Progress in understanding the biology of arbuscular mycorrhizal fungi is hampered by the limited number of species that can be
successfully propagated and studied in vitro. We report the establishment of monoxenic cultures of Glomus etunicatum in association
with excised Ri T-DNA transformed carrot roots. The fungus can be propagated in vitro using monoxenically formed resting spores
and/or colonized root fragments. Modified White's medium buffered with 10 mM MES (pH 6) or MOPSO (pH 6·5) was most
optimal for the host root growth as well as for G. etunicatum spore germination and mycorrhiza formation. The number of resting
spores formed in vitro correlated positively with the length of roots occupied by arbuscular mycorrhizal structures, including
arbuscules and vesicles. Spores first appeared in dual cultures within two weeks of root inoculation. Sporulation was asynchronous
and continued until root senescence. Under applied culture conditions, spores achieved mature appearance within 5–7 d after their
initiation. Approximately 6% of monoxenic spores were aborted at different stages of their development. Although G. etunicatum
spores formed in vitro exhibited general morphological and anatomical similarity to soil-borne inoculum, they were significantly
smaller and had thicker spore walls than their soil-borne counterparts. Caution should, therefore, be exercised in utilizing the in vitro
system as a model of growth and development of glomalean fungi in soil.