Immunolocalization of hydrophobin HYDPt-1 from the ectomycorrhizal basidiomycete Pisolithus tinctorius during colonization of Eucalyptus globulus roots

A review of the role of biosurfactants in the biodegradation of hydrophobic organopollutants: production, mode of action, biosynthesis and applications

The increasing influence of human activity and industrialization has adversely impacted the environment via pollution with organic contaminants, which are minimally soluble in water. These hydrophobic organopollutants may be present in sediment, water or biota and have created concern due to their toxic effects in mammals. The ability of microorganisms to degrade pollutants makes their use the most effective, inexpensive and ecofriendly method for environmental remediation. Microorganisms have the ability to produce natural surfactants (biosurfactants) that increase the bioavailability of hydrophobic organopollutants, which enables their use as carbon and energy sources. Due to microbial diversity in production, and the biodegradability, nontoxicity, stability and specific activity of the surfactants, the use of microbial surfactants has the potential to overcome problems associated with contamination by hydrophobic organopollutants.This review provides an overview of the current state of knowledge regarding microbial surfactant production, mode of action in the biodegradation of hydrophobic organopollutants and biosynthetic pathways as well as their applications using emergent strategy tools to remove organopollutants from the environment. It is also specified for the first time that biosurfactants are produced either as growth-associated products or secondary metabolites, and are produced in different amounts by a wide range of microorganisms.

Identification and characterization of a hydrophobin Vmh3 from Pleurotus ostreatus

Hydrophobins (HPs) are relatively small surface-active proteins of fungal origin. Being an industrially important protein, isolation of new molecules from GRAS (Generally Regarded as Safe) strains like mushrooms is the need of the time. In the present work, hydrophobin Vmh3-1 is isolated, purified, and identified from a culture broth and vegetative mycelia of Pleurotus ostreatus grown in a Potato dextrose broth (PDB) in static culture conditions. Purified proteins from the broth and the cell wall showed bands of 11 kDa and 17 kDa when analyzed on SDS-PAGE. Hydrophobin Vmh3-1 was identified in purified protein samples by the Orbitrap-HR-LC-MS/MS analysis with a maximum of 66% sequence coverage. The amphipathic nature of the protein was revealed by an increase in the water contact angle (WCA) of the hydrophilic surface of glass by 87% as well as a decrease in the WCA of the hydrophobic surface of Teflon by 19%. The emulsification property was tested with food-grade oils and Hexane. A maximum activity (EI ₂₄) of 87.64% was recorded for Sunflower oil. In CD (Circular dichroism) spectra, Vmh3-1 showed the typical spectra of hydrophobin with a dominance of β-sheets (51%) in the secondary structure and a minimum percentage of the α-helix (2%). The protein did not show a self-aggregating property on vigorous shaking making it suitable for numerous industrial applications. The identification of Vmh3-1 with detailed amino acid sequencing and the characterization of the protein to evaluate its potential in surface modifications for various industrial applications is demonstrated herein for the first time.

The ectomycorrhizal basidiomycete Laccaria bicolor releases a GH28 polygalacturonase that plays a key role in symbiosis establishment

In ectomycorrhiza, root penetration and colonization of the intercellular space by symbiotic hyphae is thought to rely on the mechanical force that results from hyphal tip growth, enhanced by the activity of secreted cell-wall-degrading enzymes. Here, we characterize the biochemical properties of the symbiosis-induced polygalacturonase LbGH28A from the ectomycorrhizal fungus Laccaria bicolor. The transcriptional regulation of LbGH28A was measured by quantitative PCR (qPCR). The biological relevance of LbGH28A was confirmed by generating RNA interference (RNAi)-silenced LbGH28A mutants. We localized the LbGH28A protein by immunofluorescence confocal and immunogold cytochemical microscopy in poplar ectomycorrhizal roots. Quantitative PCR confirmed the induced expression of LbGH28A during ectomycorrhiza formation. Laccaria bicolor RNAi mutants have a lower ability to establish ectomycorrhiza, confirming the key role of this enzyme in symbiosis. The purified recombinant LbGH28A has its highest activity towards pectin and polygalacturonic acid. In situ localization of LbGH28A indicates that this endopolygalacturonase is located in both fungal and plant cell walls at the symbiotic hyphal front. These findings suggest that the symbiosis-induced pectinase LbGH28A is involved in the Hartig net formation and is an important determinant for successful symbiotic colonization.

