A GAL4-like protein is involved in the switch between biotrophic and necrotrophic phases of the infection process of Colletotrichum lindemuthianum on common bean

Identifying pathogenicity-related genes in the pathogen Colletotrichum magnum causing watermelon anthracnose disease via T-DNA insertion mutagenesis

Fruit rot caused by Colletotrichum magnum is a crucial watermelon disease threatening the production and quality. To understand the pathogenic mechanism of C. magnum, we optimized the Agrobacterium tumefaciens-mediated transformation system (ATMT) for genetic transformation of C. magnum. The transformation efficiency of ATMT was an average of around 245 transformants per 100 million conidia. Southern blot analysis indicated that approximately 75% of the mutants contained a single copy of T-DNA. Pathogenicity test revealed that three mutants completely lost pathogenicity. The T-DNA integration sites (TISs) of three mutants were Identified. In mutant Cm699, the TISs were found in the intron region of the gene, which encoded a protein containing AP-2 complex subunit σ, and simultaneous gene deletions were observed. Two deleted genes encoded the transcription initiation protein SPT3 and a hypothetical protein, respectively. In mutant Cm854, the TISs were found in the 5'-flanking regions of a gene that was similar to the MYO5 encoding Myosin I of Pyricularia oryzae (78%). In mutant Cm1078, the T-DNA was integrated into the exon regions of two adjacent genes. One was 5'-3' exoribonuclease 1 encoding gene while the other encoded a WD-repeat protein retinoblastoma binding protein 4, the homolog of the MSl1 of Saccharomyces cerevisiae.

Plant-Fungi Interactions: Where It Goes?

Fungi live different lifestyles-including pathogenic and symbiotic-by interacting with living plants. Recently, there has been a substantial increase in the study of phytopathogenic fungi and their interactions with plants. Symbiotic relationships with plants appear to be lagging behind, although progressive. Phytopathogenic fungi cause diseases in plants and put pressure on survival. Plants fight back against such pathogens through complicated self-defense mechanisms. However, phytopathogenic fungi develop virulent responses to overcome plant defense reactions, thus continuing their deteriorative impacts. Symbiotic relationships positively influence both plants and fungi. More interestingly, they also help plants protect themselves from pathogens. In light of the nonstop discovery of novel fungi and their strains, it is imperative to pay more attention to plant-fungi interactions. Both plants and fungi are responsive to environmental changes, therefore construction of their interaction effects has emerged as a new field of study. In this review, we first attempt to highlight the evolutionary aspect of plant-fungi interactions, then the mechanism of plants to avoid the negative impact of pathogenic fungi, and fungal strategies to overcome the plant defensive responses once they have been invaded, and finally the changes of such interactions under the different environmental conditions.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspects global transcription factor profiles (TFomes) and their potential roles in coordinating CC and AC functions to accomplish host-specific interactions. Remarkably, we found a clear positive correlation between the sizes of TFomes and the proteomes of an organism. With the acquisition of ACs, the FOSC TFomes were larger than the other fungal genomes included in this study. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls were highly conserved. Among the 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 were most significantly expanded to 671 and 167 genes per family including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) that are involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3% including a disordered protein Ren1. RNA-Seq revealed a steady pattern of expression for conserved TF families and specific activation for AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspected global transcription factor profiles (TFomes) and their potential roles in coordinating CCs and ACs functions to accomplish host-specific pathogenicity. Remarkably, we found a clear positive correlation between the sizes of TFome and proteome of an organism, and FOSC TFomes are larger due to the acquisition of ACs. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls are highly conserved. Among 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 are most significantly expanded to 671 and 167 genes per family, including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3%, including a disordered protein Ren1. Expression profiles revealed a steady expression of conserved TF families and specific activation of AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

Genome-Wide Transcriptomic Analysis of the Effects of Infection with the Hemibiotrophic Fungus Colletotrichum lindemuthianum on Common Bean

Bean anthracnose caused by the hemibiotrophic fungus Colletotrichum lindemuthianum is one of the most important diseases of common bean (Phaseolus vulgaris) in the world. In the present study, the whole transcriptome of common bean infected with C. lindemuthianum during compatible and incompatible interactions was characterized at 48 and 72 hpi, corresponding to the biotrophy phase of the infection cycle. Our results highlight the prominent role of pathogenesis-related (PR) genes from the PR10/Bet vI family as well as a complex interplay of different plant hormone pathways including Ethylene, Salicylic acid (SA) and Jasmonic acid pathways. Gene Ontology enrichment analysis reveals that infected common bean seedlings responded by down-regulation of photosynthesis, ubiquitination-mediated proteolysis and cell wall modifications. In infected common bean, SA biosynthesis seems to be based on the PAL pathway instead of the ICS pathway, contrarily to what is described in Arabidopsis. Interestingly, ~30 NLR were up-regulated in both contexts. Overall, our results suggest that the difference between the compatible and incompatible reaction is more a question of timing and strength, than a massive difference in differentially expressed genes between these two contexts. Finally, we used RT-qPCR to validate the expression patterns of several genes, and the results showed an excellent agreement with deep sequencing.

Phaseolus vulgaris-Colletotrichum lindemuthianum Pathosystem in the Post-Genomic Era: An Update

Phaseolus vulgaris-Colletotrichum lindemuthianum is one among the oldest host and pathogen interface. Researchers have taken painstaking efforts across the world for understanding the dialogue during early and late phases of interaction. Collectively, these efforts resulted in the deluge of information that helped the researchers to underpin the interface. The latest molecular biology techniques furnished novel detection methods for the anthracnose pathogen, refined the understanding of pathogen population dynamics, and provided the insights on co-evolutionary common bean resistance and C. lindemuthianum virulence dynamics. One of the important breakthroughs came when the Phaseolus vulgaris and its corresponding anthracnose pathogen (C. lindemuthianum) genomes were decoded in 2014 and 2017, respectively. Availability of both the genomes yielded a significant genomic information that helped bean communities to fine map the economically important traits and to identify the pathogenicity determinants and effector molecules. The interface is in a continuous development as knowledge of the anthracnose resistance genes, their precise physical locations, and the identification of effector proteins; the fungus arsenals are being routinely updated. Hence, we revisited the interface and tried to provide an overview of host pathogen dialogue in the genomic era. Additionally, we compiled the sporadic information on this pathosystem from India and provided its futuristic road map to shape its research in the world and northern India, the major dry bean area in the country.

