Transformation of the arbuscular mycorrhiza Gigaspora rosea by particle bombardment

A nuclear-targeted effector of Rhizophagus irregularis interferes with histone 2B mono-ubiquitination to promote arbuscular mycorrhisation

Arguably, symbiotic arbuscular mycorrhizal (AM) fungi have the broadest host range of all fungi, being able to intracellularly colonise root cells in the vast majority of all land plants. This raises the question how AM fungi effectively deal with the immune systems of such a widely diverse range of plants. Here, we studied the role of a nuclear-localisation signal-containing effector from Rhizophagus irregularis, called Nuclear Localised Effector1 (RiNLE1), that is highly and specifically expressed in arbuscules. We showed that RiNLE1 is able to translocate to the host nucleus where it interacts with the plant core nucleosome protein histone 2B (H2B). RiNLE1 is able to impair the mono-ubiquitination of H2B, which results in the suppression of defence-related gene expression and enhanced colonisation levels. This study highlights a novel mechanism by which AM fungi can effectively control plant epigenetic modifications through direct interaction with a core nucleosome component. Homologues of RiNLE1 are found in a range of fungi that establish intimate interactions with plants, suggesting that this type of effector may be more widely recruited to manipulate host defence responses.

Agrobacterium tumefaciens-Mediated Genetic Transformation of the Ect-endomycorrhizal Fungus Terfezia boudieri

Mycorrhizal desert truffles such as Terfezia boudieri, Tirmania nivea, and Terfezia claveryi, form mycorrhizal associations with plants of the Cistaceae family. These valued truffles are still collected from the wild and not cultivated under intensive farming due to the lack of basic knowledge about their biology at all levels. Recently, several genomes of desert truffles have been decoded, enabling researchers to attempt genetic manipulations to enable cultivation. To execute such manipulations, the development of molecular tools for genes transformation into truffles is needed. We developed an Agrobacterium tumefaciens-mediated genetic transformation system in T. boudieri. This system was optimized for the developmental stage of the mycelia explants, bacterial optical density, infection and co-cultivation durations, and concentrations of the selection antibiotics. The pFPL-Rh plasmid harboring hph gene conferring hygromycin resistance as a selection marker and the red fluorescent protein gene were used as visual reporters. The optimal conditions were incubation with 200 μM of acetosyringone, attaining a bacterial optical density of 0.3 OD₆₀₀; transfer time of 45 min; and co-cultivation for 3 days. This is the first report on a transformation system for T. boudieri, and the proposed protocol can be adapted for the transformation of other important desert truffles as well as ectomycorrhizal species.

Expression profiling of fungal genes during arbuscular mycorrhiza symbiosis establishment using direct fluorescent in situ RT-PCR

Expression profiling of fungal genes in the arbuscular mycorrhiza (AM) symbiosis has been based on studies of RNA extracted from fungal tissue or mycorrhizal roots, giving only a general picture of overall transcript levels in the targeted tissues. Information about the spatial distribution of transcripts within AM fungal structures during different developmental stages is essential to a better understanding of fungal activity in symbiotic interactions with host roots and to determine molecular events involved in establishment and functioning of the AM symbiosis. The obligate biotrophic nature of AM fungi is a challenge for developing new molecular methods to identify and localize their activity in situ. The direct fluorescent in situ (DIFIS) RT-PCR procedure described here represents a novel tool for spatial mapping of AM fungal gene expression simultaneously prior to root penetration, within fungal tissues in the host root and in the extraradical stage of fungal development.In order to enhance detection sensitivity of the in situ RT-PCR technique and enable localization of low abundance mRNA, we have adopted direct fluorescent labeling of primers for the amplification step to overcome the problem of low detection associated with digoxigenin or biotin-labeled primers and to avoid the multiplicity of steps associated with immunological detection. Signal detection has also been greatly improved by eliminating autofluorescence of AM fungal and root tissues using confocal microscopy.

