Nutritional ecology of arbuscular mycorrhizal fungi

Xizang meadow degradation alters resource exchange ratio, network complexity, and biomass allocation tradeoff of arbuscular mycorrhizal symbiosis

The response of arbuscular mycorrhizal (AM) symbiosis to environmental fluctuations involves resource exchange between host plants and fungal partners, associations between different AM fungal taxa, and biomass allocation between AM fungal spore and hyphal structures; yet a systematic understanding of these responses to meadow degradation remains relatively unknown, particularly in Xizang alpine meadow. Here, we approached this knowledge gap by labeling dual isotopes of air 13CO2 and soil 15NH4Cl, computing ecological networks of AM fungal communities, and quantifying AM fungal biomass allocation among spores, intra- and extraradical hyphae. We found that the exchange ratio of photosynthate and nitrogen between plants and AM fungi increased with the increasing severity of meadow degradation, indicating greater dependence of host plants on this symbiosis for resource acquisition. Additionally, using 18S rRNA gene metabarcoding, we found that AM fungal co-occurrence networks were more complex in more degraded meadows, supporting the stress gradient hypothesis. Meadow degradation also increased AM fungal biomass allocation toward traits associated with intra- and extraradical hyphae at the expense of spores. Our findings suggest that an integrated consideration of resource exchange, ecological networks, and biomass allocation may be important for the restoration of degraded ecosystems.

Arbuscular mycorrhizal fungi reduce soil N2O emissions by altering root traits and soil denitrifier community composition

Arbuscular mycorrhizal fungi (AMF) increase the ability of plants to obtain nitrogen (N) from the soil, and thus can affect emissions of nitrous oxide (N2O), a long-lived potent greenhouse gas. However, the mechanisms underlying the effects of AMF on N2O emissions are still poorly understood, particularly in agroecosystems with different forms of N fertilizer inputs. Utilizing a mesocosm experiment in field, we examined the effects of AMF on N2O emissions via their influence on maize root traits and denitrifying microorganisms under ammonia and nitrate fertilizer input using 15N isotope tracer. Here we show that the presence of AMF alone or both maize roots and AMF increased maize biomass and their 15N uptake, root length, root surface area, and root volume, but led to a reduction in N2O emissions under both N input forms. Random forest model showed that root length and surface area were the most important predictors of N2O emissions. Additionally, the presence of AMF reduced the (nirK + nirS)/nosZ ratio by increasing the relative abundance of nirS-Bradyrhizobium and Rubrivivax with ammonia input, but reducing nosZ-Azospirillum, Cupriavidus and Rhodopseudomonas under both fertilizer input. Further, N2O emissions were significantly and positively correlated with the nosZ-type Azospirillum, Cupriavidus and Rhodopseudomonas, but negatively correlated with the nirS-type Bradyrhizobium and Rubrivivax. These results indicate that AMF reduce N2O emissions by increasing root length to explore N nutrients and altering the community composition of denitrifiers, suggesting that effective management of N fertilizer forms interacting with the rhizosphere microbiome may help mitigate N2O emissions under future N input scenarios.

Wheat dwarfing reshapes plant and fungal development in arbuscular mycorrhizal symbiosis

The introduction of Reduced height (Rht) dwarfing genes into elite wheat varieties has contributed to enhanced yield gain in high input agrosystems by preventing lodging. Yet, how modern selection for dwarfing has affected symbiosis remains poorly documented. In this study, we evaluated the response of both the plant and the arbuscular mycorrhizal fungus to plant genetic variation at a major Quantitative Trait Locus called QTL 4B2, known to harbor a Rht dwarfing gene, when forming the symbiosis. We used twelve inbred genotypes derived from a diversity base broadened durum wheat Evolutionary Pre-breeding Population and genotyped with a high-throughput Single Nucleotide Polymorphism (SNP) genotyping array. In a microcosm setup segregating roots and the extra-radical mycelium, each wheat genotype was grown with or without the presence of Rhizophagus irregularis. To characterize arbuscular mycorrhizal symbiosis, we assessed hyphal density, root colonization, spore production, and plant biomass. Additionally, we split the variation of these variables due either to genotypes or to the Rht dwarfing genes alone. The fungus exhibited greater development in the roots of Dwarf plants compared to non-Dwarf plants, showing increases of 27%, 37% and 51% in root colonization, arbuscules, and vesicles, respectively. In addition, the biomass of the extra-radical fungal structures increased by around 31% in Dwarf plants. The biomass of plant roots decreased by about 43% in mycorrhizal Dwarf plants. Interestingly, extraradical hyphal production was found to be partly genetically determined with no significant effect of Rht, as for plant biomasses. In contrast, variations in root colonization, arbuscules and extraradical spore production were explained by Rht dwarfing genes. Finally, when mycorrhizal, Dwarf plants had significantly lower total P content, pointing towards a less beneficial symbiosis for the plant and increased profit for the fungus. These results highlight the effect of Rht dwarfing genes on both root and fungal development. This calls for further research into the molecular mechanisms governing these effects, as well as changes in plant physiology, and their implications for fostering arbuscular mycorrhizal symbiosis in sustainable agrosystems.

Metagenomic analysis revealed N-metabolizing microbial response of Iris tectorum to Cr stress after colonization by arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) and plant growth-promoting bacteria enhance plant tolerance to abiotic stress and promote plant growth in contaminated soil. However, the interaction mechanism between rhizosphere microbial communities under chromium (Cr) stress remains unclear. This study conducted a greenhouse pot experiment and metagenomics analysis to reveal the comprehensive effects of the interaction between AMF (Rhizophagus intraradices) and nitrogen-N metabolizing plant growth promoters on the growth of Iris tectorum. The results showed that AMF significantly increased the biomass and nutrient levels of I. tectorum in contaminated soil and decreased the content of Cr in the soil. Metagenomics analysis revealed that the structure and composition of the rhizosphere microbial community involved in nitrogen metabolism changed significantly after inoculation with AMF under Cr stress. Functional genes related to soil nitrogen mineralization (gltB, gltD, gdhA, ureC, and glnA), nitrate reduction to ammonium (nirB, nrfA, and nasA), and soil nitrogen assimilation (NRT, nrtA, and nrtC) were up-regulated in the N-metabolizing microbial community. In contrast, the abundance of functional genes involved in denitrification (nirK and narI) was down-regulated. In addition, the inoculation of AMF regulates the synergies between the N-metabolic rhizosphere microbial communities and enhances the complexity and stability of the rhizosphere ecological network. This study provides a basis for improving plant tolerance to heavy metal stress by regulating the functional abundance of N-metabolizing plant growth-promoting bacteria through AMF inoculation. It helps to understand the potential mechanism of wetland plant remediation of Cr-contaminated soil.

Arbuscular mycorrhizal fungi inhibit necrotrophic, but not biotrophic, aboveground plant pathogens: a meta-analysis and experimental study

Microbial mutualists can profoundly modify host species ecology and evolution, by extension altering interactions with other microbial species, including pathogens. Arbuscular mycorrhizal fungi (AMF) may moderate infections by pathogens, but the direction and strength of these effects can be idiosyncratic. To assess how the introduction of AMF impacts the incidence and severity of aboveground plant diseases (i.e. 'disease impact'), we conducted a meta-analysis of 130 comparisons derived from 69 published studies. To elucidate the potential mechanisms underlying the influence of AMF on pathogens, we conducted three glasshouse experiments involving six non-woody plant species, yielded crucial data on leaf nutrient composition, plant defense compounds, and transcriptomes. Our meta-analysis revealed that the inoculation of AMF lead to a reduction in disease impact. More precisely, AMF inoculation was associated with a decrease in necrotrophic diseases, while no significant impact on biotrophic diseases. Chemical and transcriptome analyses suggested that these effects may be driven by AMF regulation of jasmonic acid and salicylic acid signaling pathways in glasshouse experiments. However, changes in plant nutritional status and secondary chemicals may also regulate disease impact. These results emphasize the importance of incorporating pathogen life history when predicting how microbial mutualisms affect disease impact.

