The differential virulence of Fusarium strains causing corneal infections and plant diseases is associated with accessory chromosome composition.

Fusarium oxysporum is a cross-kingdom pathogen. While some strains cause disseminated fusariosis and blinding corneal infections in humans, others are responsible for devastating vascular wilt diseases in plants. To better understand the distinct adaptations of F. oxysporum to animal or plant hosts, we conducted a comparative phenotypic and genetic analysis of two strains: MRL8996 (isolated from a keratitis patient) and Fol4287 (isolated from a wilted tomato [Solanum lycopersicum]). In vivo infection of mouse corneas and tomato plants revealed that, while both strains cause symptoms in both hosts, MRL8996 caused more severe corneal ulceration and perforation in mice, whereas Fol4287 induced more pronounced wilting symptoms in tomato. In vitro assays using abiotic stress treatments revealed that the human pathogen MRL8996 was better adapted to elevated temperatures, whereas the plant pathogen Fol4287 was more tolerant of osmotic and cell wall stresses. Both strains displayed broad resistance to antifungal treatment, with MRL8996 exhibiting the paradoxical effect of increased tolerance to higher concentrations of the antifungal caspofungin. We identified a set of accessory chromosomes (ACs) and protein-encoding genes with distinct transposon profiles and functions, respectively, between MRL8996 and Fol4287. Interestingly, ACs from both genomes also encode proteins with shared functions, such as chromatin remodeling and post-translational protein modifications. Our phenotypic assays and comparative genomics analyses lay the foundation for future studies correlating genotype with phenotype and for developing targeted antifungals for agricultural and clinical uses.

Conservation of the Pal/Rim Pathway in Ustilaginomycetes.

The ability of fungi to effectively sense and internalize signals related to extracellular changing environments is essential for survival. This adaptability is particularly important for fungal pathogens of humans and plants that must sense and respond to drastic environmental changes when colonizing their hosts. One of the most important physicochemical factors affecting fungal growth and development is the pH. Ascomycota fungal species possess mechanisms such as the Pal/Rim pathway for external pH sensing and adaptation. However, the conservation of this mechanism in other fungi, such as Ustilaginomycetes is still little studied. To overcome this knowledge gap, we used a comparative genomic approach to explore the conservation of the Pal/Rim pathway in the 13 best sequenced and annotated Ustilaginomycetes. Our findings reveal that the Rim proteins and the Endosomal Sorting Complex Required for Transport (ESCRT) proteins are conserved in Ustilaginomycetes. They conserve the canonical domains present in Pal/Rim and ESCRT proteins of Ascomycota. This study sheds light on the molecular mechanisms used by these fungi for responding to extracellular stresses such as the pH, and open the door to further experimentations for understanding the molecular bases of the signaling in Ustilaginomycetes.

Potential application of a fungal co-culture crude extract for the conservation of post-harvest fruits.

Fungal plant pathogens are responsible for serious losses in many economically important crop species worldwide. Due to the use of fungicides and the fungi genome plasticity, multi-drug resistant strains are emerging as a new generation of pathogens, causing an expansive range of superficial and systemic plant infections, or new opportunistic fungal pathogens for humans. The group of antagonistic fungi Trichoderma spp. has been widely used to enhance plant growth and for the control of different pathogens affecting crops. Although Neurospora crassa is not a mycoparasitic fungus, its secretion of secondary metabolites with antimicrobial activity has been described. In this work, the effect of crude extract of the monoculture of Trichoderma asperellum T8a or the co-culture with N. crassa as an inhibitory treatment against the fungal pathogens Botrytis cinerea and Fusarium solani was evaluated. The findings demonstrate that the secondary metabolites contained in the T. asperellum crude extract have a clear fungistatic activity against B. cinerea and F. solani. Interestingly, this fungistatic activity highly increases when T. asperellum is co-cultivated with the non-pathogenic fungus N. crassa. Moreover, the co-culture crude extract also showed antifungal activity on post-harvest fruits, and no toxic effects on Murine fibroblast L929 (CCL-1) and murine macrophages RAW 264.7 (TIB-71) were observed. All these results together are solid evidence of the potential of the co-culture crude extract of T. asperellum and N. crassa, as an antifungal agent against phytopathogenic fungi, or post-harvest fruits during the transportation or commercialization time.

