Gibberellin Promotes Fungal Entry and Colonization during Paris-Type Arbuscular Mycorrhizal Symbiosis in Eustoma grandiflorum.

Arbuscular mycorrhizas (AMs) are divided into two types according to morphology: Arum- and Paris-type AMs. Gibberellins (GAs) mainly inhibit the establishment of Arum-type AM symbiosis in most model plants, whereas the effects of GAs on Paris-type AM symbiosis are unclear. To provide insight into the mechanism underlying this type of symbiosis, the roles of GAs were investigated in Eustoma grandiflorum when used as the host plant for Paris-type AM establishment. Eustoma grandiflorum seedlings were inoculated with the model AM fungus, Rhizophagus irregularis, and the effects of GA and the GA biosynthesis inhibitor uniconazole-P on the symbiosis were quantitatively evaluated. Exogenous GA significantly increased hyphopodium formation at the epidermis, thus leading to the promotion of fungal colonization and arbuscule formation in the root cortex. By contrast, the suppression of GA biosynthesis and signaling attenuated fungal entry to E. grandiflorum roots. Moreover, the exudates from GA-treated roots strongly induced the hyphal branching of R. irregularis. Our results show that GA has an contrasting effect on Paris-type AM symbiosis in E. grandiflorum compared with Arum-type AM symbiosis. This finding could be explained by the differential regulation of the early colonization stage, where fungal hyphae make contact with and penetrate the epidermis.

Biogeochemical distribution of Pb and Zn forms in two calcareous soils affected by mycorrhizal symbiosis and alfalfa rhizosphere.

Using of arbuscular mycorrhizal fungi (AMF) has emerged as a new technique to alleviate the toxic metals stress through changing their chemical behavior. The present work was conducted as a factorial arrangement based on a completely randomized design to study the inoculation effects of Glomus intraradices, Glomus mosseae and Glomus etunicatum, on Pb and Zn fractions in the rhizosphere of alfalfa by using rhizobox technique in two agricultural soils with different Zn and Pb concentrations [with low (LH) and high (HH) concentration levels]. The results showed that AMF colonization promoted plant growth and lowered the shoot and root Pb and shoot Zn concentrations in the studied soils compared to uninoculated treatments. Mycorrhizal colonization significantly increased the Ca(NO3)2- extractable Zn and ORG-Zn (respectively 500 and 59.6% more than the uninoculated treatment) and decreased the OXI-Zn (20.32% less than the none inoculated treatment) in the HH soil. By contrast, mycorrhizae slightly increased the CARB, OXI and ORG-Zn forms in the LH soil compared to the uninoculation condition. In the AMF- treated HH soil, an increase was recorded in the Ca(NO3)2- extractable Pb, EXCH-Pb and CARB-Pb (respectively, 17.65, 3.09 and 14.22% compared to the none inoculated treatment) and a decrease in the OXI and ORG-Pb forms (respectively, 28.79 and 13.51% compared to the uninoculated treatment). A reverse status was observed for Pb changes in the LH soil. Depending on the contamination level, the mycorrhizal inoculation differentially affected the Pb and Zn fractions at different distances from the root surface. In the LH soil, at <5 mm distance (i.e. rhizospheric soil), the mycorrhizal inoculation decreased the CARB (about 17.99%) and OXI -Zn (about 29.63%) forms compared to bulk soil (i.e. > 5 mm distance) while ORG-Zn was increased up to 48.63%. However, Ca(NO3)2- extractable, CARB and ORG-Pb was increased in rhizosphere soil (respectively, 89.33, 3.84 and 6.14%) and OXI-Pb was decreased up to 10.36% compared to the bulk soil. In the HH soil, mycorrhizal inoculation increased the CARB and OXI-Zn (respectively, 1.76 and 5.71%) and OXI-Pb fractions (11.56%) compared to the <5 mm distances. Whereas, it reduced the Ca(NO3)2- extractable, EXCH, and ORG-Zn (Respectively, 52.70, 19.19 and 30.16%) and Ca(NO3)2- extractable, CARB and ORG-Pb (respectively, 47.18, 3.70 and 5.79%). These results revealed that depending on the soil contamination level and nature of the element, AMF colonization affects biogeochemical fractions of the metals and their accumulation in the plant tissues.