Integrated transcriptomics and metabolomics reveal specific phenolic and flavonoid accumulation in licorice (Glycyrrhiza uralensis Fisch.) induced by arbuscular mycorrhiza symbiosis under drought stress

Arbuscular mycorrhizal (AM) symbiosis can strengthen plant defense against abiotic stress, such as drought, through multiple mechanisms; however, the specialized chemical defenses induced by AM symbiosis are largely unknown. In a pot experiment, licorice (Glycyrrhiza uralensis Fisch.) inoculated with and without arbuscular mycorrhizal fungus Rhizophagus irregularis Schenck & Smith were grown under well-watered or water deficit conditions. Transcriptomic and metabolomic analyses were combined to investigate licorice root specialized metabolism induced by AM symbiosis under drought stress. Results showed that mycorrhizal plants had few dead leaves, less biomass reduction, and less differentially expressed genes and metabolite features in response to drought compared with nonmycorrhizal plants. Transcriptomic and metabolomic data revealed that mycorrhizal roots generally accumulated lignin regardless of the water regime; however, the expression of genes involved in lignin biosynthesis was significantly downregulated by drought stress in mycorrhizal plants. By contrast, AM inoculation significantly decreased specialized metabolites accumulation, including phenolics and flavonoids under well-watered conditions, whereas these decreases turned to be nonsignificant under drought stress. Moreover, these specific phenolics and flavonoids showed significant drought-induced accumulation pattern in mycorrhizal roots. These results highlight that accumulation of specific root phenolics and flavonoids may support the drought tolerance of mycorrhizal plants.

Plant-microbe symbiosis widens the habitability range of the Daisyworld

Plant-microbe symbiosis is pervasive in the Earth's ecosystems and dates back to the early land colonisation by plants. Mutualistic partnership with rhizobia bacteria and mycorrhizal fungi promotes plant nutrition, growth and diversity, impacting important ecosystem functions. However, how the global behaviour and dynamical properties of an ecosystem are modified by plant-microbe symbiosis is still unclear. To tackle this theoretical question, we resorted to the Daisyworld as a toy model of the global ecosystem. We redesigned the original model to allow accounting for seed production, spreading, germination, and seedling development to mature seed-producing plants to describe how symbiotic and non-symbiotic daisy species differ in these key processes. Using the steady-state and bifurcation analysis of this model, we demonstrate that symbiosis with microbes broadens the habitability range of the Daisyworld by enhancing plant growth and/or facilitating plant access to otherwise uninhabitable nutrient-poor regions.

Experimental duration determines the effect of arbuscular mycorrhizal fungi on plant biomass in pot experiments: A meta-analysis

Arbuscular mycorrhizal fungi (AMF) play various important roles in promoting plant growth. Numerous environmental and evolutionary factors influence the response of plants to AMF. However, the importance of the individual factors on the effects of AMF on plant biomass is not clearly understood. In this study, a meta-analysis using 1,640 observations from 639 published articles related to the influence of AMF on the plant shoot, root, and total biomass was performed; 13 different experimental setting factors that had an impact on the influence of AMF and their importance were quantitatively synthesized. The meta-analysis showed that AMF had positive effects on the plant shoot, root, and total biomass; moreover, the experimental duration, plant root-to-shoot ratio (R/S), AMF root length colonization, plant family, pot size, soil texture, and the soil pH all influenced the effects of AMF on the shoot, root, and total biomass. In addition, the plant root system and plant functional type had impacts on the effect of AMF on shoot biomass; AMF guild also impacted the effect of AMF on root biomass. Of these factors, the experimental duration, plant R/S, and pot size were the three most important predicting the effects of AMF on the plant shoot, root, and total biomass. This study comprehensively assessed the importance of the different factors that influenced the response of plants to AMF, highlighting that the experimental duration, plant R/S, and pot size should be taken into consideration in pot experiments in studies of the functions of AMF. Multiple unfavorable factors that may obscure or confound the observed functions of AMF should be excluded.

Cooperation With Arbuscular Mycorrhizal Fungi Increases Plant Nutrient Uptake and Improves Defenses Against Insects

Plants have evolved various defense mechanisms to cope with biotic and abiotic stresses. Cooperation with microorganisms, especially arbuscular mycorrhizal fungi (AMF), strengthens the defense capabilities of host plants. To explore the effect of AMF on the growth of Elymus and the defenses against locust feeding, we designed a two-compartment device to connect or cut the mycelia and roots. We used this to investigate communication cues and pathways between donor and receiver plants. We found that AMF significantly increased the nitrogen content and decreased the carbon to nitrogen (C:N) ratio of donor plants and receiver plants and the carbon content of both. After the establishment of the common mycorrhizal network (CMN) with AMF between the two chambers, inoculations of donor plants challenged by locusts caused enhancement in four defense-related enzymes, namely, lipoxygenase, polyphenol oxidase, phenylalanine ammonia lyase, and β-1,3-glucanase, in the receiver plants. The main components of volatile organic compounds emitted by receiver plants were terpenoids. The findings indicated that AMF could not only improve plant growth but also activate the defense response of plants to insect feeding. Four defense enzymes, volatile organic compounds, and carbon and nitrogen content were involved in the defense response, and the mycelial network could act as a conduit to deliver communication signals.

Application of plant-soil feedbacks in the selection of crop rotation sequences

Plant-soil feedback (PSF) can be a major driver of plant performance in communities, and this concept can be used in selecting crop rotation sequences to maximize agricultural yields. Potential benefits of using PSF in this context include nutrient use optimization, pathogen reduction, and enhancement of mutualisms between crops and microbes. Yet the contributions of these combined mechanisms are poorly understood. Here we investigated the relative contributions of these mechanisms using five major crops commonly cultivated in rotation (alfalfa, canola, maize, soybean, and wheat) under controlled conditions. We trained soil by growing each of the five crops in a "training phase," and then reciprocally planted the five crops in the trained soils in a "feedback phase." To tease out soil biota from nutrient effects, we established three treatments: "control" (trained unsterilized soil used in the feedback phases), "biota" (sterilized soil in the feedback phase inoculated with soil biota from the control treatment after the training phase), and "nutrient" (sterilized soils in both phases). Plant-soil feedback for each crop was calculated by comparing the total biomass of each crop grown in soils trained by each of the four other crops (i.e., in rotation) against total biomass in self-trained soil (i.e., monocropping). We found that PSF values varied among crop combinations in all the treatments, but such variation was the greatest in the nutrient treatment. Overall, soil biota feedback tended to be lower, whereas nutrient feedback tended to be greater compared to the unsterilized control soil, suggesting that effects of antagonistic biota outweighed those of beneficial microbes in the biota treatment, and that plants optimized nutrient uptake when the soil microbiome was absent in the nutrient treatment. Furthermore, soils in the nutrient treatment trained by the legume crops (alfalfa and soybean) tended to provide the greatest positive feedback, emphasizing the important legacy of N₂ fixers in crop rotation. Taken together, our data demonstrate how nutrients and soil biota can be integral to PSFs among crops, and that assessing PSFs under controlled conditions can serve as a basis to determine the most productive crop rotation sequences prior to field testing.

