The Colletotrichum acutatum gene encoding a putative pH-responsive transcription regulator is a key virulence determinant during fungal pathogenesis on citrus

The Rfg1 and Bcr1 transcription factors regulate acidic pH-induced filamentous growth in Candida albicans

IMPORTANCE

Candida albicans is a human commensal and frequent pathogen that encounters a wide range of pH stresses. The ability of C. albicans to adapt to changes in extracellular pH is crucial for its success in colonization and pathogenesis. The Rim101 pH sensing pathway is well known to govern neutral-alkaline pH responses in this pathogen. Here, we report a novel Rfg1-Bcr1 regulatory pathway that governs acidic pH responses and regulates filamentous growth in C. albicans. In addition, the Rim101-Phr1 pathway, cAMP signaling pathway, transcription factors Efg1 and Flo8, and hyphal-specific G1 cyclin Hgc1 cooperate with this regulation. Our findings provide new insights into the regulatory mechanism of acidic pH response in C. albicans.

Publicly Available and Validated DNA Reference Sequences Are Critical to Fungal Identification and Global Plant Protection Efforts: A Use-Case in Colletotrichum

Publicly available and validated DNA reference sequences useful for phylogeny estimation and identification of fungal pathogens are an increasingly important resource in the efforts of plant protection organizations to facilitate safe international trade of agricultural commodities. Colletotrichum species are among the most frequently encountered and regulated plant pathogens at U.S. ports-of-entry. The RefSeq Targeted Loci (RTL) project at NCBI (BioProject no. PRJNA177353) contains a database of curated fungal internal transcribed spacer (ITS) sequences that interact extensively with NCBI Taxonomy, resulting in verified name-strain-sequence type associations for >12,000 species. We present a publicly available dataset of verified and curated name-type strain-sequence associations for all available Colletotrichum species. This includes an updated GenBank Taxonomy for 238 species associated with up to 11 protein coding loci and an updated RTL ITS dataset for 226 species. We demonstrate that several marker loci are well suited for phylogenetic inference and identification. We improve understanding of phylogenetic relationships among verified species, verify or improve phylogenetic circumscriptions of 14 species complexes, and reveal that determining relationships among these major clades will require additional data. We present detailed comparisons between phylogenetic and similarity-based approaches to species identification, revealing complex patterns among single marker loci that often lead to misidentification when based on single-locus similarity approaches. We also demonstrate that species-level identification is elusive for a subset of samples regardless of analytical approach, which may be explained by novel species diversity in our dataset and incomplete lineage sorting and lack of accumulated synapomorphies at these loci.

Organic acid, a virulence factor for pathogenic fungi, causing postharvest decay in fruits

Decay due to fungal infection is a major cause of postharvest losses in fruits. Acidic fungi may enhance their virulence by locally reducing the pH of the host. Several devastating postharvest fungi, such as Penicillium spp., Botrytis cinerea, and Sclerotinia sclerotiorum, can secrete gluconic acid, oxalic acid, or citric acid. Emerging evidence suggests that organic acids secreted by acidic fungi are important virulence factors. In this review, we summarized the research progress on the biosynthesis of organic acids, the role of the pH signalling transcription factor PacC in regulating organic acid, and the action mechanism of the main organic acid secreted via postharvest pathogenic fungi during infection of host tissues. This paper systematically demonstrates the relationships between tissue acidification and postharvest fungal pathogenicity, which will motivate the study of host-pathogen interactions and provide a better understanding of virulence mechanisms of the pathogens so as to design new technical strategies to prevent postharvest diseases.

Occurrence and infective potential of Colletotrichum gloeosporioides isolates associated to Citrus limon var Eureka

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37 fungi from damaged tissues of Italian lemon were obtained.

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D1/D2, ITS1-5.8S-ITS2 and COX1 phylogenetic trees allow identify ten fungi genera.

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Colletotrichum isolates were characterized by species-specific PCR, rep-PCR and multilocus analysis.

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Colletotrichum isolates had leaves´s infection percentages between 17 to 67%.

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This the first report of C. gloeosporioides on Italian lemon in Mexico.

A collection of 37 fungi associated to Italian lemon plants with disease symptoms, was obtained. Ten genera including Aspergillus, Alternaria, Nigrospora, Lasiodiplodia, Dothideomycetes, Pleurostoma, Setosphaeria, Penicillium, Fusarium and Colletotrichum were identified by using ITS1–5.8S–ITS2, D1/D2 26S and COX1 loci. The last three genera were abundant on the damaged fruits, being Colletotrichum the more abundant (32.4 %). CaInt2 and CgInt primers support the identity of these isolates as C. gloeosporioides. Variability, inferred by rep-PCR and multilocus sequence analysis shows genetic differences among the C. gloeosporioides isolates. Infective profile evaluated in Colletotrichum isolates shows different leave infection percentages (26 to 60 %). SEM analysis showed mycelium, spores and appressoria on the leaves of selected Colletotrichum isolates. Specifically, the AL-05 and AL-13 isolates showed a high chitin deacetylase activity (CDA) peaking at 1.2 U/mg protein in AL-13. This is the first report on C. gloeosporioides infecting Italian lemon leaves in Mexico.

Special Issue "Interplay between Fungal Pathogens and Harvested Crops and Fruits"

The interplay between fungal pathogens and harvest crops is important in determining the extent of food losses following the storage and transport of crops to consumers. The specific factors modulating the activation of colonization are of key importance to determining the initiation of fungal colonization and host losses. It is clear nowadays from the wide number of transcription studies in colonized fruits that pathogenicity in postharvest produce is not only the result of activation of fungal pathogenicity factors but is significantly contributed to fruit maturity and ripening. In this editorial summary of the Special Issue "Interplay between Fungal Pathogens and Harvested Crops and Fruits", we present a short summary of future research directions on the importance of the interplay between fruit and pathogens and nine published papers (one review and eight original research papers), covering a wide range of subjects within the mechanism of pathogenicity by postharvest pathogens, including transcriptome analysis of pathogenesis, pathogenicity factors, new antifungal compounds and food toxin occurrence by pathogens. This summary may lead the reader to understand the key factors modulating pathogenicity in fruits.

