Symbiotic status, phosphate, and sucrose regulate the expression of two plasma membrane H+-ATPase genes from the mycorrhizal fungus Glomus mosseae

The Plasma Membrane H+ ATPase CsPMA2 Regulates Lipid Droplet Formation, Appressorial Development and Virulence in Colletotrichum siamense

Plasma membrane H+-ATPases (PMAs) play an important role in the pathogenicity of pathogenic fungi. Lipid droplets are important storage sites for neutral lipids in fungal conidia and hyphae and can be used by plant pathogenic fungi for infection. However, the relationship between plasma membrane H+-ATPase, lipid droplets and virulence remains unclear. Here, we characterized a plasma membrane H+-ATPase, CsPMA2, that plays a key role in lipid droplet formation, appresorial development and virulence in C. siamense. Deletion of CsPMA2 impaired C. siamense conidial size, conidial germination, appressorial development and virulence but did not affect hyphal growth. ΔCsPMA2 increased the sensitivity of C. siamense to phytic acid and oxalic acid. CsPMA2 was localized to lipids on the plasma membrane and intracellular membrane. Deletion of CsPMA2 significantly inhibited the accumulation of lipid droplets and significantly affected the contents of some species of lipids, including 12 species with decreased lipid contents and 3 species with increased lipid contents. Furthermore, low pH can inhibit CsPMA2 expression and lipid droplet accumulation. Overall, our data revealed that the plasma membrane H+-ATPase CsPMA2 is involved in the regulation of lipid droplet formation and affects appressorial development and virulence in C. siamense.

The plasma membrane H+-ATPase is critical for cell growth and pathogenicity in Penicillium digitatum

The plasma membrane H⁺-ATPase (PMA1) is a major cytosolic pH regulator and a potential candidate for antifungal drug discovery due to its fungal specificity and criticality. In this study, the function of Penicillum digitatum PMA1 was characterized through RNA interference (RNAi) and overexpression technology. The results showed that silencing the PMA1 gene reduces cell growth and pathogenicity, and increases susceptibility of P. digitatum to proton pump inhibitors (PPIs). Under scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examination, cell morphology was significantly altered in the PMA1- silenced mutant (si57). When compared with wild type (WT) and the overexpressed mutant (oe9), the cell walls of the si57 mutant were thicker and their cell membrane damage manifested particularly at sites of polarized growth. Consistent with the morphological change on the cell wall, chitin and glucan content of the cell wall of si57 were significantly lower and accompanied with increased activities of chitinase and glucanase. The lower ergosterol content in the si57 mutant then increased cell membrane permeability, ultimately leading to leakage of cytoplasmic contents such as ions, reduced sugars and soluble proteins. Furthermore, significantly decreased activity of cell wall degrading enzymes of si57 during citrus fruit infections indicates a reduced pathogenicity in this mutant. We conclude that PMA1 in P. digitatum plays an important role in maintaining pathogenesis and PMA1 could be a candidate novel fungicidal drug discovery for citrus green mold. KEY POINTS: Silencing PMA1 gene decreased the growth and pathogenicity of P. digitatum. Silencing PMA1 gene damaged cell wall and cell membrane integrity of P. digitatum. PMA1 appears to be a suitable fungicidal target against citrus green mold.

The plasma membrane H+ -ATPase FgPMA1 regulates the development, pathogenicity, and phenamacril sensitivity of Fusarium graminearum by interacting with FgMyo-5 and FgBmh2

Fusarium graminearum, as the causal agent of Fusarium head blight (FHB), not only causes yield loss, but also contaminates the quality of wheat by producing mycotoxins, such as deoxynivalenol (DON). The plasma membrane H⁺ -ATPases play important roles in many growth stages in plants and yeasts, but their functions and regulation in phytopathogenic fungi remain largely unknown. Here we characterized two plasma membrane H⁺ -ATPases: FgPMA1 and FgPMA2 in F. graminearum. The FgPMA1 deletion mutant (∆FgPMA1), but not FgPMA2 deletion mutant (∆FgPMA2), was impaired in vegetative growth, pathogenicity, and sexual and asexual development. FgPMA1 was localized to the plasma membrane, and ∆FgPMA1 displayed reduced integrity of plasma membrane. ∆FgPMA1 not only impaired the formation of the toxisome, which is a compartment where DON is produced, but also suppressed the expression level of DON biosynthetic enzymes, decreased DON production, and decreased the amount of mycelial invasion, leading to impaired pathogenicity by exclusively developing disease on inoculation sites of wheat ears and coleoptiles. ∆FgPMA1 exhibited decreased sensitivity to some osmotic stresses, a cell wall-damaging agent (Congo red), a cell membrane-damaging agent (sodium dodecyl sulphate), and heat shock stress. FgMyo-5 is the target of phenamacril used for controlling FHB. We found FgPMA1 interacted with FgMyo-5, and ∆FgPMA1 showed an increased expression level of FgMyo-5, resulting in increased sensitivity to phenamacril, but not to other fungicides. Furthermore, co-immunoprecipitation confirmed that FgPMA1, FgMyo-5, and FgBmh2 (a 14-3-3 protein) form a complex to regulate the sensitivity to phenamacril and biological functions. Collectively, this study identified a novel regulating mechanism of FgPMA1 in pathogenicity and phenamacril sensitivity of F. graminearum.

Arbuscular Mycorrhizal Fungi Alleviate Drought Stress in Trifoliate Orange by Regulating H+-ATPase Activity and Gene Expression

A feature of arbuscular mycorrhiza is enhanced drought tolerance of host plants, although it is unclear whether host H⁺-ATPase activity and gene expression are involved in the physiological process. The present study aimed to investigate the effects of an arbuscular mycorrhizal fungus (AMF), Funneliformis mosseae, on H⁺-ATPase activity, and gene expression of trifoliate orange (Poncirus trifoliata) seedlings subjected to well-watered (WW) and drought stress (DS), together with the changes in leaf gas exchange, root morphology, soil pH value, and ammonium content. Soil drought treatment dramatically increased H⁺-ATPase activity of leaf and root, and AMF inoculation further strengthened the increased effect. A plasma membrane (PM) H⁺-ATPase gene of trifoliate orange, PtAHA2 (MW239123), was cloned. The PtAHA2 expression was induced by mycorrhization in leaves and roots and also up-regulated by drought treatment in leaves of AMF-inoculated seedlings and in roots of AMF- and non-AMF-inoculated seedlings. And, the induced expression of PtAHA2 under mycorrhization was more prominent under DS than under WW. Mycorrhizal plants also showed greater photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate and better root volume and diameter than non-mycorrhizal plants under DS. AMF inoculation significantly increased leaf and root ammonium content, especially under DS, whereas it dramatically reduced soil pH value. In addition, H⁺-ATPase activity was significantly positively correlated with ammonium contents in leaves and roots, and root H⁺-ATPase activity was significantly negatively correlated with soil pH value. Our results concluded that AMF stimulated H⁺-ATPase activity and PtAHA2 gene expression in response to DS, which resulted in great nutrient (e.g., ammonium) uptake and root growth, as well as low soil pH microenvironment.

