Extraradical Mycelium Research Articles

Assessing extraradical mycelium of mycorrhizal fungi in tropical forests using armored in-growth mesh bags

The extraradical mycelium of mycorrhizal fungi is among the major carbon pools in soil that is hard to quantitatively assess in-situ. Established method of in-growth mesh bags in temperate ecosystems is difficult to apply in the tropics, where mesh bags are often damaged by termites. Here we introduce a modification of the in-growth mesh bag technique, in which mesh bags are enforced by stainless steel mesh. Its performance was tested in the Đồng Nai (Cát Tiên) National Park in Vietnam across two monsoon tropical forests, dominated by tree species associated with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. Armored in-growth mesh bags remained intact, while about 60 % of non-armored mesh bags were damaged by termites after 180 days of exposure. The biomass of extraradical mycelium of ectomycorrhizal fungi estimated by PLFA analysis was similar in the armored and non-armored mesh bags and did not differ between studied forests. However, fungal community composition slightly differed between armored and non-armored mesh bags in the ECM- but not in the AM-dominated forest. Fungal mycelium gathered in the AM-dominated forest was depleted in 15N compared to that collected in the ECM-dominated forest. Overall, our results argue for using armored mesh bags as a robust tool for harvesting the biomass of extraradical mycelium of mycorrhizal fungi in tropical ecosystems.

Micro PIXE mapping proves a differential distribution and concentration of trace elements in fungal structures of Rhizophagus intraradices

Arbuscular mycorrhizal (AM) fungi can sequester different potentially toxic elements, such as trace elements (TEs), within their structures to alleviate the toxicity for its host plant and themselves. To elucidate the role of AM fungi in TEs immobilization in the rhizosphere of host plants, it is important to know the TEs distribution in AM fungal structures. In the present study, we investigated the distribution and concentration of TEs within extraradical spores and mycelium of the AM fungus Rhizophagus intraradices, collected from the rhizosphere of Senecio bonariensis plants grown in a soil polluted with multiple TEs, by using Particle-Induced X-ray Emission with a micro-focused beam (micro PIXE). This technique enabled the simultaneous micrometric mapping of elements in a sample. The calculated values were compared with those in the polluted substrate, measured by the Wavelength Dispersive X-ray Fluorescence technique. The highest concentrations of Fe, P, Ti, Mn, Cr, Cu and Zn were found in AM fungal spores, where they were accumulated, while extraradical mycelium was enriched in Cu. Finally, we demonstrated that AM fungi can simultaneously accumulate high amounts of different TEs in their structures, thus reducing the toxicity of these elements to its host plant.

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.

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.

"Ectomycorrhizal exploration type" could be a functional trait explaining the spatial distribution of tree symbiotic fungi as a function of forest humus forms.

In European forests, most tree species form symbioses with ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi. The EM fungi are classified into different morphological types based on the development and structure of their extraradical mycelium. These structures could be root extensions that help trees to acquire nutrients. However, the relationship between these morphological traits and functions involved in soil nutrient foraging is still under debate.We described the composition of mycorrhizal fungal communities under 23 tree species in a wide range of climates and humus forms in Europe and investigated the exploratory types of EM fungi. We assessed the response of this tree extended phenotype to humus forms, as an indicator of the functioning and quality of forest soils. We found a significant relationship between the relative proportion of the two broad categories of EM exploration types (short- or long-distance) and the humus form, showing a greater proportion of long-distance types in the least dynamic soils. As past land-use and host tree species are significant factors structuring fungal communities, we showed this relationship was modulated by host trait (gymnosperms versus angiosperms), soil depth and past land use (farmland or forest).We propose that this potential functional trait of EM fungi be used in future studies to improve predictive models of forest soil functioning and tree adaptation to environmental nutrient conditions.

Strain-specific evolution and host-specific regulation of transposable elements in the model plant symbiont Rhizophagus irregularis.

