Structures of D 14 and D 14 L in the strigolactone and karrikin signaling pathways

Abstract

Strigolactones (SL s) are plant hormones that inhibit shoot branching. DWARF 14 (D 14) inhibits rice tillering and is an SL receptor candidate in the branching inhibition pathway, whereas the close homologue DWARF 14‐LIKE (D 14 L) participates in the signaling pathway of karrikins (KAR s), 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 GR 24 to D 14. Isothermal titration calorimetry measurements show a D 14–GR 24 binding affinity in the sub‐micromolar range. Similarly, bioactive KAR 1 directly binds D 14 L in the micromolar range. The crystal structure of rice D 14 shows a compact α‐/β‐fold hydrolase domain forming a deep ligand‐binding pocket capable of accommodating GR 24. Insertion of four α‐helices between β6 strand and α D helix forms the helical cap of the pocket, although the pocket is open to the solvent. The pocket contains the conserved catalytic triad S er‐H is‐A sp aligned with the oxyanion hole, suggesting hydrolase activity. Although these structural characteristics are conserved in D 14 L, the D 14 L pocket is smaller than that of D 14. The KAR‐insensitive mutation kai2‐1 is located at the prominent long β6‐α D 1 loop, which is characteristic in D 14 and D 14 L, but not in related α‐/β‐fold hydrolases.