A mycorrhiza-specific H+ -ATPase is essential for arbuscule development and symbiotic phosphate and nitrogen uptake

Most land plants can form symbiosis with arbuscular mycorrhizal (AM) fungi to enhance uptake of mineral nutrients, particularly phosphate (Pi) and nitrogen (N), from the soil. It is established that transport of Pi from interfacial apoplast into plant cells depends on the H⁺ gradient generated by the H⁺ -ATPase located on the periarbuscular membrane (PAM); however, little evidence regarding the potential link between mycorrhizal N transport and H⁺ -ATPase activity is available to date. Here, we report that a PAM-localized tomato H⁺ -ATPase, SlHA8, is indispensable for arbuscule development and mycorrhizal P and N uptake. Knockout of SlHA8 resulted in truncated arbuscule morphology, reduced shoot P and N accumulation, and decreased H⁺ -ATPase activity and acidification of apoplastic spaces in arbusculated cells. Overexpression of SlHA8 in tomato promoted both P and N uptake, and increased total colonization level, but did not affect arbuscule morphology. Heterogeneous expression of SlHA8 in the rice osha1 mutant could fully complement its defects in arbuscule development and mycorrhizal P and N uptake. Our results propose a pivotal role of the SlHA8 in energizing both the symbiotic P and N transport, and highlight the evolutionary conservation of the AM-specific H⁺ -ATPase orthologs in maintaining AM symbiosis across different mycorrhizal plant species.

Review: Arbuscular mycorrhizas as key players in sustainable plant phosphorus acquisition: An overview on the mechanisms involved

Phosphorus (P) is a poorly available macronutrient essential for plant growth and development and consequently for successful crop yield and ecosystem productivity. To cope with P limitations plants have evolved strategies for enhancing P uptake and/or improving P efficiency use. The universal 450-million-yr-old arbuscular mycorrhizal (AM) (fungus-root) symbioses are one of the most successful and widespread strategies to maximize access of plants to available P. AM fungi biotrophically colonize the root cortex of most plant species and develop an extraradical mycelium which overgrows the nutrient depletion zone of the soil surrounding plant roots. This hyphal network is specialized in the acquisition of low mobility nutrients from soil, particularly P. During the last years, molecular biology techniques coupled to novel physiological approaches have provided fascinating contributions to our understanding of the mechanisms of symbiotic P transport. Mycorrhiza-specific plant phosphate transporters, which are required not only for symbiotic P transfer but also for maintenance of the symbiosis, have been identified. The present review provides an overview of the contribution of AM fungi to plant P acquisition and an update of recent findings on the physiological, molecular and regulatory mechanisms of P transport in the AM symbiosis.

Colletotrichum higginsianum as a Model for Understanding Host⁻Pathogen Interactions: A Review

Colletotrichum higginsianum is a hemibiotrophic ascomycetous fungus that causes economically important anthracnose diseases on numerous monocot and dicot crops worldwide. As a model pathosystem, the Colletotrichum⁻Arabidopsis interaction has the significant advantage that both organisms can be manipulated genetically. The goal of this review is to provide an overview of the system and to point out recent significant studies that update our understanding of the pathogenesis of C. higginsianum and resistance mechanisms of Arabidopsis against this hemibiotrophic fungus. The genome sequence of C. higginsianum has provided insights into how genome structure and pathogen genetic variability has been shaped by transposable elements, and allows systematic approaches to longstanding areas of investigation, including infection structure differentiation and fungal⁻plant interactions. The Arabidopsis-Colletotrichum pathosystem provides an integrated system, with extensive information on the host plant and availability of genomes for both partners, to illustrate many of the important concepts governing fungal⁻plant interactions, and to serve as an excellent starting point for broad perspectives into issues in plant pathology.

Comparative transcriptome profiling of Blumeria graminis f. sp. tritici during compatible and incompatible interactions with sister wheat lines carrying and lacking Pm40

Blumeria graminis f. sp. tritici (Bgt) is an obligate biotrophic fungus that causes wheat powdery mildew, which is a devastating disease in wheat. However, little is known about the pathogenesis of this fungus, and differences in the pathogenesis of the same pathogen at various resistance levels in hosts have not been determined. In the present study, leaf tissues of both Pm40-expressing hexaploid wheat line L658 and its Pm40-deficient sister line L958 were harvested at 0 (without inoculation), 6, 12, 24, 48 and 72 hours post-inoculation (hpi) with Bgt race 15 and then subjected to RNA sequencing (RNA-seq). In addition, we also observed changes in fungal growth morphology at the aforementioned time points. There was a high correlation between percentage of reads mapped to the Bgt reference genome and biomass of the fungus within the leaf tissue during the growth process. The percentage of mapped reads of Bgt in compatible interactions was significantly higher (at the p<0.05 level) than that of reads in incompatible interactions from 24 to 72 hpi. Further functional annotations indicated that expression levels of genes encoding H+-transporting ATPase, putative secreted effector proteins (PSEPs) and heat shock proteins (HSPs) were significantly up-regulated in compatible interactions compared with these levels in incompatible interactions, particularly at 72 hpi. Moreover, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis suggested that genes involved in the endocytosis pathway were also enriched in compatible interactions. Overall, genes encoding H+-transporting ATPase, PSEPs and HSPs possibly played crucial roles in successfully establishing the pathogenesis of compatible interactions during late stages of inoculation. The study results also indicated that endocytosis is likely to play a potential role in Bgt in establishing compatible interactions.