Climate warming promotes deterministic assembly of arbuscular mycorrhizal fungal communities

Arbuscular mycorrhizal fungi (AMF) significantly contribute to plant resource acquisition and play important roles in mediating plant interactions and soil carbon (C) dynamics. However, it remains unclear how AMF communities respond to climate change. We assessed impacts of warming and precipitation alterations (30% increase or decrease) on soil AMF communities, and examined major ecological processes shaping the AMF community assemblage in a Tibetan alpine meadow. Our results showed that warming significantly increased root biomass, and available nitrogen (N) and phosphorus (P) in soil. While precipitation alterations increased AMF abundances, they did not significantly affect the composition or diversity of AMF communities. In contrast, warming altered the composition of AMF communities and reduced their Shannon-Wiener index and Pielou's evenness. In particular, warming shifted the AMF community composition in favor of Diversisporaceae over Glomeraceae, likely through its impact on soil N and P availability. In addition, AMF communities were phylogenetically random in the unwarmed control but clustered in warming plots, implying more deterministic community assembly under climate warming. Warming enhancement of root growth, N and P availability likely reduced plant C-allocation to AMF, imposing stronger environmental filtering on AMF communities. We further proposed a conceptual framework that integrates biological and geochemical processes into a mechanistic understanding of warming and precipitation changes' effects on AMF. Taken together, these results suggest that soil AMF communities may be more sensitive to warming than expected, highlighting the need to monitor their community structure and associated functional consequences on plant communities and soil C dynamics under the future warmer climate.

Increased Carbon Partitioning to Secondary Metabolites Under Phosphorus Deficiency in Glycyrrhiza uralensis Fisch. Is Modulated by Plant Growth Stage and Arbuscular Mycorrhizal Symbiosis

Phosphorus (P) is one of the macronutrients limiting plant growth. Plants regulate carbon (C) allocation and partitioning to cope with P deficiency, while such strategy could potentially be influenced by plant growth stage and arbuscular mycorrhizal (AM) symbiosis. In a greenhouse pot experiment using licorice (Glycyrrhiza uralensis) as the host plant, we investigated C allocation belowground and partitioning in roots of P-limited plants in comparison with P-sufficient plants under different mycorrhization status in two plant growth stages. The experimental results indicated that increased C allocation belowground by P limitation was observed only in non-AM plants in the early growth stage. Although root C partitioning to secondary metabolites (SMs) in the non-AM plants was increased by P limitation as expected, trade-off patterns were different between the two growth stages, with C partitioning to SMs at the expense of non-structural carbohydrates (NSCs) in the early growth stage but at the expense of root growth in the late growth stage. These changes, however, largely disappeared because of AM symbiosis, where more root C was partitioned to root growth and AM fungus without any changes in C allocation belowground and partitioning to SMs under P limitations. The results highlighted that besides assisting with plant P acquisition, AM symbiosis may alter plant C allocation and partitioning to improve plant tolerance to P deficiency.

Arbuscular Mycorrhizal Fungi and Nutrition Determine the Outcome of Competition Between Lolium multiflorum and Trifolium subterraneum

Arbuscular mycorrhizal fungi (AMF) may affect competitive plant interactions, which are considered a prevalent force in shaping plant communities. Aiming at understanding the role of AMF in the competition between two pasture species and its dependence on soil nutritional status, a pot experiment with mycorrhizal and non-mycorrhizal Lolium multiflorum and Trifolium subterraneum was conducted, with manipulation of species composition (five levels), and nitrogen (N)- and phosphorus (P)- fertilization (three levels). In the non-mycorrhizal state, interspecific competition did not play a major role. However, in the presence of AMF, Lolium was the strongest competitor, with this species being facilitated by Trifolium. While N-fertilization did not change the competitive balance, P-fertilization gave Lolium, a competitive advantage over Trifolium. The effect of AMF on the competitive outcome may be driven by differential C-P trade benefits, with Lolium modulating carbon investment in the mycorrhizal network and the arbuscule/vesicle ratio at the cost of Trifolium.

Co-inoculation with arbuscular mycorrhizal fungi differing in carbon sink strength induces a synergistic effect in plant growth

Arbuscular mycorrhizal (AM) fungi play a key role in determining ecosystem functionality. Understanding how diversity in the fungal community affects plant productivity is therefore an important question in ecology. Current research has focused on understanding the role of functional complementarity in the fungal community when the host plant faces multiple stress factors. Fewer studies, however, have investigated how variation in traits affecting nutrient exchange can impact the plant growth dynamics, even in the absence of environmental stressors. Combining experimental data and a mathematical model based on ordinary differential equations, we investigate the role played by carbon sink strength on plant productivity. We simulate and measure plant growth over time when the plant is associated with two fungal isolates with different carbon sink strength, and when the plant is in pairwise association with each of the isolates alone. Overall, our theoretical as well as our experimental results show that co-inoculation with fungi with different carbon sink strength can induce positive non-additive effects (or synergistic effects) in plant productivity. Fungi with high carbon sink strength are able to quickly establish a fungal community and increase the nutrient supply to the plant, with a consequent positive impact on plant growth rate. On the other side, fungi with low carbon sink strength inflict lower carbon costs to the host plant, and support maximal plant productivity once plant biomass is large. As AM fungi are widely used as organic fertilizers worldwide, our findings have important implications for restoration ecology and agricultural management.

Native AMF Communities in an Italian Vineyard at Two Different Phenological Stages of Vitis vinifera

Arbuscular mycorrhizal fungi (AMF) are beneficial soil microorganisms that can establish symbiotic associations with Vitis vinifera roots, resulting in positive effects on grapevine performance, both in terms of water use efficiency, nutrient uptake, and replant success. Grapevine is an important perennial crop cultivated worldwide, especially in Mediterranean countries. In Italy, Piedmont is one of the regions with the longest winemaking tradition. In the present study, we characterized the AMF communities of the soil associated or not with the roots of V. vinifera cv. Pinot Noir cultivated in a vineyard subjected to conventional management using 454 Roche sequencing technology. Samplings were performed at two plant phenological stages (flowering and early fruit development). The AMF community was dominated by members of the family Glomeraceae, with a prevalence of the genus Glomus and the species Rhizophagus intraradices and Rhizophagus irregularis. On the contrary, the genus Archaeospora was the only one belonging to the family Archaeosporaceae. Since different AMF communities occur in the two considered soils, independently from the plant phenological stage, a probable role of V. vinifera in determining the AMF populations associated to its roots has been highlighted.

Addition of high C:N crop residues to a P-limited substrate constrains the benefits of arbuscular mycorrhizal symbiosis for wheat P and N nutrition

Many aspects concerning the role of arbuscular mycorrhizal (AM) fungi in plant nutrient uptake from organic sources remain unclear. Here, we investigated the contribution of AM symbiosis to N and P uptake by durum wheat after the addition of a high C:N biomass to a P-limited soil. Plants were grown in pots in the presence or absence of a multispecies AM inoculum, with (Org) or without (Ctr) the addition of 15N-labelled organic matter (OM). A further treatment, in which 15N was applied in mineral form (Ctr+N) in the same amount as that supplied in the Org treatment, was also included. Inoculation with AM had positive effects on plant growth in both control treatments (Ctr and Ctr+N), mainly linked to an increase in plant P uptake. The addition of OM, increasing the P available in the soil for the plants, resulted in a marked decrease in the contribution of AM symbiosis to plant growth and nutrient uptake, although the percentage of mycorrhization was higher in the Org treatment than in the controls. In addition, mycorrhization drastically reduced the recovery of 15N from the OM added to the soil whereas it slightly increased the N recovery from the mineral fertiliser. This suggests that plants and AM fungi probably exert a differential competition for different sources of N available in the soil. On the whole, our results provide a contribution to a better understanding of the conditions under which AM fungi can play an effective role in mitigating the negative effects of nutritional stresses in plants.

