Increased trehalose biosynthesis in Hartig net hyphae of ectomycorrhizas

Identification of Trehalose-6-Phosphate Synthase (TPS) Genes Associated with Both Source-/Sink-Related Yield Traits and Drought Response in Rapeseed (Brassica napus L.)

Trehalose-6-phosphate synthase (TPS) is an important enzyme for the synthesis of Trehalose-6-phosphate (T6P). In addition to being a signaling regulator of carbon allocation that improves crop yields, T6P also plays essential roles in desiccation tolerance. However, comprehensive studies, such as evolutionary analysis, expression analysis, and functional classification of the TPS family in rapeseed (Brassica napus L.) are lacking. Here, we identified 35 BnTPSs, 14 BoTPSs, and 17 BrTPSs in cruciferous plants, which were classified into three subfamilies. Phylogenetic and syntenic analysis of TPS genes in four cruciferous species indicated that only gene elimination occurred during their evolution. Combined phylogenetic, protein property, and expression analysis of the 35 BnTPSs suggested that changes in gene structures might have led to changes in their expression profiles and further functional differentiation during their evolution. In addition, we analyzed one set of transcriptome data from Zhongshuang11 (ZS11) and two sets of data from extreme materials associated with source-/sink-related yield traits and the drought response. The expression levels of four BnTPSs (BnTPS6, BnTPS8, BnTPS9, and BnTPS11) increased sharply after drought stress, and three differentially expressed genes (BnTPS1, BnTPS5, and BnTPS9) exhibited variable expression patterns among source and sink tissues between yield-related materials. Our findings provide a reference for fundamental studies of TPSs in rapeseed and a framework for future functional research of the roles of BnTPSs in both yield and drought resistance.

Microbial storage and its implications for soil ecology

Organisms throughout the tree of life accumulate chemical resources, in particular forms or compartments, to secure their availability for future use. Here we review microbial storage and its ecological significance by assembling several rich but disconnected lines of research in microbiology, biogeochemistry, and the ecology of macroscopic organisms. Evidence is drawn from various systems, but we pay particular attention to soils, where microorganisms play crucial roles in global element cycles. An assembly of genus-level data demonstrates the likely prevalence of storage traits in soil. We provide a theoretical basis for microbial storage ecology by distinguishing a spectrum of storage strategies ranging from surplus storage (storage of abundant resources that are not immediately required) to reserve storage (storage of limited resources at the cost of other metabolic functions). This distinction highlights that microorganisms can invest in storage at times of surplus and under conditions of scarcity. We then align storage with trait-based microbial life-history strategies, leading to the hypothesis that ruderal species, which are adapted to disturbance, rely less on storage than microorganisms adapted to stress or high competition. We explore the implications of storage for soil biogeochemistry, microbial biomass, and element transformations and present a process-based model of intracellular carbon storage. Our model indicates that storage can mitigate against stoichiometric imbalances, thereby enhancing biomass growth and resource-use efficiency in the face of unbalanced resources. Given the central roles of microbes in biogeochemical cycles, we propose that microbial storage may be influential on macroscopic scales, from carbon cycling to ecosystem stability.

Integrative Study Supports the Role of Trehalose in Carbon Transfer From Fungi to Mycotrophic Orchid

Orchids rely on mycorrhizal symbiosis, especially in the stage of mycoheterotrophic protocorms, which depend on carbon and energy supply from fungi. The transfer of carbon from fungi to orchids is well-documented, but the identity of compounds ensuring this transfer remains elusive. Some evidence has been obtained for the role of amino acids, but there is also vague and neglected evidence for the role of soluble carbohydrates, probably trehalose, which is an abundant fungal carbohydrate. We therefore focused on the possible role of trehalose in carbon and energy transfer. We investigated the common marsh orchid (Dactylorhiza majalis) and its symbiotic fungus Ceratobasidium sp. using a combination of cultivation approaches, high-performance liquid chromatography, application of a specific inhibitor of the enzyme trehalase, and histochemical localization of trehalase activity. We found that axenically grown orchid protocorms possess an efficient, trehalase-dependent, metabolic pathway for utilizing exogenous trehalose, which can be as good a source of carbon and energy as their major endogenous soluble carbohydrates. This is in contrast to non-orchid plants that cannot utilize trehalose to such an extent. In symbiotically grown protocorms and roots of adult orchids, trehalase activity was tightly colocalized with mycorrhizal structures indicating its pronounced role in the mycorrhizal interface. Inhibition of trehalase activity arrested the growth of both symbiotically grown protocorms and trehalose-supported axenic protocorms. Since trehalose constitutes only an inconsiderable part of the endogenous saccharide spectrum of orchids, degradation of fungal trehalose likely takes place in orchid mycorrhiza. Our results strongly support the neglected view of the fungal trehalose, or the glucose produced by its cleavage as compounds transported from fungi to orchids to ensure carbon and energy flow. Therefore, we suggest that not only amino acids, but also soluble carbohydrates are transported. We may propose that the soluble carbohydrates would be a better source of energy for plant metabolism than amino acids, which is partially supported by our finding of the essential role of trehalase.

