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A strain of Acaulospora rehmii was, for the first time, successfully grown in vitro on Ri T-DNA transformed carrot roots allowing the in situ observation of Acaulospora spore development and of extraradical thin-walled hyphal swellings. The sporogenous hypha developed intercalarly along thin-walled coenocytic hyphae. The distal part of the sporogenous hypha swelled slightly as a sporiferous saccule primordium followed by the differentiation of a lateral spore primordium along the neck of the sporogenous hypha. Both structures matured simultaneously, and the sporiferous saccule began to collapse after spore maturation and complete differentiation of the spore wall. Several of the in situ observations on in vitro differentiated A. rehmii spores are concordant with previous ontogenic studies done on other Acaulospora species obtained from in vivo cultures. New and original observations on the early developmental stages of sporiferous saccules and spores and on the occurrence of small diameter intercalary hyphal swellings provide additional elements in the study of Acaulospora sporulation process and life cycle.

The study of arbuscular mycorrhizal (AM) fungi and the AM symbiosis formed with host plant roots is complicated by the biotrophic and hypogeous nature of the mycobionts involved. To overcome this, several attempts have been made during the last three decades to obtain this symbiosis in vitro. The use of root-organ cultures has proved particularly successful. In this review, we describe the method by which root-organ cultures (transformed and nontransformed) have been obtained, together with the choice of host species, inoculation techniques, and culture media. We also outline the potential use of continuous cultures and cryopreservation of in vitro produced spores for long-term germ plasm storage. Furthermore, this review highlights the considerable impact that in vitro root-organ cultures have had on studies of AM fungal morphology, taxonomy, and phylogeny and how they have improved our understanding of the processes leading to root colonization and development of the extraradical mycelium. This is supported by a summary of some of the most important findings, regarding this symbiosis, that have been made at the physiological, biochemical, and molecular levels. We also summarize results from studies between AM fungi and certain pathogenic and nonpathogenic soil microorganisms. We describe some of the limitations of this in vitro system and propose diverse avenues of AM research that can now be undertaken, including the potential use of a similar system for ectomycorrhizal research.