Arbuscular mycorrhizal (AM) fungi constitute one of the most widespread microbial communities of the rhizosphere, since they establish symbiotic associations with the roots of about 80% of plant species (Smith, Read 1997). Fossil and molecular data suggest that roots and arbuscular mycorrhizal (AM) fungi have shared a cooperative life since Devonian times (Simon et al. 1993). The success of mycorrhizas in evolution is mainly due to the central role that AM fungi play in the capture of nutrients from the soil in almost all ecosystems (Smith, Read 1997). As a consequence, they are crucial determinants of plant biodiversity, ecosystem variability and productivity of plant communities (van der Heijden et al. 1998). AM fungi are not only an essential feature of the biology and ecology of most terrestrial plants, they also interact with different classes of bacteria during their life cycles. AM fungi establish in fact interactions both with bacteria living in the rhizosphere during their extraradical phase and with endosymbiotic bacteria which live in the cytoplasm of some fungal isolates (Perotto, Bonfante 1997; Bianciotto et al. 2001).

The understanding of these multiple interactions is one of the most exciting challenges of current research in the field of molecular microbe-plant interactions and of their application in low chemical input agricultural systems.

In recent years a wealth of experimental investigations, together with the development of new technologies, has led to substantial advances in the knowledge of arbuscular mycorrhizal functioning, mostly in the field of molecular biology (Harrison 1999). Despite these recent achievements in our knowledge of the molecular basis of plant-fungal interactions, many aspects of the biology of AM fungi are still obscure and hamper a full exploitation of their potentialities as biotechnological tools. This is mostly due to their obligate biotrophic status, their multinuclear condition, and an unexpected level of genetic variability (Bonfante, Perotto 1995; Gianinazzi-Pearson 2000; Hosny et al. 1999; Lanfranco et al. 1999).

The aim of this short review is to analyze a feature which is peculiar of some AM fungi and increases the level of their genetic complexity: the presence of endobacteria living in their cytoplasm. We will provide information on their identification, phylogeny and possible functions.

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