Arbuscular mycorrhizal fungi influence host infection during epidemics in a wild plant pathosystem.

While pathogenic and mutualistic microbes are ubiquitous across ecosystems and often co-occur within hosts, how they interact to determine patterns of disease in genetically diverse wild populations is unknown. To test whether microbial mutualists provide protection against pathogens, and whether this varies among host genotypes, we conducted a field experiment in three naturally occurring epidemics of a fungal pathogen, Podosphaera plantaginis, infecting a host plant, Plantago lanceolata, in the Åland Islands, Finland. In each population, we collected epidemiological data on experimental plants from six allopatric populations that had been inoculated with a mixture of mutualistic arbuscular mycorrhizal fungi or a nonmycorrhizal control. Inoculation with arbuscular mycorrhizal fungi increased growth in plants from every population, but also increased host infection rate. Mycorrhizal effects on disease severity varied among host genotypes and strengthened over time during the epidemic. Host genotypes that were more susceptible to the pathogen received stronger protective effects from inoculation. Our results show that arbuscular mycorrhizal fungi introduce both benefits and risks to host plants, and shift patterns of infection in host populations under pathogen attack. Understanding how mutualists alter host susceptibility to disease will be important for predicting infection outcomes in ecological communities and in agriculture.

New species and reports of Cuphophyllus from northern North America compared with related Eurasian species.

This study describes four gray or brown species of Cuphophyllus (Hygrophoraceae, Agaricales), two of them new species, restricted to arctic-alpine and northern boreal zones of North America, and relates them morphologically and phylogenetically using multigene and nuc rDNA internal transcribed spacer ITS1-5.8S-ITS (ITS barcode) analyses to their similar, known counterparts. Cuphophyllus cinerellus, epitypified here, is shown to be a pan-palearctic species with sequence-confirmed collections from Fennoscandia and easternmost Asia. Occupying a similar habitat in the Nearctic is its sister species, the morphologically similar but novel C. esteriae, so far known only from eastern North America, including Greenland. Sister to the C. cinerellus-C. esteriae lineage, and known only from boreal raised Sphagnum bogs in Newfoundland, is a new medium-sized light cinereous brown species, C. lamarum. It has a yellow stipe but is phylogenetically distant from the yellow-stiped European C. flavipes and its North American sister species, Hygrophorus pseudopallidus. As cryptic speciation was discovered within C. flavipes, we lecto- and epitypify the name and transfer H. pseudopallidus to Cuphophyllus based on ITS analysis of the holotype. We also transfer the small European Hygrocybe comosa to Cuphophyllus based on morphology. Cuphophyllus hygrocyboides is reported from North America with the first sequence-confirmed collections from arctic-alpine British Columbia and Greenland. In addition, sequencing the holotype of C. subviolaceus identifies it as the sister species to the putative C. lacmus. Both species seem to have an intercontinental distribution. In total, we add new sequences to GenBank from 37 Cuphophyllus collections, including the holotypes of C. hygrocyboides and C. subviolaceus, the two new epitypes, and the two novel species.

Resident and phytometer plants host comparable rhizosphere fungal communities in managed grassland ecosystems.

Plants are known to modulate their own rhizosphere mycobiome. However, field studies that use resident plants to relate the microbiome assemblage to environmental factors such as land-use suffer from the problem that confounding factors such as plant age and performance may override the targeted effects. In contrast, the use of even-aged phytometer plants pre-cultivated under uniform conditions helps to reduce such random variation. We investigated the rhizosphere mycobiomes of phytometer and resident plants of two common grassland species, Dactylis glomerata L. s. str. and Plantago lanceolata L. along a land-use intensity gradient using ITS rRNA Illumina amplicon sequencing. Remarkably, we did not detect effects of the plant types (resident vs. phytometer plant, even though some fungal taxa exhibited plant species specificity), indicating that phytometer plants hosted a comparable rhizosphere mycobiome as resident plants. Our data indicate that the plant species harbor distinct fungal communities, with fungal richness in the rhizosphere of P. lanceolata being substantially higher than that of D. glomerata. Land-use intensity had a clear impact on the mycobiome of both plant species, with specific fungal genera showing differential tolerance to high intensities. Overall, the phytometer approach has a high potential to reveal environmental impacts on rhizosphere communities.

Soil-borne fungi influence seed germination and mortality, with implications for coexistence of desert winter annual plants.

