Out of the rivers: are some aquatic hyphomycetes plant endophytes?

Ingoldian fungi, or aquatic hyphomycetes, are asexual microfungi, mostly ascomycetes, commonly occurring in running freshwater. They grow on dead plant material, such as leaves and twigs, and play a major role in nutrient flows in stream ecosystems (Bärlocher, 1992). They were discovered and first extensively studied by Ingold (1942) and were thus named "Ingoldian" fungi. Ingold described their abundant multicellular asexual spores of sigmoid or, more typically, tetraradiate shape (Fig. 1). He recognized that they probably arose from multiple convergent evolutions, by secondary adaptation to aquatic life, as recently confirmed by molecular markers (Belliveau & Bärlocher, 2005; Baschien et al., 2006). Because of their apparent lack of sexuality, Ingoldian species were placed in asexual genera, such as Tricladium or Tetracladium, based on conidial morphology and/or mode of conidiogenesis. Unexpectedly, several lines of evidence now suggest that some Ingoldian fungi are also plant endophytes, that is, they grow in plants without producing symptoms.[...

The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi

BACKGROUND: A variety of spore discharge processes have evolved among the fungi. Those with the longest ranges are powered by hydrostatic pressure and include "squirt guns" that are most common in the Ascomycota and Zygomycota. In these fungi, fluid-filled stalks that support single spores or spore-filled sporangia, or cells called asci that contain multiple spores, are pressurized by osmosis. Because spores are discharged at such high speeds, most of the information on launch processes from previous studies has been inferred from mathematical models and is subject to a number of errors. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we have used ultra-high-speed video cameras running at maximum frame rates of 250,000 fps to analyze the entire launch process in four species of fungi that grow on the dung of herbivores. For the first time we have direct measurements of launch speeds and empirical estimates of acceleration in these fungi. Launch speeds ranged from 2 to 25 m s(-1) and corresponding accelerations of 20,000 to 180,000 g propelled spores over distances of up to 2.5 meters. In addition, quantitative spectroscopic methods were used to identify the organic and inorganic osmolytes responsible for generating the turgor pressures that drive spore discharge. CONCLUSIONS/SIGNIFICANCE: The new video data allowed us to test different models for the effect of viscous drag and identify errors in the previous approaches to modeling spore motion. The spectroscopic data show that high speed spore discharge mechanisms in fungi are powered by the same levels of turgor pressure that are characteristic of fungal hyphae and do not require any special mechanisms of osmolyte accumulation

Intraspecific Variation of the Aquatic Fungus Articulospora tetracladia: An Ubiquitous Perspective

The worldwide-distributed aquatic fungus Articulospora tetracladia Ingold is a dominant sporulating species in streams of the Northwest Iberian Peninsula. To elucidate the genetic diversity of A. tetracladia, we analyzed isolates collected from various types of plant litter or foam in streams from North and Central Portugal and North Spain, between 2000 and 2010. Genetic diversity of these fungal populations was assessed by denaturing gradient gel electrophoresis (DGGE) fingerprints and by using ITS1-5.8S-ITS2 barcodes. Moreover, ITS1-5.8S-ITS2 barcodes of A. tetracladia reported in other parts of the world (Central Europe, United Kingdom, Canada, Japan and Malaysia) were retrieved from the National Center for Biotechnology (NCBI) and the National Institute of Technology and Evaluation Biological Resource Center (NBRC) to probe into genetic diversity of A. tetracladia. PCR-DGGE of ITS2 region of 50 Iberian fungal isolates distinguished eight operational taxonomic units (OTUs), which were similar to those obtained from neighboring trees based on ITS2 gene sequences. On the other hand, ITS1-5.8S-ITS2 barcodes of 68 fungal isolates yielded nine OTUs, but five fungal isolates were not assigned to any of these OTUs. Molecular diversity was highest for OTU-8, which included only European isolates. Two haplotypes were observed within OTU-8 and OTU-9, while only one haplotype was found within each of the remaining OTUs. Malaysia did not share haplotypes with other countries. Overall results indicate that, apart from the Malaysian genotypes, A. tetracladia genotypes were geographically widespread irrespective of sampling time, sites or substrates. Furthermore, PCR-DGGE appeared to be a rapid tool for assessing intraspecific diversity of aquatic hyphomycetes

Paleogene Radiation of a Plant Pathogenic Mushroom

Background: The global movement and speciation of fungal plant pathogens is important, especially because of the economic losses they cause and the ease with which they are able to spread across large areas. Understanding the biogeography and origin of these plant pathogens can provide insights regarding their dispersal and current day distribution. We tested the hypothesis of a Gondwanan origin of the plant pathogenic mushroom genus Armillaria and the currently accepted premise that vicariance accounts for the extant distribution of the species. Methods: The phylogeny of a selection of Armillaria species was reconstructed based on Maximum Parsimony (MP), Maximum Likelihood (ML) and Bayesian Inference (BI). A timeline was then placed on the divergence of lineages using a Bayesian relaxed molecular clock approach. Results: Phylogenetic analyses of sequenced data for three combined nuclear regions provided strong support for three major geographically defined clades: Holarctic, South American-Australasian and African. Molecular dating placed the initial radiation of the genus at 54 million years ago within the Early Paleogene, postdating the tectonic break-up of Gondwana. Conclusions: The distribution of extant Armillaria species is the result of ancient long-distance dispersal rather than vicariance due to continental drift. As these finding are contrary to most prior vicariance hypotheses for fungi, our result

Septation of Infectious Hyphae Is Critical for Appressoria Formation and Virulence in the Smut Fungus Ustilago Maydis

Differentiation of hyphae into specialized infection structures, known as appressoria, is a common feature of plant pathogenic fungi that penetrate the plant cuticle. Appressorium formation in U. maydis is triggered by environmental signals but the molecular mechanism of this hyphal differentiation is largely unknown. Infectious hyphae grow on the leaf surface by inserting regularly spaced retraction septa at the distal end of the tip cell leaving empty sections of collapsed hyphae behind. Here we show that formation of retraction septa is critical for appressorium formation and virulence in U. maydis. We demonstrate that the diaphanous-related formin Drf1 is necessary for actomyosin ring formation during septation of infectious hyphae. Drf1 acts as an effector of a Cdc42 GTPase signaling module, which also consists of the Cdc42-specific guanine nucleotide exchange factor Don1 and the Ste20-like kinase Don3. Deletion of drf1, don1 or don3 abolished formation of retraction septa resulting in reduced virulence. Appressorium formation in these mutants was not completely blocked but infection structures were found only at the tip of short filaments indicating that retraction septa are necessary for appressorium formation in extended infectious hyphae. In addition, appressoria of drf1 mutants penetrated the plant tissue less frequently