An STE12 gene identified in the mycorrhizal fungus Glomus intraradices restores infectivity of a hemibiotrophic plant pathogen

Mechanisms of root penetration by arbuscular mycorrhizal (AM) fungi are unknown and investigations are hampered by the lack of transformation systems for these unculturable obligate biotrophs. Early steps of host infection by hemibiotrophic fungal phytopathogens, sharing common features with those of AM fungal colonization, depend on the transcription factor STE12. Using degenerated primers and rapid amplification of cDNA ends, we isolated the full-length cDNA of an STE12-like gene, GintSTE, from Glomus intraradices and profiled GintSTE expression by real-time and in situ RT-PCR. GintSTE activity and function were investigated by heterologous complementation of a yeast ste12Delta mutant and a Colletotrichum lindemuthianum clste12Delta mutant. * Sequence data indicate that GintSTE is similar to STE12 from hemibiotrophic plant pathogens, especially Colletotrichum spp. Introduction of GintSTE into a noninvasive mutant of C. lindemuthianum restored fungal infectivity of plant tissues. GintSTE expression was specifically localized in extraradicular fungal structures and was up-regulated when G. intraradices penetrated roots of wild-type Medicago truncatula as compared with an incompatible mutant. Results suggest a possible role for GintSTE in early steps of root penetration by AM fungi, and that pathogenic and symbiotic fungi may share common regulatory mechanisms for invasion of plant tissues.

Spatial monitoring of gene activity in extraradical and intraradical developmental stages of arbuscular mycorrhizal fungi by direct fluorescent in situ RT-PCR

Gene expression profiling based on tissue extracts gives only limited information about genes associated with complex developmental processes such as those implicated in fungal interactions with plant roots during arbuscular mycorrhiza development and function. To overcome this drawback, a direct fluorescent in situ RT-PCR methodology was developed for spatial mapping of gene expression in different presymbiotic and symbiotic structures of an arbuscular mycorrhizal fungus. Transcript detection was optimized by targeting the LSU rRNA gene of Glomus intraradices and monitoring expression of a stearoyl-CoA-desaturase gene that is consistently expressed at high levels in spores, hyphae, arbuscules and vesicles. This method was further validated by localizing expression of fungal peptidylprolyl isomerase and superoxide dismutase genes, which are expressed to different extents in fungal structures. Direct fluorescent in situ RT-PCR offers new perspectives for the sensitive analysis of fungal developmental processes that occur during functional differentiation in symbiotic arbuscular mycorrhiza interactions.

Expression of the fluorescence markers DsRed and GFP fused to a nuclear localization signal in the arbuscular mycorrhizal fungus Glomus intraradices

Here, arbuscular mycorrhizal (AM) fungi were monitored in vivo introducing the fluorescent reporters DsRed and GFP (green fluorescent protein) in Glomus intraradices using a biolistic approach and Agrobacterium tumefaciens-mediated transformation. Both reporter genes were fused to the nuclear localization signal of the Aspergillus nidulans transcription factor StuA to target fluorescence to nuclei. Expression of DsRed was driven by two Glomus mosseae promoters highly expressed during early symbiosis, GmPMA1 and GmFOX2, while expression of GFP was driven by the A. nidulans gpd promoter. All promoters worked in G. intraradices as well as in A. nidulans. Red and green fluorescence was localized to nuclei of G. intraradices spores and hyphae 3 d after bombardment. However, expression was transient. The efficiency of the Agrobacterium-mediated transformation was very low. These results indicate that the biolistic method allows the expression of foreign DNA into G. intraradices with high frequency, but it is insufficient to render stable transformants. DsRed vs GFP is a more appropriate living reporter to be used in G. intraradices because of the lower autofluorescence in the red channel but targeted to the nucleus both marker genes can be visualized. This is the first report of fluorescent marker expression in an AM fungus driven by arbuscular mycorrhizal promoters.

Imaging mycorrhizal fungal transformants that express EGFP during ericoid endosymbiosis

Ericoid endomycorrhizal fungi form intracellular associations with the epidermal root cells of plants belonging to Ericales. In natural environments, these fungi increase the ability of their host plants to colonise soils polluted with toxic metals, although the underlying mechanisms are not clearly understood. Genetic transformation is a powerful tool to study the function of specific genes involved in the interaction of symbiotic fungi with the host plants and with the environment. Here, we investigated the possibility to genetically transform an ericoid endomycorrhizal strain. A metal tolerant mycorrhizal Oidiodendron maius strain isolated from a contaminated area was chosen to develop the transformation system. Two different protocols were used: protoplasts and Agrobacterium-mediated transformation. Stable transformants were obtained with both techniques. They remained competent for mycorrhizal formation and GFP-transformed fungi were visualised in planta. This is the first report of stable transformation of an ericoid endomycorrhizal fungus. The protocol set up could represent a good starting point for the identification of genes important in the ericoid mycorrhiza formation and in the understanding of how this symbiosis is established and functions. The success in the genetic transformation of this strain will allow us to better define its potential use in bioremediation strategies.

