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Arsenic (As) contamination poses significant environmental and health concerns globally, particularly in regions with high exposure levels due to anthropogenic activities. As phytoremediation, particularly through the hyperaccumulator fern Pteris vittata, offers a promising approach to mitigate arsenic pollution. Bacteria and mycorrhizal fungi colonizing P. vittata roots are involved in As metabolism and resistance and plant growth promotion under stressful conditions. A total of 45 bacterial strains were isolated from bulk soil and the rhizosphere of mycorrhizal P. vittata growing in an industrial As-polluted site. Bacteria were characterized by their plant-beneficial traits, tolerance to sodium arsenate and arsenite, and the occurrence of As-resistant genes. This study highlights differences between the culturable fraction of the microbiota associated with the rhizosphere of mycorrhizal P. vittata plants and the bulk soil. Moreover, several strains showing arsenate tolerance up to 600 mM were isolated. All the bacterial strains possessed arsC genes, and about 70% of them showed arrA genes involved in the anaerobic arsenate respiration pathway. The possible exploitation of such bacterial strains in strategies devoted to the assisted phytoremediation of arsenic highlights the importance of such a study in order to develop effective in situ phytoremediation strategies. [ABSTRACT FROM AUTHOR]

The survival and productivity of potato plantlets transplanted from in vitro to commercial hydroponic minituber culture are crucial. Mycorrhizal fungi are an efficient tool to enhance potato nutrient uptake, survival, and productivity under these conditions. This study aimed to assess the effect of mycorrhizal fungi on improving the yield of minitubers and nutrient uptake in potatoes transferred from the in vitro environment to the greenhouse. This study was conducted in a greenhouse in Hamadan province, I.R. Iran, in 2013. The experimental design was factorial, including treatments with and without inoculation with the mycorrhizal fungus Rhizophagus irregularis and varying phosphorus (P) concentrations in Hoagland's solution (100%, 75%, 50%, and 25% of its maximum content, i.e. 0.965, 0.482, 0.241, and 0.12 mM H2PO4-) with three replications on potato plantlets transferred from in vitro to pots. The results indicated that the highest colonization percentage was achieved with 25% P and inoculation. The highest number of minitubers (8.9), total minituber weight per plant (53.2 grams), plant height (54.6 cm), minituber (0.38%) and leaf (0.363%) P content, minituber (33 mg/kg) and leaf (65 mg/kg) manganese (Mn) content, and minituber (38 mg/kg) and leaf (65 mg/kg) zinc (Zn) content were obtained with inoculation and 75% P of Hoagland nutrient solution, which was statistically comparable to the inoculation with 100% P treatment. In summary, inoculating potato plantlets with mycorrhiza can enhance minituber yield and nutrient uptake and potentially replace approximately 50% of the P requirement in hydroponic solutions. [ABSTRACT FROM AUTHOR]

ABSTRACT Interactions between arbuscular mycorrhizal fungi (AMF), plants, and the soil microbial community have the potential to increase the availability and uptake of phosphorus (P) and nitrogen (N) in agricultural systems. Nutrient exchange between plant roots, AMF, and the adjacent soil microbes occurs at the interface between roots colonized by mycorrhizal fungi and soil, referred to as the mycorrhizosphere. Research on the P exchange focuses on plant–AMF or AMF–microbe interactions, lacking a holistic view of P exchange between the plants, AMF, and other microbes. Recently, N exchange at both interfaces revealed the synergistic role of AMF and bacterial community in N uptake by the host plant. Here, we highlight work carried out on each interface and build upon it by emphasizing research involving all members of the tripartite network. Both nutrient systems are challenging to study due to the complex chemical and biological nature of the mycorrhizosphere. We discuss some of the effective methods to identify important nutrient processes and the tripartite members involved in these processes. The extrapolation of in vitro studies into the field is often fraught with contradiction and noise. Therefore, we also suggest some approaches that can potentially bridge the gap between laboratory-generated data and their extrapolation to the field, improving the applicability and contextual relevance of data within the field of mycorrhizosphere interactions. Overall, we argue that the research community needs to adopt a holistic tripartite approach and that we have the means to increase the applicability and accuracy of in vitro data in the field.

