Branched absorbing structures (BAS): a feature of the extraradical mycelium of symbiotic arbuscular mycorrhizal fungi

The present work describes the morphogenesis and cytological characteristics of 'branched absorbing structures' (BAS, formely named arbuscule-like structures, ALS), small groups of dichotomous hyphae formed by the extraradical mycelium of arbuscular mycorrhizal (AM) fungi. Monoxenic cultures of the AM fungus Glomus intraradices Smith & Schenck and tomato (Lycopersicum esculentum Mill.) roots allowed the continuous, non-destructive study of BAS development. These structures were not observed in axenic cultures of the fungus under different nutritional conditions or in unsuccessful (asymbiotic) monoxenic cultures. However, extraradical mycelium of G. intraradices formed BAS immediately after fungal penetration of the host root and establishment of the symbiosis. The average BAS development time was 7 d under our culture conditions, after which they degenerated, becoming empty septate structures. Certain BAS were closely associated with spore formation, appearing at the spore's substending hypha. Branches of these spore-associated BAS (spore-BAS) usually formed spores. Electron microscopy studies revealed that BAS and arbuscules show several ultrastructural similarities. The possible role of BAS in nutrient uptake by the mycorrhizal plant is discussed.