Functional Characterization of a Hexose Transporter from Root Endophyte Piriformospora indica

Understanding the mechanism of photosynthate transfer at symbiotic interface by fungal monosaccharide transporter is of substantial importance. The carbohydrate uptake at the apoplast by the fungus is facilitated by PiHXT5 hexose transporter in root endophytic fungus Piriformospora indica. The putative PiHXT5 belongs to MFS superfamily with 12 predicted transmembrane helices. It possess sugar transporter PFAM motif (PF0083) and MFS superfamily domain (PS50850). It contains the signature tags related to glucose transporter GLUT1 of human erythrocyte. PiHXT5 is regulated in response to mutualism as well as glucose concentration. We have functionally characterized PiHXT5 by complementation of hxt-null mutant of Saccharomyces cerevisiae EBY.VW4000. It is involved in transport of multiple sugars ranging from D-glucose, D-fructose, D-xylose, D-mannose, D-galactose with decreasing affinity. The uncoupling experiments indicate that it functions as H(+)/glucose co-transporter. Further, pH dependence analysis suggests that it functions maximum between pH 5 and 6. The expression of PiHXT5 is dependent on glucose concentration and was found to be expressed at low glucose levels (1 mM) which indicate its role as a high affinity glucose transporter. Our study on this sugar transporter will help in better understanding of carbon metabolism and flow in this agro-friendly fungus.

A Genetic Screen for Pathogenicity Genes in the Hemibiotrophic Fungus Colletotrichum higginsianum Identifies the Plasma Membrane Proton Pump Pma2 Required for Host Penetration

We used insertional mutagenesis by Agrobacterium tumefaciens mediated transformation (ATMT) to isolate pathogenicity mutants of Colletotrichum higginsianum. From a collection of 7200 insertion mutants we isolated 75 mutants with reduced symptoms. 19 of these were affected in host penetration, while 17 were affected in later stages of infection, like switching to necrotrophic growth. For 16 mutants the location of T-DNA insertions could be identified by PCR. A potential plasma membrane H⁺-ATPase Pma2 was targeted in five independent insertion mutants. We genetically inactivated the Ku80 component of the non-homologous end-joining pathway in C. higginsianum to establish an efficient gene knockout protocol. Chpma2 deletion mutants generated by homologous recombination in the ΔChku80 background form fully melanized appressoria but entirely fail to penetrate the host tissue and are non-pathogenic. The ChPMA2 gene is induced upon appressoria formation and infection of A. thaliana. Pma2 activity is not important for vegetative growth of saprophytically growing mycelium, since the mutant shows no growth penalty under these conditions. Colletotrichum higginsianum codes for a closely related gene (ChPMA1), which is highly expressed under most growth conditions. ChPMA1 is more similar to the homologous yeast genes for plasma membrane pumps. We propose that expression of a specific proton pump early during infection may be common to many appressoria forming fungal pathogens as we found ChPMA2 orthologs in several plant pathogenic fungi.

ESTs analysis reveals putative genes involved in symbiotic seed germination in Dendrobium officinale

Dendrobiumofficinale (Orchidaceae) is one of the world's most endangered plants with great medicinal value. In nature, D. officinale seeds must establish symbiotic relationships with fungi to germinate. However, the molecular events involved in the interaction between fungus and plant during this process are poorly understood. To isolate the genes involved in symbiotic germination, a suppression subtractive hybridization (SSH) cDNA library of symbiotically germinated D. officinale seeds was constructed. From this library, 1437 expressed sequence tags (ESTs) were clustered to 1074 Unigenes (including 902 singletons and 172 contigs), which were searched against the NCBI non-redundant (NR) protein database (E-value cutoff, e(-5)). Based on sequence similarity with known proteins, 579 differentially expressed genes in D. officinale were identified and classified into different functional categories by Gene Ontology (GO), Clusters of orthologous Groups of proteins (COGs) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. The expression levels of 15 selected genes emblematic of symbiotic germination were confirmed via real-time quantitative PCR. These genes were classified into various categories, including defense and stress response, metabolism, transcriptional regulation, transport process and signal transduction pathways. All transcripts were upregulated in the symbiotically germinated seeds (SGS). The functions of these genes in symbiotic germination were predicted. Furthermore, two fungus-induced calcium-dependent protein kinases (CDPKs), which were upregulated 6.76- and 26.69-fold in SGS compared with un-germinated seeds (UGS), were cloned from D. officinale and characterized for the first time. This study provides the first global overview of genes putatively involved in D. officinale symbiotic seed germination and provides a foundation for further functional research regarding symbiotic relationships in orchids.

How membranes shape plant symbioses: signaling and transport in nodulation and arbuscular mycorrhiza

As sessile organisms that cannot evade adverse environmental conditions, plants have evolved various adaptive strategies to cope with environmental stresses. One of the most successful adaptations is the formation of symbiotic associations with beneficial microbes. In these mutualistic interactions the partners exchange essential nutrients and improve their resistance to biotic and abiotic stresses. In arbuscular mycorrhiza (AM) and in root nodule symbiosis (RNS), AM fungi and rhizobia, respectively, penetrate roots and accommodate within the cells of the plant host. In these endosymbiotic associations, both partners keep their plasma membranes intact and use them to control the bidirectional exchange of signaling molecules and nutrients. Intracellular accommodation requires the exchange of symbiotic signals and the reprogramming of both interacting partners. This involves fundamental changes at the level of gene expression and of the cytoskeleton, as well as of organelles such as plastids, endoplasmic reticulum (ER), and the central vacuole. Symbiotic cells are highly compartmentalized and have a complex membrane system specialized for the diverse functions in molecular communication and nutrient exchange. Here, we discuss the roles of the different cellular membrane systems and their symbiosis-related proteins in AM and RNS, and we review recent progress in the analysis of membrane proteins involved in endosymbiosis.

Kinetics of NH (4) (+) uptake by the arbuscular mycorrhizal fungus Rhizophagus irregularis

The kinetics and energetics of (15)NH (4) (+) uptake by the extraradical mycelium of the arbuscular mycorrhizal fungus Rhizophagus irregularis were investigated. (15)NH (4) (+) uptake increased with increasing substrate concentration over the concentration range of 0.002 to 25 mM. Eadie-Hofstee plots showed that ammonium (NH (4) (+) ) uptake over this range was biphasic. At concentrations below 100 μM, NH (4) (+) uptake fits a Michaelis-Menten curve, typical of the activity of a saturable high-affinity transport system (HATS). At concentrations above 1 mM, NH (4) (+) influx showed a linear response typical of a nonsaturable low-affinity transport system (LATS). Both transport systems were dependent on external pH. The HATS and, to a lesser extent, the LATS were inhibited by the ionophore carbonylcyanide m-chlorophenylhydrazone (CCCP) and the ATP-synthesis inhibitor 2,4-dinitrophenol. These data indicate that the two NH (4) (+) transport systems of R. irregularis are dependent on metabolic energy and on the electrochemical H(+) gradient. The HATS- and the LATS-mediated (15)NH (4) (+) influxes were also regulated by acetate. This first report of the existence of active high- and low-affinity NH4(+) transport systems in the extraradical mycelium of an arbuscular mycorrhizal fungus and provides novel information on the mechanisms underlying mycosymbiont uptake of nitrogen from the soil environment.

