Arbuscular Mycorrhizal Fungus Rhizophagus Irregularis Research Articles

Transcriptome responses in wheat roots to colonization by the arbuscular mycorrhizal fungus Rhizophagus irregularis.

The influence of arbuscular mycorrhizal (AM) colonization on the expression of genes in the roots of wheat (Triticum aestivum L.) at the transcriptome level is largely unknown. A pot experiment was established to study the responses of the transcriptome profile in the roots of wheat to colonization by the AM fungus Rhizophagus irregularis using high through-put sequencing methods. The results indicated that the expression of 11,746 genes was regulated by AM colonization, and 64.7% of them were up-regulated genes. 1106 genes were only expressed in roots colonized by AM fungi, and 108 genes were only expressed in non-mycorrhizal roots. The differentially expressed genes (DEGs) were primarily distributed on the 2B, 3B, 2A, 2D, and 5B chromosomes of wheat. The DEGs (including both up- and down- regulated) mainly located on membranes, and functioned in nucleotide binding and transferase activity during cellular protein modification and biosynthetic processes. The data revealed that AM colonization up-regulated genes involved in the phenylpropanoid biosynthesis pathway and transcription factors which play vital roles in protecting plants from biotic or abiotic stresses. A number of key genes involved in molecular signal biosynthesis and recognition, epidermal cell colonization and arbuscule formation, carbon and nutrients exchange during AM symbiosis were found. All the ammonium transporter (AMT), iron-phytosiderophore transporter, boron, zinc, and magnesium transporter genes found in our study were up-regulated DEGs. One new AM-specific induced AMT and three new AM-specific induced nitrate transporter (NRT) genes were found in the roots of wheat colonized by AM fungi, even though a negative growth response of wheat to AM colonization occurred. The present study provided new information which is important for understanding the mechanisms behind the development and function of the symbiosis between wheat and AM fungi.

High levels of arbuscular mycorrhizal fungus colonization on Medicago truncatula reduces plant suitability as a host for pea aphids (Acyrthosiphon pisum).

This study sheds light on a poorly understood area in insect–plant–microbe interactions, focusing on aphid probing and feeding behavior on plants with varying levels of arbuscular mycorrhizal (AM) fungus root colonization. It investigates a commonly occurring interaction of three species: pea aphid Acyrthosiphon pisum, barrel medic Medicago truncatula, and the AM fungus Rhizophagus irregularis, examining whether aphid‐feeding behavior changes when insects feed on plants at different levels of AM fungus colonization (42% and 84% root length colonized). Aphid probing and feeding behavior was monitored throughout 8 h of recording using the electrical penetration graph (EPG) technique, also, foliar nutrient content and plant growth were measured. Summarizing, aphids took longer to reach their 1st sustained phloem ingestion on the 84% AM plants than on the 42% AM plants or on controls. Less aphids showed phloem ingestion on the 84% AM plants relative to the 42% AM plants. Shoots of the 84% AM plants had higher percent carbon (43.7%) relative to controls (40.5%), and the 84% AM plants had reduced percent nitrogen (5.3%) relative to the 42% AM plants (6%). In conclusion, EPG and foliar nutrient data support the hypothesis that modifications in plant anatomy (e.g., thicker leaves), and poor food quality (reduced nitrogen) in the 84% AM plants contribute to reduced aphid success in locating phloem and ultimately to differences in phloem sap ingestion. This work suggests that M. truncatula plants benefit from AM symbiosis not only because of increased nutrient uptake but also because of reduced susceptibility to aphids.

The arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 41833 increases the phosphorus uptake and biomass of Medicago truncatula, a benzo[a]pyrene-tolerant plant species.