Intra-species genetic variability drives carbon metabolism and symbiotic host interactions in the ectomycorrhizal fungus Pisolithus microcarpus

Ectomycorrhizal (ECM) fungi are integral to boreal and temperate forest ecosystem functioning and nutrient cycling. ECM fungi, however, originate from diverse saprotrophic lineages and the impacts of genetic variation across species, and especially within a given ECM species, on function and interactions with the environment is not well understood. Here, we explore the extent of intra-species variation between four isolates of the ECM fungus Pisolithus microcarpus, in terms of gene regulation, carbon metabolism and growth, and interactions with a host, Eucalyptus grandis. We demonstrate that, while a core response to the host is maintained by all of the isolates tested, they have distinct patterns of gene expression and carbon metabolism, resulting in the differential expression of isolate-specific response pathways in the host plant. Together, these results highlight the importance of using a wider range of individuals within a species to understand the broader ecological roles of ECM fungi and their host interactions.

Composite poplars: a novel tool for ectomycorrhizal research

Composite poplars were used for ectomycorrhiza formation. Structurally normal mycorrhizas of transgenic roots revealed better fungal sugar support. Targeting fluorescent proteins to peroxisomes allowed easy in planta visualization of successful transformation. A bottle neck in ectomycorrhizal research is the time demand for generation of transgenic plants. An alternative strategy for such root-centered research might be the formation of the so-called composite plants, where transgenic roots are formed by non-transgenic shoots. We have developed an Agrobacterium rhizogenes-mediated root transformation protocol using axenic Populus tremula × tremuloides and P. tremula × alba cuttings. When comparing four different bacterial strains, A. rhizogenes K599 turned out to be the most suitable for poplar transformation. Transgenic roots revealed only minor hairy root phenotype when plants were grown on agar plates with synthetic growth medium in the absence of a sugar source. When using different ectomycorrhizal fungi, formation of ectomycorrhizas by transgenic roots of composite poplars was not affected and mycorrhizas were anatomically indistinguishable from mycorrhizas of non-transgenic roots. Elevated trehalose content and marker gene expression, however, pointed towards somewhat better fungal carbon nutrition in ectomycorrhizas of transgenic compared to non-transgenic roots. Cell wall autofluorescence of poplar fine roots is an issue that can limit the use of fluorescent proteins as visual markers for in planta analysis, especially after ectomycorrhiza formation. By targeting marker proteins to peroxisomes, sensitive fluorescence detection, easily distinguishable from cell wall autofluorescence, was obtained for both poplar fine roots and ectomycorrhizas.

Genome-wide analysis of the Solanum tuberosum (potato) trehalose-6-phosphate synthase (TPS) gene family: evolution and differential expression during development and stress

BACKGROUND

Trehalose-6-phosphate synthase (TPS) serves important functions in plant desiccation tolerance and response to environmental stimuli. At present, a comprehensive analysis, i.e. functional classification, molecular evolution, and expression patterns of this gene family are still lacking in Solanum tuberosum (potato).

