A research team led by Prof. LIU Xuncheng at the South China Botanical Garden of the Chinese Academy of Sciences has shed new light on how plants fine-tune their response to far-red light. Their findings were recently published in the journal Molecular Plant. This study reveals that the stability of Arabidopsis' far-red light receptor, phytochrome A (phyA), is dynamically regulated by lysine acetylation—a post-translational modification. Histone Deacetylase 2 (HDT2)—a lysine deacetylase—plays a key role in promoting phyA degradation and governing seedling development under specific light conditions.
For years, however, the precise molecular mechanisms controlling phyA stability remained unclear. Lysine acetylation—a conserved post-translational modification—provided a potential clue, but while prior research focused on its role in histone regulation (histones package DNA), little was known about non-histone acetylation (e.g., on photoreceptors like phyA) and its impact on photomorphogenesis.
To address this challenge, the team employed a combination of immunoblot analysis and systematic acetylomic profiling—techniques that map modifications to specific proteins and amino acid sites.
Immunoblotting revealed a sharp drop in overall protein acetylation levels when dark-grown seedlings were exposed to light, indicating a broad light-induced deacetylation response.
Acetylomic profiling further pinpointed proteins and lysine residues affected by this shift. For phyA, the team identified four conserved lysine acetylation sites: K65, K296, K536, and K744. Among these, K65 and K744 showed the most dramatic deacetylation when seedlings were exposed to light.
To test the functional significance of these sites, the researchers generated transgenic Arabidopsis lines expressing modified versions of phyA: one mimicking a permanently acetylated state at K65 (K65Q) and another mimicking a deacetylated state (K65R).
Their experiments showed that only the acetylation status of K65 affected phyA's biological activity. Further analysis revealed that K65 also serves as a critical ubiquitination site—a modification that tags proteins for degradation via the 26S proteasome, a cellular "recycling system."
Mutating K65 (either to mimic acetylation or block it) significantly reduced phyA's ubiquitination and degradation rates. This, in turn, disrupted the accumulation of HY5—a downstream transcription factor central to light signaling—and altered the expression of light-responsive genes.
Using co-immunoprecipitation (Co-IP) assays, the team confirmed a direct link: light-induced deacetylation at K65 promotes phyA ubiquitination, ultimately leading to its breakdown via the 26S proteasome. This marks the first time scientists have identified a "deacetylation-ubiquitination cascade" as a requirement for light-induced phyA degradation.
The team next sought to identify the enzyme responsible for phyA's light-induced deacetylation. Protein-protein interaction assays showed that HDT2 binds specifically to phyA in the cell nucleus after light exposure.
Biochemical tests confirmed HDT2's role: it catalyzes the deacetylation of phyA at the K65 site following illumination, directly promoting phyA's ubiquitination and degradation.
Building on these biochemical findings, genetic experiments further validated this: Overexpressing HDT2 accelerated phyA breakdown in light-exposed seedlings, whereas knocking out the HDT2 gene suppressed phyA degradation.
Collectively, these results demonstrated that HDT2 is a key player in phyA-mediated far-red light signaling.
This study opens new avenues for understanding plant light signaling and could inform future research aimed at optimizing crop growth and stress resilience.
86-10-68597521 (day)
86-10-68597289 (night)
52 Sanlihe Rd., Xicheng District,
Beijing, China (100864)