Optimizing the Agrobacterium tumifaciens mediated transformation conditions in Colletotrichum lindemuthianum: A step forward to unravel the functions of pathogenicity arsenals

Colletotrichum lindemuthianum is a hemibiotrophic fungal pathogen that causes bean anthracnose and it is rated among the top 10 important diseases infecting beans. Currently our knowledge on molecular mechanisms underlying C. lindemuthianum pathogenesis is limited. About five pathogenicity genes have been identified in C. lindemuthianum using Restricted Enzyme Mediated Integration (REMI) and the transformation using Agroinfection has not been optimized. In this study, a series of experiments were conducted to optimize the key parameters affecting the Agrobacterium tumefaciens- mediated transformation (ATMT) for C. lindemuthianum. The transformation efficiency increased with increase in spore concentration and co-cultivation time. However, the optimum conditions that yielded significant number of transformants were 10⁶ ml^-1 spore concentration, co-cultivation time of 72 h, incubation at 25ºC and using a cellulose membrane filter for the co-cultivation. The optimized protocol resulted in establishment of large mutant library (2400). A few mutants were melanin deficient and a few were unable to produce conidia. To determine the altered pathogenicity, two new approaches such as detached leaf and twig techniques proved reliable and require fewer resources to screen the large mutant libraries in a short time. Among the 1200 transformants tested for virulence, 90% transformants were pathogenically similar to wild type (race 2047), 96 and 24 were reduced and impaired, respectively. The altered avirulent transformants can prove vital for understanding the missing link between growth and developmental stages of pathogen with virulence. This platform will help to develop strategies to determine the potential pathogenicity genes and to decipher molecular mechanisms of host-pathogen interactions in more detail.

A novel transcription factor negatively regulates antioxidant response, cell wall integrity and virulence in the fungal insect pathogen, Beauveria bassiana

Genomic data have identified a class of fungal specific transcription factors (FsTFs) that are thought to regulate unique aspects of fungal gene expression, although the functions of many of these proteins remain unknown. Here, a novel FsTF (BbStf1), which features a leucine zipper dimerization domain and a fungal transcription factor regulatory middle homology region, was characterized in Beauveria bassiana, a filamentous insect fungal pathogen. Transcriptional activation and nuclear localization were experimentally confirmed for BbStf1. Disruption of Bbstf1 resulted in increased tolerance to oxidative stress and cell wall perturbation, accompanied by increased peroxidase (POD) and superoxide dismutase (SOD) activities and ratio of reduced/oxidized glutathione (GSH/GSSG), and by thickened cell wall and altered composition. Gene expression profile analysis revealed that transcription patterns of antioxidant enzyme and cell wall integrity-involved genes were altered in the ∆Bbstf1, including some BbStf1-targeted genes clarified with evidence. The ∆Bbstf1 strain displayed greater virulence to Galleria mellonella in the bioassays through both topical infection and intrahaemocoel injection due to more rapid proliferation in the haemocoel as compared to the wild-type strain. Altogether, BbStf1 acts as a negative regulator of antioxidant response, cell wall integrity and virulence in B. bassiana.

A novel effector CfEC92 of Colletotrichum fructicola contributes to glomerella leaf spot virulence by suppressing plant defences at the early infection phase

The ascomycete fungus Colletotrichum fructicola causes diseases on a broad range of plant species. On susceptible cultivars of apple, it induces severe early defoliation and fruit spots, named glomerella leaf spot (GLS), but the mechanisms of pathogenicity have remained elusive. Phytopathogens exhibit small secreted effectors to advance host infection by manipulating host immune reactions. We report the identification and characterization of CfEC92, an effector required for C. fructicola virulence. CfEC92 is a Colletotrichum-specific small secreted protein that suppresses BAX-triggered cell death in Nicotiana benthamiana. Accumulation of the gene transcript was barely detectable in conidia or vegetative hyphae, but was highly up-regulated in appressoria formed during early apple leaf infection. Gene deletion mutants were not affected in vegetative growth, appressorium formation, or appressorium-mediated cellophane penetration. However, the mutants were significantly reduced in virulence toward apple leaves and fruits. Microscopic examination indicated that infection by the deletion mutants elicited elevated deposition of papillae at the penetration sites, and formation of infection vesicles and primary hyphae was retarded. Signal peptide activity, subcellular localization, and cell death-suppressive activity (without signal peptide) assays suggest that CfEC92 could be secreted and perform virulence functions inside plant cells. RNA sequencing and quantitative reverse transcription PCR results confirmed that the deletion mutants triggered elevated host defence reactions. Our results strongly support the interpretation that CfEC92 contributes to C. fructicola virulence as a plant immunity suppressor at the early infection phase.

The histidine kinase slnCl1 of Colletotrichum lindemuthianum as a pathogenicity factor against Phaseolus vulgaris L

Colletotrichum lindemuthianum, the causal agent of anthracnose, is responsible for significant damage in the common bean (Phaseolus vulgaris L.). Unraveling the genetic mechanisms involved in the plant/pathogen interaction is a powerful approach for devising efficient methods to control this disease. In the present study, we employed the Restriction Enzyme-Mediated Integration (REMI) methodology to identify the gene slnCl1, encoding a histidine kinase protein, as involved in pathogenicity. The mutant strain, MutCl1, generated by REMI, showed an insertion in the slnCl1 gene, deficiency of the production and melanization of appressoria, as well as the absence of pathogenicity on bean leaves when compared with the wild-type strain. The slnCl1 gene encodes a histidine kinase class IV called SlnCl1 showing identity of 97% and 83% with histidine kinases from Colletotrichum orbiculare and Colletotrichum gloesporioides, respectively. RNA interference was used for silencing the histidine kinase gene and confirm slnCl1 as a pathogenicity factor. Furthermore, we identified four major genes involved in the RNA interference-mediated gene silencing in Colletotrichum spp. and demonstrated the functionality of this process in C. lindemuthianum. Silencing of the EGFP reporter gene and slnCl1 were demonstrated using qPCR. This work reports for the first time the isolation and characterization of a HK in C. lindemuthianum and the occurrence of gene silencing mediated by RNA interference in this organism, demonstrating its potential use in the functional characterization of pathogenicity genes.

The Colletotrichum acutatum Species Complex as a Model System to Study Evolution and Host Specialization in Plant Pathogens

Colletotrichum spp. infect a wide diversity of hosts, causing plant diseases on many economically important crops worldwide. The genus contains approximately 189 species organized into at least 11 major phylogenetic lineages, also known as species complexes. The Colletotrichum acutatum species complex is a diverse yet relatively closely related group of plant pathogenic fungi within this genus. Within the species complex we find a wide diversity of important traits such as host range and host preference, mode of reproduction and differences in the strategy used to infect their hosts. Research on fungal comparative genomics have attempted to find correlations in these traits and patterns of gene family evolution but such studies typically compare fungi from different genera or even different fungal Orders. The C. acutatum species complex contains most of this diversity within a group of relatively closely related species. This Perspective article presents a review of the current knowledge on C. acutatum phylogeny, biology, and pathology. It also demonstrates the suitability of C. acutatum for the study of gene family evolution on a fine scale to uncover evolutionary events in the genome that are associated with the evolution of phenotypic characters important for host interactions.

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.