Foliar Roundup application has minor effects on the compositional and functional diversity of soil microorganisms in a short-term greenhouse experiment.

The herbicide Roundup (and glyphosate, its active ingredient) is extensively used for weed control on a worldwide scale. It is absorbed after foliar application and quickly translocated inside the plant. In this study, we investigated the effects of Roundup speed, a commercial glyphosate formulation, on the structural composition (dominance of microbial groups, phospholipid fatty acid analysis - PLFA) and functional diversity (use of carbon sources, Multiple Substrate Induced Respiration - MSIR) of soil microorganisms. We specifically aimed at understanding the potential impact of biotic interactions on herbicide effects and included plants, earthworms, and endomycorrhizal fungi in the experimental setup. For this, we grew clover (Trifolium repens) in the greenhouse and added mycorrhizal inoculum (Glomus mosseae) and earthworms (Lumbricus terrestris) to the pots. Two weeks after foliar Roundup application and subsequent plant death, the pots were destructively sampled. The application resulted in a significant increase of microbial respiration (SIR) by approximately 30%. A multivariate analysis of the MSIR data exhibited small but significant differences between the microbial communities of treated and untreated pots, while no significant difference was apparent for the PLFA data. Bacterial PLFAs generally decreased following herbicide application, while mycorrhizal and fungal PLFAs were not affected. We did not find a consistent difference between the fatty acid markers of gram negative and gram positive bacteria. For all investigated parameters, there were highly significant differences between the upper (0-5 cm depth) and lower (5-10 cm) soil layers. The fact that rooting density differed by a factor of 3.5 between the two layers indicated that herbicide effects were especially pronounced in the clover rhizosphere and were likely due to changes in root exudate composition. We found significant, though very small, interactions between Roundup and other experimental factors (especially mycorrhizal inoculum).

Two herbicides, two fungicides and spore-associated bacteria affect Funneliformis mosseae extraradical mycelium structural traits and viability.

The extraradical mycelium (ERM) produced by arbuscular mycorrhizal fungi is fundamental for the maintenance of biological fertility in agricultural soils, representing an important inoculum source, together with spores and mycorrhizal root fragments. Its viability and structural traits, such as density, extent and interconnectedness, which are positively correlated with the growth and nutrition of host plants, may be affected by different agronomic practices, including the use of pesticides and by different mycorrhizospheric communities. This work, carried out using a whole-plant experimental model system, showed that structural traits of ERM, such as length and density, were strongly decreased by the herbicides dicamba and glufosinolate and the fungicides benomyl and fenhexamid, while anastomosis frequency and hyphal branching were differentially modulated by singly inoculated mycorrhizospheric bacteria, depending on their identity.

Understanding interaction effect of arbuscular mycorrhizal fungi in rice under elevated carbon dioxide conditions.

Arbuscular mycorrhizal fungi (AMF), particularly the Glomerales group, play a paramount role in plant nutrient uptake, and abiotic and biotic stress management in rice, but recent evidence revealed that elevated CO2 concentration considerably reduces the Glomerales group in soil. In view of this, the present study was initiated to understand the interaction effect of native Glomerales species application in rice plants (cv. Naveen) under elevated CO2 concentrations (400 ± 10, 550 ± 20, and 700 ± 20 ppm) in open-top chambers. Three different modes of application of the AMF inoculum were evaluated, of which, combined application of AMF at the seedling production and transplanting stages showed increased AMF colonization, which significantly improved grain yield by 25.08% and also increased uptake of phosphorus by 18.2% and nitrogen by 49.5%, as observed at 700-ppm CO2 concentration. Organic acids secretion in rice root increased in AMF-inoculated plants exposed to 700-ppm CO2 concentration. To understand the overall effect of CO2 elevation on AMF interaction with the rice plant, principal component and partial least square regression analysis were performed, which found both positive and negative responses under elevated CO2 concentration.

Proteomic analysis of the response of Funnelifor mismosseae/Medicago sativa to atrazine stress.

