Genetics of Arbuscular Mycorrhizal Fungi

Dear Colleagues,

Arbuscular mycorrhizal (AM) fungi (Glomeromycota) form the foundation of terrestrial life as they form a crucial mutualistic symbiosis with the roots or root-like organs of the vast majority (~80%) of all land plants. This ancient symbiosis dates back at least ~450 million years and has enabled the colonisation of land by plants. AM fungi provide their host plants access to scarce minerals, such as phosphate and micronutrients, and provide protection against biotic and abiotic stresses in exchange for photosynthetic carbon from the autotrophic host. At the heart of the AM symbiosis is the formation of a symbiotic host membrane compartment containing highly branched hyphae inside root cortical cells, termed the arbuscule, where nutrients and carbon are exchanged in a controlled manner. AM fungi have a major impact on plant community structure and diversity and they are of major (agro-) ecological importance. Understanding the genetic make-up of the fungus is essential to get insight into this fascinating symbiosis and to exploit its potential for sustainable agriculture.

AM fungi have several unique characteristics that have severely complicated molecular genetic studies. AM fungi are obligate biotrophs that need a plant to complete their life cycle. Their hyphal network forms a continuous (coenocytic) cytoplasmic compartment where numerous nuclei move and migrate. Spores typically contain several hundreds of nuclei that migrate from the hypha into the spore and subsequently divide. So there is never a stage during the AM life cycle where only one nucleus initiates the next generation. This makes the application of the concept of the genetic individual difficult. A sexual stage has never been observed in AM fungi. Therefore, these fungi are considered to be ancient asexual organisms. On the other hand, hyphae of different (related) fungal ‘individuals’ of the same species can fuse and exchange nuclei by anastomosis. Even more striking is the observation that there is substantial nuclear (but not mitochondrial) genetic variation contained within spores or mycelia. Individual nuclei can be different (heterokaryotic), so the genome space of a single AM fungus is in fact the equivalent of multiple (diverse) nuclear genomes, called nucleotypes. Furthermore, segregation of genetically different nuclei from a single individual has been observed. This genetic make-up allows for nuclear conflict, as the fitness of individual nuclei depends on the environment (spores, mycelia) in which they reside, and as nuclear fitness may not be identical to fitness of the nuclear population contained with mycelia or spores. Such nuclear conflict may threaten its mutualistic role. Interestingly, the resulting genetic variation among offspring appears to strongly affect plant-growth promoting aspects and efficiency of the symbiosis. This highlights the need to better understand the genetic make-up of these fungi and how it interplays with the genetic make-up of plants.

Recently, in a major breakthrough, the first genome sequence of Rhizophagus irregularis has been revealed, which now offers a unique starting point to address some of these intriguing questions, such as: How is genetic variation within AM fungi maintained? Is there a sexual or parasexual cycle in AM fungi? How does speciation and cohesion within species occur in the absence of a sexual cycle? Are there still functional or degenerated mating-type genes? Is there evidence for gene transfer from the fungus to the plant genome? How are these fungi able to (intracellularly) colonize nearly all land plants? Is there selection on preferential nucleotypes by the host plants, and how does this selection work? How does the genetic composition of the fungus affect plant performance? Can we manipulate the genetic composition of the AM fungi and thereby design more beneficial mycorrhizal symbioses?

In this Special Issue we invite you to contribute your original research or review articles that deal with the genetic make-up of the fungus and how it interplays with the genetic make-up of the host plants, and impacts plant performance. We hope that this Special Issue will stimulate research into this fascinating aspect of AM biology.

Prof. Dr. Thomas W. Kuyper
Dr. Erik Limpens
Dr. Rene Geurts
Guest Editors

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• Genome analyses of AM fungi
• Transcriptome analysis of AM fungi and host plant cells
• Population genetics of AM fungi
• Inter and Intraspecific genetic variation in AM fungi (genotyping)
• Nuclear conflict and levels of selection
• Segregation and selection of nuclei (nucleotypes)
• Hetero- versus homokaryosis
• Linking genotype to phenotype and its ecological implications
• Limits to anastomosis-forming capability
• Meiosis and Recombination in AM fungi (sexual/parasexual cycles)
• Behaviour of nuclei within and between AM individuals, including cell biological analyses
• Host sanctioning or partner fidelity feedback; what drives mutualism at a genetic level?
• Developing technologies (transformation of AM fungi, reverse genetic tools, laser microdissection, etc.) to study the genetic make-up of AM fungi
• Adaptation in AM fungi
• Designer mycorrhizas
• Interplay between AM fungal and plant genotypes