The accumulation of phosphorus (P) in plants increases their biomass and resistance/tolerance to organic pollutants. Both characteristics are mandatory for the utilization of plants in phytoremediation. Arbuscular mycorrhizal (AM) fungi improve plant P nutrition, and thus growth. However, only a few studies have focused on the dynamics of inorganic P (Pi) uptake in AM fungal-colonized plants in the presence of organic pollutants. Indeed, most of the results so far were obtained after harvesting the plants, thus by evaluating P concentration and content at a single time point. Here, we investigated the effects of the AM fungus Rhizophagus irregularis MUCL 41833 on the short-term Pi uptake dynamics of Medicago truncatula plants grown in the presence of benzo[a]pyrene (B[a]P), a polyaromatic hydrocarbon (PAH) frequently found in polluted soils. The study was conducted using a non-destructive circulatory semi-hydroponic cultivation system to investigate the short-term Pi depletion from a nutrient solution and as a corollary, the Pi uptake by the AM fungal-colonized and non-colonized plants. The growth, P concentration, and content of plants were also evaluated at harvest. The presence of B[a]P neither impacted the development of the AM fungus in the roots nor the plant growth and Pi uptake, suggesting a marked tolerance of both organisms to B[a]P pollution. A generally higher Pi uptake coupled with a higher accumulation of P in shoots and roots was noticed in AM fungal-colonized plants as compared to the non-colonized controls, irrespective of the presence or absence of B[a]P. Therefore, fungal-colonized plants showed the best growth. Furthermore, the beneficial effect provided by the presence of the AM fungus in roots was similar in presence or absence of B[a]P, thus opening the door for potential utilization in phytomanagement of PAH-polluted soils.

The Effects of Arbuscular Mycorrhizal Fungal Colonisation on Nutrient Status, Growth, Productivity, and Canker Resistance of Apple (Malus pumila).

We assess whether arbuscular mycorrhizal fungi (AMF) improve growth, nutritional status, phenology, flower and fruit production, and disease resistance in woody perennial crops using apple (Malus pumila) as a study system. In a fully factorial experiment, young trees were grown for 3 years with or without AMF (Funneliformis mosseae and Rhizophagus irregularis), and with industrial standard fertiliser applications or restricted fertiliser (10% of standard). We use two commercial scions (Dabinett and Michelin) and rootstocks (MM111 and MM106). Industrial standard fertiliser applications reduced AMF colonisation and root biomass, potentially increasing drought sensitivity. Mycorrhizal status was influenced by above ground genotypes (scion type) but not rootstocks, indicating strong interactions between above and below ground plant tissue. The AMF inoculation significantly increased resistance to Neonectria ditissima, a globally economically significant fungal pathogen of apple orchards, but did not consistently alter leaf nutrients, growth, phenology or fruit and flower production. This study significantly advances understanding of AMF benefits to woody perennial crops, especially increased disease resistance which we show is not due to improved tree nutrition or drought alleviation. Breeding programmes and standard management practises can limit the potential for these benefits.

Growth benefits provided by different arbuscular mycorrhizal fungi to Plantago lanceolata depend on the form of available phosphorus

Strains of arbuscular mycorrhizal fungi (AMF) differ markedly in the growth benefits they provide to plants. We investigated whether these differences depend on the chemical form of inorganic phosphorus. The closely related AMF Glomus custos and Rhizophagus irregularis were compared using Plantago lanceolata as the host plant, grown in quartz sand with either soluble orthophosphate or sparingly soluble hydroxyapatite as a sole source of phosphorus. In a growth experiment with AMF-inoculated plants in a climate chamber, sampling at 3-wk intervals enabled a detailed time-resolved analysis of shoot and root phosphorus concentrations and growth performance of P. lanceolata. The ability of AMF to enhance plant growth and deliver phosphate depended strongly on the identity of the available phosphorus source. In orthophosphate-amended substrate only modest differences in plant growth performance (dry matter accumulation and allocation, phosphorus acquisition) were observed between the two AMFs, despite evident AMF root colonization as shown by strain-specific mtLSU qPCR analysis. The treatment with hydroxyapatite however, created stringent growth-limiting conditions and significantly increased the growth benefit provided by R. irregularis over G. custos and the non-mycorrhizal treatment. Plants with R. irregularis could acquire much more phosphorus from apatite compared to G. custos. There were also differences in shoot-to-root dry matter allocation and plant tissue phosphorus concentrations between the R. irregularis and G. custos treatments. Our observations suggest that in experiments on the symbiosis between plants and mycorrhizal fungi more attention should be paid to the chemical form of phosphorus in soil.

Arbuscular mycorrhizal fungi: intraspecific diversity and pangenomes.

