Stabilization of the MAPK–Epigenetic Signaling Axis Underlies the Protective Effect of Thyme Oil Against Cadmium Stress in Root Meristem Cells of Vicia faba

Cadmium (Cd) induces oxidative stress and disrupts nuclear organization and chromatin-associated metabolic processes in plant cells. Therefore, identifying natural, biodegradable, non-bioaccumulative compounds that enhance plant tolerance to heavy metals is crucial. We hypothesized that Cd exposure (175 µM CdCl2, 24 h) activates mitogen-activated protein kinases (MAPKs), triggering defined epigenetic modifications that lead to transcriptional repression, and that thyme oil (TO; 0.03% (v/v), emulsified) mitigates these effects by stabilizing chromatin organization. We analyzed nuclear MAPK (p44/42) activation, global DNA methylation (5-methylcytosine; 5-mC), and selected histone modifications as key components of early stress signaling and epigenetic regulation. We found that Cd exposure doubled global 5-mC levels and caused pronounced alterations in histone marks, including decreases in H3K4Me2 (~34%), H3T45Ph (~48%), and H4K5Ac, accompanied by strong increases in H3K9Ac (~57%) and H3K56Ac (~148%). These changes were associated with chromatin condensation and reduced transcriptional activity. In contrast, co-treatment with TO maintained MAPK activity and epigenetic parameters close to control levels, preventing chromatin compaction and transcriptional repression. Together, these findings indicate that TO stabilizes the nuclear signaling–epigenetic interface under Cd stress and represents a promising bioprotective strategy. This work provides the first demonstration that TO modulates both MAPK activation and Cd-induced histone modifications in plants.