Multilocus genotyping of arbuscular mycorrhizal fungi and marker suitability for population genetics

In vitro Propagation of Arbuscular Mycorrhizal Fungi May Drive Fungal Evolution

Transformed root cultures (TRC) are used to mass produce arbuscular mycorrhizal (AM) fungal propagules in vitro. These propagules are then used in research, agriculture, and ecological restoration. There are many examples from other microbial systems that long-term in vitro propagation leads to domesticated strains that differ genetically and functionally. Here, we discuss potential consequences of in TRC propagation on AM fungal traits, and how this may affect their functionality. We examine weather domestication of AM fungi has already happened and finally, we explore whether it is possible to overcome TRC-induced domestication.

Mycorrhizal response in crop versus wild plants

We proposed a theoretical framework predicting mutualistic outcomes for the arbuscular mycorrhizal (AM) symbiosis based on host provenance (crop versus wild). To test the framework, we grew two isolates of Rhizoglomus irregulare (commercial versus an isolate locally isolated), with five crop plants and five wild plants endemic to the region that co-occur with the locally sourced fungus. While inoculation with either isolate had no effect on plant biomass, it decreased leaf P content, particularly for wild plants. All plants associating with the commercial fungus had lower leaf P. Overall, our data shows that wild plants may be more sensitive to differences in mutualistic quality among fungal isolates.

Site specificity in establishment of a commercial arbuscular mycorrhizal fungal inoculant

While establishment and persistence of arbuscular mycorrhizal (AM) fungal inoculants in agricultural fields are critical to their success, little is known about how farming practices can affect their establishment in field. We developed a probe assay specific to a commercial AM fungal inoculant (Rhizoglomus irregulare DAOM197198) and tested its establishment among different grain cropping practices in the field. Establishment of the fungus was not related to cropping, or inoculation practices. Instead, establishment was site specific over the two growing seasons. Our results show that it is not yet possible to predict inoculation success in the field and use of biofertilizers requires further research under field conditions to identify key factors involved in establishment and persistence.

Beneficial associations between Brassicaceae plants and fungal endophytes under nutrient-limiting conditions: evolutionary origins and host-symbiont molecular mechanisms

Brassicaceae plants have lost symbiotic interactions with mutualistic mycorrhizal fungi, but, nonmycorrhizal Brassicaceae associate with diverse taxonomic groups of mutualistic root-endophytic fungi. Distantly related fungal endophytes of Brassicaceae plants transfer phosphorus to the hosts and promote plant growth, thereby suggesting that the beneficial function was independently acquired via convergent evolution. These beneficial interactions appear tightly regulated by the tryptophan-derived secondary metabolite pathway, which specifically developed in Brassicaceae. Importantly, phosphate availability and types of colonizing microbes appear to influence the metabolite pathway. Thus, endophytes of Brassicaceae may have evolved to adapt to the Brassicaceae-specific traits. Future comparative functional analyses among well-defined endophytic fungi and their relatives with distinct life strategies and host plants will help understand the mechanisms that establish and maintain beneficial interactions.

Inoculation effects on root-colonizing arbuscular mycorrhizal fungal communities spread beyond directly inoculated plants

Inoculation with arbuscular mycorrhizal fungi (AMF) may improve plant performance at disturbed sites, but inoculation may also suppress root colonization by native AMF and decrease the diversity of the root-colonizing AMF community. This has been shown for the roots of directly inoculated plants, but little is known about the stability of inoculation effects, and to which degree the inoculant and the inoculation-induced changes in AMF community composition spread into newly emerging seedlings that were not in direct contact with the introduced propagules. We addressed this topic in a greenhouse experiment based on the soil and native AMF community of a post-mining site. Plants were cultivated in compartmented pots with substrate containing the native AMF community, where AMF extraradical mycelium radiating from directly inoculated plants was allowed to inoculate neighboring plants. The abundances of the inoculated isolate and of native AMF taxa were monitored in the roots of the directly inoculated plants and the neighboring plants by quantitative real-time PCR. As expected, inoculation suppressed root colonization of the directly inoculated plants by other AMF taxa of the native AMF community and also by native genotypes of the same species as used for inoculation. In the neighboring plants, high abundance of the inoculant and the suppression of native AMF were maintained. Thus, we demonstrate that inoculation effects on native AMF propagate into plants that were not in direct contact with the introduced inoculum, and are therefore likely to persist at the site of inoculation.

Fungal Mating in the Most Widespread Plant Symbionts?

Arbuscular mycorrhizal fungi (AMF) are relevant plant symbionts whose hyphae and spores carry hundreds of coexisting nuclei with supposedly divergent genomes but no sign of sexual reproduction. This unusual biology suggested that conventional fungal mating is not amendable to optimize strains for plant growth, but recent evidence of sexual-related nuclear inheritance in these organisms is now challenging this widespread notion. Here, we outline our knowledge of AMF genetics within a historical context, and discuss how past and new information in this area changed our understanding of AMF biology. We also highlight the mating-related processes in AMF, and propose new research avenues and approaches that could lead to a better application of these organisms for agricultural and environmental practices.

