애기장대 히스톤 탈아세틸화 효소 HDA9의 개화 및 발아 관련 기능에 대한 연구

Abstract

Posttranslational acetylation of histones is reversibly regulated by histone deacetylases (HDACs). Despite the evident significances of HDACs in Arabidopsis development, the biological roles and underlying molecular mechanisms of many HDACs are yet to be elucidated. In this study, I revealed the biological role of the RPD3/HDA1-class histone deacetylase HDA9 in resetting histone acetylation levels during active transcription to maintain proper transcription activity in two major phase transition of plants

seed germination and flowering. Loss-of-function in HDA9 flowered early under non-inductive short-day (SD) condition and showed increased expression of the floral integrator, FT and floral activator, AGL19. The hda9 mutation increased histone H3 acetylation and RNA polymerase II occupancy at AGL19 chromatin but not FT during active transcription. In addition, HDA9 directly targeted AGL19, and AGL19 expression was higher in SD than LD condition. The agl19 mutation is epistatic to the hda9 mutation, masking the early flowering and increased FT expression of hda9. Taken together, my data indicates that HDA9 prevents precocious flowering in SD by curbing the hyper-activation of AGL19, an upstream activator of FT, through resetting local chromatin environment. Epigenetic regulation network through HAT and HDAC is known to play crucial roles in seed development. Timing of seed germination is controlled by various environmental factors in order to initiate a successful new life cycle under favorable environment. Light is the most critical environmental factor to promote seed germination. Light-induced germination process involves the perception of light mainly by phytochrome B (phyB) and degradation of the germination repressor PHYTOCHROME INTERACTING FACTOR (PIF1) resulted from its interaction with phyB. Through this study, I found out that HDA9 adds a new layer of regulation for phyB-dependent germination process. Loss-of-HDA9 activity caused rapid germination after red-light pulse treatment and under continuous white light. The expressin of HECs, previously known repressors of PIF1 transcription activity was also increased in the hda9 mutant. Epistatic analysis between the hda9 mutant and hec1hec2 RNAi showed that rapid seed germination of the hda9 mutant was caused by the increased HECs expression. Histone H3 acetylation level and RNA polymerase II occupancy at HECs were more elevated in hda9-1 than in wt after red light pulse but not after far-red light pulse. The direct association of HDA9 with HECs chromatin was also observed after red light pulse but not after far-red light pulse. Furthermore, HDA9 also affect the expression of GA-INSENSITIVE (GAI) and REPRESSOR OF GA1-3 (RGA/RGA1), downstream target genes of PIF1. Taken together, my results indicate that HDA9 plays a role in the prevention of the hyper light-sensitive germination by inhibiting the hyper-activation of HECs transcription by light through deacetylating HEC chromatin during active transcription. Thus, HDA9 acts as a fine-tuning mechanism of phyB-dependent germination ensuring the beginning of germination under proper light condition. In conclusion, throughout my research, I focused on the identification of the novel roles of HDA9 during seed germination and flowering. The role of HDA9 in transcription, unlike the conventional idea of HDACs is to modulate the transcription activity of target chromatin (AGL19 and HECs) by resetting the landscape of chromatin during active transcription.