Mycorrhizae-Herbivore Interactions: Population and Community Consequences

The mycorrhizal symbiosis alters the plant defence strategy in a model legume plant

Arbuscular mycorrhizal (AM) symbiosis modulates plant-herbivore interactions. Still, how it shapes the overall plant defence strategy and the mechanisms involved remain unclear. We investigated how AM symbiosis simultaneously modulates plant resistance and tolerance to a shoot herbivore, and explored the underlying mechanisms. Bioassays with Medicago truncatula plants were used to study the effect of the AM fungus Rhizophagus irregularis on plant resistance and tolerance to Spodoptera exigua herbivory. By performing molecular and chemical analyses, we assessed the impact of AM symbiosis on herbivore-triggered phosphate (Pi)- and jasmonate (JA)-related responses. Upon herbivory, AM symbiosis led to an increased leaf Pi content by boosting the mycorrhizal Pi-uptake pathway. This enhanced both plant tolerance and herbivore performance. AM symbiosis counteracted the herbivore-triggered JA burst, reducing plant resistance. To disentangle the role of the mycorrhizal Pi-uptake pathway in the plant's response to herbivory, we used the mutant line ha1-2, impaired in the H⁺ -ATPase gene HA1, which is essential for Pi-uptake via the mycorrhizal pathway. We found that mycorrhiza-triggered enhancement of herbivore performance was compromised in ha1-2 plants. AM symbiosis thus affects the defence pattern of M. truncatula by altering resistance and tolerance simultaneously. We propose that the mycorrhizal Pi-uptake pathway is involved in the modulation of the plant defence strategy.

Insect-Microorganism Interaction Has Implicates on Insect Olfactory Systems

Olfaction plays an essential role in various insect behaviors, including habitat selection, access to food, avoidance of predators, inter-species communication, aggregation, and reproduction. The olfactory process involves integrating multiple signals from external conditions and internal physiological states, including living environments, age, physiological conditions, and circadian rhythms. As microorganisms and insects form tight interactions, the behaviors of insects are constantly challenged by versatile microorganisms via olfactory cues. To better understand the microbial influences on insect behaviors via olfactory cues, this paper summarizes three different ways in which microorganisms modulate insect behaviors. Here, we deciphered three interesting aspects of microorganisms-contributed olfaction: (1) How do volatiles emitted by microorganisms affect the behaviors of insects? (2) How do microorganisms reshape the behaviors of insects by inducing changes in the synthesis of host volatiles? (3) How do symbiotic microorganisms act on insects by modulating behaviors?

Plant Growth-Promoting Rhizobacteria Enhance Defense of Strawberry Plants Against Spider Mites

Plants mediate interactions between below- and above-ground microbial and animal communities. Microbial communities of the rhizosphere commonly include mutualistic symbionts such as mycorrhizal fungi, rhizobia and free-living plant growth-promoting rhizobacteria (PGPR) that may influence plant growth and/or its defense system against aboveground pathogens and herbivores. Here, we scrutinized the effects of three PGPR, Azotobacter chroococcum, Azospirillum brasilense, and Pseudomonas brassicacearum, on life history and population dynamics of two-spotted spider mites, Tetranychus urticae, feeding on aboveground tissue of strawberry plants, and examined associated plant growth and physiology parameters. Our experiments suggest that these three species of free-living rhizobacteria strengthen the constitutive, and/or induce the direct, anti-herbivore defense system of strawberry plants. All three bacterial species exerted adverse effects on life history and population dynamics of T. urticae and positive effects on flowering and physiology of whole strawberry plants. Spider mites, in each life stage and in total, needed longer time to develop on PGPR-treated plants and had lower immature survival rates than those fed on chemically fertilized and untreated plants. Reduced age-specific fecundity, longer developmental time and lower age-specific survival rates of mites feeding on rhizobacteria treated plants reduced their intrinsic rate of increase as compared to mites feeding on chemically fertilized and control plants. The mean abundance was lower in spider mite populations feeding on PGPR-treated strawberries than in those feeding on chemically fertilized and untreated plants. We argue that the three studied PGPR systemically strengthened and/or induced resistance in above-ground plant parts and enhanced the level of biochemical anti-herbivore defense. This was probably achieved by inducing or upregulating the production of secondary plant metabolites, such as phenolics, flavonoids and anthocyanins, which were previously shown to be involved in induced systemic resistance of strawberry plants. Overall, our study emphasizes that PGPR treatment can be a favorable strawberry plant cultivation measure because providing essential nutrients needed for proper plant growth and at the same time decreasing the life history performance and population growth of the notorious herbivorous pest T. urticae.

The effects of mycorrhizal colonization on phytophagous insects and their natural enemies in soybean fields

The use of belowground microorganisms in agriculture, with the aim to stimulate plant growth and improve crop yields, has recently gained interest. However, few studies have examined the effects of microorganism inoculation on higher trophic levels in natural conditions. We examined how the diversity of phytophagous insects and their natural enemies responded to the field-inoculation of soybean with a model arbuscular mycorrhizal fungus (AMF), Rhizophagus irregularis, combined with a nitrogen-fixing bacterium, Bradyrhizobium japonicum, and a plant growth-promoting bacterium, Bacillus pumilus. We also investigate if the absence or presence of potassium fertilizer can affect this interaction. We found an increase in the abundance of piercing-sucking insects with the triple inoculant irrespective of potassium treatment, whereas there were no differences among treatments for other insect groups. A decrease in the abundance of the soybean aphid, Aphis glycines, with the double inoculant Rhizophagus + Bradyrhizobium was observed in potassium enriched plots and in the abundance of Empoasca spp. with potassium treatment independent of inoculation type. Although it was not possible to discriminate the mycorrhization realized by inoculum from that of the indigenous AMF in the field, we confirmed global negative effects of overall mycorrhizal colonization on the abundance of phytophagous piercing-sucking insects, phytophagous chewing insects, and the alpha diversity of phytophagous insects. In perspective, the use of AMF/Rhizobacteria inoculants in the field should focus on the identity and performance of strains to better understand their impact on insects.