Contributions of Ultrastructural Studies to the Knowledge of Filamentous Fungi Biology and Fungi-Plant Interactions

Since the first experiments in 1950s, transmission electron microscopy (TEM) observations of filamentous fungi have contributed extensively to understand their structure and to reveal the mechanisms of apical growth. Additionally, also in combination with the use of affinity techniques (such as the gold complexes), several aspects of plant-fungal interactions were elucidated. Nowadays, after the huge of information obtained from -omics techniques, TEM studies and ultrastructural observations offer the possibility to support these data, considering that the full comprehension of the mechanisms at the basis of fungal morphogenesis and the interaction with other organisms is closely related to a detailed knowledge of the structural features. Here, the contribution of these approaches on fungal biology is illustrated, focusing both on hyphae cell ultrastructure and infection structures of pathogenic and mycorrhizal fungi. Moreover, a concise appendix of methods conventionally used for the study of fungal ultrastructure is provided.

KEYLINK: towards a more integrative soil representation for inclusion in ecosystem scale models. I. review and model concept

The relatively poor simulation of the below-ground processes is a severe drawback for many ecosystem models, especially when predicting responses to climate change and management. For a meaningful estimation of ecosystem production and the cycling of water, energy, nutrients and carbon, the integration of soil processes and the exchanges at the surface is crucial. It is increasingly recognized that soil biota play an important role in soil organic carbon and nutrient cycling, shaping soil structure and hydrological properties through their activity, and in water and nutrient uptake by plants through mycorrhizal processes. In this article, we review the main soil biological actors (microbiota, fauna and roots) and their effects on soil functioning. We review to what extent they have been included in soil models and propose which of them could be included in ecosystem models. We show that the model representation of the soil food web, the impact of soil ecosystem engineers on soil structure and the related effects on hydrology and soil organic matter (SOM) stabilization are key issues in improving ecosystem-scale soil representation in models. Finally, we describe a new core model concept (KEYLINK) that integrates insights from SOM models, structural models and food web models to simulate the living soil at an ecosystem scale.

An ectomycorrhizal fungus alters sensitivity to jasmonate, salicylate, gibberellin, and ethylene in host roots

The phytohormones jasmonate, gibberellin, salicylate, and ethylene regulate an interconnected reprogramming network integrating root development with plant responses against microbes. The establishment of mutualistic ectomycorrhizal symbiosis requires the suppression of plant defense responses against fungi as well as the modification of root architecture and cortical cell wall properties. Here, we investigated the contribution of phytohormones and their crosstalk to the ontogenesis of ectomycorrhizae (ECM) between grey poplar (Populus tremula x alba) roots and the fungus Laccaria bicolor. To obtain the hormonal blueprint of developing ECM, we quantified the concentrations of jasmonates, gibberellins, and salicylate via liquid chromatography-tandem mass spectrometry. Subsequently, we assessed root architecture, mycorrhizal morphology, and gene expression levels (RNA sequencing) in phytohormone-treated poplar lateral roots in the presence or absence of L. bicolor. Salicylic acid accumulated in mid-stage ECM. Exogenous phytohormone treatment affected the fungal colonization rate and/or frequency of Hartig net formation. Colonized lateral roots displayed diminished responsiveness to jasmonate but regulated some genes, implicated in defense and cell wall remodelling, that were specifically differentially expressed after jasmonate treatment. Responses to salicylate, gibberellin, and ethylene were enhanced in ECM. The dynamics of phytohormone accumulation and response suggest that jasmonate, gibberellin, salicylate, and ethylene signalling play multifaceted roles in poplar L. bicolor ectomycorrhizal development.

