Metal pollution poses significant ecological and economic concerns for many countries, resulting from anthropogenic activities such as intensive farming, mining, and other industrial sectors. Many of these metals can be toxic, affecting not only plant and animal nutrition but also human health. Phytoremediation of metal-polluted soils is now regarded as one of the most promising nature-based solutions for removing metals from contaminated environments. It can be enhanced by plant inoculation with beneficial microorganisms, such as arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR). For about two decades, the combined use of PGPR and AMF has attracted interest. This review summarizes the studies carried out on this subject, highlighting the complementary mechanisms of these two types of microbes and their synergistic effects, which improve the plant’s mineral nutrition and tolerance to heavy metals, as well as better metal neutralization through stabilization in the plant’s aerial and root organs and in the soil. Among these mechanisms, AMF intervene by mobilizing essential minerals due to their external mycelium, which explores a large volume of soil. AMF also contribute to reducing soil erosion through the soil-binding capacity of their extraradical mycelium and glomalin production, which enhances soil aggregation and stability. These symbionts contribute efficiently to metal toxicity alleviation in plants. PGPR can improve plant growth through various mechanisms, including hormone production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, nitrogen fixation, and the secretion of different chelating substances. Metals can be neutralized by a variety of processes, including binding, biosorption, transformation, and immobilization. Mycorrhiza helper bacteria are associated with AMF and can stimulate their mycelial growth, spore production, and spore germination, thus increasing mycorrhizal colonization. The selection of bacteria and AMF for phytoremediation purposes should be based on these different complementary properties. Furthermore, genomic and transcriptomic studies may be utilized to identify the most active genes in terms of their positive effects on the plant and phytoremediation mechanisms. This approach enables a more rigorous selection of strains. Field experiments with co-inoculation of AMF and bacteria are rare at present and need to be developed in different edaphic and climatic conditions.

Diversisporales comprises species with worldwide distribution that produce glomoid, otosporoid, or tricisporoid spores. The recent reorganization of the order recognizes two families, Diversisporaceae and Corymbiglomeraceae, comprising one and five genera, respectively. Several Glomeromycota specimens collected in northern and southeastern Mexico and in French Polynesian atolls were characterized using both morphological and molecular analyses. Phylogenetic inference revealed that they represent new members of the Diversisporales, supporting the reorganization of the genus Redeckera into three independent lineages: Albocarpum gen. nov., with A. arenaceum sp. nov., A. leptohyphum sp. nov., and A. fulvum comb. nov., Pulvinocarpum pulvinatum gen. et comb. nov., and Redeckera, which retains five species, including R. varelae sp. nov. In addition, we described Melanocarpum mexicanum gen. et sp. nov. and Diversispora papillosa sp. nov. A broader phylogeny, based on eDNA ribosomal sequences and representative of Diversisporales species, including the newly described taxa, further supported the split of Redeckera and suggested three additional clades likely corresponding to a new family and two new genera, awaiting the discovery of representative morphospecies for formal description. Using eDNA sequences metadata, the occurrences of the newly described taxa were mapped, allowing the recognition of distribution patterns, mostly in the pantropical zone, distinguishing widespread and rare species, and suggesting possible endemisms. Finally, the coexistence of species forming large sporocarps (A. fulvum and A. leptohyphum) alongside species forming spores in loose aggregates (A. arenaceum) prompted us to propose a possible sporulation dimorphism hypothesis in Albocarpum, an argument previously raised to explain the nested placement of Corymbiglomus and Paracorymbiglomus within the Redeckera clade.

Diversispora cerifera and Diversispora succinacia are new arbuscular mycorrhizal fungal species that have been isolated and propagated from spores extracted from rhizosphere soils of native vegetation that had naturally established from seeds on a nickel mine tailing test basin in New Caledonia. Interestingly, these species were not recorded from ultramafic soils of maquis vegetation endemic to New Caledonia surrounding the tailing basin. In greenhouse trap and single-species cultures, the fungi produced numerous spores, which were formed terminally or intercalary on subtending hyphae. Spores of D. cerifera are white-yellow with a waxy appearance and 70–100–120 µm in diameter; spores of D. succinacia are translucent, amber in color, and 60–80–110 µm in diameter; both species have three spore wall layers. A phylogenetic analysis placed D. cerifera in a clade sister to D. succinacia. The same analysis showed that the sister species of D. succinacia is D. sabulosa.