CUR(E)ating a new approach to study fungal effectors and enhance undergraduate education through authentic research.

Course-based Undergraduate Research Experiences (CUREs) integrate active, discovery-based learning into undergraduate curricula, adding tremendous value to Biochemistry and Molecular Biology (BMB) education. There are multiple challenges in transforming a research project into a CURE, such as the readiness of students, the time commitment of the instructor, and the productivity of the research. In this article, we report a CURE course developed and offered in the University of Massachusetts Amherst BMB Department since 2018 that addresses these challenges. Our CURE focuses on fungal effectors which are proteins secreted by a destructive pathogenic fungus Fusarium oxysporum, one of the top five most devastating plant pathogens. By studying this group of proteins, students are connected to real-world problems and participate in the search for potential solutions. A 3-week "standard Boot Camp" is implemented to help students familiarize themselves with all basic techniques and boost their confidence. Next, molecular cloning, a versatile technique with modularity and repeatability, is used as the bedrock of the course. Our past 5 years of experience have confirmed that we have developed a novel and feasible CURE protocol. Measurable progress documented by students who took this course includes stimulated active learning and increased career trajectory to pursue hypothesis-based research to address societal needs. In addition, data generated through the course advance ongoing lab research. Collectively, we encourage the implementation of CURE among research-intensive faculty to provide a more inclusive research experience to undergraduate students, an important element in predicting career success.

Fungal volatile organic compounds: mechanisms involved in their sensing and dynamic communication with plants.

Microbial volatile organic compounds (MVOCs) are mixtures of gas-phase hydrophobic carbon-based molecules produced by microorganisms such as bacteria and fungi. They can act as airborne signals sensed by plants being crucial players in triggering signaling cascades influencing their secondary metabolism, development, and growth. The role of fungal volatile organic compounds (FVOCs) from beneficial or detrimental species to influence the physiology and priming effect of plants has been well studied. However, the plants mechanisms to discern between FVOCs from friend or foe remains significantly understudied. Under this outlook, we present an overview of the VOCs produced by plant-associate fungal species, with a particular focus on the challenges faced in VOCs research: i) understanding how plants could perceive FVOCs, ii) investigating the differential responses of plants to VOCs from beneficial or detrimental fungal strains, and finally, iii) exploring practical aspects related to the collection of VOCs and their eco-friendly application in agriculture.

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.

CUR(E)ating a New Approach to Study Fungal Effectors and Enhance Undergraduate Education through Authentic Research.

Course-based Undergraduate Research Experiences (CUREs) integrate active, discovery-based learning into undergraduate curriculums, adding tremendous value to Biochemistry and Molecular Biology (BMB) education. There are multiple challenges in transforming a research project into a CURE, such as the readiness of students, the time commitment of the instructor, and the productivity of the research. In this article, we report a CURE course developed and offered in the University of Massachusetts Amherst BMB Department since 2018 that addresses these challenges. Our CURE focuses on fungal effectors which are proteins secreted by a destructive pathogenic fungus Fusarium oxysporum , one of the top five most devastating plant pathogens. By studying this group of proteins, students are connected to real-world problems and participate in the search for potential solutions. A three-week "standard Bootcamp" is implemented to help students familiarize themselves with all basic techniques and boost their confidence. Next, molecular cloning, a versatile technique with modularity and repeatability, is used as the bedrock of the course. Our past five years of experience have confirmed that we have developed a novel and feasible CURE protocol. Measurable progress documented by students who took this course includes stimulated active learning and increased career trajectory to pursue hypothesis-based research to address societal needs. In addition, data generated through the course advance ongoing lab research. Collectively, we encourage the implementation of CURE among research-intensive faculty to provide a more inclusive research experience to all students, an important element in predicting career success.

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.

Differential Colonization of the Plant Vasculature Between Endophytic Versus Pathogenic Fusarium oxysporum Strains.

