Although >80% of species form arbuscular mycorrhizal symbioses, the roles of plant hormones in the associations are still being determined. Foo et al. critically review recent progress in our understanding of the roles of strigolactones, auxin, abscisic acid, ethylene, jasmonic acid and salicylic acid in mycorrhizal symbiosis, and present new evidence for a novel role for gibberellins in mycorrhizal development. Using gibberellin- and DELLA-deficient mutants of pea they show that bioactive gibberellins have a negative role in arbuscule formation. In contrast, a brassinosteriod-deficient pea mutant provides no indication of a role for these compounds in mycorrhizal development.