Evolution of unexpected diversity in a putative mating type locus and its correlation with genome variability reveals likely asexuality in the model mycorrhizal fungus Rhizophagus irregularis.

BACKGROUND: Arbuscular mycorrhizal fungi (AMF) form mutualistic partnerships with approximately 80% of plant species. AMF, and their diversity, play a fundamental role in plant growth, driving plant diversity, and global carbon cycles. Knowing whether AMF are sexual or asexual has fundamental consequences for how they can be used in agricultural applications. Evidence for and against sexuality in the model AMF, Rhizophagus irregularis, has been proposed. The discovery of a putative mating-type locus (MAT locus) in R. irregularis, and the previously suggested recombination among nuclei of a dikaryon R. irregularis isolate, potentially suggested sexuality. Unless undergoing frequent sexual reproduction, evolution of MAT-locus diversity is expected to be very low. Additionally, in sexual species, MAT-locus evolution is decoupled from the evolution of arbitrary genome-wide loci. RESULTS: We studied MAT-locus diversity of R. irregularis. This was then compared to diversification in a phosphate transporter gene (PTG), that is not involved in sex, and to genome-wide divergence, defined by 47,378 single nucleotide polymorphisms. Strikingly, we found unexpectedly high MAT-locus diversity indicating that either it is not involved in sex, or that AMF are highly active in sex. However, a strongly congruent evolutionary history of the MAT-locus, PTG and genome-wide arbitrary loci allows us to reject both the hypothesis that the MAT-locus is involved in mating and that the R. irregularis lineage is sexual. CONCLUSION: Our finding shapes the approach to developing more effective AMF strains and is highly informative as it suggests that introduced strains applied in agriculture will not exchange DNA with native populations.

Evolutionary history of arbuscular mycorrhizal fungi and genomic signatures of obligate symbiosis.

BACKGROUND: The colonization of land and the diversification of terrestrial plants is intimately linked to the evolutionary history of their symbiotic fungal partners. Extant representatives of these fungal lineages include mutualistic plant symbionts, the arbuscular mycorrhizal (AM) fungi in Glomeromycota and fine root endophytes in Endogonales (Mucoromycota), as well as fungi with saprotrophic, pathogenic and endophytic lifestyles. These fungal groups separate into three monophyletic lineages but their evolutionary relationships remain enigmatic confounding ancestral reconstructions. Their taxonomic ranks are currently fluid. RESULTS: In this study, we recognize these three monophyletic linages as phyla, and use a balanced taxon sampling and broad taxonomic representation for phylogenomic analysis that rejects a hard polytomy and resolves Glomeromycota as sister to a clade composed of Mucoromycota and Mortierellomycota. Low copy numbers of genes associated with plant cell wall degradation could not be assigned to the transition to a plant symbiotic lifestyle but appears to be an ancestral phylogenetic signal. Both plant symbiotic lineages, Glomeromycota and Endogonales, lack numerous thiamine metabolism genes but the lack of fatty acid synthesis genes is specific to AM fungi. Many genes previously thought to be missing specifically in Glomeromycota are either missing in all analyzed phyla, or in some cases, are actually present in some of the analyzed AM fungal lineages, e.g. the high affinity phosphorus transporter Pho89. CONCLUSION: Based on a broad taxon sampling of fungal genomes we present a well-supported phylogeny for AM fungi and their sister lineages. We show that among these lineages, two independent evolutionary transitions to mutualistic plant symbiosis happened in a genomic background profoundly different from that known from the emergence of ectomycorrhizal fungi in Dikarya. These results call for further reevaluation of genomic signatures associated with plant symbiosis.

Genetic and functional traits limit the success of colonisation by arbuscular mycorrhizal fungi in a tomato wild relative.

