Negative regulation of floral transition in Arabidopsis by HOS15-PWR-HDA9 complex.

Arabidopsis HOS15/PWR/HDA9 repressor complex, which is similar to the TBL1/NcoR1/HDAC complex in animals, plays a well-known role in epigenetic regulation. PWR and HDA9 have been reported to interact with each other and modulate the flowering time by repressing expression, whereas HOS15 and HDA9, together with the photoperiodic evening complex, regulate flowering time through repression of GI transcription. However, the role of the HOS15/PWR/HDA9 core repressor complex as a functional unit in the regulation of flowering time is yet to be explored. In this study, we reported that the loss-of-function triple mutant accumulates higher transcript levels of and exhibits an early flowering phenotype similar to those of , , and single mutants. Interestingly, the accumulation of HOS15 in the nucleus was drastically reduced in and mutants. As a result, HOS15 could not perform its role in histone deacetylation or interaction with H3 in the nucleus. Furthermore, HOS15 is also associated with the same region of the promoter known for PWR-HDA9 binding. The acetylation level of the promoter was increased in the mutant, similar to the and mutants. Therefore, our findings reveal that the HOS15/PWR/HDA9 repressor complex deacetylates the promoter region of , thereby negatively regulating transcription, which leads to early flowering in Arabidopsis.