CaNRT2.1 Is Required for Nitrate but Not Nitrite Uptake in Chili Pepper Pathogen Colletotrichum acutatum

Chili peppers are an important food additive used in spicy cuisines worldwide. However, the yield and quality of chilis are threatened by anthracnose disease caused by Colletotrichum acutatum. Despite the impact of C. acutatum on chili production, the genes involved in fungal development and pathogenicity in this species have not been well characterized. In this study, through T-DNA insertional mutagenesis, we identified a mutant strain termed B7, which is defective for the growth of C. acutatum on a minimal nutrient medium. Our bioinformatics analysis revealed that a large fragment DNA (19.8 kb) is deleted from the B7 genome, thus resulting in the deletion of three genes, including CaGpiP1 encoding a glycosylphosphatidyl-inisotol (GPI)-anchored protein, CaNRT2.1 encoding a membrane-bound nitrate/nitrite transporter, and CaRQH1 encoding a RecQ helicase protein. In addition, T-DNA is inserted upstream of the CaHP1 gene encoding a hypothetical protein. Functional characterization of CaGpiP1, CaNRT2.1, and CaHP1 by targeted gene disruption and bioassays indicated that CaNRT2.1 is responsible for the growth-defective phenotype of B7. Both B7 and CaNRT2.1 mutant strains cannot utilize nitrate as nitrogen sources, thus restraining the fungal growth on a minimal nutrient medium. In addition to CaNRT2.1, our results showed that CaGpiP1 is a cell wall-associated GPI-anchored protein. However, after investigating the functions of CaGpiP1 and CaHP1 in fungal pathogenicity, growth, development and stress tolerance, we were unable to uncover the roles of these two genes in C. acutatum. Collectively, in this study, our results identify the growth-defective strain B7 via T-DNA insertion and reveal the critical role of CaNRT2.1 in nitrate transportation for the fungal growth of C. acutatum.

pH effect on strain-specific transcriptomes of the take-all fungus

The soilborne fungus Gaeumannomyces tritici (G. tritici) causes the take-all disease on wheat roots. Ambient pH has been shown to be critical in different steps of G. tritici life cycle such as survival in bulk soil, saprophytic growth, and pathogenicity on plants. There are however intra-specific variations and we previously found two types of G. tritici strains that grow preferentially either at acidic pH or at neutral/alkaline pH; gene expression involved in pH-signal transduction pathway and pathogenesis was differentially regulated in two strains representative of these types. To go deeper in the description of the genetic pathways and the understanding of this adaptative mechanism, transcriptome sequencing was achieved on two strains (PG6 and PG38) which displayed opposite growth profiles in two pH conditions (acidic and neutral). PG6, growing better at acidic pH, overexpressed in this condition genes related to cell proliferation. In contrast, PG38, which grew better at neutral pH, overexpressed in this condition genes involved in fatty acids and amino acid metabolisms, and genes potentially related to pathogenesis. This strain also expressed stress resistance mechanisms at both pH, to assert a convenient growth under various ambient pH conditions. These differences in metabolic pathway expression between strains at different pH might buffer the effect of field or soil variation in wheat fields, and explain the success of the pathogen.

The pH-Responsive Transcription Factor PacC Governs Pathogenicity and Ochratoxin A Biosynthesis in Aspergillus carbonarius

Pathogenic fungi must respond effectively to changes in environmental pH for successful host colonization, virulence and toxin production. Aspergillus carbonarius is a mycotoxigenic pathogen with the ability to colonize many plant hosts and secrete ochratoxin A (OTA). In this study, we characterized the functions and addressed the role of PacC-mediated pH signaling in A. carbonarius pathogenicity using designed pacC gene knockout mutant. ΔAcpacC mutant displayed an acidity-mimicking phenotype, which resulted in impaired fungal growth at neutral/alkaline pH, accompanied by reduced sporulation and conidial germination compared to the wild type (WT) strain. The ΔAcpacC mutant was unable to efficiently acidify the growth media as a direct result of diminished gluconic and citric acid production. Furthermore, loss of AcpacC resulted in a complete inhibition of OTA production at pH 7.0. Additionally, ΔAcpacC mutant exhibited attenuated virulence compared to the WT toward grapes and nectarine fruits. Reintroduction of pacC gene into ΔAcpacC mutant restored the WT phenotype. Our results demonstrate important roles of PacC of A. carbonarius in OTA biosynthesis and in pathogenicity by controlling transcription of genes important for fungal secondary metabolism and infection.

The pH-responsive PacC transcription factor plays pivotal roles in virulence and patulin biosynthesis in Penicillium expansum

The PacC (loss or reduction in phosphatase activity at acid but not at alkaline pH [Pac]) transcription factor regulates environmental adaptation, secondary metabolism and virulence in many fungal pathogens. Here, we report the functions of PacC in Penicillium expansum, a postharvest pathogenic fungus in horticultural crops, and ascertain that the gene expression and proteolytic processing of PePacC are strictly pH-dependent. Loss of PePacC resulted in an obvious decrease in growth and conidiation of P. expansum cultured in both acidic and alkaline conditions. The ΔPePacC mutant lost the ability of patulin production at pH values above 6.0 because expressions of all the genes in patulin cluster were significantly down-regulated. Additionally, virulence of the ΔPePacC mutant was obviously reduced in pear and apple fruits. Proteome analysis revealed that PePacC could function as an activator or repressor for different target proteins, including calreticulin (PeCRT) and sulfate adenylyltransferase (PeSAT), which were further proved to be involved in virulence of P. expansum. Our results demonstrate important roles for PePacC in patulin biosynthesis via limiting expressions of the genes in the cluster, and in pathogenesis via mediating a known virulence factor glucose oxidase (PeGOD) and new virulence factors, such as PeCRT and PeSAT.

Differences in the Characteristics and Pathogenicity of Colletotrichum camelliae and C. fructicola Isolated From the Tea Plant [Camellia sinensis (L.) O. Kuntze]

Colletotrichum, the causative agent of anthracnose, is an important pathogen that invades the tea plant (Camellia sinensis). In this study, 38 isolates were obtained from the diseased leaves of tea plants collected in different areas of Zhejiang Province, China. A combination of multigene (ITS, ACT, GAPDH, TUB2, CAL, and GS) and morphology analyses showed that the 38 strains belonged to two different species, namely, C. camelliae (CC), and C. fructicola (CF). Pathogenicity tests revealed that CC was more invasive than CF. In vitro inoculation experiments demonstrated that CC formed acervuli at 72 hpi and developed appressoria on wound edges, but CF did not develop these structures. Under treatment with catechins and caffeine, the growth inhibition rates of CF were remarkably higher than those of CC, indicating that the nonpathogenic species CF was more vulnerable to catechins and caffeine. Growth condition testing indicated that CF grew at a wide temperature range of 15-35°C and that the optimum temperature for CC growth was 25°C. Growth of both CC and CF did not differ between acidic and weakly alkaline environments (pH 5-8), but the growth of CC was significantly reduced at pH values of 9 and 10. Furthermore, the PacC/RIM101 gene, which associated with pathogenicity, was identified from CC and CF genomes, and its expression was suppressed in the hyphae of both species under pH value of 5 and 10, and much lower expression level was detected in CC than that in CF at pH 6. These results indicated that temperature has more important effect than pH for the growth of two Colletotrichum species. In conclusion, the inhibition by secondary metabolite is an important reason why the pathogenicity by CC and CF are different to tea plant, although the environmental factors including pH and temperature effect the growth of two Colletotrichum species.