RESULTS

In this study, a comprehensive analysis of the TPS gene family was conducted in potato. A total of eight putative potato TPS genes (StTPSs) were identified by searching the latest potato genome sequence. The amino acid identity among eight StTPSs varied from 59.91 to 89.54%. Analysis of dN/dS ratios suggested that regions in the TPP (trehalose-6-phosphate phosphatase) domains evolved faster than the TPS domains. Although the sequence of the eight StTPSs showed high similarity (2571-2796 bp), their gene length is highly differentiated (3189-8406 bp). Many of the regulatory elements possibly related to phytohormones, abiotic stress and development were identified in different TPS genes. Based on the phylogenetic tree constructed using TPS genes of potato, and four other Solanaceae plants, TPS genes could be categorized into 6 distinct groups. Analysis revealed that purifying selection most likely played a major role during the evolution of this family. Amino acid changes detected in specific branches of the phylogenetic tree suggests relaxed constraints might have contributed to functional divergence among groups. Moreover, StTPSs were found to exhibit tissue and treatment specific expression patterns upon analysis of transcriptome data, and performing qRT-PCR.

CONCLUSIONS

This study provides a reference for genome-wide identification of the potato TPS gene family and sets a framework for further functional studies of this important gene family in development and stress response.

Cloning and expression of trehalose-6-phosphate synthase 1 from Rhizopus oryzae

Trehalose is a reducing disaccharide acting as a protectant against environmental stresses in many organisms. In fungi, Trehalose-6-phosphate synthase 1 (TPS1) plays a key role in the biosynthesis of trehalose. In this study, a full-length cDNA from Rhizopus oryzae encoding TPS1 (designated as RoTPS1) was isolated. The RoTPS1 cDNA is composed of 2505 nucleotides and encodes a protein of 834 amino acids with a molecular mass of 97.8 kDa. The amino acid sequence of RoTPS1 has a relatively high homology with the TPS1s in several other filamentous fungi. RoTPS1 was cloned into Saccharomyces cerevisiae and secretively expressed.

The dual nature of trehalose in citrus canker disease: a virulence factor for Xanthomonas citri subsp. citri and a trigger for plant defence responses

Xanthomonas citri subsp. citri (Xcc) is a bacterial pathogen that causes citrus canker in susceptible Citrus spp. The Xcc genome contains genes encoding enzymes from three separate pathways of trehalose biosynthesis. Expression of genes encoding trehalose-6-phosphate synthase (otsA) and trehalose phosphatase (otsB) was highly induced during canker development, suggesting that the two-step pathway of trehalose biosynthesis via trehalose-6-phosphate has a function in pathogenesis. This pathway was eliminated from the bacterium by deletion of the otsA gene. The resulting XccΔotsA mutant produced less trehalose than the wild-type strain, was less resistant to salt and oxidative stresses, and was less able to colonize plant tissues. Gene expression and proteomic analyses of infected leaves showed that infection with XccΔotsA triggered only weak defence responses in the plant compared with infection with Xcc, and had less impact on the host plant's metabolism than the wild-type strain. These results suggested that trehalose of bacterial origin, synthesized via the otsA-otsB pathway, in Xcc, plays a role in modifying the host plant's metabolism to its own advantage but is also perceived by the plant as a sign of pathogen attack. Thus, trehalose biosynthesis has both positive and negative consequences for Xcc. On the one hand, it enables this bacterial pathogen to survive in the inhospitable environment of the leaf surface before infection and exploit the host plant's resources after infection, but on the other hand, it is a tell-tale sign of the pathogen's presence that triggers the plant to defend itself against infection.

Metagenome sequence of Elaphomyces granulatus from sporocarp tissue reveals Ascomycota ectomycorrhizal fingerprints of genome expansion and a Proteobacteria-rich microbiome

Many obligate symbiotic fungi are difficult to maintain in culture, and there is a growing need for alternative approaches to obtaining tissue and subsequent genomic assemblies from such species. In this study, the genome of Elaphomyces granulatus was sequenced from sporocarp tissue. The genome assembly remains on many contigs, but gene space is estimated to be mostly complete. Phylogenetic analyses revealed that the Elaphomyces lineage is most closely related to Talaromyces and Trichocomaceae s.s. The genome of E. granulatus is reduced in carbohydrate-active enzymes, despite a large expansion in genome size, both of which are consistent with what is seen in Tuber melanosporum, the other sequenced ectomycorrhizal ascomycete. A large number of transposable elements are predicted in the E. granulatus genome, especially Gypsy-like long terminal repeats, and there has also been an expansion in helicases. The metagenome is a complex community dominated by bacteria in Bradyrhizobiaceae, and there is evidence to suggest that the community may be reduced in functional capacity as estimated by KEGG pathways. Through the sequencing of sporocarp tissue, this study has provided insights into Elaphomyces phylogenetics, genomics, metagenomics and the evolution of the ectomycorrhizal association.

