Strigolactones, brassinosteroids and autoregulation of nodulation

The symbiosis between legumes and N-fixing rhizobia bacteria is energy intensive, and as a result plants regulate nodulation via hormones and mechanisms such as the autoregulation of nodulation (AON) system. Foo et al. investigate interactions between the AON system and two hormones recently shown to promote nodulation, strigolactones and brassinosteroids. They find that double-mutant plants of pea (Pisum sativum) that are disrupted in elements of the AON pathway and that are also strigolactone- or brassinosteroid-deficient display supernodulating AON mutant phenotypes. Strigolactone production is not consistently affected in the AON mutants, and the results indicate that strigolactones and brassinosteroids do not act downstream of the AON genes examined. They argue that it is likely that these hormones act independently of the AON system to promote nodule formation.