Overexpressing <i>OsCCT23</i> Delays Heading Date and Increases Grain Yield by Activating <i>Ghd7</i> in Rice

Heading date and panicle architecture are pivotal traits that significantly influence rice yield. Here, we identified a gene OsCCT23 encoding a CCT domain-containing protein that delays heading by over 40 days and increases grain yield by 60-104% through overexpressing. Two types transcripts OsCCT23L and OsCCT23S were isolated by 5'RACE, and transgenic events demonstrated that the effect of the predominant transcript OsCCT23S, encoding an 81-aa protein without the B-box domain, is comparable to OsCCT23. OsCCT23 is predominantly expressed in leaves and follows a diurnal expression pattern with a peak at dawn. Overexpressing OsCCT23 upregulated the floral repressor Ghd7 and downregulated the floral inducer RID1, consequently led to the downregulation of Ehd1, Hd3a and RFT1. Additionally, it regulates the expression of certain circadian clock-related genes, including OsGI and OsTOC1. RNA in situ hybridisation analysis confirmed that OsCCT23 activates the expression of Ghd7 in the panicle branch meristem. OsCCT23 suppresses the expression of four OsCKX genes including Gn1a, which associate with cytokinin accumulation in panicles. Natural variation in OsCCT23 promoter identified by eGWAS associates its mRNA abundance and rice heading date. Consequently, OsCCT23 substantially delays heading and significantly increases grain yield, making it highly valuable for rice breeding.