The genomics of obligate (and nonobligate) biotrophs

The massive 340 megabase genome of Anisogramma anomala, a biotrophic ascomycete that causes eastern filbert blight of hazelnut

BACKGROUND

The ascomycete fungus Anisogramma anomala causes Eastern Filbert Blight (EFB) on hazelnut (Corylus spp.) trees. It is a minor disease on its native host, the American hazelnut (C. americana), but is highly destructive on the commercially important European hazelnut (C. avellana). In North America, EFB has historically limited commercial production of hazelnut to west of the Rocky Mountains. A. anomala is an obligately biotrophic fungus that has not been grown in continuous culture, rendering its study challenging. There is a 15-month latency before symptoms appear on infected hazelnut trees, and only a sexual reproductive stage has been observed. Here we report the sequencing, annotation, and characterization of its genome.

RESULTS

The genome of A. anomala was assembled into 108 scaffolds totaling 342,498,352 nt with a GC content of 34.46%. Scaffold N50 was 33.3 Mb and L50 was 5. Nineteen scaffolds with lengths over 1 Mb constituted 99% of the assembly. Telomere sequences were identified on both ends of two scaffolds and on one end of another 10 scaffolds. Flow cytometry estimated the genome size of A. anomala at 370 Mb. The genome exhibits two-speed evolution, with 93% of the assembly as AT-rich regions (32.9% GC) and the other 7% as GC-rich (57.1% GC). The AT-rich regions consist predominantly of repeats with low gene content, while 90% of predicted protein coding genes were identified in GC-rich regions. Copia-like retrotransposons accounted for more than half of the genome. Evidence of repeat-induced point mutation (RIP) was identified throughout the AT-rich regions, and two copies of the rid gene and one of dim-2, the key genes in the RIP mutation pathway, were identified in the genome. Consistent with its homothallic sexual reproduction cycle, both MAT1-1 and MAT1-2 idiomorphs were found. We identified a large suite of genes likely involved in pathogenicity, including 614 carbohydrate active enzymes, 762 secreted proteins and 165 effectors.

CONCLUSIONS

This study reveals the genomic structure, composition, and putative gene function of the important pathogen A. anomala. It provides insight into the molecular basis of the pathogen's life cycle and a solid foundation for studying EFB.

Arbuscular mycorrhizal fungi inhibit necrotrophic, but not biotrophic, aboveground plant pathogens: a meta-analysis and experimental study

Microbial mutualists can profoundly modify host species ecology and evolution, by extension altering interactions with other microbial species, including pathogens. Arbuscular mycorrhizal fungi (AMF) may moderate infections by pathogens, but the direction and strength of these effects can be idiosyncratic. To assess how the introduction of AMF impacts the incidence and severity of aboveground plant diseases (i.e. 'disease impact'), we conducted a meta-analysis of 130 comparisons derived from 69 published studies. To elucidate the potential mechanisms underlying the influence of AMF on pathogens, we conducted three glasshouse experiments involving six non-woody plant species, yielded crucial data on leaf nutrient composition, plant defense compounds, and transcriptomes. Our meta-analysis revealed that the inoculation of AMF lead to a reduction in disease impact. More precisely, AMF inoculation was associated with a decrease in necrotrophic diseases, while no significant impact on biotrophic diseases. Chemical and transcriptome analyses suggested that these effects may be driven by AMF regulation of jasmonic acid and salicylic acid signaling pathways in glasshouse experiments. However, changes in plant nutritional status and secondary chemicals may also regulate disease impact. These results emphasize the importance of incorporating pathogen life history when predicting how microbial mutualisms affect disease impact.

Spray-induced gene silencing to identify powdery mildew gene targets and processes for powdery mildew control

Spray-induced gene silencing (SIGS) is an emerging tool for crop pest protection. It utilizes exogenously applied double-stranded RNA to specifically reduce pest target gene expression using endogenous RNA interference machinery. In this study, SIGS methods were developed and optimized for powdery mildew fungi, which are widespread obligate biotrophic fungi that infect agricultural crops, using the known azole-fungicide target cytochrome P450 51 (CYP51) in the Golovinomyces orontii-Arabidopsis thaliana pathosystem. Additional screening resulted in the identification of conserved gene targets and processes important to powdery mildew proliferation: apoptosis-antagonizing transcription factor in essential cellular metabolism and stress response; lipid catabolism genes lipase a, lipase 1, and acetyl-CoA oxidase in energy production; and genes involved in manipulation of the plant host via abscisic acid metabolism (9-cis-epoxycarotenoid dioxygenase, xanthoxin dehydrogenase, and a putative abscisic acid G-protein coupled receptor) and secretion of the effector protein, effector candidate 2. Powdery mildew is the dominant disease impacting grapes and extensive powdery mildew resistance to applied fungicides has been reported. We therefore developed SIGS for the Erysiphe necator-Vitis vinifera system and tested six successful targets identified using the G. orontii-A. thaliana system. For all targets tested, a similar reduction in powdery mildew disease was observed between systems. This indicates screening of broadly conserved targets in the G. orontii-A. thaliana pathosystem identifies targets and processes for the successful control of other powdery mildew fungi. The efficacy of SIGS on powdery mildew fungi makes SIGS an exciting prospect for commercial powdery mildew control.

A chromosome-scale genome assembly of the grape powdery mildew pathogen Erysiphe necator reveals its genomic architecture and previously unknown features of its biology

Erysiphe necator is an obligate fungal pathogen that causes grape powdery mildew, globally the most important disease on grapevines. Previous attempts to obtain a quality genome assembly for this pathogen were hindered by its high repetitive DNA content. Here, chromatin conformation capture (Hi-C) with long-read PacBio sequencing was combined to obtain a chromosome-scale assembly and a high-quality annotation for E. necator isolate EnFRAME01. The resulting 81.1 Mb genome assembly is 98% complete and consists of 34 scaffolds, 11 of which represent complete chromosomes. All chromosomes contain large centromeric-like regions and lack synteny to the 11 chromosomes of the cereal PM pathogen Blumeria graminis. Further analysis of their composition showed that repeats and transposable elements (TEs) occupy 62.7% of their content. TEs were almost evenly interspersed outside centromeric and telomeric regions and massively overlapped with regions of annotated genes, suggesting that they could have a significant functional impact. Abundant gene duplicates were observed as well, particularly in genes encoding candidate secreted effector proteins. Moreover, younger in age gene duplicates exhibited more relaxed selection pressure and were more likely to be located physically close in the genome than older duplicates. A total of 122 genes with copy number variations among six isolates of E. necator were also identified and were enriched in genes that were duplicated in EnFRAME01, indicating they may reflect an adaptive variation. Taken together, our study illuminates higher-order genomic architectural features of E. necator and provides a valuable resource for studying genomic structural variations in this pathogen. IMPORTANCE Grape powdery mildew caused by the ascomycete fungus Erysiphe necator is economically the most important and recurrent disease in vineyards across the world. The obligate biotrophic nature of E. necator hinders the use of typical genetic methods to elucidate its pathogenicity and adaptation to adverse conditions, and thus comparative genomics has been a major method to study its genome biology. However, the current reference genome of E. necator isolate C-strain is highly fragmented with many non-coding regions left unassembled. This incompleteness prohibits in-depth comparative genomic analyses and the study of genomic structural variations (SVs) that are known to affect several aspects of microbial life, including fitness, virulence, and host adaptation. By obtaining a chromosome-scale genome assembly and a high-quality gene annotation for E. necator, we reveal the organization of its chromosomal content, unearth previously unknown features of its biology, and provide a reference for studying genomic SVs in this pathogen.

Elucidating the Obligate Nature and Biological Capacity of an Invasive Fungal Corn Pathogen

Tar spot is a devasting corn disease caused by the obligate fungal pathogen Phyllachora maydis. Since its initial identification in the United States in 2015, P. maydis has become an increasing threat to corn production. Despite this, P. maydis has remained largely understudied at the molecular level, due to difficulties surrounding its obligate lifestyle. Here, we generated a significantly improved P. maydis nuclear and mitochondrial genome, using a combination of long- and short-read technologies, and also provide the first transcriptomic analysis of primary tar spot lesions. Our results show that P. maydis is deficient in inorganic nitrogen utilization, is likely heterothallic, and encodes for significantly more protein-coding genes, including secreted enzymes and effectors, than previous determined. Furthermore, our expression analysis suggests that, following primary tar spot lesion formation, P. maydis might reroute carbon flux away from DNA replication and cell division pathways and towards pathways previously implicated in having significant roles in pathogenicity, such as autophagy and secretion. Together, our results identified several highly expressed unique secreted factors that likely contribute to host recognition and subsequent infection, greatly increasing our knowledge of the biological capacity of P. maydis, which have much broader implications for mitigating tar spot of corn. [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.

