Conidiogenesis in Oidiodendron periconioides and ultrastructure of ericoid mycorrhizas formed with Rhododendron brachycarpum

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Abstract
Oidiodendron periconioides was isolated from hair roots of seedlings of Rhododendron brachycarpum growing in pots containing a commercial peat moss in Japan. The unusual process of conidiogenesis in O. periconioides involved the formation of conidia from vesicle-like swellings that developed from vegetative hyphae or from the apex of a mononematous conidiophore. Scanning electron microscope examinations of mature conidia revealed that they were regularly dimpled, giving them the characteristic and previously reported radiate pattern of sculpturing visible with the light microscope. Oidiodendron periconioides formed ericoid mycorrhizas that were morphologically and ultrastructurally similar to previously studied ericoid systems involving Oidiodendron maius and Hymenoscyphus ericae, and a few other species of Rhododendron. Rough-walled hyphae grew appressed to the epidermal cells of young hair roots, penetrated the external walls via narrow penetration tubes, and invaginated the plasma membrane of the host cell during the process of forming a complex system of hyphal loops. Infection was restricted to the zone proximal to the active region of the meristem. Hyphal complexes were short-lived and degenerated as the epidermal cells matured and died. The physiological role of O. periconioides as a mycorrhizal endophyte is unknown. Key words: ericoid mycorrhiza, Oidiodendron periconioides, Rhododendron brachycarpum, conidiogenesis.

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