Nuclear factor OsNF-YB4 promotes flowering by negatively regulating the floral repressor gene Ghd7 in rice

Flowering time or heading date is a critical agronomic trait of rice and is regulated by numerous genes, including several genes encoding nuclear factor YB (NF-YB) in rice, NF-YB11 is one of the genes well known to be involved in the process, delaying flowering under long-day (LD) conditions but promoting flowering under short-day (SD) conditions. In this study, we identified another NF-YB gene, OsNF-YB4. Overexpression of OsNF-YB4 promoted rice flowering under both natural long-day (NLD) and natural short-day (NSD) conditions, whereas suppression or loss-of-function of this gene delayed flowering. The transcription of OsNF-YB4 exhibited an obvious circadian pattern and was induced by light under both LD and SD conditions. Expression analyses of flowering regulators in the photoperiodic flowering pathway demonstrated that up-regulation of OsNF-YB4 resulted in down-regulation of floral repressor Grain number, plant height and heading date 7 (Ghd7), and thus activating the Early heading date 1 (Ehd1)-mediated flowering pathway. Besides, OsNF-YB4 was observed to bind to the specific CCAAT-box regions in the Ghd7 promoter in vitro and interact with GHD7 in yeast. All these evidences support that OsNF-YB4 functions as a flowering promoter by negatively regulating the expression of floral repressor Ghd7 in rice photoperiodic flowering-time regulatory network.