Contents Summary 1129 I. Introduction 1129 II. Intraspecific phenotypic variation and the plant host 1130 III. High inter-isolate genetic diversity in model AMF 1130 IV. Genome diversity within the model AM fungus Rhizophagus irregularis 1131 V. Pangenomes and the future of AMF ecological genomics 1131 Acknowledgements 1133 References 1133 SUMMARY: Arbuscular mycorrhizal fungi (AMF) are ubiquitous plant symbionts with an intriguing population biology. Conspecific AMF strains can vary substantially at the genetic and phenotypic levels, leading to direct and quantifiable variation in plant growth. Recent studies have shown that high intraspecific diversity is very common in AMF, and not only found in model species. Studies have also revealed how the phenotype of conspecific isolates varies depending on the plant host, highlighting the functional relevance of intraspecific phenotypic plasticity for the AMF ecology and mycorrhizal symbiosis. Recent work has also demonstrated that conspecific isolates of the model AMF Rhizophagus irregularis harbor large and highly variable pangenomes, highlighting the potential role of intraspecific genome diversity for the ecological adaptation of these symbionts.

Signal beyond nutrient, fructose, exuded by an arbuscular mycorrhizal fungus triggers phytate mineralization by a phosphate solubilizing bacterium

Cooperation is a prevalent phenomenon in nature and how it originates and maintains is a fundamental question in ecology. Many efforts have been made to understand cooperation between individuals in the same species, while the mechanisms enabling cooperation between different species are less understood. Here, we investigated under strict in vitro culture conditions if the exchange of carbon and phosphorus is pivotal to the cooperation between the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis and the phosphate solubilizing bacterium (PSB) Rahnella aquatilis. We observed that fructose exuded by the AMF stimulated the expression of phosphatase genes in the bacterium as well as the rate of phosphatase release into the growth medium by regulating its protein secretory system. The phosphatase activity was subsequently increased, promoting the mineralization of organic phosphorus (i.e., phytate) into inorganic phosphorus, stimulating simultaneously the processes involved in phosphorus uptake by the AMF. Our results demonstrated for the first time that fructose not only is a carbon source, but also plays a role as a signal molecule triggering bacteria-mediated organic phosphorus mineralization processes. These results highlighted the molecular mechanisms by which the hyphal exudates play a role in maintaining the cooperation between AMF and bacteria.

Reducing Water Availability Impacts the Development of the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis MUCL 41833 and Its Ability to Take Up and Transport Phosphorus Under in Vitro Conditions.

Climate change scenarios predict a higher variability in rainfall and an increased risk of water deficits during summers for the coming decades. For this reason, arbuscular mycorrhizal fungi (AMF) and their mitigating effects on drought stress in plants are increasingly considered in crop management. However, the impact of a decrease in water availability on the development of AMF and their ability to take up and transport inorganic phosphorus (Pi) to their hosts remain poorly explored. Here, Medicago truncatula plantlets were grown in association with Rhizophagus irregularis MUCL 41833 in bi-compartmented Petri plates. The system consisted in associating the plant and AMF in a root compartment (RC), allowing only the hyphae to extend in a root-free hyphal compartment (HC). Water availability in the HC was then lowered by increasing the concentration of polyethylene glycol-8000 (PEG-8000) from 0 to 10, 25, and 50 g L-1 (corresponding to a slight decrease in water potential of -0.024, -0.025, -0.030, and -0.056 Mpa, respectively). Hyphal growth, spore production and germination were severely impaired at the lowest water availability. The dynamics of Pi uptake by the AMF was also impacted, although total Pi uptake evaluated after 24 h stayed unchanged. The percentage of metabolically active extraradical hyphae remained above 70%. Finally, at the lowest water availability, a higher P concentration was observed in the shoots of M. truncatula. At reduced water availability, the extraradical mycelium (ERM) development was impacted, potentially limiting its capacity to explore a higher volume of soil. Pi uptake was slowed down but not prevented. The sensitivity of R. irregularis MUCL 41833 to a, even small, decrease in water availability contrasted with several studies reporting tolerance of AMF to drought. This suggests a species or strain-dependent effect and support the necessity to compare the impact of water availability on morpho-anatomy, nutrient uptake and transport capacities of other, potentially more drought-tolerant (e.g., isolated from dry environments) AMF.

