Phosphate uptake, transport and transfer by the arbuscular mycorrhizal fungus Glomus intraradices is stimulated by increased carbohydrate availability

Rhizoglomus variabile and Nanoglomus plukenetiae, Native to Peru, Promote Coffee Growth in Western Amazonia

Coffee (Coffea arabica) is among the world's most economically important crops. Coffee was shown to be highly dependent on arbuscular mycorrhizal fungi (AMF) in traditionally managed coffee plantations in the tropics. The objective of this study was to assess AMF species richness in coffee plantations of four provinces in Perú, to isolate AMF isolates native to these provinces, and to test the effects of selected indigenous AMF strains on coffee growth. AMF species were identified by morphological tools on the genus level, and if possible further to the species level. Two native species, Rhizoglomus variabile and Nanoglomus plukenetiae, recently described from the Peruvian mountain ranges, were successfully cultured in the greenhouse on host plants. In two independent experiments, both species were assessed for their ability to colonize coffee seedlings and improve coffee growth over 135 days. A total of 35 AMF morphospecies were identified from 12 plantations. The two inoculated species effectively colonized coffee roots, which resulted in 3.0-8.6 times higher shoot, root and total biomass, when compared to the non-mycorrhizal controls. R. variabile was superior to N. plukenetiae in all measured parameters, increasing shoot, root, and total biomass dry weight by 4.7, 8.6 and 5.5 times, respectively. The dual inoculation of both species, however, did not further improve plant growth, when compared to single-species inoculations. The colonization of coffee by either R. variabile or N. plukenetiae strongly enhances coffee plant growth. R. variabile, in particular, offers enormous potential for improving coffee establishment and productivity. Assessment of further AMF species, including species from other AMF families should be considered for optimization of coffee growth promotion, both alone and in combination with R. variabile.

Mycorrhizal mycelium as a global carbon pool

For more than 400 million years, mycorrhizal fungi and plants have formed partnerships that are crucial to the emergence and functioning of global ecosystems. The importance of these symbiotic fungi for plant nutrition is well established. However, the role of mycorrhizal fungi in transporting carbon into soil systems on a global scale remains under-explored. This is surprising given that ∼75% of terrestrial carbon is stored belowground and mycorrhizal fungi are stationed at a key entry point of carbon into soil food webs. Here, we analyze nearly 200 datasets to provide the first global quantitative estimates of carbon allocation from plants to the mycelium of mycorrhizal fungi. We estimate that global plant communities allocate 3.93 Gt CO₂e per year to arbuscular mycorrhizal fungi, 9.07 Gt CO₂e per year to ectomycorrhizal fungi, and 0.12 Gt CO₂e per year to ericoid mycorrhizal fungi. Based on this estimate, 13.12 Gt of CO₂e fixed by terrestrial plants is, at least temporarily, allocated to the underground mycelium of mycorrhizal fungi per year, equating to ∼36% of current annual CO₂ emissions from fossil fuels. We explore the mechanisms by which mycorrhizal fungi affect soil carbon pools and identify approaches to increase our understanding of global carbon fluxes via plant-fungal pathways. Our estimates, although based on the best available evidence, are imperfect and should be interpreted with caution. Nonetheless, our estimations are conservative, and we argue that this work confirms the significant contribution made by mycorrhizal associations to global carbon dynamics. Our findings should motivate their inclusion both within global climate and carbon cycling models, and within conservation policy and practice.

Arbuscular mycorrhizal fungal communities with contrasting life-history traits influence host nutrient acquisition

Life-history traits differ substantially among arbuscular mycorrhizal (AM) fungal families, potentially affecting hyphal nutrient acquisition efficiency, host nutrition, and thereby plant health and ecosystem function. Despite these implications, AM fungal community life-history strategies and community trait diversity effects on host nutrient acquisition are poorly understood. To address this knowledge gap, we grew sudangrass with AM fungal communities representing contrasting life-history traits and diversity: either (1) five species in the AM family Gigasporaceae, representing competitor traits, (2) five Glomerales species, representing ruderal traits, or (3) a mixed-trait community combining all ten AM fungal species. After 12 weeks, we measured above and belowground plant biomass and aboveground nutrient uptake and concentration. Overall, AM fungal colonization increased host nutrition, biomass, and foliar δ⁵nitrogen enrichment compared to the uncolonized control. Between the single-trait communities, the Glomeraceae community generally outperformed the Gigasporaceae community in host nutrition and plant growth, increasing plant phosphorus (P) uptake 1.5 times more than the Gigasporaceae community. We saw weak evidence for a synergistic effect of the mixed community, which was only higher for plant P concentration (1.26 times higher) and root colonization (1.26 times higher) compared to the single-trait communities. However, this higher P concentration did not translate to more P uptake or the highest plant biomass for the mixed community. These findings demonstrate that the AM symbiosis is affected by community differences at high taxonomic levels and provide insight into how different AM fungal communities and their associated traits affect host nutrition for fast-growing plant species.

Organic acid, phosphate, sulfate and ammonium co-metabolism releasing insoluble phosphate by Klebsiella aerogenes to simultaneously stabilize lead and cadmium

Here, a novel phosphate-solubilizing bacterium (PSB), Klebsiella aerogenes Wn was applied to develop an environmental-friendly method to simultaneously stabilize Pb and Cd. The maximum dissolved phosphate was up to 701.36 mg/L by the strain Wn. The high performance liquid chromatography (HPLC) and Pearson correlation analyses showed that the acetic acid produced by the strain Wn was significantly positively associated with the released phosphate. Moreover, 100% of 500 mg/L of Pb and 100 mg/L of Cd were simultaneously stabilized in the classical NBRIP medium and the major products were Pb₅(PO₄)₃Cl, Ca_7.7Cd_0.8(PO₄)₈(H₂O)_2.4 and CdS, respectively. In addition, the bacterial genome and transcriptome analyses showed that the pentose phosphate pathway (PPP), pyruvate metabolism pathway, thiamine metabolic pathway, sulfate reduction and ammonium bio-transformation were coupled to promote releasing insoluble phosphate and stabilizing Pb and Cd. In the metabolism networks, the critical genes of gcd, aceE, thiE, thiS and cysH, etc. were significantly up-regulated. Our results are beneficial to deeper understand the molecular mechanisms of releasing insoluble inorganic phosphate by PSBs and develop a technology prototype to simultaneously stabilize Pb and Cd using the PSBs.

