Architecture and developmental dynamics of the external mycelium of the arbuscular mycorrhizal fungusGlomus intraradicesgrown under monoxenic conditions

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Abstract
The structural development of arbuscular mycorrhiza extraradical mycelium is difficult to follow in soil-based systems. The use of dual arbuscular mycorrhizal fungi/in vitro root organ cultures (monoxenic AM cultures) allowed the nondestructive study of hyphal development following establishment of the symbiosis. The present study shows that the extraradical spreading of the arbuscular mycorrhizal fungus Glomus intraradices grown monoxenically with tomato roots can be divided into three stages: (i) proliferation of runner hyphae acting as conducting channels, which divide dichotomously and extend the fungal colony radially; (ii) development of arbuscule-like structures, which are formed at regular intervals along the runner hyphae and which might play a preferential role in nutrient uptake; and (iii) formation of spores in zones already colonized by runner hyphae and arbuscule-like structures. The development of the mycorrhiza is accompanied by changes in the pH of the medium. In particular, pH decreases in zones of the medium in which a high number of arbuscular mycorrhizal fungal spores are formed. The intricate architecture shown by the extraradical mycelium highlights the potential for enhanced nutrient uptake by mycorrhizal roots, and their role in the maintainance and amelioration of soil structure.