Auxins and other phytohormones as signals in arbuscular mycorrhiza formation

The symbiosis of Medicago truncatula, as well as that of other plant species with arbuscular mycorrhizal fungi, relies in its development on the concerted action of different plant hormones. Plant organs develop in a certain manner upon the accumulation of auxin, for example. Lateral root formation during arbuscular mycorrhiza (AM) formation might follow such a pattern as well. However, other plant hormones are also involved in the establishment of a mycorrhizal symbiosis. Some are associated with initial signals, or the development of a functional mycorrhiza, but others are involved in transient defense responses. Therefore, a focus will be placed on the role of hormones during different stages of mycorrhiza development. Also, it will be highlighted wherever there is an interaction between the different hormones. Since a lot of work has been done on plant species other than M. truncatula, occasional references will be made to other plant–AM systems and observations concerning the role of hormones therein.