Laser microdissection unravels cell-type-specific transcription in arbuscular mycorrhizal roots, including CAAT-box transcription factor gene expression correlating with fungal contact and spread

Arbuscular mycorrhizae (AM) are the most widespread symbioses on Earth, promoting nutrient supply of most terrestrial plant species. To unravel gene expression in defined stages of Medicago truncatula root colonization by AM fungi, we here combined genome-wide transcriptome profiling based on whole mycorrhizal roots with real-time reverse transcription-PCR experiments that relied on characteristic cell types obtained via laser microdissection. Our genome-wide approach delivered a core set of 512 genes significantly activated by the two mycorrhizal fungi Glomus intraradices and Glomus mossae. Focusing on 62 of these genes being related to membrane transport, signaling, and transcriptional regulation, we distinguished whether they are activated in arbuscule-containing or the neighboring cortical cells harboring fungal hyphae. In addition, cortical cells from nonmycorrhizal roots served as a reference for gene expression under noncolonized conditions. Our analysis identified 25 novel arbuscule-specific genes and 37 genes expressed both in the arbuscule-containing and the adjacent cortical cells colonized by fungal hyphae. Among the AM-induced genes specifying transcriptional regulators were two members encoding CAAT-box binding transcription factors (CBFs), designated MtCbf1 and MtCbf2. Promoter analyses demonstrated that both genes were already activated by the first physical contact between the symbionts. Subsequently, and corresponding to our cell-type expression patterns, they were progressively up-regulated in those cortical areas colonized by fungal hyphae, including the arbuscule-containing cells. The encoded CBFs thus represent excellent candidates for regulators that mediate a sequential reprogramming of root tissues during the establishment of an AM symbiosis.

A versatile monosaccharide transporter that operates in the arbuscular mycorrhizal fungus Glomus sp is crucial for the symbiotic relationship with plants

For more than 400 million years, plants have maintained a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi. This evolutionary success can be traced to the role of these fungi in providing plants with mineral nutrients, particularly phosphate. In return, photosynthates are given to the fungus, which support its obligate biotrophic lifestyle. Although the mechanisms involved in phosphate transfer have been extensively studied, less is known about the reciprocal transfer of carbon. Here, we present the high-affinity Monosaccharide Transporter2 (MST2) from Glomus sp with a broad substrate spectrum that functions at several symbiotic root locations. Plant cell wall sugars can efficiently outcompete the Glc uptake capacity of MST2, suggesting they can serve as alternative carbon sources. MST2 expression closely correlates with that of the mycorrhiza-specific Phosphate Transporter4 (PT4). Furthermore, reduction of MST2 expression using host-induced gene silencing resulted in impaired mycorrhiza formation, malformed arbuscules, and reduced PT4 expression. These findings highlight the symbiotic role of MST2 and support the hypothesis that the exchange of carbon for phosphate is tightly linked. Unexpectedly, we found that the external mycelium of AM fungi is able to take up sugars in a proton-dependent manner. These results imply that the sugar uptake system operating in this symbiosis is more complex than previously anticipated.

The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea involves early and systemic signalling events

Most plants form root symbioses with arbuscular mycorrhizal (AM) fungi, which provide them with phosphate and other nutrients. High soil phosphate levels are known to affect AM symbiosis negatively, but the underlying mechanisms are not understood. This report describes experimental conditions which triggered a novel mycorrhizal phenotype under high phosphate supply: the interaction between pea and two different AM fungi was almost completely abolished at a very early stage, prior to the formation of hyphopodia. As demonstrated by split-root experiments, down-regulation of AM symbiosis occurred at least partly in response to plant-derived signals. Early signalling events were examined with a focus on strigolactones, compounds which stimulate pre-symbiotic fungal growth and metabolism. Strigolactones were also recently identified as novel plant hormones contributing to the control of shoot branching. Root exudates of plants grown under high phosphate lost their ability to stimulate AM fungi and lacked strigolactones. In addition, a systemic down-regulation of strigolactone release by high phosphate supply was demonstrated using split-root systems. Nevertheless, supplementation with exogenous strigolactones failed to restore root colonization under high phosphate. This observation does not exclude a contribution of strigolactones to the regulation of AM symbiosis by phosphate, but indicates that they are not the only factor involved. Together, the results suggest the existence of additional early signals that may control the differentiation of hyphopodia.

Plugging into the network: belowground connections between germlings and extraradical mycelium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs; nevertheless their spores can germinate in the absence of host plants. Such inconsistent behavior is balanced by diverse survival strategies. The ability of AM fungal hyphae to fuse might represent a fundamental survival strategy because germlings could plug into compatible mycorrhizal networks, thus gaining access to plant-derived carbon before asymbiotic growth arrest. An in vivo experimental system was used to grow extraradical mycelium produced by Glomus mosseae colonizing three different plant species and germlings of the same isolate. After symbiotic and asymbiotic mycelia came into contact we showed that germling hyphae fused with symbiotic network hyphae and established protoplasm connections with nuclei occurring in fusion bridges. The frequency of anastomoses between germling and symbiotic hyphae was 4.9-23.9%. Prefusion and postfusion incompatible responses, with protoplasm withdrawal in interacting hyphae, were evident in some hyphal contacts. Given the multigenomic nature of AMF, the mingling of germling nuclei with those of the mycorrhizal network through perfect fusions might represent a means for the maintenance of genetic diversity in the absence of sexual recombination.

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.

Molecular trait indicators: moving beyond phylogeny in arbuscular mycorrhizal ecology

Arbuscular mycorrhizal (AM) fungi form symbiotic associations with the roots of most plants, thereby mediating nutrient and carbon fluxes, plant performance, and ecosystem dynamics. Although considerable effort has been expended to understand the keystone ecological position of AM symbioses, most studies have been limited in scope to recording organism occurrences and identities, as determined from morphological characters and (mainly) ribosomal sequence markers. In order to overcome these restrictions and circumvent the shortcomings of culture- and phylogeny-based approaches, we propose a shift toward plant and fungal protein-encoding genes as more immediate indicators of mycorrhizal contributions to ecological processes. A number of candidate target genes, involved in the uptake of phosphorus and nitrogen, carbon cycling, and overall metabolic activity, are proposed. We discuss the advantages and disadvantages of future protein-encoding gene marker and current (phylo-) taxonomic approaches for studying the impact of AM fungi on plant growth and ecosystem functioning. Approaches based on protein-encoding genes are expected to open opportunities to advance the mechanistic understanding of ecological roles of mycorrhizas in natural and managed ecosystems.

