In vivo observations on the behavior and fate of nuclei of two arbuscular mycorrhizal fungi grown under non-symbiotic and symbiotic conditions.

BAGO, BERTA¹, WARREN ZIPFEL², REBECCA M. WILLIAMS², WATT W. WEBB² & YVES PICHE³.

¹ERRC, USDA/ARS 600 E. Mermaid Ln, Wyndmoor, PA, 19038 USA, ²Applied and Engineering Physics, Cornell University, NY, USA, ³CRBF, Université Laval, Québec, CANADA.

Two arbuscular mycorrhizal (AM) fungi were grown either in axenic or in monoxenic (dual AM fungal/root organ culture) conditions, and the behavior of their extraradical nuclei studied with 2-photon microscopy. This new technical approach allows long term cytological studies in living specimens. The movement of DAPI-stained nuclei along AM fungal hyphae was thus recorded.
When in symbiosis with a host root (monoxenic conditions), most nuclei of the AM fungal extraradical mycelium appeared laterally and at regular intervals, as if precise areas of the coenocytic hypha were under the control of assigned nuclei. Besides these lateral, static nuclei, other more rounded-shaped migrated along the central core of the hypha.
Under axenic conditions, germ-tubes issued from resting spores displayed round (migrating) or oval-shaped (static) nuclei all along their hyphal length. When the specimen did not yet show apical septation, some (rare) germ-tube areas, delimited by septa and containing distorted, much brighter fluorescent nuclei were also found. Specimens which had initiated the septation process showed a higher number of such a cytoplasmic "degrading" areas, which appeared randomly along the germ tube, alternating with areas containing normal-looking nuclei. Completely-septated specimens lacked nuclei along their empty germ-tubes. However, highly fluorescent chromatin debris often appeared collapsed between close septa. The existence of a random autolytic/apoptotic process in AM fungal germ-tubes as a consequence of the absence of a host root signal for mycorrhiza formation is proposed.