Histone deacetylase 9 modulates the acetylation dynamics of phototropin 1 to fine-tune phototropic responses in plants

Phototropism is essential for optimizing plant growth and development, with the blue light receptor phototropin 1 (phot1) acting as the primary photoreceptor. Although downstream components of phot1-mediated phototropic signaling have been extensively studied, the upstream regulatory mechanisms controlling phot1 activity remain elusive. Here, we demonstrate that lysine acetylation dynamically modifies phot1 under both dark and light conditions. Functional analyses using site-directed mutagenesis of acetylated lysines revealed that acetylation regulates phot1' s light-induced autophosphorylation and kinase activity. Genetic screening of histone deacetylase (HDAC) mutants identified HDA9 as a key regulator of phototropism, which physically interacts with phot1 to modulate its acetylation and phosphorylation levels in response to light. Importantly, we pinpointed K636 as the critical acetylation site targeted by HDA9, linking deacetylation to phot1 activation. Our findings establish a regulatory paradigm in which HDA9-mediated deacetylation fine-tunes phot1 phosphorylation dynamics to control phototropic responses. This acetylation-phosphorylation crosstalk suggests to be evolutionarily conserved, underscoring its broad significance in light signaling. Our study provides mechanistic insights into how antagonistic post-translational modifications precisely regulate photoreceptor sensitivity and signal transduction in plants.