Arsenic (As) contamination poses significant environmental and health concerns globally, particularly in regions with high exposure levels due to anthropogenic activities. As phytoremediation, particularly through the hyperaccumulator fern Pteris vittata, offers a promising approach to mitigate arsenic pollution. Bacteria and mycorrhizal fungi colonizing P. vittata roots are involved in As metabolism and resistance and plant growth promotion under stressful conditions. A total of 45 bacterial strains were isolated from bulk soil and the rhizosphere of mycorrhizal P. vittata growing in an industrial As-polluted site. Bacteria were characterized by their plant-beneficial traits, tolerance to sodium arsenate and arsenite, and the occurrence of As-resistant genes. This study highlights differences between the culturable fraction of the microbiota associated with the rhizosphere of mycorrhizal P. vittata plants and the bulk soil. Moreover, several strains showing arsenate tolerance up to 600 mM were isolated. All the bacterial strains possessed arsC genes, and about 70% of them showed arrA genes involved in the anaerobic arsenate respiration pathway. The possible exploitation of such bacterial strains in strategies devoted to the assisted phytoremediation of arsenic highlights the importance of such a study in order to develop effective in situ phytoremediation strategies.

Primary production in terrestrial ecosystems is sustained by plants, microbiota, and fungi, which are the major organic matter providers in the root zone, setting in motion the soil food webs. Predators like soil amoebae voraciously feed on bacteria, fungi, and microbial eukaryotes releasing the nutrients sequestered in their biomass. Early food web setting up is crucial for seedling nutrition and its further development after establishment. Mycorrhizal fungi are more than phosphorus providers, and we wonder what their role is in structuring the predators' trophic groups in the root zone. We evaluated the effect of Rhizophagus intraradices inoculated in Zea mays (mycorrhizosphere), on the structuration of amoebae trophic groups along vertical and horizontal (3, 6, and 9 cm) soil distribution when compared to un-inoculated plants, after 20 days in microcosms. Amoebae species richness was highest in non-mycorrhizal seedlings in the root zone at 6- to 9-cm depth, and 3 cm away from plants. More bacterial species are needed when plants are devoid of mycorrhiza, and their influence is constrained 3 cm away from roots. Higher diversity of trophic groups was recorded at mycorrhizal seedlings and at the compartment influenced by the mycelium at 6- to 9-cm depth. The highest bacterivorous diversity, higher number of rare species and protozoa-eating amoebae, and the absence of fungivorous group recorded at the mycorrhizosphere of Z. mays, indicate that the community was very different from the non-mycorrhizal plants. We conclude that the arbuscular mycorrhizal fungus exerts significant changes on the community of trophic groups of amoebae. [ABSTRACT FROM AUTHOR]

Collimonads are well-adapted to nutrient-poor environments. They are known to hydrolyse chitin, produce antifungal metabolites, weather minerals, and are effective biocontrol agents protecting plants from fungal diseases. The production of N -acyl homoserine lactones (AHLs) was suggested to be a conserved trait of collimonads, but little is known about the genes that underlie this production or the genes that are controlled by AHLs. To improve our understanding of the role of AHLs in the ecology of collimonads, we carried out transcriptomic analyses, combined with chemical and functional assays, on strain Collimonas pratensis PMB3(1). The main AHLs produced by this strain were identified as 3-hydroxy-hexa- and octa-noyl-homoserine lactone. Genome analysis permitted to identify putative genes coding for the autoinducer synthase (colI) and cognate transcriptional regulator (colR). The ability to produce AHLs was lost in Δ colI and Δ colR mutants. Functional assays revealed that the two mutants metabolized glucose, formate, oxalate, and leucine better than the wild-type (WT) strain. Transcriptome sequencing analyses revealed an up-regulation of different metabolic pathways and of motility in the QS-mutants compared to the WT strain. Overall, our results provide insights into the role of the AHL-dependent regulation system of Collimonas in environment colonization, metabolism readjustment, and microbial interactions. [ABSTRACT FROM AUTHOR]