Arbuscular mycorrhizal fungi increase grain zinc concentration and modify the expression of root ZIP transporter genes in a modern barley (Hordeum vulgare) cultivar

The positive effects of arbuscular mycorrhizal fungi (AMF) on the zinc (Zn) nutrition of a number of cereal species has been demonstrated, but for Hordeum vulgare (barley), this has been scarcely investigated. Zn is taken up by ZIP transporters in the roots, and several barley ZIP transporter genes are up-regulated under Zn deficient conditions.We grew a modern cultivar of barley (cv. Compass) at five different soil Zn concentrations ranging from no addition through to a toxic concentration. The plants were either inoculated with the AMF Rhizophagus irregularis, or mock-inoculated. At harvest, measurements of biomass, tissue Zn concentration, and expression of ZIP transporter genes were taken.Inoculation of barley with AMF resulted in improved grain and straw Zn concentrations, especially at low soil Zn concentrations, but did not increase the biomass of the plants. Of the five HvZIP genes tested that are up-regulated under low Zn conditions, one gene (HvZIP13) was significantly up-regulated by mycorrhizal colonisation at the lowest Zn treatment. Two other ZIP genes were down-regulated in mycorrhizal plants under low soil Zn.Inoculation with AMF has an effect on ZIP transporter genes in the roots of barley plants. Furthermore, AMF may be more useful for improving quality of barley grain in terms of Zn concentrations, rather than improving yield.

An improved method for Agrobacterium rhizogenes-mediated transformation of tomato suitable for the study of arbuscular mycorrhizal symbiosis

BackgroundSolanum lycopersicum, an economically important crop grown worldwide, has been used as a model for the study of arbuscular mycorrhizal (AM) symbiosis in non-legume plants for several years and several cDNA array hybridization studies have revealed specific transcriptomic profiles of mycorrhizal tomato roots. However, a method to easily screen candidate genes which could play an important role during tomato mycorrhization is required.ResultsWe have developed an optimized procedure for composite tomato plant obtaining achieved through Agrobacterium rhizogenes-mediated transformation. This protocol involves the unusual in vitro culture of composite plants between two filter papers placed on the culture media. In addition, we show that DsRed is an appropriate molecular marker for the precise selection of cotransformed tomato hairy roots. S. lycopersicum composite plant hairy roots appear to be colonized by the AM fungus Rhizophagus irregularis in a manner similar to that of normal roots, and a modified construct useful for localizing the expression of promoters putatively associated with mycorrhization was developed and tested.ConclusionsIn this study, we present an easy, fast and low-cost procedure to study AM symbiosis in tomato roots.

Arbuscular Mycorrhizal Symbiosis Modulates Antioxidant Response and Ion Distribution in Salt-Stressed Elaeagnus angustifolia Seedlings.

Elaeagnus angustifolia L. is a drought-resistant species. Arbuscular mycorrhizal symbiosis is considered to be a bio-ameliorator of saline soils that can improve salinity tolerance in plants. The present study investigated the effects of inoculation with the arbuscular mycorrhizal fungus Rhizophagus irregularis on the biomass, antioxidant enzyme activities, and root, stem, and leaf ion accumulation of E. angustifolia seedlings grown during salt stress conditions. Salt-stressed mycorrhizal seedlings produced greater root, stem, and leaf biomass than the uninoculated stressed seedlings. In addition, the seedlings colonized by R. irregularis showed notably higher activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) in the leaves of the mycorrhizal seedlings in response to salinity compared to those of the non-mycorrhizal seedlings. Mycorrhizal seedlings not only significantly increased their ability to acquire K+, Ca2+, and Mg2+, but also maintained higher K+:Na+ ratios in the leaves and lower Ca2+:Mg2+ ratios than non-mycorrhizal seedlings during salt stress. These results suggest that the salt tolerance of E. angustifolia seedlings could be enhanced by R. irregularis. The arbuscular mycorrhizal symbiosis could be a promising method to restore and utilize salt-alkaline land in northern China.