To understand whether domestication had an impact on susceptibility and responsiveness to arbuscular mycorrhizal fungi (AMF) in tomato (Solanum lycopersicum), we investigated two tomato cultivars ("M82" and "Moneymaker") and a panel of wild relatives including S. neorickii, S. habrochaites and S. pennellii encompassing the whole Lycopersicon clade. Most genotypes revealed good AM colonisation levels when inoculated with the AMF Funneliformis mosseae. By contrast, both S. pennellii accessions analysed showed a very low colonisation, but with normal arbuscule morphology, and a negative response in terms of root and shoot biomass. This behaviour was independent of fungal identity and environmental conditions. Genomic and transcriptomic analyses revealed in S. pennellii the lack of genes identified within QTLs for AM colonisation, a limited transcriptional reprogramming upon mycorrhization and a differential regulation of strigolactones and AM-related genes compared to tomato. Donor plants experiments indicated that the AMF could represent a cost for S. pennellii: F. mosseae could extensively colonise the root only when it was part of a mycorrhizal network, but a higher mycorrhization led to a higher inhibition of plant growth. These results suggest that genetics and functional traits of S. pennellii are responsible for the limited extent of AMF colonisation.

Rhizophagus irregularis and biochar can synergistically improve the physiological characteristics of saline-alkali resistance of switchgrass.

Inoculation of arbuscular mycorrhizal fungi (AMF) or biochar (BC) application can improve photosynthesis and promote plant growth under saline-alkali stress. However, little is known about the effects of the two combined on growth and physiological characteristics of switchgrass under saline-alkali stress. This study examined the effects of four treatments: (1) no AMF inoculation and no biochar addition (control), (2) biochar (BC) alone, (3) AMF (Rhizophagus irregularis, Ri) alone, and (4) the combination of both (BC+Ri) on the plant biomass, antioxidant enzymes, chlorophyll, and photosynthetic parameters of switchgrass under saline-alkali stress. The results showed that the above-ground, belowground and total biomass of switchgrass in the BC+Ri treatment group was significantly higher (+136.7%, 120.2% and 132.4%, respectively) than in other treatments compared with Control. BC+Ri treatment significantly increased plant leaves' relative chlorophyll content, antioxidant enzyme activity, and photosynthesis parameters. It is worth noting that the transpiration rate, stomatal conductance, net photosynthetic rate, PSII efficiency and other photosynthetic-related indexes of the BC+Ri treatment group were the highest (38% to 54% higher than other treatments). The fitting results of light response and CO2 response curves showed that the light saturation point, light compensation point, maximum carboxylation rate and maximum electron transfer rate of switchgrass in the Ri+BC treatment group were the highest. In conclusion, biochar combined with Ri has potential beneficial effects on promoting switchgrass growth under saline-alkali stress and improving the activity of antioxidant enzymes and photosynthetic characteristics of plants.

Divergent colonization traits, convergent benefits: different species of arbuscular mycorrhizal fungi alleviate Meloidogyne incognita damage in tomato.

Arbuscular mycorrhizal fungi (AMF) can increase plant tolerance and/or resistance to pests such as the root-knot nematode Meloidogyne incognita. However, the ameliorative effects may depend on AMF species. The aim of this work was therefore to evaluate whether four AMF species differentially affect plant performance in response to M. incognita infection. Tomato plants grown in greenhouse conditions were inoculated with four different AMF isolates (Claroideoglomus claroideum, Funneliformis mosseae, Gigaspora margarita, and Rhizophagus intraradices) and infected with 100 second stage juveniles of M. incognita at two different times: simultaneously or 2 weeks after the inoculation with AMF. After 60 days, the number of galls, egg masses, and reproduction factor of the nematodes were assessed along with plant biomass, phosphorus (P), and nitrogen concentrations in roots and shoots and root colonization by AMF. Only the simultaneous nematode inoculation without AMF caused a large reduction in plant shoot biomass, while all AMF species were able to ameliorate this effect and improve plant P uptake. The AMF isolates responded differently to the interaction with nematodes, either increasing the frequency of vesicles (C. claroideum) or reducing the number of arbuscules (F. mosseae and Gi. margarita). AMF inoculation did not decrease galls; however, it reduced the number of egg masses per gall in nematode simultaneous inoculation, except for C. claroideum. This work shows the importance of biotic stress alleviation associated with an improvement in P uptake and mediated by four different AMF species, irrespective of their fungal root colonization levels and specific interactions with the parasite.

Soil compaction reversed the effect of arbuscular mycorrhizal fungi on soil hydraulic properties.