Evidence for the sexual origin of heterokaryosis in arbuscular mycorrhizal fungi

Sexual reproduction is ubiquitous among eukaryotes, and fully asexual lineages are extremely rare. Prominent among ancient asexual lineages are the arbuscular mycorrhizal fungi (AMF), a group of plant symbionts with a multinucleate cytoplasm. Genomic divergence among co-existing nuclei was proposed to drive the evolutionary success of AMF in the absence of sex(1), but this hypothesis has been contradicted by recent genome analyses that failed to find significant genetic diversity within an AMF isolate(2,3). Here, we set out to resolve issues surrounding the genome organization and sexual potential of AMF by exploring the genomes of five isolates of Rhizophagus irregularis, a model AMF. We find that genetic diversity in this species varies among isolates and is structured in a homo-dikaryon-like manner usually linked with the existence of a sexual life cycle. We also identify a putative AMF mating-type locus, containing two genes with structural and evolutionary similarities with the mating-type locus of some Dikarya. Our analyses suggest that this locus may be multi-allelic and that AMF could be heterothallic and bipolar. These findings reconcile opposing views on the genome organization of these ubiquitous plant symbionts and open avenues for strain improvement and environmental application of these organisms.

The largest subunit of RNA polymerase II as a new marker gene to study assemblages of arbuscular mycorrhizal fungi in the field

Due to the potential of arbuscular mycorrhizal fungi (AMF, Glomeromycota) to improve plant growth and soil quality, the influence of agricultural practice on their diversity continues to be an important research question. Up to now studies of community diversity in AMF have exclusively been based on nuclear ribosomal gene regions, which in AMF show high intra-organism polymorphism, seriously complicating interpretation of these data. We designed specific PCR primers for 454 sequencing of a region of the largest subunit of RNA polymerase II gene, and established a new reference dataset comprising all major AMF lineages. This gene is known to be monomorphic within fungal isolates but shows an excellent barcode gap between species. We designed a primer set to amplify all known lineages of AMF and demonstrated its applicability in combination with high-throughput sequencing in a long-term tillage experiment. The PCR primers showed a specificity of 99.94% for glomeromycotan sequences. We found evidence of significant shifts of the AMF communities caused by soil management and showed that tillage effects on different AMF taxa are clearly more complex than previously thought. The high resolving power of high-throughput sequencing highlights the need for quantitative measurements to efficiently detect these effects.

Long-term tracing of Rhizophagus irregularis isolate BEG140 inoculated on Phalaris arundinacea in a coal mine spoil bank, using mitochondrial large subunit rDNA markers

During the last decade, the application of arbuscular mycorrhizal fungi (AMF) as bioenhancers has increased significantly. However, until now, it has been difficult to verify the inoculation success in terms of fungal symbiont establishment in roots of inoculated plants because specific fungal strains could not be detected within colonized roots. Using mitochondrial large subunit ribosomal DNA, we show that Rhizophagus irregularis (formerly known as Glomus intraradices) isolate BEG140 consists of two different haplotypes. We developed nested PCR assays to specifically trace each of the two haplotypes in the roots of Phalaris arundinacea from a field experiment in a spoil bank of a former coal mine, where BEG140 was used as inoculant. We revealed that despite the relatively high diversity of native R. irregularis strains, R. irregularis BEG140 survived and proliferated successfully in the field experiment and was found significantly more often in the inoculated than control plots. This work is the first one to show tracing of an inoculated AMF isolate in the roots of target plants and to verify its survival and propagation in the field. These results will have implications for basic research on the ecology of AMF at the intraspecific level as well as for commercial users of mycorrhizal inoculation.

Nonself vegetative fusion and genetic exchange in the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi (AMF) form symbioses with the majority of plants and form extensive underground hyphal networks simultaneously connecting the roots of different plant species. No empirical evidence exists for either anastomosis between genetically different AMF or genetic exchange.Five isolates of one population of Glomus intraradices were used to study anastomosis between hyphae of germinating spores. We show that genetically distinct AMF, from the same field, anastomose, resulting in viable cytoplasmic connections through which genetic exchange could potentially occur.Pairs of genetically different isolates were then co-cultured in an in vitro system.Freshly produced spores were individually germinated to establish new cultures.Using several molecular tools, we show that genetic exchange occurred between genetically different AMF. Specific genetic markers from each parent were transmitted to the progeny. The progeny were viable, forming symbioses with plant roots. The phenotypes of some of the progeny were significantly different from either parent.Our results indicate that considerable promiscuity could occur in these fungi because nine out of 10 combinations of different isolates anastomosed. The ability to perform genetic crosses between AMF experimentally lays a foundation for understanding the genetics and evolutionary biology of these important plants symbionts.

Recombination in Glomus intraradices, a supposed ancient asexual arbuscular mycorrhizal fungus

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are important symbionts of most plant species, promoting plant diversity and productivity. This symbiosis is thought to have contributed to the early colonisation of land by plants. Morphological stasis over 400 million years and the lack of an observed sexual stage in any member of the phylum Glomeromycota led to the controversial suggestion of AMF being ancients asexuals. Evidence for recombination in AMF is contradictory.

RESULTS

We addressed the question of recombination in the AMF Glomus intraradices by sequencing 11 polymorphic nuclear loci in 40 morphologically identical isolates from one field. Phylogenetic relationships among genotypes showed a reticulate network pattern providing a rationale to test for recombination. Five statistical tests predicted multiple recombinant regions in the genome of a core set of isolates. In contrast, five clonal lineages had fixed a large number of differences.

CONCLUSION

Our data show that AMF from one field have undergone recombination but that clonal lineages coexist. This finding has important consequences for understanding AMF evolution, co-evolution of AMF and plants and highlights the potential for commercially introduced AMF inoculum recombining with existing local populations. Finally, our results reconcile seemingly contradictory studies on whether AMF are clonal or form recombining populations.