The occupancy of Ghd7 to the transcriptional activation domain of Hd1 leads to functional conversion of Hd1 from promoting to suppressing heading under long-day conditions in rice

Hd1 alone constantly promotes heading both under LD and SD conditions in rice. But it suppresses heading in the presence of Ghd7 under LD conditions. It is not clear how Ghd7 makes Hd1 function conversion. To address this question, both Hd1 and Ghd7 were truncated for protein interaction analysis. Ghd7-TS (the terminal amino acids 243–257 of Ghd7) and Hd1-ZN (the zinc finger domain of Hd1) were verified as the interaction domains between Hd1 and Ghd7. Moreover, Hd1(243–337) was demonstrated as the primary transcriptional activation domain of Hd1. The interaction domain edited alleles Hd1^▵ZN and Ghd7^▵TS kept a partial function in regulating heading date but lost the interaction ability. The mutants Hd1^▵ZNGhd7 or Hd1Ghd7^▵TS showed a much earlier heading date than the wildtype Hd1Ghd7 mainly due to the elimination of interaction effect. The length of non-specific amino acids appended near the Ghd7-TS region is highly correlated with Hd1 transcriptional repression, suggesting that Ghd7 inhibits Hd1 transcriptional activity probably through a steric hindrance effect by targeting its activation domain, in turn reducing the expression of Ehd1, Hd3a, and RFT1, and ultimately delaying heading. These findings provide new insights into the photoperiodic flowering mechanism and the flexibility to breed varieties with fine differences in heading date by utilizing the edited Hd1^▵ZN or Ghd7^▵TS alleles.