Transcription Factor Repertoire of Necrotrophic Fungal Phytopathogen Ascochyta rabiei: Predominance of MYB Transcription Factors As Potential Regulators of Secretome

Transcription factors (TFs) are the key players in gene expression and their study is highly significant for shedding light on the molecular mechanisms and evolutionary history of organisms. During host-pathogen interaction, extensive reprogramming of gene expression facilitated by TFs is likely to occur in both host and pathogen. To date, the knowledge about TF repertoire in filamentous fungi is in infancy. The necrotrophic fungus Ascochyta rabiei, that causes destructive Ascochyta blight (AB) disease of chickpea (Cicer arietinum), demands more comprehensive study for better understanding of Ascochyta-legume pathosystem. In the present study, we performed the genome-wide identification and analysis of TFs in A. rabiei. Taking advantage of A. rabiei genome sequence, we used a bioinformatic approach to predict the TF repertoire of A. rabiei. For identification and classification of A. rabiei TFs, we designed a comprehensive pipeline using a combination of BLAST and InterProScan software. A total of 381 A. rabiei TFs were predicted and divided into 32 fungal specific families of TFs. The gene structure, domain organization and phylogenetic analysis of abundant families of A. rabiei TFs were also carried out. Comparative study of A. rabiei TFs with that of other necrotrophic, biotrophic, hemibiotrophic, symbiotic, and saprotrophic fungi was performed. It suggested presence of both conserved as well as unique features among them. Moreover, cis-acting elements on promoter sequences of earlier predicted A. rabiei secretome were also identified. With the help of published A. rabiei transcriptome data, the differential expression of TF and secretory protein coding genes was analyzed. Furthermore, comprehensive expression analysis of few selected A. rabiei TFs using quantitative real-time polymerase chain reaction revealed variety of expression patterns during host colonization. These genes were expressed in at least one of the time points tested post infection. Overall, this study illustrates the first genome-wide identification and analysis of TF repertoire of A. rabiei. This work would provide the basis for further studies to dissect role of TFs in the molecular mechanisms during A. rabiei-chickpea interactions.

Transcriptome Profiling of the Phaseolus vulgaris - Colletotrichum lindemuthianum Pathosystem

Bean (Phaseolus vulgaris) anthracnose caused by the hemi-biotrophic pathogen Colletotrichum lindemuthianum is a major factor limiting production worldwide. Although sources of resistance have been identified and characterized, the early molecular events in the host-pathogen interface have not been investigated. In the current study, we conducted a comprehensive transcriptome analysis using Illumina sequencing of two near isogenic lines (NILs) differing for the presence of the Co-1 gene on chromosome Pv01 during a time course following infection with race 73 of C. lindemuthianum. From this, we identified 3,250 significantly differentially expressed genes (DEGs) within and between the NILs over the time course of infection. During the biotrophic phase the majority of DEGs were up regulated in the susceptible NIL, whereas more DEGs were up-regulated in the resistant NIL during the necrotrophic phase. Various defense related genes, such as those encoding PR proteins, peroxidases, lipoxygenases were up regulated in the resistant NIL. Conversely, genes encoding sugar transporters were up-regulated in the susceptible NIL during the later stages of infection. Additionally, numerous transcription factors (TFs) and candidate genes within the vicinity of the Co-1 locus were differentially expressed, suggesting a global reprogramming of gene expression in and around the Co-1 locus. Through this analysis, we reduced the previous number of candidate genes reported at the Co-1 locus from eight to three. These results suggest the dynamic nature of P. vulgaris-C. lindemuthianum interaction at the transcriptomic level and reflect the role of both pathogen and effector triggered immunity on changes in plant gene expression.

Genome and transcriptome analysis of the fungal pathogen Fusarium oxysporum f. sp. cubense causing banana vascular wilt disease

BACKGROUND

The asexual fungus Fusarium oxysporum f. sp. cubense (Foc) causing vascular wilt disease is one of the most devastating pathogens of banana (Musa spp.). To understand the molecular underpinning of pathogenicity in Foc, the genomes and transcriptomes of two Foc isolates were sequenced.

METHODOLOGY/PRINCIPAL FINDINGS

Genome analysis revealed that the genome structures of race 1 and race 4 isolates were highly syntenic with those of F. oxysporum f. sp. lycopersici strain Fol4287. A large number of putative virulence associated genes were identified in both Foc genomes, including genes putatively involved in root attachment, cell degradation, detoxification of toxin, transport, secondary metabolites biosynthesis and signal transductions. Importantly, relative to the Foc race 1 isolate (Foc1), the Foc race 4 isolate (Foc4) has evolved with some expanded gene families of transporters and transcription factors for transport of toxins and nutrients that may facilitate its ability to adapt to host environments and contribute to pathogenicity to banana. Transcriptome analysis disclosed a significant difference in transcriptional responses between Foc1 and Foc4 at 48 h post inoculation to the banana 'Brazil' in comparison with the vegetative growth stage. Of particular note, more virulence-associated genes were up regulated in Foc4 than in Foc1. Several signaling pathways like the mitogen-activated protein kinase Fmk1 mediated invasion growth pathway, the FGA1-mediated G protein signaling pathway and a pathogenicity associated two-component system were activated in Foc4 rather than in Foc1. Together, these differences in gene content and transcription response between Foc1 and Foc4 might account for variation in their virulence during infection of the banana variety 'Brazil'.

CONCLUSIONS/SIGNIFICANCE

Foc genome sequences will facilitate us to identify pathogenicity mechanism involved in the banana vascular wilt disease development. These will thus advance us develop effective methods for managing the banana vascular wilt disease, including improvement of disease resistance in banana.

Overexpression of a novel biotrophy-specific Colletotrichum truncatum effector, CtNUDIX, in hemibiotrophic fungal phytopathogens causes incompatibility with their host plants

The hemibiotrophic fungus Colletotrichum truncatum causes anthracnose disease on lentils and a few other grain legumes. It shows initial symptomless intracellular growth, where colonized host cells remain viable (biotrophy), and then switches to necrotrophic growth, killing the colonized host plant tissues. Here, we report a novel effector gene, CtNUDIX, from C. truncatum that is exclusively expressed during the late biotrophic phase (before the switch to necrotrophy) and elicits a hypersensitive response (HR)-like cell death in tobacco leaves transiently expressing the effector. CtNUDIX homologs, which contain a signal peptide and a Nudix hydrolase domain, may be unique to hemibiotrophic fungal and fungus-like plant pathogens. CtNUDIX lacking a signal peptide or a Nudix motif failed to induce cell death in tobacco. Expression of CtNUDIX:eGFP in tobacco suggested that the fusion protein might act on the host cell plasma membrane. Overexpression of CtNUDIX in C. truncatum and the rice blast pathogen, Magnaporthe oryzae, resulted in incompatibility with the hosts lentil and barley, respectively, by causing an HR-like response in infected host cells associated with the biotrophic invasive hyphae. These results suggest that C. truncatum and possibly M. oryzae elicit cell death to signal the transition from biotrophy to necrotrophy.

