The hydrolase receptor DWARF14 is required for zaxinone perception in plants

Strigolactones (SLs) are plant hormones involved in plant growth, stress response and plant-organism interactions. DWARF14 (D14) is a a/ß hydrolase involved in the perception of SLs in plants. Zaxinone has been reported as a plant endogenous apocarotenoid that modulates growth and biotic interactions, however, the molecular basis of its perception remains elusive. Here we identified a new D14 ligand, the apocarotenoid zaxinone. We showed that hundreds of genes, and specifically CAROTENOID CLEAVAGE DEOXYGENASE7 (CCD7) and CCD8, require At/OsD14, and at lower extent MORE AXILLARY BRANCHING2 (MAX2)/D3, for their transcriptional induction. We find that zaxinone and D14 physically interact in vitro with lower affinity than GR24; the resolution of the AtD14-zaxinone complex, by X-ray crystallography, provided additional evidence that zaxinone antagonizes SL by binding D14. Zaxinone treatments in d14 mutants revealed that At/OsD14 are essential to trigger the induction of CCD7 and CCD8 genes, SL content (roots and exudates) and Striga seed germination validating their interaction in planta. Moreover, OsD14 is required to trigger the zaxinone growth-promoting effect in roots. These findings reveal a perception mechanism for the hormone candidate zaxinone through the SL receptor D14 to regulate transcription of SL genes, SL content and growth in higher plants. Overall design: To investigate the function of zaxinone on gene expression, we treated 5-weeks old Arabidopsis plants, including wt col-0, d14, kai2 and max2 mutants, by applying this compound in the hydroponic media. Root samples were harvested after 6 hours of treatment.