Soil-borne fungi influence coexistence of plant species in mesic environments, but much less is known about their effects on demographic processes relevant to coexistence in arid and semi-arid systems. We isolated 43 fungal strains that naturally colonize seeds of an invasive winter annual (Brassica tournefortii) in the Sonoran Desert, and evaluated the impact of 18 of them on seed germination and mortality of B. tournefortii and a co-occurring native annual (Plantago ovata) under simulated summer and winter temperatures. Fungi isolated from B. tournefortii seeds impacted germination and mortality of seeds of both plant species in vitro. Seed responses reflected host-specific effects by fungi, the degree of which differed significantly between the strains, and depended on the temperature. In the winter temperature, ten fungal strains increased or reduced seed germination, but substantial seed mortality due to fungi was not observed. Two strains increased germination of P. ovata more strongly than B. tournefortii. In the summer temperature, fungi induced both substantial seed germination and mortality, with ten strains demonstrating host-specificity. Under natural conditions, host-specific effects of fungi on seed germination may further differentiate plant species niche in germination response, with a potential of promoting coexistence. Both host-specific and non-host-specific effects of fungi on seed loss may induce polarizing effects on plant coexistence depending on the ecological context. The coexistence theory provides a clear framework to interpret these polarizing effects. Moreover, fungi pathogenic to both plant species could induce host-specific germination, which challenges the theoretical assumption of density-independent germination response. These implications from an in vitro study underscore the need to weave theoretical modeling, reductive empirical experiments, and natural observations to illuminate effects of soil-borne fungi on coexistence of annual plant species in variable desert environments.

Black Sigatoka in bananas: Ecoclimatic suitability and disease pressure assessments.

Black leaf streak disease, or black Sigatoka, is caused by the fungus Pseudocercospora fijiensis, and has been identified as a major constraint to global production of banana and plantain. We fitted a climatic niche model (CLIMEX) for P. fijiensis to gain an understanding of the patterns of climate suitability, and hence hazard from this disease. We then calibrated the climate suitability patterns against the results of an expert elicitation of disease pressure patterns. We found a moderately strong non-linear relationship between modelled climate suitability for P.°fijiensis and the expert ratings for disease pressure. The strength of the relationship provides a cross-validation between the CLIMEX model and the expert elicitation process. The bulk of global banana production experiences high potential threat from P. fijiensis, and the higher yielding areas for banana and plantain production are at greatest threat. By explicitly considering the role of irrigation we have been able to identify how strategic irrigation could be used to support banana production in areas that are at low risk from P. fijiensis.

Variation and correlations between sexual, asexual and natural enemy resistance life-history traits in a natural plant pathogen population.

BACKGROUND: Understanding the mechanisms by which diversity is maintained in pathogen populations is critical for epidemiological predictions. Life-history trade-offs have been proposed as a hypothesis for explaining long-term maintenance of variation in pathogen populations, yet the empirical evidence supporting trade-offs has remained mixed. This is in part due to the challenges of documenting successive pathogen life-history stages in many pathosystems. Moreover, little is understood of the role of natural enemies of pathogens on their life-history evolution. RESULTS: We characterize life-history-trait variation and possible trade-offs in fungal pathogen Podosphaera plantaginis infecting the host plant Plantago lanceolata. We measured the timing of both asexual and sexual stages, as well as resistance to a hyperparasite of seven pathogen strains that vary in their prevalence in nature. We find significant variation among the strains in their life-history traits that constitute the infection cycle, but no evidence for trade-offs among pathogen development stages, apart from fast pathogen growth coninciding with fast hyperparasite growth. Also, the seemingly least fit pathogen strain was the most prevalent in the nature. CONCLUSIONS: We conclude that in the nature environmental variation, and interactions with the antagonists of pathogens themselves may maintain variation in pathogen populations.

Variable opportunities for outcrossing result in hotspots of novel genetic variation in a pathogen metapopulation.

Many pathogens possess the capacity for sex through outcrossing, despite being able to reproduce also asexually and/or via selfing. Given that sex is assumed to come at a cost, these mixed reproductive strategies typical of pathogens have remained puzzling. While the ecological and evolutionary benefits of outcrossing are theoretically well-supported, support for such benefits in pathogen populations are still scarce. Here, we analyze the epidemiology and genetic structure of natural populations of an obligate fungal pathogen, Podosphaera plantaginis. We find that the opportunities for outcrossing vary spatially. Populations supporting high levels of coinfection -a prerequisite of sex - result in hotspots of novel genetic diversity. Pathogen populations supporting coinfection also have a higher probability of surviving winter. Jointly our results show that outcrossing has direct epidemiological consequences as well as a major impact on pathogen population genetic diversity, thereby providing evidence of ecological and evolutionary benefits of outcrossing in pathogens.