There are complex interactions between mycorrhizal helper bacteria (MHBs) and ectomycorrhizal (ECM) fungi, with MHBs promoting mycorrhizal synthesis and ECM fungi regulating plant rhizobacterial colonization, diversity, and function. In this study, to investigate whether the ECM fungus Hymenochaete sp. Rl affects the survival and colonization of the MHB strain Bacillus pumilus HR10 in the rhizosphere, the biomass of B. pumilus HR10 was measured in the rhizosphere and mycorrhizosphere. In addition, extracts of Hymenochaete sp. Rl and Pinus thunbergii were evaluated for their effect on B. pumilus HR10 colonization (growth, sporulation, biofilm formation, extracellular polysaccharide and extracellular protein contents, flagellar motility, and expression of colonization-related genes). The results showed that inoculation of Hymenochaete sp. Rl significantly increased the biomass of B. pumilus HR10 in the rhizosphere; however, while extracts of Hymenochaete sp. Rl and P. thunbergii did not affect the biomass or spore formation of HR10, they did affect its biofilm formation, extracellular polysaccharide and extracellular protein production, and flagellar motility. Furthermore, the addition of symbiont extracts affected the expression of chemotaxis-related genes in HR10. When the extracts were added separately, the expression of srf genes in HR10 increased; when the extracts were added simultaneously, the expression of the flagellin gene fliG in HR10 increased, but there was no significant effect on the expression of srf genes, consistent with the results on biofilm production. Thus, Hymenochaete sp. Rl and P. thunbergii roots had a positive effect on colonization by B. pumilus HR10 at the rhizosphere level through their secretions. [ABSTRACT FROM AUTHOR]

There are complex interactions between mycorrhizal helper bacteria (MHBs) and ectomycorrhizal (ECM) fungi, with MHBs promoting mycorrhizal synthesis and ECM fungi regulating plant rhizobacterial colonization, diversity, and function. In this study, to investigate whether the ECM fungus Hymenochaete sp. Rl affects the survival and colonization of the MHB strain Bacillus pumilus HR10 in the rhizosphere, the biomass of B. pumilus HR10 was measured in the rhizosphere and mycorrhizosphere. In addition, extracts of Hymenochaete sp. Rl and Pinus thunbergii were evaluated for their effect on B. pumilus HR10 colonization (growth, sporulation, biofilm formation, extracellular polysaccharide and extracellular protein contents, flagellar motility, and expression of colonization-related genes). The results showed that inoculation of Hymenochaete sp. Rl significantly increased the biomass of B. pumilus HR10 in the rhizosphere; however, while extracts of Hymenochaete sp. Rl and P. thunbergii did not affect the biomass or spore formation of HR10, they did affect its biofilm formation, extracellular polysaccharide and extracellular protein production, and flagellar motility. Furthermore, the addition of symbiont extracts affected the expression of chemotaxis-related genes in HR10. When the extracts were added separately, the expression of srf genes in HR10 increased; when the extracts were added simultaneously, the expression of the flagellin gene fliG in HR10 increased, but there was no significant effect on the expression of srf genes, consistent with the results on biofilm production. Thus, Hymenochaete sp. Rl and P. thunbergii roots had a positive effect on colonization by B. pumilus HR10 at the rhizosphere level through their secretions.

Interactions between arbuscular mycorrhizal fungi (AMF), plants, and the soil microbial community have the potential to increase the availability and uptake of phosphorus (P) and nitrogen (N) in agricultural systems. Nutrient exchange between plant roots, AMF, and the adjacent soil microbes occurs at the interface between roots colonized by mycorrhizal fungi and soil, referred to as the mycorrhizosphere. Research on the P exchange focuses on plant-AMF or AMF-microbe interactions, lacking a holistic view of P exchange between the plants, AMF, and other microbes. Recently, N exchange at both interfaces revealed the synergistic role of AMF and bacterial community in N uptake by the host plant. Here, we highlight work carried out on each interface and build upon it by emphasizing research involving all members of the tripartite network. Both nutrient systems are challenging to study due to the complex chemical and biological nature of the mycorrhizosphere. We discuss some of the effective methods to identify important nutrient processes and the tripartite members involved in these processes. The extrapolation of in vitro studies into the field is often fraught with contradiction and noise. Therefore, we also suggest some approaches that can potentially bridge the gap between laboratory-generated data and their extrapolation to the field, improving the applicability and contextual relevance of data within the field of mycorrhizosphere interactions. Overall, we argue that the research community needs to adopt a holistic tripartite approach and that we have the means to increase the applicability and accuracy of in vitro data in the field., Competing Interests: The authors declare no conflict of interest.

