Biology subjects

Oekmen, B.

Publications and source records attributed to Oekmen, B..

7 recordsLinked to original sources

bioRxiv2026

Infection of maize by Ustilago maydis remodels the phyllosphere microbiome and requires the activity of antimicrobial effectors

Plant-associated microbial communities play a critical role in plant health and disease resistance, but the mechanisms which reshape these communities during pathogen infection are poorly understood. In this study, we investigated how infection of maize by the smut fungus Ustilago maydis is functionally linked with the bacterial phyllosphere microbiome and explored the role of an antimicrobial effector GH25 in fungal infection. Using a combination of culture-dependent and culture-independent approaches, we compared the leaf microbiomes of infected and uninfected plants. We observed a significant increase in microbial abundance and pronounced shifts in community composition and identified distinct health-associated (HCom) and disease-associated (DCom) bacterial communities. To assess whether U. maydis directly manipulates the microbiome, we tested the antimicrobial activity of the antimicrobial effector GH25 against isolated strains. Notably, all HCom bacteria were sensitive to GH25 and co-inoculation of HCom bacteria with a U. maydis {Delta}gh25 knockout mutant significantly reduced fungal virulence. In contrast, DCom exhibited minimal sensitivity to U. maydis and did not affect the virulence of U. maydis {Delta}gh25. Genome-scale metabolic community modelling coupled with functional profiling revealed infection-associated shifts in predicted metabolic potential, consistent with U. maydis induced leaf tumors being strong sink tissues. Together, this work shows that U. maydis infection reshapes the maize phyllosphere microbiome through a combination of effector-mediated antimicrobial activity and host metabolic reprogramming.

microbiology↗

bioRxiv2023

The fungal pathogen Ustilago maydis targets the maize corepressor TPL2 to modulate host transcription for tumorigenesis

Ustilago maydis is a biotrophic fungus that causes tumor formation on all aerial parts of maize. U. maydis secretes effector proteins during penetration and colonization to successfully overcome the plant immune response and reprogram host physiology to promote infection. In this study, we functionally characterized the U. maydis effector protein Topless (TPL) interacting protein 6 (Tip6). We found that Tip6 interacts with the N-terminus of ZmTPL2 through its two EAR (Ethylene-responsive element binding factor-associated amphiphilic repression) motifs. We show that the EAR motifs are essential for the virulence function of Tip6 and critical for altering the nuclear distribution pattern of ZmTPL2. We propose that Tip6 mimics the recruitment of ZmTPL2 by plant repressor proteins, thus disrupting host transcriptional regulation. We show that a large group of AP2/ERF B1 subfamily transcription factors are misregulated in the presence of Tip6. Our study suggests a regulatory mechanism where the U. maydis effector Tip6 utilizes repressive domains to recruit the corepressor ZmTPL2 to disrupt the transcriptional networks of the host plant.

plant biology↗

bioRxiv2023

A conserved extracellular ribonuclease with broad-spectrum cytotoxic activity enables smut fungi to compete with host-associated bacteria

Ribotoxins are secreted ribonucleases that specifically target and cleave the universally conserved sarcin-ricin loop sequence of rRNA, which leads to inhibition of protein biosynthesis and subsequently to cell death. We have identified and characterized a secreted Ribo1 protein of plant pathogenic smut fungi. Heterologous expression in different model systems showed that smut Ribo1 has cytotoxic activity against bacteria, yeast, host and non-host plants. Recombinant expression of Ribo1 in Nicotiana benthamiana induced plant cell death; however, an active site mutant induced cell death only when expressed as a secreted protein. In the maize smut Ustilago maydis, transcription of Ribo1 is specifically induced in early infection stages. While a knock-out mutant revealed that Ribo1 is dispensable for U. maydis virulence, the overexpression of Ribo1 in-planta had a strong dominant negative effect on virulence and induced host defense responses including cell death. This suggests a function of Ribo1 during the epiphytic development rather than for invasive colonization of the host. Accordingly, in presence of the biocontrol bacteria Pantoea sp., which were isolated from maize leaves, the ribo1 knock-out mutant was significantly impaired in virulence. Together, we conclude that Ribo1 enables smut fungi to compete with host-associated bacteria during epiphytic development.

plant biology↗

bioRxiv2022

A conserved enzyme of smut fungi facilitates cell-to-cell movement in the plant bundle sheath

The smut fungi are one of the largest groups of fungal plant pathogens, causing disease in all cereal crops. They directly penetrate their hosts and establish a biotrophic interaction. During colonization of the plant, smut fungi secrete a wide range of effector proteins, which suppress plant immunity and modulate cellular functions as well as development of the host, thereby determining the pathogens life-style and virulence potential. The conserved effector Erc1 (enzyme required for cell-to-cell movement) contributes to virulence of the corn smut Ustilago maydis in maize leaves, but not on the tassel. Erc1 binds to host cell wall components and has a 1,3-{beta}-glucanase activity, which is required to attenuate {beta}-glucan-induced defense responses in host leaves. Confocal microscopy revealed that Erc1 has a cell type-specific virulence function, being necessary for fungal cell-to-cell movement in the plant bundle sheath. This cell type-specific virulence function of Erc1 is fully conserved in the barley pathogen Ustilago hordei, which has a functionally conserved Erc1 orthologue. Thus, Erc1 is an enzymatically active core virulence factor with a cell type-specific virulence function in different hosts, which is important for cell-to-cell movement during host colonization of pathogenic smut fungi.