Global negative effects of nutrient enrichment on arbuscular mycorrhizal fungi, plant diversity and ecosystem multifunctionality

Despite widespread anthropogenic nutrient enrichment, it remains unclear how nutrient enrichment influences plant-arbuscular mycorrhizal fungi (AMF) symbiosis and ecosystem multifunctionality at the global scale. Here, we conducted a meta-analysis to examine the worldwide effects of nutrient enrichment on AMF and plant diversity and ecosystem multifunctionality using data of field experiments from 136 papers. Our analyses showed that nutrient addition simultaneously decreased AMF diversity and abundance belowground and plant diversity aboveground at the global scale. The decreases in AMF diversity and abundance associated with nutrient addition were more pronounced with increasing experimental duration, mean annual temperature (MAT) and mean annual precipitation (MAP). Nutrient addition-induced changes in soil pH and available phosphorus (P) predominantly regulated the responses of AMF diversity and abundance. Furthermore, AMF diversity correlated with ecosystem multifunctionality under nutrient addition worldwide. Our findings identify the negative effects of nutrient enrichment on AMF and plant diversity and suggest that AMF diversity is closely linked with ecosystem function. This study offers an important advancement in our understanding of plant-AMF interactions and their likely responses to ongoing global change.

Arbuscular Mycorrhiza Mediates Efficient Recycling From Soil to Plants of Nitrogen Bound in Chitin

Symbiosis between plants and arbuscular mycorrhizal (AM) fungi, involving great majority of extant plant species including most crops, is heavily implicated in plant mineral nutrition, abiotic and biotic stress tolerance, soil aggregate stabilization, as well as shaping soil microbiomes. The latter is particularly important for efficient recycling from soil to plants of nutrients such as phosphorus and nitrogen (N) bound in organic forms. Chitin is one of the most widespread polysaccharides on Earth, and contains substantial amounts of N (>6% by weight). Chitin is present in insect exoskeletons and cell walls of many fungi, and can be degraded by many prokaryotic as well as eukaryotic microbes normally present in soil. However, the AM fungi seem not to have the ability to directly access N bound in chitin molecules, thus relying on microbes in their hyphosphere to gain access to this nutrient-rich resource in the process referred to as organic N mineralization. Here we show, using data from two pot experiments, both including root-free compartments amended with 15N-labeled chitin, that AM fungi can channel substantial proportions (more than 20%) of N supplied as chitin into their plants hosts within as short as 5 weeks. Further, we show that overall N losses (leaching and/or volatilization), sometimes exceeding 50% of the N supplied to the soil as chitin within several weeks, were significantly lower in mycorrhizal as compared to non-mycorrhizal pots. Surprisingly, the rate of chitin mineralization and its N utilization by the AM fungi was at least as fast as that of green manure (clover biomass), based on direct 15N labeling and tracing. This efficient N recycling from soil to plant, observed in mycorrhizal pots, was not strongly affected by the composition of AM fungal communities or environmental context (glasshouse or outdoors, additional mineral N supply to the plants or not). These results indicate that AM fungi in general can be regarded as a critical and robust soil resource with respect to complex soil processes such as organic N mineralization and recycling. More specific research is warranted into the exact molecular mechanisms and microbial players behind the observed patterns.

Effects of Native Arbuscular Mycorrhizae Isolated on Root Biomass and Secondary Metabolites of Salvia miltiorrhiza Bge

Arbuscular mycorrhiza fungi (AMFs) are a group of soil-dwelling fungi that form symbiotic associations with plants, to mediate the secondary metabolism and production of active ingredients in aromatic and medicinal plants. Currently, there is little research on Salvia miltiorrhiza Bge. inoculation with native AMFs and the concomitant effects on growth and secondary metabolites. In this study, S. miltiorrhiza was treated with eight AMFs, i.e., Glomus formosanum; Gl. tenebrosum; Septoglomus constrictum; Funneliformis geosporum; Rhizophagus manihotis; Ambispora gerdemanii; Acaulospora laevis; Ac. tuberculata, to investigate the influence of AMF inoculation on biomass and secondary production under greenhouse conditions in S. miltiorrhiza roots. The results showed that mycorrhiza formation rates were between 54.83 and 86.10%. Apart from Ac. laevis and Gl. tenebrosum treatment, the roots biomass of the other treatment groups was effectively increased, and the fresh and dry weight of the plant inoculated with Fu. geosporum were increased by 86.76 and 86.95%, respectively. Specifically, AMF treatments also impacted on phenolic acids production; inoculation with both Fu. geosporum or Ac. laevis significantly reduced total phenolic acids, whereas the other treatments effectively increased these levels, of which Gl. formosanum generated significant levels. Most AMF-plant symbiotic experiments facilitated phenolic acid accumulation in the secondary metabolites of S. miltiorrhiza (except Ac. laevis). This study showed that most native AMFs inoculation with S. miltiorrhiza promoted roots growth and increased secondary metabolites production (especially phenolic acids). Going forward, inoculation of native AMF is a promising method to improve the quality and yield of S. miltiorrhiza and should be considered during production.

Higher temperatures and lower annual rainfall do not restrict, directly or indirectly, the mycorrhizal colonization of barley (Hordeum vulgare L.) under rainfed conditions

Whereas the role of arbuscular mycorrhizal fungi (AMF) in plant growth improvement has been well described in agroecosystems, little is known about the effect of environmental factors on AMF root colonization status of barley, the fourth most important cereal crop all over the world. In order to understand the influence of environmental factors, such as climatic and soil physico-chemical properties, on the spontaneous mycorrhizal ability of barley (Hordeum vulgare L.), a field investigation was conducted in 31 different sites in sub-humid, upper and middle semi-arid areas of Northern Tunisia. Mycorrhizal root colonization of H. vulgare varied considerably among sites. Principal component analysis showed that barley mycorrhization is influenced by both climatic and edaphic factors. A partial least square structural equation modelling (PLS-SEM) revealed that 39% (R²) of the total variation in AMF mycorrhizal rate of barley roots was mainly explained by chemical soil properties and climatic characteristics. Whereas barley root mycorrhizal rates were inversely correlated with soil organic nitrogen (ON), available phosphorus amounts (P), altitude (Z), average annual rainfall (AAR), they were directly correlated with soil pH and temperature. Our results indicated that AMF root colonization of barley was strongly related to climatic characteristics than chemical soil properties. The current study highlights the importance of the PLS-SEM to understand the interactions between climate, soil properties and AMF symbiosis of barley in field conditions.

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.

AMF communities associated to Vitis vinifera in an Italian vineyard subjected to integrated pest management at two different phenological stages

Vitis vinifera L. is an economically important crop that can be influenced by soil microorganisms, including arbuscular mycorrhizal fungi (AMF), that establish symbiotic associations with its roots. AMF have beneficial effects on grapevine performance improving water use efficiency and replant success. Most grapevine varieties are susceptible to various diseases, and integrated pest management (IPM) is one of the emerging approaches to perform pest control. In the present study, we examined the AMF communities present in the soil associated to the roots of V. vinifera cv. Pinot Noir (comparing them to those present in a soil not affected by grapevine roots), in a vineyard subjected to IPM at two different phenological stages, using 454 Roche sequencing technology. We proposed a new approach to analyze sequencing data. Most of the taxa were included in the family Glomeraceae. In particular, Glomus sp. Rhizophagus sp. and Septoglomus viscosum were present. The family Archeosporaceae was represented only by the genus Archeospora sp. Different AMF communities were found in the two soils and the importance of the phenological stage in regulating AMF biodiversity was assessed.