Arbuscular Mycorrhizal Fungi Alter the Community Structure of Ammonia Oxidizers at High Fertility via Competition for Soil NH4.

Nitrification represents a central process in the cycling of nitrogen (N) which in high-fertility habitats can occasionally be undesirable. Here, we explore how arbuscular mycorrhiza (AM) impacts nitrification when N availability is not limiting to plant growth. We wanted to test which of the mechanisms that have been proposed in the literature best describes how AM influences nitrification. We manipulated the growth settings of Plantago lanceolata so that we could control the mycorrhizal state of our plants. AM induced no changes in the potential nitrification rates or the estimates of ammonium oxidizing (AO) bacteria. However, we could observe a moderate shift in the community of ammonia-oxidizers, which matched the shift we saw when comparing hyphosphere to rhizosphere soil samples and mirrored well changes in the availability of ammonium in soil. We interpret our results as support that it is competition for N that drives the interaction between AM and AO. Our experiment sheds light on an understudied interaction which is pertinent to typical management practices in agricultural systems.

Multiscale patterns and drivers of arbuscular mycorrhizal fungal communities in the roots and root-associated soil of a wild perennial herb.

Arbuscular mycorrhizal (AM) fungi form diverse communities and are known to influence above-ground community dynamics and biodiversity. However, the multiscale patterns and drivers of AM fungal composition and diversity are still poorly understood. We sequenced DNA markers from roots and root-associated soil from Plantago lanceolata plants collected across multiple spatial scales to allow comparison of AM fungal communities among neighbouring plants, plant subpopulations, nearby plant populations, and regions. We also measured soil nutrients, temperature, humidity, and community composition of neighbouring plants and nonAM root-associated fungi. AM fungal communities were already highly dissimilar among neighbouring plants (c. 30 cm apart), albeit with a high variation in the degree of similarity at this small spatial scale. AM fungal communities were increasingly, and more consistently, dissimilar at larger spatial scales. Spatial structure and environmental drivers explained a similar percentage of the variation, from 7% to 25%. A large fraction of the variation remained unexplained, which may be a result of unmeasured environmental variables, species interactions and stochastic processes. We conclude that AM fungal communities are highly variable among nearby plants. AM fungi may therefore play a major role in maintaining small-scale variation in community dynamics and biodiversity.

Effect of spatial connectivity on host resistance in a highly fragmented natural pathosystem.

Both theory and experimental evolution studies predict migration to influence the outcome of antagonistic coevolution between hosts and their parasites, with higher migration rates leading to increased diversity and evolutionary potential. Migration rates are expected to vary in spatially structured natural pathosystems, yet how spatial structure generates variation in coevolutionary trajectories across populations occupying the same landscape has not been tested. Here, we studied the effect of spatial connectivity on host evolutionary potential in a natural pathosystem characterized by a stable Plantago lanceolata host network and a highly dynamic Podosphaera plantaginis parasite metapopulation. We designed a large inoculation experiment to test resistance of five isolated and five well-connected host populations against sympatric and allopatric pathogen strains, over 4 years. Contrary to our expectations, we did not find consistently higher resistance against sympatric pathogen strains in the well-connected populations. Instead, host local adaptation varied considerably among populations and through time with greater fluctuations observed in the well-connected populations. Jointly, our results suggest that in populations where pathogens have successfully established, they have the upper hand in the coevolutionary arms race, but hosts may be better able to respond to pathogen-imposed selection in the well-connected than in the isolated populations. Hence, the ongoing and extensive fragmentation of natural habitats may increase vulnerability to diseases.

Xanthomonas campestris pv. musacearum: a major constraint to banana, plantain and enset production in central and east Africa over the past decade.