Fungal Secondary Metabolites and Small RNAs Enhance Pathogenicity during Plant-Fungal Pathogen Interactions

Fungal plant pathogens use proteinaceous effectors as well as newly identified secondary metabolites (SMs) and small non-coding RNA (sRNA) effectors to manipulate the host plant's defense system via diverse plant cell compartments, distinct organelles, and many host genes. However, most molecular studies of plant-fungal interactions have focused on secreted effector proteins without exploring the possibly equivalent functions performed by fungal (SMs) and sRNAs, which are collectively known as "non-proteinaceous effectors". Fungal SMs have been shown to be generated throughout the plant colonization process, particularly in the early biotrophic stages of infection. The fungal repertoire of non-proteinaceous effectors has been broadened by the discovery of fungal sRNAs that specifically target plant genes involved in resistance and defense responses. Many RNAs, particularly sRNAs involved in gene silencing, have been shown to transmit bidirectionally between fungal pathogens and their hosts. However, there are no clear functional approaches to study the role of these SM and sRNA effectors. Undoubtedly, fungal SM and sRNA effectors are now a treasured land to seek. Therefore, understanding the role of fungal SM and sRNA effectors may provide insights into the infection process and identification of the interacting host genes that are targeted by these effectors. This review discusses the role of fungal SMs and sRNAs during plant-fungal interactions. It will also focus on the translocation of sRNA effectors across kingdoms, the application of cross-kingdom RNA interference in managing plant diseases and the tools that can be used to predict and study these non-proteinaceous effectors.

A facultative ectomycorrhizal association is triggered by organic nitrogen

Increasing nitrogen (N) deposition often tends to negatively impact the functions of belowground ectomycorrhizal networks, although the exact molecular mechanisms underlying this trait are still unclear. Here, we assess how the root-associated fungus Clitopilus hobsonii establishes an ectomycorrhiza-like association with its host tree Populus tomentosa and how this interaction is favored by organic N over mineral N. The establishment of a functional symbiosis in the presence of organic N promotes plant growth and the transfer of ¹⁵N from the fungus to above ground plant tissues. Genomic traits and in planta transcriptional signatures suggest that C. hobsonii may have a dual lifestyle with saprotrophic and mutualistic traits. For example, several genes involved in the digestion of cellulose and hemicellulose are highly expressed during the interaction, whereas the expression of multiple copies of pectin-digesting genes is tightly controlled. Conversely, the nutritional mutualism is dampened in the presence of ammonium (NH₄⁺) or nitrate (NO₃^-). Increasing levels of NH₄⁺ led to a higher expression of pectin-digesting genes and a continuous increase in hydrogen peroxide production in roots, whereas the presence of NO₃^- resulted in toxin production. In summary, our results suggest that C. hobsonii is a facultative ectomycorrhizal fungus. Access to various forms of N acts as an on/off switch for mutualism caused by large-scale fungal physiological remodeling. Furthermore, the abundance of pectin-degrading enzymes with distinct expression patterns during functional divergence after exposure to NH₄⁺ or organic N is likely to be central to the transition from parasitism to mutualism.

A Global Transcriptome and Co-expression Analysis Reveals Robust Host Defense Pathway Reprogramming and Identifies Key Regulators of Early Phases of Cicer-Ascochyta Interactions

Ascochyta blight (AB) caused by the filamentous fungus Ascochyta rabiei is a major threat to global chickpea production. The mechanisms underlying chickpea response to A. rabiei remain elusive to date. Here, we investigated the comparative transcriptional dynamics of AB-resistant and -susceptible chickpea genotypes upon A. rabiei infection, to understand the early host defense response. Our findings revealed that AB-resistant plants underwent rapid and extensive transcriptional reprogramming compared with a susceptible host. At the early stage (24 h postinoculation [hpi]), mainly cell-wall remodeling and secondary metabolite pathways were highly activated, while differentially expressed genes related to signaling components, such as protein kinases, transcription factors, and hormonal pathways, show a remarkable upsurge at 72 hpi, especially in the resistant genotype. Notably, our data suggest an imperative role of jasmonic acid, ethylene, and abscisic acid signaling in providing immunity against A. rabiei. Furthermore, gene co-expression networks and modules corroborated the importance of cell-wall remodeling, signal transduction, and phytohormone pathways. Hub genes such as MYB14, PRE6, and MADS-SOC1 discovered in these modules might be the master regulators governing chickpea immunity. Overall, we not only provide novel insights for comprehensive understanding of immune signaling components mediating AB resistance and susceptibility at early Cicer-Ascochyta interactions but, also, offer a valuable resource for developing AB-resistant chickpea. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Evolution of pathogenicity in obligate fungal pathogens and allied genera

Obligate fungal pathogens (ascomycetes and basidiomycetes) and oomycetes are known to cause diseases in cereal crop plants. They feed on living cells and most of them have learned to bypass the host immune machinery. This paper discusses some of the factors that are associated with pathogenicity drawing examples from ascomycetes, basidiomycetes and oomycetes, with respect to their manifestation in crop plants. The comparisons have revealed a striking similarity in the three groups suggesting convergent pathways that have arisen from three lineages independently leading to an obligate lifestyle. This review has been written with the intent, that new information on adaptation strategies of biotrophs, modifications in pathogenicity strategies and population dynamics will improve current strategies for breeding with stable resistance.

De Novo Long-Read Whole-Genome Assemblies and the Comparative Pan-Genome Analysis of Ascochyta Blight Pathogens Affecting Field Pea

Ascochyta Blight (AB) is a major disease of many cool-season legumes globally. In field pea, three fungal pathogens have been identified to be responsible for this disease in Australia, namely Peyronellaea pinodes, Peyronellaea pinodella and Phoma koolunga. Limited genomic resources for these pathogens have been generated, which has hampered the implementation of effective management strategies and breeding for resistant cultivars. Using Oxford Nanopore long-read sequencing, we report the first high-quality, fully annotated, near-chromosome-level nuclear and mitochondrial genome assemblies for 18 isolates from the Australian AB complex. Comparative genome analysis was performed to elucidate the differences and similarities between species and isolates using phylogenetic relationships and functional diversity. Our data indicated that P. pinodella and P. koolunga are heterothallic, while P. pinodes is homothallic. More homology and orthologous gene clusters are shared between P. pinodes and P. pinodella compared to P. koolunga. The analysis of the repetitive DNA content showed differences in the transposable repeat composition in the genomes and their expression in the transcriptomes. Significant repeat expansion in P. koolunga’s genome was seen, with strong repeat-induced point mutation (RIP) activity being evident. Phylogenetic analysis revealed that genetic diversity can be exploited for species marker development. This study provided the much-needed genetic resources and characterization of the AB species to further drive research in key areas such as disease epidemiology and host–pathogen interactions.

The unsung roles of microbial secondary metabolite effectors in the plant disease cacophony

Plants counter disease with an array of responses to styme pathogen ingress. In contrast to this cacophony, plant pathogens orchestrate a finely tuned repertoire of virulence mechanisms in their attempt to cause disease. One such example is the production of secondary metabolite effectors (SMEs). Despite many attempts to functionally categorize SMEs, their many roles in plant disease have proven they march to the beat of their producer's drum. Some lesser studied features of SMEs in plant disease include self-resistance (SR) and manipulation of the microbiome to enhance pathogen virulence. SR can be accomplished in three general compositions, with the first being the transport of the SME to a benign location; the second being modification of the SME so it cannot harm the producer; and the third being metabolic regulation of the SME or the producer homolog of the SME target. SMEs may also play an interlude prior to disease by shaping the plant microbial community, allowing producers to better establish themselves. Taken together, SMEs are integral players in the phytopathology canon.

Microbiota manipulation through the secretion of effector proteins is fundamental to the wealth of lifestyles in the fungal kingdom

Fungi are well-known decomposers of organic matter that thrive in virtually any environment on earth where they encounter wealths of other microbes. Some fungi evolved symbiotic lifestyles, including pathogens and mutualists, that have mostly been studied in binary interactions with their hosts. However, we now appreciate that such interactions are greatly influenced by the ecological context in which they take place. While establishing their symbioses, fungi not only interact with their hosts, but also with the host-associated microbiota. Thus, they target the host and its associated microbiota as a single holobiont. Recent studies have shown that fungal pathogens manipulate the host microbiota by means of secreted effector proteins with selective antimicrobial activity to stimulate disease development. In this review we discuss the ecological contexts in which such effector-mediated microbiota manipulation is relevant for the fungal lifestyle and argue that this is not only relevant for pathogens of plants and animals, but beneficial in virtually any niche where fungi occur. Moreover, we reason that effector-mediated microbiota manipulation likely evolved already in fungal ancestors that encountered microbial competition long before symbiosis with land plants and mammalian animals evolved. Thus, we claim that effector-mediated microbiota manipulation is fundamental to fungal biology.