Maturation of nematode-induced galls in Medicago truncatula is related to water status and primary metabolism modifications

Root-knot nematodes are obligatory plant parasitic worms that establish and maintain an intimate relationship with their host plants. During a compatible interaction, these nematodes induce the redifferentiation of root cells into multinucleate and hypertrophied giant cells (GCs). These metabolically active feeding cells constitute the exclusive source of nutrients for the nematode. We analyzed the modifications of water status, ionic content and accumulation of metabolites in development and mature galls induced by Meloidogyne incognita and in uninfected roots of Medicago truncatula plants. Water potential and osmotic pressure are significantly modified in mature galls compared to developing galls and control roots. Ionic content is significantly modified in galls compared to roots. Principal component analyses of metabolite content showed that mature gall metabolism is significantly modified compared to developing gall metabolism. The most striking differences were the three-fold increase of trehalose content associated to the five-fold diminution in glucose concentration in mature galls. Gene expression analysis showed that trehalose accumulation was, at least, partially linked to a significantly lower expression of the trehalase gene in mature galls. Our results point to significant modifications of gall physiology during maturation.

Trehalose metabolism in plants

Trehalose is a quantitatively important compatible solute and stress protectant in many organisms, including green algae and primitive plants. These functions have largely been replaced by sucrose in vascular plants, and trehalose metabolism has taken on new roles. Trehalose is a potential signal metabolite in plant interactions with pathogenic or symbiotic micro-organisms and herbivorous insects. It is also implicated in responses to cold and salinity, and in regulation of stomatal conductance and water-use efficiency. In plants, as in other eukaryotes and many prokaryotes, trehalose is synthesized via a phosphorylated intermediate, trehalose 6-phosphate (Tre6P). A meta-analysis revealed that the levels of Tre6P change in parallel with sucrose, which is the major product of photosynthesis and the main transport sugar in plants. We propose the existence of a bi-directional network, in which Tre6P is a signal of sucrose availability and acts to maintain sucrose concentrations within an appropriate range. Tre6P influences the relative amounts of sucrose and starch that accumulate in leaves during the day, and regulates the rate of starch degradation at night to match the demand for sucrose. Mutants in Tre6P metabolism have highly pleiotropic phenotypes, showing defects in embryogenesis, leaf growth, flowering, inflorescence branching and seed set. It has been proposed that Tre6P influences plant growth and development via inhibition of the SNF1-related protein kinase (SnRK1). However, current models conflict with some experimental data, and do not completely explain the pleiotropic phenotypes exhibited by mutants in Tre6P metabolism. Additional explanations for the diverse effects of alterations in Tre6P metabolism are discussed.

A fluorometric assay for trehalose in the picomole range

BACKGROUND

Trehalose is a non-reducing disaccharide that is used as an osmolyte, transport sugar, carbon reserve and stress protectant in a wide range of organisms. In plants, trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is thought to be a signal of sucrose status. Trehalose itself may play a role in pathogenic and symbiotic plant-microbe interactions, in responses to abiotic stress and in developmental signalling, but its precise functions are unknown. A major obstacle to investigating its function is the technical difficulty of measuring the very low levels of trehalose usually found in plant tissues, as most of the established trehalose assays lack sufficient specificity and/or sensitivity.

RESULTS

A kinetic assay for trehalose was established using recombinant Escherichia coli cytoplasmic trehalase (treF), which was shown to be highly specific for trehalose. Hydrolysis of trehalose to glucose is monitored fluorometrically and the trehalose content of the tissue extract is determined from an internal calibration curve. The assay is linear for 0.2-40 pmol trehalose, and recoveries of trehalose were ≥88%. A. thaliana Col-0 rosettes contain about 20-30 nmol g-1FW of trehalose, increasing to about 50-60 nmol g-1FW in plants grown at 8°C. Trehalose is not correlated with sucrose content, whereas a strong correlation between Tre6P and sucrose was confirmed. The trehalose contents of ear inflorescence primordia from the maize ramosa3 mutant and wild type plants were 6.6±2.6 nmol g-1FW and 19.0±12.7 nmol g-1FW, respectively. The trehalose:Tre6P ratios in the ramosa3 and wild-type primordia were 2.43±0.85 and 6.16±3.45, respectively.