Dissecting Phaseolus vulgaris innate immune system against Colletotrichum lindemuthianum infection

Background

The genus Colletotrichum is one of the most economically important plant pathogens, causing anthracnose on a wide range of crops including common beans (Phaseolus vulgaris L.). Crop yield can be dramatically decreased depending on the plant cultivar used and the environmental conditions. This study aimed to identify potential genetic components of the bean immune system to provide environmentally friendly control measures against this fungus.

Methodology and Principal Findings

As the common bean is not amenable to reverse genetics to explore functionality and its genome is not fully curated, we used putative Arabidopsis orthologs of bean expressed sequence tag (EST) to perform bioinformatic analysis and experimental validation of gene expression to identify common bean genes regulated during the incompatible interaction with C. lindemuthianum. Similar to model pathosystems, Gene Ontology (GO) analysis indicated that hormone biosynthesis and signaling in common beans seem to be modulated by fungus infection. For instance, cytokinin and ethylene responses were up-regulated and jasmonic acid, gibberellin, and abscisic acid responses were down-regulated, indicating that these hormones may play a central role in this pathosystem. Importantly, we have identified putative bean gene orthologs of Arabidopsis genes involved in the plant immune system. Based on experimental validation of gene expression, we propose that hypersensitive reaction as part of effector-triggered immunity may operate, at least in part, by down-regulating genes, such as FLS2-like and MKK5-like, putative orthologs of the Arabidopsis genes involved in pathogen perception and downstream signaling.

Conclusions/Significance

We have identified specific bean genes and uncovered metabolic processes and pathways that may be involved in the immune response against pathogens. Our transcriptome database is a rich resource for mining novel defense-related genes, which enabled us to develop a model of the molecular components of the bean innate immune system regulated upon pathogen attack.

The hemibiotrophic cacao pathogen Moniliophthora perniciosa depends on a mitochondrial alternative oxidase for biotrophic development

The tropical pathogen Moniliophthora perniciosa causes witches' broom disease in cacao. As a hemibiotrophic fungus, it initially colonizes the living host tissues (biotrophic phase), and later grows over the dead plant (necrotrophic phase). Little is known about the mechanisms that promote these distinct fungal phases or mediate the transition between them. An alternative oxidase gene (Mp-aox) was identified in the M. perniciosa genome and its expression was analyzed througout the fungal life cycle. In addition, the effects of inhibitors of the cytochrome-dependent respiratory chain (CRC) and alternative oxidase (AOX) were evaluated on the in vitro development of M. perniciosa. Larger numbers of Mp-aox transcripts were observed in the biotrophic hyphae, which accordingly showed elevated sensitivity to AOX inhibitors. More importantly, the inhibition of CRC prevented the transition from the biotrophic to the necrotrophic phase, and the combined use of a CRC and AOX inhibitor completely halted fungal growth. On the basis of these results, a novel mechanism is presented in which AOX plays a role in the biotrophic development of M. perniciosa and regulates the transition to its necrotrophic stage. Strikingly, this model correlates well with the infection strategy of animal pathogens, particularly Trypanosoma brucei, which uses AOX as a strategy for pathogenicity.

Plant defense mechanisms are activated during biotrophic and necrotrophic development of Colletotricum graminicola in maize

Hemibiotrophic plant pathogens first establish a biotrophic interaction with the host plant and later switch to a destructive necrotrophic lifestyle. Studies of biotrophic pathogens have shown that they actively suppress plant defenses after an initial microbe-associated molecular pattern-triggered activation. In contrast, studies of the hemibiotrophs suggest that they do not suppress plant defenses during the biotrophic phase, indicating that while there are similarities between the biotrophic phase of hemibiotrophs and biotrophic pathogens, the two lifestyles are not analogous. We performed transcriptomic, histological, and biochemical studies of the early events during the infection of maize (Zea mays) with Colletotrichum graminicola, a model pathosystem for the study of hemibiotrophy. Time-course experiments revealed that mRNAs of several defense-related genes, reactive oxygen species, and antimicrobial compounds all begin to accumulate early in the infection process and continue to accumulate during the biotrophic stage. We also discovered the production of maize-derived vesicular bodies containing hydrogen peroxide targeting the fungal hyphae. We describe the fungal respiratory burst during host infection, paralleled by superoxide ion production in specific fungal cells during the transition from biotrophy to a necrotrophic lifestyle. We also identified several novel putative fungal effectors and studied their expression during anthracnose development in maize. Our results demonstrate a strong induction of defense mechanisms occurring in maize cells during C. graminicola infection, even during the biotrophic development of the pathogen. We hypothesize that the switch to necrotrophic growth enables the fungus to evade the effects of the plant immune system and allows for full fungal pathogenicity.

Cataloging proteins putatively secreted during the biotrophy-necrotrophy transition of the anthracnose pathogen Colletotrichum truncatum

Hemibiotrophic phytopathogenic fungi cause devastating diseases in agronomically important crops. These fungal pathogens exploit a stealth bi-phasic infection strategy to colonize host plants. Their morphological and nutritional transition from biotrophy (characterized by voluminous intracellular primary hyphae) to necrotrophy (characterized by thin secondary hyphae) known as the biotrophy-necrotrophy switch (hemibiotrophy) is critical in symptom and disease development. To establish successful hemibiotrophic parasitism, pathogens likely secrete suites of proteins at the switch that constitute the biotrophy-necrotrophy switch secretome. To catalogue such proteins, a directional cDNA library was constructed from mRNA isolated from infected Lens culinaris leaflet tissues displaying the switch of Colletotrichum truncatum, and 5000 expressed sequence tags (ESTs) were generated. Four potential groups (hydrolytic enzymes, cell envelope-associated proteins [CEAPs], candidate effectors and proteins with diverse functions) were identified from pathogen-derived ESTs. Expression profiling of transcripts encoding CEAPs and candidate effectors in an infection time-course revealed that the majority of these transcripts were expressed or induced during the necrotrophic phase and repressed during the biotrophic phase of in planta colonization, indicating the massive accumulation of proteins at the switch. Taken together, our data suggest that the hemibiotrophic mode of fungal proliferation entails complex interactions of a pathogen with its host wherein the pathogen requires live host cells prior to switching to the necrotrophic phase. The microbial proteins employed during pathogenesis are likely to have defined roles at specific stages of pathogenesis.

EST mining identifies proteins putatively secreted by the anthracnose pathogen Colletotrichum truncatum

Background

Colletotrichum truncatum is a haploid, hemibiotrophic, ascomycete fungal pathogen that causes anthracnose disease on many economically important leguminous crops. This pathogen exploits sequential biotrophic- and necrotrophic- infection strategies to colonize the host. Transition from biotrophy to a destructive necrotrophic phase called the biotrophy-necrotrophy switch is critical in symptom development. C. truncatum likely secretes an arsenal of proteins that are implicated in maintaining a compatible interaction with its host. Some of them might be transition specific.