Arbuscular mycorrhizal fungi (AMF) typically provide a wide range of nutritional benefits to their host plants, and their role in plant water uptake, although still controversial, is often cited as one of the hallmarks of this symbiosis. Less attention has been dedicated to other effects relating to water dynamics that the presence of AMF in soils may have. Evidence that AMF can affect soil hydraulic properties is only beginning to emerge. In one of our recent experiments with dwarf tomato plants, we serendipitously found that the arbuscular mycorrhizal fungus (Rhizophagus irregularis 'PH5') can slightly but significantly reduce water holding capacity (WHC) of the substrate (a sand-zeolite-soil mixture). This was further investigated in a subsequent experiment, but there we found exactly the opposite effect as mycorrhizal substrate retained more water than did the non-mycorrhizal substrate. Because the same substrate was used and other conditions were mostly comparable in the two experiments, we explain the contrasting results by different substrate compaction, most likely caused by different pot shapes. It seems that in compacted substrates, AMF may have no effect upon or even decrease the substrates' WHC. On the other hand, the AMF hyphae interweaving the pores of less compacted substrates may increase the capillary movement of water throughout such substrates and cause slightly more water to remain in the pores after the free water has drained. We believe that this phenomenon is worthy of mycorrhizologists' attention and merits further investigation as to the role of AMF in soil hydraulic properties.

Absence of Gigasporales and rarity of spores in a hot desert revealed by a multimethod approach.

Hot deserts impose extreme conditions on plants growing in arid soils. Deserts are expanding due to climate change, thereby increasing the vulnerability of ecosystems and the need to preserve them. Arbuscular mycorrhizal fungi (AMF) improve plant fitness by enhancing plant water/nutrient uptake and stress tolerance. However, few studies have focused on AMF diversity and community composition in deserts, and the soil and land use parameters affecting them. This study aimed to comprehensively describe AMF ecological features in a 5,000 km2 arid hyperalkaline region in AlUla, Saudi Arabia. We used a multimethod approach to analyse over 1,000 soil and 300 plant root samples of various species encompassing agricultural, old agricultural, urban and natural ecosystems. Our method involved metabarcoding using 18S and ITS2 markers, histological techniques for direct AMF colonization observation and soil spore extraction and observation. Our findings revealed a predominance of AMF taxa assigned to Glomeraceae, regardless of the local conditions, and an almost complete absence of Gigasporales taxa. Land use had little effect on the AMF richness, diversity and community composition, while soil texture, pH and substantial unexplained stochastic variance drove these compositions in AlUla soils. Mycorrhization was frequently observed in the studied plant species, even in usually non-mycorrhizal plant taxa (e.g. Amaranthaceae, Urticaceae). Date palms and Citrus trees, representing two major crops in the region, however, displayed a very low mycorrhizal frequency and intensity. AlUla soils had a very low concentration of spores, which were mostly small. This study generated new insight on AMF and specific behavioral features of these fungi in arid environments.

Unraveling the diversity of hyphal explorative traits among Rhizophagus irregularis genotypes.

Differences in functioning among various genotypes of arbuscular mycorrhizal (AM) fungi can determine their fitness under specific environmental conditions, although knowledge of the underlying mechanisms still is very fragmented. Here we compared seven homokaryotic isolates (genotypes) of Rhizophagus irregularis, aiming to characterize the range of intraspecific variability with respect to hyphal exploration of organic nitrogen (N) resources, and N supply to plants. To this end we established two experiments (one in vitro and one in open pots) and used 15N-chitin as the isotopically labeled organic N source. In Experiment 1 (in vitro), mycelium of all AM fungal genotypes transferred a higher amount of 15N to the plants than the passive transfer of 15N measured in the non-mycorrhizal (NM) controls. Noticeably, certain genotypes (e.g., LPA9) showed higher extraradical mycelium biomass production but not necessarily greater 15N acquisition than the others. Experiment 2 (in pots) highlighted that some of the AM fungal genotypes (e.g., MA2, STSI) exhibited higher rates of targeted hyphal exploration of chitin-enriched zones, indicative of distinct N exploration patterns from the other genotypes. Importantly, there was a high congruence of hyphal exploration patterns between the two experiments (isolate STSI always showing highest efficiency of hyphal exploration and isolate L23/1 being consistently the lowest), despite very different (micro) environmental conditions in the two experiments. This study suggests possible strategies that AM fungal genotypes employ for efficient N acquisition, and how to measure them. Implications of such traits for local mycorrhizal community assembly still need to be understood.

