Nicolás Marro

Summary: Arbuscular mycorrhizal fungi (AMF) are key drivers of plant growth and nutrition, shaping the relationship between plant diversity and ecosystem productivity. In agroecosystems, AMF generally benefit crops but often have neutral or even negative effects on weeds, yet the mechanisms underlying these contrasting interactions remain poorly understood. In this Viewpoint, we propose a plant community‐level framework to investigate interactions between multiple crop and weed species and diverse AMF taxa, focusing on chemically mediated communication via root exudates, particularly flavonoids (FLVs) and strigolactones (SLs). These compounds can act as ‘cry for help’ signals that recruit beneficial soil microorganisms to alleviate environmental stress. We found that their composition varies widely among plant families, with crops typically producing more diverse and functionally distinctive FLV profiles than weeds. Similar patterns, though less documented, appear for SLs. Different FLV subclasses elicit contrasting AMF responses, influencing spore germination, hyphal growth, and root colonization. Notably, FLVs with stronger positive effects on AMF are more common in crops, whereas those with neutral effects tend to dominate in weeds. Our results are consistent with the idea that such molecular cues may shape AMF recruitment and could potentially feed back into plant community dynamics, although this hypothesis should be explicitly tested.

The study of how phenotypic selection by plant mutualists and antagonists varies within ecosystems is crucial for evaluating potential plant adaptations to changing environments, such as urban areas. Here, we describe the interaction intensity of plants with their mutualistic and antagonistic partners, as well as the selective landscapes in one natural and one urban site. We measured direct phenotypic selection on plant traits relevant to three interaction partners: floral tube length, resistance to herbivores and arbuscular mycorrhizal fungi (AMF) colonization. Significant direct phenotypic selection on the three interaction traits was detected in the natural site. We observed disruptive selection on floral tube length, with the two fitness optima closely matching the proboscis peak lengths of the main pollinator species. In addition, we observed stabilizing selection by arbuscular mycorrhizal fungi, favouring plants with around 11% of arbuscular colonization. This result supports the idea that plants may autoregulate mean intraradical arbuscular colonization to maximize fitness relative to carbon allocation to arbuscular mycorrhizal fungi. Finally, we detected directional selection benefiting plants with high resistance to herbivores. Our results highlight the importance of adopting a multispecies, context-dependent approach to better understand the complex ecological interactions that drive plant evolution.

Traditionally, characterisation and comparison of AMF communities has been carried out by morphological identification of asexual spores in soil. In recent decades, molecular methods such as soil metabarcoding have become more popular than morphological identification of spores, but direct comparisons of the efficiency of both approaches have been rare. In this study, we compared AMF communities in soil samples from vegetable farms using both morphological and molecular methods (internal transcribed spacer, ITS, markers). In addition, we performed a systematic literature search and retrieved nine studies that analysed AMF communities using both approaches in the same soil samples, mostly in agroecosystems. Our results show that AMF communities determined by morphological spore-based identification are different than those determined by molecular genetic markers, but not as often claimed. In some cases, the morphological spore-based characterisation of spores revealed more diverse glomeromycotan communities. Moreover, in several cases the spore-based methods recovered taxa that the molecular methods did not, while in other cases the opposite was observed. The field and literature-based results of this study indicate that for a comprehensive and exhaustive characterisation of AMF communities it is necessary to combine both approaches. However, if the aim is to compare communities under different environmental conditions, both approaches provide comparable patterns.

Conventional agriculture has been suggested to promote less mutualistic arbuscular mycorrhizal fungi (AMF). The main aim of this study was to test this assumption by a detailed functional analysis of the plant mycorrhizal benefits and costs. A cross-inoculation experiment was established with Plantago lanceolata as a host plant and inocula of AMF sourced from four pairs of conventionally managed arable fields and neighbouring grasslands. Mycorrhizal effects were determined for a range of plant parameters including fluxes of isotopically labelled phosphorus (P), nitrogen (N) and carbon (C), and related to root colonization and composition of the different AMF communities. The association of P. lanceolata with arable-field inocula was less beneficial in terms of plant growth promotion and it also led to more pronounced P accumulation in plant biomass, as compared to grassland inocula. Furthermore, arable-field AMF increased ¹⁵N depletion in soil and ¹⁵N transfer to shoots, and induced higher ¹³C drain to soil. These differential functional parameters were related to consistent compositional differences between arable-soil and grassland AMF communities in the roots. Differential effects of the AMF inocula on N and C partitioning in the soil–plant system suggest faster foraging for nutrients by arable-soil AMF and higher demand for C, which are characteristics associated with ruderal AMF. This implies that arable-soil AMF may be less beneficial in conditions of plant growth limitation by C than the grassland AMF.

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

Summary: The great majority of plants gain access to soil nutrients and enhance their performance under stressful conditions through symbiosis with arbuscular mycorrhizal fungi (AMF). The benefits that AMF confer vary among species and taxonomic groups. However, a comparative analysis of the different benefits among AMF has not yet been performed. We conducted a global meta‐analysis of recent studies testing the benefits of individual AMF species and main taxonomic groups in terms of plant performance (growth and nutrition). Separately, we examined AMF benefits to plants facing biotic (pathogens, parasites, and herbivores) and abiotic (drought, salinity, and heavy metals) stress. AMF had stronger positive effects on phosphorus nutrition than on plant growth and nitrogen nutrition and the effects on the growth of plants facing biotic and abiotic stresses were similarly positive. While the AMF taxonomic groups showed positive effects on plant performance either with or without stress, Diversisporales were the most beneficial to plants without stress and Gigasporales to plants facing biotic stress. Our results provide a comprehensive analysis of the benefits of different AMF species and taxonomic groups on plant performance and useful insights for their management and use as bio‐inoculants for agriculture and restoration.