BACKGROUND: Arbuscular mycorrhizal (AM) fungi form symbiotic associations with host plants can protect host plants against diverse biotic and abiotic stresses, and promote biodegradation of various contaminants. However, the molecular mechanisms of how the arbuscular mycorrhizal fungi and host plant association on atrazine stress were still poorly understood. To better characterize how arbuscular mycorrhizal fungi and host plant interactions increase atrazine stress, we performed physiological and proteomic analysis of Funneliformis mosseae (mycorrhizal fungi) and Medicago sativa (alfalfa) association under atrazine stress. RESULTS: The results showed that in the Arbuscular mycorrhizal, protective enzymes were up regulated and the malondialdehyde content increased relative to those of non-mycorrhizal M.sativa. We also examined the atrazine degradation rates within the nutrient solution, and a 44.43% reduction was observed with the mycorrhizal M.sativa, with 30.83% of the reduction attributed to F. mosseae. The accumulation content in root and stem of mycorrhizal M.sativa were obviously increased 11.89% and 16.33% than those of non- mycorrhizal M.sativa. The activity of PPO, POD, CAT and SOD in mycorrhizal M.sativa were obviously higher than non mycorrhizal M.sativa under atrazine stess. We identified differential root proteins using isobaric tags for relative and absolute quantization coupled with liquid chromatography-mass spectrometry, with 533 proteins identified (276 unregulated and 257 downregulated). The differentially expressed proteins were further examined using GO, BLAST comparisons, and a literature inquiry and were classified into the categories of atrazine degradation (37.1%); atrazine stress response (28.6%); plant immune responses (14.3%); translation, synthesis, and processing (10%); and signal transduction and biological processes (10%). Furthermore, we identified glycosyl transferase, glutathione S-transferase, laccase, cytochrome P450 monooxygenase, peroxidase, and other proteins closely related to the degradation process. CONCLUSIONS: Mycorrhizal Medicago showed improved atrazine degradation within the culturing medium and increased atrazine enrichment in the roots and stems. Additionally, AMF increased the plant root response to atrazine, with relevant enzymes up regulated and toxic effects alleviated. Overall, the findings of this study show that AMF played an important role in easing atrazine stress in plants and contributed to atrazine remediation and further contributed to the understanding of the molecular mechanism associated with atrazine stresses and potential mycorrhizal contributions in M.sativa.

Placing arbuscular mycorrhizal fungi on the risk assessment test battery of plant protection products (PPPs).

Arbuscular mycorrhizal fungi (AMF) are mutualistic symbionts considered a key group in soil systems involved in the provision of several ecosystem services. Recently they have been listed by EFSA as organisms to be included in the test battery for the risk assessment of plant protection product (PPPs). This study aimed to contribute to improve the ISO Protocol (ISO 10832: 2009) by assessing the feasibility of using other AMF species under different test conditions. Overall, results showed that AMF species Gigaspora albida and Rhizophagus clarus (selected out of five AMF species) are suitable to be used in spore germination tests using the ISO protocol (14 days incubation with sand or artificial soil as substrate) to test PPPs. However, several modifications to the protocol were made in order to accommodate the use of the tested isolates, namely the incubation temperature (28 °C instead of 24 °C) and the change of reference substance (boric acid instead of cadmium nitrate). The need for these changes, plus the results obtained with the three fungicides tested (chlorothalonil, mancozeb and metalaxyl-M) and comparisons made with literature on the relevance of the origin of AMF isolates in dictating the adequate test conditions, emphasize the importance of adjusting test conditions (AMF species/isolates and test temperature) when assessing effects for prospective risk assessment targeting different climatic zones. So, further studies should be conducted with different AMF species and isolates from different climatic regions, in order to better define which species/isolate and test conditions should be used to assess effects of a particular PPP targeting a given climatic zone.

Transcriptional profiling of arbuscular mycorrhizal roots exposed to high levels of phosphate reveals the repression of cell cycle-related genes and secreted protein genes in Rhizophagus irregularis.