Isolation and sequence analysis of a beta-tubulin gene from arbuscular mycorrhizal fungi

A full-length beta-tubulin gene has been cloned and sequenced from Gigaspora gigantea and Glomus clarum, two arbuscular mycorrhizal fungi (AMF) species in the phylum Glomeromyota. The gene in both species is organized into five exons and four introns. Both genes are 94.9% similar and encode a 447 amino acid protein. In comparison with other fungal groups, the amino acid sequence is most similar to that of fungi in the Chytridiomycota. The codon usage of the gene in both AMF species is broad and biased in favor of an A or a T in the third position. The four introns varied in length from 87 to 168 bp for G. gigantea and from 90 to 136 bp for G. clarum. Of all fungi in which full-length sequences have been published, only AMF do not have an intron before codon 174. The introns positioned at codons 174 and 257 in AMF match the position of different introns in beta-tubulin genes of some Zygomycete, Basidiomycete, and Ascomycete fungi. The 5' and 3' splice site consensus sequences are similar to those found in introns of most fungi. Sequence analysis from single-strand conformation polymorphism analysis confirmed the presence of two beta-tubulin gene copies in G. clarum, but only one copy was evident in G. gigantea based on Southern hybridization analysis.

A mycorrhizal-specific ammonium transporter from Lotus japonicus acquires nitrogen released by arbuscular mycorrhizal fungi

In mycorrhizal associations, the fungal partner assists its plant host by providing nitrogen (N) in addition to phosphate. Arbuscular mycorrhizal (AM) fungi have access to inorganic or organic forms of N and translocate them via arginine from the extra- to the intraradical mycelium, where the N is transferred to the plant without any carbon skeleton. However, the molecular form in which N is transferred, as well as the involved mechanisms, is still under debate. NH(4)(+) seems to be the preferential transferred molecule, but no plant ammonium transporter (AMT) has been identified so far. Here, we offer evidence of a plant AMT that is involved in N uptake during mycorrhiza symbiosis. The gene LjAMT2;2, which has been shown to be the highest up-regulated gene in a transcriptomic analysis of Lotus japonicus roots upon colonization with Gigaspora margarita, has been characterized as a high-affinity AMT belonging to the AMT2 subfamily. It is exclusively expressed in the mycorrhizal roots, but not in the nodules, and transcripts have preferentially been located in the arbusculated cells. Yeast (Saccharomyces cerevisiae) mutant complementation has confirmed its functionality and revealed its dependency on acidic pH. The transport experiments using Xenopus laevis oocytes indicated that, unlike other plant AMTs, LjAMT2;2 transports NH(3) instead of NH(4)(+). Our results suggest that the transporter binds charged ammonium in the apoplastic interfacial compartment and releases the uncharged NH(3) into the plant cytoplasm. The implications of such a finding are discussed in the context of AM functioning and plant phosphorus uptake.

Symbiosis-related plant genes modulate molecular responses in an arbuscular mycorrhizal fungus during early root interactions

To gain further insight into the role of the plant genome in arbuscular mycorrhiza (AM) establishment, we investigated whether symbiosis-related plant genes affect fungal gene expression in germinating spores and at the appressoria stage of root interactions. Glomus intraradices genes were identified in expressed sequence tag libraries of mycorrhizal Medicago truncatula roots by in silico expression analyses. Transcripts of a subset of genes, with predicted functions in transcription, protein synthesis, primary or secondary metabolism, or of unknown function, were monitored in spores and germinating spores and during interactions with roots of wild-type or mycorrhiza-defective (Myc-) mutants of M. truncatula. Not all the fungal genes were active in quiescent spores but all were expressed when G. intraradices spores germinated in wild-type M. truncatula root exudates or when appressoria or arbuscules were formed in association with wild-type M. truncatula roots. Most of the fungal genes were upregulated or induced at the stage of appressorium development. Inactivation of the M. truncatula genes DMI1, DMI2/MtSYM2, or DMI3/MtSYM13 was associated with altered fungal gene expression (nonactivation or inhibition), modified appressorium structure, and plant cell wall responses, providing first evidence that cell processes modified by symbiosis-related plant genes impact on root interactions by directly modulating AM fungal activity.

Proton pump OsA8 is linked to phosphorus uptake and translocation in rice

The plasma membrane (PM) proton pump ATPases (H(+)-ATPases) are involved in almost all aspects of biology. They are plant specific and several members of this family are supposed to play a key role in nutrient acquisition. At present, only some members of this gene family in plants have been characterized. However, no nutrient uptake associated H(+)-ATPase gene in rice has been functionally analysed. It is reported here that OsA8, a typical PM H(+)-ATPases gene that was predominantly expressed in roots of rice, is down-regulated by nutrient deficiency. The Osa8 mutant had a relatively smaller size and root to shoot biomass ratio, but higher ATPase activity than its wild-type counterparts under phosphorus (P) starvation conditions. Knockout of OsA8 affected the expression of several OsA genes and the high affinity phosphate transporter, OsPT6, and resulted in a higher P concentration in the roots and a lower amount of P in the shoots. These analyses demonstrate that OsA8 not only influences the uptake of P by roots, but also the translocation of P from the roots to the shoots in rice.

Unraveling the network: Novel developments in the understanding of signaling and nutrient exchange mechanisms in the arbuscular mycorrhizal symbiosis

The arbuscular mycorrhhiza (AM) symbiosis involves an intricate network of signaling and biochemical pathways designed to ensure that a beneficial relationship is established between the plant and fungal partners as a result of a mutual nutrient exchange. Emerging data has been recently published to explain why the relationship is not always fair, as observed in prevalent parasitic AM relationships in which the plant host receives no phosphorus (P) in exchange for carbon (C) delivered to the fungus. The theory behind this unorthodox view of the AM relationship, together with the description of other recent developments in nutrient mobilization as well as in key aspects of the bi-directional signaling that culminates in the symbiotic association, is the subject of this review.

pH signature for the responses of arbuscular mycorrhizal fungi to external stimuli

Environmental and developmental signals can elicit differential activation of membrane proton (H(+)) fluxes as one of the primary responses of plant and fungal cells. In recent work,1 we could determine that during the presymbiotic growth of arbuscular mycorrhizal (AM) fungi specific domains of H(+) flux are activated by clover root factors, namely host root exudates or whole root system. Consequently, activation on hyphal growth and branching were observed and the role of plasma membrane H(+)-ATPase was investigated. The specific inhibitors differentially abolished most of hyphal H(+) effluxes and fungal growth. As this enzyme can act in signal transduction pathways, we believe that spatial and temporal oscillations of the hyphal H(+) fluxes could represent a pH signature for both early events of the AM symbiosis and fungal ontogeny.

