Rhizophagus irregularis, the model fungus in arbuscular mycorrhiza research, forms dimorphic spores

Abstract

Summary Rhizophagus irregularis is the model species for arbuscular mycorrhizal fungi (AMF) research and the most widely propagated species for commercial plant biostimulants. Using asymbiotic and symbiotic cultivation systems initiated from single spores, advanced microscopy, Sanger sequencing of the glomalin gene, and PacBio sequencing of the partial 45S rRNA gene, we show that four strains of R. irregularis produce spores of two distinct morphotypes, one corresponding to the morphotype described in the R. irregularis protologue and the other having the phenotype of R. fasciculatus. The two spore morphs are easily distinguished by spore colour, thickness of the subtending hypha, thickness of the second wall layer, lamination of the innermost layer, and the dextrinoid reaction of the two outer spore wall layers to Melzer's reagent. The glomalin gene of the two spore morphs is identical and that of the PacBio sequences of the partial SSU‐ITS‐LSU region (2780 bp) obtained from single spores of the R. cf fasciculatus morphotype has a median pairwise similarity of 99.8% (SD = 0.005%) to the rDNA ribotypes of R. irregularis DAOM 197198. Based on these results, we conclude that the model AMF species R. irregularis is dimorphic, which has caused taxonomic confusion in culture collections and possibly in AMF research.

Mycorrhizal research now: from the micro‐ to the macro‐scale

“…The life history of arbuscular mycorrhizal fungi provides further insights. Sporulation of the model fungus Rhizophagus irregularis yields spores of two different morphologies, including those matching the phenotypes of Rhizophagus fasciculatus in the case of at least four isolates, as described by Kokkoris et al (2024). Lofgren et al (2024Lofgren et al ( , in this issue pp.…”

Hyphal exploration strategies and habitat modification of an arbuscular mycorrhizal fungus in microengineered soil chips

“…The sporulation we observed in openings of the chip design visibly clogged passages and would have prevented any movement of organisms, water or gas across to the adjacent pore space. Irregularly shaped spores of R. irregularis have been documented in soils, including variable globose, oblong and irregular, sometimes even knobby shapes (INVAM, Walker, 2013) and a recent discovery has showed that R. irregularis spores may be dimorphic (Kokkoris et al, 2023). While it is recognised that different AMF species produce a wide diversity of spore shapes and sizes (Smith and Read, 2008;Aguilar-Trigueros et al, 2019), the potential implications of spores being formed into the shape of the micro scale environment has not previously been examined.…”

Three new species of arbuscular mycorrhizal fungi (Glomeromycota) and Acaulospora gedanensis revised

“…Compared to 45S sequences, the taxonomic resolution of the rpb1 sequences is higher, and their use reduces the risk of creating potential phylogenetic artifacts due to paralogous sequences because rpb1 is a single-copy gene in arbuscular mycorrhizal fungi (Stockinger et al, 2014 ). Other single-copy marker loci recommended for use in reconstructing AMF phylogenies are the ß -tubulin and glomalin genes (Corradi et al, 2004a , b ; Msiska and Morton, 2009 ; Magurno et al, 2019 ; Kokkoris et al, 2023 ). However, lower resolution and poor support at the order level of the ß -tubulin phylogenies, compared to those reconstructed from SSU, LSU, and rpb1 sequences (Msiska and Morton, 2009 ) and the relatively small number of sequences for the AMF glomalin gene (Magurno et al, 2019 ; Kokkoris et al, 2023 ), have made these two protein-coding loci of little use in taxonomic analyses of large numbers of Glomeromycota representatives.…”