Enemy or ally: a genomic approach to elucidate the lifestyle of Phyllosticta citrichinaensis

Members of the fungal genus Phyllosticta can colonize a variety of plant hosts, including several Citrus species such as Citrus sinensis (orange), Citrus limon (lemon), and Citrus maxima (pomelo). Some Phyllosticta species have the capacity to cause disease, such as Citrus Black Spot, while others have only been observed as endophytes. Thus far, genomic differences underlying lifestyle adaptations of Phyllosticta species have not yet been studied. Furthermore, the lifestyle of Phyllosticta citrichinaensis is ambiguous, as it has been described as a weak pathogen but Koch's postulates may not have been established and the presence of this species was never reported to cause any crop or economic losses. Here, we examined the genomic differences between pathogenic and endophytic Phyllosticta spp. colonizing Citrus and specifically aimed to elucidate the lifestyle of Phyllosticta citrichinaensis. We found several genomic differences between species of different lifestyles, including groups of genes that were only present in pathogens or endophytes. We also observed that species, based on their carbohydrate active enzymes, group independent of their phylogenetic association, and this clustering correlated with trophy prediction. Phyllosticta citrichinaensis shows an intermediate lifestyle, sharing genomic and phenotypic attributes of both pathogens and endophytes. We thus present the first genomic comparison of multiple citrus-colonizing pathogens and endophytes of the genus Phyllosticta, and therefore provide the basis for further comparative studies into the lifestyle adaptations within this genus.

Annotation survey and life-cycle transcriptomics of transcription factors in rust fungi (Pucciniales) identify a possible role for cold shock proteins in dormancy exit

Fungi of the order Pucciniales are obligate plant biotrophs causing rust diseases. They exhibit a complex life cycle with the production of up to five spore types, infection of two unrelated hosts and an overwintering stage. Transcription factors (TFs) are key regulators of gene expression in eukaryote cells. In order to better understand genetic programs expressed during major transitions of the rust life cycle, we surveyed the complement of TFs in fungal genomes with an emphasis on Pucciniales. We found that despite their large gene numbers, rust genomes have a reduced repertoire of TFs compared to other fungi. The proportions of C2H2 and Zinc cluster -two of the most represented TF families in fungi- indicate differences in their evolutionary relationships in Pucciniales and other fungal taxa. The regulatory gene family encoding cold shock protein (CSP) showed a striking expansion in Pucciniomycotina with specific duplications in the order Pucciniales. The survey of expression profiles collected by transcriptomics along the life cycle of the poplar rust fungus revealed TF genes related to major biological transitions, e.g. response to environmental cues and host infection. Particularly, poplar rust CSPs were strongly expressed in basidia produced after the overwintering stage suggesting a possible role in dormancy exit. Expression during transition from dormant telia to basidia confirmed the specific expression of the three poplar rust CSP genes. Their heterologous expression in yeast improved cell growth after cold stress exposure, suggesting a probable regulatory function when the poplar rust fungus exits dormancy. This study addresses for the first time TF and regulatory genes involved in developmental transition in the rust life cycle opening perspectives to further explore molecular regulation in the biology of the Pucciniales.

Sugar conundrum in plant-pathogen interactions: roles of invertase and sugar transporters depend on pathosystems

It has been increasingly recognized that CWIN (cell wall invertase) and sugar transporters including STP (sugar transport protein) and SWEET (sugar will eventually be exported transporters) play important roles in plant-pathogen interactions. However, the information available in the literature comes from diverse systems and often yields contradictory findings and conclusions. To solve this puzzle, we provide here a comprehensive assessment of the topic. Our analyses revealed that the regulation of plant-microbe interactions by CWIN, SWEET, and STP is conditioned by the specific pathosystems involved. The roles of CWINs in plant resistance are largely determined by the lifestyle of pathogens (biotrophs versus necrotrophs or hemibiotrophs), possibly through CWIN-mediated salicylic acid or jasmonic acid signaling and programmed cell death pathways. The up-regulation of SWEETs and STPs may enhance or reduce plant resistance, depending on the cellular sites from which pathogens acquire sugars from the host cells. Finally, plants employ unique mechanisms to defend against viral infection, in part through a sugar-based regulation of plasmodesmatal development or aperture. Our appraisal further calls for attention to be paid to the involvement of microbial sugar metabolism and transport in plant-pathogen interactions, which is an integrated but overlooked component of such interactions.

Two Cladosporium Fungi with Opposite Functions to the Chinese White Wax Scale Insect Have Different Genome Characters

Insects encounter infection of microorganisms, and they also harbor endosymbiosis to participate in nutrition providing and act as a defender against pathogens. We previously found the Chinese white wax scale insect, Ericerus pela, was infected and killed by Cladosporium sp. (pathogen). We also found it harbored Cladosporium sp. (endogensis). In this study, we cultured these two Cladosporium fungi and sequenced their genome. The results showed Cladosporium sp. (endogensis) has a larger genome size and more genes than Cladosporium sp. (pathogen). Pan-genome analysis showed Cladosporium sp. (endogensis)-specific genes enriched in pathways related to nutrition production, such as amino acid metabolism, carbohydrate metabolism, and energy metabolism. These pathways were absent in that of Cladosporium sp. (pathogen). Gene Ontology analysis showed Cladosporium sp. (pathogen)-specific genes enriched in the biosynthesis of asperfuranone, emericellamide, and fumagillin. These terms were not found in that of Cladosporium sp. (endogensis). Pathogen Host Interactions analysis found Cladosporium sp. (endogensis) had more genes related to loss of pathogenicity and reduced virulence than Cladosporium sp. (pathogen). Cytotoxicity assay indicated Cladosporium sp. (pathogen) had cytotoxicity, while Cladosporium sp. (endogensis) had no cytotoxicity. These characters reflect the adaptation of endosymbiosis to host-restricted lifestyle and the invader of the entomopathogen to the host.

Comparative genome analysis of plant ascomycete fungal pathogens with different lifestyles reveals distinctive virulence strategies

Background

Pathogens have evolved diverse lifestyles and adopted pivotal new roles in both natural ecosystems and human environments. However, the molecular mechanisms underlying their adaptation to new lifestyles are obscure. Comparative genomics was adopted to determine distinct strategies of plant ascomycete fungal pathogens with different lifestyles and to elucidate their distinctive virulence strategies.

Results

We found that plant ascomycete biotrophs exhibited lower gene gain and loss events and loss of CAZyme-encoding genes involved in plant cell wall degradation and biosynthesis gene clusters for the production of secondary metabolites in the genome. Comparison with the candidate effectome detected distinctive variations between plant biotrophic pathogens and other groups (including human, necrotrophic and hemibiotrophic pathogens). The results revealed the biotroph-specific and lifestyle-conserved candidate effector families. These data have been configured in web-based genome browser applications for public display (http://lab.malab.cn/soft/PFPG). This resource allows researchers to profile the genome, proteome, secretome and effectome of plant fungal pathogens.

Conclusions

Our findings demonstrated different genome evolution strategies of plant fungal pathogens with different lifestyles and explored their lifestyle-conserved and specific candidate effectors. It will provide a new basis for discovering the novel effectors and their pathogenic mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08165-1.

Antagonism to Plant Pathogens by Epichloë Fungal Endophytes-A Review

Epichloë is a genus of filamentous fungal endophytes that has co-evolved with cool-season grasses with which they form long-term, symbiotic associations. The most agriculturally important associations for pasture persistence for grazing livestock are those between asexual vertically transmitted Epichloë strains and the pasture species, perennial ryegrass, and tall fescue. The fungus confers additional traits to their host grasses including invertebrate pest deterrence and drought tolerance. Selected strains of these mutualistic endophytes have been developed into highly efficacious biocontrol products and are widely utilized within the Americas, Australia, and New Zealand for pasture persistence. Less publicized is the antagonism Epichloë endophytes display towards multiple species of saprophytic and pathogenic microbes. This opinion piece will review the current literature on antimicrobial properties exhibited by this genus of endophyte and discuss the reasons why this trait has historically remained a research curiosity rather than a trait of commercial significance.

