In the rhizosphere, strigolactones not only act as crucial signalling molecules in the communication of plants with parasitic
weeds and arbuscular mycorrhiza, but they also play a key role in regulating different aspects of the root system.
Here we investigated how strigolactones influence the root architecture of Medicago truncatula. We provide evidence
that addition of the synthetic strigolactone analogue GR24 has an inhibitory effect on the lateral root density. Moreover,
treatment with GR24 of Sinorhizobium meliloti-inoculated M. truncatula plants affects the nodule number both positively
and negatively, depending on the concentration. Plants treated with 0.1 μM GR24 had a slightly increased number
of nodules, whereas concentrations of 2 and 5 μM strongly reduced it. This effect was independent of the autoregulation
of nodulation mechanism that is controlled by SUPER NUMERIC NODULE. Furthermore, we demonstrate that
GR24 controls the nodule number through crosstalk with SICKLE-dependent ethylene signalling. Additionally, because
the expression of the nodulation marker EARLY NODULATION11 was strongly reduced in GR24-treated plants, we concluded
that strigolactones influence nodulation at a very early stage of the symbiotic interaction.