In vitro propagation and life cycle of the arbuscular mycorrhizal fungus Glomus etunicatum

Progress in understanding the biology of arbuscular mycorrhizal fungi is ha mpered 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 res ting spores and/or colonized root fragments. Modified White's medium buffer ed with 10 mM MES (pH 6) or MOPSO (pH 6.5) was most optimal for the host ro ot growth as well as for G. etunicatum spore germination and mycorrhiza for mation. The number of resting spores formed in vitro correlated positively with the length of roots occupied by arbuscular mycorrhizal structures, inc luding arbuscules and vesicles. Spores first appeared in dual cultures with in two weeks of root inoculation. Sporulation was asynchronous and continue d 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. Alt hough G. etunicatum spores formed in vitro exhibited general morphological and anatomical similarity to soil-borne inoculum, they were significantly s maller and had thicker spore walls than their soil-borne counterparts. Caut ion should, therefore, be exercised in utilizing the in vitro system as a m odel of growth and development of glomalean fungi in soil.