Cross-species analysis between the maize smut fungi Ustilago maydis and Sporisorium reilianum highlights the role of transcriptional change of effector orthologs for virulence and disease

Tip of the iceberg? Three novel TOPLESS-interacting effectors of the gall-inducing fungus Ustilago maydis

Ustilago maydis is a biotrophic pathogen causing smut disease in maize. It secretes a cocktail of effector proteins, which target different host proteins during its biotrophic stages in the host plant. One such class of proteins we identified previously is TOPLESS (TPL) and TOPLESS-RELATED (TPR) transcriptional corepressors. Here, we screened 297 U. maydis effector candidates for their ability to interact with maize TPL protein RAMOSA 1 ENHANCER LOCUS 2 LIKE 2 (RELK2) and their ability to induce auxin signaling and thereby identified three novel TPL-interacting protein effectors (Tip6, Tip7, and Tip8). Structural modeling and mutational analysis allowed the identification of TPL-interaction motifs of Tip6 and Tip7. In planta interaction between Tip6 and Tip7 with RELK2 occurs mainly in nuclear compartments, whereas Tip8 colocalizes with RELK2 in a compartment outside the nucleus. Overexpression of Tip8 in nonhost plants leads to cell death, indicating recognition of the effector or its activity. By performing infection assays with single and multideletion mutants of U. maydis, we demonstrate a positive role of Tip6 and Tip7 in U. maydis virulence. Transcriptional profiling of maize leaves infected with Tip effector mutants in comparison with SG200 strain suggests Tip effector activities are not merely redundant.

Novel Secreted Effectors Conserved Among Smut Fungi Contribute to the Virulence of Ustilago maydis

Fungal pathogens deploy a set of molecules (proteins, specialized metabolites, and sRNAs), so-called effectors, to aid the infection process. In comparison to other plant pathogens, smut fungi have small genomes and secretomes of 20 Mb and around 500 proteins, respectively. Previous comparative genomic studies have shown that many secreted effector proteins without known domains, i.e., novel, are conserved only in the Ustilaginaceae family. By analyzing the secretomes of 11 species within Ustilaginaceae, we identified 53 core homologous groups commonly present in this lineage. By collecting existing mutants and generating additional ones, we gathered 44 Ustilago maydis strains lacking single core effectors as well as 9 strains containing multiple deletions of core effector gene families. Pathogenicity assays revealed that 20 of these 53 mutant strains were affected in virulence. Among the 33 mutants that had no obvious phenotypic changes, 13 carried additional, sequence-divergent, structurally similar paralogs. We report a virulence contribution of seven previously uncharacterized single core effectors and of one effector family. Our results help to prioritize effectors for understanding U. maydis virulence and provide genetic resources for further characterization. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The fungal pathogen Ustilago maydis targets the maize corepressor RELK2 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 RELK2 through its two Ethylene-responsive element binding factor-associated amphiphilic repression (EAR) motifs. We show that the EAR motifs are essential for the virulence function of Tip6 and critical for altering the nuclear distribution pattern of RELK2. We propose that Tip6 mimics the recruitment of RELK2 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 RELK2 to disrupt the transcriptional networks of the host plant.

A conserved extracellular Ribo1 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 nonhost 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 knockout 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. Our findings suggest a function of Ribo1 during the epiphytic development rather than for invasive colonization of the host. Accordingly, in the presence of the biocontrol bacteria Pantoea sp., which were isolated from maize leaves, the ribo1 knockout mutant was significantly impaired in virulence. Together, we conclude that Ribo1 enables smut fungi to compete with host-associated bacteria during epiphytic development.

A transcriptional activator effector of Ustilago maydis regulates hyperplasia in maize during pathogen-induced tumor formation

Ustilago maydis causes common smut in maize, which is characterized by tumor formation in aerial parts of maize. Tumors result from the de novo cell division of highly developed bundle sheath and subsequent cell enlargement. However, the molecular mechanisms underlying tumorigenesis are still largely unknown. Here, we characterize the U. maydis effector Sts2 (Small tumor on seedlings 2), which promotes the division of hyperplasia tumor cells. Upon infection, Sts2 is translocated into the maize cell nucleus, where it acts as a transcriptional activator, and the transactivation activity is crucial for its virulence function. Sts2 interacts with ZmNECAP1, a yet undescribed plant transcriptional activator, and it activates the expression of several leaf developmental regulators to potentiate tumor formation. On the contrary, fusion of a suppressive SRDX-motif to Sts2 causes dominant negative inhibition of tumor formation, underpinning the central role of Sts2 for tumorigenesis. Our results not only disclose the virulence mechanism of a tumorigenic effector, but also reveal the essential role of leaf developmental regulators in pathogen-induced tumor formation.