Approaches to functional genomics in filamentous fungi

The study of gene function in filamentous fungi is a field of research that has made great advances in very recent years. A number of transformation and gene manipulation strategies have been developed and applied to a diverse and rapidly expanding list of economically important filamentous fungi and oomycetes. With the significant number of fungal genomes now sequenced or being sequenced, functional genomics promises to uncover a great deal of new information in coming years. This review discusses recent advances that have been made in examining gene function in filamentous fungi and describes the advantages and limitations of the different approaches.

Transformation of Fusarium oxysporum by particle bombardment and characterisation of the resulting transformants expressing a GFP transgene

Fusarium is the causative agent of a variety of economically significant vascular wilt diseases of vegetables, flowers and field crops. The completion of the first Fusarium genome and the availability of an EST database now provides a platform for both forward and reverse genetic approaches to ascribe gene function in this phytopathogen. To underpin these strategies effective gene transfer procedures will be required. Here we describe an efficient and robust procedure for Fusarium oxysporum transformation based on particle bombardment. We utilised this procedure to introduce a chimeric gene comprised of the Aspergillus nidulans Pgdp promoter fused to a GFP reporter gene. A transformation efficiency of 45 transformants per mug of plasmid DNA was routinely achieved. The Pgdp promoter directed strong cytoplasmic expression of the GFP marker in transformed F. oxysporum monitored via fluorescence and confocal microscopy. A pathogenicity assay undertaken on Arabidopsis seedlings with selected transformants revealed that virulence was retained following transformation. Moreover, in a similar fashion to wild-type F. oxysporum, these transformants activated three distinct Arabidopsis defence gene promoter::luciferase fusions, which defined specific defence gene subsets.

Partner communication in the arbuscular mycorrhizal interaction

During 400 million years of genome–genome interaction, plants and arbuscular mycorrhizal (AM) fungi have become highly interdependent, both ecologically and physiologically. As a result, the differentiation of a functional mycorrhiza is a multistep process requiring the active participation of both partners. During the presymbiotic stage of the AM interaction, some active molecules present in root exudates rapidly induce several fungal genes, in addition to stimulating important cellular and metabolic functions in the fungus, such as mitochondrial biogenesis and respiration. As a result of this activation, the fungus can use its lipidic reserves and reach further developmental stages. Subsequently, the fungus produces factors that induce new gene expression in roots. The fact that the partners of the AM symbiosis exchange such "pheromonal" active molecules during the presymbiotic stage of their interaction suggests the existence of other cross-signaling molecules during the symbiotic stage. These later signals might be involved in activating fungal fatty acid synthesis and sugar uptake or be responsible for specific plant gene induction. Now the challenge is to characterize the chemical nature and the exact role of these fungal and plant regulators in the AM symbiosis.Key words: arbuscular mycorrhizal symbiosis, signaling, root exudates, Myc factor, respiration, lipid metabolism.

Isolation and sequence analysis of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol & Gerd.) Gerdemann & Trappe 3-phosphoglycerate kinase (PGK) gene promoter region

The Glomus mosseae 3-phosphoglycerate kinase (GmPGK) gene promoter has been isolated from a phage genomic library and represents one of the few promoter elements to be isolated and analysed from these symbiotic fungi. The analysis revealed the presence of several motifs which are found in the promoter region of other fungal PGK genes. In particular, DNA sequences homologous to segments of the S. cerevisiae and Rhizopus niveus upstream activating elements (UAS). The importance of these UAS sequences in regulating carbon source in PGK genes is known and the presence of two carbon source regulated UAS sequences in the GmPGK gene promoter and its role in the biology of AM fungi is discussed briefly.

Arbuscular mycorrhiza on root-organ cultures

The study of arbuscular mycorrhizal (AM) fungi and the AM symbiosis formed with host plant roots is complicated by the biotrophic and hypogeous nature of the mycobionts involved. To overcome this, several attempts have been made during the last three decades to obtain this symbiosis in vitro. The use of root-organ cultures has proved particularly successful. In this review, we describe the method by which root-organ cultures (transformed and nontransformed) have been obtained, together with the choice of host species, inoculation techniques, and culture media. We also outline the potential use of continuous cultures and cryopreservation of in vitro produced spores for long-term germ plasm storage. Furthermore, this review highlights the considerable impact that in vitro root-organ cultures have had on studies of AM fungal morphology, taxonomy, and phylogeny and how they have improved our understanding of the processes leading to root colonization and development of the extraradical mycelium. This is supported by a summary of some of the most important findings, regarding this symbiosis, that have been made at the physiological, biochemical, and molecular levels. We also summarize results from studies between AM fungi and certain pathogenic and nonpathogenic soil microorganisms. We describe some of the limitations of this in vitro system and propose diverse avenues of AM research that can now be undertaken, including the potential use of a similar system for ectomycorrhizal research.Key words: arbuscular mycorrhiza, root-organ cultures, Glomales, in vitro, root symbioses, source of inoculum, cryopreservation, intraradical and extraradical mycelium, mycorrhizosphere.