Transposable elements (TEs) are repetitive DNA that can create genome structure and regulation variability. The genome of Rhizophagus irregularis, a widely studied arbuscular mycorrhizal fungus (AMF), comprises ∼50% repetitive sequences that include TEs. Despite their abundance, two-thirds of TEs remain unclassified, and their regulation among AMF life stages remains unknown. Here, we aimed to improve our understanding of TE diversity and regulation in this model species by curating repeat datasets obtained from chromosome-level assemblies and by investigating their expression across multiple conditions. Our analyses uncovered new TE superfamilies and families in this model symbiont and revealed significant differences in how these sequences evolve both within and between R. irregularis strains. With this curated TE annotation, we also found that the number of upregulated TE families in colonized roots is 4 times higher than in the extraradical mycelium, and their overall expression differs depending on the plant host. This work provides a fine-scale view of TE diversity and evolution in model plant symbionts and highlights their transcriptional dynamism and specificity during host-microbe interactions. We also provide Hidden Markov Model profiles of TE domains for future manual curation of uncharacterized sequences (https://github.com/jordana-olive/TE-manual-curation/tree/main).

The arbuscular mycorrhizal fungus Rhizophagus irregularis uses the copper exporting ATPase RiCRD1 as a major strategy for copper detoxification

Arbuscular mycorrhizal (AM) fungi establish a mutualistic symbiosis with most land plants. AM fungi regulate plant copper (Cu) acquisition both in Cu deficient and polluted soils. Here, we report characterization of RiCRD1, a Rhizophagus irregularis gene putatively encoding a Cu transporting ATPase. Based on its sequence analysis, RiCRD1 was identified as a plasma membrane Cu + efflux protein of the P1B1-ATPase subfamily. As revealed by heterologous complementation assays in yeast, RiCRD1 encodes a functional protein capable of conferring increased tolerance against Cu. In the extraradical mycelium, RiCRD1 expression was highly up-regulated in response to high concentrations of Cu in the medium. Comparison of the expression patterns of different players of metal tolerance in R. irregularis under high Cu levels suggests that this fungus could mainly use a metal efflux based-strategy to cope with Cu toxicity. RiCRD1 was also expressed in the intraradical fungal structures and, more specifically, in the arbuscules, which suggests a role for RiCRD1 in Cu release from the fungus to the symbiotic interface. Overall, our results show that RiCRD1 encodes a protein which could have a pivotal dual role in Cu homeostasis in R. irregularis, playing a role in Cu detoxification in the extraradical mycelium and in Cu transfer to the apoplast of the symbiotic interface in the arbuscules.

Fungal metabolism and free amino acid content may predict nitrogen transfer to the host plant in the ectomycorrhizal relationship between Pisolithus spp. and Eucalyptus grandis.

Ectomycorrhizal (ECM) fungi are crucial for tree nitrogen (N) nutrition; however, mechanisms governing N transfer from fungal tissues to the host plant are not well understood. ECM fungal isolates, even from the same species, vary considerably in their ability to support tree N nutrition, resulting in a range of often unpredictable symbiotic outcomes. In this study, we used isotopic labelling to quantify the transfer of N to the plant host by isolates from the ECM genus Pisolithus, known to have significant variability in colonisation and transfer of nutrients to a host. We considered the metabolic fate of N acquired by the fungi and found that the percentage of plant N acquired through symbiosis significantly correlated to the concentration of free amino acids in ECM extra-radical mycelium. Transcriptomic analyses complemented these findings with isolates having high amino acid content and N transfer showing increased expression of genes related to amino acid transport and catabolic pathways. These results suggest that fungal N metabolism impacts N transfer to the host plant in this interaction and that relative N transfer may be possible to predict through basic biochemical analyses.

Cadmium uptake and mycorrhization by cacao clones in agroforestry and monoculture systems of Peruvian Amazon

The production system influences the cadmium (Cd) content and mycorrhization in cocoa plantations. The objective of this study was to determine the effects of different production systems on Cd uptake and the presence of mycorrhizas in cacao clones in field conditions, in the Peruvian Amazon. Twelve subplots of 108 m2 were selected in representative cocoa cultivation systems under agroforestry (AF) and monoculture (MON), with the cocoa clones ICS and CCN. Significant differences and data distribution were evaluated using ANOVA, principal component analysis, and Tukey's tests. Mycorrhizal colonization was higher in the AF_ICS system (71.11%) while the length of the extraradical mycelium was higher in the AF_CCN system (17.23%). The highest Cd content in soils was found under the AF_CCN and AF_ICS systems, both with 0.39 mg kg-1. The Cd content in cacao roots, leaves, and beans were higher in the MON_CCN system with 1.87, 2.06, and 1.12 mg kg-1 respectively. Cocoa monocultures (with both clones) generally showed lower levels of mycorrhizal colonization than agroforestry systems, which in turn (also for both clones) presented higher Cd content in beans, even exceeding the limit established by the world health authorities.