Until recent past, arbuscular mycorrhizal fungi (AMF) were generally used as single species inoculants carefully selected to enhance plant growth. However, in ecological perspective, it is expected that a mixture of different AMF species would be more efficient to occupy the ecological niches of the symbiosis and would adapt better to environmental fluctuations. A limited number of studies have reported complementary effects of different AMF taxa on plant development. Experiments were performed in different conditions. In most cases, combinations of AMF showed better results than single species. Particularly, clear results were obtained in ultramafic soils characterized by multiple stress factors: low level of main mineral elements, high concentrations of potentially toxic metals (Ni, Co, Cr, Mn), and critically low Ca/Mg ratio. These studies reported synergistic effects of mixed isolates belonging to different families. The most performant mix showed quantities of P and K absorbed per plant more than ten times higher than non-inoculated plants and more than two times higher than the most performant AMF isolate. Inversely, the translocation factors for potentially toxic metals (particularly Ni and Co) were reduced three to ten times. Mathematical modelization of the effects of 12 AMF inoculants, including 6 mixes on plant growth, revealed that the predicted value of the performance based on the properties of the different AMF was very high (R2 = 0.90; p < 0.00001), indicating that the improvement of plant biomass was nearly totally explained as a resultant of the complementary effects of the five isolates. Field experiments in natural conditions and commercial trials confirmed the greenhouse results obtained in sterilized soils and revealed large positive effects on plant development of selected AMF mixes. For such success, it is suggested that the isolates used for the mixed inoculants must belong to different families and must be selected for their synergistic effects on plant development.

Our knowledge about New Caledonian serpentine ecosystems has increased greatly during the past half-century, mainly thanks to Jaffre's group. However, research on soil microflora and plant symbionts started only in the nineties and was mainly published during the last two decades. We aim to synthesize these studies, focusing particularly on arbuscular mycorrhizal fungi (AMF). Research on AMF consists firstly of a global and inventory approach aiming to produce a basic but essential lacking knowledge. These studies showed that AMF are abundant in ultramafic soils and concerned nearly all plant species of these ecosystems. Even Ni-hyperaccumulator plants and sedges, generally considered non-mycorrhizal, were found to be functionally colonized by AMF in New Caledonian ultramafic soils. The adaptation of AMF communities to the extreme conditions of these soils led to high levels of metal tolerance (particularly to Ni) and noticeable originality of the taxa. The influence of these symbionts on plant growth and adaptation was assessed in greenhouse and field conditions. An accurate selection of AMF isolates that improve plant growth, and plant metal tolerance was performed. It was demonstrated that combinations of AMF isolates with complementary functional traits showed highly synergistic effects on plant development. Finally, a partnership with a biotechnological company led to the production of an efficient commercial inoculant now used in the ecological restoration of mine-degraded areas. Today studies are focused mainly on the additive effects of AMF and mycorrhiza-helper bacteria.

Examination of fungal specimens collected in the Atlantic rain forest ecosystems of Northeast Brazil revealed many potentially new epigeous and semihypogeous glomerocarp-producing species of the phylum Glomeromycota. Among them were two fungi that formed unorganized epigeous glomerocarps with glomoid spores of almost identical morphology. The sole structure that distinguished the two fungi was the laminate layer 2 of their three-layered spore wall, which in spores of the second fungus crushed in PVLG-based mountants contracted and, consequently, transferred into a crown-like structure. Surprisingly, phylogenetic analyses of sequences of the 18S-ITS-28S nuc rDNA and the rpb1 gene indicated that these glomerocarps represent two strongly divergent undescribed species in the family Glomeraceae. The analyses placed the first in the genus Dominikia, and the second in a sister clade to the monospecific generic clade Kamienskia with Kamienskia bistrata. The first species was described here as Dominikia glomerocarpica sp. nov. Because D. glomerocarpica is the first glomerocarp-forming species in Dominikia, the generic description of this genus was emended. The very large phylogenetic distance and the fundamental morphological differences between the second species and K. bistrata suggested us to introduce a new genus, here named as Epigeocarpum gen. nov., and name the new species Epigeocarpum crypticum sp. nov. In addition, our analyses also focused on an arbuscular mycorrhizal fungus originally described as Rhizophagus neocaledonicus, later transferred to the genus Rhizoglomus. The analyses indicated that this species does not belong to any of these two genera but represents a new clade at the rank of genus in the Glomeraceae, here described as Silvaspora gen. nov.

Research on arbuscular mycorrhizal fungi (AMF) in ultramafic soils has principally focused on ecological restoration whereas little attention has been given to agriculture. The present study aims at understanding if mixtures of different AMF species from a New Caledonian ultramafic soil induced a better plant fitness and lower contents of potentially toxic metals in aerial parts of a crop plant than single species inoculants. Sorghum vulgare plants were inoculated using six AMF species separately and in different mixtures of these species in a glasshouse experiment in ultramafic soil. The tested inocula showed very different effects on plant fitness. Results highlight that, when sorghum is grown in ultramafic soil, AMF mixes were more efficient than single species inoculation in alleviating abiotic stresses by reducing translocation of potentially toxic metals to the aerial part of the plant and thereby improving the fitness of the plants. Our findings point out the utility of arbuscular mycorrhizal biotechnology for agricultural ultramafic soils.

