Arbuscular mycorrhizal (AM) fungi are obligately biotrophic organisms that
live symbiotically with the roots of most plants. The establishment of a fu
nctional symbiosis between AM fungi and host plants involves a sequence of
recognition events leading to the morphological and physiological integrati
on of the two symbionts. The developmental switches in the fungi are trigge
red by host signals which induce changes in gene expression and a process l
eading to unequivocal recognition 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 remaining 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 functional symbiosis. The aim of this present review
is to highlight present-day knowledge of the fungal developmental switches
involved in the process of host/non-host discrimination. The following stag
es of the life cycle of AM fungi are analysed in detail: spore germination,
presymbiotic mycelial growth, differential branching pattern and chemotrop
ism, appressorium formation, root colonization.