Plant xylem colonization is the hallmark of vascular wilt diseases caused by phytopathogens within the Fusarium oxysporum species complex. Recently, xylem colonization has also been reported among endophytic F. oxysporum strains, resulting in some uncertainty. This study compares xylem colonization processes by pathogenic versus endophytic strains in Arabidopsis thaliana and Solanum lycopersicum, using Arabidopsis pathogen Fo5176, tomato pathogen Fol4287, and the endophyte Fo47, which can colonize both plant hosts. We observed that all strains were able to advance from epidermis to endodermis within 3 days postinoculation (dpi) and reached the root xylem at 4 dpi. However, this shared progression was restricted to lateral roots and the elongation zone of the primary root. Only pathogens reached the xylem above the primary-root maturation zone (PMZ). Related to the distinct colonization patterns, we also observed stronger induction of callose at the PMZ and lignin deposition at primary-lateral root junctions by the endophyte in both plants. This observation was further supported by stronger induction of Arabidopsis genes involved in callose and lignin biosynthesis during the endophytic colonization (Fo47) compared with the pathogenic interaction (Fo5176). Moreover, both pathogens encode more plant cell wall-degrading enzymes than the endophyte Fo47. Therefore, observed differences in callose and lignin deposition could be the combination of host production and the subsequent fungal degradation. In summary, this study demonstrates spatial differences between endophytic and pathogenic colonization, strongly suggesting that further investigations of molecular arm-races are needed to understand how plants differentiate friend from foe. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Chitosan Is Necessary for the Structure of the Cell Wall, and Full Virulence of Ustilago maydis.

Smut fungi comprise a large group of biotrophic phytopathogens infecting important crops, such as wheat and corn. U. maydis is a plant pathogenic fungus responsible for common smut in maize and teocintle. Through our analysis of the transcriptome of the yeast-to-mycelium dimorphic transition at acid pH, we determined the number of genes encoding chitin deacetylases of the fungus, and observed that the gene encoding one of them (UMAG_11922; CDA1) was the only one up-regulated. The mutation of this gene and the analysis of the mutants revealed that they contained reduced amounts of chitosan, were severely affected in their virulence, and showed aberrant mycelial morphology when grown at acid pH. When the CDA1 gene was reinserted into the mutants by the use of an autonomous replication plasmid, virulence and chitosan levels were recovered in the retro mutant strains, indicating that the CDA1 gene was involved in these features. These data revealed that chitosan plays a crucial role in the structure and morphogenesis of the cell wall during mycelial development of the fungus, and that in its absence, the cell wall becomes altered and is unable to support the stress imposed by the defense mechanism mounted on by the plant host during the infection process.

Tec1, a member of the TEA transcription factors family, is involved in virulence and basidiocarp development in Ustilago maydis.

The life cycle of Ustilago maydis involves alternation of a haploid saprophytic yeast-like stage and a dikaryotic hyphal virulent form. Under in vitro conditions, basidiocarps are formed. Analysis of the transcriptional network of basidiocarp formation revealed the possible involvement of a Tec transcription factor (Tec1, UMAG_02835) in the process. In some Ascomycota, Tec factors are involved in mycelial formation, pathogenesis, and interaction with other regulatory elements, but their role in Basidiomycota species is almost unknown. Accordingly, we proceeded to determine the role of this gene in U. maydis by its mutation. Tec1 was found to be a crucial factor for normal mating, basidiocarp development, and virulence, all of the functions related to the dikaryotic stage dependent of the b genes, whereas dimorphism and resistance to different stress conditions occurring in the haploid stage were not affected in tec1 mutants. The observation that mutants showed a low residual wild-type phenotype suggests the presence of a secondary mechanism that partially compensates the loss of Tec1.

Invertases in Phytophthora infestans Localize to Haustoria and Are Programmed for Infection-Specific Expression.

