Histone Deacetylase Complex 1 facilitates hypocotyl elongation by attenuating transcriptional and epigenetic responses via a PIF4-dependent process.

Photomorphogenesis is a key transitional response occurring when young seedlings are exposed to light. Upon red-light perception, this process depends on the nuclear translocation of the photoreceptor Phytochrome B (phyB) and on its interaction with downstream components. Histone Deacetylase Complex 1 (HDC1) is a member of the histone deacetylation complex (HDAC) that regulates the sensitivity of Arabidopsis seedlings to environmental cues. Here we show that HDC1 is a positive regulator of hypocotyl elongation primarily when plants are exposed to red-light conditions. By employing protein interaction assays and mutant combinations, we demonstrate that HDC1 can directly interact with PIF4 and regulate its abundance. Chromatin immunoprecipitation combined with next generation sequencing (ChIP-seq) revealed that HDC1 can modulate the expression of growth promoting genes primarily by deacetylating their promoter regions and that HDC1 is required for PIF4 association over DNA targets. Taken together, our findings further explore the connection between light signalling pathway and chromatin state and assign another function to a member of the HDAC complex in plants.