Biotic and abiotic factors shape arbuscular mycorrhizal fungal communities associated with the roots of the widespread fern Botrychium lunaria (Ophioglossaceae)

Arbuscular mycorrhizal fungi (AMF) play central roles in terrestrial ecosystems by interacting with both above and belowground communities as well as by influencing edaphic properties. The AMF communities associated with the roots of the fern Botrychium lunaria (Ophioglossaceae) were sampled in four transects at 2400 m a.s.l. in the Swiss Alps and analyzed using metabarcoding. Members of five Glomeromycota genera were identified across the 71 samples. Our analyses revealed the existence of a core microbiome composed of four abundant Glomus operational taxonomic units (OTUs), as well as a low OTU turnover between samples. The AMF communities were not spatially structured, which contrasts with most studies on AMF associated with angiosperms. pH, microbial connectivity and humus cover significantly shaped AMF beta diversity but only explained a minor fraction of variation in beta diversity. AMF OTUs associations were found to be significant by both cohesion and co-occurrence analyses, suggesting a role for fungus-fungus interactions in AMF community assembly. In particular, OTU co-occurrences were more frequent between different genera than among the same genus, rising the hypothesis of functional complementarity among the AMF associated to B. lunaria. Altogether, our results provide new insights into the ecology of fern symbionts in alpine grasslands.

HyLength: a semi-automated digital image analysis tool for measuring the length of roots and fungal hyphae of dense mycelia

In plant-fungus phenotyping, determining fungal hyphal and plant root lengths by digital image analysis can reduce labour and increase data reproducibility. However, the degree of software sophistication is often prohibitive and manual measuring is still used, despite being very time-consuming. We developed the HyLength tool for measuring the lengths of hyphae and roots in in vivo and in vitro systems. The HyLength was successfully validated against manual measures of roots and fungal hyphae obtained from all systems. Compared with manual methods, the HyLength underestimated Medicago sativa roots in the in vivo system and Rhizophagus irregularis hyphae in the in vitro system by about 12 cm per m and allowed to save about 1 h for a single experimental unit. As regards hyphae of R. irregularis in the in vivo system, the HyLength overestimated the length by about 21 cm per m compared with manual measures, but time saving was up to 20.5 h per single experimental unit. Finally, with hyphae of Aspergillus oryzae, the underestimation was about 8 cm per m with a time saving of about 10 min for a single germinating spore. By benchmarking the HyLength against the AnaMorf plugin of the ImageJ/Fiji, we found that the HyLength performed better for dense fungal hyphae, also strongly reducing the measuring time. The HyLength can allow measuring the length over a whole experimental unit, eliminating the error due to sub-area selection by the user and allowing processing a high number of samples. Therefore, we propose the HyLength as a useful freeware tool for measuring fungal hyphae of dense mycelia.

Field Inoculation of Bread Wheat with Rhizophagus irregularis under Organic Farming: Variability in Growth Response and Nutritional Uptake of Eleven Old Genotypes and A Modern Variety

Arbuscular mycorrhizal fungi (AMF) promote crop growth and yield by increasing N and P uptake and disease resistance, but the role of field AMF inoculation on the uptake of micronutrients, such as Fe and Zn, and accumulation in plant edible portions is still not clarified. Therefore, we studied the effect of field inoculation with Rhizophagus irregularis in an organic system on 11 old genotypes and a modern variety of bread wheat. Inoculation increased root colonization, root biomass and shoot Zn concentration at early stage and grain Fe concentration at harvest, while it did not modify yield. Genotypes widely varied for shoot Zn concentration at early stage, and for plant height, grain yield, Zn and protein concentration at harvest. Inoculation differentially modified root AMF community of the genotypes Autonomia B, Frassineto and Bologna. A higher abundance of Rhizophagus sp., putatively corresponding to the inoculated isolate, was only proved in Frassineto. The increase of plant growth and grain Zn content in Frassineto is likely linked to the higher R. irregularis abundance. The AMF role in increasing micronutrient uptake in grain was proved. This supports the introduction of inoculation in cereal farming, if the variable response of wheat genotypes to inoculation is considered.

Inorganic N addition replaces N supplied to switchgrass (Panicum virgatum) by arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) provide many benefits in agroecosystems including improved soil tilth, carbon sequestration, and water and nutrient transfer to plants. AMF are known to affect plant nitrogen (N) dynamics and transfer N to plants, but there have been few studies addressing whether the amount of N transferred to plants by AMF is agronomically relevant. We used δ¹⁵ N natural abundance methods and δ¹⁵ N mass balance equations to estimate the amount of plant N derived from AMF transfer in perennial grasses managed for bioenergy production under different N addition treatments (0, 56, and 196 kg N/ha). Differentiation of δ¹⁵ N among plant, soil N, and AMF pools was higher than anticipated leading to calculations of 34-55% of plant N transferred by AMF in the treatments receiving no N addition to 6-22% of plant N transferred to plants in high-N addition treatments. AMF extra-radical hyphae biomass was significantly reduced in the high-N (196 kg N/ha) addition treatments, which was negatively correlated to enriched plant δ¹⁵ N. Our results suggest that N addition decreases AMF N transfer to plants. When N was limiting to plant growth, AMF supplied agronomically significant amounts of plant N, and a higher proportion of overall plant N. Because differentiation between N pools was greater than expected, stable isotope measurements can be used to estimate N transfer to AMF plant hosts.

Slope Position Rather Than Thinning Intensity Affects Arbuscular Mycorrhizal Fungi (AMF) Community in Chinese Fir Plantations

Background and Objectives: Arbuscular mycorrhizal fungi (AMF) play a crucial role in individual plant capability and whole ecosystem sustainability. Chinese fir, one of the most widely planted tree species in southern China, forms associations with AMF. However, it is still unclear what impacts thinning management applied to Chinese fir plantations has on the structure and diversity of soil AMF communities. This research attempts to bridge this knowledge gap. Materials and Methods: A thinning experiment was designed on different slope positions in Chinese fir plantations to examine the impacts of slope position and thinning intensity on colonization, diversity, and community composition of AMF. Results: Our research showed that the altitudinal slope position had significant effects on colonization, diversity, and community composition of AMF in Chinese fir plantations. In addition, the interaction between slope position and thinning intensity had significant effects on AMF diversity. Colonization by AMF on the lower slope position was significantly higher than on the upper slope position, while AMF diversity on the upper slope position was higher than on the middle and lower slope positions. Glomus was the most abundant genus in all slope positions, especially on the middle and lower slope positions. The relative abundance of Diversispora was significantly different among slope positions with absolute dominance on the upper slope position. Scutellospora was uniquely found on the upper slope position. Furthermore, soil Mg and Mn contents and soil temperature positively affected AMF community composition at the operational taxonomic unit (OTU) level. Conclusions: These findings suggested that slope position should be considered in the management of Chinese fir plantations. Furthermore, both chemical fertilization and AMF augmentation should be undertaken on upper hill slope positions as part of sustainable management practices for Chinese fir plantations.

Differences in Root Nitrogen Uptake Between Tropical Lowland Rainforests and Oil Palm Plantations

Conversion of lowland tropical rainforests to intensely fertilized agricultural land-use systems such as oil palm (Elaeis guineensis) plantations leads to changes in nitrogen (N) cycling. Although soil microbial-driven N dynamics has been largely studied, the role of the plant as a major component in N uptake has rarely been considered. We address this gap by comparing the root N contents and uptake in lowland rainforests with that in oil palm plantations on Sumatra, Indonesia. To this aim, we applied ¹⁵N-labeled ammonium to intact soil, measured the ¹⁵N recovery in soil and roots, and calculated the root relative N uptake efficiency for 10 days after label application. We found that root N contents were by one third higher in the rainforest than oil palm plantations. However, ¹⁵N uptake efficiency was similar in the two systems. This finding suggests that lower N contents in oil palm roots were likely caused by plant internal utilization of the absorbed N (e.g., N export to fruit bunches) than by lower ability to take up N from the soil. ¹⁵N recovery in roots was primarily driven by the amount of root biomass, which was higher in oil palm plantation than rainforest. The oil palms unveiled a high capacity to acquire N, offering the possibility of enhancing sustainable plantation management by reducing N fertilizer application.