Results

A directional cDNA library was constructed from mRNA isolated from infected Lens culinaris leaflet tissues displaying the biotrophy-necrotrophy switch of C. truncatum and 5000 expressed sequence tags (ESTs) with an average read of > 600 bp from the 5-prime end were generated. Nearly 39% of the ESTs were predicted to encode proteins of fungal origin and among these, 162 ESTs were predicted to contain N-terminal signal peptides (SPs) in their deduced open reading frames (ORFs). The 162 sequences could be assembled into 122 tentative unigenes comprising 32 contigs and 90 singletons. Sequence analyses of unigenes revealed four potential groups: hydrolases, cell envelope associated proteins (CEAPs), candidate effectors and other proteins. Eleven candidate effector genes were identified based on features common to characterized fungal effectors, i.e. they encode small, soluble (lack of transmembrane domain), cysteine-rich proteins with a putative SP. For a selected subset of CEAPs and candidate effectors, semiquantitative RT-PCR showed that these transcripts were either expressed constitutively in both in vitro and in planta or induced during plant infection. Using potato virus X (PVX) based transient expression assays, we showed that one of the candidate effectors, i. e. contig 8 that encodes a cerato-platanin (CP) domain containing protein, unlike CP proteins from other fungal pathogens was unable to elicit a hypersensitive response (HR).

Conclusions

The current study catalogues proteins putatively secreted at the in planta biotrophy-necrotrophy transition of C. truncatum. Some of these proteins may have a role in establishing compatible interaction with the host plant.

Transcriptome analysis shows differential gene expression in the saprotrophic to parasitic transition of Pochonia chlamydosporia

Expression profiles were identified in the fungus Pochonia chlamydosporia, a biological control agent of plant parasitic nematodes, through a cDNA-amplified fragment length polymorphism approach. Two isolates with different host ranges, IMI 380407 and IMI 331547, were assayed in conditions of saprotrophic-to-parasitic transition, through in vitro assays. Gene expression profiles from three different nutritional conditions and four sampling times were generated, with eggs of host nematodes Globodera pallida and Meloidogyne incognita. Expression of transcripts changed in RNA fingerprints obtained under different nutritional stresses (starvation in presence/absence of eggs, or rich growth media). Transcript derived fragments (TDFs) obtained from the expression profiles corresponded to 6,800 products. A subset was sequenced and their expression profile confirmed through RT PCR. A total of 57 TDFs were selected for further analysis, based on similarities to translated or annotated sequences. Genes expressed during egg parasitism for both IMI 380407 and IMI 331547 were involved in metabolic functions, cellular signal regulation, cellular transport, regulation of gene expression, DNA repair, and other unknown functions. Multivariate analysis of TDF expression showed three groups for IMI 380407 and one for IMI 331547, each characterized by expression of genes related to eggs parasitism. Common amplification profiles among TDF clusters from both isolates also reflected a pool of constitutive genes, not affected by the nutritional conditions and nematode associations, related to general metabolic functions. The differential expression of parasitism related genes suggest a network of induced/repressed products, playing a role in fungal signaling and infection, with partial overlaps in host infection and parasitism traits.

Three highly similar formate dehydrogenase genes located in the vicinity of the B4 resistance gene cluster are differentially expressed under biotic and abiotic stresses in Phaseolus vulgaris

In higher plants, formate dehydrogenase (FDH, EC1.2.1.2.) catalyzes the NAD-linked oxidation of formate to CO(2), and FDH transcript accumulation has been reported after various abiotic stresses. By sequencing a Phaseolus vulgaris BAC clone encompassing a CC-NBS-LRR gene rich region of the B4 resistance gene cluster, we identified three FDH-encoding genes. FDH is present as a single copy gene in the Arabidopsis thaliana genome, and public database searches confirm that FDH is a low copy gene in plant genomes, since only 33 FDH homologs were identified from 27 plant species. Three independent prediction programs (Predotar, TargetP and Mitoprot) used on this large subset of 33 plant FDHs, revealed that mitochondrial localization of FDH might be the rule in higher plants. A phylogenetic analysis suggests a scenario of local FDH gene duplication in an ancestor of the Phaseoleae followed by another more recent duplication event after bean/soybean divergence. The expression levels of two common bean FDH genes under different treatments were investigated by quantitative RT-PCR analysis. FDH genes are differentially up-regulated after biotic and abiotic stresses (infection with the fungus Colletotrichum lindemuthianum, and dark treatment, respectively). The present study provides the first report of FDH transcript accumulation after biotic stress, suggesting the involvement of FDH in the pathogen resistance process.

A secreted effector protein (SNE1) from Phytophthora infestans is a broadly acting suppressor of programmed cell death

Evasion or active suppression of host defenses are critical strategies employed by biotrophic phytopathogens and hemibiotrophs whose infection mechanism includes sequential biotrophic and necrotrophic stages. Although defense suppression by secreted effector proteins has been well studied in bacteria, equivalent systems in fungi and oomycetes are poorly understood. We report the characterization of SNE1 (suppressor of necrosis 1), a gene encoding a secreted protein from the hemibiotrophic oomycete Phytophthora infestans that is specifically expressed at the transcriptional level during biotrophic growth within the host plant tomato (Solanum lycopersicum). Using transient expression assays, we show that SNE1 suppresses the action of secreted cell death-inducing effectors from Phytophthora that are expressed during the necrotrophic growth phase, as well as programmed cell death mediated by a range of Avr-R protein interactions. We also report that SNE1 contains predicted NLS motifs and translocates to the plant nucleus in transient expression studies. A conceptual model is presented in which the sequential coordinated secretion of antagonistic effectors by P. infestans first suppresses, but then induces, host cell death, thereby providing a highly regulated means to control the transition from biotrophy to necrotrophy.

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.

Identification of virulence genes in Fusarium oxysporum f. sp. lycopersici by large-scale transposon tagging

Forward genetic screens are efficient tools for the dissection of complex biological processes, such as fungal pathogenicity. A transposon tagging system was developed in the vascular wilt fungus Fusarium oxysporum f. sp. lycopersici by inserting the novel modified impala element imp160::gfp upstream of the Aspergillus nidulans niaD gene, followed by transactivation with a constitutively expressed transposase. A collection of 2072 Nia(+) revertants was obtained from reporter strain T12 and screened for alterations in virulence, using a rapid assay for invasive growth on apple slices. Seven strains exhibited reduced virulence on both apple slices and intact tomato plants. Five of these were true revertants showing the re-insertion of imp160::gfp within or upstream of predicted coding regions, whereas the other two showed either excision without re-insertion or no excision. Linkage between imp160::gfp insertion and virulence phenotype was determined in four transposon-tagged loci using targeted deletion in the wild-type strain. Knockout mutants in one of the genes, FOXG_00016, displayed significantly reduced virulence, and complementation of the original revertant with the wild-type FOXG_00016 allele fully restored virulence. FOXG_00016 has homology to the velvet gene family of A. nidulans. The high rate of untagged virulence mutations in the T12 reporter strain appears to be associated with increased genetic instability, possibly as a result of the transactivation of endogenous transposable elements by the constitutively expressed transposase.