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.

Proton (H+) flux signature for the presymbiotic development of the arbuscular mycorrhizal fungi

Ion dynamics are important for cell nutrition and growth in fungi and plants. Here, the focus is on the relationship between the hyphal H(+) fluxes and the control of presymbiotic growth and host recognition by arbuscular mycorrhizal (AM) fungi. Fluxes of H(+) around azygopores and along lateral hyphae of Gigaspora margarita during presymbiotic growth, and their regulation by phosphate (P) and sucrose (Suc), were analyzed with an H(+)-specific vibrating probe. Changes in hyphal H(+) fluxes were followed after induction by root exudates (RE) or by the presence Trifolium repens roots. Differential sensitivity to P-type ATPase inhibitors (orthovanadate or erythrosin B) suggests an asymmetric distribution or activation of H(+)-pump isoforms along the hyphae of the AM fungi. Concentration of P and Suc affected the hyphal H(+) fluxes and growth rate. However, further increases in H+ efflux and growth rate were observed when the fungus was growing close to clover roots or pretreated with RE. The H(+) flux data correlate with those from polarized hyphal growth analyses, suggesting that spatial and temporal alterations of the hyphal H(+)fluxes are regulated by nutrient availability and might underlie a pH signaling elicitation by host RE during the early events of the AM symbiosis.

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.

Structural characterization and molecular identification of arbuscular mycorrhiza morphotypes of Alzatea verticillata (Alzateaceae), a prominent tree in the tropical mountain rain forest of South Ecuador

The vast majority of the highly diverse trees in the tropical mountain rain forest of South Ecuador form arbuscular mycorrhizas, and previous molecular investigations revealed a high diversity of fungi. In this study, we present a first trial to link fungal DNA-sequences with defined morphotypes characterized on the basis of partly new mycelial features obtained from field material of one tree species, Alzatea verticillata. Fine roots were halved lengthwise to study the mycelium anatomy on one half and to obtain fungal nuclear rDNA coding for the small subunit rRNA of Glomeromycota from the other half. Light microscopy revealed conspicuously large amounts of mycelium attaching to the surface of the rootlets. The mycelium formed fine- or large-branched appressoria-like plates, vesicles of regular or irregular shape, and very fine, multibranched structures ensheathed by septate hyphae. These previously undescribed features of the supraradical mycelia combined with intraradical mycelium structures were used for distinguishing of four main morphogroups and subordinate 14 morphotypes. DNA sequences of Glomus group A, Acaulospora and Gigaspora, were obtained and linked to three morphogroups. Two sequence types within Glomus group A could be tentatively associated to subordinate morphotypes.

Laser microdissection reveals that transcripts for five plant and one fungal phosphate transporter genes are contemporaneously present in arbusculated cells

The establishment of a symbiotic interaction between plant roots and arbuscular mycorrhizal (AM) fungi requires both partners to undergo significant morphological and physiological modifications which eventually lead to reciprocal beneficial effects. Extensive changes in gene expression profiles recently have been described in transcriptomic studies that have analyzed the whole mycorrhizal root. However, because root colonization by AM fungi involves different cell types, a cell-specific gene expression pattern is likely to occur. We have applied the laser microdissection (LMD) technology to investigate expression profiles of both plant and fungal genes in Lycopersicon esculentum roots colonized by Glomus mosseae. A protocol to harvest arbuscule-containing cells from paraffin sections of mycorrhizal roots has been developed using a Leica AS LMD system. RNA of satisfactory quantity and quality has been extracted for molecular analysis. Transcripts for plant phosphate transporters (LePTs), selected as molecular markers for a functional symbiosis, have been detected by reverse-transcriptase polymerase chain reaction assays and associated to distinct cell types, leading to novel insights into the distribution of LePT mRNAs. In fact, the transcripts of the five phosphate transporters (PTs) have been detected contemporaneously in the same arbusculated cell population, unlike from the neighboring noncolonized cells. In addition, fungal H(+)ATPase (GmHA5) and phosphate transporter (GmosPT) mRNAs were found exclusively in arbusculated cells. The discovery that five plant and one fungal PT genes are consistently expressed inside the arbusculated cells provides a new scenario for plant-fungus nutrient exchanges.

Early legume responses to inoculation with Rhizobium sp. NGR234

Interactions between legumes and rhizobia are controlled by the sequential exchange of symbiotic signals. Two different techniques, 2D-PAGE electrophoresis and differential display were used to study the effects of rhizobial signals on legume development. Application of variously substituted lipo-oligo-saccharidic Nod-factors to roots of Vigna unguiculata resulted in changes in the phosphorylation patterns of microsomal proteins. Reliable amino-acid sequences were obtained for one Nod-factor enhanced protein which was highly homologous to the 57-kDa subunit from Arabidopsis thaliana vacuolar membrane H(+)-ATPase. Immuno-blotting techniques demonstrated that Nod-factors cause rapid and massive increases of this enzyme in treated roots, suggesting that H(+)-ATPases play symbiotic roles. Concomitantly, we used differential display (DD) techniques on mRNA isolated from root-hairs to analyse early root responses to NGR234. Significant matches of several DD clones to known sequences were found. Clone D2.62 was homologous to a multitude of receptor kinases including S receptor-like kinases of A. thaliana and clone D4.1 showed similarities to Lotus japonicus phosphatidylinositol transfer-like protein III and late nodulin 16. Independent confirmatory analyses of these differentially expressed clones indicated expression at very low levels.

Fungal and plant gene expression in arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizas (AMs) are a unique example of symbiosis between two eukaryotes, soil fungi and plants. This association induces important physiological changes in each partner that lead to reciprocal benefits, mainly in nutrient supply. The symbiosis results from modifications in plant and fungal cell organization caused by specific changes in gene expression. Recently, much effort has gone into studying these gene expression patterns to identify a wider spectrum of genes involved. We aim in this review to describe AM symbiosis in terms of current knowledge on plant and fungal gene expression profiles.