Interannual dynamics of Tuber melanosporum and fungal communities in productive black truffle orchards amended with truffle nests.

Truffle growers devote great efforts to improve black truffle productivity, developing agronomic practices such as 'truffle nests' (peat amendments that are supplemented with truffle spore inoculum). It has been hypothesized that improved fruiting associated with nests is linked to stimulation of truffle mycelia previously established in soil or to changes generated in soil fungal community. To assess this, we used real-time PCR to quantify black truffle extraradical mycelium during 2 years after nests installation. We also characterized the fungal community via high-throughput amplicon sequencing of the ITS region of rRNA genes. We found that neither the abundance of truffle mycelium in nests nor in the soil-nest interphase was higher than in the bulk soil, which indicates that nests do not improve mycelial growth. The fungal community in nests showed lower richness and Shannon index and was compositionally different from that of soil, which suggests that nests may act as an open niche for fungal colonization that facilitates truffle fruiting. The ectomycorrhizal fungal community showed lower richness in nests. However, no negative relationships between amount of truffle mycelium and reads of other ectomycorrhizal fungi were found, thus countering the hypothesis that ectomycorrhizal competition plays a role in the nest effect.

Mycorrhizal Colonization of Wheat by Intact Extraradical Mycelium of Mn-Tolerant Native Plants Induces Different Biochemical Mechanisms of Protection.

Soil with excess Mn induces toxicity and impairs crop growth. However, with the development in the soil of an intact extraradical mycelia (ERM) from arbuscular mycorrhizal fungi (AMF) symbiotic to native Mn-tolerant plants, wheat growth is promoted due to a stronger AMF colonization and subsequent increased protection against Mn toxicity. To determine the biochemical mechanisms of protection induced by this native ERM under Mn toxicity, wheat grown in soil from previously developed Lolium rigidum (LOL) or Ornithopus compressus (ORN), both strongly mycotrophic plants, was compared to wheat grown in soil from previously developed Silene gallica (SIL), a non-mycotrophic plant. Wheat grown after LOL or ORN had 60% higher dry weight, ca. two-fold lower Mn levels and almost double P contents. Mn in the shoots was preferentially translocated to the apoplast along with Mg and P. The activity of catalase increased; however, guaiacol peroxidase (GPX) and superoxide dismutase (SOD) showed lower activities. Wheat grown after ORN differed from that grown after LOL by displaying slightly higher Mn levels, higher root Mg and Ca levels and higher GPX and Mn-SOD activities. The AMF consortia established from these native plants can promote distinct biochemical mechanisms for protecting wheat against Mn toxicity.

Effects of arbuscular mycorrhizae and extraradical mycelium of subtropical tree species on soil nitrogen mineralization and enzyme activities.

Through symbiosis with plants, arbuscular mycorrhizal (AM) fungi effectively improve the availability of soil nitrogen (N). However, the mechanism through which AM and associated extraradical mycelium affect soil N mineralization remains unknow. We carried out an in situ soil culture experiment by using in-growth cores in plantations of three subtropical tree species, Cunninghamia lanceolata, Schima superba, and Liquidambar formosana. We measured soil physical and chemical properties, net N mineralization rate, and the activities of four kinds of hydrolase (leucine aminopeptidase (LAP), β-1,4-N-acetylglucosaminidase (NAG), β-1,4-glucosidase (βG), cellobiohydrolase (CB)) and two kinds of oxidases (polyphenol oxidase (POX) and peroxidase (PER)) involved in soil organic matter (SOM) mineralization in treatments of mycorrhiza (with absorbing roots and hyphae), hyphae (hyphae only), and control (mycorrhiza-free). The results showed that mycorrhizal treatments significantly affected soil total carbon and pH but did not affect N mineralization rates and all enzymatic activities. Tree species significantly affected net ammonification rate, net N mineralization rate and activities of NAG, βG, CB, POX and PER. The net N mineralization rate and enzyme activities in the C. lanceolata stand were significantly higher than that in monoculture broad-leaved stands of either S. superba or L. formosana. There was no interactive effect of mycorrhizal treatment and tree species on any of soil properties, nor on enzymatic activities or net N mineralization rates. Soil pH was negatively and significantly correlated with five kinds of enzymatic activities except for LAP, while net N mineralization rate significantly correlated with ammonium nitrogen content, available phosphorus content, and the activity level of βG, CB, POX, and PER. In conclusion, there was no difference in enzymatic activities and N mineralization rates between rhizosphere and hyphosphere soils of three subtropical tree species in the whole growing season. The activity of particular carbon cycle-related enzymes was closely related to soil N mineralization rate. It is suggested that differences in litter quality and root functional traits among different tree species affect soil enzyme activities and N mineralization rates through organic matter inputs and shaping soil condition.