pH-Signaling Transcription Factor AopacC Regulates Ochratoxin A Biosynthesis in Aspergillus ochraceus

In Aspergillus and Penicillium species, an essential pH-response transcription factor pacC is involved in growth, pathogenicity, and toxigenicity. To investigate the connection between ochratoxin A (OTA) biosynthesis and ambient pH, the AopacC in Aspergillus ochraceus was functionally characterized using a loss-of-function mutant. The mycelium growth was inhibited under pH 4.5 and 10.0, while the sporulation increased under alkaline condition. A reduction of mycelium growth and an elevation of sporulation was observed in Δ AopacC mutant. Compared to neutral condition, OTA contents were respectively reduced by 71.6 and 79.8% under acidic and alkaline conditions. The expression of AopacC increased with the elevated pH, and deleting AopacC dramatically decreased OTA production and biosynthetic genes Aopks expression. Additionally, the Δ AopacC mutant exhibited attenuated infection ability toward pear fruits. These results suggest that AopacC is an alkaline-induced regulator responsible for growth and OTA biosynthesis in A. ochraceus and this regulatory mechanism might be pH-dependent.

Adaptation to the Host Environment by Plant-Pathogenic Fungi

Many fungi can live both saprophytically and as endophyte or pathogen inside a living plant. In both environments, complex organic polymers are used as sources of nutrients. Propagation inside a living host also requires the ability to respond to immune responses of the host. We review current knowledge of how plant-pathogenic fungi do this. First, we look at how fungi change their global gene expression upon recognition of the host environment, leading to secretion of effectors, enzymes, and secondary metabolites; changes in metabolism; and defense against toxic compounds. Second, we look at what is known about the various cues that enable fungi to sense the presence of living plant cells. Finally, we review literature on transcription factors that participate in gene expression in planta or are suspected to be involved in that process because they are required for the ability to cause disease.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Gene family expansions and contractions are associated with host range in plant pathogens of the genus Colletotrichum

BACKGROUND

Many species belonging to the genus Colletotrichum cause anthracnose disease on a wide range of plant species. In addition to their economic impact, the genus Colletotrichum is a useful model for the study of the evolution of host specificity, speciation and reproductive behaviors. Genome projects of Colletotrichum species have already opened a new era for studying the evolution of pathogenesis in fungi.

RESULTS

We sequenced and annotated the genomes of four strains in the Colletotrichum acutatum species complex (CAsc), a clade of broad host range pathogens within the genus. The four CAsc proteomes and secretomes along with those representing an additional 13 species (six Colletotrichum spp. and seven other Sordariomycetes) were classified into protein families using a variety of tools. Hierarchical clustering of gene family and functional domain assignments, and phylogenetic analyses revealed lineage specific losses of carbohydrate-active enzymes (CAZymes) and proteases encoding genes in Colletotrichum species that have narrow host range as well as duplications of these families in the CAsc. We also found a lineage specific expansion of necrosis and ethylene-inducing peptide 1 (Nep1)-like protein (NLPs) families within the CAsc.

CONCLUSIONS

This study illustrates the plasticity of Colletotrichum genomes, and shows that major changes in host range are associated with relatively recent changes in gene content.

Diacylglycerol acyl transferase: A pathogenicity related gene in Colletotrichum gloeosporioides

To gain more insight into the molecular mechanisms of Colletotrichum gloeosporioides pathogenesis, restriction enzyme-mediated integration (REMI) mutagenesis identified the mutants of C. gloeosporioides impaired in pathogenicity. Transformants screened for defects in pathogenicity using detached leaves and fruits. Of the 20 REMI transformants tested, two mutants (H4 and H7) showed reduced pathogenicity on leaves of apple, kiwi, mango, peach, and fruits of guava, apple, and capsicum. One tagged gene from the genome sequence of mutant H4 was recovered by inverse PCR. Sequence analysis of the tagged site in mutant H4 revealed insertion in diacylglycerol acyltransferase gene which encodes diacylglycerol acyltransferase enzyme, catalyzing the steps involved in the biosynthesis of triacylglycerol, an important component of biological membranes and source of energy. Therefore, tagging of diacylglycerol acyltransferase gene in mutant H4 resulted in reduced pathogenicity, indicating possible role of this gene in pathogenicity of C. gloeosporioides.

Genome-wide gene expression dynamics of the fungal pathogen Dothistroma septosporum throughout its infection cycle of the gymnosperm host Pinus radiata

We present genome-wide gene expression patterns as a time series through the infection cycle of the fungal pine needle blight pathogen, Dothistroma septosporum, as it invades its gymnosperm host, Pinus radiata. We determined the molecular changes at three stages of the disease cycle: epiphytic/biotrophic (early), initial necrosis (mid) and mature sporulating lesion (late). Over 1.7 billion combined plant and fungal reads were sequenced to obtain 3.2 million fungal-specific reads, which comprised as little as 0.1% of the sample reads early in infection. This enriched dataset shows that the initial biotrophic stage is characterized by the up-regulation of genes encoding fungal cell wall-modifying enzymes and signalling proteins. Later necrotrophic stages show the up-regulation of genes for secondary metabolism, putative effectors, oxidoreductases, transporters and starch degradation. This in-depth through-time transcriptomic study provides our first snapshot of the gene expression dynamics that characterize infection by this fungal pathogen in its gymnosperm host.

How Does Host Carbon Concentration Modulate the Lifestyle of Postharvest Pathogens during Colonization?

Postharvest pathogens can penetrate fruit by breaching the cuticle or directly through wounds, and they show disease symptoms only long after infection. During ripening and senescence, the fruit undergo physiological processes accompanied by a decline in antifungal compounds, which allows the pathogen to activate a mechanism of secretion of small effector molecules that modulate host environmental pH. These result in the activation of genes under their optimal pH conditions, enabling the fungus to use a specific group of pathogenicity factors at each particular pH. New research suggests that carbon availability in the environment is a key factor triggering the production and secretion of small pH-modulating molecules: ammonia and organic acids. Ammonia is secreted under limited carbon and gluconic acid under excess carbon. This mini review describes our most recent knowledge of the mechanism of activation of pH-secreted molecules and their contribution to colonization by postharvest pathogens to facilitate the transition from quiescence to necrotrophic lifestyle.

