovery of pathogenicity genes in the crucifer anthracnose fungus Colletotrichum higginsianum, using random insertional mutagenesis

Agrobacterium tumefaciens-mediated transformation (ATMT) was used for random insertional mutagenesis to identify pathogenicity genes in the hemibiotrophic fungus Colletotrichum higginsianum. A high-throughput primary infection assay on Arabidopsis thaliana seedlings allowed the rapid screening of 8,850 transformants. Forty mutants showing reproducible pathogenicity defects on Arabidopsis and Brassica plants were obtained, and their infection phenotypes were characterized microscopically. Six mutants were impaired in appressorial melanization, fifteen had reduced penetration ability, 14 induced host papillae or hypersensitive cell death, and five were affected in the transition from biotrophy to necrotrophy. Southern blot analysis showed 58% of the transformants had single-site T-DNA integrations. Right-border flanking sequences were recovered from 12 mutants by inverse polymerase chain reaction (PCR) or thermal asymmetric interlaced PCR and were used to isolate the tagged genes from a genomic library. The putative pathogenicity genes encoded homologs of a major facilitator superfamily phosphate transporter, importin-beta2, ornithine decarboxylase, beta-1,3(4)-glucanase, ATP-binding endoribonuclease, carbamoyl-phosphate synthetase, and the polyprotein precursor of N-acetylglutamate kinase and N-acetylglutamyl-phosphate reductase. Six further loci were homologous to proteins of unknown function. None of these genes were previously implicated in the pathogenicity of any Colletotrichum species. The results demonstrate that ATMT is an effective tool for gene discovery in this model pathogen.

Genomes, free radicals and plant cell invasion: recent developments in plant pathogenic fungi

This review describes current advances in our understanding of fungal-plant interactions. The widespread application of whole genome sequencing to a diverse range of fungal species has allowed new insight into the evolution of fungal pathogenesis and the definition of the gene inventories associated with important plant pathogens. This has also led to functional genomic approaches to carry out large-scale gene functional analysis. There has also been significant progress in understanding appressorium-mediated plant infection by fungi and its underlying genetic basis. The nature of biotrophic proliferation of fungal pathogens in host tissue has recently revealed new potential mechanisms for cell-to-cell movement by invading pathogens.

SSD1 is integral to host defense peptide resistance in Candida albicans

Candida albicans is usually a harmless human commensal. Because inflammatory responses are not normally induced by colonization, antimicrobial peptides are likely integral to first-line host defense against invasive candidiasis. Thus, C. albicans must have mechanisms to tolerate or circumvent molecular effectors of innate immunity and thereby colonize human tissues. Prior studies demonstrated that an antimicrobial peptide-resistant strain of C. albicans, 36082(R), is hypervirulent in animal models versus its susceptible counterpart (36082(S)). The current study aimed to identify a genetic basis for antimicrobial peptide resistance in C. albicans. Screening of a C. albicans genomic library identified SSD1 as capable of conferring peptide resistance to a susceptible surrogate, Saccharomyces cerevisiae. Sequencing confirmed that the predicted translation products of 36082(S) and 36082(R) SSD1 genes were identical. However, Northern analyses corroborated that SSD1 is expressed at higher levels in 36082(R) than in 36082(S). In isogenic backgrounds, ssd1Delta/ssd1Delta null mutants were significantly more susceptible to antimicrobial peptides than parental strains but had equivalent susceptibilities to nonpeptide stressors. Moreover, SSD1 complementation of ssd1Delta/ssd1Delta mutants restored parental antimicrobial peptide resistance phenotypes, and overexpression of SSD1 conferred enhanced peptide resistance. Consistent with these in vitro findings, ssd1 null mutants were significantly less virulent in a murine model of disseminated candidiasis than were their parental or complemented strains. Collectively, these results indicate that SSD1 is integral to C. albicans resistance to host defense peptides, a phenotype that appears to enhance the virulence of this organism in vivo.