SLs are known to increase hyphal branching of AM fungi (Akiyama et al., 2005; Besserer et al., 2006) and this biological response can be measured in vitro to characterize the activity of SLs, SL analogs and mimics (Akiyama et al., 2010; Mori et al., 2016). However, a causal link between this branching response and symbiosis has not been established. Here, we used a different assay in which SL-deficient mutants of M. truncatula are inoculated with the AM fungus Rhizophagus irregularis. The roots of these mutants are hardly colonized, likely due to deficient stimulation of the AM fungus. Treatment with (±)- GR 24 can increase the number of root infection units (Fig. 7), and thus this test can be used as a bioassay to assess the effect of SL mimics on AM symbiotic ability. (±)-Contalactone applied at 100 nM was able to enhance significantly the level of root colonization by R. irregularis, although the activity was slightly lower than that of (±)- GR 24 (Fig. 7). Thus, in addition to the effects on plants reported above, (±)-contalactone also shows significant bioactivity on symbiotic fungi.