The genetics of prominent plant symbionts: the arbuscular mycorrhizal fungi
Nicolas Corradi
The arbuscular mycorrhizal symbiosis: a keystone terrestrial mutualism The roots of most known land plants associate with
biology. Specifically, these fungi form large and unsegmented underground hyphal networks whereby several thousands (potentially millions) of nuclei co-exist within one large syncytium (Figure 1). In contrast to other multinucleate fungi, there is also no known stage in the AMF life cycle where only one (or two) nuclei are found in one cell or in individual spores. Furthermore, there is no formal observation of sexual reproduction in these organisms. The continuing co-existence of nuclei within AMF spores and mycelium, together with their apparent lack of sexual reproduction, has led to the emergence of a long-held belief that AMF carry unique ‘non- Mendelian’ genetics and that these fungi never underwent sexual reproduction. The advent of genomics, and in particular single-nucleus sequencing, has ultimately challenged those hypotheses. The community now overwhelmingly agrees that AMF genetics follow patterns seen in other fungi. Furthermore, data have shown that these fungi have found ways to generate genetic diversity either via sexual (meiotic) or parasexual (somatic) recombination (or both). Genome data and single-nucleus sequencing revealed that AMF strains separate into two ‘genetic categories’. Specifically, some strains of the model species Rhizophagus irregularis carry a nuclear organisation in which all co-existing nuclei are genetically homogeneous (AMF homokaryons), while few others carry thousands of co-existing nuclei deriving from two parental strains (AMF heterokaryons). The existence of homo-heterokaryotic stages resembles transitions seen between clonal and sexual strains in sexual fungi (particularly basidiomycetes). Indeed, a genetic locus
underground fungi called arbuscular mycorrhizal fungi (AMF). In this mycorrhizal (meaning fungi within roots) association, the fungal hyphae emerge from AMF spores and penetrate the plant root cortex, thereby producing tree-like structures called arbuscules that give these fungi their name. These unique structures represent the site of nutrient exchange between the symbiotic partners, and through these the fungus helps plants improve uptake of soil nutrients in exchange for carbon sources fixed photosynthetically by the plant; primarily lipids and sugars. Generally, though not always, AMF are beneficial for plant hosts and crops. In particular, the arbuscular mycorrhizal (AM) symbiosis has been shown to improve plant fitness and crop yield and can result in increased protection against fungal plant pathogens (such as the head blight agent Fusarium spp.). Moreover, AMF taxonomic diversity increases plant biodiversity and ecosystem productivity. Consequently, AMF are keystone mutualists in terrestrial ecosystems, and it is thus unsurprising to see a growing number of industries specialising in their production to use them as bio-stimulants in agriculture, forestry or plant nursery practices. The genetics and genomics of multinucleate arbuscular mycorrhizal fungi Besides their obvious benefits to plants in terrestrial ecosystems, AMF are known for their unusual cellular
96 Microbiology Today October 2022 | microbiologysociety.org
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