Imbalanced Regulation of Fungal Nutrient Transports According to Phosphate Availability in a Symbiocosm Formed by Poplar, Sorghum, and Rhizophagus irregularis

In arbuscular mycorrhizal (AM) symbiosis, key components of nutrient uptake and exchange are specialized transporters that facilitate nutrient transport across membranes. As phosphate is a nutrient and a regulator of nutrient exchanges, we investigated the effect of P availability to extraradical mycelium (ERM) on both plant and fungus transcriptomes and metabolomes in a symbiocosm system. By perturbing nutrient exchanges under the control of P, our objectives were to identify new fungal genes involved in nutrient transports, and to characterize in which extent the fungus differentially modulates its metabolism when interacting with two different plant species. We performed transportome analysis on the ERM and intraradical mycelium of the AM fungus Rhizophagus irregularis associated to Populus trichocarpa and Sorghum bicolor under high and low P availability in ERM, using quantitative RT-PCR and Illumina mRNA-sequencing. We observed that mycorrhizal symbiosis induces expression of specific phosphate and ammonium transporters in both plants. Furthermore, we identified new AM-inducible transporters and showed that a subset of phosphate transporters is regulated independently of symbiotic nutrient exchange. mRNA-Sequencing revealed that the fungal transportome was not similarly regulated in the two host plant species according to P availability. Mirroring this effect, many plant carbohydrate transporters were down-regulated in P. trichocarpa mycorrhizal root tissue. Metabolome analysis revealed further that AM root colonization led to a modification of root primary metabolism under low and high P availability and to a decrease of primary metabolite pools in general. Moreover, the down regulation of the sucrose transporters suggests that the plant limits carbohydrate long distance transport (i.e. from shoot to the mycorrhizal roots). By simultaneous uptake/reuptake of nutrients from the apoplast at the biotrophic interface, plant and fungus are both able to control reciprocal nutrient fluxes.

Transcriptome Analysis of Wheat Roots Reveals a Differential Regulation of Stress Responses Related to Arbuscular Mycorrhizal Fungi and Soil Disturbance

Symbioses with soil microorganisms are central in shaping the diversity and productivity of land plants and provide protection against a diversity of stresses, including metal toxicity. Arbuscular mycorrhizal fungi (AMF) can form extensive extraradical mycelial networks (ERM), which are very efficient in colonizing a new host. We quantified the responses of transcriptomes of wheat and one AMF partner, Rhizoglomus irregulare, to soil disturbance (Undisturbed vs. Disturbed) and to two different preceding mycotrophic species (Ornithopus compressus and Lolium rigidum). Soil disturbance and preceding plant species engender different AMF communities in wheat roots, resulting in a differential tolerance to soil manganese (Mn) toxicity. Soil disturbance negatively impacted wheat growth under manganese toxicity, probably due to the disruption of the ERM, and activated a large number of stress and starvation-related genes. The O. compressus treatment, which induces a greater Mn protection in wheat than L. rigidum, activated processes related to cellular division and growth, and very few related to stress. The L. rigidum treatment mostly induced genes that were related to oxidative stress, disease protection, and metal ion binding. R. irregulare cell division and molecular exchange between nucleus and cytoplasm were increased by O. compressus. These findings are highly relevant for sustainable agricultural systems, when considering a fit-for-purpose symbiosis.

Contrasting Nitrogen Fertilisation Rates Alter Mycorrhizal Contribution to Barley Nutrition in a Field Trial

Controlled environment studies show that arbuscular mycorrhizal fungi (AMF) may contribute to plant nitrogen (N) uptake, but the role of these near-ubiquitous symbionts in crop plant N nutrition under natural field conditions remains largely unknown. In a field trial, we tested the effects of N fertilisation and barley (Hordeum vulgare L.) cultivar identity on the contribution of AMF to barley N uptake using 15N tracers added to rhizosphere soil compartments. AMF were shown capable of significantly increasing plant 15N acquisition from root exclusion zones, and this was influenced by nitrogen addition type, N fertiliser application rate and barley cultivar identity. Our data demonstrate a previously overlooked potential route of crop plant N uptake which may be influenced substantially and rapidly in response to shifting agricultural management practices.

Arbuscular mycorrhizal fungi alter the food utilization, growth, development and reproduction of armyworm (Mythimna separata) fed on Bacillus thuringiensis maize

BACKGROUND

The cultivation of Bt maize (maize genetically modified with Bacillus thuringiensis) continues to expand globally. Arbuscular mycorrhizal fungi (AMF), an important kind of microorganism closely related to soil fertility and plant nutrition, may influence the ecological risk of target lepidopteran pests in Bt crops.

METHODS

In this study, transgenic Bt maize (Line IE09S034 with Cry1Ie vs. its parental line of non-Bt maize cv. Xianyu335) was inoculated with a species of AMF, Glomus caledonium (GC). Its effects on the food utilization, reproduction and development of armyworm, Mythimna separata, were studied in a potted experiment from 2017 to 2018.

RESULTS

GC inoculation increased the AMF colonization of both modified and non-modified maize, and also increased the grain weight per plant and 1,000-grain weight of modified and non-modified maize. However, the cultivation of Bt maize did not significantly affect the AMF colonization. The feeding of M. separata with Bt maize resulted in a notable decrease in RCR (relative consumption rate), RGR (relative growth rate), AD (approximate digestibility), ECD (efficiency of conversion of digested food) and ECI (efficiency of conversion of ingested food) parameters in comparison to those observed in larvae fed with non-Bt maize in 2017 and 2018, regardless of GC inoculation. Furthermore, remarkable prolongation of larval life span and decreases in the rate of pupation, weight of pupa, rate of eclosion, fecundity and adult longevity of M. separata were observed in the Bt treatment regardless of GC inoculation during the two-year experiment. Also, when M. separata was fed with Bt maize, a significant prolongation of larval life and significant decreases in the pupal weight, fecundity and adult longevity of M. separata were observed when inoculated with GC. However, it was just the opposite for larvae fed with non-Bt maize that was inoculated with GC. The increased percentage of larval life-span, the decreased percentages of the food utilization, and the other indexes of reproduction, growth, and development of M. separata fed on Bt maize relative to non-Bt maize were all visibly lower when under GC inoculation in contrast to the CK.

DISCUSSION

It is presumed that Bt maize has a marked adverse impact on M. separata development, reproduction and feeding, especially when in combination with the GC inoculation. Additionally, GC inoculation favors the effectiveness of Bt maize against M. separata larvae by reducing their food utilization ability, which negatively affects the development and reproduction of the armyworm. Thus, Bt maize inoculated with AMF (here, GC) can reduce the severe threats arising of armyworms, and hence the AMF inoculation may play an important ecological functions in the field of Bt maize ecosystem, with potentially high control efficiency for the target lepidopteran pests.

Nitrogen fertilization altered arbuscular mycorrhizal fungi abundance and soil erosion of paddy fields in the Taihu Lake region of China

Arbuscular mycorrhizal (AM) fungi were of importance in mitigating soil erosion, which was highly influenced by biotic and abiotic factors, such as host plant growth and soil nutrient. To investigate the impact of nitrogen (N) fertilization on seasonal variance in AM colonization and soil erosion, we conducted a field experiment with rice cultivation under four N fertilizer levels (0 kg N ha^-1, 270 kg N ha^-1, 300 kg N ha^-1, and 375 kg N ha^-1 plus organic fertilizers) in the Taihu Lake region, China. We investigated AM colonization before rice transplantation, during rice growth, and after rice harvest. We also assessed soil splash erosion of intact soil cores sampled at tillering and after rice harvest. We found that AM colonization (indicated by percentage of root length colonization) varied from 15 to 73%, which was attributed to rice growth, N fertilization, and their interaction. Soil loss due to splash erosion was cut down by organic N fertilizer at tillering, while higher inorganic N fertilization significantly increased soil loss after rice harvest. Additionally, we found significantly negative relationships of AM colonization to soil loss but positive relationships to soil aggregate stability. We highlighted the potential role of AM fungi in decreasing soil erosion and suggested that high N fertilization should be considered carefully when seeking after high yields.