In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces [e.g., rhizosphere, detritusphere, (bio)-pores, and aggregate surfaces]. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment. [ABSTRACT FROM AUTHOR]

In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces [e.g., rhizosphere, detritusphere, (bio)-pores, and aggregate surfaces]. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment.

Arbuscular mycorrhizal fungi living in the soil closely collaborate with plants in their root zone and play very important role in their evolution. Their symbiosis stimulates plant growth and resistance to different environmental stresses. Plant root system, extended by mycelium of arbuscular mycorrhizal fungi, has better capability to reach the water and dissolved nutrients from a much larger volume of soil. This could solve the problem of imminent depletion of phosphate stock, affect plant fertilisation, and contribute to sustainable production of foods, feeds, biofuel, and raw materials. Expanded plant root systems reduce erosion of soil, improve soil quality, and extend the diversity of soil microflora. On the other hand, symbiosis with plants affects species diversity of arbuscular mycorrhizal fungi and increased plant diversity supports diversity of fungi. This review summarizes the importance of arbuscular mycorrhizal fungi in relation to beneficial potential of their symbiosis with plants, and their function in the ecosystem.

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.

The implementation of sustainable agriculture encompasses practices enhancing the activity of beneficial soil microorganisms, able to modulate biogeochemical soil cycles and to affect soil fertility. Among them, arbuscular mycorrhizal fungi (AMF) establish symbioses with the roots of most food crops and play a key role in nutrient uptake and plant protection from biotic and abiotic stresses. Such beneficial services, encompassing improved crop performances, and soil resources availability, are the outcome of the synergistic action of AMF and the vast communities of mycorrhizospheric bacteria living strictly associated with their mycelium and spores, most of which showing plant growth promoting (PGP) activities, such as the ability to solubilize phosphate and produce siderophores and indole acetic acid (IAA). One of the strategies devised to exploit AMF benefits is represented by the inoculation of selected isolates, either as single species or in a mixture. Here, for the first time, the microbiota associated with a commercial AMF inoculum was identified and characterized, using a polyphasic approach, i.e., a combination of culture-dependent analyses and metagenomic sequencing. Overall, 276 bacterial genera were identified by Illumina high-throughput sequencing, belonging to 165 families, 107 orders, and 23 phyla, mostly represented by Proteobacteria and Bacteroidetes. The commercial inoculum harbored a rich culturable heterotrophic bacterial community, whose populations ranged from 2.5 to 6.1 × 106 CFU/mL. The isolation of functional groups allowed the selection of 36 bacterial strains showing PGP activities. Among them, 14 strains showed strong IAA and/or siderophores production and were affiliated with Actinomycetales (Microbacterium trichotecenolyticum, Streptomyces deccanensis/scabiei), Bacillales (Bacillus litoralis, Bacillus megaterium), Enterobacteriales (Enterobacter), Rhizobiales (Rhizobium radiobacter). This work demonstrates for the first time that an AMF inoculum, obtained following industrial production processes, is home of a large and diverse community of bacteria with important functional PGP traits, possibly acting in synergy with AMF and providing additional services and benefits. Such bacteria, available in pure culture, could be utilized, individually and/or in multispecies consortia with AMF, as biofertilizers and bioenhancers in sustainable agroecosystems, aimed at minimizing the use of chemical fertilizers and pesticides, promoting primary production, and maintaining soil health and fertility.

