Cellular organization in germ tube tips of Gigaspora and its phylogenetic implications

A bird's-eye view of the endoplasmic reticulum in filamentous fungi

SUMMARYThe endoplasmic reticulum (ER) is one of the most extensive organelles in eukaryotic cells. It performs crucial roles in protein and lipid synthesis and Ca2+ homeostasis. Most information on ER types, functions, organization, and domains comes from studies in uninucleate animal, plant, and yeast cells. In contrast, there is limited information on the multinucleate cells of filamentous fungi, i.e., hyphae. We provide an analytical review of existing literature to categorize different types of ER described in filamentous fungi while emphasizing the research techniques and markers used. Additionally, we identify the knowledge gaps that need to be resolved better to understand the structure-function correlation of ER in filamentous fungi. Finally, advanced technologies that can provide breakthroughs in understanding the ER in filamentous fungi are discussed.

Quercetin and 1-methyl-2-oxindole mimic root signaling that promotes spore germination and mycelial growth of Gigaspora margarita

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs, and the difficulty of growing them in asymbiotic or monoxenic (AMF + root) conditions limits research and their large-scale production as biofertilizer. We hypothesized that a combination of flavanols and strigolactones can mimic complex root signaling during the presymbiotic stages of AMF. We evaluated the germination, mycelial growth, branching, and auxiliary cell clusters formation by Gigaspora margarita during the presymbiotic stage in the presence (or absence) of transformed Cichorium intybus roots in basal culture medium enriched with glucose, a flavonol (quercetin or biochanin A) and a strigolactone analogue (1-Methyl-2-oxindole or indole propionic acid). With quercetin (5 µM), methyl oxindole (2.5 nM), and glucose (8.2 g/L) in the absence of roots, the presymbiotic mycelium of G. margarita grew without cytoplasmic retraction and produced auxiliary cells over 71 days similar to presymbiotic mycelium in the presence of roots but without glucose, strigolactones, and flavonols. Our results indicate that glucose and a specific combination of certain concentrations of a flavonol and a strigolactone might be used in asymbiotic or monoxenic liquid or semisolid cultures to stimulate AMF inoculant bioprocesses.

Fungicides With Contrasting Mode of Action Differentially Affect Hyphal Healing Mechanism in Gigaspora sp. and Rhizophagus irregularis

Fungicides are widely used in conventional agriculture to control fungal diseases, but may also affect non-target microorganisms such as arbuscular mycorrhizal (AM) fungi. These root symbionts develop extended mycelial networks within the soil via mechanisms such as anastomosis that indistinctly concerns intact and damaged hyphae, the latter being named hyphal healing mechanism (HHM). The HHM differs between Glomeraceae and Gigasporaceae. However, the effects of fungicides on this mechanism in unknown. Here, the impact of azoxystrobin, pencycuron, flutolanil, and fenpropimorph at 0.02 and 2 mg L-1 were tested in vitro on the HHM of Gigaspora sp. MUCL 52331 and Rhizophagus irregularis MUCL 41833, and repair events visualized carefully under a dissecting bright-field light microscope. Azoxystrobin was the more detrimental for both AM fungi at 2 mg L-1, while fenpropimorph impacted only R. irregularis (stimulating at low and inhibiting at high concentration). Conversely, flutolanil and pencycuron did not impact any of the two AM fungi. The mechanisms involved remains to be elucidated, but perturbation in the still-to-be firmly demonstrated spitzenkörper or in sterols content as well as a process of hormesis are possible avenues that deserve to be explored in view of a rationale management of chemicals to control fungal pathogens without harming the beneficial AM fungi.

Subcellular structure and behaviour in fungal hyphae

This work briefly surveys the diversity of selected subcellular characteristics in hyphal tip cells of the fungal kingdom (Mycota). Hyphae are filamentous cells that grow by tip extension. It is a highly polarised mechanism that requires a robust secretory system for the delivery of materials (e.g. membrane, proteins, cell wall materials) to sites of cell growth. These events result it the self-assembly of a Spitzenkörper (Spk), found most often in the Basidiomycota, Ascomycota, and Blastocladiomycota, or an apical vesicle crescent (AVC), present in the most Mucoromycota and Zoopagomycota. The Spk is a complex apical body composed of secretory vesicles, cytoskeletal elements, and signaling proteins. The AVC appears less complex, though little is known of its composition other than secretory vesicles. Both bodies influence hyphal growth and morphogenesis. Other factors such as cytoskeletal functions, endocytosis, cytoplasmic flow, and turgor pressure are also important in sustaining hyphal growth. Clarifying subcellular structures, functions, and behaviours through bioimagining analysis are providing a better understanding of the cell biology and phylogenetic relationships of fungi. LAY DESCRIPTION: Fungi are most familiar to the public as yeast, molds, and mushrooms. They are eukaryotic organisms that inhabit diverse ecological niches around the world and are critical to the health of ecosystems performing roles in decomposition of organic matter and nutrient recycling (Heath, 1990). Fungi are heterotrophs, unlike plants, and comprise the most successful and diverse phyla of eukaryotic microbes, interacting with all other forms of life in associations that range from beneficial (e.g., mycorrhizae) to antagonistic (e.g., pathogens). Some fungi can be parasitic or pathogenic on plants (e.g., Cryphonectria parasitica, Magnaporthe grisea), insects (e.g., Beauveria bassiana, Cordyceps sp.), invertebrates (e.g., Drechslerella anchonia), vertebrates (e.g., Coccidioides immitis, Candia albicans) and other fungi (e.g., Trichoderma viride, Ampelomyces quisqualis). The majority of fungi, however, are saprophytes, obtaining nutrition through the brake down of non-living organic matter.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