Rapid Transfer of Plant Photosynthates to Soil Bacteria via Ectomycorrhizal Hyphae and Its Interaction With Nitrogen Availability

Plant roots release recent photosynthates into the rhizosphere, accelerating decomposition of organic matter by saprotrophic soil microbes ("rhizosphere priming effect") which consequently increases nutrient availability for plants. However, about 90% of all higher plant species are mycorrhizal, transferring a significant fraction of their photosynthates directly to their fungal partners. Whether mycorrhizal fungi pass on plant-derived carbon (C) to bacteria in root-distant soil areas, i.e., incite a "hyphosphere priming effect," is not known. Experimental evidence for C transfer from mycorrhizal hyphae to soil bacteria is limited, especially for ectomycorrhizal systems. As ectomycorrhizal fungi possess enzymatic capabilities to degrade organic matter themselves, it remains unclear whether they cooperate with soil bacteria by providing photosynthates, or compete for available nutrients. To investigate a possible C transfer from ectomycorrhizal hyphae to soil bacteria, and its response to changing nutrient availability, we planted young beech trees (Fagus sylvatica) into "split-root" boxes, dividing their root systems into two disconnected soil compartments. Each of these compartments was separated from a litter compartment by a mesh penetrable for fungal hyphae, but not for roots. Plants were exposed to a 13C-CO2-labeled atmosphere, while 15N-labeled ammonium and amino acids were added to one side of the split-root system. We found a rapid transfer of recent photosynthates via ectomycorrhizal hyphae to bacteria in root-distant soil areas. Fungal and bacterial phospholipid fatty acid (PLFA) biomarkers were significantly enriched in hyphae-exclusive compartments 24 h after 13C-CO2-labeling. Isotope imaging with nanometer-scale secondary ion mass spectrometry (NanoSIMS) allowed for the first time in situ visualization of plant-derived C and N taken up by an extraradical fungal hypha, and in microbial cells thriving on hyphal surfaces. When N was added to the litter compartments, bacterial biomass, and the amount of incorporated 13C strongly declined. Interestingly, this effect was also observed in adjacent soil compartments where added N was only available for bacteria through hyphal transport, indicating that ectomycorrhizal fungi were acting on soil bacteria. Together, our results demonstrate that (i) ectomycorrhizal hyphae rapidly transfer plant-derived C to bacterial communities in root-distant areas, and (ii) this transfer promptly responds to changing soil nutrient conditions.

Dynamics of arbuscular mycorrhizal fungal community structure and functioning along a nitrogen enrichment gradient in an alpine meadow ecosystem

Nitrogen (N) availability is increasing dramatically in many ecosystems, but the influence of elevated N on the functioning of arbuscular mycorrhizal (AM) fungi in natural ecosystems is not well understood. We measured AM fungal community structure and mycorrhizal function simultaneously across an experimental N addition gradient in an alpine meadow that is limited by N but not by phosphorus (P). AM fungal communities at both whole-plant-community (mixed roots) and single-plant-species (Elymus nutans roots) scales were described using pyro-sequencing, and the mycorrhizal functioning was quantified using a mycorrhizal-suppression treatment in the field (whole-plant-community scale) and a glasshouse inoculation experiment (single-plant-species scale). Nitrogen enrichment progressively reduced AM fungal abundance, changed AM fungal community composition, and shifted mycorrhizal functioning towards parasitism at both whole-plant-community and E. nutans scales. N-induced shifts in AM fungal community composition were tightly linked to soil N availability and/or plant species richness, whereas the shifts in mycorrhizal function were associated with the communities of specific AM fungal lineages. The observed changes in both AM fungal community structure and functioning across an N enrichment gradient highlight that N enrichment of ecosystems that are not P-limited can induce parasitic mycorrhizal functioning and influence plant community structure and ecosystem sustainability.

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.

Responses of arbuscular mycorrhizal fungi to long-term inorganic and organic nutrient addition in a lowland tropical forest

Improved understanding of the nutritional ecology of arbuscular mycorrhizal (AM) fungi is important in understanding how tropical forests maintain high productivity on low-fertility soils. Relatively little is known about how AM fungi will respond to changes in nutrient inputs in tropical forests, which hampers our ability to assess how forest productivity will be influenced by anthropogenic change. Here we assessed the influence of long-term inorganic and organic nutrient additions and nutrient depletion on AM fungi, using two adjacent experiments in a lowland tropical forest in Panama. We characterised AM fungal communities in soil and roots using 454-pyrosequencing, and quantified AM fungal abundance using microscopy and a lipid biomarker. Phosphorus and nitrogen addition reduced the abundance of AM fungi to a similar extent, but affected community composition in different ways. Nutrient depletion (removal of leaf litter) had a pronounced effect on AM fungal community composition, affecting nearly as many OTUs as phosphorus addition. The addition of nutrients in organic form (leaf litter) had little effect on any AM fungal parameter. Soil AM fungal communities responded more strongly to changes in nutrient availability than communities in roots. This suggests that the 'dual niches' of AM fungi in soil versus roots are structured to different degrees by abiotic environmental filters, and biotic filters imposed by the plant host. Our findings indicate that AM fungal communities are fine-tuned to nutrient regimes, and support future studies aiming to link AM fungal community dynamics with ecosystem function.

Arbuscular mycorrhizal fungi enhance antioxidant defense in the leaves and the retention of heavy metals in the roots of maize

In this study, we investigated the effects of the arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae and Diversispora spurcum on the growth, antioxidant physiology, and uptake of phosphorus (P), sulfur (S), lead (Pb), zinc (Zn), cadmium (Cd), and arsenic (As) by maize (Zea mays L.) grown in heavy metal-polluted soils though a potted plant experiment. F. mosseae significantly increased the plant chlorophyll a content, height, and biomass; decreased the H₂O₂ and malondialdehyde (MDA) contents; and enhanced the superoxide dismutase (SOD) and catalase (CAT) activities and the total antioxidant capacity (T-AOC) in maize leaves; this effect was not observed with D. spurcum. Both F. mosseae and D. spurcum promoted the retention of heavy metals in roots and increased the uptake of Pb, Zn, Cd, and As, and both fungi restricted heavy metal transfer, resulting in decreased Pb, Zn, and Cd contents in shoots. Therefore, the fungi reduced the translocation factors for heavy metal content (TF) and uptake (TF') in maize. Additionally, F. mosseae promoted P and S uptake by shoots, and D. spurcum increased P and S uptake by roots. Moreover, highly significant negative correlations were found between antioxidant capacity and the H₂O₂, MDA, and heavy metal contents, and there was a positive correlation with the biomass of maize leaves. These results suggested that AMF alleviated plant toxicity and that this effect was closely related to antioxidant activation in the maize leaves and increased retention of heavy metals in the roots.

Manganese binding to Rubisco could drive a photorespiratory pathway that increases the energy efficiency of photosynthesis

Most plants, contrary to popular belief, do not waste over 30% of their photosynthate in a futile cycle called photorespiration. Rather, the photorespiratory pathway generates additional malate in the chloroplast that empowers many energy-intensive chemical reactions, such as those involved in nitrate assimilation. Thus, the balance between carbon fixation and photorespiration determines the plant carbon-nitrogen balance and protein concentrations. Plant protein concentrations, in turn, depend not only on the relative concentrations of carbon dioxide and oxygen in the chloroplast but also on the relative activities of magnesium and manganese, which are metals that associate with several key enzymes in the photorespiratory pathway and alter their function. Understanding the regulation of these processes is critical for sustaining food quality under rising CO2 atmospheres.