Rhizophagus Irregularis regulates flavonoids metabolism in paper mulberry roots under cadmium stress.

Broussonetia papyrifera is widely found in cadmium (Cd) contaminated areas, with an inherent enhanced flavonoids metabolism and inhibited lignin biosynthesis, colonized by lots of symbiotic fungi, such as arbuscular mycorrhizal fungi (AMF). However, the physiological and molecular mechanisms by which Rhizophagus irregularis, an AM fungus, regulates flavonoids and lignin in B. papyrifera under Cd stress remain unclear. Here, a pot experiment of B. papyrifera inoculated and non-inoculated with R. irregularis under Cd stress was carried out. We determined flavonoids and lignin concentrations in B. papyrifera roots by LC-MS and GC-MS, respectively, and measured the transcriptional levels of flavonoids- or lignin-related genes in B. papyrifera roots, aiming to ascertain the key components of flavonoids or lignin, and key genes regulated by R. irregularis in response to Cd stress. Without R. irregularis, the concentrations of eriodictyol, quercetin and myricetin were significantly increased under Cd stress. The concentrations of eriodictyol and genistein were significantly increased by R. irregularis, while the concentration of rutin was significantly decreased. Total lignin and lignin monomer had no alteration under Cd stress or with R. irregularis inoculation. As for flavonoids- or lignin-related genes, 26 genes were co-regulated by Cd stress and R. irregularis. Among these genes, BpC4H2, BpCHS8 and BpCHI5 were strongly positively associated with eriodictyol, indicating that these three genes participate in eriodictyol biosynthesis and were involved in R. irregularis assisting B. papyrifera to cope with Cd stress. This lays a foundation for further research revealing molecular mechanisms by which R. irregularis regulates flavonoids synthesis to enhance tolerance of B. papyrifera to Cd stress.

Mycorrhizal Symbiosis Can Change the Composition of Secondary Metabolites in Fruits of Solanum nigrum L.

Solanum nigrum is a common weed in arable land, while being used in traditional medicine around the world due to its remarkable levels of valuable secondary metabolites. Agronomic and biological techniques can alter the production of a specific metabolite by influencing plant growth and metabolism. The effects of colonization with three arbuscular mycorrhizal fungi (AMF), including Funneliformis mosseae, Rhizoglomus intraradices, and Rhizoglomus fasciculatum, on the chemical composition of S. nigrum fruits were evaluated by gas chromatography-mass spectrometry (GC-MS) analysis. More than 100 different chemical constituents were evaluated by GC-MS. Our study revealed that the levels of phenols (quinic acid), benzenes (hydroquinone), sulfur-containing compounds, lactone and carboxylic acids were improved by R. intraradices. In contrast, hydroxymethylfurfural increased by 68 % in R. fasciculatum inoculated with uninoculated S. nigrum plants, and this species was also the most efficient in inducing sugar compounds (D-galactose, lactose, and melezitose). Our results suggest that AMF colonization is an effective biological strategy that can alter the chemical composition and improve the medicinal properties of S. nigrum.

Different behavior of two strains of the arbuscular mycorrhizal fungus Rhizophagus intraradices on Senecio bonariensis Hook. & Arn. against heavy metal soil pollution: a pilot-scale test.

Arbuscular mycorrhizal fungi (AMF) have different biological mechanisms to alleviate stressful conditions in heavy metals (HMs) polluted soil. These mechanisms were widely assessed under controlled/greenhouse conditions, but scarcely studied at pilot or territory scale. The aim of this study was to evaluate the response of two Rhizophagus intraradices strains isolated from soils with different histories of pollution, in association with Senecio bonariensis plants, growing in an engineering vegetal depuration module filled with artificially HMs polluted substrate. Plants inoculated with GC3 strain uptook low amounts of HMs and translocated them to shoot biomass. Heavy metals (Mg, Zn, Mn, Cr, Cu and Ni) and macronutrients (Ca, K, S and P) were accumulated in roots of S. bonariensis when inoculated with GB8 strain, limiting their translocation to the shoot. Uninoculated plants showed high translocation of all studied elements to shoot tissues. Concluding, tested R. intraradices strains have exhibited different phytoprotection mechanisms under extremely toxic concentrations of HMs. Moreover, the development of the assay at such a high Technological Readiness Level represents a novel contribution in this field of study.