CmpacC regulates mycoparasitism, oxalate degradation and antifungal activity in the mycoparasitic fungus Coniothyrium minitans

The PacC/Rim101 pH-responsive transcription factor is an important pathogenicity element for many plant-pathogenic fungi. In this study, we investigated the roles of a PacC homologue, CmpacC, in the mycoparasitic fungus Coniothyrium minitans. CmpacC was confirmed to have the transcriptional activation activity by the transcriptional activation test in Saccharomyces cerevisiae. Disruption of CmpacC resulted in impaired fungal responses to ambient pH. Compared to the wild-type, the CmpacC-disruption mutant ΔCmpacC-29 was significantly suppressed for activities of chitinase and β-1,3-glucanase at pH 5 and 7, consistent with reduced expression levels of Cmch1 and Cmg1 coding for the two enzymes respectively. However, the mutant displayed acidity-mimicking phenotypes such as improved oxalate degradation and increased antifungal activity at pH 6 or higher. Improved efficacy in oxalate degradation by ΔCmpacC-29 was consistent with the enhanced expression level of Cmoxdc1 coding for oxalate decarboxylase. CmpacC transcriptional activation of Cmch1 and Cmg1 and repression of Cmoxdc1 were verified by the presence of the PacC/Rim101 consensus binding-motifs in gene promoter regions and by the promoter DNA-binding assays. This study suggests that CmpacC plays an activator role in regulation of C. minitans mycoparasitism, whereas plays a repressor role in regulation of oxalate degradation and possibly antifungal activity of C. minitans.

Expression of Five Endopolygalacturonase Genes and Demonstration that MfPG1 Overexpression Diminishes Virulence in the Brown Rot Pathogen Monilinia fructicola

Monilinia fructicola is a devastating pathogen on stone fruits, causing blossom blight and fruit rot. Little is known about pathogenic mechanisms in M. fructicola and related Monilinia species. In this study, five endopolygalacturonase (endo-PG) genes were cloned and functionally characterized in M. fructicola. Quantitative reverse-transcriptase PCR (qRT-PCR) revealed that the five MfPG genes are differentially expressed during pathogenesis and in culture under various pH regimes and carbon and nitrogen sources. MfPG1 encodes the major endo-PG and is expressed to significantly higher levels compared to the other four MfPGs in culture and in planta. MfPG1 function during pathogenesis was evaluated by examining the disease phenotypes and gene expression patterns in M. fructicola MfPG1-overexpressing strains and in strains carrying the β-glucuronidase (GUS) reporter gene fused with MfPG1 (MfPG1-GUS). The MFPG1-GUS reporter was expressed in situ in conidia and hyphae following inoculation of flower petals, and qRT-PCR analysis confirmed MfPG1 expression during pathogenesis. MfPG1-overexpressing strains produced smaller lesions and higher levels of reactive oxygen species (ROS) on the petals of peach and rose flowers than the wild-type strain, suggesting that MfPG1 affecting fungal virulence might be in part resulted from the increase of ROS in the Prunus–M. fructicola interactions.

Transcriptomic analysis of the GCN5 gene reveals mechanisms of the epigenetic regulation of virulence and morphogenesis in Ustilago maydis

Chromatin in the eukaryotic nucleus is highly organized in the form of nucleosomes where histones wrap DNA. This structure may be altered by some chemical modifications of histones, one of them, acetylation by histone acetyltransferases (HATs) that originates relaxation of the nucleosome structure, providing access to different transcription factors and other effectors. In this way, HATs regulate cellular processes including DNA replication, and gene transcription. Previously, we isolated Ustilago maydis mutants deficient in the GCN5 HAT that are avirulent, and grow constitutively as mycelium. In this work, we proceeded to identify the genes differentially regulated by GCN5, comparing the transcriptomes of the mutant and the wild type using microarrays, to analyse the epigenetic control of virulence and morphogenesis. We identified 1203 genes, 574 positively and 629 negatively regulated in the wild type. We found that genes belonging to different categories involved in pathogenesis were downregulated in the mutant, and that genes involved in mycelial growth were negatively regulated in the wild type, offering a working hypothesis on the epigenetic control of virulence and morphogenesis of U. maydis. Interestingly, several differentially regulated genes appeared in clusters, suggesting a common regulation. Some of these belonged to pathogenesis or secondary metabolism.

The Cryptococcus neoformans alkaline response pathway: identification of a novel rim pathway activator

The Rim101/PacC transcription factor acts in a fungal-specific signaling pathway responsible for sensing extracellular pH signals. First characterized in ascomycete fungi such as Aspergillus nidulans and Saccharomyces cerevisiae, the Rim/Pal pathway maintains conserved features among very distantly related fungi, where it coordinates cellular adaptation to alkaline pH signals and micronutrient deprivation. However, it also directs species-specific functions in fungal pathogens such as Cryptococcus neoformans, where it controls surface capsule expression. Moreover, disruption of the Rim pathway central transcription factor, Rim101, results in a strain that causes a hyper-inflammatory response in animal infection models. Using targeted gene deletions, we demonstrate that several genes encoding components of the classical Rim/Pal pathway are present in the C. neoformans genome. Many of these genes are in fact required for Rim101 activation, including members of the ESCRT complex (Vps23 and Snf7), ESCRT-interacting proteins (Rim20 and Rim23), and the predicted Rim13 protease. We demonstrate that in neutral/alkaline pH, Rim23 is recruited to punctate regions on the plasma membrane. This change in Rim23 localization requires upstream ESCRT complex components but does not require other Rim101 proteolysis components, such as Rim20 or Rim13. Using a forward genetics screen, we identified the RRA1 gene encoding a novel membrane protein that is also required for Rim101 protein activation and, like the ESCRT complex, is functionally upstream of Rim23-membrane localization. Homologs of RRA1 are present in other Cryptococcus species as well as other basidiomycetes, but closely related genes are not present in ascomycetes. These findings suggest that major branches of the fungal Kingdom developed different mechanisms to sense and respond to very elemental extracellular signals such as changing pH levels.

Phylogenic analysis revealed an expanded C₂H₂-homeobox subfamily and expression profiles of C₂H₂ zinc finger gene family in Verticillium dahliae

C2H2 zinc finger (CZF) proteins are a major class of transcription factors that play crucial roles in fungal growth, development, various stress responses, and virulence. Little genome-wide data is available regarding the roles of CZF proteins in Verticillium dahliae, a destructive pathogen that causes vascular wilt disease in more than 200 plant species. We identified a total of 79 typical CZF genes in V. dahliae. Comparative analysis revealed that four plant pathogenic fungi, V. dahliae, Fusarium oxysporum, Magnaporthe oryzae, and Botrytis cinerea, have comparable numbers of predicted CZF genes with similar characteristics. Phylogenetic analysis identified a C2H2-homeobox subfamily in V. dahliae containing seven genes with similar gene structures. V. dahliae and F. oxysporum (Hypocreomycetidae) have more genes of this subfamily than M. oryzae (Sordariomycetidae) and B. cinerea (Leotiomycetes). Furthermore, gene-expression analysis of the smoke tree wilt fungus V. dahliae strain XS11 using digital gene-expression profiling and RT-qPCR revealed that a number of CZF genes were differentially expressed during microsclerotia formation, nutritional starvation, and simulated in planta conditions. Furthermore, the expression profiles revealed that some CZF genes were overrepresented during multiple stages, indicating that they might play diverse roles. Our results provide useful information concerning the functions of CZF genes in microsclerotia formation, nutritional stress responses, and pathogenicity in V. dahliae, and form a basis for future functional studies of these genes.