The arbuscular mycorrhizal symbiosis: a molecular review of the fungal dimension

Mycorrhizal associations vary widely in structure and function, but the most common interaction is the arbuscular mycorrhizal (AM) symbiosis. This interaction is formed between the roots of over 80% of all terrestrial plant species and Zygomycete fungi from the Order Glomales. These fungi are termed AM fungi and are obligate symbionts which form endomycorrhizal symbioses. This symbiosis confers benefits directly to the host plant's growth and development through the acquisition of P and other mineral nutrients from the soil by the fungus. In addition, they may also enhance the plant's resistance to biotic and abiotic stresses. These beneficial effects of the AM symbiosis occur as a result of a complex molecular dialogue between the two symbiotic partners. Identifying the molecules involved in the dialogue is a prerequisite for a greater understanding of the symbiosis. Ongoing research attempts to understand the underlying dialogue and concomitant molecular changes occurring in the plant and the fungus during the establishment of a functioning AM symbiosis. This paper focuses on the molecular approaches being used to study AM fungal genes being expressed in the symbiotic and asymbiotic stages of its lifecycle. In addition, the importance of studying these fungi, in relation to understanding plant processes, is discussed briefly.

Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?

Plant cells engage in mutualistic and parasitic endosymbioses with a wide variety of microorganisms, ranging from Gram-negative (Rhizobium, Nostoc) and Gram-positive bacteria (Frankia), to oomycetes (Phytophthora), Chytridiomycetes, Zygomycetes (arbuscular mycorrhizal fungi) and true fungi (Erysiphe, ascomycete; Puccinia, basidiomycete). Endosymbiosis is characterised by the 'symbiosome', a compartment within host cells in which the symbiotic microorganism is either partially or completely enclosed by a host-derived membrane. The analysis of plant mutants indicates that the genetic requirements for the interaction with rhizobia and arbuscular mycorrhiza fungi are partially overlapping. The extent to which plants use similar or identical developmental programs for the intracellular accommodation of different microorganisms is, however, not clear. For example, plant cells actively weaken their cell wall to facilitate bacterial colonisation, whereas penetration by fungal symbionts appears not to be assisted in this manner. Moreover, different transport requirements are imposed on the symbiotic interface of different interactions indicating that additional system-specific components are likely to exist.

MOLECULAR AND CELLULAR ASPECTS OF THE ARBUSCULAR MYCORRHIZAL SYMBIOSIS

Arbuscular mycorrhizae are symbiotic associations formed between a wide range of plant species including angiosperms, gymnosperms, pteridophytes, and some bryophytes, and a limited range of fungi belonging to a single order, the Glomales. The symbiosis develops in the plant roots where the fungus colonizes the apoplast and cells of the cortex to access carbon supplied by the plant. The fungal contribution to the symbiosis is complex, but a major aspect includes the transfer of mineral nutrients, particularly phosphate from the soil to the plant. Development of this highly compatible association requires the coordinate molecular and cellular differentiation of both symbionts to form specialized interfaces over which bi-directional nutrient transfer occurs. Recent insights into the molecular events underlying these aspects of the symbiosis are discussed.

Chitin synthase genes in the arbuscular mycorrhizal fungus Glomus versiforme: full sequence of a gene encoding a class IV chitin synthase

Chitin synthase genes of the arbuscular mycorrhizal fungus Glomus versiforme were sought in an investigation of the molecular basis of fungal growth. Three DNA fragments (Gvchs1, Gvchs2 and Gvchs3) corresponding to the conserved regions of distinct chitin synthase (chs) genes were amplified by means of the polymerase chain reaction (PCR) with two sets of degenerate primers. Gvchs1 and Gvchs2 encode two class I chitin synthases, whereas Gvchs3 encodes a class IV chitin synthase. A genomic library was used to obtain the Gvchs3 complete gene (1194 amino acids), which shows a very close similarity to the class IV chitin synthase from Neurospora crassa.