The development of arbuscular mycorrhiza (AM) is strongly suppressed under high-phosphate (Pi) conditions. To investigate AM fungal responses during the suppression of AM by high Pi, we performed an RNA-seq analysis of Rhizophagus irregularis colonizing Lotus japonicus roots at different levels of Pi (20, 100, 300, and 500 µM). AM fungal colonization decreased markedly under high-Pi conditions. In total, 163 fungal genes were differentially expressed among the four Pi treatments. Among these genes, a cell cycle-regulatory gene, cyclin-dependent kinase CDK1, and several DNA replication- and mitosis-related genes were repressed under high-Pi conditions. More than 20 genes encoding secreted proteins were also downregulated by high-Pi conditions, including the strigolactone-induced putative secreted protein 1 gene that enhances AM fungal colonization. In contrast, the expression of genes related to aerobic respiration and transport in R. irregularis were largely unaffected. Our data suggest that high Pi suppresses the expression of genes associated with fungal cell cycle progression or that encode secreted proteins that may be required for intercellular hyphal growth and arbuscule formation. However, high Pi has little effect on the transcriptional regulation of the primary metabolism or transport in preformed fungal structures.

Gibberellins interfere with symbiosis signaling and gene expression and alter colonization by arbuscular mycorrhizal fungi in Lotus japonicus.

Arbuscular mycorrhiza is a mutualistic plant-fungus interaction that confers great advantages for plant growth. Arbuscular mycorrhizal (AM) fungi enter the host root and form symbiotic structures that facilitate nutrient supplies between the symbionts. The gibberellins (GAs) are phytohormones known to inhibit AM fungal infection. However, our transcriptome analysis and phytohormone quantification revealed GA accumulation in the roots of Lotus japonicus infected with AM fungi, suggesting that de novo GA synthesis plays a role in arbuscular mycorrhiza development. We found pleiotropic effects of GAs on the AM fungal infection. In particular, the morphology of AM fungal colonization was drastically altered by the status of GA signaling in the host root. Exogenous GA treatment inhibited AM hyphal entry into the host root and suppressed the expression of Reduced Arbuscular Mycorrhization1 (RAM1) and RAM2 homologs that function in hyphal entry and arbuscule formation. On the other hand, inhibition of GA biosynthesis or suppression of GA signaling also affected arbuscular mycorrhiza development in the host root. Low-GA conditions suppressed arbuscular mycorrhiza-induced subtilisin-like serine protease1 (SbtM1) expression that is required for AM fungal colonization and reduced hyphal branching in the host root. The reduced hyphal branching and SbtM1 expression caused by the inhibition of GA biosynthesis were recovered by GA treatment, supporting the theory that insufficient GA signaling causes the inhibitory effects on arbuscular mycorrhiza development. Most studies have focused on the negative role of GA signaling, whereas our study demonstrates that GA signaling also positively interacts with symbiotic responses and promotes AM colonization of the host root.

Integrated multi-omics analysis supports role of lysophosphatidylcholine and related glycerophospholipids in the Lotus japonicus-Glomus intraradices mycorrhizal symbiosis.

Interaction of plant roots with arbuscular mycorrhizal fungi (AMF) is a complex trait resulting in cooperative interactions among the two symbionts including bidirectional exchange of resources. To study arbuscular mycorrhizal symbiosis (AMS) trait variation in the model plant Lotus japonicus, we performed an integrated multi-omics analysis with a focus on plant and fungal phospholipid (PL) metabolism and biological significance of lysophosphatidylcholine (LPC). Our results support the role of LPC as a bioactive compound eliciting cellular and molecular response mechanisms in Lotus. Evidence is provided for large interspecific chemical diversity of LPC species among mycorrhizae with related AMF species. Lipid, gene expression and elemental profiling emphasize the Lotus-Glomus intraradices interaction as distinct from other arbuscular mycorrhizal (AM) interactions. In G. intraradices, genes involved in fatty acid (FA) elongation and biosynthesis of unsaturated FAs were enhanced, while in Lotus, FA synthesis genes were up-regulated during AMS. Furthermore, FAS protein localization to mitochondria suggests FA biosynthesis and elongation may also occur in AMF. Our results suggest the existence of interspecific partitioning of PL resources for generation of LPC and novel candidate bioactive PLs in the Lotus-G. intraradices symbiosis. Moreover, the data advocate research with phylogenetically diverse Glomeromycota species for a broader understanding of the molecular underpinnings of AMS.