A Role of AREB in the Regulation of PACC-Dependent Acid-Expressed-Genes and Pathogenicity of Colletotrichum gloeosporioides

Gene expression regulation by pH in filamentous fungi and yeasts is controlled by the PACC/RIM101 transcription factor. In Colletotrichum gloeosporioides, PACC is known to act as positive regulator of alkaline-expressed genes, and this regulation was shown to contribute to fungal pathogenicity. PACC is also a negative regulator of acid-expressed genes, however; the mechanism of downregulation of acid-expressed genes by PACC and their contribution to C. gloeosporioides pathogenicity is not well understood. RNA sequencing data analysis was employed to demonstrate that PACC transcription factor binding sites (TFBS) are significantly overrepresented in the promoter of PACC-upregulated, alkaline-expressed genes. In contrast, they are not overrepresented in the PACC-downregulated, acid-expressed genes. Instead, acid-expressed genes showed overrepresentation of AREB GATA TFBS in C. gloeosporioides and in homologs of five other ascomycetes genomes. The areB promoter contains PACC TFBS; its transcript was upregulated at pH 7 and repressed in ΔpacC. Furthermore, acid-expressed genes were found to be constitutively upregulated in ΔareB during alkalizing conditions. The areB mutants showed significantly reduced ammonia secretion and pathogenicity on tomato fruit. Present results indicate that PACC activates areB expression, thereby conditionally repressing acid-expressed genes and contributing critically to C. gloeosporioides pathogenicity.

Global aspects of pacC regulation of pathogenicity genes in Colletotrichum gloeosporioides as revealed by transcriptome analysis

Colletotrichum gloeosporioides alkalinizes its surroundings during colonization of host tissue. The transcription factor pacC is a regulator of pH-controlled genes and is essential for successful colonization. We present here the sequence assembly of the Colletotrichum fruit pathogen and use it to explore the global regulation of pathogenicity by ambient pH. The assembled genome size was 54 Mb, encoding 18,456 genes. Transcriptomes of the wild type and ΔpacC mutant were established by RNA-seq and explored for their global pH-dependent gene regulation. The analysis showed that pacC upregulates 478 genes and downregulates 483 genes, comprising 5% of the fungal genome, including transporters, antioxidants, and cell-wall-degrading enzymes. Interestingly, gene families with similar functionality are both up- and downregulated by pacC. Global analysis of secreted genes showed significant pacC activation of degradative enzymes at alkaline pH and during fruit infection. Select genes from alkalizing-type pathogen C. gloeosporioides and from acidifying-type pathogen Sclerotinia sclerotiorum were verified by quantitative reverse-transcription polymerase chain reaction analysis at different pH values. Knock out of several pacC-activated genes confirmed their involvement in pathogenic colonization of alkalinized surroundings. The results suggest a global regulation by pacC of key pathogenicity genes during pH change in alkalinizing and acidifying pathogens.

The pH signaling transcription factor PacC is required for full virulence in Penicillium digitatum

Penicillium digitatum is the most important postharvest pathogen of citrus fruits. Along disease progression, the infected citrus peel tissue is acidified due to the accumulation of organic acids. So far, relatively little is known about the environmental factors that regulate pathogenicity in this fungus. In this study, the role of the pH signaling transcription factor PacC in the pathogenesis of P. digitatum was investigated. We identified the pacC ortholog (PdpacC) in P. digitatum and found that its transcript levels were elevated under alkaline conditions (pH ≥ 7) in vitro, as well as during the infection of citrus fruits in spite of the low pH (about 3.0 to 3.5) of the macerated tissue. Na(+) and pectin also induced the expression of PdpacC. Disruption of PdpacC resulted in impaired mycelial growth under neutral or alkaline pH conditions and on synthetic medium supplemented with pectin as the sole carbon source, and attenuated virulence towards citrus fruits. Introducing the full length of PdpacC into the ΔPdpacC mutant restored all these phenotypes. The expression of the polygalacturonase gene Pdpg2 and pectin lyase gene Pdpnl1 in P. digitatum was upregulated in the wild type strain but not or weakly upregulated in the ΔPdpacC mutant during infection. Disruption of Pdpg2 also resulted in attenuated virulence of P. digitatum towards citrus fruits. Collectively, we conclude that PdPacC plays an important role in pathogenesis of P. digitatum via regulation of the expression of cell wall degradation enzyme genes, such as Pdpg2 and Pdpnl1.

Similar and distinct roles of NADPH oxidase components in the tangerine pathotype of Alternaria alternata

The fungal nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) complex, which has been implicated in the production of low-level reactive oxygen species (ROS), contains mainly NoxA, NoxB (gp91(phox) homologues) and NoxR (p67(phox) homologue). Here, we report the developmental and pathological functions of NoxB and NoxR in the tangerine pathotype of Alternaria alternata. Loss-of-function genetics revealed that all three Nox components are required for the accumulation of cellular hydrogen peroxide (H₂O₂). Alternaria alternata strains lacking NoxA, NoxB or NoxR also displayed an increased sensitivity to H₂O₂ and many ROS-generating oxidants. These phenotypes are highly similar to those previously seen for the Δyap1 mutant lacking a YAP1 transcriptional regulator and for the Δhog1 mutant lacking a HOG1 mitogen-activated protein (MAP) kinase, implicating a possible link among them. A fungal strain carrying a NoxA NoxB or NoxA NoxR double mutation was more sensitive to the test compounds than the strain mutated at a single gene, implicating a synergistic function among Nox components. The ΔnoxB mutant strain failed to produce any conidia; both ΔnoxA and ΔnoxR mutant strains showed a severe reduction in sporulation. Mutant strains carrying defective NoxB had higher chitin content than the wild-type and were insensitive to calcofluor white, Congo red and the fungicides vinclozolin and fludioxonil. Virulence assays revealed that all three Nox components are required for the elaboration of the penetration process. The inability to penetrate the citrus host, observed for Δnox mutants, could be overcome by wounding and by reacquiring a dominant Nox gene. The A. alternata NoxR did not influence the expression of NoxB, but negatively regulated NoxA. Importantly, the expression of both YAP1 and HOG1 genes, whose products are involved in resistance to ROS, was down-regulated in fungi carrying defective NoxA, NoxB or NoxR. Our results highlight the requirement of Nox in ROS resistance and provide insights into its critical role in regulating both YAP1 and HOG1 in A. alternata.