Genome assembly of Scorias spongiosa and comparative genomics provide insights into ecological adaptation of honeydew-dependent sooty mould fungi

Sooty moulds are fungi of economic importance and with unique lifestyle mainly growing on insect honeydew. However, the lack of genomic data hinders investigation of genetic mechanisms underlying their ecological adaptation. With long-read sequencing technology, we generated the genome of Scorias spongiosa, an extraordinary sooty mould fungus associated with honeydew of colony aphids and producing large fruiting bodies. A 24.21 Mb high-quality genome assembly with a N50 length of 3.37 Mb was obtained. The genome contained 7758 protein coding genes, 97.13% of which were homologous to known genes, and approximately 0.29 Mb repeat sequences. Comparative genomics showed S. spongiosa lost relatively more gene families and contained fewer species-specific genes and gene families, with many CAZyme families and sugar transporters reduced or absent. This study not only promotes understanding of the ecological adaptation of sooty moulds, but also provides valuable genomic data resource for future comparative genomic and genetic studies.

Biogeography and Ecology of Magnaporthales: A Case Study

The order Magnaporthales belongs to Sordariomycetes, Ascomycota. Magnaporthales includes five families, namely Ceratosphaeriaceae, Pseudohalonectriaceae, Ophioceraceae, Pyriculariaceae, and Magnaporthaceae. Most Magnaporthales members are found in Poaceae plants and other monocotyledonous herbaceous plants ubiquitously as plant pathogens or endophytic fungi, and some members are found in decaying wood or dead grass as saprophytic fungi. Therefore, studying the biogeography and ecology of Magnaporthales is of great significance. Here, we described the biodiversity of endophytic Magnaporthales fungi from Poaceae at three latitudes in China and conducted a meta-analysis of the geography and ecology of Magnaporthales worldwide. We found that Magnaporthales is a dominant order in the endophytic fungi of Poaceae. More than half of the endophytic Magnaporthales fungi have a taxonomically uncertain placement. Notably, few endophytic fungi are grouped in the clusters with known saprophytic or pathogenic Magnaporthales fungi, indicating that they may have saprophytic and parasitic differentiation in nutritional modes and lifestyles. The meta-analysis revealed that most species of Magnaporthales have characteristic geographical, host, and tissue specificity. The geographical distribution of the three most studied genera, namely Gaeumannomyces, Magnaporthiopsis, and Pyricularia, in Magnaporthales may depend on the distribution of their hosts. Therefore, studies on the endophytic fungal Magnaporthales from monocotyledonous plants, including Poaceae, in middle and low latitudes will deepen our understanding of the biogeography and ecology of Magnaporthales.

The use of flow cytometry for fungal nuclear DNA quantification

Genome size information is sparse across fungi, with information being available for less than 2000 species. So far, most records have been obtained using static, microscope-based cytometry methods or derived from genome sequencing projects. Flow cytometry is now considered the state-of-the-art method for obtaining genome size measurements, and appropriate methods and DNA standards are available, enabling the analysis of most genome size ranges in a rapid, robust and inexpensive way. The average fungal genome size is 60 Mbp, but sizes vary across phylogeny, ranging from 2.2 (Encephalitozoon romaleae) to 3706 Mbp (Jafnea semitosta). In several fungal clades, genome size expansion seems to accompany evolution either to plant mutualism or to plant parasitism (particularly biotrophy), and fungi that interact with plants seem to have larger genomes than saprobes and those that interact with animals. Whereas flow cytometry for nuclear DNA quantification is routinely employed in plant sciences for genome size and ploidy studies, its use in fungal biology is still infrequent. Appropriate standards, methods and best practices are described here, with the aim of stimulating a more generalized and widespread use of flow cytometry for fungal genome size measurement.

Near-Chromosome-Level Genome Assembly of the Dark Septate Endophyte Laburnicola rhizohalophila: A Model for Investigating Root-Fungus Symbiosis

The novel DSE Laburnicola rhizohalophila (Pleosporales, Ascomycota) is frequently found in the halophytic seepweed (Suaeda salsa). In this article, we report a near-chromosome-level hybrid assembly of this fungus using a combination of short-read Illumina data to polish assemblies generated from long-read Nanopore data. The reference genome for L. rhizohalophila was assembled into 26 scaffolds with a total length of 64.0 Mb and a N50 length of 3.15 Mb. Of them, 17 scaffolds approached the length of intact chromosomes, and 5 had telomeres at one end only. A total of 10,891 gene models were predicted. Intriguingly, 27.5 Mb of repeat sequences that accounted for 42.97% of the genome was identified, and long terminal repeat retrotransposons were the most frequent known transposable elements, indicating that transposable element proliferation contributes to its increased genome size. BUSCO analyses using the Fungi_odb10 data set showed that 95.0% of genes were complete. In addition, 292 carbohydrate active enzymes, 33 secondary metabolite clusters, and 84 putative effectors were identified in silico. The resulting high-quality assembly and genome features are not only an important resource for further research on understanding the mechanism of root-fungi symbiotic interactions but will also contribute to comparative analyses of genome biology and evolution within Pleosporalean species.

The Arabidopsis PHD-finger protein EDM2 has multiple roles in balancing NLR immune receptor gene expression

Plant NLR-type receptors serve as sensitive triggers of host immunity. Their expression has to be well-balanced, due to their interference with various cellular processes and dose-dependency of their defense-inducing activity. A genetic “arms race” with fast-evolving pathogenic microbes requires plants to constantly innovate their NLR repertoires. We previously showed that insertion of the COPIA-R7 retrotransposon into RPP7 co-opted the epigenetic transposon silencing signal H3K9me2 to a new function promoting expression of this Arabidopsis thaliana NLR gene. Recruitment of the histone binding protein EDM2 to COPIA-R7-associated H3K9me2 is required for optimal expression of RPP7. By profiling of genome-wide effects of EDM2, we now uncovered additional examples illustrating effects of transposons on NLR gene expression, strongly suggesting that these mobile elements can play critical roles in the rapid evolution of plant NLR genes by providing the “raw material” for gene expression mechanisms. We further found EDM2 to have a global role in NLR expression control. Besides serving as a positive regulator of RPP7 and a small number of other NLR genes, EDM2 acts as a suppressor of a multitude of additional NLR genes. We speculate that the dual functionality of EDM2 in NLR expression control arose from the need to compensate for fitness penalties caused by high expression of some NLR genes by suppression of others. Moreover, we are providing new insights into functional relationships of EDM2 with its interaction partner, the RNA binding protein EDM3/AIPP1, and its target gene IBM1, encoding an H3K9-demethylase.

We previously found the Arabidopsis thaliana PHD-finger protein EDM2 to serve as a chromatin-associated factor controlling expression of the NLR-type immune receptor gene RPP7. EDM2 binds to the transposon-silencing signal H3K9me2 and affects levels of this epigenetic mark at various loci. By genome-wide profiling of transcript- and H3K9me2-levels we now found EDM2 to have a broader role in controlling NLR gene expression. In order to mitigate fitness costs caused by its promoting effects on RPP7 expression and that of several other NLR genes, EDM2 seems to suppress expression of many additional members of this gene family. This observation is in line with multiple reports demonstrating the need for balanced expression of NLRs, which can substantially reduce overall plant fitness, but need to be present at certain minimal levels to confer sufficient immune protection. Our previous results demonstrated that the influence of EDM2 on RPP7 expression was co-opted to this immune receptor gene by the insertion of an EDM2-controlled transposon. Here, we are providing additional examples for transposon-associated effects on NLR gene expression, suggesting that these mobile elements play an important role for NLR genes by equipping members of this rapidly evolving gene family with regulatory mechanisms needed for balanced expression.

A Global Survey of Carbohydrate Esterase Families 1 and 10 in Oomycetes

Carbohydrate-active enzymes (CAZymes) are a cornerstone in the phytopathogenicity of filamentous microbes. CAZymes are required for every step of a successful infection cycle-from penetration, to nutrient acquisition (during colonization), to exit and dispersal. Yet, CAZymes are not a unique feature of filamentous pathogens. They are found across eukaryotic genomes and including, for example, saprotrophic relatives of major pathogens. Comparative genomics and functional analyses revealed that CAZyme content is shaped by a multitude of factors, including utilized substrate, lifestyle, and host preference. Yet, family size alone says little about usage. Indeed, in a previous study, we found that genes putatively coding for the CAZyme families of carbohydrate esterase (CE)1 and CE10, while not specifically enriched in number, were suggested to have lifestyle-specific gene expression patterns. Here, we used comparative genomics and a clustering approach to understand how the repertoire of the CE1- and CE10-encoding gene families is shaped across oomycete evolution. These data are combined with comparative transcriptomic analyses across homologous clusters within the gene families. We find that CE1 and CE10 have been reduced in number in biotrophic oomycetes independent of the phylogenetic relationship of the biotrophs to each other. The reduction in CE1 is different from that observed for CE10: While in CE10 specific clusters of homologous sequences show convergent reduction, CE1 reduction is caused by species-specific losses. Comparative transcriptomics revealed that some clusters of CE1 or CE10 sequences have a higher expression than others, independent of the species composition within them. Further, we find that CE1- and CE10-encoding genes are mainly induced in plant pathogens and that some homologous genes show lifestyle-specific gene expression levels during infection, with hemibiotrophs showing the highest expression levels.