Seasonal variation in the response of arbuscular mycorrhizal fungi to grazing intensity

Despite existing evidence of pronounced seasonality in arbuscular mycorrhizal (AM) fungal communities, little is known about the ecology of AM fungi in response to grazing intensity in different seasons. Here, we assessed AM fungal abundance, represented by soil hyphal length density (HLD), mycorrhizal root colonization intensity (MI), and arbuscule intensity (AI) throughout three seasons (spring, summer, autumn) in a farm-scale field experiment in typical, grazed steppe vegetation in northern China. Seven levels of field-manipulated, grazing intensities had been maintained for over 13 years within two topographies, flat and slope. We also measured soil nutrients and carbon content throughout the growing season to investigate whether seasonal variation in AM fungal abundance was related to seasonal shifts in soil resource availability along the grazing gradient. We further examined the association between AM fungal metrics in the different grazing treatments through the growing season. Our results showed a pronounced seasonal shift in HLD but there was no clear seasonality in MI and AI. HLD was significantly negatively related to grazing intensity over the course of the growing season from spring to autumn. However, MI and AI were related negatively to grazing intensity only in spring. In addition, differential responses of AM fungal abundance to grazing intensity at the two topographical sites were detected. No strong evidence was found for associations between AM fungal abundance and soil resource availability. Moreover, AM fungal internal and external abundance were correlated positively under the different grazing intensities throughout the growing season. Overall, our study suggests that external AM fungal structures in soil were more responsive to seasonal variation and grazing than internal structures in roots. The findings also suggest that early grazing may be detrimental to AM fungal root colonization of newly emerged plants.

Unraveling spatiotemporal variability of arbuscular mycorrhizal fungi in a temperate grassland plot

Soils provide a heterogeneous environment varying in space and time; consequently, the biodiversity of soil microorganisms also differs spatially and temporally. For soil microbes tightly associated with plant roots, such as arbuscular mycorrhizal fungi (AMF), the diversity of plant partners and seasonal variability in trophic exchanges between the symbionts introduce additional heterogeneity. To clarify the impact of such heterogeneity, we investigated spatiotemporal variation in AMF diversity on a plot scale (10 × 10 m) in a grassland managed at low intensity in southwest Germany. AMF diversity was determined using 18S rDNA pyrosequencing analysis of 360 soil samples taken at six time points within a year. We observed high AMF alpha- and beta-diversity across the plot and at all investigated time points. Relationships were detected between spatiotemporal variation in AMF OTU richness and plant species richness, root biomass, minimal changes in soil texture and pH. The plot was characterized by high AMF turnover rates with a positive spatiotemporal relationship for AMF beta-diversity. However, environmental variables explained only ≈20% of the variation in AMF communities. This indicates that the observed spatiotemporal richness and community variability of AMF was largely independent of the abiotic environment, but related to plant properties and the cooccurring microbiome.

Mycorrhiza-Induced Resistance in Potato Involves Priming of Defense Responses Against Cabbage Looper (Noctuidae: Lepidoptera)

Most plants form mutualistic associations with arbuscular mycorrhizal (AM) fungi that are ubiquitous in soils. Through this symbiosis, plants can withstand abiotic and biotic stresses. The underlying molecular mechanisms involved in mediating mycorrhiza-induced resistance against insects needs further research, and this is particularly true for potato (Solanum tuberosum L. (Solanales: Solanaceae)), which is the fourth most important crop worldwide. In this study, the tripartite interaction between potato, the AM fungus Rhizophagus irregularis (Glomerales: Glomeraceae), and cabbage looper (Trichoplusia ni Hübner) (Lepidoptera: Noctuidae) was examined to determine whether potato exhibits mycorrhiza-induced resistance against this insect. Plant growth, insect fitness, AM fungal colonization of roots, and transcript levels of defense-related genes were measured in shoots and roots after 5 and 8 d of herbivory on mycorrhizal and nonmycorrhizal plants. AM fungal colonization of roots did not have an effect on potato growth, but root colonization levels increased by herbivory. Larval weight gain was reduced after 8 d of feeding on mycorrhizal plants compared with nonmycorrhizal plants. Systemic upregulation of Allene Oxide Synthase 1 (AOS1), 12-Oxo-Phytodienoate Reductase 3 (OPR3) (jasmonic acid pathway), Protease Inhibitor Type I (PI-I) (anti-herbivore defense), and Phenylalanine Ammonia Lyase (PAL) transcripts (phenylpropanoid pathway) was found during the tripartite interaction. Together, these findings suggest that potato may exhibit mycorrhiza-induced resistance to cabbage looper by priming anti-herbivore defenses aboveground. This study illustrates how mycorrhizal potato responds to herbivory by a generalist-chewing insect and serves as the basis for future studies involving tripartite interactions with other pests.

