Hd1 function conversion in regulating heading is dependent on gene combinations of Ghd7, Ghd8, and Ghd7.1 under long-day conditions in rice

Ghd7, Ghd8, Ghd7.1, and Hd1 are all pleiotropic genes regulating heading date, plant height, and yield in rice. Early studies showed that Hd1 promoted heading under short-day conditions (SD) and delayed heading under long-day conditions (LD). Recent studies found that Hd1 also promoted heading in some genetic backgrounds under LD. In this study, we developed a series of near-isogenic lines for Ghd7, Ghd8, Ghd7.1, and Hd1 in the Zhenshan 97 (ZS97) and Minghui 63 (MH63) backgrounds and recorded their heading dates. In the ZS97 background, Ghd7 alone triggered the conversion of Hd1 function from promoting to suppressing heading under LD. Ghd8 alone and Ghd7.1 alone did not promote but enhanced Ghd7-mediated Hd1 function conversion. In the MH63 background, Ghd7 alone and Ghd7.1 alone did not convert Hd1 function under LD, but they jointly promoted Hd1 function conversion. Conversion of Hd1 function occurred only under LD but not SD. Transcript analysis showed that downregulation or upregulation of Ehd1 and Hd3a by Hd1 in the lines under LD was determined by genetic background. In summary, multiple gene combinations of Ghd7, Ghd7.1, and Ghd8 and unknown genes caused conversion of Hd1 function. Moreover, the different gene combinations had a big difference in photoperiod sensitivities. These findings lay a solid foundation for further unveiling the molecular mechanism of Hd1 function conversion and provide guidance for heading date improvement in rice.