The Genome of Peronospora belbahrii Reveals High Heterozygosity, a Low Number of Canonical Effectors, and TC-Rich Promoters

Along with Plasmopara destructor, Peronosopora belbahrii has arguably been the economically most important newly emerging downy mildew pathogen of the past two decades. Originating from Africa, it has started devastating basil production throughout the world, most likely due to the distribution of infested seed material. Here, we present the genome of this pathogen and results from comparisons of its genomic features to other oomycetes. The assembly of the nuclear genome was around 35.4 Mbp in length, with an N₅₀ scaffold length of around 248 kbp and an L₅₀ scaffold count of 46. The circular mitochondrial genome consisted of around 40.1 kbp. From the repeat-masked genome, 9,049 protein-coding genes were predicted, out of which 335 were predicted to have extracellular functions, representing the smallest secretome so far found in peronosporalean oomycetes. About 16% of the genome consists of repetitive sequences, and, based on simple sequence repeat regions, we provide a set of microsatellites that could be used for population genetic studies of P. belbahrii. P. belbahrii has undergone a high degree of convergent evolution with other obligate parasitic pathogen groups, reflecting its obligate biotrophic lifestyle. Features of its secretome, signaling networks, and promoters are presented, and some patterns are hypothesized to reflect the high degree of host specificity in Peronospora species. In addition, we suggest the presence of additional virulence factors apart from classical effector classes that are promising candidates for future functional studies.

Adaptation of the metabolomics profile of rice after Pyricularia oryzae infection

Pyricularia oryzae (P. oryzae), one of the most devastating fungal pathogens, is the cause of blast disease in rice. Infection with a blast fungus induces biological responses in the host plant that lead to its survival through the termination or suppression of pathogen growth, and metabolite compounds play vital roles in plant interactions with a wide variety of other organisms. Numerous studies have indicated that rice has a multi-layered plant immune system that includes pre-developed (e.g., cell wall and phytoanticipins), constitutive and inducible (phytoalexins) defence barriers against stresses. Significant progress towards understanding the basis of the molecular mechanisms underlying the defence responses of rice to P. oryzae has been achieved. Nonetheless, even though the important metabolites in the responses of rice to pathogens have been identified, their exact mechanisms and their contributions to plant immunity against blast fungi have not been elucidated. The purpose of this review is to summarize and discuss recent advances towards the understanding of the integrated metabolite variations in rice after P. oryzae invasion.

Stealth Pathogens: The Sooty Blotch and Flyspeck Fungal Complex

Sooty blotch and flyspeck (SBFS) fungi produce superficial, dark-colored colonies on fruits, stems, and leaves of many plant genera. These blemishes are economically damaging on fruit, primarily apple and pear, because they reduce the sale price of fresh fruit. Fungicide spray programs can control SBFS but are costly and impair human and environmental health; thus, less chemically intensive management strategies are needed. Although the scientific study of SBFS fungi began nearly 200 years ago, recent DNA-driven studies revealed an unexpectedly diverse complex: more than 100 species in 30 genera of Ascomycota and Basidiomycota. Analysis of evolutionary phylogenetics and phylogenomics indicates that the evolution of SBFS fungi from plant-penetrating ancestors to noninvasive ectophytic parasites was accompanied by a massive contraction of pathogenicity-related genes, including plant cell wall-degrading enzymes and effectors, and an expansion of cuticle-degradation genes. This article reviews progress in understanding SBFS taxonomy and ecology and improving disease management. We also highlight recent breakthroughs in reconstructing the evolutionary origins of these unusual plant pathogens and delineating adaptations to their ectophytic niche.

The lichen symbiosis re-viewed through the genomes of Cladonia grayi and its algal partner Asterochloris glomerata

BACKGROUND

Lichens, encompassing 20,000 known species, are symbioses between specialized fungi (mycobionts), mostly ascomycetes, and unicellular green algae or cyanobacteria (photobionts). Here we describe the first parallel genomic analysis of the mycobiont Cladonia grayi and of its green algal photobiont Asterochloris glomerata. We focus on genes/predicted proteins of potential symbiotic significance, sought by surveying proteins differentially activated during early stages of mycobiont and photobiont interaction in coculture, expanded or contracted protein families, and proteins with differential rates of evolution.

RESULTS

A) In coculture, the fungus upregulated small secreted proteins, membrane transport proteins, signal transduction components, extracellular hydrolases and, notably, a ribitol transporter and an ammonium transporter, and the alga activated DNA metabolism, signal transduction, and expression of flagellar components. B) Expanded fungal protein families include heterokaryon incompatibility proteins, polyketide synthases, and a unique set of G-protein α subunit paralogs. Expanded algal protein families include carbohydrate active enzymes and a specific subclass of cytoplasmic carbonic anhydrases. The alga also appears to have acquired by horizontal gene transfer from prokaryotes novel archaeal ATPases and Desiccation-Related Proteins. Expanded in both symbionts are signal transduction components, ankyrin domain proteins and transcription factors involved in chromatin remodeling and stress responses. The fungal transportome is contracted, as are algal nitrate assimilation genes. C) In the mycobiont, slow-evolving proteins were enriched for components involved in protein translation, translocation and sorting.

CONCLUSIONS

The surveyed genes affect stress resistance, signaling, genome reprogramming, nutritional and structural interactions. The alga carries many genes likely transferred horizontally through viruses, yet we found no evidence of inter-symbiont gene transfer. The presence in the photobiont of meiosis-specific genes supports the notion that sexual reproduction occurs in Asterochloris while they are free-living, a phenomenon with implications for the adaptability of lichens and the persistent autonomy of the symbionts. The diversity of the genes affecting the symbiosis suggests that lichens evolved by accretion of many scattered regulatory and structural changes rather than through introduction of a few key innovations. This predicts that paths to lichenization were variable in different phyla, which is consistent with the emerging consensus that ascolichens could have had a few independent origins.

Metabolic Model of the Phytophthora infestans-Tomato Interaction Reveals Metabolic Switches during Host Colonization

The oomycete pathogen Phytophthora infestans causes potato and tomato late blight, a disease that is a serious threat to agriculture. P. infestans is a hemibiotrophic pathogen, and during infection, it scavenges nutrients from living host cells for its own proliferation. To date, the nutrient flux from host to pathogen during infection has hardly been studied, and the interlinked metabolisms of the pathogen and host remain poorly understood. Here, we reconstructed an integrated metabolic model of P. infestans and tomato (Solanum lycopersicum) by integrating two previously published models for both species. We used this integrated model to simulate metabolic fluxes from host to pathogen and explored the topology of the model to study the dependencies of the metabolism of P. infestans on that of tomato. This showed, for example, that P. infestans, a thiamine auxotroph, depends on certain metabolic reactions of the tomato thiamine biosynthesis. We also exploited dual-transcriptome data of a time course of a full late blight infection cycle on tomato leaves and integrated the expression of metabolic enzymes in the model. This revealed profound changes in pathogen-host metabolism during infection. As infection progresses, P. infestans performs less de novo synthesis of metabolites and scavenges more metabolites from tomato. This integrated metabolic model for the P. infestans-tomato interaction provides a framework to integrate data and generate hypotheses about in planta nutrition of P. infestans throughout its infection cycle.IMPORTANCE Late blight disease caused by the oomycete pathogen Phytophthora infestans leads to extensive yield losses in tomato and potato cultivation worldwide. To effectively control this pathogen, a thorough understanding of the mechanisms shaping the interaction with its hosts is paramount. While considerable work has focused on exploring host defense mechanisms and identifying P. infestans proteins contributing to virulence and pathogenicity, the nutritional strategies of the pathogen are mostly unresolved. Genome-scale metabolic models (GEMs) can be used to simulate metabolic fluxes and help in unravelling the complex nature of metabolism. We integrated a GEM of tomato with a GEM of P. infestans to simulate the metabolic fluxes that occur during infection. This yields insights into the nutrients that P. infestans obtains during different phases of the infection cycle and helps in generating hypotheses about nutrition in planta.

