Structures of <scp>D</scp>14 and <scp>D</scp>14<scp>L</scp> 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.