Hyphosphere interactions between Rhizophagus irregularis and Rahnella aquatilis promote carbon-phosphorus exchange at the peri-arbuscular space in Medicago truncatula

Arbuscular mycorrhizal (AM) fungi form a continuum between roots and soil. One end of this continuum is comprised of the highly intimate plant-fungus interface with intracellular organelles for nutrient exchange, while on the other end the fungus interacts with bacteria to compensate for the AM fungus' inability to take up organic nutrients from soil. How both interfaces communicate in this highly complex tripartite mutualism is widely unknown. Here, the effects of phosphate-solubilizing bacteria (PSB) Rahnella aquatilis dwelling at the surface of the extraradical hyphae of Rhizophagus irregularis was analysed based on the expression of genes involved in C-P exchange at the peri-arbuscular space (PAS) in Medicago truncatula. The interaction between AM fungus and PSB resulted in an increase in uptake and transport of Pi along the extraradical hyphae and its transfer from AM fungus to plant. In return, this was remunerated by a transfer of C from plant to AM fungus, improving the C-P exchange at the PAS. These results demonstrated that a microorganism (i.e., a PSB) developing at the hyphosphere interface can affect the C-P exchange at the PAS between plant and AM fungus, suggesting a fine-tuned communication operated between three organisms via two distantly connected interfaces.

Years of sand fixation with Caragana korshinskii drive the enrichment of its rhizosphere functional microbes by accumulating soil N

C. korshinskii is one of the most widely-planted sand-fixing legumes in northwest China and exploring its rhizosphere microbiome is of great ecological importance. However, the effect of long-term sand fixation on the composition, diversity, and underlying functions of microbes in the C. korshinskii rhizosphere in dryland ecosystems remain unclear. Here, we performed high-throughput sequencing using a 16S rRNA (absolute quantification) and bacterial functional annotation of prokaryotic taxa (FAPROTAX) analysis and an ITS (relative quantification) and fungal functional guild (FUNGuild) analysis to investigate the C. korshinskii rhizosphere microbiome and metabolic functional groups at different sand-fixing ages (six years, CK6; twelve years, CK12; and eighteen years, CK18) and determined the physicochemical properties of the rhizosphere soil. Results showed that the key bacterial taxa of the rhizosphere were significantly more abundant in CK18 than in CK12 and CK6 at the phylum-class-genus level, and that fungal Glomeromycota was also significantly more abundant in the CK18 rhizosphere compared to CK12 and CK6. Among these bacterial taxa, the enrichment effect of key, functional, genus-level species of bacteria was the most obvious, including Rhizobium, Ensifer, Neorhizobium, Mesorhizobium, Streptomyces, Sphingomonas, and Flavobacterium, which are N-fixing and/or phosphate-solubilizing groups. The significant improvement seen in the physicochemical properties of the CK18 rhizosphere soil, including the higher total nitrogen (TN), available nitrogen (AN), pH, electrical conductivity (EC), higher N:P ratio, and lower C:N ratio, all demonstrated the relationship between the rhizosphere microbes and soil carbon (C) and nitrogen (N) cycling. A redundancy analysis (RDA) of different taxonomic levels indicated a close positive relationship between rhizosphere microbes and AN. In addition, the functional groups of the C. korshinskii rhizosphere bacteria were closely related to soil AN and were mainly composed of chemoheterotrophy and aerobic chemoheterotrophy. A Spearman correlation analysis revealed that these functional groups were mainly identified from bacterial Actinobacteria, Proteobacteria, Verrucomicrobia, Bacteroidetes, and fungal Glomeromycota. Our study provides evidence that the rhizosphere microbes of C. korshinskii are closely related to the accumulation of N in the restoration of desert ecosystems, and that the ecological functional processes they are involved in mainly involve C and N cycles, which play an important role in desertification reversal.

The fine-tuning of mycorrhizal pathway in sorghum depends on both nitrogen-phosphorus availability and the identity of the fungal partner

Sorghum is an important worldwide source of food, feed and fibres. Like most plants, it forms mutualistic symbioses with arbuscular mycorrhizal fungi (AMF), but the nutritional basis of mycorrhiza-responsiveness is largely unknown. Here, we investigated the transcriptional and physiological responses of sorghum to two different AMF species, Rhizophagus irregularis and Funneliformis mosseae, under 16 different conditions of nitrogen (N) and phosphorus (P) supply. Our experiment reveals fine-scale differences between two AMF species in the nutritional interactions with sorghum plants. Physiological and gene expression patterns (ammonium transporters: AMT; phosphate transporters: PHT) indicate the existence of generalist or specialist mycorrhizal pathway. While R. irregularis switched on the mycorrhizal pathway independently of the plant nutritional status, F. mosseae influenced the mycorrhizal pathway depending on the N-to-P plant ratio and soil supply. The differences between both AMF species suggest some AMT and PHT as ideal candidates to develop markers for improving efficiency of nutrient acquisition in sorghum under P and N limitation, and for the selection of plant genotypes.

Arbuscular Mycorrhiza and Nitrification: Disentangling Processes and Players by Using Synthetic Nitrification Inhibitors

Both plants and their associated arbuscular mycorrhizal (AM) fungi require nitrogen (N) for their metabolism and growth. This can result in both positive and negative effects of AM symbiosis on plant N nutrition. Either way, the demand for and efficiency of uptake of mineral N from the soil by mycorrhizal plants are often higher than those of nonmycorrhizal plants. In consequence, the symbiosis of plants with AM fungi exerts important feedbacks on soil processes in general and N cycling in particular. Here, we investigated the role of the AM symbiosis in N uptake by Andropogon gerardii from an organic source (¹⁵N-labeled plant litter) that was provided beyond the direct reach of roots. In addition, we tested if pathways of ¹⁵N uptake from litter by mycorrhizal hyphae were affected by amendment with different synthetic nitrification inhibitors (dicyandiamide [DCD], nitrapyrin, or 3,4-dimethylpyrazole phosphate [DMPP]). We observed efficient acquisition of ¹⁵N by mycorrhizal plants through the mycorrhizal pathway, independent of nitrification inhibitors. These results were in stark contrast to ¹⁵N uptake by nonmycorrhizal plants, which generally took up much less ¹⁵N, and the uptake was further suppressed by nitrapyrin or DMPP amendments. Quantitative real-time PCR analyses showed that bacteria involved in the rate-limiting step of nitrification, ammonia oxidation, were suppressed similarly by the presence of AM fungi and by nitrapyrin or DMPP (but not DCD) amendments. On the other hand, abundances of ammonia-oxidizing archaea were not strongly affected by either the AM fungi or the nitrification inhibitors. IMPORTANCE Nitrogen is one of the most important elements for all life on Earth. In soil, N is present in various chemical forms and is fiercely competed for by various microorganisms as well as plants. Here, we address competition for reduced N (ammonia) between ammonia-oxidizing prokaryotes and arbuscular mycorrhizal fungi. These two functionally important groups of soil microorganisms, participating in nitrification and plant mineral nutrient acquisition, respectively, have often been studied in separation in the past. Here, we showed, using various biochemical and molecular approaches, that the fungi systematically suppress ammonia-oxidizing bacteria to an extent similar to that of some widely used synthetic nitrification inhibitors, whereas they have only a limited impact on abundance of ammonia-oxidizing archaea. Competition for free ammonium is a plausible explanation here, but it is also possible that the fungi produce some compounds acting as so-called biological nitrification inhibitors.