Mutations in DMI3 and SUNN modify the appressorium-responsive root proteome in arbuscular mycorrhiza

Modification of the Medicago truncatula root proteome during the early stage of arbuscular mycorrhizal symbiosis was investigated by comparing, using two-dimensional electrophoresis, the protein patterns obtained from non-inoculated roots and roots synchronized for Glomus intraradices appressorium formation. This approach was conducted in wild-type (J5), mycorrhiza-defective (TRV25, dmi3), and autoregulation-defective (TR122, sunn) M. truncatula genotypes. The groups of proteins that responded to appressorium formation were further compared between wild-type and mutant genotypes; few overlaps and major differences were recorded, demonstrating that mutations in DMI3 and SUNN modified the appressorium-responsive root proteome. Except for a chalcone reductase, none of the differentially displayed proteins that could be identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry previously was known as appressorium responsive. A DMI3-dependent increased accumulation of signal transduction-related proteins (dehydroascorbate reductase, cyclophilin, and actin depolymerization factor) was found to precede mycorrhiza establishment. Differences in the accumulation of proteins related to plant defense reactions, cytoskeleton rearrangements, and auxin signaling upon symbiont contact were recorded between wild-type and hypermycorrhizal genotypes, pointing to some putative pathways by which SUNN may regulate very early arbuscule formation.

Transcriptional snapshots provide insights into the molecular basis of arbuscular mycorrhiza in the model legume Medicago truncatula

The arbuscular mycorrhizal (AM) association between terrestrial plants and soil fungi of the phylum Glomeromycota is the most widespread beneficial plant-microbe interaction on earth. In the course of the symbiosis, fungal hyphae colonise plant roots and supply limiting nutrients, in particular phosphorus, in exchange for carbon compounds. Owing to the obligate biotrophy of mycorrhizal fungi and the lack of genetic systems to study them, targeted molecular studies on AM symbioses proved to be difficult. With the emergence of plant genomics and the selection of suitable models, an application of untargeted expression profiling experiments became possible. In the model legume Medicago truncatula, high-throughput expressed sequence tag (EST)-sequencing in conjunction with in silico and experimental transcriptome profiling provided transcriptional snapshots that together defined the global genetic program activated during AM. Owing to an asynchronous development of the symbiosis, several hundred genes found to be activated during the symbiosis cannot be easily correlated with symbiotic structures, but the expression of selected genes has been extended to the cellular level to correlate gene expression with specific stages of AM development. These approaches identified marker genes for the AM symbiosis and provided the first insights into the molecular basis of gene expression regulation during AM.

Evolution of the P-type II ATPase gene family in the fungi and presence of structural genomic changes among isolates of Glomus intraradices

BACKGROUND

The P-type II ATPase gene family encodes proteins with an important role in adaptation of the cell to variation in external K+, Ca2+ and Na2+ concentrations. The presence of P-type II gene subfamilies that are specific for certain kingdoms has been reported but was sometimes contradicted by discovery of previously unknown homologous sequences in newly sequenced genomes. Members of this gene family have been sampled in all of the fungal phyla except the arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota), which are known to play a key-role in terrestrial ecosystems and to be genetically highly variable within populations. Here we used highly degenerate primers on AMF genomic DNA to increase the sampling of fungal P-Type II ATPases and to test previous predictions about their evolution. In parallel, homologous sequences of the P-type II ATPases have been used to determine the nature and amount of polymorphism that is present at these loci among isolates of Glomus intraradices harvested from the same field.

RESULTS

In this study, four P-type II ATPase sub-families have been isolated from three AMF species. We show that, contrary to previous predictions, P-type IIC ATPases are present in all basal fungal taxa. Additionally, P-Type IIE ATPases should no longer be considered as exclusive to the Ascomycota and the Basidiomycota, since we also demonstrate their presence in the Zygomycota. Finally, a comparison of homologous sequences encoding P-type IID ATPases showed unexpectedly that indel mutations among coding regions, as well as specific gene duplications occur among AMF individuals within the same field.

CONCLUSION

On the basis of these results we suggest that the diversification of P-Type IIC and E ATPases followed the diversification of the extant fungal phyla with independent events of gene gains and losses. Consistent with recent findings on the human genome, but at a much smaller geographic scale, we provided evidence that structural genomic changes, such as exonic indel mutations and gene duplications are less rare than previously thought and that these also occur within fungal populations.

Isolation, cloning and expression analysis of EcPMA1, a putative plasma membrane H+ -ATPase transporter gene from the biotrophic pathogenic fungus Erysiphe cichoracearum

Little is known at the molecular level about the transporters involved in nutrient transfer in the plant/powdery mildew interaction. A PCR-based approach was used to identify and isolate a partial-length cDNA coding for an isoform of the plasma membrane H+ -ATPase (EcPMA1) in the biotrophic pathogenic fungus Erysiphe cichoracearum. Southern analysis suggests that EcPMA1 exists as a single-copy gene. Sequence analysis indicated a high similarity of EcPMA1 to other fungal H+ -ATPases. Expression of EcPMA1 increases in infected Arabidopsis leaves as the disease progresses, correlating with the growth of the pathogen.

Identification of membrane-associated proteins regulated by the arbuscular mycorrhizal symbiosis

A sub-cellular proteomic approach was carried out to monitor membrane-associated protein modifications in response to the arbuscular mycorrhizal (AM) symbiosis. Membrane proteins were extracted from Medicago truncatula roots either inoculated or not with the AM fungus Glomus intraradices. Comparative two-dimensional electrophoresis revealed that 36 spots were differentially displayed in response to the fungal colonization including 15 proteins induced, 3 up-regulated and 18 down-regulated. Among them, seven proteins were found to be commonly down-regulated in AM-colonized and phosphate-fertilized roots. Twenty-five spots out of the 36 of interest could be identified by matrix assisted laser desorption/ionisation-time of flight and/or tandem mass spectrometry analyses. Excepting an acid phosphatase and a lectin, none of them was previously reported as being regulated during AM symbiosis. In addition, this proteomic approach allowed us for the first time to identify AM fungal proteins in planta.

Expression profiles of a phosphate transporter gene (GmosPT) from the endomycorrhizal fungus Glomus mosseae

Arbuscular mycorrhizal (AM) fungi have long been shown to successfully contribute to phosphate uptake by plant roots. The first step of the fungus-mediated uptake is carried out by fungal membrane Pi transporters (PT) that transfer Pi from the soil into the extraradical hyphae. In the present work we report the identification and characterisation of a PT gene from Glomus mosseae, an AM fungus important for natural and agricultural ecosystems. Degenerate primers and rapid amplification of cDNA ends-polymerase chain reaction (PCR) allowed us to obtain a sequence (GmosPT) showing a highly significant similarity with GiPT and GvPT, the only two other PT genes already isolated from AM fungi. Reverse transcriptase-PCR experiments were carried out to study GmosPT expression profiles in structures corresponding to different fungal life stages (quiescent and germinated sporocarps, intraradical and extraradical hyphae) and in extra- and intraradical hyphae exposed to high and low Pi concentrations. GmosPT showed an expression pattern similar to GiPT, the Glomus intraradices PT gene, since its transcript was more abundant in the extraradical mycelium treated with micromolar Pi levels. In addition, the intraradical mycelium also showed a significant GmosPT expression level that was independent from external Pi concentrations. This finding opens new questions about the role and functioning of high-affinity PT in AM fungi.