Mycorrhiza-feeding soil invertebrates in two coniferous forests traced with 13C labelling.

Mycorrhizal fungi represent a potentially abundant carbon resource for soil animals, but their role in soil food webs remains poorly understood. To detect taxa that are trophically linked to the extraradical mycelium of mycorrhizal fungi, we used stable isotope (13C) labelling of whole trees in combination with the in-growth mesh bag technique in two coniferous forests. This allowed us to detect the flux of carbon in the mycelium of mycorrhizal fungi, and consequently in the tissues of soil invertebrates. The mycorrhizal fungal genera constituted 93.5% of reads in mycelium samples from the in-growth mesh bags. All mycelium from in-growth mesh bags and about 32% of the invertebrates sampled (in total 11 taxa) received the 13C label after 45days of exposure. The extent of feeding of soil invertebrates on the mycelium of mycorrhizal fungi depended on the taxonomic affinity of the animals. The strongest trophic link to the mycorrhiza-derived carbon was detected in Isotomidae (Collembola) and Oppiidae (Oribatida). The label was also observed in the generalist predators, indicating the propagation of mycorrhiza-derived carbon into the higher trophic levels of the soil food web. Higher 13C labelling in the tissues of euedaphic Collembola and Oribatida compared to atmobiotic and hemiedaphic families indicates the importance of mycorrhizal fungi as a food resource for invertebrates in deeper soil horizons.

The effect of plant mycotrophy and soil disturbance on soil microbial activity

AbstractIn cropping systems, the choices adopted for the tillage system used and plants cultivated can strongly influence the soil microbial population and its functional profile. Arbuscular mycorrhizal fungi are an important component of soil microbiome and their mutualistic symbiosis with the majority of higher plants grant the latter a wide range of benefits. The extraradical mycelium developed by these fungi expands the volume of soil influenced and harbours a diversity of microbes establishing a distinct environment of complementary interactions. We assessed how growing plants with different levels of mycotrophy modifies the biological activity profile in the soil under Mn toxicity and whether this is modified by soil disturbance. Following mycotrophic plants, soil contained a more active microbiome than after the non‐mycotrophic plants, as expressed by higher values of soil basal respiration or dehydrogenase activity. Additionally, the count of phosphorus solubilizes and activity of phosphatase were greater after mycotrophic plants. Even among mycotrophic plants, different profiles of biological activity can be distinguished after growing a legume or grass. ERM disruption by soil disturbance decreased most of the parameters studied and for phosphatase activity and P solubilizers in a more significant way. These results indicate that even under Mn toxicity, the microbiome associated with AMF symbiosis following mycotrophic plants growth presented a higher biological activity and had a differential biological response towards the stress imposed by soil disturbance, when compared with the microbiome associated with non‐mycotrophic roots.

Aboveground herbivory does not affect mycorrhiza-dependent nitrogen acquisition from soil but inhibits mycorrhizal network-mediated nitrogen interplant transfer in maize.