Arbuscular mycorrhizal fungi enhanced the growth, phosphorus uptake and Pht expression of olive (Olea europaea L.) plantlets

Olive (Olea europaea L.) is a highly mycotrophic species that has been introduced and cultivated in China for half a century. The arbuscular mycorrhizal fungi (AMF) is extremely valuable as a kind of biofertilizer to promote the health and vigor of olive plants. However, it is still unclear how native AMF impact growth and mineral nutrients, especially phosphorus absorption in the area where olive trees were introduced in China. In the present study, through a pot experiment, the effects of native AMF on the growth, phosphorus uptake and expression levels of four phosphate transporter genes (Pht) of olive plantlets were characterized. We found that (1) typical AMF colonization was observed within the roots of inoculated olive plantlets, and the growth of plantlets was significantly promoted; (2) some indigenous consortia (AMF1 and AMF2) notably promoted the absorption of phosphorus, fertilizers significantly increased the foliar content of nitrogen, and both AMF inoculation and fertilization had no significant effect on the uptake of potassium; and (3) AMF inoculation enhanced the expression of phosphate transporter genes in inoculated olive roots. This work demonstrates the effectiveness of native AMF on the cultivation of robust olive plantlets and highlights the role of AMF in increasing phosphorus uptake. There is great potential in using native AMF consortia as inoculants for the production of healthy and robust olive plantlets.

Effect of contrasting phosphorus levels on nitrous oxide and carbon dioxide emissions from temperate grassland soils

Agricultural practices such as repeated fertilization impact carbon (C), nitrogen (N) and phosphorus (P) cycling and their relationships in the plant–soil continuum, which could have important implications for the magnitude of greenhouse gas emissions. However, little is known about the effect of C and N additions under contrasting soil P availability status on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions. In this study, we conducted a field-based experiment that investigated the impact of long-term (23 years) P management (no (P0, 0 kg P ha⁻¹), low (P15, 15 kg P ha⁻¹) and high (P45, 45 kg P ha⁻¹) P inputs) on N₂O and CO₂ emissions following two C + N application events in two managed grassland ecosystems with loam and sandy loam soils. The magnitude of fluxes varied between the soil P availability levels. Cumulative N₂O emission was significantly higher in P0 soils (1.08 ± 0.09 g N₂O-N m⁻²) than P45 soils (0.63 ± 0.03 g N₂O-N m⁻²), with the loam soil (1.04 ± 0.04 g N₂O-N m⁻²) producing significantly higher emissions than the sandy loam soil (0.88 ± 0.05 g N₂O-N m⁻²). We conclude that P-limitation stimulates N₂O emissions, whereas P-enrichment promotes soil respiration in these temperate grassland sites. Our findings inform effective nutrient management strategies underpinning optimized use of N and P inputs to agricultural soils as mitigation measures for both food security and reducing greenhouse gas emissions.

Genome-resolved metagenomics identifies the particular genetic traits of phosphate-solubilizing bacteria in agricultural soil

Bacteria play a key role in phosphate solubilization, but related genome-centric research on agricultural microbiomes is scarce. Here, we reconstructed 472 metagenome-assembled genomes (MAGs) covering agricultural soils from six long-term field trials across China. A total of 79 MAGs contained gcd encoding quinoprotein glucose dehydrogenase (GCD), which is the key biomarker for phosphate solubilization. Our findings showed that all GCD-MAGs represent potentially novel species, with gcd copy numbers varying from 1 to 10 per genome. Large genome size, a high ratio of glycosyl hydrolase genes, and increased capacity for carbohydrate utilization were specific traits of GCD-MAGs. Notably, the gcd copy number showed a significant and positive correlation with genome size. Generated using a machine learning approach, our findings were validated in a dataset of 692 genotypes covering the 18 bacterial families to which the 79 GCD-MAGs belong. Our results improve the knowledge of both the diversity and the genetic composition of phosphate-solubilizing bacteria. In particular, they reveal a genomic link between phosphate solubilization capacity and increased potential for carbohydrate metabolism, which may accelerate targeted engineering and improve management practices for sustainable agriculture.

Plant nitrogen nutrition: The roles of arbuscular mycorrhizal fungi

Nitrogen (N) is the most abundant mineral nutrient required by plants, and crop productivity depends heavily on N fertilization in many soils. Production and application of N fertilizers consume huge amounts of energy and substantially increase the costs of agricultural production. Excess N compounds released from agricultural systems are also detrimental to the environment. Thus, increasing plant N uptake efficiency is essential for the development of sustainable agriculture. Arbuscular mycorrhizal (AM) fungi are beneficial symbionts of most terrestrial plants that facilitate plant nutrient uptake and increase host resistance to diverse environmental stresses. AM association is an endosymbiotic process that relies on the differentiation of both host plant roots and AM fungi to create novel contact interfaces within the cells of plant roots. AM plants have two pathways for nutrient uptake: either direct uptake via the root hairs and root epidermis, or indirectly through AM fungal hyphae into root cortical cells. Over the last few years, great progress has been made in deciphering the molecular mechanisms underlying the AM-mediated modulation of nutrient uptake processes, and a growing number of fungal and plant genes responsible for the uptake of nutrients from soil or transfer across the fungi-root interface have been identified. Here, we mainly summarize the recent advances in N uptake, assimilation, and translocation in AM symbiosis, and also discuss how N interplays with C and P in modulating AM development, as well as the synergies between AM fungi and soil microbial communities in N uptake.