Multi-omics characterization of the necrotrophic mycoparasite Saccharomycopsis schoenii

Pathogenic yeasts and fungi are an increasing global healthcare burden, but discovery of novel antifungal agents is slow. The mycoparasitic yeast Saccharomycopsis schoenii was recently demonstrated to be able to kill the emerging multi-drug resistant yeast pathogen Candida auris. However, the molecular mechanisms involved in the predatory activity of S. schoenii have not been explored. To this end, we de novo sequenced, assembled and annotated a draft genome of S. schoenii. Using proteomics, we confirmed that Saccharomycopsis yeasts have reassigned the CTG codon and translate CTG into serine instead of leucine. Further, we confirmed an absence of all genes from the sulfate assimilation pathway in the genome of S. schoenii, and detected the expansion of several gene families, including aspartic proteases. Using Saccharomyces cerevisiae as a model prey cell, we honed in on the timing and nutritional conditions under which S. schoenii kills prey cells. We found that a general nutrition limitation, not a specific methionine deficiency, triggered predatory activity. Nevertheless, by means of genome-wide transcriptome analysis we observed dramatic responses to methionine deprivation, which were alleviated when S. cerevisiae was available as prey, and therefore postulate that S. schoenii acquired methionine from its prey cells. During predation, both proteomic and transcriptomic analyses revealed that S. schoenii highly upregulated and translated aspartic protease genes, probably used to break down prey cell walls. With these fundamental insights into the predatory behavior of S. schoenii, we open up for further exploitation of this yeast as a biocontrol yeast and/or source for novel antifungal agents.

Advances in understanding obligate biotrophy in rust fungi

Contents Summary 1190 I. Introduction 1190 II. Rust fungi: a diverse and serious threat to agriculture 1191 III. The different facets of rust life cycles and unresolved questions about their evolution 1191 IV. The biology of rust infection 1192 V. Rusts in the genomics era: the ever-expanding list of candidate effector genes 1195 VI. Functional characterization of rust effectors 1197 VII. Putting rusts to sleep: Pucciniales research outlooks 1201 Acknowledgements 1202 References 1202 SUMMARY: Rust fungi (Pucciniales) are the largest group of plant pathogens and represent one of the most devastating threats to agricultural crops worldwide. Despite the economic importance of these highly specialized pathogens, many aspects of their biology remain obscure, largely because rust fungi are obligate biotrophs. The rise of genomics and advances in high-throughput sequencing technology have presented new options for identifying candidate effector genes involved in pathogenicity mechanisms of rust fungi. Transcriptome analysis and integrated bioinformatics tools have led to the identification of key genetic determinants of host susceptibility to infection by rusts. Thousands of genes encoding secreted proteins highly expressed during host infection have been reported for different rust species, which represents significant potential towards understanding rust effector function. Recent high-throughput in planta expression screen approaches (effectoromics) have pushed the field ahead even further towards predicting high-priority effectors and identifying avirulence genes. These new insights into rust effector biology promise to inform future research and spur the development of effective and sustainable strategies for managing rust diseases.

Niche-specific metabolic adaptation in biotrophic and necrotrophic oomycetes is manifested in differential use of nutrients, variation in gene content, and enzyme evolution

The use of host nutrients to support pathogen growth is central to disease. We addressed the relationship between metabolism and trophic behavior by comparing metabolic gene expression during potato tuber colonization by two oomycetes, the hemibiotroph Phytophthora infestans and the necrotroph Pythium ultimum. Genes for several pathways including amino acid, nucleotide, and cofactor biosynthesis were expressed more by Ph. infestans during its biotrophic stage compared to Py. ultimum. In contrast, Py. ultimum had higher expression of genes for metabolizing compounds that are normally sequestered within plant cells but released to the pathogen upon plant cell lysis, such as starch and triacylglycerides. The transcription pattern of metabolic genes in Ph. infestans during late infection became more like that of Py. ultimum, consistent with the former's transition to necrotrophy. Interspecific variation in metabolic gene content was limited but included the presence of γ-amylase only in Py. ultimum. The pathogens were also found to employ strikingly distinct strategies for using nitrate. Measurements of mRNA, ¹⁵N labeling studies, enzyme assays, and immunoblotting indicated that the assimilation pathway in Ph. infestans was nitrate-insensitive but induced during amino acid and ammonium starvation. In contrast, the pathway was nitrate-induced but not amino acid-repressed in Py. ultimum. The lack of amino acid repression in Py. ultimum appears due to the absence of a transcription factor common to fungi and Phytophthora that acts as a nitrogen metabolite repressor. Evidence for functional diversification in nitrate reductase protein was also observed. Its temperature optimum was adapted to each organism's growth range, and its K_m was much lower in Py. ultimum. In summary, we observed divergence in patterns of gene expression, gene content, and enzyme function which contribute to the fitness of each species in its niche.

A key feature of disease is the pathogen's consumption of host metabolites to support its growth and multiplication. Understanding how host nutrients are used by pathogens may lead to strategies for limiting disease, for example by developing inhibitors of metabolic pathways needed for pathogen growth. Feeding strategies of plant pathogens range between two extremes: necrotrophs kill host cells and consume the released nutrients, while biotrophs do not injure host cells but instead acquire nutrients from extracellular spaces in the plant. In this study, a comparison was made between the metabolism of Phytophthora infestans (the infamous Irish Famine pathogen) and Pythium ultimum during potato tuber colonization. These microbes have close evolutionary histories, but while Py. ultimum is a necrotroph, Ph. infestans is a biotroph for most of the disease cycle. It was discovered that distinct patterns of metabolic gene expression, gene content, and enzyme behavior underlie these lifestyles. For example, genes for utilizing compounds that are normally stored within plant cells were expressed more by Py. ultimum, while Ph. infestans appeared to synthesize more biosubstances from precursors. Several differences in carbon and nitrogen metabolism were linked to variation in enzyme content and gene expression regulators in the two species.

Genome sequencing and transcript analysis of Hemileia vastatrix reveal expression dynamics of candidate effectors dependent on host compatibility

Coffee leaf rust caused by the fungus Hemileia vastatrix is one of the most important leaf diseases of coffee plantations worldwide. Current knowledge of the H. vastatrix genome is limited and only a small fraction of the total fungal secretome has been identified. In order to obtain a more comprehensive understanding of its secretome, we aimed to sequence and assemble the entire H. vastatrix genome using two next-generation sequencing platforms and a hybrid assembly strategy. This resulted in a 547 Mb genome of H. vastatrix race XXXIII (Hv33), with 13,364 predicted genes that encode 13,034 putative proteins with transcriptomic support. Based on this proteome, 615 proteins contain putative secretion peptides, and lack transmembrane domains. From this putative secretome, 111 proteins were identified as candidate effectors (EHv33) unique to H. vastatrix, and a subset consisting of 17 EHv33 genes was selected for a temporal gene expression analysis during infection. Five genes were significantly induced early during an incompatible interaction, indicating their potential role as pre-haustorial effectors possibly recognized by the resistant coffee genotype. Another nine genes were significantly induced after haustorium formation in the compatible interaction. Overall, we suggest that this fungus is able to selectively mount its survival strategy with effectors that depend on the host genotype involved in the infection process.

Powdery Mildews Are Characterized by Contracted Carbohydrate Metabolism and Diverse Effectors to Adapt to Obligate Biotrophic Lifestyle

Powdery mildew is a widespread plant disease caused by obligate biotrophic fungal pathogens involving species-specific interactions between host and parasite. To gain genomic insights into the underlying obligate biotrophic mechanisms, we analyzed 15 microbial genomes covering powdery and downy mildews and rusts. We observed a genome-wide, massive contraction of multiple gene families in powdery mildews, such as enzymes in the carbohydrate metabolism pathway, when compared with ascomycete phytopathogens, while the fatty acid metabolism pathway maintained its integrity. We also observed significant differences in candidate secreted effector protein (CSEP) families between monocot and dicot powdery mildews, perhaps due to different selection forces. While CSEPs in monocot mildews are likely subject to positive selection causing rapid expansion, CSEP families in dicot mildews are shrinking under strong purifying selection. Our results not only illustrate obligate biotrophic mechanisms of powdery mildews driven by gene family evolution in nutrient metabolism, but also demonstrate how the divergence of CSEPs between monocot and dicot lineages might contribute to species-specific adaption.

Ericoid mycorrhizal fungi and their genomes: another side to the mycorrhizal symbiosis?

Contents Summary 1141 I. Introduction 1141 II. The ericoid mycorrhizal lifestyle 1141 III. Lessons from the mycorrhizal fungal genomes 1142 IV. ERM fungi: a discordant voice in the mycorrhizal choir 1143 V. An endophytic niche for ERM fungi 1144 VI. Specialised vs unspecialised mycorrhizal fungi? 1145 VII. Conclusions and perspectives 1145 Acknowledgements 1146 References 1146 SUMMARY: The genome of an organism bears the signature of its lifestyle, and organisms with similar life strategies are expected to share common genomic traits. Indeed, ectomycorrhizal and arbuscular mycorrhizal fungi share some genomic traits, such as the expansion of gene families encoding taxon-specific small secreted proteins, which are candidate effectors in the symbiosis, and a very small repertoire of plant cell wall-degrading enzymes. A large gene family coding for candidate effectors was also revealed in ascomycetous ericoid mycorrhizal (ERM) fungi, but these fungal genomes are characterised by a very high number of genes encoding degradative enzymes, mainly acting on plant cell wall components. We suggest that the genomic signature of ERM fungi mirrors a versatile life strategy, which allows them to occupy several ecological niches.