Influence of leaf damage by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimić) on mycorrhiza of Aesculus hippocastanum L

In many parts of Europe, the white horse chestnut (Aesculus hippocastanum L.) has been attacked by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimić), which causes premature leaf dieback. A. hippocastanum L. establishes mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi. This study involved a comparison of mature A. hippocastanum individuals susceptible to C. ohridella and individuals resistant to this insect after a one-time treatment with a chemical preparation injected into the tree trunks 7 years before the investigation began. Concentration of macronutrients in soil and the activity of soil nonspecific dehydrogenase did not differ between soils under canopies of the treated and untreated trees. Concentrations of C and N were significantly higher in leaves of the treated than those of the untreated trees. The infestation by C. ohridella and defoliation of leaves of the untreated trees did not significantly influence the frequency and intensity of AM colonization compared to the chemically treated trees, although a tendency towards higher average AM colonization of roots of the untreated trees, infested by the herbivores, than roots of the non-infested trees was observed. The results also indicated a tendency for higher biomass of fine roots per soil volume under the trees treated against C. ohridella than under the trees invaded by the insect.

Biodiversity of arbuscular mycorrhizal fungi and ecosystem function

Contents Summary 1059 I. Introduction: pathways of influence and pervasiveness of effects 1060 II. AM fungal richness effects on ecosystem functions 1062 III. Other dimensions of biodiversity 1062 IV. Back to basics - primary axes of niche differentiation by AM fungi 1066 V. Functional diversity of AM fungi - a role for biological stoichiometry? 1067 VI. Past, novel and future ecosystems 1068 VII. Opportunities and the way forward 1071 Acknowledgements 1072 References 1072 SUMMARY: Arbuscular mycorrhizal (AM) fungi play important functional roles in ecosystems, including the uptake and transfer of nutrients, modification of the physical soil environment and alteration of plant interactions with other biota. Several studies have demonstrated the potential for variation in AM fungal diversity to also affect ecosystem functioning, mainly via effects on primary productivity. Diversity in these studies is usually characterized in terms of the number of species, unique evolutionary lineages or complementary mycorrhizal traits, as well as the ability of plants to discriminate among AM fungi in space and time. However, the emergent outcomes of these relationships are usually indirect, and thus context dependent, and difficult to predict with certainty. Here, we advocate a fungal-centric view of AM fungal biodiversity-ecosystem function relationships that focuses on the direct and specific links between AM fungal fitness and consequences for their roles in ecosystems, especially highlighting functional diversity in hyphal resource economics. We conclude by arguing that an understanding of AM fungal functional diversity is fundamental to determine whether AM fungi have a role in the exploitation of marginal/novel environments (whether past, present or future) and highlight avenues for future research.

Mycorrhizal interactions do not influence plant-herbivore interactions in populations of Clarkia xantiana ssp. xantiana spanning from center to margin of the geographic range

Multispecies interactions can be important to the expression of phenotypes and in determining patterns of individual fitness in nature. Many plants engage in symbiosis with arbuscular mycorrhizal fungi (AMF), but the extent to which AMF modulate other species interactions remains poorly understood. We examined multispecies interactions among plants, AMF, and insect herbivores under drought stress using a greenhouse experiment and herbivore choice assays. The experiment included six populations of Clarkia xantiana (Onagraceae), which span a complex environmental gradient in the Southern Sierra Nevada of California. Clarkia xantiana's developing fruits are commonly attacked by grasshoppers at the end of the growing season, and the frequency of attack is more common in populations from the range center than range margin. We found that AMF negatively influenced all metrics of plant growth and reproduction across all populations, presumably because plants supplied carbon to AMF but did not benefit substantially from resources potentially supplied by the AMF. The fruits of plants infected with AMF did not differ from those without AMF in their resistance to grasshoppers. There was significant variation among populations in damage from herbivores but did not reflect the center-to-margin pattern of herbivory observed in the field. In sum, our results do not support the view that AMF interactions modulate plant-herbivore interactions in this system.

Long-term effects of grazing and topography on extra-radical hyphae of arbuscular mycorrhizal fungi in semi-arid grasslands

Grazing and topography have drastic effects on plant communities and soil properties. These effects are thought to influence arbuscular mycorrhizal (AM) fungi. However, the simultaneous impacts of grazing pressure (sheep ha^-1) and topography on plant and soil factors and their relationship to the production of extra-radical AM hyphae are not well understood. Our 10-year study assessed relationships between grazing, plant species richness, aboveground plant productivity, soil nutrients, edaphic properties, and AM hyphal length density (HLD) in different topographic areas (flat or sloped). We found HLD linearly declined with increasing grazing pressure (1.5-9.0 sheep ha^-1) in sloped areas, but HLD was greatest at moderate grazing pressure (4.5 sheep ha^-1) in flat areas. Structural equation modeling indicates grazing reduces HLD by altering soil nutrient dynamics in sloped areas, but non-linearly influences HLD through plant community and edaphic changes in flat areas. Our findings highlight how topography influences key plant and soil factors, thus regulating the effects of grazing pressure on extra-radical hyphal production of AM fungi in grasslands. Understanding how grazing and topography influence AM fungi in semi-arid grasslands is vital, as globally, severe human population pressure and increasing demand for food aggravate the grazing intensity in grasslands.

Complex effects of mammalian grazing on extramatrical mycelial biomass in the Scandes forest-tundra ecotone

Mycorrhizal associations are widespread in high-latitude ecosystems and are potentially of great importance for global carbon dynamics. Although large herbivores play a key part in shaping subarctic plant communities, their impact on mycorrhizal dynamics is largely unknown. We measured extramatrical mycelial (EMM) biomass during one growing season in 16-year-old herbivore exclosures and unenclosed control plots (ambient), at three mountain birch forests and two shrub heath sites, in the Scandes forest-tundra ecotone. We also used high-throughput amplicon sequencing for taxonomic identification to investigate differences in fungal species composition. At the birch forest sites, EMM biomass was significantly higher in exclosures (1.36 ± 0.43 g C/m2) than in ambient conditions (0.66 ± 0.17 g C/m2) and was positively influenced by soil thawing degree-days. At the shrub heath sites, there was no significant effect on EMM biomass (exclosures: 0.72 ± 0.09 g C/m2; ambient plots: 1.43 ± 0.94). However, EMM biomass was negatively related to Betula nana abundance, which was greater in exclosures, suggesting that grazing affected EMM biomass positively. We found no significant treatment effects on fungal diversity but the most abundant ectomycorrhizal lineage/cortinarius, showed a near-significant positive effect of herbivore exclusion (p = .08), indicating that herbivory also affects fungal community composition. These results suggest that herbivory can influence fungal biomass in highly context-dependent ways in subarctic ecosystems. Considering the importance of root-associated fungi for ecosystem carbon balance, these findings could have far-reaching implications.