The oomycete Phytophthora infestans, the causal agent of potato and tomato blight, expresses two extracellular invertases. Unlike typical fungal invertases, the P. infestans genes are not sucrose induced or glucose repressed but instead appear to be under developmental control. Transcript levels of both genes were very low in mycelia harvested from artificial medium but high in preinfection stages (sporangia, zoospores, and germinated cysts), high during biotrophic growth in leaves and tubers, and low during necrotrophy. Genome-wide analyses of metabolic enzymes and effectors indicated that this expression profile was fairly unusual, matched only by a few other enzymes, such as carbonic anhydrases and a few RXLR effectors. Genes for other metabolic enzymes were typically downregulated in the preinfection stages. Overall metabolic gene expression during the necrotrophic stage of infection clustered with artificial medium, while the biotrophic phase formed a separate cluster. Confocal microscopy of transformants expressing green fluorescent protein (GFP) fusions indicated that invertase protein resided primarily in haustoria during infection. This localization was not attributable to haustorium-specific promoter activity. Instead, the N-terminal regions of proteins containing signal peptides were sufficient to deliver proteins to haustoria. Invertase expression during leaf infection was linked to a decline in apoplastic sucrose, consistent with a role of the enzymes in plant pathogenesis. This was also suggested by the discovery that invertase genes occur across multiple orders of oomycetes but not in most animal pathogens or a mycoparasite.IMPORTANCE Oomycetes cause hundreds of diseases in economically and environmentally significant plants. How these microbes acquire host nutrients is not well understood. Many oomycetes insert specialized hyphae called haustoria into plant cells, but unlike their fungal counterparts, a role in nutrition has remained unproven. The discovery that Phytophthora invertases localize to haustoria provides the first strong evidence that these structures participate in feeding. Since regions of proteins containing signal peptides targeted proteins to the haustorium-plant interface, haustoria appear to be the primary machinery for secreting proteins during biotrophic pathogenesis. Although oomycete invertases were acquired laterally from fungi, their expression patterns have adapted to the Phytophthora lifestyle by abandoning substrate-level regulation in favor of developmental control, allowing the enzymes to be produced in anticipation of plant colonization. This study highlights how a widely distributed hydrolytic enzyme has evolved new behaviors in oomycetes.

Molecular Mechanisms Involved in the Multicellular Growth of Ustilaginomycetes.

Multicellularity is defined as the developmental process by which unicellular organisms became pluricellular during the evolution of complex organisms on Earth. This process requires the convergence of genetic, ecological, and environmental factors. In fungi, mycelial and pseudomycelium growth, snowflake phenotype (where daughter cells remain attached to their stem cells after mitosis), and fruiting bodies have been described as models of multicellular structures. Ustilaginomycetes are Basidiomycota fungi, many of which are pathogens of economically important plant species. These fungi usually grow unicellularly as yeasts (sporidia), but also as simple multicellular forms, such as pseudomycelium, multicellular clusters, or mycelium during plant infection and under different environmental conditions: Nitrogen starvation, nutrient starvation, acid culture media, or with fatty acids as a carbon source. Even under specific conditions, Ustilago maydis can form basidiocarps or fruiting bodies that are complex multicellular structures. These fungi conserve an important set of genes and molecular mechanisms involved in their multicellular growth. In this review, we will discuss in-depth the signaling pathways, epigenetic regulation, required polyamines, cell wall synthesis/degradation, polarized cell growth, and other cellular-genetic processes involved in the different types of Ustilaginomycetes multicellular growth. Finally, considering their short life cycle, easy handling in the laboratory and great morphological plasticity, Ustilaginomycetes can be considered as model organisms for studying fungal multicellularity.

Multicellular growth of the Basidiomycota phytopathogen fungus Sporisorium reilianum induced by acid conditions.

Fungi are considered model organisms for the analysis of important phenomena of eukaryotes. For example, some of them have been described as models to understand the phenomenon of multicellularity acquisition by different unicellular organisms phylogenetically distant. Interestingly, in this work, we describe the multicellular development in the model fungus S. reilianum. We observed that Sporisorium reilianum, a Basidiomycota cereal pathogen that at neutral pH grows with a yeast-like morphology during its saprophytic haploid stage, when incubated at acid pH grew in the form of multicellular clusters. The multicellularity observed in S. reilianum was of clonal type, where buds of "stem" cells growing as yeasts remain joined by their cell wall septa, after cytokinesis. The elaboration and analysis of a regulatory network of S. reilianum showed that the putative zinc finger transcription factor CBQ73544.1 regulates a number of genes involved in cell cycle, cellular division, signal transduction pathways, and biogenesis of cell wall. Interestingly, homologous of these genes have been found to be regulated during Saccharomyces cerevisiae multicellular growth. In adddition, some of these genes were found to be negatively regulated during multicellularity of S. reilianum. With these data, we suggest that S. reilianum is an interesting model for the study of multicellular development.

Functional analysis of the MAPK pathways in fungi.