Abstract Numerous 13CO 2 labeling studies have traced the flow of carbon from fresh plant exudates into rhizosphere bacterial communities. However, the succession of the uptake of carbon leaving the roots by distinct rhizosphere microbiota has rarely been resolved between microbial kingdoms. This can provide valuable insights on the niche partitioning of primary rhizodeposit consumption, as well as on community interactions in plant-derived carbon flows in soil. Here, we have traced the flow of fresh plant assimilates to rhizosphere microbiota of maize (Zea mays L.) by rRNA-stable isotope probing (SIP). Carbon flows involving bacteria, unicellular fungi, as well as protists were observed over 5 and 8 days. Surprisingly, labeling of Paraglomerales and several bacteria including Opitutus, Mucliaginibacter and Massilia spp. was especially apparent in soil surrounding the strict rhizosphere after 5 d. This highlights the central role of arbuscular mycorrhizal fungi (AMF) as a shunt for fresh plant assimilates to soil microbes not directly influenced by root exudation. Distinct trophic webs involving different flagellates, amoeba and ciliates were also observed in rhizosphere and surrounding soil, while labeling of filamentous saprotrophic Ascomycota or Basidiomycota was not apparent. This challenges the proposed "sapro-rhizosphere" concept and demonstrates the utility of rRNA-SIP to disentangle inter-kingdom microbial relationships in the rhizosphere. Graphical abstract Image 1 Highlights • rRNA-SIP was used to trace plant-derived carbon flows in the rhizosphere of maize. • 13C-labeling of mycorrhiza was stronger in surrounding soil than in the rhizosphere. • Labelling of bacteria incl. Opitutus spp. was also stronger in surrounding soil. • AMF are a shunt of plant assimilates to microbes outside the strict rhizosphere. [ABSTRACT FROM AUTHOR]

The implementation of sustainable agriculture encompasses practices enhancing the activity of beneficial soil microorganisms, able to modulate biogeochemical soil cycles and to affect soil fertility. Among them, arbuscular mycorrhizal fungi (AMF) establish symbioses with the roots of most food crops and play a key role in nutrient uptake and plant protection from biotic and abiotic stresses. Such beneficial services, encompassing improved crop performances, and soil resources availability, are the outcome of the synergistic action of AMF and the vast communities of mycorrhizospheric bacteria living strictly associated with their mycelium and spores, most of which showing plant growth promoting (PGP) activities, such as the ability to solubilize phosphate and produce siderophores and indole acetic acid (IAA). One of the strategies devised to exploit AMF benefits is represented by the inoculation of selected isolates, either as single species or in a mixture. Here, for the first time, the microbiota associated with a commercial AMF inoculum was identified and characterized, using a polyphasic approach, i.e., a combination of culture-dependent analyses and metagenomic sequencing. Overall, 276 bacterial genera were identified by Illumina high-throughput sequencing, belonging to 165 families, 107 orders, and 23 phyla, mostly represented by Proteobacteria and Bacteroidetes. The commercial inoculum harbored a rich culturable heterotrophic bacterial community, whose populations ranged from 2.5 to 6.1 × 106 CFU/mL. The isolation of functional groups allowed the selection of 36 bacterial strains showing PGP activities. Among them, 14 strains showed strong IAA and/or siderophores production and were affiliated with Actinomycetales (Microbacterium trichotecenolyticum, Streptomyces deccanensis/scabiei), Bacillales (Bacillus litoralis, Bacillus megaterium), Enterobacteriales (Enterobacter), Rhizobiales (Rhizobium radiobacter). This work demonstrates for the first time that an AMF inoculum, obtained following industrial production processes, is home of a large and diverse community of bacteria with important functional PGP traits, possibly acting in synergy with AMF and providing additional services and benefits. Such bacteria, available in pure culture, could be utilized, individually and/or in multispecies consortia with AMF, as biofertilizers and bioenhancers in sustainable agroecosystems, aimed at minimizing the use of chemical fertilizers and pesticides, promoting primary production, and maintaining soil health and fertility. [ABSTRACT FROM AUTHOR]

The concept of food quality, traditionally based on nutritional and sensory properties, has recently acquired an additional meaning, referring to the health-promoting properties of plant products, that are ascribed to plant secondary metabolites called phytochemicals, primarily represented by polyphenolic compounds and glucosinolates. The diversity and content of phytochemicals in plant products are affected by different variables, such as plant genotype, agronomic factors, and arbuscular mycorrhizal fungi (AMF), which establish mycorrhizal symbioses with most crops, including cereals, legumes, vegetables, fruit trees, sunflower, cotton, and sugarcane. AMF and associated bacteria enhance plant growth and health, and affect the production of polyphenols and carotenoids, and the activity of antioxidant enzymes. The production of health-promoting phytochemicals was shown to be differentially modulated by different AMF isolates and bacterial strains, in several food plants, i.e., tomato, lettuce, strawberry, artichoke, maize, grapevine, sunflower. Here, we provide an overview of recent studies concerning the multiple roles played by AMF and associated bacteria in the modulation of the biosynthesis of plant secondary metabolites with health-promoting activity, and discuss the development of designed multifunctional consortia to be used in sustainable agriculture.