Adaptation to pH and role of PacC in the rice blast fungus Magnaporthe oryzae

Fungi are known to adapt to pH partly via specific activation of the Pal signaling pathway and subsequent gene regulation through the transcription factor PacC. The role of PacC in pathogenic fungi has been explored in few species, and each time its partaking in virulence has been found. We studied the impact of pH and the role of PacC in the biology of the rice pathogen Magnaporthe oryzae. Conidia formation and germination were affected by pH whereas fungal growth and appressorium formation were not. Growth in vitro and in planta was characterized by alkalinization and ammonia accumulation in the surrounding medium. Expression of the MoPACC gene increased when the fungus was placed under alkaline conditions. Except for MoPALF, expression of the MoPAL genes encoding the pH-signaling components was not influenced by pH. Deletion of PACC caused a progressive loss in growth rate from pH 5 to pH 8, a loss in conidia production at pH 8 in vitro, a loss in regulation of the MoPALF gene, a decreased production of secreted lytic enzymes and a partial loss in virulence towards barley and rice. PacC therefore plays a significant role in M. oryzae’s biology, and pH is revealed as one component at work during interaction between the fungus and its host plants.

Modelling the activation of alkaline pH response transcription factor PacC in Aspergillus nidulans: involvement of a negative feedback loop

Alkaline pH adaptation represents an important environmental stress response in Aspergillus nidulans. It is mediated by the pal signalling pathway and the PacC transcription factor. Although studied extensively experimentally, the activation mechanism of PacC has not been quantified, and it is not clear how this activation is regulated. Here, by constructing mathematical models, we first show that the pattern of PacC activation observed in previously published experiments cannot be explained based on existing knowledge about PacC activation. Extending the model with a negative feedback loop is necessary to produce simulation results that are consistent with the data, suggesting the existence of a negative feedback loop in the PacC activation process. This extended model is then validated against published measurements for cells with drug treatment and mutant cells. Furthermore, we investigate the role of an intermediate form of PacC in the PacC activation process, and propose experiments that can be used to test our predictions. Our work illustrates how mathematical models can be used to uncover regulatory mechanisms in the transcription regulation, and generate hypotheses that guide further laboratory investigations.

Identifying pathogenicity genes in the rubber tree anthracnose fungus Colletotrichum gloeosporioides through random insertional mutagenesis

To gain more insight into the molecular mechanisms of Colletotrichum gloeosporioides pathogenesis, Agrobacterium tumefaciens-mediated transformation (ATMT) was used to identify mutants of C. gloeosporioides impaired in pathogenicity. An ATMT library of 4128 C. gloeosporioides transformants was generated. Transformants were screened for defects in pathogenicity with a detached copper brown leaf assay. 32 mutants showing reproducible pathogenicity defects were obtained. Southern blot analysis showed 60.4% of the transformants had single-site T-DNA integrations. 16 Genomic sequences flanking T-DNA were recovered from mutants by thermal asymmetric interlaced PCR, and were used to isolate the tagged genes from the genome sequence of wild-type C. gloeosporioides by Basic Local Alignment Search Tool searches against the local genome database of the wild-type C. gloeosporioides. One potential pathogenicity genes encoded calcium-translocating P-type ATPase. Six potential pathogenicity genes had no known homologs in filamentous fungi and were likely to be novel fungal virulence factors. Two putative genes encoded Glycosyltransferase family 28 domain-containing protein and Mov34/MPN/PAD-1 family protein, respectively. Five potential pathogenicity genes had putative function matched with putative protein of other Colletotrichum species. Two known C. gloeosporioides pathogenicity genes were also identified, the encoding Glomerella cingulata hard-surface induced protein and C. gloeosporioides regulatory subunit of protein kinase A gene involved in cAMP-dependent PKA signal transduction pathway.

PacC and pH-dependent transcriptome of the mycotrophic fungus Trichoderma virens

BACKGROUND

In fungi, environmental pH is an important signal for development, and successful host colonization depends on homeostasis. Surprisingly, little is known regarding the role of pH in fungal-fungal interactions. Species of Trichoderma grow as soil saprobes but many are primarily mycotrophic, using other fungi as hosts. Therefore, Trichoderma spp. are studied for their potential in biocontrol of plant diseases. Particularly in alkaline soil, pH is a critical limiting factor for these biofungicides, whose optimal growth pH is 4-6. Gaining an understanding of pH adaptability is an important step in broadening the activity spectrum of these economically important fungi.

RESULTS

We studied the pH-responsive transcription factor PacC by gene knockout and by introduction of a constitutively active allele (pacCc). ΔpacC mutants exhibited reduced growth at alkaline pH, while pacCc strains grew poorly at acidic pH. In plate confrontation assays ΔpacC mutants showed decreased ability to compete with the plant pathogens Rhizoctonia solani and Sclerotium rolfsii. The pacCc strain exhibited an overgrowth of R. solani that was comparable to the wild type, but was unable to overgrow S. rolfsii. To identify genes whose expression is dependent on pH and pacC, we designed oligonucleotide microarrays from the transcript models of the T. virens genome, and compared the transcriptomes of wild type and mutant cultures exposed to high or low pH. Transcript levels from several functional classes were dependent on pacC, on pH, or on both. Furthermore, the expression of a set of pacC-dependent genes was increased in the constitutively-active pacCc strain, and was pH-independent in some, but not all cases.

CONCLUSIONS

PacC is important for biocontrol-related antagonism of other fungi by T. virens. As much as 5% of the transcriptome is pH-dependent, and of these genes, some 25% depend on pacC. Secondary metabolite biosynthesis and ion transport are among the relevant gene classes. We suggest that ΔpacC mutants may have lost their full biocontrol potential due to their inability to adapt to alkaline pH, to perceive ambient pH, or both. The results raise the novel possibility of genetically manipulating Trichoderma in order to improve adaptability and biocontrol at alkaline pH.

Quiescent and necrotrophic lifestyle choice during postharvest disease development

Insidious fungal infections by postharvest pathogens remain quiescent during fruit growth until, at a particular phase during fruit ripening and senescence, the pathogens switch to the necrotrophic lifestyle and cause decay. During ripening, fruits undergo physiological processes, such as activation of ethylene biosynthesis, cuticular changes, and cell-wall loosening-changes that are accompanied by a decline of antifungal compounds, both those that are preformed and those that are inducible secondary metabolites. Pathogen infection of the unripe host fruit initiates defensive signal-transduction cascades, culminating in accumulation of antifungal proteins that limit fungal growth and development. In contrast, development of the same pathogens during fruit ripening and storage activates a substantially different signaling network, one that facilitates aggressive fungal colonization. This review focuses on responses induced by the quiescent pathogens of postharvest diseases in unripe host fruits. New genome-scale experimental approaches have begun to delineate the complex and multiple networks of host and pathogen responses activated to maintain or to facilitate the transition from the quiescent to the necrotrophic lifestyle.