The Mitogen-Activated Protein Kinase (MAPK) signaling pathways constitute one of the most important and evolutionarily conserved mechanisms for the perception of extracellular information in all the eukaryotic organisms. The MAPK pathways are involved in the transfer to the cell of the information perceived from extracellular stimuli, with the final outcome of activation of different transcription factors that regulate gene expression in response to them. In all species of fungi, the MAPK pathways have important roles in their physiology and development; e.g. cell cycle control, mating, morphogenesis, response to different stresses, resistance to UV radiation and to temperature changes, cell wall assembly and integrity, degradation of cellular organelles, virulence, cell-cell signaling, fungus-plant interaction, and response to damage-associated molecular patterns (DAMPs). Considering the importance of the phylogenetically conserved MAPK pathways in fungi, an updated review of the knowledge on them is discussed in this article. This information reveals their importance, their distribution in fungal species evolutionarily distant and with different lifestyles, their organization and function, and the interactions occurring between different MAPK pathways, and with other signaling pathways, for the regulation of the most complex cellular processes.

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.

Regulation of the expression of the whole genome of Ustilago maydis by a MAPK pathway.

The operation of mitogen-activated protein kinase (MAPK) signal transduction pathways is one of the most important mechanisms for the transfer of extracellular information into the cell. These pathways are highly conserved in eukaryotic organisms. In fungi, MAPK pathways are involved in the regulation of a number of cellular processes such as metabolism, homeostasis, pathogenesis and cell differentiation and morphogenesis. Considering the importance of pathways, in the present work we proceeded to identify all the genes that are regulated by the signal transduction pathway involved in mating, pathogenesis and morphogenesis of Ustilago maydis. Accordingly we made a comparison between the transcriptomes from a wild-type strain and an Ubc2 mutant affected in the interacting protein of this pathway by use of microarrays. By this methodology, we identified 939 genes regulated directly or indirectly by the MAPK pathway. Of them, 432 were positively, and 507 were negatively found regulated. By functional grouping, genes encoding cyclin-dependent kinases, transcription factors, proteins involved in signal transduction, in synthesis of wall and cell membrane, and involved in dimorphism were identified as differentially regulated. These data reveal the importance of these global studies, and the large (and unsuspected) number of functions of the fungus under the control of this MAPK, providing clues to the possible mechanisms involved.

Transcriptomic analysis of the dimorphic transition of Ustilago maydis induced in vitro by a change in pH.

Dimorphism is the property of fungi to grow as budding yeasts or mycelium, depending on the environmental conditions. This phenomenon is important as a model of differentiation in eukaryotic organisms, and since a large number of fungal diseases are caused by dimorphic fungi, its study is important for practical reasons. In this work, we examined the transcriptome during the dimorphic transition of the basidiomycota phytopathogenic fungus Ustilago maydis using microarrays, utilizing yeast and mycelium monomorphic mutants as controls. This way, we thereby identified 154 genes of the fungus that are specifically involved in the dimorphic transition induced by a pH change. Of these, 82 genes were up-regulated, and 72 were down-regulated. Differential categorization of these genes revealed that they mostly belonged to the classes of metabolism, cell cycle and DNA processing, transcription and protein fate, transport and cellular communication, stress, cell differentiation and biogenesis of cellular components, while a significant number of them corresponded to unclassified proteins. The data reported in this work are important for our understanding of the molecular bases of dimorphism in U. maydis, and possibly of other fungi.

Transcriptomic analysis of Ustilago maydis infecting Arabidopsis reveals important aspects of the fungus pathogenic mechanisms.

Transcriptomic and biochemical analyses of the experimental pathosystem constituted by Ustilago maydis and Arabidopsis thaliana were performed. Haploid or diploid strains of U. maydis inoculated in A. thaliana plantlets grew on the surface and within the plant tissues in the form of mycelium, inducing chlorosis, anthocyanin formation, malformations, necrosis and adventitious roots development, but not teliospores. Symptoms were more severe in plants inoculated with the haploid strain which grew more vigorously than the diploid strain. RNA extracted at different times post-infection was used for hybridization of one-channel microarrays that were analyzed focusing on the fungal genes involved in the general pathogenic process, biogenesis of the fungal cell wall and the secretome. In total, 3,537 and 3,299 genes were differentially expressed in the haploid and diploid strains, respectively. Differentially expressed genes were related to different functional categories and many of them showed a similar regulation occurring in U. maydis infecting maize. Our data suggest that the haploid strain behaves as a necrotrophic pathogen, whereas the diploid behaves as a biotrophic pathogen. The results obtained are evidence of the usefulness of the U. maydis-A. thaliana pathosystem for the analysis of the pathogenic mechanisms of U. maydis.