Arbuscular mycorrhizal fungi (AMF) are considered biofertilizers for sustainable agriculture due to their ability to facilitate plant uptake of important mineral elements, such as nitrogen (N). However, plant mycorrhiza-dependent N uptake and interplant transfer may be highly context-dependent, and whether it is affected by aboveground herbivory remains largely unknown. Here, we used 15N labeling and tracking to examine the effect of aboveground insect herbivory by Spodoptera frugiperda on mycorrhiza-dependent N uptake in maize (Zea mays L.). To minimize consumption differences and 15N loss due to insect chewing, insect herbivory was simulated by mechanical wounding and oral secretion of S. frugiperda larvae. Inoculation with Rhizophagus irregularis (Rir) significantly improved maize growth, and N/P uptake. The 15N labeling experiment showed that maize plants absorbed N from soils via the extraradical mycelium of mycorrhizal fungi and from neighboring plants transferred by common mycorrhizal networks (CMNs). Simulated aboveground leaf herbivory did not affect mycorrhiza-mediated N acquisition from soil. However, CMN-mediated N transfer from neighboring plants was blocked by leaf simulated herbivory. Our findings suggest that aboveground herbivory inhibits CMN-mediated N transfer between plants but does not affect N acquisition from soil solutions via extraradical mycorrhizal mycelium.

Do ectomycorrhizal exploration types reflect mycelial foraging strategies?

Ectomycorrhizal exploration types are commonly assumed to denote spatial foraging patterns and resource-related niches of extraradical mycelia. However, empirical evidence of the consistency of foraging strategies within exploration types is lacking. Here, we analysed ectomycorrhizal foraging patterns by incubating root-excluding ingrowth mesh bags filled with six different substrates in mature Picea abies forests. High-throughput sequencing was used to characterise ectomycorrhizal fungal communities in the mesh bags and on adjacent fine roots after one growing season. Contrary to expectations, many ectomycorrhizal genera of exploration types that are thought to produce little extraradical mycelium colonised ingrowth bags extensively, whereas genera commonly associated with ample mycelial production occurred sparsely in ingrowth bags relative to their abundance on roots. Previous assumptions about soil foraging patterns of exploration types do not seem to hold. Instead, we propose that variation in the proliferation of extraradical mycelium is related to intergeneric differences in mycelial longevity and the mobility of targeted resources.

Effect of arbuscular mycorrhiza on germination and initial growth of Cinchona officinalis L. (Rubiaceae)

Cinchona officinalis, known locally as cascarilla or cinchona, is a plant species native to South America. It was used as a source of quinine to combat malaria in the 17th century. The species is threatened by various anthropogenic activities. Further, the propagation of the species depends on seed dispersal and its germination capacity. Therefore, it is necessary to conserve and propagate this species. Because C. officinalis seeds have a low germination capacity, we determined the effect of arbuscular mycorrhizae (AM) on their germination and growth. A randomized design was employed with two treatments, one treated with mycorrhizae (CM) and another without mycorrhizae (SM). For each treatment, three replicates of 100 seeds were used. Germination, growth, and fungal characteristics were evaluated. In germination parameters, the CM treatment showed better performance, but the improvement was statistically insignificant. However, the application of AM significantly improved seedling height (cm), root length (cm), leaf area (cm2), and root number by 53.52, 28.72, 29.73, and 61.66%, respectively. Likewise, mycorrhization intensity (%), mycorrhization frequency (%), and extraradical mycelium length (cm) in the CM treatment were 37.13, 3.44, and 174.97% higher compared to the SM treatment, respectively. Therefore, the use of AM fungi proves to be advantageous in the propagation of C. officinalis, and these results provide a basis for the largescale and sustainable propagation of this species.

Subcellular Element Distribution in Shoots of Wheat Grown in an Acidic Soil with Native AMF Extraradical Mycelium

Soil acidity can reduce crop growth by increasing bioavailable soil Al, Fe, and/or Mn to toxic levels. The presence of an intact extraradical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF), developed by the native Ornithopus compressus in the acidic soil, can increase wheat growth and prevent symptoms of Mn toxicity. To understand the protective effect of the intact ERM of this native plant on wheat element balance and distribution, in the present study, shoot Al, Fe, K, Zn, Na, and Si levels and their subcellular partitioning were determined by inductively coupled plasma mass spectrometry (ICP-MS), for the first time, for this system. In undisturbed soil, where an intact ERM structure is maintained, wheat shoot growth was promoted, probably due to faster root mycorrhizal colonization. The levels of potentially toxic Al and Fe were reduced, the proportions of the macronutrient K and micronutrient Zn were higher in the symplast, and the Na proportion increased in the vacuole, while Si increased in the apoplast. Overall, the undisturbed soil from O. compressus treatment appeared to influence the uptake and distribution of essential and beneficial elements, as a strategy to reduce the negative effect of soil acidity on wheat growth. Understanding the dynamics of element distribution influenced by stress-adapted AMF on wheat growth can provide more sustainable approaches to intensive agriculture.