Arbuscular mycorrhizal fungi (AMF) are beneficial soil microorganisms establishing mutualistic symbioses with the roots of the most important food crops and playing key roles in the maintenance of long-term soil fertility and health. The great inter- and intra-specific AMF diversity can be fully exploited by selecting AMF inocula on the basis of their colonization ability and efficiency, which are affected by fungal and plant genotypes and diverse environmental variables. The multiple services provided by AMF are the result of the synergistic activities of the bacterial communities living in the mycorrhizosphere, encompassing nitrogen fixation, P solubilization, and the production of phytohormones, siderophores, and antibiotics. The tripartite association among host plants, mycorrhizal symbionts, and associated bacteria show beneficial emerging properties which could be efficiently exploited in sustainable agriculture. Further in-depth studies, both in microcosms and in the field, performed on different AMF species and isolates, should evaluate their colonization ability, efficiency, and resilience. Transcriptomic studies can reveal the expression levels of nutrient transporter genes in fungal absorbing hyphae in the presence of selected bacterial strains. Eventually, newly designed multifunctional microbial consortia can be utilized as biofertilizers and biostimulants in sustainable and innovative production systems.

Plants are key contributors to ecosystem services delivered by green roofs in cities including stormwater capture, temperature regulation, and wildlife habitat. As a result, current research has primarily focused on their growth in relationship to extensive green roof (e.g., substrates

Declining trends in soil health under continuous monoculture systems of winter wheat are a concern for sustainable production in the Southern Great Plains of the US. This study was conducted to evaluate the long-term implementation of conservation tillage in combination with nitrogen treatments and summer cover crop (cowpeas) rotations with winter wheat, for their legacy effects on soil health attributes of microbial communities and soil organic carbon (SOC). Microbial biomass and composition were estimated, along with soil physico-chemical parameters in the soil profile during the annual rotation cycle of wheat and cover crops. Positive legacy effects of cover crop rotations were evident, as arbuscular mycorrhizal fungi (AMF) biomass during the wheat-growing season was significantly higher in cover crop treatments (by around 30-70%) compared to summer fallow treatment. Some dominant taxons such as Acidobacteria, Actinobacteria , Proteobacteria (>70% of prokaryotic relative abundance) and Ascomycota (>50% of fungal relative abundance) were detected in all experimental treatments. Microbial composition did not significantly change at phylum level, although some reorganization at OTU level was evident throughout the soil profile, mostly because of nitrogen treatments. Several Glomeromycota OTUs were significantly altered by soil depth and by nitrogen fertilization suggest distinct mycorhizosphere interactions in subsurface soil than the surface soil. Tillage treatment did not significantly alter the microbial abundance and their diversity. Differences in microbial biomass-C concentration among experimental treatments did not result in a change in SOC concentrations within the soil profile. Results of this study demonstrated that summer cowpea appeared to be an effective cover crop for enhancing beneficial microbial biomass and expansion of the mycorrhizosphere to deeper soil layers. Cover crop rotations appeared to be a suitable option for rapidly enhancing soil health in winter wheat production systems. [ABSTRACT FROM AUTHOR]

This Mycorrhiza issue groups topical papers based on presentations and discussions at the Mycorrhizal Microbiomes session at 9th International Conference on Mycorrhiza, Prague, Czech Republic, August 2017. The five articles that appear in this special issue advance the field of mycorrhizal microbiomes, not simply by importing ideas from an emerging area, but by using them to inform rich and methodologically grounded research. The aim of this special issue is to explore the interactions between mycorrhizal fungi and surrounding complex environments from a distinct but complementary point of view, highlighting the large spectrum of unknowns that still need to be explored. In this editorial, we first introduce the level of knowledge in this thematic area, then describe major results from the five manuscripts and characterise their importance to mycorrhizal research, and finally discuss the developing topics in this rapidly emerging thematic area. [ABSTRACT FROM AUTHOR]