plant biology↗

bioRxiv2021

High nucleotide substitution rates associated with retrotransposon proliferation drive dynamic secretome evolution in smut pathogens

Transposable elements (TEs) play a pivotal role in shaping diversity in eukaryotic genomes. The covered smut pathogen on barley, Ustilago hordei, encountered a recent genome expansion. Using long reads, we assembled genomes of 6 U. hordei strains and 3 sister species, to study this genome expansion. We found that larger genome sizes can mainly be attributed to a higher genome fraction of long terminal repeat retrotransposons (LTR-RTs). In the studied smut genomes, LTR-RTs fractions are the largest in U. hordei and are positively correlated to the mating-type locus sizes, which is up to [~]560 kb in U. hordei. Furthermore, LTR-RTs were found to be associated with higher nucleotide substitution levels, as these higher levels occur more clustered in smut species with a recent LTR-RT proliferation. Moreover, genes in genome regions with higher nucleotide substitution levels generally reside closer to LTR-RTs than other genome regions. Genome regions with many nucleotide substitutions encountered an especially high fraction of CG substitutions, which is not observed for LTR-RT sequences. The high nucleotide substitution levels particularly accelerate the evolution of secretome genes, as their more flexible nature results that substitutions often lead to amino acid alterations. ImportanceGenomic alteration can be generated through various means, in which transposable elements (TEs) can play a pivotal role. Their mobility causes mutagenesis in itself and can disrupt the function of the sequences they insert into. Indirectly, they also impact genome evolution as their repetitive nature facilitates non-homologous recombination. Furthermore, TEs have been linked to specific epigenetic genome organizations. We report a recent TE proliferation in the genome of the barley covered smut fungus, Ustilago hordei. This proliferation is associated with a distinct nucleotide substitution regime that has a higher rate and a higher fraction of CG substitutions. This different regime shapes the evolution of genes in subjected genome regions. Our findings highlight that TEs may influence the error-rate of DNA polymerase in a hitherto unknown fashion.

evolutionary biology↗

bioRxiv2020

The Ustilago hordei-barley interaction is a versatile system to characterize fungal effectors

Obligate biotrophic fungal pathogens, such as Blumeria graminis and Puccinia graminis, are amongst the most devastating plant pathogens, causing dramatic yield losses in many economically important crops worldwide. However, a lack of reliable tools for the efficient genetic transformation has hampered studies into the molecular basis of their virulence/pathogenicity. In this study, we present the U. hordei-barley pathosystem as a model to characterize effectors from different plant pathogenic fungi. We have generated U. hordei solopathogenic strains, which form infectious filaments without presence of compatible mating partner. Solopathogenic strains are suitable as heterologous expression system for fungal virulence factors. A highly efficient Crispr/Cas9 gene editing system is made available for U. hordei. In addition, U. hordei infection structures during barley colonization were analyzed by transmission electron microscopy, which shows that U. hordei forms intracellular infection structures sharing high similarity to haustoria formed by obligate rust and powdery mildew fungi. Thus, U. hordei has high potential as a fungal expression platform for functional studies of heterologous effector proteins in barley.

plant biology↗

bioRxiv2018

Pseudozyma saprotrophic yeasts have retained a large effector arsenal, including functional Pep1 orthologs

The basidiomycete smut fungi are predominantly plant parasitic, causing severe losses in some crops. Most species feature a saprotrophic haploid yeast stage, and several smut fungi are only known from this stage, with some isolated from habitats without suitable hosts, e.g. from Antarctica. Thus, these species are generally believed to be apathogenic, but recent findings that some of these might have a plant pathogenic sexual counterpart, casts doubts on the validity of this hypothesis. Here, four Pseudozyma genomes were re-annotated and compared to published smut pathogens and the well-characterised effector gene Pep1 from these species was checked for its ability to complement a Pep1 deletion strain of Ustilago maydis. It was found that 113 high-confidence putative effector proteins were conserved among smut and Pseudozyma genomes. Among these were several validated effector proteins, including Pep1. By genetic complementation we show that Pep1 homologs from the supposedly apathogenic yeasts restore virulence in Pep1-deficient mutants Ustilago maydis. Thus, it is concluded that Pseudozyma species have retained a suite of effectors. This hints at the possibility that Pseudozyma species have kept an unknown plant pathogenic stage for sexual recombination or that these effectors have positive effects when colonising plant surfaces.

evolutionary biology↗