Arbuscular mycorrhiza improve growth, nitrogen uptake, and nitrogen use efficiency in wheat grown under elevated CO2.

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO2 in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO2 concentrations (400 and 700 ppm) for 10 weeks. A (15)N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO2 increased AM fungal colonization. Under CO2 elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO2 elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, (15)N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO2 enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO2.

The intracellular delivery of TAT-aequorin reveals calcium-mediated sensing of environmental and symbiotic signals by the arbuscular mycorrhizal fungus Gigaspora margarita.

Arbuscular mycorrhiza (AM) is an ecologically relevant symbiosis between most land plants and Glomeromycota fungi. The peculiar traits of AM fungi have so far limited traditional approaches such as genetic transformation. The aim of this work was to investigate whether the protein transduction domain of the HIV-1 transactivator of transcription (TAT) protein, previously shown to act as a potent nanocarrier for macromolecule delivery in both animal and plant cells, may translocate protein cargoes into AM fungi. We evaluated the internalization into germinated spores of Gigaspora margarita of two recombinant TAT fusion proteins consisting of either a fluorescent (GFP) or a luminescent (aequorin) reporter linked to the TAT peptide. Both TAT-fused proteins were found to enter AM fungal mycelia after a short incubation period (5-10 min). Ca2+ measurements in G. margarita mycelia pre-incubated with TAT-aequorin demonstrated the occurrence of changes in the intracellular free Ca2+ concentration in response to relevant stimuli, such as touch, cold, salinity, and strigolactones, symbiosis-related plant signals. These data indicate that the cell-penetrating properties of the TAT peptide can be used as an effective strategy for intracellularly delivering proteins of interest and shed new light on Ca2+ homeostasis and signalling in AM fungi.

Relationship between genetic variability in Rhizophagus irregularis and tolerance to saline conditions.

Reclamation of saline soils produced by extraction of bitumen from oil sands is challenging. The main objective of this study was to select a salt-tolerant arbuscular mycorrhizal (AM) fungal isolate that could, in the future, be used to pre-inoculate plants used in reclamation of saline substrates produced by oil sand industry. To achieve this, the effects of NaCl, Na(2)SO(4), and saline release water from composite tailings (CT) on hyphal growth of two AM fungal isolates from non-saline (Rhizophagus irregularis DAOM 181602, Rhizophagus sp. DAOM 227023) and three isolates of R. irregularis isolated from saline or sodic soils (DAOM 234181, DAOM241558, and DAOM241559) were tested in vitro. Pre-symbiotic hyphal growth of the five isolates, in absence of a host plant, decreased with increasing salt stress and no spores germinated in CT. The symbiotic extraradical phase of the four isolates of R. irregularis developed well in saline media compared to the Rhizophagus sp. Nevertheless, fungal development of the four R. irregularis isolates differed in saline media indicating phenotypic variations between isolates.

Investigating the simultaneous effect of chitosan and arbuscular mycorrhizal fungi on growth, phenolic compounds, PAL enzyme activity and lipid peroxidation in Salvia nemorosa L.

