CHANGES IN ACTIN FILAMENT ARRAYS IN PROTOCORM CELLS OF THE ORCHID SPECIES, SPIRANTHES-SINENSIS, INDUCED BY THE SYMBIOTIC FUNGUS CERATOBASIDIUM-CORNIGERUM

Seeds of the terrestrial orchid, Spiranthes sinensis, were germinated in vitro in association with the symbiotic fungus, Ceratobasidium corn igerum. Resulting colonized protocorms were prepared for light microsc opy, transmission electron microscopy, and fluorescence labelling of a ctin filaments for examination with laser scanning confocal microscopy . Fungal hyphae invaded the suspensor end of embryos, formed typical h yphal coils (pelotons) within parenchyma cells, and then underwent lys is resulting in degraded hyphal masses. Hyphae and hyphal masses were enveloped by host-derived membrane. Changes in actin filament arrays a ccompanied fungal colonization. Uncolonized cells had a network of act in 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, m ost of the cortical network disappeared and instead actin filaments ra diated from the periphery of developing pelotons towards the cell wall . Degraded hyphal masses also had actin filament arrays associated wit h them, again radiating toward the cell periphery; a network of cortic al actin filaments reappeared in the protocorm cell cytoplasm at this stage. Actin filaments did not appear to have a close physical associa tion with fungal hyphae except in the epidermal hairs that developed f rom protocorms; this differs from our previous observations on microtu bules in this system.