JA-Modulated OsbHLH091 Drives Lodicule Cell Expansion to Initiate and Synchronize Diurnal Floret Opening in Rice

Asynchronous floret opening hinders hybrid rice seed production, particularly between indica and japonica subspecies. Although jasmonate (JA) signaling regulates diurnal floret opening time (DFOT), the mechanism underlying structural changes in the lodicule as a swellable organ driving floret opening remains unclear. Here, we identified OsbHLH091 as a lodicule-specific transcription factor mediating JA-induced floret opening. The null mutants of OsbHLH091 exhibit cleistogamy (closed flowers) without defects in floral organ, while OsbHLH091 overexpression accelerates floral opening. Mechanistically, OsbHLH091 activates cell expansion genes, including EXPANSINs and CELLULOSE SYNTHASEs, and such activity is repressed by JAZ proteins, the JA signaling inhibitors. Increasing JA before anthesis degrades JAZs, which releases OsbHLH091 to trigger lodicule expansion. Natural variation in OsbHLH091 contributes to DFOT differences between subspecies: the japonica haplotype interacts more strongly with JAZs, thus delaying opening. Together, as a lodicule-specific regulator, OsbHLH091 not only reveals a previously unknown mechanism controlling floret opening but also provides a precise tool to synchronize DFOT across subspecies, enabling efficient indica-japonica hybrid breeding.