www.newphytologist.org 175 Research 5Blackwell Science, Ltd Patterns of below-ground plant interconnections established by means of arbuscular mycorrhizal networks Manuela Giovannetti 1 , Cristiana Sbrana 2, *, Luciano Avio 2 and Patrizia Strani 1, * 1 Dipartimento di Chimica e Biotecnologie Agrarie, Università di Pisa, Italy; 2 Istituto di Biologia e Biotecnologia Agraria, C. N. R., Sezione di Pisa, Via del Borghetto 80, 56124 Pisa, Italy Summary • The ability of arbuscular mycorrhizal (AM) networks originating from plants of different species, genera and families to become interconnected by means of hyphal anastomoses was assessed. • An in vivo two-dimensional experimental model system was used to reveal the occurrence of linkages between contiguous mycorrhizal networks spreading from Allium porrum root systems and those originating from Daucus carota , Gossypium hirsutum , Lactuca sativa , Solanum melongena , colonized by Glomus mosseae . • Percentages of hyphal contacts leading to anastomosis between extraradical networks originating from different plant species ranged from 44% in the pairing A. porrum – S. melongena to 49% in A. porrum – G. hirsutum . DAPI and Sytox stainings detected nuclei in the middle of fusion bridges connecting different mycorrhizal networks. • Present data suggest that, by means of anastomoses, AM fungal mycelium would potentially create an indefinitely large network interconnecting different plants in a community, and that, in the absence of sexual recombination, the intermingling of nuclei in extraradical mycelium may provide endless opportunities for the exchange of genetic material. Key words: anastomosis, extraradical mycorrhizal networks, Glomus mosseae , nuclear exchange, plant linkages. New Phytologist (2004) 164 : 175 –181 © New Phytologist (2004) doi : 10.1111/j.1469-8137.2004.01145.x Author for correspondence: M. Giovannetti Tel: +39-050-9719324 Fax: +39-050-571562 Email: mgiova@agr.unipi.it Received: 19 March 2004 Accepted: 05 April 2004 Introduction The majority of land plants establish mutualistic symbioses with arbuscular mycorrhizal (AM) fungi (Glomeromycota), which play a central role in soil fertility and plant nutrition and in the maintenance of stability and biodiversity within plant communities (Smith & Read, 1997). AM fungi develop extraradical hyphae which, spreading from mycorrhizal roots into the surrounding soil, absorb and translocate mineral nutrients to the host plant (Harrison & van Buuren, 1995; Smith & Read, 1997; Smith et al., 2000) and are thought to be active in mediating nutrient transfer in nature (Chiariello et al. 1982; Francis & Read, 1984; Grime et al., 1987; Watkins et al., 1996; Graves et al., 1997; Lerat et al., 2002). Such extensive below-ground mycelial networks, due to the lack of host specificity of AM fungi, are able to link together different host plants by means of hyphae growing into the soil and establishing mycorrhizal symbioses with the diverse plant species with which they come into contact (Graves et al., 1997; Read, 1998; Van der Heijden et al., 1998). So far, the hypothesis that different mycorrhizal networks may become interconnected by means of hyphal fusions has not been considered. Some studies provided qualitative information on the architecture and developmental dynamics of AM extraradical mycelium, by using root observation chambers and in vitro dual systems (Friese & Allen, 1991; Bago et al., 1998). Recent investigations utilized an in vivo two-dimensional experi- mental model system to visualize and quantify intact AM extraradical networks originating from individual plant root systems, and showed their extremely high interconnectedness due to the widespread occurrence of anastomoses (Giovannetti et al., 2001). Here, we applied the same experimental system *These authors contributed equally to this work