Toxin-producing endosymbionts shield pathogenic fungus against micropredators

The phytopathogenic fungus Rhizopus microsporus harbours a bacterial endosymbiont (Mycetohabitans rhizoxinica) for the production of the toxin rhizoxin, the causative agent of rice seedling blight. This toxinogenic bacterial-fungal alliance is, however, not restricted to the plant disease, but has been detected in numerous environmental isolates from geographically distinct sites covering all five continents. Yet, the ecological role of rhizoxin beyond rice seedling blight has been unknown. Here we show that rhizoxin serves the fungal host in fending off protozoan and metazoan predators. Fluorescence microscopy and co-culture experiments with the fungivorous amoeba Protostelium aurantium revealed that ingestion of R. microsporus spores is toxic to P. aurantium. This amoebicidal effect is caused by the bacterial rhizoxin congener rhizoxin S2, which is also lethal towards the model nematode Caenorhabditis elegans. By combining stereomicroscopy, automated image analyses, and quantification of nematode movement we show that the fungivorous nematode Aphelenchus avenae actively feeds on R. microsporus that is lacking endosymbionts, while worms co-incubated with symbiotic R. microsporus are significantly less lively. This work uncovers an unexpected ecological role of rhizoxin as shield against micropredators. This finding suggests that predators may function an evolutionary driving force to maintain toxin-producing endosymbionts in non-pathogenic fungi.

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Biodegradation of the Allelopathic Chemical Pterostilbene by a Sphingobium sp. Strain from the Peanut Rhizosphere

Applied and Environmental Microbiology ◽

10.1128/aem.02154-18 ◽

2018 ◽

Vol 85 (5) ◽

Cited By ~ 2

Author(s):

Ri-Qing Yu ◽

Zohre Kurt ◽

Fei He ◽

Jim C. Spain

Keyword(s):

Pathogenic Fungi ◽

Bacterial Degradation ◽

Ecological Role ◽

Microbial Infection ◽

Catabolic Pathway ◽

Content Type ◽

Dead End ◽

Initial Cleavage ◽

Bacterial Interference

ABSTRACT Many plants produce allelopathic chemicals, such as stilbenes, to inhibit pathogenic fungi. The degradation of allelopathic compounds by bacteria associated with the plants would limit their effectiveness, but little is known about the extent of biodegradation or the bacteria involved. Screening of tissues and rhizosphere of peanut (Arachis hypogaea) plants revealed substantial enrichment of bacteria able to grow on resveratrol and pterostilbene, the most common stilbenes produced by the plants. Investigation of the catabolic pathway in Sphingobium sp. strain JS1018, isolated from the rhizosphere, indicated that the initial cleavage of pterostilbene was catalyzed by a carotenoid cleavage oxygenase (CCO), which led to the transient accumulation of 4-hydroxybenzaldehyde and 3,5-dimethoxybenzaldehyde. 4-Hydroxybenzaldehyde was subsequently used for the growth of the isolate, while 3,5-dimethoxybenzaldehyde was further converted to a dead-end metabolite with a molecular weight of 414 (C24H31O6). The gene that encodes the initial oxygenase was identified in the genome of strain JS1018, and its function was confirmed by heterologous expression in Escherichia coli. This study reveals the biodegradation pathway of pterostilbene by plant-associated bacteria. The prevalence of such bacteria in the rhizosphere and plant tissues suggests a potential role of bacterial interference in plant allelopathy. IMPORTANCE Pterostilbene, an analog of resveratrol, is a stilbene allelochemical produced by plants to inhibit microbial infection. As a potent antioxidant, pterostilbene acts more effectively than resveratrol as an antifungal agent. Bacterial degradation of this plant natural product would affect the allelopathic efficacy and fate of pterostilbene and thus its ecological role. This study explores the isolation and abundance of bacteria that degrade resveratrol and pterostilbene in peanut tissues and rhizosphere, the catabolic pathway for pterostilbene, and the molecular basis for the initial cleavage of pterostilbene. If plant allelopathy is an important process in agriculture and management of invasive plants, the ecological role of bacteria that degrade the allelopathic chemicals must be equally important.

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Secreted TAL effectors protect symbiotic bacteria from entrapment within fungal hyphae

10.1101/2020.03.28.013177 ◽

2020 ◽

Cited By ~ 1

Author(s):

Ingrid Richter ◽

Zerrin Uzum ◽

Claire E. Stanley ◽

Nadine Moebius ◽

Timothy P. Stinear ◽

...

