Changes in actin filament arrays in protocorm cells of the orchid species, Spiranthes sinensis, induced by the symbiotic fungus Ceratobasidium cornigerum

Abstract

Seeds of the terrestrial orchid, Spiranthes sinensis, were germinated in vitro in association with the symbiotic fungus, Ceratobasidium cornigerum. Resulting colonized protocorms were prepared for light microscopy, transmission electron microscopy, and fluorescence labelling of actin filaments for examination with laser scanning confocal microscopy. Fungal hyphae invaded the suspensor end of embryos, formed typical hyphal coils (pelotons) within parenchyma cells, and then underwent lysis resulting in degraded hyphal masses. Hyphae and hyphal masses were enveloped by host-derived membrane. Changes in actin filament arrays accompanied fungal colonization. Uncolonized cells had a network of actin filaments and actin bundles (cables) located in the cortical region of the cell cytoplasm; some of these were associated with the nucleus and amyloplasts. Although actin filament arrays were still present in protocorm cell cytoplasm during fungal entry and peloton formation, most of the cortical network disappeared and instead actin filaments radiated from the periphery of developing pelotons towards the cell wall. Degraded hyphal masses also had actin filament arrays associated with them, again radiating toward the cell periphery; a network of cortical actin filaments reappeared in the protocorm cell cytoplasm at this stage. Actin filaments did not appear to have a close physical association with fungal hyphae except in the epidermal hairs that developed from protocorms; this differs from our previous observations on microtubules in this system. Key words: actin, actin filaments, orchids, mycorrhizas, laser scanning confocal microscopy.