CONCLUSION

The fluorometric assay is highly specific for trehalose and sensitive enough to measure the trehalose content of very small amounts of plant tissue. Chilling induced a 2-fold accumulation of trehalose in A. thaliana rosettes, but the levels were too low to make a substantial quantitative contribution to osmoregulation. Trehalose is unlikely to function as a signal of sucrose status. The abnormal inflorescence branching phenotype of the maize ramosa3 mutant might be linked to a decrease in trehalose levels in the inflorescence primordia or a downward shift in the trehalose:Tre6P ratio.

Trehalose metabolism is activated upon chilling in grapevine and might participate in Burkholderia phytofirmans induced chilling tolerance

During the last decade, there has been growing interest in the role of trehalose metabolism in tolerance to abiotic stress in higher plants, especially cold stress. So far, this metabolism has not yet been studied in Vitis vinifera L., despite the economic importance of this crop. The goal of this paper was to investigate the involvement of trehalose metabolism in the response of grapevine to chilling stress, and to compare the response in plants bacterised with Burkholderia phytofirmans strain PsJN, a plant growth-promoting rhizobacterium that confers grapevine chilling tolerance, with mock-inoculated plants. In silico analysis revealed that the V. vinifera L. genome contains genes encoding the enzymes responsible for trehalose synthesis and degradation. Transcript analysis showed that these genes were differentially expressed in various plant organs, and we also characterised their response to chilling. Both trehalose and trehalose 6-phosphate (T6P) were present in grapevine tissues and showed a distinct pattern of accumulation upon chilling. Our results suggest a role for T6P as the main active molecule in the metabolism upon chilling, with a possible link with sucrose metabolism. Furthermore, plants colonised by B. phytofirmans and cultivated at 26°C accumulated T6P and trehalose in stems and leaves at concentrations similar to non-bacterised plants exposed to chilling temperatures for 1 day. Overall, our data suggest that T6P and trehalose accumulate upon chilling stress in grapevine and might participate in the resistance to chilling stress conferred by B. phytofirmans.

Genomic profiling of carbohydrate metabolism in the ectomycorrhizal fungus Tuber melanosporum

• Primary carbohydrate metabolism plays a special role related to carbon/nitrogen exchange, as well as metabolic support of fruiting body development, in ectomycorrhizal macrofungi. In this study, we used information retrieved from the recently sequenced Tuber melanosporum genome, together with transcriptome analysis data and targeted validation experiments, to construct the first genome-wide catalogue of the proteins supporting carbohydrate metabolism in a plant-symbiotic ascomycete. • More than 100 genes coding for enzymes of the glycolysis, pentose phosphate, tricarboxylic acid, glyoxylate and methylcitrate pathways, glycogen, trehalose and mannitol metabolism and cell wall precursor were annotated. Transcriptional regulation of these pathways in different stages of the T. melanosporum lifecycle was investigated using whole-genome oligoarray expression data together with real-time reverse transcription-polymerase chain reaction analysis of selected genes. • The most significant results were the identification of methylcitrate cycle genes and of an acid invertase, the first enzyme of this kind to be described in a plant-symbiotic filamentous fungus. • A subset of transcripts coding for trehalose, glyoxylate and methylcitrate enzymes was up-regulated in fruiting bodies, whereas genes involved in mannitol and glycogen metabolism were preferentially expressed in mycelia and ectomycorrhizas, respectively. These data indicate a high degree of lifecycle stage specialization for particular branches of carbohydrate metabolism in T. melanosporum.