Genome-Wide Analysis of Nutrient Signaling Pathways Conserved in Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal (AM) fungi form a mutualistic symbiosis with a majority of terrestrial vascular plants. To achieve an efficient nutrient trade with their hosts, AM fungi sense external and internal nutrients, and integrate different hierarchic regulations to optimize nutrient acquisition and homeostasis during mycorrhization. However, the underlying molecular networks in AM fungi orchestrating the nutrient sensing and signaling remain elusive. Based on homology search, we here found that at least 72 gene components involved in four nutrient sensing and signaling pathways, including cAMP-dependent protein kinase A (cAMP-PKA), sucrose non-fermenting 1 (SNF1) protein kinase, target of rapamycin kinase (TOR) and phosphate (PHO) signaling cascades, are well conserved in AM fungi. Based on the knowledge known in model yeast and filamentous fungi, we outlined the possible gene networks functioning in AM fungi. These pathways may regulate the expression of downstream genes involved in nutrient transport, lipid metabolism, trehalase activity, stress resistance and autophagy. The RNA-seq analysis and qRT-PCR results of some core genes further indicate that these pathways may play important roles in spore germination, appressorium formation, arbuscule longevity and sporulation of AM fungi. We hope to inspire further studies on the roles of these candidate genes involved in these nutrient sensing and signaling pathways in AM fungi and AM symbiosis.

Mycorrhizal fungi control phosphorus value in trade symbiosis with host roots when exposed to abrupt 'crashes' and 'booms' of resource availability

Biological market theory provides a conceptual framework to analyse trade strategies in symbiotic partnerships. A key prediction of biological market theory is that individuals can influence resource value - meaning the amount a partner is willing to pay for it - by mediating where and when it is traded. The arbuscular mycorrhizal symbiosis, characterised by roots and fungi trading phosphorus and carbon, shows many features of a biological market. However, it is unknown if or how fungi can control phosphorus value when exposed to abrupt changes in their trade environment. We mimicked an economic 'crash', manually severing part of the fungal network (Rhizophagus irregularis) to restrict resource access, and an economic 'boom' through phosphorus additions. We quantified trading strategies over a 3-wk period using a recently developed technique that allowed us to tag rock phosphate with fluorescing quantum dots of three different colours. We found that the fungus: compensated for resource loss in the 'crash' treatment by transferring phosphorus from alternative pools closer to the host root (Daucus carota); and stored the surplus nutrients in the 'boom' treatment until root demand increased. By mediating from where, when and how much phosphorus was transferred to the host, the fungus successfully controlled resource value.

Temporal tracking of quantum-dot apatite across in vitro mycorrhizal networks shows how host demand can influence fungal nutrient transfer strategies

Arbuscular mycorrhizal fungi function as conduits for underground nutrient transport. While the fungal partner is dependent on the plant host for its carbon (C) needs, the amount of nutrients that the fungus allocates to hosts can vary with context. Because fungal allocation patterns to hosts can change over time, they have historically been difficult to quantify accurately. We developed a technique to tag rock phosphorus (P) apatite with fluorescent quantum-dot (QD) nanoparticles of three different colors, allowing us to study nutrient transfer in an in vitro fungal network formed between two host roots of different ages and different P demands over a 3-week period. Using confocal microscopy and raster image correlation spectroscopy, we could distinguish between P transfer from the hyphae to the roots and P retention in the hyphae. By tracking QD-apatite from its point of origin, we found that the P demands of the younger root influenced both: (1) how the fungus distributed nutrients among different root hosts and (2) the storage patterns in the fungus itself. Our work highlights that fungal trade strategies are highly dynamic over time to local conditions, and stresses the need for precise measurements of symbiotic nutrient transfer across both space and time.

Species-specific synergistic effects of two plant growth-promoting microbes on green roof plant biomass and photosynthetic efficiency

Rhizophagus irregularis, an arbuscular mycorrhizal fungus, and Bacillus amyloliquefaciens, a bacterium, are microorganisms that promote plant growth. They associate with plant roots and facilitate nutrient absorption by their hosts, increase resistance against pathogens and pests, and regulate plant growth through phytohormones. In this study, eight local plant species in Finland (Antennaria dioica, Campanula rotundifolia, Fragaria vesca, Geranium sanguineum, Lotus corniculatus, Thymus serpyllum, Trifolium repens, and Viola tricolor) were inoculated with R. irregularis and/or B. amyloliquefaciens in autoclaved substrates to evaluate the plant growth-promoting effects of different plant/microbe combinations under controlled conditions. The eight plant species were inoculated with R. irregularis, B. amyloliquefaciens, or both microbes or were not inoculated as a control. The impact of the microbes on the plants was evaluated by measuring dry shoot weight, colonization rate by the arbuscular mycorrhizal fungus, bacterial population density, and chlorophyll fluorescence using a plant phenotyping facility. Under dual inoculation conditions, B. amyloliquefaciens acted as a "mycorrhiza helper bacterium" to facilitate arbuscular mycorrhizal fungus colonization in all tested plants. In contrast, R. irregularis did not demonstrate reciprocal facilitation of the population density of B. amyloliquefaciens. Dual inoculation with B. amyloliquefaciens and R. irregularis resulted in the greatest increase in shoot weight and photosynthetic efficiency in T. repens and F. vesca.

Signal beyond nutrient, fructose, exuded by an arbuscular mycorrhizal fungus triggers phytate mineralization by a phosphate solubilizing bacterium

Cooperation is a prevalent phenomenon in nature and how it originates and maintains is a fundamental question in ecology. Many efforts have been made to understand cooperation between individuals in the same species, while the mechanisms enabling cooperation between different species are less understood. Here, we investigated under strict in vitro culture conditions if the exchange of carbon and phosphorus is pivotal to the cooperation between the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis and the phosphate solubilizing bacterium (PSB) Rahnella aquatilis. We observed that fructose exuded by the AMF stimulated the expression of phosphatase genes in the bacterium as well as the rate of phosphatase release into the growth medium by regulating its protein secretory system. The phosphatase activity was subsequently increased, promoting the mineralization of organic phosphorus (i.e., phytate) into inorganic phosphorus, stimulating simultaneously the processes involved in phosphorus uptake by the AMF. Our results demonstrated for the first time that fructose not only is a carbon source, but also plays a role as a signal molecule triggering bacteria-mediated organic phosphorus mineralization processes. These results highlighted the molecular mechanisms by which the hyphal exudates play a role in maintaining the cooperation between AMF and bacteria.