Differential gene expression between the biotrophic-like and saprotrophic mycelia of the witches' broom pathogen Moniliophthora perniciosa

Moniliophthora perniciosa is a hemibiotrophic fungus that causes witches' broom disease (WBD) in cacao. Marked dimorphism characterizes this fungus, showing a monokaryotic or biotrophic phase that causes disease symptoms and a later dikaryotic or saprotrophic phase. A combined strategy of DNA microarray, expressed sequence tag, and real-time reverse-transcriptase polymerase chain reaction analyses was employed to analyze differences between these two fungal stages in vitro. In all, 1,131 putative genes were hybridized with cDNA from different phases, resulting in 189 differentially expressed genes, and 4,595 reads were clusterized, producing 1,534 unigenes. The analysis of these genes, which represent approximately 21% of the total genes, indicates that the biotrophic-like phase undergoes carbon and nitrogen catabolite repression that correlates to the expression of phytopathogenicity genes. Moreover, downregulation of mitochondrial oxidative phosphorylation and the presence of a putative ngr1 of Saccharomyces cerevisiae could help explain its lower growth rate. In contrast, the saprotrophic mycelium expresses genes related to the metabolism of hexoses, ammonia, and oxidative phosphorylation, which could explain its faster growth. Antifungal toxins were upregulated and could prevent the colonization by competing fungi. This work significantly contributes to our understanding of the molecular mechanisms of WBD and, to our knowledge, is the first to analyze differential gene expression of the different phases of a hemibiotrophic fungus.

The hemibiotrophic lifestyle of Colletotrichum species

Colletotrichum species infect several economically important crop plants. To establish a compatible parasitic interaction, a specialized infection cell, the melanized appressorium, is differentiated on the cuticle of the host. After penetration, an infection vesicle and primary hyphae are formed. These structures do not kill the host cell and show some similarities with haustoria formed by powdery mildews and rust fungi. Therefore, this stage of infection is called biotrophic. Later in the infection process, necrotrophic secondary hyphae spread within and kill the host tissue. The lifestyle of Colletotrichum species is called hemibiotrophic, as biotrophic and necrotrophic developmental stages are sequentially established. As most Colletotrichum species are accessible to molecular techniques, genes can be identified and functionally characterized. Here we demonstrate that Agrobacterium tumefaciens-mediated transformation is a well-suited method for tagging of genes mediating compatibility in the Colletotrichum graminicola-maize interaction.

Detached and attached Arabidopsis leaf assays reveal distinctive defense responses against hemibiotrophic Colletotrichum spp

The agriculturally important genus Colletotrichum is an emerging model pathogen for studying defense in Arabidopsis. During the process of screening for novel pathogenic Colletotrichum isolates on Arabidopsis, we found significant differences in defense responses between detached and attached leaf assays. A near-adapted isolate Colletotrichum linicola A1 could launch a typical infection only on detached, but not attached, Arabidopsis leaves. Remarkably, resistance gene-like locus RCH1-mediated resistance in intact plants also was compromised in detached leaves during the attacks with the virulent reference isolate C. higginsianum. The differences in symptom development between the detached leaf and intact plant assays were further confirmed on defense-defective mutants following inoculation with C. higginsianum, where the greatest inconsistency occurred on ethylene-insensitive mutants. In intact Arabidopsis plants, both the salicylic acid- and ethylene-dependent pathways were required for resistance to C. higginsianum and were associated with induced expression of pathogenesis-related genes PR1 and PDF1.2. In contrast, disease symptom development in detached leaves appeared to be uncoupled from these defense pathways and more closely associated with senescence: an observation substantiated by coordinated gene expression analysis and disease symptom development, and chemically and genetically mimicking senescence.

Fow2, a Zn(II)2Cys6-type transcription regulator, controls plant infection of the vascular wilt fungus Fusarium oxysporum

The filamentous fungus Fusarium oxysporum is a soil-borne parasite that causes vascular wilts in a wide variety of crops by directly penetrating roots and colonizing the vascular tissue. In previous work, we generated the non-pathogenic mutant B137 of the melon wilt pathogen F. oxysporum f. sp. melonis by using restriction enzyme-mediated integration (REMI) mutagenesis. Molecular characterization of B137 revealed that this mutant has a single-copy plasmid insertion in a gene, designated FOW2, which encodes a putative transcription regulator belonging to the Zn(II)2Cys6 family. The REMI mutant B137 and other FOW2-targeted mutants completely lost pathogenicity, but were not impaired in vegetative growth and conidiation in cultures. Microscopic observation of infection behaviours of green fluorescent protein (GFP)-marked wild-type and mutant strains revealed that the mutants were defective in their abilities to invade roots and colonize plant tissues. FOW2 is conserved in F. oxysporum pathogens that infect different plants. The FOW2-targeted mutants of the tomato wilt pathogen F. oxysporum f. sp. lycopersici also lost pathogenicity. Nuclear localization of Fow2 was verified using strains expressing Fow2-GFP and GFP-Fow2 fusion proteins. These data strongly suggest that FOW2 encodes a transcription regulator controlling the plant infection capability of F. oxysporum pathogens.

A critical assessment of Agrobacterium tumefaciens-mediated transformation as a tool for pathogenicity gene discovery in the phytopathogenic fungus Leptosphaeria maculans

We evaluated the usefulness and robustness of Agrobacterium tumefaciens-mediated transformation (ATMT) as a high-throughput transformation tool for pathogenicity gene discovery in the filamentous phytopathogen Leptosphaeria maculans. Thermal asymmetric interlaced polymerase chain reaction allowed us to amplify the left border (LB) flanking sequence in 135 of 400 transformants analysed, and indicated a high level of preservation of the T-DNA LB. In addition, T-DNA preferentially integrated as a single copy in gene-rich regions of the fungal genome, with a probable bias towards intergenic and/or regulatory regions. A total of 53 transformants out of 1388 (3.8%) showed reproducible pathogenicity defects when inoculated on cotyledons of Brassica napus, with diverse altered phenotypes. Co-segregation of the altered phenotype with the T-DNA integration was observed for 6 of 12 transformants crossed. If extrapolated to the whole collection, this indicates that 1.9% of the collection actually corresponds to tagged pathogenicity mutants. The preferential insertion into gene-rich regions along with the high ratio of tagged mutants renders ATMT a tool of choice for large-scale gene discovery in L. maculans.