Considering the importance of Salvia nemorosa L. in the pharmaceutical and food industries, and also beneficial approaches of arbuscular mycorrhizal fungi (AMF) symbiosis and the use of bioelicitors such as chitosan to improve secondary metabolites, the aim of this study was to evaluate the performance of chitosan on the symbiosis of AMF and the effect of both on the biochemical and phytochemical performance of this plant and finally introduced the best treatment. Two factors were considered for the factorial experiment: AMF with four levels (non-inoculated plants, Funneliformis mosseae, Rhizophagus intraradices and the combination of both), and chitosan with six levels (0, 50, 100, 200, 400 mg L-1 and 1% acetic acid). Four months after treatments, the aerial part and root length, the levels of lipid peroxidation, H2O2, phenylalanine ammonia lyase (PAL) activity, total phenol and flavonoid contents and the main secondary metabolites (rosmarinic acid and quercetin) in the leaves and roots were determined. The flowering stage was observed in R. intraradices treatments and the highest percentage of colonization (78.87%) was observed in the treatment of F. mosseae × 400 mg L-1 chitosan. Furthermore, simultaneous application of chitosan and AMF were more effective than their separate application to induce phenolic compounds accumulation, PAL activity and reduce oxidative compounds. The cluster and principal component analysis based on the measured variables indicated that the treatments could be classified into three clusters. It seems that different treatments in different tissues have different effects. However, in an overview, it can be concluded that 400 mg L-1 chitosan and F. mosseae × R. intraradices showed better results in single and simultaneous applications. The results of this research can be considered in the optimization of this medicinal plant under normal conditions and experiments related to abiotic stresses in the future.

The arbuscular mycorrhizal symbiosis influences sulfur starvation responses of Medicago truncatula.

Arbuscular mycorrhizal (AM) symbiosis is a mutualistic interaction that occurs between the large majority of vascular plants and fungi of the phylum Glomeromycota. In addition to other nutrients, sulfur compounds are symbiotically transferred from AM fungus to host plants; however, the physiological importance of mycorrhizal-mediated sulfur for plant metabolism has not yet been determined. We applied different sulfur and phosphate fertilization treatments to Medicago truncatula and investigated whether mycorrhizal colonization influences leaf metabolite composition and the expression of sulfur starvation-related genes. The expression pattern of sulfur starvation-related genes indicated reduced sulfur starvation responses in mycorrhizal plants grown at 1 mM phosphate nutrition. Leaf metabolite concentrations clearly showed that phosphate stress has a greater impact than sulfur stress on plant metabolism, with no demand for sulfur at strong phosphate starvation. However, when phosphate nutrition is high enough, mycorrhizal colonization reduces sulfur stress responses, probably as a result of symbiotic sulfur uptake. Mycorrhizal colonization is able to reduce sulfur starvation responses in M. truncatula when the plant's phosphate status is high enough that sulfur starvation is of physiological importance. This clearly shows the impact of mycorrhizal sulfur transfer on plant metabolism.

Effects of prometryn and acetochlor on arbuscular mycorrhizal fungi and symbiotic system.

UNLABELLED: Prometryn and acetochlor are common herbicides widely used to control weeds in agricultural systems. The impacts of the two herbicides on spore germination, hyphal elongation, the biomass and malondialdehyde content of carrot hairy roots were investigated using a strict in vitro cultivation system associating the Ri T-DNA-transferred carrot hairy roots with Glomus etunicatum. Alternatively, root colonization, daughter spore production and the proportion of hyphae with succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities were also investigated. No significant impact on spore germination was noted in the presence of acetochlor at all three concentrations tested, while a significant decrease was observed with prometryn only at the highest concentration. Moreover, an inverse correlation was identified between herbicides concentrations and G. etunicatum root colonization and spore production as well as hyphal SDH and ALP activity, with a positive correlation identified among these four factors. Both herbicides exerted negative effects on the arbuscular mycorrhizal (AM) fungus and symbiosis at increasing concentrations, with prometryn apparently more toxic than acetochlor. Furthermore, the AM symbiotic system was shown to improve biomass, reduce malondialdehyde accumulation and ease lipid peroxidation in carrot hairy roots and decrease damage in host plants, thus enhancing plant tolerance to adverse conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: In this study, the effect of prometryn and acetochlor on the physiology and metabolic activities of the AM fungus Glomus etunicatum were investigated. Our findings demonstrate for the first time, the impact of the two herbicides at three concentrations (0.1, 1 and 10 mg l(-1)) on transformed carrot hairy roots/AM fungus association under strict in vitro culture conditions, which may guide the application of the two herbicides in modern agriculture.

Ultrastructural evidence for AMF mediated salt stress mitigation in Trigonella foenum-graecum.