Glomus versiforme and intercropping with Sphagneticola calendulacea decrease Cd accumulation in maize.

Little information is available on the influence of the compound use of intercropping (IN) and arbuscular mycorrhizal fungus (AMF) on Cd accumulation and the expression of Cd transporter genes in two intercropped plants. A pot experiment was conducted to study the influences of IN and AMF-Glomus versiforme on growth and Cd uptake of two intercropped plants-maize and Cd hyperaccumulator Sphagneticola calendulacea, and the expression of Cd transporter genes in maize in Cd-polluted soils. IN, AMF and combined treatments of IN and AMF (IN + AMF) obviously improved biomass, photosynthesis and total antioxidant capacities of two plants. Moreover, single and compound treatments of IN and AMF evidently reduced Cd contents in maize, and the greatest decreases appeared in the compound treatment. However, Cd contents of S. calendulacea in IN, AMF and IN + AMF groups were notably improved. Furthermore, the single and compound treatments of IN and AMF significantly downregulated the expression levels of Nramp1, HMA1, ABCC1 and ABCC10 in roots and leaves, and the largest decreases were observed in the combined treatment. Our work first revealed that the combined use of IN and AMF appeared to have a synergistic effect on decreasing Cd content by downregulating the expression of Cd transporter genes in maize.

Water stress protection by the arbuscular mycorrhizal fungus Rhizoglomus irregulare involves physiological and hormonal responses in an organ-specific manner.

Arbuscular mycorrhizal fungi may alleviate water stress in plants. Although several protection mechanisms have already been described, little information is available on how these fungi influence the hormonal response to water stress at an organ-specific level. In this study, we evaluated the physiological and hormonal responses to water stress in above and below-ground tissues of the legume grass Trifolium repens colonized by the arbuscular mycorrhizal fungus Rhizoglomus irregulare. Plants were subjected to progressive water stress and recovery. Different leaf and root physiological parameters, as well as phytohormone levels, were quantified. Water-stressed mycorrhizal plants showed an improved water status and no photoinhibition compared to uncolonized individuals, while some stress markers like α-tocopherol and malondialdehyde content, an indicator of the extent of lipid peroxidation, transiently increased in roots, but not in leaves. Water stress protection exerted by mycorrhiza appeared to be related to a differential root-to-shoot redox signaling, probably mediated by jasmonates, and mycorrhization enhanced the production of the cytokinin trans-zeatin in both roots and leaves. Overall, our results suggest that mycorrhization affects physiological, redox and hormonal responses to water stress at an organ-specific level, which may eventually modulate the final protection of the host from water stress.

Functional analysis of the OsNPF4.5 nitrate transporter reveals a conserved mycorrhizal pathway of nitrogen acquisition in plants.

Low availability of nitrogen (N) is often a major limiting factor to crop yield in most nutrient-poor soils. Arbuscular mycorrhizal (AM) fungi are beneficial symbionts of most land plants that enhance plant nutrient uptake, particularly of phosphate. A growing number of reports point to the substantially increased N accumulation in many mycorrhizal plants; however, the contribution of AM symbiosis to plant N nutrition and the mechanisms underlying the AM-mediated N acquisition are still in the early stages of being understood. Here, we report that inoculation with AM fungus Rhizophagus irregularis remarkably promoted rice (Oryza sativa) growth and N acquisition, and about 42% of the overall N acquired by rice roots could be delivered via the symbiotic route under N-NO3- supply condition. Mycorrhizal colonization strongly induced expression of the putative nitrate transporter gene OsNPF4.5 in rice roots, and its orthologs ZmNPF4.5 in Zea mays and SbNPF4.5 in Sorghum bicolor OsNPF4.5 is exclusively expressed in the cells containing arbuscules and displayed a low-affinity NO3- transport activity when expressed in Xenopus laevis oocytes. Moreover, knockout of OsNPF4.5 resulted in a 45% decrease in symbiotic N uptake and a significant reduction in arbuscule incidence when NO3- was supplied as an N source. Based on our results, we propose that the NPF4.5 plays a key role in mycorrhizal NO3- acquisition, a symbiotic N uptake route that might be highly conserved in gramineous species.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis.