The plant nitrogen mobilization promoted by Colletotrichum lindemuthianum in Phaseolus leaves depends on fungus pathogenicity

Nitrogen plays an essential role in the nutrient relationship between plants and pathogens. Some studies report that the nitrogen-mobilizing plant metabolism that occurs during abiotic and biotic stress could be a 'slash-and-burn' defence strategy. In order to study nitrogen recycling and mobilization in host plants during pathogen attack and invasion, the Colletotrichum lindemuthianum/Phaseolus vulgaris interaction was used as a model. C. lindemuthianum is a hemibiotroph that causes anthracnose disease on P. vulgaris. Non-pathogenic mutants and the pathogenic wild-type strain were used to compare their effects on plant metabolism. The deleterious effects of infection were monitored by measuring changes in chlorophyll, protein, and amino acid concentrations. It was shown that amino acid composition changed depending on the plant-fungus interaction and that glutamine accumulated mainly in the leaves infected by the pathogenic strain. Glutamine accumulation correlated with the accumulation of cytosolic glutamine synthetase (GS1 alpha) mRNA. The most striking result was that the GS1 alpha gene was induced in all the fungus-infected leaves, independent of the strain used for inoculation, and that GS1 alpha expression paralleled the PAL3 and CHS defence gene expression. It is concluded that a role of GS1 alpha in plant defence has to be considered.

A fungal family of transcriptional regulators: the zinc cluster proteins

The trace element zinc is required for proper functioning of a large number of proteins, including various enzymes. However, most zinc-containing proteins are transcription factors capable of binding DNA and are named zinc finger proteins. They form one of the largest families of transcriptional regulators and are categorized into various classes according to zinc-binding motifs. This review focuses on one class of zinc finger proteins called zinc cluster (or binuclear) proteins. Members of this family are exclusively fungal and possess the well-conserved motif CysX(2)CysX(6)CysX(5-12)CysX(2)CysX(6-8)Cys. The cysteine residues bind to two zinc atoms, which coordinate folding of the domain involved in DNA recognition. The first- and best-studied zinc cluster protein is Gal4p, a transcriptional activator of genes involved in the catabolism of galactose in the budding yeast Saccharomyces cerevisiae. Since the discovery of Gal4p, many other zinc cluster proteins have been characterized; they function in a wide range of processes, including primary and secondary metabolism and meiosis. Other roles include regulation of genes involved in the stress response as well as pleiotropic drug resistance, as demonstrated in budding yeast and in human fungal pathogens. With the number of characterized zinc cluster proteins growing rapidly, it is becoming more and more apparent that they are important regulators of fungal physiology.

Over-expression of a cacao class I chitinase gene in Theobroma cacao L. enhances resistance against the pathogen, Colletotrichum gloeosporioides

Theobroma cacao L. plants over-expressing a cacao class I chitinase gene (TcChi1) under the control of a modified CaMV-35S promoter were obtained by Agrobacterium-mediated transformation of somatic embryo cotyledons. Southern blot analysis confirmed insertion of the transgene in eight independent lines. High levels of TcChi1 transgene expression in the transgenic lines were confirmed by northern blot analysis. Chitinase activity levels were measured using an in vitro fluorometric assay. The transgene was expressed at varying levels in the different transgenic lines with up to a sixfold increase of endochitinase activity compared to non-transgenic and transgenic control plants. The in vivo antifungal activity of the transgene against the foliar pathogen Colletotrichum gloeosporioides was evaluated using a cacao leaf disk bioassay. The assay demonstrated that the TcChi1 transgenic cacao leaves significantly inhibited the growth of the fungus and the development of leaf necrosis compared to controls when leaves were wound inoculated with 5,000 spores. These results demonstrate for the first time the utility of the cacao transformation system as a tool for gene functional analysis and the potential utility of the cacao chitinase gene for increasing fungal pathogen resistance in cacao.

Sequence analysis and expression pattern of MGTA1 gene in rice blast pathogen Magnaporthe grisea

MGTA1, a putative fungal Zn(II)(2)Cys(6) transcriptional activator-encoding gene, was isolated from rice blast pathogen Magnaporthe grisea, which is homologous to CLTA1 from Colletotrichum lindemuthianum with 51% identity at protein level. MGTA1 cassette contains a 2370 bp open reading frame, consisting of 6 exons, coding a 790 amino acid peptide. MGTA1 gene exists as a single copy in genomes of 7 strains of M. grisea, and is expressed in tip hyphae, conidia, and mature appressoria of strain Guy11.

Arabidopsis pathology breathes new life into the necrotrophs-vs.-biotrophs classification of fungal pathogens

SUMMARY Fungal plant pathologists have for many decades attempted to classify pathogens into groups called necrotrophs, biotrophs and, more recently, hemibiotrophs. Although these terms are well known and frequently used, disagreements about which pathogens fall into which classes, as well as the precise definition of these terms, has conspired to limit their usefulness. Dogmas concerning the properties of the classes have been progressively eroded. However, the genetic analysis of disease resistance, particularly in the model plant Arabidopsis thaliana, has provided a biologically meaningful division based on whether defence against fungal pathogens is controlled via the salicylate or jasmonate/ethylene pathways. This mode-of-defence division distinguishes necrotrophs and biotrophs but it limits the biotroph class to pathogens that possess haustoria. The small number and limited range of pathogens that infect Arabidopsis means that several interesting questions are still unanswered. Do hemibiotrophs represents a distinct class or a subclass of the necrotrophs? Does the division apply to other plant families and particularly to cereals? and does this classification help us understand the intricacies of either fungal pathogenicity or plant defence?

A novel Arabidopsis-Colletotrichum pathosystem for the molecular dissection of plant-fungal interactions

The ability of a Colletotrichum sp., originally isolated from Brassica campestris, to infect Arabidopsis thaliana was examined. Sequence analysis of the internal transcribed spacer (ITS)1, 5.8S RNA gene and ITS2 regions of ribosomal (r)DNA showed the pathogen to be Colletotrichum destructivum. The host range was broad, including many cruciferous plants and some legumes. At 25 degrees C, all A. thaliana accessions tested were susceptible to the Brassica isolates of C. destructivum; however, at 15 degrees C, the accession Ws-2 showed a temperature-dependant resistance, in which single epidermal cells underwent a rapid hypersensitive response. Legume isolates of C. destructivum were unable to infect A. thaliana and induced deposition of callose papillae at sites of attempted penetration. In compatible interactions, C. destructivum showed a two-stage, hemibiotrophic infection process. The initial biotrophic phase was associated with large, intracellular primary hyphae and was confined to one epidermal cell; whereas, in the subsequent necrotrophic phase, narrow secondary hyphae extensively colonized the tissue and conidia were produced in acervuli. An efficient transformation system was established for C. destructivum, using Agrobacterium-mediated transfer of DNA. The ability to genetically manipulate both partners in the interaction is an important advantage, and the Arabidopsis-Colletotrichum pathosystem should provide a valuable new model for dissecting plant-fungal interactions.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

Characterization of a fusarium 2-gene cluster involved in trichothecene C-8 modification

The Fusarium trichothecenes T-2 toxin and deoxynivalenol (DON) are potent inhibitors of eukaryotic protein synthesis and are a significant agricultural problem. Three coregulated loci are required for T-2 toxin synthesis by Fusarium sporotrichioides. The core-trichothecene gene cluster consists of 12 genes (Tri3-Tri14) while the second locus consists of a single gene (Tri101). The third locus was recently partially described and encodes 1-2 biosynthetic enzymes and a putative regulatory gene. Here, we describe a detailed characterization of this locus. Located adjacent to Tri1 is Tri16, which is required for esterification of the C-8 hydroxyl. A putative regulatory gene, also adjacent to Tri1, is not required for T-2 toxin synthesis. The genomic sequence of Fusarium graminearum (a DON producer) contains a putative functional Tri1 and a nonfunctional Tri16. The presence of the Tri16 pseudogene is consistent with the chemical structure of DON, which has a C-8 keto group rather than the C-8 ester of T-2 toxin.

