Structures of D14 and D14L in the strigolactone and karrikin signaling pathways

Strigolactones (SLs) are plant hormones that inhibit shoot branching. DWARF14 (D14) inhibits rice tillering and is an SL receptor candidate in the branching inhibition pathway, whereas the close homologue DWARF14-LIKE (D14L) participates in the signaling pathway of karrikins (KARs), which are derived from burnt vegetation as smoke stimulants of seed germination. We provide the first evidence for direct binding of the bioactive SL analogue GR24 to D14. Isothermal titration calorimetry measurements show a D14GR24 binding affinity in the sub-micromolar range. Similarly, bioactive KAR1 directly binds D14L in the micromolar range. The crystal structure of rice D14 shows a compact a-/beta-fold hydrolase domain forming a deep ligand-binding pocket capable of accommodating GR24. Insertion of four a-helices between beta 6 strand and aD helix forms the helical cap of the pocket, although the pocket is open to the solvent. The pocket contains the conserved catalytic triad Ser-His-Asp aligned with the oxyanion hole, suggesting hydrolase activity. Although these structural characteristics are conserved in D14L, the D14L pocket is smaller than that of D14. The KAR-insensitive mutation kai2-1 is located at the prominent long beta 6-aD1 loop, which is characteristic in D14 and D14L, but not in related a-/beta-fold hydrolases.