A phylum-level phylogenetic classification of zygomycete fungi based on genome-scale data

Zygomycete fungi were classified as a single phylum, Zygomycota, based on sexual reproduction by zygospores, frequent asexual reproduction by sporangia, absence of multicellular sporocarps, and production of coenocytic hyphae, all with some exceptions. Molecular phylogenies based on one or a few genes did not support the monophyly of the phylum, however, and the phylum was subsequently abandoned. Here we present phylogenetic analyses of a genome-scale data set for 46 taxa, including 25 zygomycetes and 192 proteins, and we demonstrate that zygomycetes comprise two major clades that form a paraphyletic grade. A formal phylogenetic classification is proposed herein and includes two phyla, six subphyla, four classes and 16 orders. On the basis of these results, the phyla Mucoromycota and Zoopagomycota are circumscribed. Zoopagomycota comprises Entomophtoromycotina, Kickxellomycotina and Zoopagomycotina; it constitutes the earliest diverging lineage of zygomycetes and contains species that are primarily parasites and pathogens of small animals (e.g. amoeba, insects, etc.) and other fungi, i.e. mycoparasites. Mucoromycota comprises Glomeromycotina, Mortierellomycotina, and Mucoromycotina and is sister to Dikarya. It is the more derived clade of zygomycetes and mainly consists of mycorrhizal fungi, root endophytes, and decomposers of plant material. Evolution of trophic modes, morphology, and analysis of genome-scale data are discussed.

The large (134.9 kb) mitochondrial genome of the glomeromycete Funneliformis mosseae

Funneliformis mosseae is among the most ecologically and economically important glomeromycete species and occurs both in natural and disturbed areas in a wide range of habitats and climates. In this study, we report the sequencing of the complete mitochondrial (mt) genome of F. mosseae isolate FL299 using 454 pyrosequencing and Illumina HiSeq technologies. This mt genome is a full-length circular chromosome of 134,925 bp, placing it among the largest mitochondrial DNAs (mtDNAs) in the fungal kingdom. A comparative analysis with publically available arbuscular mycorrhizal fungal mtDNAs revealed that the mtDNA of F. mosseae FL299 contained a very large number of insertions contributing to its expansion. The gene synteny was completely reshuffled compared to previously published glomeromycotan mtDNAs and several genes were oriented in an anti-sense direction. Furthermore, the presence of different types of introns and insertions in rnl (14 introns) made this gene very distinctive in Glomeromycota. The presence of alternative genetic codes in both initiation (GUG) and termination (UGA) codons was another new feature in this mtDNA compared to previously published glomeromycotan mt genomes. The phylogenetic analysis inferred from the analysis of 14 protein mt genes confirmed the position of the Glomeromycota clade as a sister group of Mortierellomycotina. This mt genome is the largest observed so far in Glomeromycota and the first mt genome within the Funneliformis clade, providing new opportunities to better understand their evolution and to develop molecular markers.

Hyphal tip cytoplasmic organization in four zygomycetous fungi

We have examined the hyphal tip structure in four zygomycetous fungi: Mortierella verticillata (Mortierellales), Coemansia reversa (Kickxellales), Mucor indicus and Gilbertella persicaria (Mucorales) using both light and transmission electron microscopy. We have used cryofixation and freeze-substitution methods to preserve fungal hyphae for transmission electron microscopy, which yielded improved preservation of ultrastructural details. Our research has confirmed studies that described the accumulation of secretory vesicles as a crescent at the hyphal apex (i.e. the apical vesicle crescent [AVC]) and provided a more detailed understanding of the vesicle populations. In addition, we have been able to observe the behavior of the AVC during hyphal growth in M. indicus and G. persicaria.

Cytology and molecular phylogenetics of Monoblepharidomycetes provide evidence for multiple independent origins of the hyphal habit in the Fungi

The evolution of filamentous hyphae underlies an astounding diversity of fungal form and function. We studied the cellular structure and evolutionary origins of the filamentous form in the Monoblepharidomycetes (Chytridiomycota), an early-diverging fungal lineage that displays an exceptional range of body types, from crescent-shaped single cells to sprawling hyphae. To do so, we combined light and transmission electron microscopic analyses of hyphal cytoplasm with molecular phylogenetic reconstructions. Hyphae of Monoblepharidomycetes lack a complex aggregation of secretory vesicles at the hyphal apex (i.e. Spitzenkörper), have centrosomes as primary microtubule organizing centers and have stacked Golgi cisternae instead of tubular/fenestrated Golgi equivalents. The cytoplasmic distribution of actin in Monoblepharidomycetes is comparable to the arrangement observed previously in other filamentous fungi. To discern the origins of Monoblepharidomycetes hyphae, we inferred a phylogeny of the fungi based on 18S and 28S ribosomal DNA sequence data with maximum likelihood and Bayesian inference methods. We focused sampling on Monoblepharidomycetes to infer intergeneric relationships within the class and determined 78 new sequences. Analyses showed class Monoblepharidomycetes to be monophyletic and nested within Chytridiomycota. Hyphal Monoblepharidomycetes formed a clade sister to the genera without hyphae, Harpochytrium and Oedogoniomyces. A likelihood ancestral state reconstruction indicated that hyphae arose independently within the Monoblepharidomycetes lineage and in at least two other lineages. Cytological differences among monoblepharidalean and other fungal hyphae are consistent with these convergent origins.