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

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

Dynamic preferential allocation to arbuscular mycorrhizal fungi explains fungal succession and coexistence

Evidence accumulates about the role of arbuscular mycorrhizal (AM) fungi in shaping plant communities, but little is known about the factors determining the biomass and coexistence of several types of AM fungi in a plant community. Here, using a consumer-resource framework that treats the relationship between plants and fungi as simultaneous, reciprocal exploitation, we investigated what patterns of dynamic preferential plant carbon allocation to empirically-defined fungal types (on-going partner choice) would be optimal for plants, and how these patterns depend on successional dynamics. We found that ruderal AM fungi can dominate under low steady-state nutrient availability, and competitor AM fungi can dominate at higher steady-state nutrient availability; these are conditions characteristic of early and late succession, respectively. We also found that dynamic preferential allocation alone can maintain a diversity of mutualists, suggesting that on-going partner choice is a new coexistence mechanism for mutualists. Our model can therefore explain both mutualist coexistence and successional strategy, providing a powerful tool to derive testable predictions.

Engineering crop nutrient efficiency for sustainable agriculture

Increasing crop yields can provide food, animal feed, bioenergy feedstocks and biomaterials to meet increasing global demand; however, the methods used to increase yield can negatively affect sustainability. For example, application of excess fertilizer can generate and maintain high yields but also increases input costs and contributes to environmental damage through eutrophication, soil acidification and air pollution. Improving crop nutrient efficiency can improve agricultural sustainability by increasing yield while decreasing input costs and harmful environmental effects. Here, we review the mechanisms of nutrient efficiency (primarily for nitrogen, phosphorus, potassium and iron) and breeding strategies for improving this trait, along with the role of regulation of gene expression in enhancing crop nutrient efficiency to increase yields. We focus on the importance of root system architecture to improve nutrient acquisition efficiency, as well as the contributions of mineral translocation, remobilization and metabolic efficiency to nutrient utilization efficiency.

External hyphae of Rhizophagus irregularis DAOM 197198 are less sensitive to low pH than roots in arbuscular mycorrhizae: evidence from axenic culture system

The growth of plant roots and arbuscular mycorrhizal fungi (AMF) can be inhibited by low pH; however, it is largely unknown which is more sensitive to low pH. This study aimed to compare the physiological and molecular responses of external hyphae (EH) and roots to low pH in terms of growth, development and functioning. We established AM symbiosis in a two-compartmented system (root compartment, RC; hyphal compartment, HC) using AMF and transformed hairy roots and exposed them to pH 6.5 and/or pH 4.5. The results showed that pH 4.5 significantly decreased root cell viability, while EH at pH 6.5 attenuated the effect. In either RC or HC, pH 4.5 reduced biomass, P content, colonization, ALP activity in roots, and ALP activity and polyphosphate accumulation in EH. GintPT expression in EH was inhibited by pH 4.5 in HC but not in RC. The expression of mycorrhiza-responsive LePTs was significantly reduced by the lower colonization due to decreased pH in either RC or HC, while the expression of non-mycorrhiza-responsive LePTs was not affected. Variation partitioning analysis indicated that EH was less sensitive to low pH than roots. The interactions between roots and EH under low pH stress merit further investigation.

Fungal root symbionts of high-altitude vascular plants in the Himalayas

Arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) form symbiotic relationships with plants influencing their productivity, diversity and ecosystem functions. Only a few studies on these fungi, however, have been conducted in extreme elevations and none over 5500 m a.s.l., although vascular plants occur up to 6150 m a.s.l. in the Himalayas. We quantified AMF and DSE in roots of 62 plant species from contrasting habitats along an elevational gradient (3400-6150 m) in the Himalayas using a combination of optical microscopy and next generation sequencing. We linked AMF and DSE communities with host plant evolutionary history, ecological preferences (elevation and habitat type) and functional traits. We detected AMF in elevations up to 5800 m, indicating it is more constrained by extreme conditions than the host plants, which ascend up to 6150 m. In contrast, DSE were found across the entire gradient up to 6150 m. AMF diversity was unimodally related to elevation and positively related to the intensity of AMF colonization. Mid-elevation steppe and alpine plants hosted more diverse AMF communities than plants from deserts and the subnival zone. Our results bring novel insights to the abiotic and biotic filters structuring AMF and DSE communities in the Himalayas.

Increasing phosphorus concentration in the extraradical hyphae of Rhizophagus irregularis DAOM 197198 leads to a concomitant increase in metal minerals

Plants associated with arbuscular mycorrhizal fungi (AMF) acquire phosphorus via roots and extraradical hyphae. How soil P level affects P accumulation within hyphae and how P in hyphae influences the accumulation of metal minerals remains little explored. A bi-compartmented in vitro cultivation system separating a root compartment (RC), containing a Ri T-DNA transformed carrot root associated to the AMF Rhizophagus irregularis DAOM 197198, from a hyphal compartment (HC), containing only the extraradical hyphae, was used. The HC contained a liquid growth medium (i.e., the modified Strullu-Romand medium containing P in the form of KH₂PO₄) without (0 μM) or adjusted to 35, 100, and 700 μM of KH₂PO₄. The accumulation of P and metal minerals (Ca, Mg, K, Na, Fe, Cu, Mn) within extraradical hyphae and AMF-colonized roots, and the expression of the phosphate transporter gene GintPT were assessed. The expression of GintPT in the extraradical hyphae did not differ in absence of KH₂PO₄ or in presence of 35 and 100 μM KH₂PO₄ in the HC but was markedly reduced in presence of 700 μM KH₂PO₄. Hyphal P concentration was significantly lowest in absence of KH₂PO₄, intermediate at 35 and 100 μM KH₂PO₄ and significantly highest in presence of 700 μM KH₂PO₄ in the HC. The concentrations of K, Mg, and Na were positively associated with the concentration of P in the extraradical hyphae developing in the HC. Similarly, P concentration in extraradical hyphae in the HC was related to P concentration in the growth medium and influenced the concentration of K, Mg, and Na. The accumulation of the metal mineral K, Mg, and Na in the extraradical hyphae developing in the HC was possibly related to their function in neutralizing the negative charges of PolyP accumulated in the hyphae.