Molecular and cell biology of arbuscular mycorrhizal symbiosis

The roots of most extant plants are able to become engaged in an interaction with a small group of fungi of the fungal order Glomales (Glomeromycota). This interaction-arbuscular mycorrhizal (AM) symbiosis-is the evolutionary precursor of most other mutualistic root-microbe associations. The molecular analysis of this interaction can elucidate basic principles regarding such associations. This review summarizes our present knowledge about cellular and molecular aspects of AM. Emphasis is placed on morphological changes in colonized cells, transfer of nutrients between both interacting partners, and plant defence responses. Similarities to and differences from other associations of plant and microorganisms are highlighted regarding defence reactions and signal perception.

Citrate secretion coupled with the modulation of soybean root tip under aluminum stress. Up-regulation of transcription, translation, and threonine-oriented phosphorylation of plasma membrane H+-ATPase

The aluminum (Al)-induced secretion of citrate has been regarded as an important mechanism for Al resistance in soybean (Glycine max). However, the mechanism of how Al induces citrate secretion remains unclear. In this study, we investigated the regulatory role of plasma membrane H+-ATPase on the Al-induced secretion of citrate from soybean roots. Experiments performed with plants grown in full nutrient solution showed that Al-induced activity of plasma membrane H+-ATPase paralleled secretion of citrate. Vanadate and fusicoccin, an inhibitor and an activator, respectively, of plasma membrane H+-ATPase, exerted inhibitory and stimulatory effects on the Al-induced secretion of citrate. Higher activity of plasma membrane H+-ATPase coincided with more citrate secretion in Al-resistant than Al-sensitive soybean cultivars. These results suggested that the effects of Al stress on citrate secretion were mediated via modulation of the activity of plasma membrane H+-ATPase. The relationship between the Al-induced secretion of citrate and the activity of plasma membrane H+-ATPase was further demonstrated by analysis of plasma membrane H+-ATPase transgenic Arabidopsis (Arabidopsis thaliana). When plants were grown on Murashige and Skoog medium containing 30 microM Al (9.1 microM Al3+ activity), transgenic plants exuded more citrate compared with wild-type Arabidopsis. Results from real-time reverse transcription-PCR and immunodetection analysis indicated that the increase of plasma membrane H+-ATPase activity by Al is caused by transcriptional and translational regulation. Furthermore, plasma membrane H+-ATPase activity and expression were higher in an Al-resistant cultivar than in an Al-sensitive cultivar. Al activated the threonine-oriented phosphorylation of plasma membrane H+-ATPase in a dose- and time-dependent manner. Taken together, our results demonstrated that up-regulation of plasma membrane H+-ATPase activity was associated with the secretion of citrate from soybean roots.

Overlaps in the transcriptional profiles of Medicago truncatula roots inoculated with two different Glomus fungi provide insights into the genetic program activated during arbuscular mycorrhiza

Arbuscular mycorrhiza (AM) is a widespread symbiotic association between plants and fungal microsymbionts that supports plant development under nutrient-limiting and various stress conditions. In this study, we focused on the overlapping genetic program activated by two commonly studied microsymbionts in addition to identifying AM-related genes. We thus applied 16,086 probe microarrays to profile the transcriptome of the model legume Medicago truncatula during interactions with Glomus mosseae and Glomus intraradices and specified a total of 201 plant genes as significantly coinduced at least 2-fold, with more than 160 being reported as AM induced for the first time. Several hundred genes were additionally up-regulated during a sole interaction, indicating that the plant genetic program activated in AM to some extent depends on the colonizing microsymbiont. Genes induced during both interactions specified AM-related nitrate, ion, and sugar transporters, enzymes involved in secondary metabolism, proteases, and Kunitz-type protease inhibitors. Furthermore, coinduced genes encoded receptor kinases and other components of signal transduction pathways as well as AM-induced transcriptional regulators, thus reflecting changes in signaling. By the use of reporter gene expression, we demonstrated that one member of the AM-induced gene family encoding blue copper binding proteins (MtBcp1) was both specifically and strongly up-regulated in arbuscule-containing regions of mycorrhizal roots. A comparison of the AM expression profiles to those of nitrogen-fixing root nodules suggested only a limited overlap between the genetic programs orchestrating root endosymbioses.

Fungal elicitation of signal transduction-related plant genes precedes mycorrhiza establishment and requires the dmi3 gene in Medicago truncatula

Suppressive subtractive hybridization and expressed sequence tag sequencing identified 29 plant genes which are upregulated during the appressorium stage of mycorrhiza establishment between Medicago truncatula J5 (Myc+) and Glomus mosseae. Eleven genes coding plant proteins with predicted functions in signal transduction, transcription, and translation were investigated in more detail for their relation to early events of symbiotic interactions. Expression profiling showed that the genes are activated not only from the appressorium stage up to the fully established symbiosis in the Myc+ genotype of M. truncatula, but also when the symbionts are not in direct cell contact, suggesting that diffusible fungal molecules (Myc factors) play a, role in the induction of a signal-transduction pathway. Transcript accumulation in roots of a mycorrhiza-defective Myc- dmi3 mutant of M. truncatula is not modified by appressorium formation or diffusible fungal molecules, indicating that the signal transduction pathway is required for a successful G. mosseae-M. truncatula interaction leading to symbiosis development. The symbiotic nodulating bacterium Sinorhizobium meliloti does not activate the 11 genes, which supposes early discrimination by plant roots between the microbial symbionts.

Functional genomics of arbuscular mycorrhiza: decoding the symbiotic cell programme

More extensive insight into plant genes involved in the symbiotic programme of arbuscular mycorrhiza is presently being achieved by global approaches that aim to discover novel genes or subsets of genes that are essential to cell programmes in the different steps of plant–fungal interactions. The strategy of functional genomics based on large-scale differential RNA expression analyses (differential-display reverse transcriptase - PCR), electronic Northerns, suppressive subtractive hybridization, DNA chips) is presented, with a focus on arbuscular mycorrhiza in Pisum sativum and Medicago truncatula. The most recent knowledge about gene networks that are modulated in roots during arbuscular establishment and functioning is discussed.Key words: arbuscular mycorrhiza, symbiotic programme, gene expression, pea, annual alfalfa.