The ecological restoration of nickel mining-degraded areas in New Caledonia is strongly limited by low availability of soil mineral nutrients, metal toxicity, and slow growth rates of native plant species. In order to improve plant growth for restoration programs, special attention was paid to interactions between plant and soil microorganisms. In this study, we evaluated the influence of inoculation with Curtobacterium citreum BE isolated from a New Caledonian ultramafic soil on arbuscular mycorrhizal symbiosis and growth of Tetraria comosa , an endemic sedge used in restoration programs . A greenhouse experiment on ultramafic substrate was conducted with an inoculum comprising two arbuscular mycorrhizal fungi (AMF) species isolated from New Caledonian ultramafic soils: Rhizophagus neocaledonicus and Claroideoglomus etunicatum . The effects on plant growth of the AMF and C. citreum BE inoculated separately were not significant, but their co-inoculation significantly enhanced the dry weight of T. comosa compared with the non-inoculated control. These differences were positively correlated with mycorrhizal colonization which was improved by C. citreum BE. Compared with the control, co-inoculated plants were characterized by better mineral nutrition, a higher Ca/Mg ratio, and lower metal translocation. However, for Ca/Mg ratio and metal translocation, there were no significant differences between the effects of AMF inoculation and co-inoculation.

Inoculation of arbuscular mycorrhizal fungi (AMF) as plant growth promoters has mostly been conducted using single-species inoculum. In this study, we investigated whether co-inoculation of different native AMF species induced an improvement of plant growth in an ultramafic soil. We analyzed the effects of six species of AMF from a New Caledonian ultramafic soil on plant growth and nutrition, using mono-inoculations and mixtures comprising different numbers of AMF species, in a greenhouse experiment. The endemic Metrosideros laurifolia was used as a host plant. Our results suggest that, when the plant faced multiple abiotic stress factors (nutrient deficiencies and high concentrations of different heavy metals), co-inoculation of AMF belonging to different families was more efficient than mono-inoculation in improving biomass, mineral nutrition, Ca/Mg ratio, and tolerance to heavy metals of plants in ultramafic soil. This performance suggested functional complementarity between distantly related AMF. Our findings will have important implications for restoration ecology and mycorrhizal biotechnology applied to ultramafic soils.

Four new species of arbuscular mycorrhizal (AM) fungi (Glomeromycota) were isolated from the rhizosphere of endemic metallophytic plants in ultramafic soils in New Caledonia (South Pacific) and propagated on Sorghum vulgare . Acaulospora saccata and A. fragilissima are placed in the Acaulosporaceae, Scutellospora ovalis in the Gigasporaceae, and Rhizophagus neocaledonicus in the Glomeraceae. The novelty of these species is supported by morphological characters of spores and phylogenetic analyses of sequences of the rDNA region, comprising partial small subunit rRNA gene, the internal transcribed spacers, 5.8S rRNA gene, and the partial large subunit rRNA gene. New Caledonia is known for its high degree of endemism in plants, which is due to its geographic position and geological history. This is the first taxonomic study exploring local Glomeromycota of this island, which may help to address the question of possible AMF endemism in future studies.

A new arbuscular mycorrhizal, Paraglomus-like, fungal species (Glomeromycotina) was found and propagated in single-species cultures established from spores coming from Oman, Greece, Tunisia, and New Caledonia. The hyaline spores of the fungus are small, 17-67 mu m diam when globose. Their spore wall consists of an evanescent, 1.0-2.3 mu m thick, short-lived outer layer and a laminate, smooth, 3.8-7.3 mu m thick inner layer, of which none shows amyloid or dextrinoid reaction in Melzer's reagent. Mycorrhizal structures (arbuscules and hyphae without vesicles) of the fungus stained slightly in Trypan blue, thus like those of the formerly described Paraglomus spp., in which the histochemical feature has been recognized. However, in the phylogenies gained from analyses of nrDNA sequences, the fungus formed a distinct lineage in a basal position relative to and highly diverged from that with P. majewskii and that with the other Paraglomus spp. of known molecular phylogeny. Comparisons of similarity of sequences and the spore wall structure of the new fungus with those of P. majewskii and the other Paraglomus spp. and the positions of clades of the three taxa relative to those of the other taxa of the Glomeromycotina suggested transferring P. majewskii to a new genus in the Paraglomeraceae and describing the new fungus as a new species of a new genus in a new family of the order Paraglomerales.

Arbuscular mycorrhizal fungi (AMF, Glomeromycota) are mutualistic symbionts associated with majority of land plants. These fungi play an important role in plant growth, but their taxonomic identification remains a challenge for academic research, culture collections and inoculum producers who need to certify their products. Identification of these fungi was traditionally performed based on their spore morphology. DNA sequence data have successfully been used to study the evolutionary relationships of AMF, develop molecular identification tools and assess their diversity in the environment. However, these methods require considerable expertise and are not well-adapted for “routine” quality control of culture collections and inoculum production. Here, we show that Matrix-Assisted Laser Desorption Ionisation Time of Flight Mass Spectrometry proteomic-based biotyping is a highly efficient approach for AMF identification. Nineteen isolates belonging to fourteen species, seven genera and five families were clearly differentiated by MALDI biotyping at the species level, and intraspecific differentiation was achieved for the majority. AMF identification by MALDI biotyping could be highly useful, not only for research but also in agricultural and environmental applications. Fast, accurate and inexpensive molecular mass determination and the possibility of automation make MALDI-TOF-MS a real alternative to conventional morphological and molecular methods for AMF identification.