Symbiosis of Arbuscular Mycorrhizal Fungi and Robinia pseudoacacia L. Improves Root Tensile Strength and Soil Aggregate Stability

Robinia pseudoacacia L. (black locust) is a widely planted tree species on Loess Plateau for revegetation. Due to its symbiosis forming capability with arbuscular mycorrhizal (AM) fungi, we explored the influence of arbuscular mycorrhizal fungi on plant biomass, root morphology, root tensile strength and soil aggregate stability in a pot experiment. We inoculated R. pseudoacacia with/without AM fungus (Rhizophagus irregularis or Glomus versiforme), and measured root colonization, plant growth, root morphological characters, root tensile force and tensile strength, and parameters for soil aggregate stability at twelve weeks after inoculation. AM fungi colonized more than 70% plant root, significantly improved plant growth. Meanwhile, AM fungi elevated root morphological parameters, root tensile force, root tensile strength, Glomalin-related soil protein (GRSP) content in soil, and parameters for soil aggregate stability such as water stable aggregate (WSA), mean weight diameter (MWD) and geometric mean diameter (GMD). Root length was highly correlated with WSA, MWD and GMD, while hyphae length was highly correlated with GRSP content. The improved R. pseudoacacia growth, root tensile strength and soil aggregate stability indicated that AM fungi could accelerate soil fixation and stabilization with R. pseudoacacia, and its function in revegetation on Loess Plateau deserves more attention.

The effector candidate repertoire of the arbuscular mycorrhizal fungus Rhizophagus clarus

Background

Arbuscular mycorrhizal fungi (AMF) form an ecologically important symbiosis with more than two thirds of studied land plants. Recent studies of plant-pathogen interactions showed that effector proteins play a key role in host colonization by controlling the plant immune system. We hypothesise that also for symbiotic-plant interactions the secreted effectome of the fungus is a major component of communication and the conservation level of effector proteins between AMF species may be indicative whether they play a fundamental role.

Results

In this study, we used a bioinformatics pipeline to predict and compare the effector candidate repertoire of the two AMF species, Rhizophagus irregularis and Rhizophagus clarus. Our in silico pipeline revealed a list of 220 R. irregularis candidate effector genes that create a valuable information source to elucidate the mechanism of plant infection and colonization by fungi during AMF symbiotic interaction. While most of the candidate effectors show no homologies to known domains or proteins, the candidates with homologies point to potential roles in signal transduction, cell wall modification or transcription regulation. A remarkable aspect of our work is presence of a large portion of the effector proteins involved in symbiosis, which are not unique to each fungi or plant species, but shared along the Glomeromycota phylum. For 95 % of R. irregularis candidates we found homologs in a R. clarus genome draft generated by Illumina high-throughput sequencing. Interestingly, 9 % of the predicted effectors are at least as conserved between the two Rhizophagus species as proteins with housekeeping functions (similarity > 90 %). Therefore, we state that this group of highly conserved effector proteins between AMF species may play a fundamental role during fungus-plant interaction.

Conclusions

We hypothesise that in symbiotic interactions the secreted effectome of the fungus might be an important component of communication. Identification and functional characterization of the primary AMF effectors that regulate symbiotic development will help in understanding the mechanisms of fungus-plant interaction.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2422-y) contains supplementary material, which is available to authorized users.

Biological trade and markets

Cooperation between organisms can often be understood, like trade between merchants, as a mutually beneficial exchange of services, resources or other 'commodities'. Mutual benefits alone, however, are not sufficient to explain the evolution of trade-based cooperation. First, organisms may reject a particular trade if another partner offers a better deal. Second, while human trade often entails binding contracts, non-human trade requires unwritten 'terms of contract' that 'self-stabilize' trade and prevent cheating even if all traders strive to maximize fitness. Whenever trading partners can be chosen, market-like situations arise in nature that biologists studying cooperation need to account for. The mere possibility of exerting partner choice stabilizes many forms of otherwise cheatable trade, induces competition, facilitates the evolution of specialization and often leads to intricate forms of cooperation. We discuss selected examples to illustrate these general points and review basic conceptual approaches that are important in the theory of biological trade and markets. Comparing these approaches with theory in economics, it turns out that conventional models-often called 'Walrasian' markets-are of limited relevance to biology. In contrast, early approaches to trade and markets, as found in the works of Ricardo and Cournot, contain elements of thought that have inspired useful models in biology. For example, the concept of comparative advantage has biological applications in trade, signalling and ecological competition. We also see convergence between post-Walrasian economics and biological markets. For example, both economists and biologists are studying 'principal-agent' problems with principals offering jobs to agents without being sure that the agents will do a proper job. Finally, we show that mating markets have many peculiarities not shared with conventional economic markets. Ideas from economics are useful for biologists studying cooperation but need to be taken with caution.

Regulation of resource exchange in the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi are one of the most important groups of plant symbionts. These fungi provide mineral nutrients to plants in exchange for carbon. Although substantial amounts of resources are exchanged, the factors that regulate trade in the AM symbiosis are poorly understood. Recent evidence for the reciprocally regulated exchange of resources by AM fungi and plants has led to the suggestion that these symbioses operate according to biological market dynamics, in which interactions are viewed from an economic perspective, and the most beneficial partners are favoured. Here we present five arguments that challenge the importance of reciprocally regulated exchange, and thereby market dynamics, for resource exchange in the AM symbiosis, and suggest that such reciprocity is only found in a subset of symbionts, under specific conditions. We instead propose that resource exchange in the AM symbiosis is determined by competition for surplus resources, functional diversity and sink strength.

High functional diversity within species of arbuscular mycorrhizal fungi is associated with differences in phosphate and nitrogen uptake and fungal phosphate metabolism

Plant growth responses following colonization with different isolates of a single species of an arbuscular mycorrhizal (AM) fungus can range from highly beneficial to detrimental, but the reasons for this high within-species diversity are currently unknown. To examine whether differences in growth and nutritional benefits are related to the phosphate (P) metabolism of the fungal symbiont, the effect of 31 different isolates from 10 AM fungal morphospecies on the P and nitrogen (N) nutrition of Medicago sativa and the P allocation among different P pools was examined. Based on differences in the mycorrhizal growth response, high, medium, and low performance isolates were distinguished. Plant growth benefit was positively correlated to the mycorrhizal effect on P and N nutrition. High performance isolates increased plant biomass by more than 170 % and contributed substantially to both P and N nutrition, whereas the effect of medium performance isolates particularly on the N nutrition of the host was significantly lower. Roots colonized by high performance isolates were characterized by relatively low tissue concentrations of inorganic P and short-chain polyphosphates and a high ratio between long- to short-chain polyphosphates. The high performance isolates belonged to different morphospecies and genera, indicating that the ability to contribute to P and N nutrition is widespread within the Glomeromycota and that differences in symbiotic performance and P metabolism are not specific for individual fungal morphospecies.