Cell-free expression of natively folded hydrophobins

Hydrophobins are a family of cysteine-rich proteins unique to filamentous fungi. The proteins are produced in a soluble form but self-assemble into organised amphipathic layers at hydrophilic:hydrophobic interfaces. These layers contribute to transitions between wet and dry environments, spore dispersal and attachment to surfaces for growth and infection. Hydrophobins are characterised by four disulphide bonds that are critical to their structure and function. Thus, obtaining correctly folded, soluble and functional hydrophobins directly from bacterial recombinant expression is challenging and in most cases, initial denaturation from inclusion bodies followed by oxidative refolding are required to obtain folded proteins. Here, we report the use of cell-free expression with E. coli cell lysate to directly obtain natively folded hydrophobins. All six of the hydrophobins tested could be expressed after optimisation of redox conditions. For some hydrophobins, the inclusion of the disulfide isomerase DsbC further enhanced expression levels. We are able to achieve a yield of up to 1 mg of natively folded hydrophobin per mL of reaction. This has allowed the confirmation of the correct folding of hydrophobins with the use of ¹⁵N-cysteine and ¹⁵N-¹H nuclear magnetic resonance experiments within 24 h of starting from plasmid stocks.

Effect of gene copy number and chaperone coexpression on recombinant hydrophobin HFBI biosurfactant production in Pichia pastoris

Hydrophobins are small highly surface-active fungal proteins with potential as biosurfactants in a wide array of applications. However, practical implementation of hydrophobins at large scale has been hindered by low recombinant yields. In this study, the effects of increasing hydrophobin gene copy number and overexpressing endoplasmic reticulum resident chaperone proteins Kar2p, Pdi1p, and Ero1p were explored as a means to enhance recombinant yields of the class II hydrophobin HFBI in the eukaryotic expression host Pichia pastoris. One-, 2-, and 3-copy-HFBI strains were attained using an in vitro multimer ligation approach, with strains displaying copy number stability following subsequent transformations as measured by quantitative polymerase chain reaction. Increasing HFBI copy number alone had no effect on increasing HFBI secretion, but increasing copy number in concert with chaperone overexpression synergistically increased HFBI secretion. Overexpression of PDI1 or ERO1 caused insignificant changes in HFBI secretion in 1- and 2-copy strains, but a statistically significant HFBI secretion increase in 3-copy strain. KAR2 overexpression consistently resulted in enhanced HFBI secretion in all copy number strains, with 3-copy-HFBI secreting 22±1.6 fold more than the 1-copy-HFBI/no chaperone strain. The highest increase was seen in 3-copy-HFBI/Ero1p overexpressing strain with 30±4.0 fold increase in HFBI secretion over 1-copy-HFBI/no chaperone strain. This corresponded to an expression level of approximately 330 mg/L HFBI in the 5 ml small-scale format used in this study.

The ectomycorrhizal basidiomycete Laccaria bicolor releases a secreted β-1,4 endoglucanase that plays a key role in symbiosis development

In ectomycorrhiza, root ingress and colonization of the apoplast by colonizing hyphae is thought to rely mainly on the mechanical force that results from hyphal tip growth, but this could be enhanced by secretion of cell-wall-degrading enzymes, which have not yet been identified. The sole cellulose-binding module (CBM1) encoded in the genome of the ectomycorrhizal Laccaria bicolor is linked to a glycoside hydrolase family 5 (GH5) endoglucanase, LbGH5-CBM1. Here, we characterize LbGH5-CBM1 gene expression and the biochemical properties of its protein product. We also immunolocalized LbGH5-CBM1 by immunofluorescence confocal microscopy in poplar ectomycorrhiza. We show that LbGH5-CBM1 expression is substantially induced in ectomycorrhiza, and RNAi mutants with a decreased LbGH5-CBM1 expression have a lower ability to form ectomycorrhiza, suggesting a key role in symbiosis. Recombinant LbGH5-CBM1 displays its highest activity towards cellulose and galactomannans, but no activity toward L. bicolor cell walls. In situ localization of LbGH5-CBM1 in ectomycorrhiza reveals that the endoglucanase accumulates at the periphery of hyphae forming the Hartig net and the mantle. Our data suggest that the symbiosis-induced endoglucanase LbGH5-CBM1 is an enzymatic effector involved in cell wall remodeling during formation of the Hartig net and is an important determinant for successful symbiotic colonization.