Transcription factor Amr1 induces melanin biosynthesis and suppresses virulence in Alternaria brassicicola

Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. Several A. brassicicola genes have been characterized as affecting pathogenesis of Brassica species. To study regulatory mechanisms of pathogenesis, we mined 421 genes in silico encoding putative transcription factors in a machine-annotated, draft genome sequence of A. brassicicola. In this study, targeted gene disruption mutants for 117 of the transcription factor genes were produced and screened. Three of these genes were associated with pathogenesis. Disruption mutants of one gene (AbPacC) were nonpathogenic and another gene (AbVf8) caused lesions less than half the diameter of wild-type lesions. Unexpectedly, mutants of the third gene, Amr1, caused lesions with a two-fold larger diameter than the wild type and complementation mutants. Amr1 is a homolog of Cmr1, a transcription factor that regulates melanin biosynthesis in several fungi. We created gene deletion mutants of Δamr1 and characterized their phenotypes. The Δamr1 mutants used pectin as a carbon source more efficiently than the wild type, were melanin-deficient, and more sensitive to UV light and glucanase digestion. The AMR1 protein was localized in the nuclei of hyphae and in highly melanized conidia during the late stage of plant pathogenesis. RNA-seq analysis revealed that three genes in the melanin biosynthesis pathway, along with the deleted Amr1 gene, were expressed at low levels in the mutants. In contrast, many hydrolytic enzyme-coding genes were expressed at higher levels in the mutants than in the wild type during pathogenesis. The results of this study suggested that a gene important for survival in nature negatively affected virulence, probably by a less efficient use of plant cell-wall materials. We speculate that the functions of the Amr1 gene are important to the success of A. brassicicola as a competitive saprophyte and plant parasite.

The molecular pathogenicity of Fusarium keratitis: a fungal transcriptional regulator promotes hyphal penetration of the cornea

PURPOSE

The pathogenic mechanisms of fungal infection during human keratomycosis were investigated in an ex vivo corneal model that used strains of Fusarium oxysporum differing in the production of a fungal transcription factor.

METHODS

A pacC loss-of-function mutant and a pacC dominant-activating mutant were constructed from a wild-type isolate of F. oxysporum, and the 3 strains were characterized by in vitro growth kinetics. Twenty-seven human donor corneas maintained in tissue culture were superficially scarified and topically inoculated with the wild-type, the pacC loss-of-function mutant, or the pacC dominant-activating strains. Relative hyphal invasion into the stroma was compared histopathologically in corneal sections.

RESULTS

F. oxysporum strains demonstrated comparable exponential growth rates in vitro. Wild-type F. oxysporum invaded into the corneal tissue within 1 day and penetrated through the anterior stroma during the next 4 days. The pacC loss-of-function mutant invaded explanted corneas significantly less than the wild-type strain on day 1 (P < 0.0001) and on day 3 (P = 0.0003). The pacC dominant-activating strain adhered and penetrated explanted corneas similar to the wild-type strain.

CONCLUSIONS

The PacC pathway regulating the transcription of fungal genes allows fungal adaptation to the ocular surface and enables invasion of the injured cornea by F. oxysporum.

The pH regulatory factor Pac1 regulates Tri gene expression and trichothecene production in Fusarium graminearum

Fungi manage the adaptation to extra-cellular pH through the PacC transcription factor, a key component of the pH regulatory system. PacC regulates the production of various secondary metabolites in filamentous fungi. In the important cereal pathogen Fusarium graminearum, the production of trichothecene is induced only under acidic pH conditions. Here, we examined the role of the PacC homologue from F. graminearum, FgPac1, on the regulation of trichothecene production. An FgΔPac1 deletion mutant was constructed in F. graminearum which showed a reduced development under neutral and alkaline pH, increased sensitivity to H(2)O(2) and an earlier Tri gene induction and toxin accumulation at acidic pH. A strain expressing the FgPac1(c) constitutively active form of Pac1 exhibited a strongly repressed Tri gene expression and reduced toxin accumulation at acidic pH. These results demonstrate that Pac1 negatively regulates Tri gene expression and toxin production in F. graminearum.

PacC in the nematophagous fungus Clonostachys rosea controls virulence to nematodes

Nematophagous fungi are commonly used as biological control agents of plant and animal parasitic nematodes. However, relatively little is known of the environmental attributes conferring pathogenicity in these fungi. In this report, we investigated the role of PacC-mediated pH response in the pathogenesis of the nematophagous fungus Clonostachys rosea. We identified a pacC orthologue from this fungus and found that its transcript was elevated in C. rosea during the early stage of its infection of nematode. Disruption of pacC resulted in slowed growth at alkaline pH, altered filamentation, reduced conidiation and attenuated virulence to nematodes. The expression of an extracellular serine protease PrC, a putative virulence factor, was downregulated in the pacC mutants. The PrC transcript levels were significantly higher under alkaline growth conditions than under acidic growth conditions. Promoter activity analysis and electrophoretic mobility shift assay indicated that the regulation of PrC by pH via the PacC pathway occurred at the transcriptional level. In conclusion, PacC functions as a positive regulator of virulence to nematodes in C. rosea.

The FUS3 MAPK signaling pathway of the citrus pathogen Alternaria alternata functions independently or cooperatively with the fungal redox-responsive AP1 regulator for diverse developmental, physiological and pathogenic processes

Alternaria alternata, the fungus that causes citrus brown spot, invades its hosts primarily through the production and action of a host-selective ACT toxin that kills citrus cells prior to invasion. In this study, we show that, in the tangerine pathotype of A. alternata, a mitogen-activated protein kinase (MAPK)-mediated signaling pathway governs a number of biological functions, either separately or in a cooperative manner, with the AaAP1 gene encoding a transcription regulator. The reported MAPK is encoded by the AaFUS3 gene, which we show to be necessary for conidial development, resistance to copper fungicides, melanin biosynthesis, and particularly, for elaboration of the penetration process. In contrast, AaFUS3 negatively controls salt tolerance and production of several hydrolytic enzymes. AaFUS3 has no apparent role in the biosynthesis of host-selective toxin or in resistance to oxidative stress. Both AaAP1 and AaFUS3 are required for fungal resistance to 2,3,5-triiodobenzoic acid (TIBA), 2-chloro-5-hydroxypyridine (CHP), diethyl maleate (DEM), and many pyridine-containing compounds. A strain with mutations in both AaAP1 and AaFUS3 displayed an increased sensitivity to these compounds. Expression of the AaAP1 and AaFUS3 genes and phosphorylation of AaFUS3 were also induced by CHP, DEM, or TIBA. Expression of two genes coding for a putative MFS transporter was coordinately regulated by AaAP1 and AaFUS3. The AaAP1::sGFP (synthetic green fluorescent protein) fusion protein became localized in the nucleus in response to CHP or TIBA. Inactivation of the AaAP1 gene, however, promoted phosphorylation of AaFUS3. Taken together, our results indicate that A. alternata utilizes specialized or synergistic regulatory interactions between the AP1 and MAPK signaling pathways for diverse physiological functions.