Chemoprofile and functional diversity of fungal and bacterial endophytes and role of ecofactors - A review

Endophytes represent a hidden world within plants. Almost all plants that are studied harbor one or more endophytes, which help their host to survive against pathogens and changing adverse environmental conditions. Fungal and bacterial endophytes with distinct ecological niches show important biological activities and ecological functions. Their unique physiological and biochemical characteristics lead to the production of niche specific secondary metabolites that may have pharmacological potential. Identification of specific secondary metabolites in adverse environment can also help us in understanding mechanisms of host tolerance against stress condition such as biological invasions, salt, drought, temperature. These metabolites include micro as well as macromolecules, which they produce through least studied yet surprising mechanisms like xenohormesis, toxin-antitoxin system, quorum sensing. Therefore, future studies should focus on unfolding all the underlying molecular mechanisms as well as the impact of physical and biochemical environment of a specific host over endophytic function and metabolite elicitation. Need of the hour is to reshape the focus of research over endophytes and scientifically drive their ecological role toward prospective pharmacological as well as eco-friendly biological applications. This may help to manage these endophytes especially from untapped ecoregions as a useful undying biological tool to meet the present challenges as well as lay a strong and logical basis for any impending challenges.

Arbuscular mycorrhizal fungus responses to disturbance are context-dependent

Anthropogenic disturbance is one of the most important forces shaping soil ecosystems. While organisms that live in the soil, such as arbuscular mycorrhizal (AM) fungi, are sensitive to disturbance, their response is not always predictable. Given the range of disturbance types and differences among AM fungi in their growth strategies, the unpredictability of the responses of AM fungi to disturbance is not surprising. We investigated the role of disturbance type (i.e., soil disruption, agriculture, host perturbation, and chemical disturbance) and fungus identity on disturbance response in the AM symbiosis. Using meta-analysis, we found evidence for differential disturbance response among AM fungal species, as well as evidence that particular fungal species are especially susceptible to certain disturbance types, perhaps because of their life history strategies.

Interrelated effects of mycorrhiza and free-living nitrogen fixers cascade up to aboveground herbivores

Aboveground plant performance is strongly influenced by belowground microorganisms, some of which are pathogenic and have negative effects, while others, such as nitrogen-fixing bacteria and arbuscular mycorrhizal fungi, usually have positive effects. Recent research revealed that belowground interactions between plants and functionally distinct groups of microorganisms cascade up to aboveground plant associates such as herbivores and their natural enemies. However, while functionally distinct belowground microorganisms commonly co-occur in the rhizosphere, their combined effects, and relative contributions, respectively, on performance of aboveground plant-associated organisms are virtually unexplored. Here, we scrutinized and disentangled the effects of free-living nitrogen-fixing (diazotrophic) bacteria Azotobacter chroococcum (DB) and arbuscular mycorrhizal fungi Glomus mosseae (AMF) on host plant choice and reproduction of the herbivorous two-spotted spider mite Tetranychus urticae on common bean plants Phaseolus vulgaris. Additionally, we assessed plant growth, and AMF and DB occurrence and density as affected by each other. Both AMF alone and DB alone increased spider mite reproduction to similar levels, as compared to the control, and exerted additive effects under co-occurrence. These effects were similarly apparent in host plant choice, that is, the mites preferred leaves from plants with both AMF and DB to plants with AMF or DB to plants grown without AMF and DB. DB, which also act as AMF helper bacteria, enhanced root colonization by AMF, whereas AMF did not affect DB abundance. AMF but not DB increased growth of reproductive plant tissue and seed production, respectively. Both AMF and DB increased the biomass of vegetative aboveground plant tissue. Our study breaks new ground in multitrophic belowground-aboveground research by providing first insights into the fitness implications of plant-mediated interactions between interrelated belowground fungi-bacteria and aboveground herbivores.

Putative linkages between below- and aboveground mutualisms during alien plant invasions

Evidence of the fundamental role of below-aboveground links in controlling ecosystem processes is mostly based on studies done with soil herbivores or mutualists and aboveground herbivores. Much less is known about the links between belowground and aboveground mutualisms, which have been studied separately for decades. It has not been until recently that these mutualisms-mycorrhizas and legume-rhizobia on one hand, and pollinators and seed dispersers on the other hand-have been found to influence each other, with potential ecological and evolutionary consequences. Here we review the mechanisms that may link these two-level mutualisms, mostly reported for native plant species, and make predictions about their relevance during alien plant invasions. We propose that alien plants establishing effective mutualisms with belowground microbes might improve their reproductive success through positive interactions between those mutualists and pollinators and seed dispersers. On the other hand, changes in the abundance and diversity of soil mutualists induced by invasion can also interfere with below-aboveground links for native plant species. We conclude that further research on this topic is needed in the field of invasion ecology as it can provide interesting clues on synergistic interactions and invasional meltdowns during alien plant invasions.