Carbon allocation in ectomycorrhizal plants at limited optimal N supply: an attempt aat unraveling conflicting theories

With regard to mycorrhiza, conflicting theories try to explain how the balance between fungal demand for carbohydrates and the plant’s needs for nutrients varies, resulting in conflicting predictions. In order to evaluate current concepts, we investigated some metabolic parameters, which are indicative for plant carbon allocation in response to mycorrhization at limited and optimal N supply. Pinus pinaster seedlings were inoculated with living or dead (control) cultures of Pisolithus tinctorius, supplied with ammonium at 4 (limiting) or 7% d−1 (non-limiting) N relative addition rate (RARN), and followed development for 29 days. Mycorrhizal colonization of roots was quantified by the determination of ergosterol. A series of enzymes (sucrose and trehalose metabolism, anaplerosis) and metabolites (soluble carbohydrate, including trehalose; fructose 2,6 bisphosphate, free amino acids) relevant in the C/N exchange between symbionts, and in the carbon allocation and sink strength within the plant were assayed for 2-day-intervals for up to 14 days, and at 5-day-intervals for the rest of the experiment. The first 10 days reflected the establishment of mycorrhizal interaction, and the carbon allocation to the root was higher in M plants independent of N supply. Following this period, carbon allocation became N-related, higher at low, and lower at high N supply. The belowground C investment of M plants was dependent on N availability, but not on N gain. Finally, increased belowground C allocation was accompanied by a shift from plant to fungal metabolism.

Effect of co-inoculation with mycorrhiza and rhizobia on the nodule trehalose content of different bean genotypes

Studies on Rhizobium-legume symbiosis show that trehalose content in nodules under drought stress correlates positively with an increase in plant tolerance to this stress. Fewer reports describe trehalose accumulation in mycorrhiza where, in contrast with rhizobia, there is no flux of carbohydrates from the microsymbiont to the plant. However, the trehalose dynamics in the Mycorrhiza-Rhizobium-Legume tripartite symbiosis is unknown. The present study explores the role of this tripartite symbiosis in the trehalose content of nodules grown under contrasting moisture conditions. Three wild genotypes (P. filiformis, P. acutifolis and P. vulgaris) and two commercial genotypes of Phaseolus vulgaris (Pinto villa and Flor de Mayo) were used. Co-inoculation treatments were conducted with Glomus intraradices and a mixture of seven native rhizobial strains, and trehalose content was determined by GC/MS. The results showed a negative effect of mycorrhizal inoculation on nodule development, as mycorrhized plants showed fewer nodules and lower nodule dry weight compared to plants inoculated only with Rhizobium. Mycorrhizal colonization was also higher in plants inoculated only with Glomus as compared to plants co-inoculated with both microsymbionts. In regard to trehalose, co-inoculation negatively affects its accumulation in the nodules of each genotype tested. However, the correlation analysis showed a significantly positive correlation between mycorrhizal colonization and nodule trehalose content.

Effect of co-inoculation with mycorrhiza and rhizobia on the nodule trehalose content of different bean genotypes

Studies on Rhizobium-legume symbiosis show that trehalose content in nodules under drought stress correlates positively with an increase in plant tolerance to this stress. Fewer reports describe trehalose accumulation in mycorrhiza where, in contrast with rhizobia, there is no flux of carbohydrates from the microsymbiont to the plant. However, the trehalose dynamics in the Mycorrhiza-Rhizobium-Legume tripartite symbiosis is unknown. The present study explores the role of this tripartite symbiosis in the trehalose content of nodules grown under contrasting moisture conditions. Three wild genotypes (P. filiformis, P. acutifolis and P. vulgaris) and two commercial genotypes of Phaseolus vulgaris (Pinto villa and Flor de Mayo) were used. Co-inoculation treatments were conducted with Glomus intraradices and a mixture of seven native rhizobial strains, and trehalose content was determined by GC/MS. The results showed a negative effect of mycorrhizal inoculation on nodule development, as mycorrhized plants showed fewer nodules and lower nodule dry weight compared to plants inoculated only with Rhizobium. Mycorrhizal colonization was also higher in plants inoculated only with Glomus as compared to plants co-inoculated with both microsymbionts. In regard to trehalose, co-inoculation negatively affects its accumulation in the nodules of each genotype tested. However, the correlation analysis showed a significantly positive correlation between mycorrhizal colonization and nodule trehalose content.