The Hydrophobin HYTLO1 Secreted by the Biocontrol Fungus Trichoderma longibrachiatum Triggers a NAADP-Mediated Calcium Signalling Pathway in Lotus japonicus

Trichoderma filamentous fungi are increasingly used as biocontrol agents and plant biostimulants. Growing evidence indicates that part of the beneficial effects is mediated by the activity of fungal metabolites on the plant host. We have investigated the mechanism of plant perception of HYTLO1, a hydrophobin abundantly secreted by Trichoderma longibrachiatum, which may play an important role in the early stages of the plant-fungus interaction. Aequorin-expressing Lotus japonicus suspension cell cultures responded to HYTLO1 with a rapid cytosolic Ca2+ increase that dissipated within 30 min, followed by the activation of the defence-related genes MPK3, WRK33, and CP450. The Ca2+-dependence of these gene expression was demonstrated by using the extracellular Ca2+ chelator EGTA and Ned-19, a potent inhibitor of the nicotinic acid adenine dinucleotide phosphate (NAADP) receptor in animal cells, which effectively blocked the HYTLO1-induced Ca2+ elevation. Immunocytochemical analyses showed the localization of the fungal hydrophobin at the plant cell surface, where it forms a protein film covering the plant cell wall. Our data demonstrate the Ca2+-mediated perception by plant cells of a key metabolite secreted by a biocontrol fungus, and provide the first evidence of the involvement of NAADP-gated Ca2+ release in a signalling pathway triggered by a biotic stimulus.

The Hydrophobin-Like OmSSP1 May Be an Effector in the Ericoid Mycorrhizal Symbiosis

Mutualistic and pathogenic plant-colonizing fungi use effector molecules to manipulate the host cell metabolism to allow plant tissue invasion. Some small secreted proteins (SSPs) have been identified as fungal effectors in both ectomycorrhizal and arbuscular mycorrhizal fungi, but it is currently unknown whether SSPs also play a role as effectors in other mycorrhizal associations. Ericoid mycorrhiza is a specific endomycorrhizal type that involves symbiotic fungi mostly belonging to the Leotiomycetes (Ascomycetes) and plants in the family Ericaceae. Genomic and RNASeq data from the ericoid mycorrhizal fungus Oidiodendron maius led to the identification of several symbiosis-upregulated genes encoding putative SSPs. OmSSP1, the most highly symbiosis up-regulated SSP, was found to share some features with fungal hydrophobins, even though it lacks the Pfam hydrophobin domain. Sequence alignment with other hydrophobins and hydrophobin-like fungal proteins placed OmSSP1 within Class I hydrophobins. However, the predicted features of OmSSP1 may suggest a distinct type of hydrophobin-like proteins. The presence of a predicted signal peptide and a yeast-based signal sequence trap assay demonstrate that OmSSP1 is secreted. OmSSP1 null-mutants showed a reduced capacity to form ericoid mycorrhiza with Vaccinium myrtillus roots, suggesting a role as effectors in the ericoid mycorrhizal interaction.

Recent Insights on Biological and Ecological Aspects of Ectomycorrhizal Fungi and Their Interactions

The roots of most terrestrial plants are colonized by mycorrhizal fungi. They play a key role in terrestrial environments influencing soil structure and ecosystem functionality. Around them a peculiar region, the mycorrhizosphere, develops. This is a very dynamic environment where plants, soil and microorganisms interact. Interest in this fascinating environment has increased over the years. For a long period the knowledge of the microbial populations in the rhizosphere has been limited, because they have always been studied by traditional culture-based techniques. These methods, which only allow the study of cultured microorganisms, do not allow the characterization of most organisms existing in nature. The introduction in the last few years of methodologies that are independent of culture techniques has bypassed this limitation. This together with the development of high-throughput molecular tools has given new insights into the biology, evolution, and biodiversity of mycorrhizal associations, as well as, the molecular dialog between plants and fungi. The genomes of many mycorrhizal fungal species have been sequenced so far allowing to better understanding the lifestyle of these fungi, their sexual reproduction modalities and metabolic functions. The possibility to detect the mycelium and the mycorrhizae of heterothallic fungi has also allowed to follow the spatial and temporal distributional patterns of strains of different mating types. On the other hand, the availability of the genome sequencing from several mycorrhizal fungi with a different lifestyle, or belonging to different groups, allowed to verify the common feature of the mycorrhizal symbiosis as well as the differences on how different mycorrhizal species interact and dialog with the plant. Here, we will consider the aspects described before, mainly focusing on ectomycorrhizal fungi and their interactions with plants and other soil microorganisms.