CLNR1, the AREA/NIT2-like global nitrogen regulator of the plant fungal pathogen Colletotrichum lindemuthianum is required for the infection cycle

Nitrogen starvation is generally assumed to be encountered by biotrophic and hemibiotrophic plant fungal pathogens at the beginning of their infection cycle. We tested whether nitrogen starvation constitutes a cue regulating genes that are required for pathogenicity of Colletotrichum lindemuthianum, a fungal pathogen of common bean. The clnr1 (C. lindemuthianumnitrogen regulator 1) gene, the areA/nit-2 orthologue of C. lindemuthianum, was isolated. The predicted CLNR1 protein exhibits high amino acid sequence similarities with the AREA and NIT2 global fungal nitrogen regulators. Targeted clnr1- mutants are unable to use a wide array of nitrogen sources, indicating that clnr1 is the C. lindemuthianum major nitrogen regulatory gene. The clnr1- mutants are non-pathogenic, although few anthracnose lesions seldom occur on whole plantlets. Surprisingly, cytological analysis reveals that the clnr1- mutants are not disturbed from the penetration stage until the end of the biotrophic phase, but that they are impaired during the setting up of the necrotrophic phase. Thus, through CLNR1, nitrogen starvation constitutes a cue for the regulation of genes that are compulsory for this stage of the C. lindemuthianum infection process. Additionally, clnr1- mutants complemented with the Aspergillus nidulans areA gene are fully pathogenic, indicating that areA is able to activate the C. lindemuthianum suited genes, normally under the control of clnr1.

Class V chitin synthase determines pathogenesis in the vascular wilt fungus Fusarium oxysporum and mediates resistance to plant defence compounds

Chitin, a beta-1,4-linked polysaccharide of N-acetylglucosamine, is a major structural component of fungal cell walls. Fungi have multiple classes of chitin synthases that catalyse N-acetylglucosamine polymerization. Here, we demonstrate the requirement for a class V chitin synthase during host infection by the vascular wilt pathogen Fusarium oxysporum. The chsV gene was identified in an insertional mutagenesis screen for pathogenicity mutants. ChsV has a putative myosin motor and a chitin synthase domain characteristic of class V chitin synthases. The chsV insertional mutant and a gene replacement mutant of F. oxysporum display morphological abnormalities such as hyphal swellings that are indicative of alterations in cell wall structure and can be partially restored by osmotic stabilizer. The mutants are unable to infect and colonize tomato plants or to grow invasively on tomato fruit tissue. They are also hypersensitive to plant antimicrobial defence compounds such as the tomato phytoanticipin alpha-tomatine or H2O2. Reintroduction of a functional chsV copy into the mutant restored the growth phenotype of the wild-type strain. These data suggest that F. oxysporum requires a specific class V chitin synthase for pathogenesis, most probably to protect itself against plant defence mechanisms.

Plant infection and the establishment of fungal biotrophy

To exploit plants as living substrates, biotrophic fungi have evolved remarkable variations of their tubular cells, the hyphae. They form infection structures such as appressoria, penetration hyphae and infection hyphae to invade the plant with minimal damage to host cells. To establish compatibility with the host, controlled secretory activity and distinct interface layers appear to be essential. Colletotrichum species switch from initial biotrophic to necrotrophic growth and are amenable to mutant analysis and molecular studies. Obligate biotrophic rust fungi can form the most specialized hypha: the haustorium. Gene expression and immunocytological studies with rust fungi support the idea that the haustorium is a transfer apparatus for the long-term absorption of host nutrients.

Ultrastructural Characterization of Infection and Colonization of Maize Leaves by Colletotrichum graminicola, and by a C. graminicola Pathogenicity Mutant

ABSTRACT Observations were made of the ultrastructure of infection and colonization of leaves of a susceptible maize inbred by Colletotrichum graminicola and by a C. graminicola pathogenicity mutant. The mutant causes no symptoms on either maize leaves or stalks. Prior evidence suggested that it is deficient in production of signal peptidase, responsible for cleavage of signal peptides from proteins destined for transport through the endoplasmic reticulum. There was no significant difference in the process of infection or colonization by the mutant and wild-type strains up to 48 h after inoculation. Both the mutant and the wild type produced globose, melanized appressoria within 24 h after inoculation on the host surface. By 36 h, both strains had penetrated the host epidermal cells directly. The host cells frequently formed papillae in response to appressoria, but these were not usually successful in preventing fungal ingress in either case. Penetration was followed by formation of irregularly shaped, swollen infection hyphae. Infection hyphae of both strains grew biotrophically for a relatively short time (less than 12 h). One or more hyphal branches was produced from each infection hypha, and these invaded adjacent mesophyll cells. Both strains of the fungus grew cell-to-cell, setting up new biotrophic interactions in each cell, between 36 and 48 h after inoculation. Papillae were frequently formed by the mesophyll cells, but these were not successful in preventing fungal ingress. The first noticeable difference between the mutant and the wild type was related to their interaction with mesophyll cells. Cells invaded by the wild type died relatively quickly, whereas those infected by the mutant appeared to survive longer. The most dramatic difference between the mutant and wild type occurred when the mutant completely failed to make a transition to necrotrophic growth, while the wild type made that switch at 48 to 72 h after inoculation. The mutant may be unable to secrete sufficient quantities of one or more proteins that are necessary to support the switch between biotrophy and necrotrophy.

Signal and nutrient exchange at biotrophic plant-fungus interfaces

Biotrophic interfaces are formed in mutualistic and parasitic plant-fungus interactions. They result from coordinated developmental programs in both partners and represent specialized platforms for the exchange of information and nutritional metabolites. New data on the establishment and the components of functional interfaces have been obtained in a number of ways. First, by isolation of symbiotically defective mutants; second, by characterization of new genes and their products; and, third, by the identification and localization of components of biotrophic interfaces, such as cell-wall proteins, H+-ATPases and nutrient transporters.