TAXONOMY: Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; currently classified as X. campestris pv. musacearum (Xcm). However, fatty acid methyl ester analysis and genetic and genomic evidence suggest that this pathogen is X. vasicola and resides in a separate pathovar. ISOLATION AND DETECTION: Xcm can be isolated on yeast extract peptone glucose agar (YPGA), cellobiose cephalexin agar and yeast extract tryptone sucrose agar (YTSA) complemented with 5-fluorouracil, cephalexin and cycloheximide to confer semi-selectivity. Xcm can also be identified using direct antigen coating enzyme-linked immunosorbent assay (DAC-ELISA), species-specific polymerase chain reaction (PCR) using GspDm primers and lateral flow devices that detect latent infections. HOST RANGE: Causes Xanthomonas wilt on plants belonging to the Musaceae, primarily banana (Musa acuminata), plantain (M. acuminata × balbisiana) and enset (Ensete ventricosum). DIVERSITY: There is a high level of genetic homogeneity within Xcm, although genome sequencing has revealed two major sublineages. SYMPTOMS: Yellowing and wilting of leaves, premature fruit ripening and dry rot, bacterial exudate from cut stems. DISTRIBUTION: Xcm has only been found in African countries, namely Burundi, Ethiopia, Democratic Republic of the Congo, Kenya, Rwanda, Tanzania and Uganda. ECOLOGY AND EPIDEMIOLOGY: Xcm is transmitted by insects, bats, birds and farming implements. Long-distance dispersal of the pathogen is by the transportation of latently infected plants into new areas. MANAGEMENT: The management of Xcm has relied on cultural practices that keep the pathogen population at tolerable levels. Biotechnology programmes have been successful in producing resistant banana plants. However, the deployment of such genetic material has not as yet been achieved in farmers' fields, and the sustainability of transgenic resistance remains to be addressed.

Ancylobacter pratisalsi sp. nov. with plant growth promotion abilities from the rhizosphere of Plantago winteri Wirtg.

A Gram-negative bacterium, designated E130T, was isolated from rhizospheric soil of Plantago winteri Wirtg. from a natural salt meadow as part of an investigation on rhizospheric bacteria from salt-resistant plant species and evaluation of their plant growth-promoting abilities. Cells were rods, non-motile, aerobic, and oxidase and catalase positive, grew in a temperature range of between 4 and 37 °C, and in the presence of 0.5-5 % NaCl (w/v). Based on 16S rRNA gene sequence analysis, strain E130T is affiliated within the genus Ancylobacter, sharing the highest similarity with Ancylobacter rudongensis DSM 17131T (97.6 %), Ancylobacter defluvii CCUG 63806T (97.5 %) and Ancylobacter dichloromethanicus DSM 21507T (97.4 %). The DNA G+C content of strain E130T was 65.1 mol%. Its respiratory quinones were Q-9 and Q-10 and its major polar lipids comprised phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine and unidentified phospholipid. Major fatty acids of the strains E130T were C12 : 0, C16 : 0, C18 : 1ω7c and C19 : 0cycloω8c. The DNA-DNA relatedness of E130T to A. rudongensis DSM 17131T, A. defluvii CCUG 63806T and A. dichloromethanicus DSM 21507T was 29.2, 21.2 and 32.2 % respectively. On the basis of our polyphasic taxonomic study the new isolate represents a novel species, for which the name Ancylobacter pratisalsi sp. nov. is proposed. The type strain is E130T (LMG 29367T=DSM 102029T).

Host resistance and pathogen aggressiveness are key determinants of coinfection in the wild.

Coinfection, whereby the same host is infected by more than one pathogen strain, may favor faster host exploitation rates as strains compete for the same limited resources. Hence, coinfection is expected to have major consequences for pathogen evolution, virulence, and epidemiology. Theory predicts genetic variation in host resistance and pathogen infectivity to play a key role in how coinfections are formed. The limited number of studies available has demonstrated coinfection to be a common phenomenon, but little is known about how coinfection varies in space, and what its determinants are. Our aim is to understand how variation in host resistance and pathogen infectivity and aggressiveness contribute to how coinfections are formed in the interaction between fungal pathogen Podosphaera plantaginis and Plantago lanceolata. Our phenotyping study reveals that more aggressive strains are more likely to form coinfections than less aggressive strains in the natural populations. In the natural populations most of the variation in coinfection is found at the individual plant level, and results from a common garden study confirm the prevalence of coinfection to vary significantly among host genotypes. These results show that genetic variation in both the host and pathogen populations are key determinants of coinfection in the wild.

Influence of arbuscular mycorrhizal stage and plant age on the performance of a generalist aphid.