Unearthing the roots of ectomycorrhizal symbioses

During the diversification of Fungi and the rise of conifer-dominated and angiosperm- dominated forests, mutualistic symbioses developed between certain trees and ectomycorrhizal fungi that enabled these trees to colonize boreal and temperate regions. The evolutionary success of these symbioses is evident from phylogenomic analyses that suggest that ectomycorrhizal fungi have arisen in approximately 60 independent saprotrophic lineages, which has led to the wide range of ectomycorrhizal associations that exist today. In this Review, we discuss recent genomic studies that have revealed the adaptations that seem to be fundamental to the convergent evolution of ectomycorrhizal fungi, including the loss of some metabolic functions and the acquisition of effectors that facilitate mutualistic interactions with host plants. Finally, we consider how these insights can be integrated into a model of the development of ectomycorrhizal symbioses.

Cell wall remodeling in mycorrhizal symbiosis: a way towards biotrophism

Cell walls are deeply involved in the molecular talk between partners during plant and microbe interactions, and their role in mycorrhizae, i.e., the widespread symbiotic associations established between plant roots and soil fungi, has been investigated extensively. All mycorrhizal interactions achieve full symbiotic functionality through the development of an extensive contact surface between the plant and fungal cells, where signals and nutrients are exchanged. The exchange of molecules between the fungal and the plant cytoplasm takes place both through their plasma membranes and their cell walls; a functional compartment, known as the symbiotic interface, is thus defined. Among all the symbiotic interfaces, the complex intracellular interface of arbuscular mycorrhizal (AM) symbiosis has received a great deal of attention since its first description. Here, in fact, the host plasma membrane invaginates and proliferates around all the developing intracellular fungal structures, and cell wall material is laid down between this membrane and the fungal cell surface. By contrast, in ectomycorrhizae (ECM), where the fungus grows outside and between the root cells, plant and fungal cell walls are always in direct contact and form the interface between the two partners. The organization and composition of cell walls within the interface compartment is a topic that has attracted widespread attention, both in ecto- and endomycorrhizae. The aim of this review is to provide a general overview of the current knowledge on this topic by integrating morphological observations, which have illustrated cell wall features during mycorrhizal interactions, with the current data produced by genomic and transcriptomic approaches.

Genome-wide analysis of cell wall-related genes in Tuber melanosporum

A genome-wide inventory of proteins involved in cell wall synthesis and remodeling has been obtained by taking advantage of the recently released genome sequence of the ectomycorrhizal Tuber melanosporum black truffle. Genes that encode cell wall biosynthetic enzymes, enzymes involved in cell wall polysaccharide synthesis or modification, GPI-anchored proteins and other cell wall proteins were identified in the black truffle genome. As a second step, array data were validated and the symbiotic stage was chosen as the main focus. Quantitative RT-PCR experiments were performed on 29 selected genes to verify their expression during ectomycorrhizal formation. The results confirmed the array data, and this suggests that cell wall-related genes are required for morphogenetic transition from mycelium growth to the ectomycorrhizal branched hyphae. Labeling experiments were also performed on T. melanosporum mycelium and ectomycorrhizae to localize cell wall components.

Phylogenetic, genomic organization and expression analysis of hydrophobin genes in the ectomycorrhizal basidiomycete Laccaria bicolor

Hydrophobins are morphogenetic, small secreted hydrophobic fungal proteins produced in response to changing development and environmental conditions. These proteins are important in the interaction between certain fungi and their hosts. In mutualistic ectomycorrhizal fungi several hydrophobins form a subclass of mycorrhizal-induced small secreted proteins that are likely to be critical in the formation of the symbiotic interface with host root cells. In this study, two genomes of the ectomycorrhizal basidiomycete Laccaria bicolor strains S238N-H82 (from North America) and 81306 (from Europe) were surveyed to construct a comprehensive genome-wide inventory of hydrophobins and to explore their characteristics and roles during host colonization. The S238N-H82 L. bicolor hydrophobin gene family is composed of 12 genes while the 81306 strain encodes nine hydrophobins, all corresponding to class I hydrophobins. The three extra hydrophobin genes encoded by the S238N-H82 genome likely arose via gene duplication and are bordered by transposon rich regions. Expression profiles of the hydrophobin genes of L. bicolor varied greatly depending on life stage (e.g. free living mycelium vs. root colonization) and on the host root environment. We conclude from this study that the complex diversity and range of expression profiles of the Laccaria hydrophobin multi-gene family have likely been a selective advantage for this mutualist in colonizing a wide range of host plants.