Nitrogen and carbon/nitrogen dynamics in arbuscular mycorrhiza: the great unknown

Many studies have established that arbuscular mycorrhizal fungi transfer N to the host plant. However, the role and importance of arbuscular mycorrhiza (AM) in plant N nutrition is still uncertain, as are the C/N interactions within the symbiosis. Published reports provide differing, and often contradictory, results that are difficult to combine in a coherent framework. This review explores questions such as: What makes the difference between a positive and a negative effect of AM on plant N nutrition? Is the mycorrhizal N response (MNR) correlated to the mycorrhizal growth response (MGR), and how or under which conditions? Is the MNR effect on plant growth C mediated? Is plant C investment on fungal growth related to N needs or N benefit? How is the N for C trade between symbionts regulated? The patternless nature of current knowledge is made evident, and possible reasons for this are discussed.

Host diversity affects the abundance of the extraradical arbuscular mycorrhizal network

Arbuscular mycorrhizal fungi (AMF) can form complex networks in the soil that connect different host plants. Previous studies have focused on the effects of these networks on individual hosts and host communities. However, very little is known about how different host species affect the success of the fungal network itself. Given the potentially strong selection pressure against hosts that invest in a fungal network which benefits their competitors, we predict that the presence of multiple host species negatively affects the growth of the extraradical network. We designed an experiment using an in vitro culture approach to investigate the effect of different hosts (carrot, chichory and medicago) on the formation of a common mycelial network. In vitro root cultures, each inoculated with their own fungal network, were grown in a double split plate design with two host compartments and a common central compartment where fungal networks could form. We found that the size of fungal networks differs depending on the social environment of the host. When host species were propagated in a mixed species environment, the fungal abundance was significantly reduced compared to monoculture predictions. Our work demonstrates how host-to-host conflict can influence the abundance of the fungal partner.

Effect of arbuscular mycorrhizal fungi on growth and on micronutrient and macronutrient uptake and allocation in olive plantlets growing under high total Mn levels

The reported work was designed to increase knowledge about the role of arbuscular mycorrhizal fungi (AMF) on the phytoavailability and allocation of some of the principal macroelements and microelements in young potted olive plants growing in a soil presenting high levels of manganese (Mn), taken from an experimental olive field. A greenhouse trial was performed using self-rooted cuttings of Ascolana tenera, Nocellara del Belice and Carolea cultivars inoculated or not with two mycorrhizal inocula (commercial vs native). Molecular characterization of the indigenous AMF indicated that the species found in the experimental soil were different from those present in the commercial inoculum. The important incidence of AMF on P uptake was confirmed with generally double the concentration in mycorrhizal olive plants as compared to non-mycorrhizal controls, irrespective of genotype and inocula. Furthermore, apart from promoting plant growth (from 1.7- to 5-fold), the symbiosis reduced Mn concentrations from 43 to 83%. The observed differences depended on the cultivar and the inoculum, with native AMF being more effective probably as a result of their adaptation to the experimental soil. No clear direct relationship was found between AMF inoculation and other elements analysed.

Nutrient transfer in plant-fungal symbioses

Almost all plant species form symbioses with soil fungi, and nutrient transfer to plants is largely mediated through this partnership. Studies of fungal nutrient transfer to plants have largely focused on the transfer of limiting soil nutrients, such as nitrogen and phosphorous, by mycorrhizal fungi. However, certain fungal endophytes, such as Metarhizium and Beauveria, are also able to transfer nitrogen to their plant hosts. Here, we review recent studies that have identified genes and their encoded transporters involved in the movement of nitrogen, phosphorous, and nonlimiting soil nutrients between symbionts. These recent advances in our understanding could lead to applications in agricultural and horticultural settings, and to the development of model fungal systems that could further elucidate the role of fungi in these symbioses.

Polyphosphate accumulation is driven by transcriptome alterations that lead to near-synchronous and near-equivalent uptake of inorganic cations in an arbuscular mycorrhizal fungus

Arbuscular mycorrhizal (AM) fungi accumulate a massive amount of phosphate as polyphosphate to deliver to the host, but the underlying physiological and molecular mechanisms have yet to be elucidated. In the present study, the dynamics of cationic components during polyphosphate accumulation were investigated in conjunction with transcriptome analysis. Rhizophagus sp. HR1 was grown with Lotus japonicus under phosphorus-deficient conditions, and extraradical mycelia were harvested after phosphate application at prescribed intervals. Levels of polyphosphate, inorganic cations and amino acids were measured, and RNA-Seq was performed on the Illumina platform. Phosphate application triggered not only polyphosphate accumulation but also near-synchronous and near-equivalent uptake of Na(+) , K(+) , Ca(2+) and Mg(2+) , whereas no distinct changes in the levels of amino acids were observed. During polyphosphate accumulation, the genes responsible for mineral uptake, phosphate and nitrogen metabolism and the maintenance of cellular homeostasis were up-regulated. The results suggest that inorganic cations play a major role in neutralizing the negative charge of polyphosphate, and these processes are achieved by the orchestrated regulation of gene expression. Our findings provide, for the first time, a global picture of the cellular response to increased phosphate availability, which is the initial process of nutrient delivery in the associations.

Colonization of olive trees (Olea europaea L.) with the arbuscular mycorrhizal fungus Glomus sp. modified the glycolipids biosynthesis and resulted in accumulation of unsaturated fatty acids

The influence of arbuscular mycorrhizal (AM) fungi colonization on photosynthesis, mineral nutrition, the amount of phospholipids and glycolipids in the leaves of olive (Olea europaea L.) trees was investigated. After six months of growth, the rate of photosynthesis, carboxylation efficiency, transpiration and stomatal conductance in mycorrhizal (M) plants was significantly higher than that of non-mycorrhizal (NM) plants. The inoculation treatment increased the foliar P and Mg but not N. The amount of glycolipids in the leaves of M plants was significantly higher than that of NM plants. However, the amount of phospholipids in the leaves of M plants was not significantly different to that in the leaves of NM plants. Also, we observed a significant increase in the level of α-linolenic acid (C18:3ω3) in glycolipids of M plants. This work supports the view that increased glycolipids level in the leaves of M plants could be involved, at least in part, in the beneficial effects of mycorrhizal colonization on photosynthesis performance of olive trees. To our knowledge, this is the first report on the effect of AM fungi on the amount of glycolipids in the leaves of mycorrhizal plants.

