Cytokinin Delays Flowering in Rice By Inhibiting Ehd1 Protein

We studied a role of cytokinin in controlling flowering time in rice. Level of trans-zeatin, an active cytokinin, peaked at 24 days after germination and reduced to a low level at a stage when florigens are generated. To elucidate whether exogenous cytokinin influences flowering time, we applied 6-benzylaminopurine (6-BAP) to plants at the stage when the flowering signals were produced. The cytokinin treatment delayed flowering time by 8 to 9 days compared to mock treated plants. In rice, a type-B response regulator (RR), Ehd1, is a major inducer of florigen gene expression. Ehd1 transcript was not altered by the treatment but levels of Hd3a and RFT1 transcripts were significantly reduced in response to the hormone. These results suggest that exogenous cytokinin reduces florigen genes expression by inhibiting Ehd1 protein. The cytokinin-inducible type-A RR, OsRR1, physically interacted with Ehd1. In addition, OsRR1-overexpressing plants showed a late-flowering phenotype. These observations suggest that cytokinins delay flowering by increasing the levels of OsRR1, which inhibits Ehd1 activity by forming an inactive complex.