Soybean metabolites regulated in root hairs in response to the symbiotic bacterium Bradyrhizobium japonicum

Nodulation of soybean (Glycine max) root hairs by the nitrogen-fixing symbiotic bacterium Bradyrhizobium japonicum is a complex process coordinated by the mutual exchange of diffusible signal molecules. A metabolomic study was performed to identify small molecules produced in roots and root hairs during the rhizobial infection process. Metabolites extracted from roots and root hairs mock inoculated or inoculated with B. japonicum were analyzed by gas chromatography-mass spectrometry and ultraperformance liquid chromatography-quadrupole time of flight-mass spectrometry. These combined approaches identified 2,610 metabolites in root hairs. Of these, 166 were significantly regulated in response to B. japonicum inoculation, including various (iso)flavonoids, amino acids, fatty acids, carboxylic acids, and various carbohydrates. Trehalose was among the most strongly induced metabolites produced following inoculation. Subsequent metabolomic analyses of root hairs inoculated with a B. japonicum mutant defective in the trehalose synthase, trehalose 6-phosphate synthase, and maltooligosyltrehalose synthase genes showed that the trehalose detected in the inoculated root hairs was primarily of bacterial origin. Since trehalose is generally considered an osmoprotectant, these data suggest that B. japonicum likely experiences osmotic stress during the infection process, either on the root hair surface or within the infection thread.

Trehalose and plant stress responses: friend or foe?

The disaccharide trehalose is involved in stress response in many organisms. However, in plants, its precise role remains unclear, although some data indicate that trehalose has a protective role during abiotic stresses. By contrast, some trehalose metabolism mutants exhibit growth aberrations, revealing potential negative effects on plant physiology. Contradictory effects also appear under biotic stress conditions. Specifically, trehalose is essential for the infectivity of several pathogens but at the same time elicits plant defense. Here, we argue that trehalose should not be regarded only as a protective sugar but rather like a double-faced molecule and that further investigation is required to elucidate its exact role in stress tolerance in plants.

Fungal carbohydrate support in the ectomycorrhizal symbiosis: a review

Ectomycorrhizal (ECM) symbiosis is a mutualistic interaction between certain soil fungi and fine roots of perennial plants, mainly forest trees, by which both partners become capable of efficiently colonising nutrient-limited environments. The success of this interaction is reflected in the dominance of ECM forest ecosystems in the Northern hemisphere. Apart from their economic importance (wood production), forest ecosystems are essential for large-scale carbon sequestration, leading to substantial reductions in anthropogenic CO(2) release. The biological function of ECM symbiosis is the exchange of fungus-derived mineral nutrients for plant-derived carbohydrates. Improved plant nutrition as a result of this interaction, however, has a price. Together with their fungal partner, root systems of ECM plants can receive about half of the photosynthetically fixed carbon. To enable such a strong carbohydrate sink, the monosaccharide uptake capacity and carbohydrate flux through glycolysis and intermediate carbohydrate storage pools (trehalose and/or mannitol) of mycorrhizal fungi is strongly increased at the plant-fungus interface. Apart from their function as a carbohydrate store, trehalose/mannitol are additionally considered to be involved in carbon allocation within the fungal colony. Dependent on the fungal species involved in the symbiosis, regulation and fine-tuning of fungal carbohydrate uptake and metabolism seems to be controlled either by developmental mechanisms or by the apoplastic sugar content. As a consequence of the increased carbohydrate demand in symbiosis, trees increase their photosynthetic capacity. In addition, host plants control and restrict carbohydrate flux towards their partner to avoid fungal parasitism. The mechanisms behind this phenomenon are still largely unknown but rates of local sucrose hydrolysis and hexose uptake by rhizodermal cells are thought to restrict fungal carbohydrate nutrition under certain conditions (e.g., reduced fungal nutrient export).