Convergence in mycorrhizal fungal communities due to drought, plant competition, parasitism, and susceptibility to herbivory: consequences for fungi and host plants

Plants and mycorrhizal fungi influence each other's abundance, diversity, and distribution. How other biotic interactions affect the mycorrhizal symbiosis is less well understood. Likewise, we know little about the effects of climate change on the fungal component of the symbiosis or its function. We synthesized our long-term studies on the influence of plant parasites, insect herbivores, competing trees, and drought on the ectomycorrhizal fungal communities associated with a foundation tree species of the southwestern United States, pinyon pine (Pinus edulis), and described how these changes feed back to affect host plant performance. We found that drought and all three of the biotic interactions studied resulted in similar shifts in ectomycorrhizal fungal community composition, demonstrating a convergence of the community towards dominance by a few closely related fungal taxa. Ectomycorrhizal fungi responded similarly to each of these stressors resulting in a predictable trajectory of community disassembly, consistent with ecological theory. Although we predicted that the fungal communities associated with trees stressed by drought, herbivory, competition, and parasitism would be poor mutualists, we found the opposite pattern in field studies. Our results suggest that climate change and the increased importance of herbivores, competitors, and parasites that can be associated with it, may ultimately lead to reductions in ectomycorrhizal fungal diversity, but that the remaining fungal community may be beneficial to host trees under the current climate and the warmer, drier climate predicted for the future.

Contrasting impacts of defoliation on root colonization by arbuscular mycorrhizal and dark septate endophytic fungi of Medicago sativa

Individual plants typically interact with multiple mutualists and enemies simultaneously. Plant roots encounter both arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi, while the leaves are exposed to herbivores. AMF are usually beneficial symbionts, while the functional role of DSE is largely unknown. Leaf herbivory may have a negative effect on root symbiotic fungi due to decreased carbon availability. However, evidence for this is ambiguous and no inoculation-based experiment on joint effects of herbivory on AM and DSE has been done to date. We investigated how artificial defoliation impacts root colonization by AM (Glomus intraradices) and DSE (Phialocephala fortinii) fungi and growth of Medicago sativa host in a factorial laboratory experiment. Defoliation affected fungi differentially, causing a decrease in arbuscular colonization and a slight increase in DSE-type colonization. However, the presence of one fungal species had no effect on colonization by the other or on plant growth. Defoliation reduced plant biomass, with this effect independent of the fungal treatments. Inoculation by either fungal species reduced root/shoot ratios, with this effect independent of the defoliation treatments. These results suggest AM colonization is limited by host carbon availability, while DSE may benefit from root dieback or exudation associated with defoliation. Reductions in root allocation associated with fungal inoculation combined with a lack of effect of fungi on plant biomass suggest DSE and AMF may be functional equivalent to the plant within this study. Combined, our results indicate different controls of colonization, but no apparent functional consequences between AM and DSE association in plant roots in this experimental setup.

Impact of mycorrhization on the abundance, growth and leaf nutrient status of ferns along a tropical elevational gradient

Mycorrhizal fungi are crucial for the ecological success of land plants, providing their hosts with nutrients in exchange for organic C. However, not all plants are mycorrhizal, especially ferns, of which about one-third of the species lack this symbiosis. Because the mycorrhizal status is evolutionarily ancestral, this lack of mycorrhizae must have ecological advantages, but what these advantages are and how they affect the competitive ability of non-mycorrhizal plants under natural conditions is currently unknown. To address this uncertainty, we studied terrestrial fern assemblages and species abundances as well as their mycorrhization status, leaf nutrient concentration and relative annual growth along an elevational gradient in the Ecuadorian Andes (500-4,000 m). We surveyed the mycorrhizal status of 375 root samples belonging to 85 species, and found mycorrhizae in 89% of the samples. The degree of mycorrhization decreased with elevation but was unrelated to soil nutrients. Species with mycorrhizae were significantly more abundant than non-mycorrhizal species, but non-mycorrhizal species had significantly higher relative growth and concentrations of leaf N, P, Mg, and Ca. Our study thus shows that despite lower abundances, non-mycorrhizal fern species did not appear to be limited in their growth or nutrient supply relative to mycorrhizal ones. As a basis for future studies, we hypothesize that non-mycorrhizal fern species may be favoured in special microhabitats of the forest understory with high soil nutrient or water availability, or that the ecological benefit of mycorrhizae is not related to nutrient uptake but rather to, for example, pathogen resistance.

Elevated CO2 spurs reciprocal positive effects between a plant virus and an arbuscular mycorrhizal fungus

Plants form ubiquitous associations with diverse microbes. These interactions range from parasitism to mutualism, depending partly on resource supplies that are being altered by global change. While many studies have considered the separate effects of pathogens and mutualists on their hosts, few studies have investigated interactions among microbial mutualists and pathogens in the context of global change. Using two wild grass species as model hosts, we grew individual plants under ambient or elevated CO(2), and ambient or increased soil phosphorus (P) supply. Additionally, individuals were grown with or without arbuscular mycorrhizal inoculum, and after 2 wk, plants were inoculated or mock-inoculated with a phloem-restricted virus. Under elevated CO(2), mycorrhizal association increased the titer of virus infections, and virus infection reciprocally increased the colonization of roots by mycorrhizal fungi. Additionally, virus infection decreased plant allocation to root biomass, increased leaf P, and modulated effects of CO(2) and P addition on mycorrhizal root colonization. These results indicate that plant mutualists and pathogens can alter each other's success, and predict that these interactions will respond to increased resource availability and elevated CO(2). Together, our findings highlight the importance of interactions among multiple microorganisms for plant performance under global change.