Host plant quality for herbivores is modulated by different factors including symbiosis with soil organisms, such as arbuscular mycorrhiza (AM), as well as plant age. However, the role of the developmental stage of the AM in such plant-microbe-herbivore interactions has been neglected. To investigate the effects of AM stage and plant age on aphid performance, individuals of the generalist Myzus persicae were reared on leaves of non-mycorrhized (NM) or mycorrhized (AM) Plantago lanceolata plants at two time points, on young plants (with a minor established AM) and on older plants (with a well-established AM), respectively. Various performance traits were measured in the first aphid generation. Additionally, the body mass of the offspring was recorded to determine effects of previous infestation on the next generation. At the end of the herbivore experiments, plant carbon (C), nitrogen (N) and phosphorus (P) as well as leaf mass per area (LMA) were analyzed as measures of plant quality. Developmental performance traits of the aphids were either affected by AM and time point (nymph body mass at day 6) or by the interaction of both (relative growth rate). However, body mass at day 10 and reproductive performance traits were lower on older plants, independent of AM treatment. In line with these results, host plant quality changed little due to AM but strongly with age, with decreases in leaf N and P, but increases in C and LMA. Furthermore, nymphs gained a higher body mass when feeding on previously infested plants compared to their parents that started to feed on non-infested hosts, likely due to an aphid-induced modification in host traits. In summary, our results indicate that effects of both the developmental stage of AM and the plant age on aphid performance change throughout aphid development and between generations, attributable to shifts in plant quality.

Spatial variation in soil biota mediates plant adaptation to a foliar pathogen.

Theory suggests that below-ground spatial heterogeneity may mediate host-parasite evolutionary dynamics and patterns of local adaptation, but this has rarely been tested in natural systems. Here, we test experimentally for the impact of spatial variation in the abiotic and biotic soil environment on the evolutionary outcome of the interaction between the host plant Plantago lanceolata and its specialist foliar pathogen Podosphaera plantaginis. Plants showed no adaptation to the local soil environment in the absence of natural enemies. However, quantitative, but not qualitative, plant resistance against local pathogens was higher when plants were grown in their local field soil than when they were grown in nonlocal field soil. This pattern was robust when extending the spatial scale beyond a single region, but disappeared with soil sterilization, indicating that soil biota mediated plant adaptation. We conclude that below-ground biotic heterogeneity mediates above-ground patterns of plant adaptation, resulting in increased plant resistance when plants are grown in their local soil environment. From an applied perspective, our findings emphasize the importance of using locally selected seeds in restoration ecology and low-input agriculture.

Local adaptation at higher trophic levels: contrasting hyperparasite-pathogen infection dynamics in the field and laboratory.

Predicting and controlling infectious disease epidemics is a major challenge facing the management of agriculture, human and wildlife health. Co-evolutionarily derived patterns of local adaptation among pathogen populations have the potential to generate variation in disease epidemiology; however, studies of local adaptation in disease systems have mostly focused on interactions between competing pathogens or pathogens and their hosts. In nature, parasites and pathogens are also subject to attack by hyperparasitic natural enemies that can severely impact upon their infection dynamics. However, few studies have investigated whether this interaction varies across combinations of pathogen-hyperparasite strains, and whether this influences hyperparasite incidence in natural pathogen populations. Here, we test whether the association between a hyperparasitic fungus, Ampelomyces, and a single powdery mildew host, Podosphaera plantaginis, varies among genotype combinations, and whether this drives hyperparasite incidence in nature. Laboratory inoculation studies reveal that genotype, genotype × genotype interactions and local adaptation affect hyperparasite infection. However, observations of a natural pathogen metapopulation reveal that spatial rather than genetic factors predict the risk of hyperparasite presence. Our results highlight how sensitive the outcome of biocontrol using hyperparasites is to selection of hyperparasite strains.

Effects of Arbuscular Mycorrhiza on Plant Chemistry and the Development and Behavior of a Generalist Herbivore.

Arbuscular mycorrhiza (AM) formed between plants and AM fungi (AMF) can alter host plant quality and thus influence plant-herbivore interactions. While AM is known to affect the development of generalist chewing-biting herbivores, AM-mediated impacts on insect behavior have been neglected until now. In this study, the effects of Rhizophagus irregularis, a generalist AMF, on phenotypic and leaf metabolic traits of Plantago major plants were investigated. Further, the influence of AM-mediated host plant modifications on the development and on seven behavioral traits of larvae of the generalist Mamestra brassicae were recorded. Tests were carried out in the third (L3) and fourth (L4) larval instar, respectively. While shoot water content, specific leaf area, and foliar concentrations of the secondary metabolite aucubin were higher in AM-treated compared to non-mycorrhized (NM) plants, lower concentrations of the primary metabolites citric acid and isocitric acid were found in leaves of AM plants. Larvae reared on AM plants gained a higher body mass and tended to develop faster than individuals reared on NM plants. However, plant treatment had no significant effect on any of the behavioral traits. Instead, differences between larvae of different ages were detected in several behavioral features, with L4 being less active and less bold than L3 larvae. The results demonstrate that AM-induced modifications of host plant quality influence larval development, whereas the behavioral phenotype seems to be more fixed at least under the tested conditions.