New insights into Phakopsora pachyrhizi infection based on transcriptome analysis in planta

Asian soybean rust (ASR) is one of the most destructive diseases affecting soybeans. The causative agent of ASR, the fungus Phakopsora pachyrhizi, presents characteristics that make it difficult to study in vitro, limiting our knowledge of plant-pathogen dynamics. Therefore, this work used leaf lesion laser microdissection associated with deep sequencing to determine the pathogen transcriptome during compatible and incompatible interactions with soybean. The 36,350 generated unisequences provided an overview of the main genes and biological pathways that were active in the fungus during the infection cycle. We also identified the most expressed transcripts, including sequences similar to other fungal virulence and signaling proteins. Enriched P. pachyrhizi transcripts in the resistant (PI561356) soybean genotype were related to extracellular matrix organization and metabolic signaling pathways and, among infection structures, in amino acid metabolism and intracellular transport. Unisequences were further grouped into gene families along predicted sequences from 15 other fungi and oomycetes, including rust fungi, allowing the identification of conserved multigenic families, as well as being specific to P. pachyrhizi. The results revealed important biological processes observed in P. pachyrhizi, contributing with information related to fungal biology and, consequently, a better understanding of ASR.

Deciphering the biology of Cryptophyllachora eurasiatica gen. et sp. nov., an often cryptic pathogen of an allergenic weed, Ambrosia artemisiifolia

A little known, unculturable ascomycete, referred to as Phyllachora ambrosiae, can destroy the inflorescences of Ambrosia artemisiifolia, an invasive agricultural weed and producer of highly allergenic pollen. The fungus often remains undetectable in ragweed populations. This work was conducted to understand its origin and pathogenesis, a prerequisite to consider its potential as a biocontrol agent. The methods used included light and transmission electron microscopy, nrDNA sequencing, phylogenetic analyses, artificial inoculations, and the examination of old herbarium and recent field specimens from Hungary, Korea, Ukraine and USA. The Eurasian and the North American specimens of this fungus were to represent two distinct, although closely related lineages that were only distantly related to other lineages within the Ascomycota. Consequently, we describe a new genus that includes Cryptophyllachora eurasiatica gen. et sp. nov. and C. ambrosiae comb. nov., respectively. The pathogenesis of C. eurasiatica was shown in A. artemisiifolia. No evidence was found for either seed-borne transmission or systemic infection. Two hypotheses were developed to explain the interaction between C. eurasiatica and A. artemisiifolia: (i) as yet undetected seed-borne transmissions and latent, systemic infections; or (ii) alternative hosts.

Functional and Expression Analyses of the Pneumocystis MAT Genes Suggest Obligate Sexuality through Primary Homothallism within Host Lungs

Fungi of the genus Pneumocystis are obligate parasites that colonize mammals’ lungs and are host species specific. Pneumocystis jirovecii and Pneumocystis carinii infect, respectively, humans and rats. They can turn into opportunistic pathogens in immunosuppressed hosts, causing severe pneumonia. Their cell cycle is poorly known, mainly because of the absence of an established method of culture in vitro. It is thought to include both asexual and sexual phases. Comparative genomic analysis suggested that their mode of sexual reproduction is primary homothallism involving a single mating type (MAT) locus encompassing plus and minus genes (matMc, matMi, and matPi; Almeida et al., mBio 6:e02250-14, 2015). Thus, each strain would be capable of sexual reproduction alone (self-fertility). However, this is a working hypothesis derived from computational analyses that is, in addition, based on the genome sequences of single isolates. Here, we tested this hypothesis in the wet laboratory. The function of the P. jirovecii and P. carinii matMc genes was ascertained by restoration of sporulation in the corresponding mutant of fission yeast. Using PCR, we found the same single MAT locus in all P. jirovecii isolates and showed that all three MAT genes are often concomitantly expressed during pneumonia. Extensive homology searches did not identify other types of MAT transcription factors in the genomes or cis-acting motifs flanking the MAT locus that could have been involved in MAT switching or silencing. Our observations suggest that Pneumocystis sexuality through primary homothallism is obligate within host lungs to complete the cell cycle, i.e., produce asci necessary for airborne transmission to new hosts.

Fungi of the genus Pneumocystis colonize the lungs of mammals. In immunosuppressed human hosts, Pneumocystis jirovecii may cause severe pneumonia that can be fatal. This disease is one of the most frequent life-threatening invasive fungal infections in humans. The analysis of the genome sequences of these uncultivable pathogens suggested that their sexual reproduction involves a single partner (self-fertilization). Here, we report laboratory experiments that support this hypothesis. The function of the three genes responsible for sexual differentiation was ascertained by the restoration of sexual reproduction in the corresponding mutant of another fungus. As predicted by self-fertilization, all P. jirovecii isolates harbored the same three genes that were often concomitantly expressed within human lungs during infection. Our observations suggest that the sexuality of these pathogens relies on the self-fertility of each isolate and is obligate within host lungs to complete the cell cycle and allow dissemination of the fungus to new hosts.

The coffee leaf rust pathogen Hemileia vastatrix: one and a half centuries around the tropics

TAXONOMY AND HISTORY

Hemileia vastatrix Berk. and Broome (Basidiomycota, Pucciniales) was described in 1869 in eastern Africa and Ceylon as the agent of coffee leaf rust and has spread to all coffee cultivation areas worldwide. Major disease outbreaks in Asia, Africa and America caused and continue to cause severe yield losses, making this the most important disease of Arabica coffee, a cash crop for many tropical and sub-tropical countries.

LIFE CYCLE AND DISEASE SYMPTOMS

Hemileia vastatrix is a hemicyclic fungus with the urediniosporic life cycle as its most important (if not only) source of inoculum. Chlorotic spots are the first macroscopic symptoms, preceding the differentiation of suprastomatal, bouquet-shaped, orange-coloured uredinia. The disease can cause yield losses of up to 35% and have a polyetic epidemiological impact on subsequent years.

DISEASE CONTROL

Although the use of fungicides is one of the preferred immediate control measures, the use of resistant cultivars is considered to be the most effective and durable disease control strategy. The discovery of 'Híbrido de Timor' provided sources of resistance that have been used in several breeding programmes and that have been proven to be effective and durable, as some have been in use for more than 30 years.

GENETIC DIVERSITY AND MOLECULAR PATHOGENICITY

Although exhibiting limited genetic polymorphism, the very large genome of H. vastatrix (c. 797 Mbp) conceals great pathological diversity, with more than 50 physiological races. Gene expression studies have revealed a very precocious activation of signalling pathways and production of putative effectors, suggesting that the plant-fungus dialogue starts as early as at the germ tube stage, and have provided clues for the identification of avr genes.

Validation of standards suitable for genome size estimation of fungi

Genome size information is fundamental to genome sequencing and may also uncover genomic aspects of evolution. Flow Cytometry, the preferred method for genome size estimation, requires suitable standards. Here we validate Inonotus hispidus, Colletotrichum acutatum and Cenococcum geophilum (41, 68 and 203Mbp), as standards for fungal genome size estimation.

Genome sequence of the ectophytic fungus Ramichloridium luteum reveals unique evolutionary adaptations to plant surface niche

BACKGROUND

Ectophytic fungi occupy the waxy plant surface, an extreme environment characterized by prolonged desiccation, nutrient limitation, and exposure to solar radiation. The nature of mechanisms that facilitate adaptation to this environment remains unclear. In this study, we sequenced the complete genome of an ectophytic fungus, Ramichloridium luteum, which colonizes the surface of apple fruit, and carried out comparative genomic and transcriptome analysis.

RESULTS

The R. luteum genome was 28.18 Mb and encoded 9466 genes containing 1.85% repetitive elements. Compared with cell-penetrating pathogens, genes encoding plant cell wall degrading enzymes (PCWDEs), PTH11-like G protein-coupled receptors (GPCRs) and effectors were drastically reduced. In contrast, genes encoding cutinases and secretory lipases were strikingly expanded, and four of nine secretory lipases were probably acquired by horizontal gene transfer from Basidiomycota. Transcriptomic analysis revealed elevated expression of genes involved in cuticle degradation (cutinase, secretory lipase) and stress responses (melanin biosynthesis, aquaporins, lysozymes and HOG pathway).

CONCLUSIONS

Taken together, our results highlight genomic features associated with evolution of surface niche adaptation by the ectophytic fungus R. luteum, namely the contraction of PCWDEs, PTH11-like GPCRs and effectors, and the expansion of cuticle degradation and stress tolerance.