Co-immunoprecipitation-Based Identification of Effector-Host Protein Interactions from Pathogen-Infected Plant Tissue

Protein-protein interactions play an essential role in host-pathogen interactions. Phytopathogens secrete a cocktail of effector proteins to suppress plant immunity and reprogram host cell metabolism in their favor. Identification and characterization of effectors and their target protein complexes by co-immunoprecipitation can help to gain a deeper understanding of the functions of individual effectors during pathogenicity and can also provide new insights into the wiring of plant signaling pathways or metabolic complexes. Here we describe a detailed protocol to perform co-immunoprecipitation of effector-target protein complexes from plant extracts with an example of the Ustilago maydis/maize pathosystem for which we also provide a fungal protoplast transformation and maize seedling infection protocols.

Many ways to TOPLESS - manipulation of plant auxin signalling by a cluster of fungal effectors

Plant biotrophic pathogens employ secreted molecules, called effectors, to suppress the host immune system and redirect the host's metabolism and development in their favour. Putative effectors of the gall-inducing maize pathogenic fungus Ustilago maydis were analysed for their ability to induce auxin signalling in plants. Using genetic, biochemical, cell-biological, and bioinformatic approaches we functionally elucidate a set of five, genetically linked effectors, called Topless (TPL) interacting protein (Tips) effectors that induce auxin signalling. We show that Tips induce auxin signalling by interfering with central corepressors of the TPL family. CRISPR-Cas9 mutants and deletion strain analysis indicate that the auxin signalling inducing subcluster effectors plays a redundant role in virulence. Although none of the Tips seem to have a conserved interaction motif, four of them bind solely to the N-terminal TPL domain and, for Tip1 and Tip4, we demonstrate direct competition with auxin/indole-3-acetic acid transcriptional repressors for their binding to TPL class of corepressors. Our findings reveal that TPL proteins, key regulators of growth-defence antagonism, are a major target of the U. maydis effectome.

Recent advances in understanding of fungal and oomycete effectors

Fungal and oomycete pathogens secrete complex arrays of proteins and small RNAs to interface with plant-host targets and manipulate plant regulatory networks to the microbes' advantage. Research on these important virulence factors has been accelerated by improved genome sequences, refined bioinformatic prediction tools, and exploitation of efficient platforms for understanding effector gene expression and function. Recent studies have validated the expectation that oomycetes and fungi target many of the same sectors in immune signaling networks, but the specific host plant targets and modes of action are diverse. Effector research has also contributed to deeper understanding of the mechanisms of effector-triggered immunity.

Induce defense response of DADS in eggplants during the biotrophic phase of Verticillium dahliae

PURPOSE

Verticillium wilt is a destructive vascular disease in eggplants. The complex defensive mechanisms of eggplant against this disease are very limited.

METHODS

Our work examined the bioactive properties of garlic allelochemical diallyl disulfide (DADS) as potential biostimulants for defense against V. dahliae in eggplant seedlings. We, therefore, foliar sprayed DADS on eggplants to study the defense response during the early biotrophic phase of V. dahliae (a hemibiotroph).

RESULTS

DADS application significantly increased root peroxidase (POD), phenylalanine-ammonia lyase (PAL) enzyme activity, and reduced H₂O₂ levels after 24 h of fungal inoculation. Salicylic acid (SA) in leaves and roots was significantly increased while, the jasmonic acid (JA), indole acetic acid (IAA), and abscisic acid (ABA) levels were decreased. The microscopic examinations of V. dahliae infection in roots displayed that the progression of infection was restricted in DADS-treated plants. Depositions of lignin and phenolic compounds such as ferulic acid, p-coumaric acid, and caffeic acid content were significantly higher in DADS-treated plants at 48 h post-inoculation. Similarly, the DADS application up-regulated pathogenesis-related (PR1, PR2, and PR5), mitogen-activated protein kinase (MPK1), and lipoxygenase (LOX) genes. Furthermore, DADS-treated plants exhibited a lower disease severity index (23.3% vs. 57.0% in controls), indicating successful defense against V. dahliae.

CONCLUSIONS

Our findings concluded that the biological function of garlic allelochemical DADS has a prominent role in the higher defense resistance of eggplants during the early infection of V. dahliae.

Isolation of Ustilago bromivora Strains from Infected Spikelets through Spore Recovery and Germination

Ustilago bromivora is a biotrophic smut fungus infecting Brachypodium sp. It is closely related to the barley-infecting smut Ustilago hordei, and related to the well-studied, gall-inducing model pathogen Ustilago maydis. Upon flowering, the spikelets of U. bromivora-infected plants are filled with black fungal spores. While it is possible to directly use this spore material to infect Brachypodium seeds, in many cases it is more useful to isolate individual strains of U. bromivora for a genetically homogenous population. This protocol describes how to collect and germinate the spores of U. bromivora on plate in order to obtain strains derived from a single cell.