Arbuscular mycorrhiza fungi colonisation stimulates uptake of inorganic nitrogen and sulphur but reduces utilisation of organic forms in tomato

Arbuscular mycorrhizal fungi (AMF) form symbioses with most plants, potentially improving their growth and nutrient assimilation activities. Cysteine (Cys) and methionine (Met) are nitrogen (N)- and sulphur (S)-containing amino acids. Compared with phosphate and N, limited attention has been paid to the role of AMF in low molecular weight organic S acquisition. To explore the uptake and relative contributions of organic and inorganic N and S to plants, and the role of AMF in S uptake, a pot study was conducted based on 14C, 35S, 13C, and 15N quad labelling (6-h labelling test after adding the labelled solution to the soil–plant system) using a mutant tomato genotype with highly decreased AMF symbiosis capacity. Tomato roots can uptake limited amounts of added Met and Cys (<1.77%) within 6 h, as indicated by the 14C and 13C labelling results, and most of them were utilised by soil microorganisms. After uptake for 6 h, 10.0–14.8% of N and 1.4–6.1% of S derived from added Cys and Met were utilised by plants, mainly in inorganic N and S forms derived from Cys and Met decomposition. Met and Cys could be important S sources (Met: 3.0–9.8%, Cys: 8.8–22.0%) for plants, however have negligible roles in N nutrition (∼1%), as most N uptake by plants was derived from soil inorganic N. The tomato uptake of inorganic S derived from Cys decomposition was much higher than that derived from Met, as higher ratios of S-Cys were released as SO42− from microorganisms during the 6-h testing periods. Even with the artificial addition of AMF, most of the added Met and Cys were utilised by Gram-negative bacteria, as indicated by 13C-PLFA biomarkers. AMF reduced host plant uptake of organic N and S, but stimulated plant N uptake from Met and Cys, which was mainly inorganic N following mineralisation. AMF not only utilise organic carbon from host plants but also capture soil organic matter to satisfy their energy demands. In the spaces where both root and AMF occur, AMF colonisation decreased tomato 35S uptake from Cys, Met, and SO42− by 24.6%, 20.6%, and 11.0% within the 6-h uptake period, respectively, when compared with that in the mutant genotype with reduced colonisation capacity; in contrast, AMF colonisation increased 35S-Cys uptake by 118.7% from areas that roots could not reach. Overall, AMF enhanced host plant N uptake but reduced organic N uptake under competition with plant roots for S in the rhizosphere, but stimulated plant S uptake by extraradical mycelia, based on this short-term pulse-chase test.

Zinc and Iron Biofortification and Accumulation of Health-Promoting Compounds in Mycorrhizal Cichorium intybus L.

The positive impact of arbuscular mycorrhizal symbionts on plant growth and health has been reported for many species, and supports their use as biofertilizers and bioenhancers. Here, the potential role of the arbuscular mycorrhizal symbiont Funneliformis mosseae in the improvement of chicory (Cichorium intybus L.) nutritional value, in terms of nutrient uptake and accumulation of health-promoting compounds, was studied using an in vivo whole-plant system, allowing both plant and fungal tissue collection. Biomass and nutrient distribution were determined in plant and extraradical mycelium, and photosynthetic pigments and fructooligosaccharide concentrations were evaluated in chicory shoots and roots. Zinc shoot concentration of mycorrhizal chicory was significantly increased, as well as the whole-plant Fe uptake, while root Cu concentration was decreased, compared with uninoculated controls. F. mosseae extraradical mycelium accumulated Cu, Zn, Mn, and Fe at high concentrations, compared with those of the host plant tissues, suggesting that it plays a double functional “scavenging-filtering” role, by its ability to balance the uptake of microelements or to limit their translocation depending on plant-soil concentrations. The higher Zn and Fe uptake by mycorrhizal plants was significantly correlated with higher carotenoid, inulin, and fructose levels, suggesting a relationship among the modulation of micronutrient uptake by mycorrhizal symbionts and the biosynthesis of health-promoting molecules by the host. Overall, data from this work may boost the implementation of arbuscular mycorrhizal fungal inoculation aimed at inducing plant biofortification and enhancement of nutritional value of plant-derived food.