Rice Zinc-Finger Homeodomain 1 (OsZHD1) Finetunes Flowering Time by Modulating the Photoperiodic Response.

Plants exhibit photoperiod sensitivity (PS), which influences flowering time in response to changes in daylength, allowing cereal crops to optimise seed production at different latitudes. Rice is a facultative short-day (SD) plant, flowering early under SD and late under long-day (LD) conditions. This study identifies the rice (Oryza sativa) Zinc-finger Homeodomain 1 (OsZHD1) as a new photoperiod regulator that modulates flowering time in response to daylength. Compared to the parental japonica cultivar, oszhd1 knockout mutants exhibited enhanced PS, resulting in flowering that occurred earlier under SD and later under LD. Conversely, transgenic rice overexpressing OsZHD1 showed a reduction in PS, resulting in delayed flowering under SD and accelerated flowering under LD. RT-qPCR analysis revealed that OsZHD1 plays a role in the Ehd1-Hd3a/RFT1 pathway for flowering induction. We found that OsZHD1 interacts with Hd1 and OsPRR37, but not with Ghd7 or DTH8. In addition, OsZHD1 binds directly to the Ehd1 promoter, potentially downregulating Ehd1 in SD and upregulating it under LD to reduce PS. SNP analysis showed that rice varieties with the japonica-type OsZHD1a allele flowered earlier than those with the indica-type OsZHD1b allele. Taken together, these results highlight a critical role of OsZHD1 in finetuning flowering time at different latitudes.