Phosphorus Acquisition Efficiency Related to Root Traits: Is Mycorrhizal Symbiosis a Key Factor to Wheat and Barley Cropping?

Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) are major crops cultivated around the world, thus playing a crucial role on human diet. Remarkably, the growing human population requires a significant increase in agricultural production in order to feed everybody. In this context, phosphorus (P) management is a key factor as it is component of organic molecules such as nucleic acids, ATP and phospholipids, and it is the most abundant macronutrient in biomass after nitrogen (N), although being one of the scarcest elements in the lithosphere. In general, P fertilization has low efficiency, as only a fraction of the applied P is acquired by roots, leaving a substantial amount to be accumulated in soil as not readily available P. Breeding for P-efficient cultivars is a relatively low cost alternative and can be done through two mechanisms: i) improving P use efficiency (PUE), and/or ii) P acquisition efficiency (PAE). PUE is related to the internal allocation/mobilization of P, and is usually represented by the amount of P accumulated per biomass. PAE relies on roots ability to acquire P from the soil, and is commonly expressed as the relative difference of P acquired under low and high P availability conditions. In this review, plant adaptations related to improved PAE are described, with emphasis on arbuscular mycorrhizal (AM) symbiosis, which is generally accepted to enhance plant P acquisition. A state of the art (1980-2018) of AM growth responses and P uptake in wheat and barley is made to discuss about the commonly accepted growth promoting effect and P increased uptake by AM fungi and the contrasting evidence about the generally accepted lack of positive responses in both plant species. Finally, the mechanisms by which AM symbiosis can affect wheat and barley PAE are discussed, highlighting the importance of considering AM functional diversity on future studies and the necessity to improve PAE definition by considering the carbon trading between all the directly related PAE traits and its return to the host plant.

Chronic fertilization of 37 years alters the phylogenetic structure of soil arbuscular mycorrhizal fungi in Chinese Mollisols

Arbuscular mycorrhizal fungi (AMF) play vital roles in sustaining soil productivity and plant communities. However, adaption and differentiation of AMF in response to commonly used fertilization remain poorly understood. In this study, we showed that the AMF community composition was primarily driven by soil physiochemical changes associated with chronic inorganic and organic fertilization of 37 years in Mollisols. High-throughput sequencing indicated that inorganic fertilizer negatively affected AMF diversity and richness, implying a reduction of mutualism in plant-AMF symbiosis; however, a reverse trend was observed for the application of inorganic fertilizer combined with manure. With regards to AMF community composition, order Glomerales was dominant, but varied significantly among different fertilization treatments. All fertilization treatments decreased family Glomeraceae and genus Funneliformis, while Rhizophagus abundance increased. Plant-growth-promoting-microorganisms of family Claroideoglomeraceae and genus Claroideoglomus were stimulated by manure application, and likely benefited pathogen suppression and phosphorus (P) acquisition. Family Gigasporaceae and genus Gigaspora were negatively correlated with available P in soil. Additionally, redundancy analysis further suggested that soil available P, organic matter and pH were the most important factors in shaping AMF community composition. These results provide strong evidence for niche differentiation of phylogenetically distinct AMF populations under different fertilization regimes. Manure likely contributes to restoration and maintenance of plant-AMF symbiosis, and the balanced fertilization would favor the growth of beneficial AMF communities as one optimized management in support of sustainable agriculture in Mollisols.

Arbuscular mycorrhizal fungal and soil microbial communities in African Dark Earths

The socio-economic values of fertile and carbon-rich Dark Earth soils are well described from the Amazon region. Very recently, Dark Earth soils were also identified in tropical West Africa, with comparable beneficial soil properties and plant growth-promoting effects. The impact of this management technique on soil microbial communities, however, is less well understood, especially with respect to the ecologically relevant group of arbuscular mycorrhizal (AM) fungi. Thus, we tested the hypotheses that (1) improved soil quality in African Dark Earth (AfDE) will increase soil microbial biomass and shift community composition and (2) concurrently increased nutrient availability will negatively affect AM fungal communities. Microbial communities were distinct in AfDE in comparison to adjacent sites, with an increased fungal:bacterial ratio of 71%, a pattern mainly related to shifts in pH. AM fungal abundance and diversity, however, did not differ despite clearly increased soil fertility in AfDE, with 3.7 and 1.7 times greater extractable P and total N content, respectively. The absence of detrimental effects on AM fungi, often seen following applications of inorganic fertilizers, and the enhanced role of saprobic fungi relevant for mineralization and C sequestration support previous assertions of this management type as a sustainable alternative agricultural practice.

Compost Addition Enhanced Hyphal Growth and Sporulation of Arbuscular Mycorrhizal Fungi without Affecting Their Community Composition in the Soil

Arbuscular mycorrhizal (AM) fungi form symbiotic associations with most crop plant species in agricultural ecosystems, and are conspicuously influenced by various agricultural practices. To understand the impact of compost addition on AM fungi, we examined effect of four compost rates (0, 11.25, 22.5, and 45 Mg/ha) on the abundance and community composition of AM fungi in seedling, flowering, and mature stage of soybean in a 1-year compost addition experiment system in Northeast China. Soybean [Glycine max (L.) Merrill] was used as test plant. Moderate (22.5 Mg/ha) and high (45 Mg/ha) levels of compost addition significantly increased AM root colonization and extraradical hyphal (ERH) density compared with control, whereas low (11.5 Mg/ha) level of compost addition did not cause significant increase in AM root colonization and ERH density. AM fungal spore density was significantly enhanced by all the compost rates compared with control. The temporal variations analysis revealed that, AM root colonization in seedling stage was significantly lower than in flowering and mature stage. Although AM fungal operational taxonomic unit richness and community composition was unaffected by compost addition, some abundant AM fungal species showed significantly different response to compost addition. In mature stage, Rhizophagus fasciculatum showed increasing trend along with compost addition gradient, whereas the opposite was observed with Paraglomus sp. In addition, AM fungal community composition exhibited significant temporal variation during growing season. Further analysis indicated that the temporal variation in AM fungal community only occurred in control treatment, but not in low, moderate, and high level of compost addition treatments. Our findings highlighted the significant effects of compost addition on AM growth and sporulation, and emphasized that growth stage is a stronger determinant than 1-year compost addition in shaping AM fungal community in black soil of Northeast China.

Arbuscular Mycorrhizal Fungi and Plant Chemical Defence: Effects of Colonisation on Aboveground and Belowground Metabolomes

Arbuscular mycorrhizal fungal (AMF) colonisation of plant roots is one of the most ancient and widespread interactions in ecology, yet the systemic consequences for plant secondary chemistry remain unclear. We performed the first metabolomic investigation into the impact of AMF colonisation by Rhizophagus irregularis on the chemical defences, spanning above- and below-ground tissues, in its host-plant ragwort (Senecio jacobaea). We used a non-targeted metabolomics approach to profile, and where possible identify, compounds induced by AMF colonisation in both roots and shoots. Metabolomics analyses revealed that 33 compounds were significantly increased in the root tissue of AMF colonised plants, including seven blumenols, plant-derived compounds known to be associated with AMF colonisation. One of these was a novel structure conjugated with a malonyl-sugar and uronic acid moiety, hitherto an unreported combination. Such structural modifications of blumenols could be significant for their previously reported functional roles associated with the establishment and maintenance of AM colonisation. Pyrrolizidine alkaloids (PAs), key anti-herbivore defence compounds in ragwort, dominated the metabolomic profiles of root and shoot extracts. Analyses of the metabolomic profiles revealed an increase in four PAs in roots (but not shoots) of AMF colonised plants, with the potential to protect colonised plants from below-ground organisms.