Arbuscular mycorrhiza formation in cordate gametophytes of two ferns, Angiopteris lygodiifolia and Osmunda japonica.

Mycorrhizal symbiosis is common among land plants including pteridophytes (monilophytes and lycophytes). In pteridophytes with diplohaplontic life cycle, mycorrhizal formations were mostly reported for sporophytes, but very few for gametophytes. To clarify the mycorrhizal association of photosynthetic gametophytes, field-collected gametophytes of Angiopteris lygodiifolia (Marattiaceae, n = 52) and Osmunda japonica (Osmundaceae, n = 45) were examined using microscopic and molecular techniques. Collected gametophytes were mostly cut into two pieces. One piece was used for light and scanning microscopic observations, and the other for molecular identification of plant species (chloroplast rbcL sequences) and mycorrhizal fungi (small subunit rDNA sequences). Microscopic observations showed that 96 % (50/52) of Angiopteris and 95 % (41/43) of Osmunda gametophytes contained intracellular hyphae with arbuscules and/or vesicles and fungal colonization was limited to the inner tissue of the thick midribs (cushion). Fungal DNA analyses showed that 92 % (48/52) of Angiopteris and 92 % (35/38) of Osmunda have sequences of arbuscular mycorrhizal fungi, which were highly divergent but all belonged to Glomus group A. These results suggest that A. lygodiifolia and O. japonica gametophytes consistently form arbuscular mycorrhizae. Mycorrhizal formation in wild fern gametophytes, based on large-scale sampling with molecular identification of host plant species, was demonstrated for the first time.