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 studied extensively, the upstream regulatory mechanisms that control phot1 activity remain to be clarified. Here, we demonstrate that lysine acetylation dynamically modifies phot1 under both dark and light conditions. Site-directed mutagenesis of acetylated lysines revealed that acetylation regulates the light-induced autophosphorylation and kinase activity of phot1. Genetic screening of histone deacetylase (HDAC) mutants identified HDA9 as a key regulator of phototropism that physically interacts with phot1, modulating its acetylation and phosphorylation levels in response to light. We pinpointed K636 as the critical acetylation site targeted by HDA9, linking deacetylation to phot1 activation. Our findings reveal a regulatory paradigm in which HDA9-mediated deacetylation fine-tunes the phosphorylation dynamics of phot1 to control phototropic responses. This acetylation-phosphorylation crosstalk appears to be evolutionarily conserved, underscoring its broad significance in light signaling. Our study provides insight into the mechanisms by which antagonistic post-translational modifications precisely regulate photoreceptor sensitivity and signal transduction in plants.