Fungal Plant Pathogenesis Mediated by Effectors

The interactions between fungi and plants encompass a spectrum of ecologies ranging from saprotrophy (growth on dead plant material) through pathogenesis (growth of the fungus accompanied by disease on the plant) to symbiosis (growth of the fungus with growth enhancement of the plant). We consider pathogenesis in this article and the key roles played by a range of pathogen-encoded molecules that have collectively become known as effectors.

Purine Acquisition and Synthesis by Human Fungal Pathogens

While members of the Kingdom Fungi are found across many of the world's most hostile environments, only a limited number of species can thrive within the human host. The causative agents of the most common invasive fungal infections are Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. During the infection process, these fungi must not only combat the host immune system while adapting to dramatic changes in temperature and pH, but also acquire sufficient nutrients to enable growth and dissemination in the host. One class of nutrients required by fungi, which is found in varying concentrations in their environmental niches and the human host, is the purines. These nitrogen-containing heterocycles are one of the most abundant organic molecules in nature and are required for roles as diverse as signal transduction, energy metabolism and DNA synthesis. The most common life-threatening fungal pathogens can degrade, salvage and synthesize de novo purines through a number of enzymatic steps that are conserved. While these enable them to adapt to the changing purine availability in the environment, only de novo purine biosynthesis is essential during infection and therefore an attractive antimycotic target.

Comparative analysis of the predicted secretomes of Rosaceae scab pathogens Venturia inaequalis and V. pirina reveals expanded effector families and putative determinants of host range

BACKGROUND

Fungal plant pathogens belonging to the genus Venturia cause damaging scab diseases of members of the Rosaceae. In terms of economic impact, the most important of these are V. inaequalis, which infects apple, and V. pirina, which is a pathogen of European pear. Given that Venturia fungi colonise the sub-cuticular space without penetrating plant cells, it is assumed that effectors that contribute to virulence and determination of host range will be secreted into this plant-pathogen interface. Thus the predicted secretomes of a range of isolates of Venturia with distinct host-ranges were interrogated to reveal putative proteins involved in virulence and pathogenicity.

RESULTS

Genomes of Venturia pirina (one European pear scab isolate) and Venturia inaequalis (three apple scab, and one loquat scab, isolates) were sequenced and the predicted secretomes of each isolate identified. RNA-Seq was conducted on the apple-specific V. inaequalis isolate Vi1 (in vitro and infected apple leaves) to highlight virulence and pathogenicity components of the secretome. Genes encoding over 600 small secreted proteins (candidate effectors) were identified, most of which are novel to Venturia, with expansion of putative effector families a feature of the genus. Numerous genes with similarity to Leptosphaeria maculans AvrLm6 and the Verticillium spp. Ave1 were identified. Candidates for avirulence effectors with cognate resistance genes involved in race-cultivar specificity were identified, as were putative proteins involved in host-species determination. Candidate effectors were found, on average, to be in regions of relatively low gene-density and in closer proximity to repeats (e.g. transposable elements), compared with core eukaryotic genes.

CONCLUSIONS

Comparative secretomics has revealed candidate effectors from Venturia fungal plant pathogens that attack pome fruit. Effectors that are putative determinants of host range were identified; both those that may be involved in race-cultivar and host-species specificity. Since many of the effector candidates are in close proximity to repetitive sequences this may point to a possible mechanism for the effector gene family expansion observed and a route to diversification via transposition and repeat-induced point mutation.

Spatiotemporal Patterns in the Airborne Dispersal of Spinach Downy Mildew

Downy mildew is the most devastating disease threatening sustainable spinach production, particularly in the organic sector. The disease is caused by the biotrophic oomycete pathogen Peronospora effusa, and the disease results in yellow lesions that render the crop unmarketable. In this study, the levels of DNA from airborne spores of P. effusa were assessed near a field of susceptible plants in Salinas, CA during the winter months of 2013-14 and 2014/15 using rotating-arm impaction spore-trap samplers that were assessed with a species-specific quantitative polymerase chain reaction (qPCR) assay. Low levels of P. effusa DNA were detectable from December through February in both winters but increased during January in both years, in correlation with observed disease incidence; sharp peaks in P. effusa DNA detection were associated with the onset of disease incidence. The incidence of downy mildew in the susceptible field displayed logistic-like dynamics but with considerable interseason variation. Analysis of the area under the disease progress curves suggested that the 2013-14 epidemic was significantly more severe than the 2014-15 epidemic. Spatial analyses indicated that disease incidence was dependent within an average range of 5.6 m, approximately equivalent to the width of three planted beds in a typical production field. The spatial distribution of spores captured during an active epidemic most closely fit a power-law distribution but could also be fit with an exponential distribution. These studies revealed two important results in the epidemiology of spinach downy mildew in California. First, they demonstrated the potential of impaction spore-trap samplers linked with a qPCR assay for indicating periods of high disease risk, as well as the detection of long-distance dispersal of P. effusa spores. Second, at the scale of individual crops, a high degree of spatial aggregation in disease incidence was revealed.

Chloroplast Redox Status Modulates Genome-Wide Plant Responses during the Non-host Interaction of Tobacco with the Hemibiotrophic Bacterium Xanthomonas campestris pv. vesicatoria

Non-host resistance is the most ample and durable form of plant resistance against pathogen infection. It includes induction of defense-associated genes, massive metabolic reprogramming, and in many instances, a form of localized cell death (LCD) at the site of infection, purportedly designed to limit the spread of biotrophic and hemibiotrophic microorganisms. Reactive oxygen species (ROS) have been proposed to act as signals for LCD orchestration. They are produced in various cellular compartments including chloroplasts, mitochondria and apoplast. We have previously reported that down-regulation of ROS build-up in chloroplasts by expression of a plastid-targeted flavodoxin (Fld) suppressed LCD in tobacco leaves inoculated with the non-host bacterium Xanthomonas campestris pv. vesicatoria (Xcv), while other defensive responses were unaffected, suggesting that chloroplast ROS and/or redox status play a major role in the progress of LCD. To better understand these effects, we compare here the transcriptomic alterations caused by Xcv inoculation on leaves of Fld-expressing tobacco plants and their wild-type siblings. About 29% of leaf-expressed genes were affected by Xcv and/or Fld. Surprisingly, 5.8% of them (1,111 genes) were regulated by Fld in the absence of infection, presumably representing pathways responsive to chloroplast ROS production and/or redox status during normal growth conditions. While the majority (∼75%) of pathogen-responsive genes were not affected by Fld, many Xcv responses were exacerbated, attenuated, or regulated in opposite direction by expression of this protein. Particularly interesting was a group of 384 genes displaying Xcv responses that were already triggered by Fld in the absence of infection, suggesting that the transgenic plants had a larger and more diversified suite of constitutive defenses against the attacking microorganism compared to the wild type. Fld modulated many genes involved in pathogenesis, signal transduction, transcriptional regulation and hormone-based pathways. Remarkable interactions with proteasomal protein degradation were observed. The results provide the first genome-wide, comprehensive picture illustrating the relevance of chloroplast redox status in biotic stress responses.

Global Transcriptome Analysis and Identification of Differentially Expressed Genes in Strawberry after Preharvest Application of Benzothiadiazole and Chitosan

The use of resistance inducers is a novel strategy to elicit defense responses in strawberry fruit to protect against preharvest and postharvest decay. However, the mechanisms behind the specific resistance inducers are not completely understood. Here, global transcriptional changes in strawberry fruit were investigated using RNA-Seq technology. Preharvest, benzothiadiazole (BTH) and chitosan were applied to the plant canopy, and the fruit were harvested at 6, 12, and 24 h post-treatment. Overall, 5,062 and 5,210 differentially expressed genes (fold change ≥ 2) were identified in these fruits under the BTH and chitosan treatments, respectively, as compared to the control expression. About 80% of these genes were differentially expressed by both elicitors. Comprehensive functional enrichment analysis highlighted different gene modulation over time for transcripts associated with photosynthesis and heat-shock proteins, according to elicitor. Up-regulation of genes associated with reprogramming of protein metabolism was observed in fruit treated with both elicitors, which led to increased storage proteins. Several genes associated with the plant immune system, hormone metabolism, systemic acquired resistance, and biotic and abiotic stresses were differentially expressed in treated versus untreated plants. The RNA-Seq output was confirmed using RT-qPCR for 12 selected genes. This study demonstrates that these two elicitors affect cell networks associated with plant defenses in different ways, and suggests a role for chloroplasts as the primary target in this modulation of the plant defense responses, which actively communicate these signals through changes in redox status. The genes identified in this study represent markers to better elucidate plant/pathogen/resistance-inducer interactions, and to plan novel sustainable disease management strategies.