Rhizosphere yeasts improve P uptake of a maize arbuscular mycorrhizal association

Plant roots associate with microorganisms to acquire phosphorus (P) from the soil including arbuscular mycorrhizal (AM) fungi and phosphate solubilizers such as yeasts. However, information on interactions between AM fungi and yeasts is limited. The objective of the present study was to examine possible cooperation between the AM fungus Rhizophagus irregularis and the rhizosphere yeasts Cryptococcus flavus and Candida railenensis in terms of maize plant growth and P uptake employing P isotope tracer monitoring. Compartmented growth units with root-hyphal (RHC) and hyphal (HC) mesh bags allowed studying interactions between R. irregularis and yeasts with and without roots. The P isotope dilution method was used to measure possible phosphate solubilization in soil by the rhizosphere yeasts with subsequent P uptake by the AM association. Plants were grown for 8 weeks in low P soil under controlled growth chamber conditions. Main results showed strong plant growth promotion after inoculation with R. irregularis and both yeast species altered the specific root length of maize. The yeasts also improved maize shoot P content, but only in mycorrhizal plants. More specifically yeasts improved AM hyphal 32P uptake, but not that of root 33P uptake. On the other hand, yeasts had no effect on extraradical growth of R. irregularis. Inoculation with yeasts did not result in P isotope dilution measured as specific P isotope activity, indicating that the reported improved P uptake from AM associations in combination with yeasts was most likely not related to P solubilization, whereas alteration in root growth and specific root length seems more likely mechanisms of interactions. We conclude that rhizosphere yeasts promote P nutrition of arbuscular mycorrhizal maize linked to improved specific root length and AM hyphal P uptake.

Chromium detoxification in arbuscular mycorrhizal symbiosis mediated by sulfur uptake and metabolism

It has been well demonstrated that arbuscular mycorrhizal (AM) symbiosis can improve plant chromium (Cr) tolerance, but the detoxification mechanisms are still unsolved. In the present study the role of AM symbiosis in plant sulfur (S) acquisition and metabolism related to Cr detoxification was investigated. Medicago truncatula plants were cultivated with/without AM fungus Rhizophagus irregularis in Cr(VI) contaminated soils at two soil S and phosphorus (P) concentrations. Sulfur uptake, transport and metabolism by plants were determined by biochemical and molecular methods. Meanwhile, Cr uptake and transformation were characterized by using spectroscopic methods, including synchrotron based X-ray absorption near edge structure (XANES) analysis. The results indicated that mycorrhizal inoculation improved plant S acquisition by up-regulating the expression of high affinity sulfate transporter genes (i.e. MtSULTR1.1, MtSULTR1.2) in plant roots. Mycorrhizal colonization also systematically regulated S transport and metabolism for Cr(VI) detoxification. XANES analysis revealed the Cr immobilization in AM roots by phosphate and histidine-like ligands rather than thiol groups. Correlation analysis further confirmed possible involvement of S metabolites in the relief of Cr induced oxidative stress. Mycorrhizal effects were more pronounced under low P condition. The study allowed a new insight into the mechanisms of enhanced plant Cr tolerance by AM symbiosis.

The effect of inoculation with arbuscular mycorrhizal fungus Rhizophagus irregularis on cytokinin content in a highly mycotrophic Medicago lupulina line under low phosphorus level in the soil

The study is focused on the elucidation of the role of cytokinins (CKs, zeatin and zeatin riboside) in the development of effective arbuscular mycorrhiza (AM) symbiosis with Medicago lupulina. An important mechanism involved in the regulation of host plant growth is supposed to be linked to the modulation of plant hormone balance. The data obtained revealed the formation of an effective AM-symbiosis (M. lupulina + Rhizophagus irregularis) under phosphorus-deficiency. At the shooting stage (35<sup>th</sup> day after sowing), it is characterized by a decrease in the root:shoot ratio, the lowering in arbuscules and vesicle abundances, but an increase in the intensity of mycelium development. Mycorrhized plants differed from the control ones by higher CK levels in both roots and leaves. Zeatin and zeatin riboside concentration exhibited uneven alterations over time. A role of mycelium in the modulation of CK balance has been discussed.