Potential of Endophytic Bacterium Paenibacillus sp. PHE-3 Isolated from Plantago asiatica L. for Reduction of PAH Contamination in Plant Tissues.

Endophytes are ubiquitous in plants, and they may have a natural capacity to biodegrade polycyclic aromatic hydrocarbons (PAHs). In our study, a phenanthrene-degrading endophytic Paenibacillus sp. PHE-3 was isolated from P. asiatica L. grown in a PAH-contaminated site. The effects of environmental variables on phenanthrene biodegradation by strain PHE-3 were studied, and the ability of strain PHE-3 to use high molecular weight PAH (HMW-PAH) as a sole carbon source was also evaluated. Our results indicated that pH value of 4.0-8.0, temperature of 30 °C-42 °C, initial phenanthrene concentration less than 100 mg·L(-1), and some additional nutrients are favorable for the biodegradation of phenanthrene by strain PHE-3. The maximum biodegradation efficiency of phenanthrene was achieved at 99.9% after 84 h cultivation with additional glutamate. Moreover, the phenanthrene biodegradation by strain PHE-3 was positively correlated with the catechol 2,3-dioxygenase activity (ρ = 0.981, p < 0.05), suggesting that strain PHE-3 had the capability of degrading HMW-PAHs. In the presence of other 2-, 3-ringed PAHs, strain PHE-3 effectively degraded HMW-PAHs through co-metabolism. The results of this study are beneficial in that the re-colonization potential and PAH degradation performance of endophytic Paenibacillus sp. PHE-3 may be applied towards reducing PAH contamination in plants.

Options of partners improve carbon for phosphorus trade in the arbuscular mycorrhizal mutualism.

The mutualism between plants and arbuscular mycorrhizal fungi (AMF) is widespread and has persisted for over 400 million years. Although this mutualism depends on fair resource exchange between plants and fungi, inequality exists among partners despite mechanisms that regulate trade. Here, we use (33) P and (14) C isotopes and a split-root system to test for preferential allocation and reciprocal rewards in the plant-AMF symbiosis by presenting a plant with two AMF that differ in cooperativeness. We found that plants received more (33) P from less cooperative AMF in the presence of another AMF species. This increase in (33) P resulted in a reduced (14) C cost per unit of (33) P from less cooperative AMF when alternative options were available. Our results indicate that AMF diversity promotes cooperation between plants and AMF, which may be an important mechanism maintaining the evolutionary persistence of and diversity within the plant-AMF mutualism.

Rhizophagus irregularis MUCL 41833 can colonize and improve P uptake of Plantago lanceolata after exposure to ionizing gamma radiation in root organ culture.

Long-lived radionuclides such as (90)Sr and (137)Cs can be naturally or accidentally deposited in the upper soil layers where they emit ß/γ radiation. Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can accumulate and transfer radionuclides from soil to plant, but there have been no studies on the direct impact of ionizing radiation on AMF. In this study, root organ cultures of the AMF Rhizophagus irregularis MUCL 41833 were exposed to 15.37, 30.35, and 113.03 Gy gamma radiation from a (137)Cs source. Exposed spores were subsequently inoculated to Plantago lanceolata seedlings in pots, and root colonization and P uptake evaluated. P. lanceolata seedlings inoculated with non-irradiated AMF spores or with spores irradiated with up to 30.35 Gy gamma radiation had similar levels of root colonization. Spores irradiated with 113.03 Gy gamma radiation failed to colonize P. lanceolata roots. P content of plants inoculated with non-irradiated spores or of plants inoculated with spores irradiated with up to 30.35 Gy gamma radiation was higher than in non-mycorrhizal plants or plants inoculated with spores irradiated with 113.03 Gy gamma radiation. These results demonstrate that spores of R. irregularis MUCL 41833 are tolerant to chronic ionizing radiation at high doses.