Fungal nutrient allocation in common mycorrhizal networks is regulated by the carbon source strength of individual host plants

Common mycorrhizal networks (CMNs) of arbuscular mycorrhizal (AM) fungi in the soil simultaneously provide multiple host plants with nutrients, but the mechanisms by which the nutrient transport to individual host plants within one CMN is controlled are unknown. Using radioactive and stable isotopes, we followed the transport of phosphorus (P) and nitrogen (N) in the CMNs of two fungal species to plants that differed in their carbon (C) source strength, and correlated the transport to the expression of mycorrhiza-inducible plant P (MtPt4) and ammonium (1723.m00046) transporters in mycorrhizal roots. AM fungi discriminated between host plants that shared a CMN and preferentially allocated nutrients to high-quality (nonshaded) hosts. However, the fungus also supplied low-quality (shaded) hosts with nutrients and maintained a high colonization rate in these plants. Fungal P transport was correlated to the expression of MtPt4. The expression of the putative ammonium transporter 1723.m00046 was dependent on the fungal nutrient supply and was induced when the CMN had access to N. Biological market theory has emerged as a tool with which the strategic investment of competing partners in trading networks can be studied. Our work demonstrates how fungal partners are able to retain bargaining power, despite being obligately dependent on their hosts.

Transcriptional regulation of phosphate transporters from Lolium perenne and its mycorrhizal symbionts in response to phosphorus supply

Phosphate (P) uptake is critical for plant growth, but to date little is known about P uptake and transport in the pasture grass Lolium perenne L. We have identified a putative P transporter (PT) from L. perenne mycorrhizal roots (LpPT1) and assessed its transcriptional regulation by soil P availability and mycorrhizal colonisation. We also investigated transcript levels of fungal PTs from the two arbuscular mycorrhizal species Rhizophagus intraradices and Funneliformis mosseae. Our analyses indicated that LpPT1 codes for a high affinity PT most likely responsible for direct P uptake from the soil. LpPT1 is highly expressed in roots of plants grown at low P, whereas high P repressed its expression. LpPT1 was not expressed in above-ground plant tissues. Colonisation with R. intraradices did not affect expression of LpPT1 significantly. Transcript levels of the R. intraradices PT were not affected by P availability but the F. mosseae PT was repressed by high P supply, particularly in intraradical hyphae. Our study could assist in deciphering the molecular mechanisms of P uptake in the pasture grass L. perenne.

Common arbuscular mycorrhizal networks amplify competition for phosphorus between seedlings and established plants

Common mycorrhizal networks (CMNs) influence competition between plants, but reports regarding their precise effect are conflicting. We studied CMN effects on phosphorus (P) uptake and growth of seedlings as influenced by various disruptions of network components. Tomato (Solanum lycopersicon) seedlings grew into established networks of Rhizophagus irregularis and cucumber (Cucumis sativus) in two experiments. One experiment studied seedling uptake of (32)P in the network in response to cutting of cucumber shoots; the other analysed seedling uptake of P and nitrogen (N) in the presence of intact or severed arbuscular mycorrhizal fungus networks and at two soil P concentrations. Pre-established and intact networks suppressed growth of tomato seedlings. Cutting of cucumber shoots mitigated P deficiency symptoms of seedlings, which obtained access to P in the extraradical mycelium and thereby showed improved growth. Solitary seedlings growing in a network patch that had been severed from the CMN also grew much better than seedlings of the corresponding CMN. Interspecific and size-asymmetric competition between plants may be amplified rather than relaxed by CMNs that transfer P to large plants providing most carbon and render small plants P deficient. It is likely that grazing or senescence of the large plants will alleviate the network-induced suppression of seedling growth.

Intense competition between arbuscular mycorrhizal mutualists in an in vitro root microbiome negatively affects total fungal abundance

The root microbiome is composed of an incredibly diverse microbial community that provides services to the plant. A major question in rhizosphere research is how species in root microbiome communities interact with each other and their host. In the nutrient mutualism between host plants and arbuscular mycorrhizal fungi (AMF), competition often leads to certain species dominating host colonization, with the outcome being dependent on environmental conditions. In the past, it has been difficult to quantify the abundance of closely related species and track competitive interactions in different regions of the rhizosphere, specifically within and outside the host. Here, we used an artificial root system (in vitro root organ cultures) to investigate intraradical (within the root) and extraradical (outside the root) competitive interactions between two closely related AMF species, Rhizophagus irregularis and Glomus aggregatum, under different phosphorus availabilities. We found that competitive interactions between AMF species reduced overall fungal abundance. R. irregularis was consistently the most abundant symbiont for both intraradical and extraradical colonization. Competition was the most intense for resources within the host, where both species negatively affected each other's abundance. We found the investment ratio (i.e. extraradical abundance/intraradical abundance) shifted for both species depending on whether competitors were present or not. Phosphorus availability did not change the outcome of these interactions. Our results suggest that studies on competitive interactions should focus on intraradical colonization dynamics and consider how changes in investment ratio are mediated by fungal species interactions.

A trait-based framework to understand life history of mycorrhizal fungi

Despite the growing appreciation for the functional diversity of arbuscular mycorrhizal (AM) fungi, our understanding of the causes and consequences of this diversity is still poor. In this opinion article, we review published data on AM fungal functional traits and attempt to identify major axes of life history variation. We propose that a life history classification system based on the grouping of functional traits, such as Grime's C-S-R (competitor, stress tolerator, ruderal) framework, can help to explain life history diversification in AM fungi, successional dynamics, and the spatial structure of AM fungal assemblages. Using a common life history classification framework for both plants and AM fungi could also help in predicting probable species associations in natural communities and increase our fundamental understanding of the interaction between land plants and AM fungi.

Protein actors sustaining arbuscular mycorrhizal symbiosis: underground artists break the silence

The roots of most land plants can enter a relationship with soil-borne fungi belonging to the phylum Glomeromycota. This symbiosis with arbuscular mycorrhizal (AM) fungi belongs to the so-called biotrophic interactions, involving the intracellular accommodation of a microorganism by a living plant cell without causing the death of the host. Although profiling technologies have generated an increasing depository of plant and fungal proteins eligible for sustaining AM accommodation and functioning, a bottleneck exists for their functional analysis as these experiments are difficult to carry out with mycorrhiza. Nonetheless, the expansion of gene-to-phenotype reverse genetic tools, including RNA interference and transposon silencing, have recently succeeded in elucidating some of the plant-related protein candidates. Likewise, despite the ongoing absence of transformation tools for AM fungi, host-induced gene silencing has allowed knockdown of fungal gene expression in planta for the first time, thus unlocking a technological limitation in deciphering the functional pertinence of glomeromycotan proteins during mycorrhizal establishment. This review is thus intended to draw a picture of our current knowledge about the plant and fungal protein actors that have been demonstrated to be functionally implicated in sustaining AM symbiosis mostly on the basis of silencing approaches.