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

Abstract

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 <i>AGL19</i> 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 <i>hos15-2/pwr/hda9</i> triple mutant accumulates higher transcript levels of <i>AGL19</i> and exhibits an early flowering phenotype similar to those of <i>hos15</i>, <i>pwr</i>, and <i>hda9</i> single mutants. Interestingly, the accumulation of HOS15 in the nucleus was drastically reduced in <i>pwr</i> and <i>hda9</i> 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 <i>AGL19</i> promoter known for PWR-HDA9 binding. The acetylation level of the <i>AGL19</i> promoter was increased in the <i>hos15-2</i> mutant, similar to the <i>pwr</i> and <i>hda9</i> mutants. Therefore, our findings reveal that the HOS15/PWR/HDA9 repressor complex deacetylates the promoter region of <i>AGL19</i>, thereby negatively regulating <i>AGL19</i> transcription, which leads to early flowering in Arabidopsis.