Keyword(s):

Microfluidic Devices ◽

Phytopathogenic Fungus ◽

Transcription Activator ◽

Rhizopus Microsporus ◽

Protective Response ◽

Tal Effectors ◽

Dynamic Interactions ◽

Fungal Host ◽

Fungal Hyphae ◽

Fungal Interactions

AbstractThe association of the agriculturally significant phytopathogenic fungus Rhizopus microsporus with the bacterial endosymbiont Burkholderia rhizoxinica is a remarkable example of bacteria controlling host physiology and reproduction. Here, we show that a group of transcription activator-like effectors (TALEs) called Burkholderia TALE-like proteins (BATs) from B. rhizoxinica are essential for the establishment of the symbiosis. Mutants lacking BAT proteins are unable to induce host sporulation. Utilising novel microfluidic devices in combination with fluorescence microscopy we observed the accumulation of BAT-deficient mutants in specific fungal side-hyphae with accompanying increased fungal re-infection. High-resolution live imaging revealed septa biogenesis at the base of infected hyphae leading to compartmental trapping of BATdeficient endobacteria. Trapped endosymbionts showed reduced intracellular survival, suggesting a protective response from the fungal host against bacteria lacking specific effectors. These findings underscore the involvement of BAT proteins in maintaining a balance between mutualism and antagonism in bacterial-fungal interactions and provide deeper insights into the dynamic interactions between bacteria and eukaryotes.

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Insights Into the Ecological Role of Pseudomonas spp. in an Ant-plant Symbiosis

Frontiers in Microbiology ◽

10.3389/fmicb.2021.621274 ◽

2021 ◽

Vol 12 ◽

Author(s):

Taise T. H. Fukuda ◽

Camila F. Pereira ◽

Weilan G. P. Melo ◽

Carla Menegatti ◽

Paulo H. M. Andrade ◽

...

Keyword(s):

Nitrogen Fixation ◽

Biochemical Characterization ◽

Antimicrobial Activities ◽

Phytopathogenic Fungus ◽

Ecological Role ◽

Bacterial Isolates ◽

Loss Of Function ◽

Symbiotic System ◽

Mutualistic Relationship

In the myrmecophytic mutualistic relationship between Azteca ants and Cecropia plants both species receive protection and exchange nutrients. The presence of microorganisms in this symbiotic system has been reported, and the symbiotic role of some fungi involved in the myrmecophytic interactions has been described. In this work we focus on bacteria within this mutualism, conducting isolations and screening for antimicrobial activities, genome sequencing, and biochemical characterization. We show that Pantoea, Rhizobium, Methylobacterium, Streptomyces and Pseudomonas are the most common cultivable genera of bacteria. Interestingly, Pseudomonas spp. isolates showed potent activity against 83% of the pathogens tested in our antimicrobial activity assays, including a phytopathogenic fungus isolated from Cecropia samples. Given the predicted nitrogen limitations associated with the fungal patches within this myrmecophyte, we performed nitrogen fixation analyses on the bacterial isolates within the Proteobacteria and show the potential for nitrogen fixation in Pseudomonas strains. The genome of one Pseudomonas strain was sequenced and analyzed. The gene cluster involved in the biosynthesis of cyclic lipodepsipeptides (CLPs) was identified, and we found mutations that may be related to the loss of function in the dual epimerization/condensation domains. The compound was isolated, and its structure was determined, corresponding to the antifungal viscosinamide. Our findings of diazotrophy and production of viscosinamide in multiple Pseudomonas isolates suggests that this bacterial genus may play an important role in the Cecropia-Azteca symbiosis.

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Vesicle-associated melanization in Cryptococcus neoformans

Microbiology ◽

10.1099/mic.0.032854-0 ◽

2009 ◽

Vol 155 (12) ◽

pp. 3860-3867 ◽

Cited By ~ 91

Author(s):

Helene C. Eisenman ◽

Susana Frases ◽

André M. Nicola ◽

Marcio L. Rodrigues ◽

Arturo Casadevall

Keyword(s):