Identification of arbuscular mycorrhiza-inducible Nitrate Transporter 1/Peptide Transporter Family (NPF) genes in rice

Arbuscular mycorrhizal fungi (AMF) colonize up to 90% of all land plants and facilitate the acquisition of mineral nutrients by their hosts. Inorganic orthophosphate (Pi) and nitrogen (N) are the major nutrients transferred from the fungi to plants. While plant Pi transporters involved in nutrient transfer at the plant-fungal interface have been well studied, the plant N transporters participating in this process are largely unknown except for some ammonium transporters (AMT) specifically assigned to arbuscule-colonized cortical cells. In plants, many nitrate transporter 1/peptide transporter family (NPF) members are involved in the translocation of nitrogenous compounds including nitrate, amino acids, peptides and plant hormones. Whether NPF members respond to AMF colonization, however, is not yet known. Here, we investigated the transcriptional regulation of 82 rice (Oryza sativa) NPF genes in response to colonization by the AMF Rhizophagus irregularis in roots of plants grown under five different nutrition regimes. Expression of the four OsNPF genes NPF2.2/PTR2, NPF1.3, NPF6.4 and NPF4.12 was strongly induced in mycorrhizal roots and depended on the composition of the fertilizer solution, nominating them as interesting candidates for nutrient signaling and exchange processes at the plant-fungal interface.

The dual benefit of arbuscular mycorrhizal fungi under soil zinc deficiency and toxicity: linking plant physiology and gene expression

Colonisation of roots by arbuscular mycorrhizal fungi (AMF) can increase plant biomass and nutrition under soil zinc (Zn) deficiency and toxicity conditions, but the genes and transporters involved in these processes are unknown. The aim here was to determine whether there is a ZIP (Zrt-, Irt-like protein) transporter gene that is differentially-regulated by mycorrhizal colonisation that may be involved in mycorrhizal Zn uptake. We grew Medicago truncatula plants at soil Zn concentrations ranging from deficient to toxic, with and without inoculation of the AMF Rhizophagus irregularis, and measured plant dry weight, shoot nutrient concentrations and the expression of phosphate, aquaporin and ZIP genes in the roots. At low and high soil Zn concentrations, there were positive biomass responses to AMF colonisation. Furthermore, at low soil Zn concentrations, MtZIP6 was highly up-regulated in the mycorrhizal plants. With increasing soil Zn concentration, expression of the AMF-induced phosphate transporter gene MtPT4 increased, and mycorrhizal colonisation was maintained. We have identified two different mechanisms by which AMF colonisation can increase plant biomass under low and high Zn stress: first, up-regulation of MtZIP6 at low soil Zn to supplement Zn uptake from the rhizosphere; and second, persistence of mycorrhizal colonisation and expression of MtPT4, which at high Zn could promote increased plant biomass and reduced tissue Zn concentration.

Short-term chromium (VI) exposure increases phosphorus uptake by the extraradical mycelium of the arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 41833

Hexavalent chromium is a potent carcinogen, while phosphorus is an essential nutrient. The role of arbuscular mycorrhizal fungi (AMF) in the uptake of P is well known and was also reported, at low levels, for Cr. However, it is unclear whether the uptake of Cr can impact the short-term uptake dynamics of P since both elements have a similar chemical structure and may thus potentially compete with each other during the uptake process. This study investigated the impact of Cr(VI) on short-term P uptake by the AMF Rhizophagus irregularis MUCL 41833 in Medicago truncatula. Bi-compartmented Petri plates were used to spatially separate a root compartment (RC) from a hyphal compartment (HC) using a whole plant in vitro culture system. The HC was supplemented with Cr(VI). Chromium(VI) as well as total Cr and P were monitored during 16 h within the HC and their concentrations determined by the end of the experiment within roots and shoots. Our results indicated that the uptake and translocation of Cr from hyphae to roots was a fast process: roots in which the extraradical mycelium (ERM) was exposed to Cr(VI) accumulated more Cr than roots of which the ERM was not exposed to Cr(VI) or was dead. Our results further confirmed that dead ERM immobilized more Cr than alive ERM. Finally our results demonstrated that the short exposure to Cr(VI) was sufficient to stimulate P uptake by the ERM and that the stimulation process began within the first 4 h of exposure.

