Fig. 1

Detection of rhizopine exudation with a rhizopine biosensor. a Chemical structures of rhizopines and related cyclitols. b NightOwl images of bioluminescence of R. leguminosarum Rlv3841/pOPS0046 rhizopine lux biosensor on the surface of M. sativa roots nodulated by S. meliloti L5-30 wild-type (rhizopine+) and S. meliloti L5-30 mosB:pK19 (rhizopine –, see Fig. 2a). Numbers in top right corners indicate number of plants tested that showed significant levels of bioluminescence (n = 5). Circle in bottom right corner contains magnified image of a root section enriched with nodules. The positions of nodules are indicated with arrowheads (scale bar, 1 cm). Colours represent luminescence intensity from 0 counts per second (cool/purple) to 250 counts per second (warm/red). c Specificity of induction of rhizopine lux biosensor with rhizopines compared to other chemically similar plant polyols, grown in UMS minimal medium with pyruvate and ammonia as carbon and nitrogen sources, supplemented with 1 mM of inducing molecule. Data are presented as relative luminescence units (rlu). Black dots indicate individual data points for each condition. d Induction curves showing the dynamic range and sensitivity of the rhizopine lux biosensor with chemically synthesised SIA 1, (−)-1l-3-O-MSI (−)-1l-2 and (+)-1d-3-O-MSI (+)-1d-2 grown in UMS minimal medium with pyruvate and ammonia as carbon and nitrogen sources. Error bars represent standard deviation of the mean of n = 3 independent replicates. All experiments were repeated at least three independent times