Transcriptional regulation of elsinochrome phytotoxin biosynthesis by an EfSTE12 activator in the citrus scab pathogen Elsinoë fawcettii

Elsinochrome (ESC), produced by the citrus pathogen Elsinoë fawcettii, is a nonhost-selective, light-dependent, polyketide-derived phytotoxin and plays a crucial role for full virulence. The biosynthesis of ESC is regulated by a wide array of environmental stimuli and is primarily governed by the pathway-specific TSF1 transcription regulator whose coding gene is clustered with the EfPKS1 gene encoding a polyketide synthase and other biosynthetic genes in the genome. In this report, an EfSTE12 gene, encoding a polypeptide resembling the yeast STE12 transcription factor, was cloned and characterized to play a role, independent of TSF1, for ESC production in E. fawcettii. The loss-of-function mutant, specifically disrupted at the EfSTE12 locus, displays reduced ESC accumulation, elevated activities for pectinase and proteolytic enzymes but unaltered in conidiation and fungal pathogenicity. Impairment of the EfSTE12 gene decreased the abundance of the EfPKS1 but not the TSF1 gene transcript. In contrast, expression of the EfSTE12 gene appears normal in the EfPKS1 or TSF1 disruptants. The results indicate that EfSTE12 is functioning for ESC biosynthesis by directly activating the biosynthetic genes without regulating the pathway-specific TSF1 regulator. The defective phenotypes were fully reverted when a functional copy of EfSTE12 was re-introduced into the disrupted mutant. A hypothetical model underlying intertwined regulatory pathways via TSF1, EfSTE12, and other potent transcriptional activators led to the ESC biosynthesis and conidiation is described.

Carboxylate transporter gene JEN1 from the entomopathogenic fungus Beauveria bassiana is involved in conidiation and virulence

Beauveria bassiana is an important entomopathogenic fungus widely used as a biological agent to control insect pests. A gene (B. bassiana JEN1 [BbJEN1]) homologous to JEN1 encoding a carboxylate transporter in Saccharomyces cerevisiae was identified in a B. bassiana transfer DNA (T-DNA) insertional mutant. Disruption of the gene decreased the carboxylate contents in hyphae, while increasing the conidial yield. However, overexpression of this transporter resulted in significant increases in carboxylates and decreased the conidial yield. BbJEN1 was strongly induced by insect cuticles and highly expressed in the hyphae penetrating insect cuticles not in hyphal bodies, suggesting that this gene is involved in the early stage of pathogenesis of B. bassiana. The bioassay results indicated that disruption of BbJEN1 significantly reduced the virulence of B. bassiana to aphids. Compared to the wild type, DeltaBbJEN1 alkalinized the insect cuticle to a reduced extent. The alkalinization of the cuticle is a physiological signal triggering the production of pathogenicity. Therefore, we identified a new factor influencing virulence, which is responsible for the alkalinization of the insect cuticle and the initiation of fungal pathogenesis in insects.

A fungal pH-responsive signaling pathway regulating Aspergillus adaptation and invasion into the cornea

PURPOSE

To investigate the role of PalB and PacC, two components of a pH-responsive signal-transduction pathway of Aspergillus nidulans, during the pathogenesis of fungal infection of the cornea.

METHODS

Fungal strains included an A. nidulans wild-type isolate (A83), loss-of-function A. nidulans mutants of the palB (B7) or pacC (C6309) genes, and reconstituted genotypic strains (B7R and C6309R). Doubling times and radial growth rates were examined under neutral and acidic conditions. Corneal virulence was assessed ex vivo by topical inoculation of scarified porcine or human corneas with A. nidulans strains maintained in buffered medium until histologic examination after days 1, 3, and 5.

RESULTS

In vitro growth kinetics were similar for A. nidulans strains in liquid medium at pH 6.0 (P = 0.24) and 7.3 (P = 0.75). The pacC mutant C6309 grew more slowly (P < 0.001) on solid medium, whereas palB and pacC rescuants had growth kinetics comparable to those of the wild-type. Wild-type A. nidulans germinated on porcine corneas and produced hyphae that progressively invaded the stroma, reaching an average maximum penetration of 56% +/- 9% at 5 days after exposure. In contrast, hyphal invasion was significantly less by mutant strains B7 (P = 0.005) and C6309 (P = 0.003). Fungal penetration by C6309 was also significantly less than the wild-type (P = 0.0005) on explanted human corneas. Both fungal rescuants showed stromal invasion similar to the wild-type.

CONCLUSIONS

Corneal invasion by filamentous hyphae is attenuated by palB and pacC mutant strains of A. nidulans. The PacC pathway is involved in regulating fungal filamentation during ex vivo Aspergillus infection of the cornea.

Candida albicans transcription factor Rim101 mediates pathogenic interactions through cell wall functions

pH-responsive transcription factors of the Rim101/PacC family govern virulence in many fungal pathogens. These family members control expression of target genes with diverse functions in growth, morphology and environmental adaptation, so the mechanistic relationship between Rim101/PacC and infection is unclear. We have focused on Rim101 from Candida albicans, which we find to be required for virulence in an oropharyngeal candidiasis model. Rim101 affects the yeast-hypha morphological transition, a major virulence requirement in disseminated infection models. However, virulence in the oropharyngeal candidiasis model is independent of the yeast-hypha transition because it is unaffected by an nrg1 mutation, which prevents formation of yeast cells. Here we have identified Rim101 target genes in an nrg1Delta/Delta mutant background and surveyed function using an overexpression-rescue approach. Increased expression of Rim101 target genes ALS3, CHT2, PGA7/RBT6, SKN1 or ZRT1 can partially restore pathogenic interaction of a rim101Delta/Delta mutant with oral epithelial cells. Four of these five genes govern cell wall structure. Our results indicate that Rim101-dependent cell wall alteration contributes to C. albicans pathogenic interactions with oral epithelial cells, independently of cell morphology.