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Differences in the effects of single and mixed species of AMF on the growth and oxidative stress defense in Lolium perenne exposed to hydrocarbons.

Arbuscular mycorrhizal fungi (AMF) are ubiquitous mutualistic plant symbionts that promote plant growth and protect them from abiotic stresses. Studies on AMF-assisted phytoremediation have shown that AMF can increase plant tolerance to the presence of hydrocarbon contaminants by improving plant nutrition status and mitigating oxidative stress. This work aimed to evaluate the impact of single and mixed-species AMF inocula (Funneliformis caledonium, Diversispora varaderana, Claroideoglomus walkeri), obtained from a contaminated environment, on the growth, oxidative stress (DNA oxidation and lipid peroxidation), and activity of antioxidative enzymes (superoxide dismutase, catalase, peroxidase) in Lolium perenne growing on a substrate contaminated with 0/0-30/120 mg phenol/polynuclear aromatic hydrocarbons (PAHs) kg-1. The assessment of AMF tolerance to the presence of contaminants was based on mycorrhizal root colonization, spore production, the level of oxidative stress, and antioxidative activity in AMF spores. In contrast to the mixed-species AMF inoculum, single AMF species significantly enhanced the growth of host plants cultured on the contaminated substrate. The effect of inoculation on the level of oxidative stress and the activity of antioxidative enzymes in plant tissues differed between the AMF species. Changes in the level of oxidative stress and the activity of antioxidative enzymes in AMF spores in response to contamination also depended on AMF species. Although the concentration of phenol and PAHs had a negative effect on the production of AMF spores, low (5/20 mg phenol/PAHs kg-1) and medium (15/60 mg phenol/PAHs kg-1) substrate contamination stimulated the mycorrhizal colonization of roots. Among the studied AMF species, F. caledonium was the most tolerant to phenol and PAHs and showed the highest potential in plant growth promotion. The results presented in this study might contribute to the development of functionally customized AMF-assisted phytoremediation strategies with indigenous AMF, more effective than commercial AMF inocula, as a result of their selection by the presence of contaminants.

Arbuscular mycorrhizal fungus facilitates ryegrass (Lolium perenne L.) growth and polychlorinated biphenyls degradation in a soil applied with nanoscale zero-valent iron.

Nanoscale zero-valent iron (nZVI) shows an excellent degradation effect on chlorinated contaminants in soil, but poses a threat to plants in combination with phytoremediation. Arbuscular mycorrhizal (AM) fungus can reduce the phyototoxicity of nZVI, but their combined impacts on polychlorinated biphenyls (PCBs) degradation and plant growth remain unclear. Here, a greenhouse pot experiment was conducted to investigate the influences of nZVI and/or Funneliformis caledonium on soil PCB degradation and ryegrass (Lolium perenne L.) antioxidative responses. The amendment of nZVI significantly reduced not only the total and homolog concentrations of PCBs in the soil, but also the ryegrass biomass as well as soil available P and root P concentrations. Moreover, nZVI significantly decreased leaf superoxide disutase (SOD) activity, while tended to decrease the protein content. In contrast, the additional inoculation of F. caledonium significantly increased leaf SOD activity and protein content, while tended to increase the catalase activity and tended to decrease the malondialdehyde content. The additional inoculation of F. caledonium also significantly increased soil alkaline phosphatase activity, and tended to increase root P concentration, but had no significantly effects on soil available P concentration, the biomass and P acquisition of ryegrass, which could be attributed to the fixation of soil available nutrients by nZVI. Additionally, F. caledonium facilitated PCB degradation in the nZVI-applied soil. Thus, AM fungus can alleviate the nZVI-induced phytotoxicity, showing great application potentials in accompany with nZVI for soil remediation.