A key role for arbuscular mycorrhiza in plant acquisition of P from sewage sludge recycled to soil

Dried and incinerated sewage sludge (SS) have the potential to be used as phosphorus (P) fertilisers. Arbuscular mycorrhizal fungi (AMF) contribute to plant P uptake; however, their role in P uptake from SS has yet to be fully explored. A compartmented pot system with an isotope pool dilution approach was used to investigate wheat (Triticum aestivum L.) P uptake from soluble P, dried SS and incinerated SS, via roots and/or AMF hyphae. Wheat was sown into an inner compartment containing a 33P label with/without AMF (Rhizophagus irregularis) inoculum. An outer soil compartment contained the P source. Compartments were separated by mesh of different sizes to allow root and/or AMF hyphal access. Plants obtained P from dried SS via AMF but there was limited evidence that plants obtained P from incinerated SS via AMF. Phosphorus uptake was significantly greater when both roots and hyphae could access dried SS than when only roots could access dried SS. We discuss the results in terms of availability of P to roots and hyphae. We conclude that AMF play an important role in wheat P acquisition from dried SS and therefore can assist in the recycling of P in waste.

Arbuscular mycorrhizal fungi and water availability affect biomass and C:N:P ecological stoichiometry in alfalfa (Medicago sativa L.) during regrowth

Ecological stoichiometry has been widely studied in terrestrial ecosystems, but these studies have been limited in terms of symbiotic association between alfalfa and arbuscular mycorrhizal fungi (AMF), especially during regrowth. To evaluate the effect of AMF on the regrowth and C:N:P stoichiometry of alfalfa (Medicago sativa L.) under well-watered and drought conditions, alfalfa plants inoculated with AMF (Rhizophagus irregularis, M), nitrogen-fixing bacteria (Sinorhizobium, R), both nitrogen-fixing bacteria and AMF or no inoculations (CK) were evaluated in a pot experiment under controlled conditions. The biomass and organic carbon (C), nitrogen (N) and phosphorus (P) nutritional status of plant leaves and roots were measured under two water treatments during regrowth. Water deficit reduced the accumulation of dry matter and the concentrations of C and N in leaves and P in roots but increased the concentrations of P in leaves and C and N in roots of alfalfa during regrowth. Compared to CK plants, inoculation significantly improved the regrowth biomass and the concentrations of C, N and P in the leaves and roots and especially increased P levels when the plant were inoculated with AMF. However, this effect of microbes on alfalfa regrowth was dependent on the soil water status. Drought reduced the C:N and C:P in the leaves and the C:N in roots, while N:P and C:P increased in the roots. Inoculation of AMF decreased the C:P and N:P in the leaves and the C:N and C:P in the roots, whereas it increased the C:N under water stress. These results indicate that AMF play a significant role in regrowth and C:N:P ecological stoichiometry after defoliation by influencing C assimilation, N and P uptake and that the responses in the leaves and the roots are opposite.

The plasma membrane proteome of Medicago truncatula roots as modified by arbuscular mycorrhizal symbiosis.

In arbuscular mycorrhizal (AM) roots, the plasma membrane (PM) of the host plant is involved in all developmental stages of the symbiotic interaction, from initial recognition to intracellular accommodation of intra-radical hyphae and arbuscules. Although the role of the PM as the agent for cellular morphogenesis and nutrient exchange is especially accentuated in endosymbiosis, very little is known regarding the PM protein composition of mycorrhizal roots. To obtain a global overview at the proteome level of the host PM proteins as modified by symbiosis, we performed a comparative protein profiling of PM fractions from Medicago truncatula roots either inoculated or not with the AM fungus Rhizophagus irregularis. PM proteins were isolated from root microsomes using an optimized discontinuous sucrose gradient; their subsequent analysis by liquid chromatography followed by mass spectrometry (MS) identified 674 proteins. Cross-species sequence homology searches combined with MS-based quantification clearly confirmed enrichment in PM-associated proteins and depletion of major microsomal contaminants. Changes in protein amounts between the PM proteomes of mycorrhizal and non-mycorrhizal roots were monitored further by spectral counting. This workflow identified a set of 82 mycorrhiza-responsive proteins that provided insights into the plant PM response to mycorrhizal symbiosis. Among them, the association of one third of the mycorrhiza-responsive proteins with detergent-resistant membranes pointed at partitioning to PM microdomains. The PM-associated proteins responsive to mycorrhization also supported host plant control of sugar uptake to limit fungal colonization, and lipid turnover events in the PM fraction of symbiotic roots. Because of the depletion upon symbiosis of proteins mediating the replacement of phospholipids by phosphorus-free lipids in the plasmalemma, we propose a role of phosphate nutrition in the PM composition of mycorrhizal roots.