M. Giovannetti (Y) 7 C. Sbrana Dipartimento di Chimica e Biotecnologie Agrarie, Università di Pisa, Via del Borghetto 80, I-56124 Pisa, Italy e-mail: mgiova6agr.unipi.it, Fax: c39-50-571562 Mycorrhiza (1998) 8 : 123–130 Q Springer-Verlag 1998 REVIEW Manuela Giovannetti 7 Cristiana Sbrana Meeting a non-host: the behaviour of AM fungi Accepted: 17 June 1998 Abstract Arbuscular mycorrhizal (AM) fungi are obli- gately biotrophic organisms that live symbiotically with the roots of most plants. The establishment of a func- tional symbiosis between AM fungi and host plants in- volves a sequence of recognition events leading to the morphological and physiological integration of the two symbionts. The developmental switches in the fungi are triggered by host signals which induce changes in gene expression and a process leading to unequivocal recog- nition between the two partners of the symbiosis. It has been calculated that about 80% of plant families from all phyla of land plants are hosts of AM fungi. The re- maining plant species are either non-mycorrhizal or hosts of mycorrhizas other than the arbuscular type. Non-host plants have been used to obtain information on the factors regulating the development of a func- tional symbiosis. The aim of this present review is to highlight present-day knowledge of the fungal develop- mental switches involved in the process of host/non- host discrimination. The following stages of the life cy- cle of AM fungi are analysed in detail: spore germina- tion, presymbiotic mycelial growth, differential branch- ing pattern and chemotropism, appressorium forma- tion, root colonization. Introduction The establishment of a functional symbiosis between arbuscular mycorrhizal (AM) fungi and host plants in- volves a sequence of recognition events leading to the morphological and physiological integration of the two symbionts. Developmental switches in the fungi are triggered by host signals which induce changes in gene expression and a process leading to unequivocal recog- nition between the two partners of the symbiosis (Smith and Read 1997). When growing in natural plant ecosystems and en- countering plant roots, AM fungi discriminate against the many surrounding non-hosts (Read 1991; Giovan- netti et al. 1994). The aim of this review is to highlight current knowledge of the key events of the AM fungi life cycle and to discuss the developmental switches in the fungi involved in the process of host/non-host dis- crimination. Which plants are non-hosts of AM fungi? It has been calculated that about 80% of plant families from all phyla of land plants are hosts of AM fungi. Non-mycorrhizal plant families have been described among these non-hosts, and non-mycorrhizal species and genera have been reported in mycorrhizal families (Hirrel et al. 1978; Trappe 1987). Experimental data obtained in a 3-year study showed the occurrence of all the different types of mycorrhizas in a Mediterranean plant ecosystem on the isle of Brioni (Croatia). The survey showed arbuscular mycorrhizas in about 75% of plant species, whith the remaining 25% represented by non-mycorrhizal plants (7%) or plants hosting mycorr- hizas other than the arbuscular type (18%) (Matosevic 1996). An interesting, thorough survey of all the data col- lected on non-mycorrhizal species in different geogra- phical areas by Tester et al. (1987) details the occur- rence of mycorrhizas in non-mycorrhizal families. Since many reports concerned field-collected samples, the au- thors stressed the importance of the method to be used for assessing the mycorrhizal status of any plant spe- cies, i.e. “... to grow plants in pot-cultures ensuring ex- posure of the roots to infective propagules”. Most hosts of non-arbuscular mycorrhizas never es- tablish functional arbuscular mycorrhizal symbioses in nature. Plants of the Orchidaceae, Ericaceae, Monotro- paceae and Pyrolaceae have been reported as exclusive