The study unveils that inoculation with arbuscular mycorrhizal fungus (Glomus intraradices Schenck and Smith) prevents salt-induced ultrastructural alterations in fenugreek (Trigonella foenum-graecum L.) plants. Mycorrhizal (M) and non-mycorrhizal (NM) fenugreek plants were subjected to four levels of NaCl (0, 50, 100, and 200 mM NaCl). Salt-induced ultrastructural changes were captured using a Transmission Electron Microscope. Effects of salt on the ultrastructure of cells include shrinkage of protoplasm, widening apoplastic space between cell wall and cell membrane, disorganization of grana in chloroplast--swelling and reduction in the number of thylakoids, disintegration of chloroplast membrane, accumulation of plastoglobules, dilation of cristae and denser matrix in mitochondria, and aggregation of chromatin in nucleus. However, the extent of salt-induced ultrastructural damage was less in M plants as compared to NM plants. Lower lipid peroxidation and electrolyte leakage in M plants also indicated less membrane damage. This reduction of ultrastructure damage is a demonstration of enhanced tolerance in M plants to salt stress. The AMF-mediated lesser damage may be due to higher osmolyte (glycinebetaine, sugars) and polyamines concentration, and more and bigger plastoglobules (higher α-tocopherol concentration) in M plants as compared to NM plants. While lower Na(+) and Cl(-) ions assures less ionic toxicity, higher osmolytes and tocopherols ensure osmotic adjustment and better capacity to scavenge free radicals generated due to salt stress, respectively.

Direct and indirect influences of 8 yr of nitrogen and phosphorus fertilization on Glomeromycota in an alpine meadow ecosystem.

We measured the influences of soil fertility and plant community composition on Glomeromycota, and tested the prediction of the functional equilibrium hypothesis that increased availability of soil resources will reduce the abundance of arbuscular mycorrhizal (AM) fungi. Communities of plants and AM fungi were measured in mixed roots and in Elymus nutans roots across an experimental fertilization gradient in an alpine meadow on the Tibetan Plateau. As predicted, fertilization reduced the abundance of Glomeromycota as well as the species richness of plants and AM fungi. The response of the glomeromycotan community was strongly linked to the plant community shift towards dominance by Elymus nutans. A reduction in the extraradical hyphae of AM fungi was associated with both the changes in soil factors and shifts in the plant community composition that were caused by fertilization. Our findings highlight the importance of soil fertility in regulating both plant and glomeromycotan communities, and emphasize that high fertilizer inputs can reduce the biodiversity of plants and AM fungi, and influence the sustainability of ecosystems.

Structure and expression profile of the phosphate Pht1 transporter gene family in mycorrhizal Populus trichocarpa.

Gene networks involved in inorganic phosphate (Pi) acquisition and homeostasis in woody perennial species able to form mycorrhizal symbioses are poorly known. Here, we describe the features of the 12 genes coding for Pi transporters of the Pht1 family in poplar (Populus trichocarpa). Individual Pht1 transporters play distinct roles in acquiring and translocating Pi in different tissues of mycorrhizal and nonmycorrhizal poplar during different growth conditions and developmental stages. Pi starvation triggered the up-regulation of most members of the Pht1 family, especially PtPT9 and PtPT11. PtPT9 and PtPT12 showed a striking up-regulation in ectomycorrhizas and endomycorrhizas, whereas PtPT1 and PtPT11 were strongly down-regulated. PtPT10 transcripts were highly abundant in arbuscular mycorrhiza (AM) roots only. PtPT8 and PtPT10 are phylogenetically associated to the AM-inducible Pht1 subfamily I. The analysis of promoter sequences revealed conserved motifs similar to other AM-inducible orthologs in PtPT10 only. To gain more insight into gene regulatory mechanisms governing the AM symbiosis in woody plant species, the activation of the poplar PtPT10 promoter was investigated and detected in AM of potato (Solanum tuberosum) roots. These results indicated that the regulation of AM-inducible Pi transporter genes is conserved between perennial woody and herbaceous plant species. Moreover, poplar has developed an alternative Pi uptake pathway distinct from AM plants, allowing ectomycorrhizal poplar to recruit PtPT9 and PtPT12 to cope with limiting Pi concentrations in forest soils.

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.