Infection by mycorrhizal fungi increases natural enemy abundance on tobacco (Nicotiana rustica)

The presence of arbuscular mycorrhizal fungi (AMF) influences plant nutrient uptake, growth, and plant defensive chemistry, thereby directly influencing multi-trophic interactions. Different fungal isolates (genotypes of the same fungal species) have been shown to differ in nutrient uptake ability. Plants infected with different AMF genotypes may vary in foliar nutrient or defensive chemical levels, potentially influencing multi-trophic interactions. Using a completely randomized design, we compared the effect of two isolates of the mycorrhizal fungus Glomus etunicatum W. N. Becker & Gerdemann on silver leaf whitefly (Bemisia argentifolii Bellows & Perring) (Hemiptera: Aleyrodidae) and parasitic wasp (Eretmocerus eremicus Rose & Zolnerowich) (Hymenoptera: Aphelinidae) abundance. Whitefly populations were not influenced by AMF infection. Parasite populations were higher on plants infected with the isolate collected from Georgia, even after controlling for whitefly abundance and plant architecture. We propose that AMF indirectly influences parasite abundance and parasitism through a change in leaf surface chemicals that affect parasitic wasps. Because of the ubiquity of and genetic variation in AMF, multi-trophic interactions are likely to be strongly influenced by belowground processes.

Effect of poplar genotypes on mycorrhizal infection and secreted enzyme activities in mycorrhizal and non-mycorrhizal roots

The impact of ectomycorrhiza formation on the secretion of exoenzymes by the host plant and the symbiont is unknown. Thirty-eight F(1) individuals from an interspecific Populus deltoides (Bartr.)×Populus trichocarpa (Torr. & A. Gray) controlled cross were inoculated with the ectomycorrhizal fungus Laccaria bicolor. The colonization of poplar roots by L. bicolor dramatically modified their ability to secrete enzymes involved in organic matter breakdown or organic phosphorus mobilization, such as N-acetylglucosaminidase, β-glucuronidase, cellobiohydrolase, β-glucosidase, β-xylosidase, laccase, and acid phosphatase. The expression of genes coding for laccase, N-acetylglucosaminidase, and acid phosphatase was studied in mycorrhizal and non-mycorrhizal root tips. Depending on the genes, their expression was regulated upon symbiosis development. Moreover, it appears that poplar laccases or phosphatases contribute poorly to ectomycorrhiza metabolic activity. Enzymes secreted by poplar roots were added to or substituted by enzymes secreted by L. bicolor. The enzymatic activities expressed in mycorrhizal roots differed significantly between the two parents, while it did not differ in non-mycorrhizal roots. Significant differences were found between poplar genotypes for all enzymatic activities measured on ectomycorrhizas except for laccases activity. In contrast, no significant differences were found between poplar genotypes for enzymatic activities of non-mycorrhizal root tips except for acid phosphatase activity. The level of enzymes secreted by the ectomycorrhizal root tips is under the genetic control of the host. Moreover, poplar heterosis was expressed through the enzymatic activities of the fungal partner.

Do arbuscular mycorrhizal fungi with contrasting life-history strategies differ in their responses to repeated defoliation?

Arbuscular mycorrhizal (AM) fungi obligatorily depend on carbon (C) resources provided via the plant and therefore fluctuations in C availability may strongly and differently affect AM fungi with different life-history strategies (LHS). In the present study, we examined the effect of repeated defoliation of in vitro grown barrel medic (Medicago truncatula) on the spore and auxiliary cell (AC) production dynamics of a presumed r-strategist (Glomus intraradices) and a presumed K-strategist (Dentiscutata reticulata). Glomus intraradices modulated the production of spores directly to C availability, showing direct investment in reproduction as expected for r-strategists. In contrast, AC production of D. reticulata was not affected after a single defoliation and thus showed higher resistance to fluctuating C levels, as expected for K-strategists. Our results demonstrate that plant defoliation affects the production of extraradical C storage structures of G. intraradices and D. reticulata differently. Our results contribute towards revealing differences in LHS among AM fungal species, a step further towards understanding their community dynamics in natural ecosystems and agroenvironments.

Fungal endophytes of native grasses decrease insect herbivore preference and performance

Endophytic fungal symbionts of grasses are well known for their protective benefit of herbivory reduction. However, the majority of studies on endophyte-grass symbioses have been conducted on economically important, agricultural species-particularly tall fescue (Lolium arundinaceum) and perennial ryegrass (Lolium perenne)-raising the hypothesis that strong benefits are the product of artificial selection. We examined whether fungal endophytes found in natural populations of native grass species deterred insect herbivores. By testing several native grass-endophyte symbiota, we examined phylogenetic signals in the effects of endophytes on insects and compared the relative importance of herbivore and symbiotum identity in the outcome of the interactions. Preference was assessed using three herbivore species [Spodoptera frugiperda (Lepidoptera), Schistocerca americana (Orthoptera), Rhopalosiphum padi (Hemiptera)] and ten native symbiota, which spanned seven grass genera. We also assessed herbivore performance in a no choice experiment for five native symbiota against S. frugiperda. We compared greenhouse and laboratory trials with natural levels of herbivory measured in experimental field populations. In all cases, we included the agronomic grass species, L. arundinaceum, to compare with results from the native grasses. Both in the field and in experimental trials, herbivores showed a significant preference for endophyte-free plant material for the majority of native grasses, with up to three times lower herbivory for endophyte-symbiotic plants; however, the degree of response depended on the identity of the herbivore species. Endophyte presence also significantly reduced performance of S. frugiperda for the majority of grass species. In contrast, the endophyte in L. arundinaceum had few significant anti-herbivore effects, except for a reduction in herbivory at one of two field sites. Our results demonstrate that the mechanisms by which native symbionts deter herbivores are at least as potent as those in model agricultural systems, despite the absence of artificial selection.