Prokaryotic expression, purification, and polyclonal antibody production of a hydrophobin from Grifola frondosa

Hydrophobins are small fungal proteins that self-assemble spontaneously at hydrophilic-hydrophobic interfaces and change the polar nature of the surfaces to which they attach. A new hydrophobin gene hgfI was identified recently from the edible mushroom Grifola frondosa. In this paper, the cloning, expression, purification, and polyclonal antibody preparation of the HGFI were described. The hgfI gene was cloned into pET-28a expression plasmid at the EcoRI and NdeI restriction sites and then transformed into Escherichia coli BL21 strain. SDSPAGE analysis showed that recombinant HGFI protein was satisfactorily expressed by optimizing the concentration and induction time of IPTG. The expressed recombinant HGFI protein was purified by electroelution because its inclusion body was insoluble in traditional processing method. After a desalting procedure with Sephadex G-25, the recombinant HGFI protein was used to immunize adult rabbits following standard protocol. ELISA and western blot analysis indicated that the produced antiserum could detect both HGFI protein expressed in the prokaryotic (E. coli) and in the eukaryotic cells (G. frondosa). Furthermore, the antiserum was used to determine the localization of HGFI protein in G. frondosa cells using an immunofluorescence technique. The results demonstrated that HGFI protein was localized in the cell wall, especially at the budding position of hypha. The polyclonal antibody against HGFI will facilitate further production and functional study of HGFI protein.

Fungal transcript pattern during the preinfection stage (12 h) of ectomycorrhiza formed between Pisolithus tinctorius and Castanea sativa roots, identified using cDNA microarrays

Transcriptional changes in Pisolithus tinctorius leading to ectomycorrhizal formation in P. tinctorius- Castanea sativa were investigated using a 12-h fungal interaction in vitro system. Using a 3107-cDNA clone microarray, 34 unique expressed sequence tags (ESTs) were found to be differentially expressed. These ESTs represent 14 known genes, 5 upregulated and 9 downregulated, and 20 orphan sequences. Some transcripts of upregulated genes (with unknown function) were previously identified in other mycorrhizal Pisolithus spp. associations. ESTs for S-adenosyl-L-homocysteine hydrolase and several orphan sequences were identified in our system. The identified transcript of downregulated genes involved hydrophobins, 5S, 18S, and 28S ribosomal RNA genes, large subunits of ribosomal RNA (mitochondrial gene), and two types of heat shock proteins. This study demonstrates the high complexity of molecular events involved in the preinfection steps and suggests the utilization of different fungal gene repertories before ectomycorrhizal formation. These data constitute a first contribution for the molecular understanding of early signaling events between P. tinctorius and C. sativa roots during ectomycorrhizal formation.

Hydrophobins: proteins with potential

Hydrophobins are self-assembling proteins of fungal origin. Their ability to self-assemble into an amphipathic membrane is of interest for many different applications, ranging from medical and technical coatings to the production of proteinaceous glue and cosmetics. Assembled hydrophobins can modify surface characteristics, thus controling the binding properties of the surface; for example, enzymes can be actively and non-covalently immobilized on electrode surfaces and medical coatings can be improved for biocompatibility. Over the past few years research on hydrophobins has contributed to a better understanding of the self-assembly process and is generating more handles to control and manipulate the process. This knowledge could have an immediate effect on production levels, which are not yet adequate, and provide the boost needed for hydrophobins to reach their full potential.

Mycorrhizas and soil structure

In addition to their well-recognized roles in plant nutrition and communities, mycorrhizas can influence the key ecosystem process of soil aggregation. Here we review the contribution of mycorrhizas, mostly focused on arbuscular mycorrhizal fungi (AMF), to soil structure at various hierarchical levels: plant community; individual root; and the soil mycelium. There are a suite of mechanisms by which mycorrhizal fungi can influence soil aggregation at each of these various scales. By extension of these mechanisms to the question of fungal diversity, it is recognized that different species or communities of fungi can promote soil aggregation to different degrees. We argue that soil aggregation should be included in a more complete 'multifunctional' perspective of mycorrhizal ecology, and that in-depth understanding of mycorrhizas/soil process relationships will require analyses emphasizing feedbacks between soil structure and mycorrhizas, rather than a uni-directional approach simply addressing mycorrhizal effects on soils. We finish the discussion by highlighting new tools, developments and foci that will probably be crucial in further understanding mycorrhizal contributions to soil structure.