Trehalose-6-phosphate synthase 1 from Metarhizium anisopliae: clone, expression and properties of the recombinant

Trehalose, an important component in fungal spores, is a disaccharide which protects against several environmental stresses, such as heat, desiccation, salt. Trehalose-6-phosphate synthase 1 (TPS1) is a subunit of trehalose synthase complex in fungi; it plays a key role in the biosynthesis of trehalose. In this study, a full-length cDNA from Metarhizium anisopliae encoding TPS1 (designated as MaTPS1) was isolated. The MaTPS1 cDNA is composed of 1836 nucleotides encoding a protein of 517 amino acids with a molecular mass of 58 kDa. The amino acid sequence has a relatively high homology with the TPS1s in several other filamentous fungi. Southern blot analysis showed that MaTPS1 gene occurs as a single copy in the M. anisopliae genome. And MaTPS1 was cloned into Pichia pastoris KM71 and secretively expressed with a histamine tag to facilitate a rapid purification of recombinant MaTPS1 (designated reTPS1). The properties of reTPS1 were examined. The K(m) value of reTPS1 for UDP-glucose and glucose-6-phosphate was 9.6 mM and 3.9 mM, respectively, and the optimal pH and temperature were about 6.5 and 40 degrees C. The enzyme was highly specific to glucose-6-phosphate for glucosyl acceptor, and its activity decreased rapidly as the concentrations of phosphate increased. The expression system will provide sufficient amounts of reTPS1 for future structural characterization of the protein and use for further investigation of MaTPS1's function.

The major pathways of carbohydrate metabolism in the ectomycorrhizal basidiomycete Laccaria bicolor S238N

The primary carbohydrate metabolism of an ectomycorrhizal fungus and its transcriptional regulation has never been characterized at the genome scale although it plays a fundamental role in the functioning of the symbiosis. In this study, the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor S238N-H82 was explored to construct a comprehensive genome-wide inventory of pathways involved in primary carbohydrate metabolism. Several genes and gene families were annotated, including those of the glycolysis, pentose phosphate pathway, tricarboxylic acid cycle, and trehalose and mannitol metabolism. The transcriptional regulation of these pathways was studied using whole-genome expression oligoarrays and quantitative polymerase chain reaction in free-living mycelium, ectomycorrhizas and fruiting bodies. Pathways of carbohydrate biosynthesis and catabolism are identical in L. bicolor compared with other sequenced saprotrophic basidiomycetes. Ectomycorrhiza and fruiting body development induced the regulation of a restricted set of transcripts of the glycolytic, mannitol and trehalose metabolisms.

The sugar porter gene family of Laccaria bicolor: function in ectomycorrhizal symbiosis and soil-growing hyphae

Formation of ectomycorrhizas, a symbiosis with fine roots of woody plants, is one way for soil fungi to overcome carbohydrate limitation in forest ecosystems. Fifteen potential hexose transporter proteins, of which 10 group within three clusters, are encoded in the genome of the ectomycorrhizal model fungus Laccaria bicolor. For 14 of them, transcripts were detectable. When grown in liquid culture, carbon starvation resulted in at least twofold higher transcript abundances for seven genes. Temporarily elevated transcript abundance after sugar addition was observed for three genes. Compared with the extraradical mycelium, ectomycorrhiza formation resulted in a strongly enhanced expression of six genes, of which four revealed their highest observed transcript abundances in symbiosis. A function as hexose importer was proven for three of them. Only three genes, of which just one was expressed at a considerable level, revealed a reduced transcript content in mycorrhizas. From gene expression patterns and import kinetics, the L. bicolor hexose transporters could be divided into two groups: those responsible for uptake of carbohydrates by soil-growing hyphae, for improved carbon nutrition, and to reduce nutrient uptake competition by other soil microorganisms; and those responsible for efficient hexose uptake at the plant-fungus interface.

The mycorrhiza helper bacteria revisited

In natural conditions, mycorrhizal fungi are surrounded by complex microbial communities, which modulate the mycorrhizal symbiosis. Here, the focus is on the so-called mycorrhiza helper bacteria (MHB). This concept is revisited, and the distinction is made between the helper bacteria, which assist mycorrhiza formation, and those that interact positively with the functioning of the symbiosis. After considering some examples of MHB from the literature, the ecological and evolutionary implications of the relationships of MHB with mycorrhizal fungi are discussed. The question of the specificity of the MHB effect is addressed, and an assessment is made of progress in understanding the mechanisms of the MHB effect, which has been made possible through the development of genomics. Finally, clear evidence is presented suggesting that some MHB promote the functioning of the mycorrhizal symbiosis. This is illustrated for three critical functions of practical significance: nutrient mobilization from soil minerals, fixation of atmospheric nitrogen, and protection of plants against root pathogens. The review concludes with discussion of future research priorities regarding the potentially very fruitful concept of MHB.