Cell Wall ◽

Cryptococcus Neoformans ◽

Extracellular Vesicles ◽

Lipid A ◽

Pathogenic Fungus ◽

Pathogenic Fungi ◽

Spherical Particles ◽

Human Pathogenic Fungus ◽

Kinetics Of

Recently, several pathogenic fungi were shown to produce extracellular vesicles that contain various components associated with virulence. In the human pathogenic fungus Cryptococcus neoformans, these components included laccase, an enzyme that catalyses melanin synthesis. Spherical melanin granules have been observed in the cell wall of C. neoformans. Given that melanin granules have dimensions that are comparable to those of extracellular vesicles, and that metazoan organisms produce melanin in vesicular structures known as melanosomes, we investigated the role of vesicles in cryptococcal melanization. Extracellular vesicles melanized when incubated with the melanin precursor l-3,4-dihydroxyphenylalanine (l-DOPA). The kinetics of substrate incorporation into cells and vesicles was analysed using radiolabelled l-DOPA. The results indicated that substrate incorporation was different for cells and isolated vesicles. Acid-generated melanin ghosts stained with lipophilic dyes, implying the presence of associated lipid. A model for C. neoformans melanization is proposed that accounts for these observations and provides a mechanism for the assembly of melanin into relatively uniform spherical particles stacked in an orderly arrangement in the cell wall.

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Active invasion of bacteria into living fungal cells

eLife ◽

10.7554/elife.03007 ◽

2014 ◽

Vol 3 ◽

Cited By ~ 57

Author(s):

Nadine Moebius ◽

Zerrin Üzüm ◽

Jan Dijksterhuis ◽

Gerald Lackner ◽

Christian Hertweck

Keyword(s):

Genome Mining ◽

Rice Seedling ◽

Rhizopus Microsporus ◽

Fungal Cell ◽

Chitinolytic Enzymes ◽

Cryo Electron Microscopy ◽

Functional Analyses ◽

Insight Into

The rice seedling blight fungus Rhizopus microsporus and its endosymbiont Burkholderia rhizoxinica form an unusual, highly specific alliance to produce the highly potent antimitotic phytotoxin rhizoxin. Yet, it has remained a riddle how bacteria invade the fungal cells. Genome mining for potential symbiosis factors and functional analyses revealed that a type 2 secretion system (T2SS) of the bacterial endosymbiont is required for the formation of the endosymbiosis. Comparative proteome analyses show that the T2SS releases chitinolytic enzymes (chitinase, chitosanase) and chitin-binding proteins. The genes responsible for chitinolytic proteins and T2SS components are highly expressed during infection. Through targeted gene knock-outs, sporulation assays and microscopic investigations we found that chitinase is essential for bacteria to enter hyphae. Unprecedented snapshots of the traceless bacterial intrusion were obtained using cryo-electron microscopy. Beyond unveiling the pivotal role of chitinolytic enzymes in the active invasion of a fungus by bacteria, these findings grant unprecedented insight into the fungal cell wall penetration and symbiosis formation.

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Clathrin Is Important for Virulence Factors Delivery in the Necrotrophic Fungus Botrytis cinerea

Frontiers in Plant Science ◽

10.3389/fpls.2021.668937 ◽

2021 ◽

Vol 12 ◽

Author(s):

Eytham Souibgui ◽

Christophe Bruel ◽

Mathias Choquer ◽

Amélie de Vallée ◽

Cindy Dieryckx ◽

...

Keyword(s):

Botrytis Cinerea ◽

Virulence Factors ◽

Essential Role ◽

Plant Pathogens ◽

Pathogenic Fungus ◽

Hydrolytic Enzymes ◽

Pathogenic Fungi ◽

Dominant Negative ◽

Intracellular Vesicles

Fungi are the most prevalent plant pathogens, causing annually important damages. To infect and colonize their hosts, they secrete effectors including hydrolytic enzymes able to kill and macerate plant tissues. These secreted proteins are transported from the Endoplasmic Reticulum and the Golgi apparatus to the extracellular space through intracellular vesicles. In pathogenic fungi, intracellular vesicles were described but their biogenesis and their role in virulence remain unclear. In this study, we report the essential role of clathrin heavy chain (CHC) in the pathogenicity of Botrytis cinerea, the agent of gray mold disease. To investigate the importance of this protein involved in coat vesicles formation in eukaryotic cells, a T-DNA insertional mutant reduced in the expression of the CHC-encoding gene, and a mutant expressing a dominant-negative form of CHC were studied. Both mutants were strongly affected in pathogenicity. Characterization of the mutants revealed altered infection cushions and an important defect in protein secretion. This study demonstrates the essential role of clathrin in the infectious process of a plant pathogenic fungus and more particularly its role in virulence factors delivery.

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