Global patterns of gene regulation associated with the development of ectomycorrhiza between birch (Betula pendula Roth.) and Paxillus involutus (Batsch) Fr

The formation of ectomycorrhizal (ECM) root tissue is characterized by distinct morphological and developmental stages, such as preinfection and adhesion, mantle, and Hartig net formation. The global pattern of gene expression during these stages in the birch (Betula pendula)-Paxillus involutus ECM association was analyzed using cDNA microarrays. In comparison with nonsymbiotic conditions, 251 fungal (from a total of 1,075) and 138 plant (1,074 in total) genes were found to be differentially regulated during the ECM development. For instance, during mantle and Hartig net development, there were several plant genes upregulated that are normally involved in defense responses during pathogenic fungal challenges. These responses were, at later stages of ECM development, found to be repressed. Other birch genes that showed differential regulation involved several homologs that usually are implicated in water permeability (aquaporins) and water stress tolerance (dehydrins). Among fungal genes differentially upregulated during stages of mantle and Hartig net formation were homologs putatively involved in mitochondrial respiration. In fully developed ECM tissue, there was an upregulation of fungal genes related to protein synthesis and the cytoskeleton assembly machinery. This study highlights complex molecular interactions between two symbionts during the development of an ECM association.

Transcript patterns associated with ectomycorrhiza development in Eucalyptus globulus and Pisolithus microcarpus

Regulated gene expression is an important mechanism for controlling ectomycorrhizal symbiosis development. This study aimed to elucidate the coordination between development of mycorrhiza and the differential gene expression in both partners. We analysed RNA levels from sequential samples of symbiotic tissues of Eucalyptus globulus bicostata and the basidiomycete Pisolithus microcarpus progressing through ectomycorrhiza development using cDNA arrays. We derived groups of coordinately expressed genes using hierarchical and nonhierarchical clustering algorithms. Five major distinct temporal patterns of induction/repression were observed with distinct groups of early, middle-, and late-transcriptionally responsive genes to symbiosis formation. At earliest stages, the differentially expressed fungal genes included cell wall symbiosis-regulated proteins, hydrophobins and mannoproteins, whereas transcripts coding for defense-related proteins were upregulated in plant tissues. Middle- and late-transcriptionally responsive genes coded enzymes of glycolysis, tricarboxylic acid cycle and amino acid biosynthesis, as well as protein synthesis, hormone metabolism and signal transduction components. This investigation confirms and extends earlier results which found that changes in morphology associated with mycorrhizal development were accompanied by changes in transcript patterns, but no ectomycorrhiza-specific genes were detected.

Five hydrophobin genes in Fusarium verticillioides include two required for microconidial chain formation

Five hydrophobin genes have been identified in the fungal corn pathogen Fusarium verticillioides. HYD1, HYD2, and HYD3 encode Class I hydrophobins. The predicted structures of Hyd1p and Hyd2p are 80% similar, while Hyd3p has an unusually small number of amino acids between the third and fourth cysteines. HYD4 and HYD5 encode Class II hydrophobins. Mutants with HYD1-5 individually deleted and a hyd1deltahyd2delta double mutant were similar to wild-type strains in the amount of disease caused in a corn seedling infection assay and in the number of microconidia produced. Microconidial chains were rare in hyd1delta and hyd2delta mutants as microconidia were present almost exclusively as false heads. Transformation of hyd1delta and hyd2delta mutants with HYD1 and HYD2, respectively, restored microconidial chain formation, but transformation with HYD1::AcGFP and HYD2::AcGFP did not complement the mutation. HYD1::AcGFP and HYD2::AcGFP localized to the outside of conidia in false heads and in chains.

Hydrophobins and the interactions between fungi and plants

Summary Hydrophobins are small proteins thought to be ubiquitous in filamentous fungi. They are usually secreted and are found on the outer surfaces of cell walls of hyphae and conidia where they mediate interactions between the fungus and the environment. We review here what is currently known about the primary and secondary structure of these proteins, as well as their post-translational modifications. We also discuss the diverse functions of hydrophobins in biology and development, with particular attention to fungi involved in pathogenesis and symbiosis.

