The ability of PDR1 OE plants to obtain faster mycorrhization levels and longer/thicker stems suggests that SL biosynthesis might be induced to support the stronger SL transport/exudation driven by PDR1. We analyzed the expression levels of two SL biosynthetic genes, DAD1 and MAX1, in three different PDR1 OE lines (Fig. S2d–f). Compared with the wild‐type, PDR1, DAD1, and MAX1 are significantly up‐regulated (Fig. 4g–i) in shoots and roots of PDR1 OE plants. This result indicates that the overexpression of the transporter induces the SL biosynthesis pathway; SL biosynthesis is feedback‐inhibited by SL; and enhanced SL export from the site of its synthesis releases this feedback inhibition. To test if SL concentrations are changed in PDR1 OE roots, we assayed SL‐inducible germination of the parasitic weed Phelipanche ramosa using P. hybrida root extracts. PDR1 OE root extracts cannot induce P. ramosa germination as strongly as the wild‐type (Fig. 3j), showing that PDR1 OE roots are partially depleted in SL as a result of increased transport to the shoot and/or exudation to the soil. The DAD1 and MAX1 expression results in roots imply that their gene expression can be used as inversely proportional readouts of SL accumulation.