Repression of flowering under a noninductive photoperiod by the HDA9-AGL19-FT module in Arabidopsis.

Posttranslational acetylation of histones is reversibly regulated by histone deacetylases (HDACs). Despite the evident significance of HDACs in Arabidopsis development, the biological roles and underlying molecular mechanisms of many HDACs are yet to be elucidated. By a reverse-genetic approach, we isolated an hda9 mutant and performed phenotypic analyses on it. In order to address the role of HDA9 in flowering, genetic, molecular, and biochemical approaches were employed. hda9 flowered early under noninductive short-day (SD) conditions and had increased expression of the floral integrator FLOWERING LOCUS T (FT) and the floral activator AGAMOUS-LIKE 19 (AGL19) compared with the wild-type. The hda9 mutation increased histone acetylation and RNA polymerase II occupancy at AGL19 but not at FT during active transcription, and the HDA9 protein directly targeted AGL19. AGL19 expression was higher under SD than under inductive long-day (LD) conditions, and an AGL19 overexpression caused a strong up-regulation of FT. A genetic analysis showed that an agl19 mutation is epistatic to the hda9 mutation, masking both the early flowering and the increased FT expression of hda9. Taken together, our data indicate that HDA9 prevents precocious flowering under SD conditions by curbing the hyperactivation of AGL19, an upstream activator of FT, through resetting the local chromatin environment.