Does mycorrhization influence herbivore-induced volatile emission in Medicago truncatula?

Symbiosis with mycorrhizal fungi substantially impacts secondary metabolism and defensive traits of colonised plants. In the present study, we investigated the influence of mycorrhization (Glomus intraradices) on inducible indirect defences against herbivores using the model legume Medicago truncatula. Volatile emission by mycorrhizal and non-mycorrhizal plants was measured in reaction to damage inflicted by Spodoptera spp. and compared to the basal levels of volatile emission by plants of two different cultivars. Emitted volatiles were recorded using closed-loop stripping and gas chromatography/mass spectrometry. The documented volatile patterns were evaluated using multidimensional scaling to visualise patterns and stepwise linear discriminant analysis to distinguish volatile blends of plants with distinct physiological status and genetic background. Volatile blends emitted by different cultivars of M. truncatula prove to be clearly distinct, whereas mycorrhization only slightly influenced herbivore-induced volatile emissions. Still, the observed differences were sufficient to create classification rules to distinguish mycorrhizal and non-mycorrhizal plants by the volatiles emitted. Moreover, the effect of mycorrhization turned out to be opposed in the two cultivars examined. Root symbionts thus seem to alter indirect inducible defences of M. truncatula against insect herbivores. The impact of this effect strongly depends on the genetic background of the plant and, hence, in part explains the highly contradictory results on tripartite interactions gathered to date.

Genetically based susceptibility to herbivory influences the ectomycorrhizal fungal communities of a foundation tree species

Although recent research indicates that herbivores interact with plant-associated microbes in complex ways, few studies have examined these interactions using a community approach. For example, the impact of herbivory on the community structure of ectomycorrhizal fungi (EMF) is not well known. The influence of host plant genetics on EMF community composition is also poorly understood. We used a study system in which susceptibility to herbivory has a genetic basis and a 20-yr insect removal experiment to examine the influence of chronic herbivory and plant genetics on the EMF community structure of Pinus edulis. We compared EMF communities of herbivore resistant trees, herbivore susceptible trees and herbivore susceptible trees from which herbivores were experimentally removed at two dates 10 yr apart. In both years sampled, resistant and susceptible trees differed significantly in EMF community composition. After 10 yr and 20 yr of herbivore removal, the EMF communities of removal trees were similar to those of susceptible trees, but different from resistant trees. The EMF community composition was more strongly influenced by innate genetic differences in plant traits associated with resistance and susceptibility to herbivory than by indirect effects of herbivory on host plant relationships with ectomycorrhizal fungi.

Endophyte-mediated resistance to herbivores depends on herbivore identity in the wild grass Festuca subverticillata

Understanding factors that affect the context dependency of species interactions has been identified as a critical research area in ecology. The presence of symbionts in host plants can be an important factor influencing the outcome of plant-insect interactions. Similarly, herbivore identity can alter the outcome of plant-symbiont interactions. Symbiotic foliar fungal endophytes confer resistance to herbivores in economically important agronomic grasses, in part through the production of alkaloids. Although endophytes are common in nature, relatively little is known about their effects on herbivores of native, wild grass species, and a recent meta-analysis suggested that endophytes are only beneficial in agronomic settings. In this study, we performed choice trials for five insect species and a greenhouse experiment with one species to assess effects of the fungal endophyte Neotyphodium sp. on herbivores of the wild grass Festuca subverticillata. In feeding trials, endophyte presence altered the preference of all five insect species tested. However, the magnitude and direction of preference varied among species, with Pterophylla camellifolia (F.), Spodoptera frugiperda (J. E. Smith), and Rhopalosiphum padi L. preferring endophyte-disinfected plants and Encoptolophus costalis (Scudder) and Romalea guttata (Houttuyn) preferring endophyte-symbiotic plants. Despite reducing insect preference, the endophyte had no significant effect on S. frugiperda performance in a no-choice greenhouse experiment and did not increase plant growth in response to this herbivore. Our results show that endophyte-mediated resistance to herbivory depends strongly on herbivore identity and suggest that the fitness consequences of endophyte symbiosis for host plants will be context dependent on the local composition of insect herbivores.

Specific bottom-up effects of arbuscular mycorrhizal fungi across a plant-herbivore-parasitoid system

The majority of plants are involved in symbioses with arbuscular mycorrhizal fungi (AMF), and these associations are known to have a strong influence on the performance of both plants and insect herbivores. Little is known about the impact of AMF on complex trophic chains, although such effects are conceivable. In a greenhouse study we examined the effects of two AMF species, Glomus intraradices and G. mosseae on trophic interactions between the grass Phleum pratense, the aphid Rhopalosiphum padi, and the parasitic wasp Aphidius rhopalosiphi. Inoculation with AMF in our study system generally enhanced plant biomass (+5.2%) and decreased aphid population growth (-47%), but there were no fungal species-specific effects. When plants were infested with G. intraradices, the rate of parasitism in aphids increased by 140% relative to the G. mosseae and control treatment. When plants were associated with AMF, the developmental time of the parasitoids decreased by 4.3% and weight at eclosion increased by 23.8%. There were no clear effects of AMF on the concentration of nitrogen and phosphorus in plant foliage. Our study demonstrates that the effects of AMF go beyond a simple amelioration of the plants' nutritional status and involve rather more complex species-specific cascading effects of AMF in the food chain that have a strong impact not only on the performance of plants but also on higher trophic levels, such as herbivores and parasitoids.

