An ultrastructural analysis of the development of mycorrhizas in Rhododendron ponticum

Abstract

The sequence of events involved in the initiation, establishment, and degeneration of the ericoid mycorrhizas of Rhododendron ponticum was followed at the ultrastructural level. Seedlings were planted in inoculated sterile soil or natural soil and harvested sequentially over a period of weeks. Their roots were fixed and examined by scanning and transmission electron microscopy. Field-collected roots were also examined for comparative purposes. In inoculated soil, surface colonisation of root epidermal cells occurs within 4 weeks of inoculation, and penetration follows immediately. The functional life of the infected epidermal cell is short, evidence of degeneration of host cytoplasm being visible within 7 weeks. Host breakdown precedes fungal degeneration, which suggests that nutrient transfer between partners must occur in the short period when both have full structural integrity. Endophyte hyphae degenerate after collapse of host cytoplasm, first becoming vacuolate and then devoid of contents. The result of this pattern of infection is that most cells of the root epidermis are dead and devoid of contents. The pattern is the same in natural soil, though each stage is delayed by 2–3 weeks. The possible relationship between the structural and functional characteristics of ericoid roots is discussed and comparisons are made with other types of endomycorrhizas.