Ecotoxicological test to assess effects of herbicides on spore germination of Rhizophagus clarus and Gigaspora albida.

Given the essential role of arbuscular mycorrhizal fungi (AMF) in soil systems and agriculture, their use as biological indicators has risen in all fields of microbiology research. However, AMF sensitivity to chemical pesticides is poorly understood in field conditions, and not explored in ecotoxicology protocols. Hence, the goal of this study was to evaluate the effects of different concentrations of glyphosate (Roundup®) and diuron+paraquat (Gramocil®) on the germination of spores of Gigaspora albida and Rhizophagus clarus in a tropical artificial soil. This study was conducted in 2019 at the Soil Ecology and Ecotoxicology Laboratory of the Universidade do Estado de Santa Catarina. The nominal concentrations of glyphosate were 0, 10, 50, 100, 250, 500, 750 and 1000 mg a.i. kg-1. For diuron+paraquat, the concentrations tested were 0, 10 + 20, 50 + 100, 100 + 200, 250 + 500, 500 + 1000, 750 + 1500 and 1000 + 2000 mg a.i. kg-1. Glyphosate did not alter germination of G. albida, but germination inhibition of R. clarus spores was of 30.8% at 1000 mg kg-1. Diuron+paraquat inhibited by 8.0% germination of G. albida, but only at the highest concentration tested. On the other hand, effects on R. clarus were detected at 50 + 100 mg kg-1 concentration and above, and inhibition was as high as 57.7% at the highest concentration evaluated. These results suggest distinct response mechanisms of Rhizophagus and Gigaspora when exposed to herbicides, with the former being more sensitive than the later.

Gibberellin Promotes Fungal Entry and Colonization during Paris-Type Arbuscular Mycorrhizal Symbiosis in Eustoma grandiflorum.

Arbuscular mycorrhizas (AMs) are divided into two types according to morphology: Arum- and Paris-type AMs. Gibberellins (GAs) mainly inhibit the establishment of Arum-type AM symbiosis in most model plants, whereas the effects of GAs on Paris-type AM symbiosis are unclear. To provide insight into the mechanism underlying this type of symbiosis, the roles of GAs were investigated in Eustoma grandiflorum when used as the host plant for Paris-type AM establishment. Eustoma grandiflorum seedlings were inoculated with the model AM fungus, Rhizophagus irregularis, and the effects of GA and the GA biosynthesis inhibitor uniconazole-P on the symbiosis were quantitatively evaluated. Exogenous GA significantly increased hyphopodium formation at the epidermis, thus leading to the promotion of fungal colonization and arbuscule formation in the root cortex. By contrast, the suppression of GA biosynthesis and signaling attenuated fungal entry to E. grandiflorum roots. Moreover, the exudates from GA-treated roots strongly induced the hyphal branching of R. irregularis. Our results show that GA has an contrasting effect on Paris-type AM symbiosis in E. grandiflorum compared with Arum-type AM symbiosis. This finding could be explained by the differential regulation of the early colonization stage, where fungal hyphae make contact with and penetrate the epidermis.

At the nexus of three kingdoms: the genome of the mycorrhizal fungus Gigaspora margarita provides insights into plant, endobacterial and fungal interactions.

As members of the plant microbiota, arbuscular mycorrhizal fungi (AMF, Glomeromycotina) symbiotically colonize plant roots. AMF also possess their own microbiota, hosting some uncultivable endobacteria. Ongoing research has revealed the genetics underlying plant responses to colonization by AMF, but the fungal side of the relationship remains in the dark. Here, we sequenced the genome of Gigaspora margarita, a member of the Gigasporaceae in an early diverging group of the Glomeromycotina. In contrast to other AMF, G. margarita may host distinct endobacterial populations and possesses the largest fungal genome so far annotated (773.104 Mbp), with more than 64% transposable elements. Other unique traits of the G. margarita genome include the expansion of genes for inorganic phosphate metabolism, the presence of genes for production of secondary metabolites and a considerable number of potential horizontal gene transfer events. The sequencing of G. margarita genome reveals the importance of its immune system, shedding light on the evolutionary pathways that allowed early diverging fungi to interact with both plants and bacteria.