Ectomycorrhiza: gene expression, metabolism and the wood-wide web

In the ectomycorrhizal symbiosis between fungi and trees, the fungus completely ensheaths the tree roots and takes over water and mineral nutrient supply, while the plant supplies photosynthate. Recent work has focussed on gene expression in the two partners, on the effects of global change and nitrogen deposition rate on the symbiosis, and on the role of mycorrhizal fungi in connecting individual plants to form a 'wood-wide web'.

Coevolution of roots and mycorrhizas of land plants

Here, the coevolution of mycorrhizal fungi and roots is assessed in the light of evidence now available, from palaeobotanical and morphological studies and the analysis of DNA-based phylogenies. The first bryophyte-like land plants, in the early Devonian (400 million years ago), had endophytic associations resembling vesicular-arbuscular mycorrhizas (VAM) even before roots evolved. Mycorrhizal evolution would have progressed from endophytic hyphae towards balanced associations where partners were interdependent due to the exchange of limiting energy and nutrient resources. Most mycorrhizas are mutualistic, but in some cases the trend for increasing plant control of fungi culminates in the exploitative mycorrhizas of achlorophyllous, mycoheterotrophic plants. Ectomycorrhizal, ericoid and orchid mycorrhizas, as well as nonmycorrhizal roots, evolved during the period of rapid angiosperm radiation in the Cretaceous. It is hypothesised that roots gradually evolved from rhizomes to provide more suitable habitats for mycorrhizal fungi and provide plants with complex branching and leaves with water and nutrients. Selection pressures have caused the morphological divergence of roots with different types of mycorrizas. Root cortex thickness and exodermis suberization are greatest in obllgately mycorrhizal plants, while nonmycorrhizal plants tend to have fine roots, with more roots hairs and relatively advanced chemical defences. Major coevolutionary trends and the relative success of plants with different root types are discussed. Contents Summary 275 I. Introduction 276 II. Mycorrhizal Fungi 276 III. The Dawn of Mycorrhizas 279 IV. Mycorrhizal Associations of Living and Extinct Plants 282 V. Evolution of Roots 288 VI. The Root as a Habitat for Fungi 290 VII. Mycorrhizal Evolution Trends 295 Acknowledgements 298 References 298.

Identification of a hydrophobin gene that is developmentally regulated in the ectomycorrhizal fungus Tricholoma terreum

The symbiosis between ectomycorrhizal fungi and trees is an essential part of forest ecology and depends entirely on the communication between the two partners for establishing and maintaining the relationship. The identification and characterization of differentially expressed genes is a step to identifying such signals and to understanding the regulation of this process. We determined the role of hydrophobins produced by Tricholoma terreum in mycorrhiza formation and hyphal development. A hydrophobin was purified from culture supernatant, and the corresponding gene was identified. The gene is expressed in aerial mycelium and in mycorrhiza. By using a heterologous antiserum directed against a hydrophobin found in the aerial mycelium of Schizophyllum commune, we detected a hydrophobin in the symbiosis between T. terreum and its native pine host Pinus sylvestris. The hydrophobin was found in aerial mycelium of the hyphal mantle and also in the Hartig net hyphae, which form the interface between both partners. Interestingly, this was not the case in the interaction of T. terreum with a host of low compatibility, the spruce Picea abies. The differential expression with respect to host was verified at the transcriptional level by competitive PCR. The differential protein accumulation pattern with respect to host compatibility seen by immunofluorescence staining can thus be attributed at least in part to transcriptional control of the hyd1 gene.

Signal and nutrient exchange at biotrophic plant-fungus interfaces

Biotrophic interfaces are formed in mutualistic and parasitic plant-fungus interactions. They result from coordinated developmental programs in both partners and represent specialized platforms for the exchange of information and nutritional metabolites. New data on the establishment and the components of functional interfaces have been obtained in a number of ways. First, by isolation of symbiotically defective mutants; second, by characterization of new genes and their products; and, third, by the identification and localization of components of biotrophic interfaces, such as cell-wall proteins, H+-ATPases and nutrient transporters.