TANSLEY-REVIEW NO-82 - STRATEGIES OF ARBUSCULAR MYCORRHIZAL FUNGI WHEN INFECTING HOST PLANTS

Arbuscular mycorrhizal (AM) fungi are ancient Zygomycetes, thought to have colonized the first land plants; today, they are associated with the roots of about 80% of plant species. The symbiosis they form is potentially valuable not only for developmental programmes based on low-input agriculture, but also as a complex experimental model, where both fungal and host plant growth are regulated. Here we review some recent progress in the area of cell and molecular biology of arbuscular mycorrhizas. Particular attention is given to strategies followed by AM fungi when, as obligate biotrophs, they establish a successful symbiosis with their host plants. Four topics are analysed: (i) parameters controlling fungal growth in the absence and presence of the host root, i.e. events of DNA replication and timing of the cell cycle; (ii) mechanical and enzymatic mechanisms which allow the fungus to colonize root tissues, maintaining host viability; (iii) morphological changes induced by AM fungi in host cells and, in particular, the formation de novo of a subcellular compartment termed the interface, and (iv) modifications of plant gene expression during fungal colonization, including those involving structural, symbiotic and defence-related genes.