Enhancement of systemic acquired resistance in rice by F-box protein D3-mediated strigolactone/karrikin signaling

Strigolactones (SLs) are butenolide-type plant hormones that play several roles in plants, such as suppressing shoot branching and promoting arbuscular mycorrhizal symbiosis. Recently, SLs have been reported to positively regulate disease resistance in plants. In this study, we analyzed the effect of the synthetic SL analog rac-4-bromodebranon (rac-4BD) on systemic acquired resistance (SAR) in rice. First, we demonstrated in vitro that rac-4BD, similar to the common SL analog rac-GR24, promotes the interaction of SL and karrikin receptor, D14 and D14-like (D14L), respectively, with signaling factor D3. Gene expression analysis and inoculation tests indicated that pretreatment with rac-4BD promotes the effect of the SAR inducer BIT. Activation of SAR was also significantly observed in the SL and karrikin signal-deficient rice mutant d3. These results suggest that D3-mediated SL/karrikin signaling by rac-4BD treatment does not directly activate rice immunity but induces a priming state in the plant that enhances SAR induction.