Arbuscular mycorrhizal fungal species suppress inducible plant responses and alter defensive strategies following herbivory

In a greenhouse experiment using Plantago lanceolata, plants grown with different arbuscular mycorrhizal (AM) fungal species differed in constitutive levels of chemical defense depending on the species of AM fungi with which they were associated. AM fungal inoculation also modified the induced chemical response following herbivory by the specialist lepidopoteran herbivore Junonia coenia, and fungal species varied in how they affected induced responses. On average, inoculation with AM fungi substantially reduced the induced chemical response as compared with sterile controls, and inoculation with a mixture of AM fungi suppressed the induced response of P. lanceolata to herbivory. These results suggest that AM fungi can exert controlling influence over plant defensive phenotypes, and a portion of the substantial variation among experimental tests of induced chemical responses may be attributable to AM fungi.

Trade-offs between arbuscular mycorrhizal fungal competitive ability and host growth promotion in Plantago lanceolata

In this study we tested for trade-offs between the benefit arbuscular mycorrhizal (AM) fungi provide for hosts and their competitive ability in host roots, and whether this potential trade-off shifts in the presence of a plant stress (herbivory). We used three species of AM fungi previously determined to vary in host growth promotion and spore production in association with host plants. We found that these AM fungal species competed for root space, and the best competitor, Scutellospora calospora, was the worst mutualist. In addition, the worst competitor, Glomus white, was the best mutualist. Competition proved to have stronger effects on fungal infection patterns than herbivory, and competitive dominance was not altered by herbivory. We found a similar pattern in a previous test of competition among AM fungi, and we discuss the implications of these results for the persistence of the mutualism and feedbacks between AM fungi and their plant hosts.

Mycorrhizal colonisation of mountain birch (Betula pubescens ssp. czerepanovii) along three environmental gradients: does life in harsh environments alter plant-fungal relationships?

Environmental stress affects ectomycorrhizal communities (ECM), but it is not known how general the detected ECM responses are. We investigated ECM fungi on roots of mountain birch, Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti, along three environmental gradients, two natural (altitude, seashore) and one human-induced (pollution), within the Kola Peninsula, NW Russia. Chlorophyll fluorescence of birch leaves indicated no environmental stress even in the conditions that were presumed most stressful in terms of abiotic environment, where the biomass and population density of birches were strongly reduced. Although neither overall ECM colonisation nor root fungal biomass showed stress-related patterns, colonisation by Cenococcum geophilum tended to decrease with abiotic stress. ECM morphotype diversity declined with abiotic stress, and along altitudinal gradient this decline was related to an increase in proportion of morphotypes with high fungal biomass. Polycormic birches had higher ECM colonisation than monocormic birches at high stress sites only. ECM morphotype diversity increased with foliar nitrogen concentration at low stress sites, but not at high stress sites. Birches with higher chlorophyll fluorescence had lower chitin concentration in their roots (indicating lower proportion of fungal structures) at high stress sites only. Our results suggest that at high stress sites (1) mechanical shelter created by polycormic trees may favour ECM fungi and (2) mountain birches maintain lower ECM diversity than at low stress sites.

Impacts of plant symbiotic fungi on insect herbivores: mutualism in a multitrophic context

We consider how fungi that form symbiotic associations with plants interact with insect herbivores attacking the same plants. Both endophytes and mycorrhizae have significant impacts on herbivores with which they are in relatively intimate contact, but weaker effects on those from which they are spatially separated. Generalist insects are usually adversely affected by the presence of endophytes and mycorrhizae, whereas specialist insects may often benefit. Effects on feeding guilds vary according to type of fungi; for example, aphids are often negatively affected by endophytes but respond positively to mycorrhizae, and leaf-chewers are usually negatively affected by both types of fungi. There is a strong taxonomic bias in the literature and many interactions remain little studied; laboratory studies predominate over field studies. Although some patterns emerge, there is a large amount of specificity and context dependency in the outcome of interactions, reflecting the influence of fungal and host genotype, fungal, host, and insect species, and environmental factors. Whereas some of the mechanisms underpinning these interactions are relatively well characterized, others remain unclear and await elucidation by molecular and metabolomic techniques.

Arbuscular mycorrhizal symbiosis increases host plant acceptance and population growth rates of the two-spotted spider mite Tetranychus urticae

Most terrestrial plants live in symbiosis with arbuscular mycorrhizal (AM) fungi. Studies on the direct interaction between plants and mycorrhizal fungi are numerous whereas studies on the indirect interaction between such fungi and herbivores feeding on aboveground plant parts are scarce. We studied the impact of AM symbiosis on host plant choice and life history of an acarine surface piercing-sucking herbivore, the polyphagous two-spotted spider mite Tetranychus urticae. Experiments were performed on detached leaflets taken from common bean plants (Phaseolus vulgaris) colonized or not colonized by the AM fungus Glomus mosseae. T. urticae females were subjected to choice tests between leaves from mycorrhizal and non-mycorrhizal plants. Juvenile survival and development, adult female survival, oviposition rate and offspring sex ratio were measured in order to estimate the population growth parameters of T. urticae on either substrate. Moreover, we analyzed the macro- and micronutrient concentration of the aboveground plant parts. Adult T. urticae females preferentially resided and oviposited on mycorrhizal versus non-mycorrhizal leaflets. AM symbiosis significantly decreased embryonic development time and increased the overall oviposition rate as well as the proportion of female offspring produced during peak oviposition. Altogether, the improved life history parameters resulted in significant changes in net reproductive rate, intrinsic rate of increase, doubling time and finite rate of increase. Aboveground parts of colonized plants showed higher concentrations of P and K whereas Mn and Zn were both found at lower levels. This is the first study documenting the effect of AM symbiosis on the population growth rates of a herbivore, tracking the changes in life history characteristics throughout the life cycle. We discuss the AM-plant-herbivore interaction in relation to plant quality, herbivore feeding type and site and the evolutionary implications in a multi-trophic context.