In this study, the bacterial populations of roots and mycospheres of the boreal pine forest ericoid plants, heather ( Calluna vulgaris), bilberry ( Vaccinium myrtillus), and lingonberry ( Vaccinium vitis-idaea), were studied by qPCR and next-generation sequencing (NGS). All bacterial communities of mycosphere soils differed from soils uncolonized by mycorrhizal mycelia. Colonization by mycorrhizal hyphae increased the total number of bacterial 16S ribosomal DNA (rDNA) gene copies in the humus but decreased the number of different bacterial operational taxonomic units (OTUs). Nevertheless, ericoid roots and mycospheres supported numerous OTUs not present in uncolonized humus. Bacterial communities in bilberry mycospheres were surprisingly similar to those in pine mycospheres but not to bacterial communities in heather and lingonberry mycospheres. In contrast, bacterial communities of ericoid roots were more similar to each other than to those of pine roots. In all sample types, the relative abundances of bacterial sequences belonging to Alphaproteobacteria and Acidobacteria were higher than the sequences belonging to other classes. Soil samples contained more Actinobacteria, Deltaproteobacteria, Opitutae, and Planctomycetia, whereas Armatimonadia, Betaproteobacteria, Gammaproteobacteria, and Sphingobacteriia were more common to roots. All mycosphere soils and roots harbored bacteria unique to that particular habitat. Our study suggests that the habitation by ericoid plants increases the overall bacterial diversity of boreal forest soils. [ABSTRACT FROM AUTHOR]

Background and aims: Arbuscular mycorrhizal (AM) hyphae represent an important route for input of plant-derived C to soil, but impacts of these inputs on microbial communities and processes are poorly understood. In this study we characterised pathways of C-flow through microbial communities associated with AM hyphae and quantified impacts on mineralisation of native SOM. Methods: Continuous, steady-state CO labelling was applied throughout the growth period (60 d) of Lolium perenne. Exclusion meshes were used to control access of roots and AM hyphae to soil, and plant-derived C was quantified within microbial PLFA and NLFA, and soil CO efflux was partitioned into plant- and soil organic matter (SOM) derived components. Results: Pathways of C-flow through hyphosphere and mycorrhizosphere communities were distinct, as was the fate of plant-derived C from AM hyphae accessing soil through 37 and 1 μm meshes. Mineralisation of native SOM was increased in all treatments, relative to unplanted controls, and this priming effect was largest for AM hyphae accessing soil through the 1 μm mesh size. Conclusions: We demonstrated that AM hyphae can strongly increase mineralisation of native SOM and identified distinct pathways of C-flow through hyphosphere communities. Our results suggest that, in addition to affecting rates of litter decomposition, AM hyphae may have a significant influence on turnover of native SOM. [ABSTRACT FROM AUTHOR]

Plant growth is highly dependent on bacteria, saprophytic and mycorrhizal fungi which facilitate the cycling and mobilization of nutrients. Over 95% of the sulfur (S) in soil is present in an organic form. Sulfate-esters and sulfonates, the major forms of organo-S in soils, arise through deposition of biological material and are transformed through subsequent humification. Fungi and bacteria release S from sulfate-esters using sulfatases, however, release of S from sulfonates is catalyzed by a bacterial multi-component mono-oxygenase system. The asfA gene is used as a key marker in this desulfonation process to study sulfonatase activity in soil bacteria identified as Variovorax, Polaromonas, Acidovorax and Rhodococcus. The rhizosphere is regarded as a hot spot for microbial activity and recent studies indicate that this is also the case for the mycorrhizosphere where bacteria may attach to the fungal hyphae capable of mobilizing organo-S. While current evidence is not showing sulfatase and sulfonatase activity in arbuscular mycorrhiza, their effect on the expression of plant host sulfate transporters is documented. A revision of the role of bacteria, fungi and the interactions between soil bacteria and mycorrhiza in plant S supply was conducted.