Coordinated chromatin modifications mediated by AREB and MYB transcription factors sustain drought tolerance in Populus.

Sustained drought tolerance in plants relies on transcriptional memory through successive stress cycles, yet the chromatin-based mechanisms underlying this memory remain unclear. Previously, we revealed that PtrMYB161 overexpression in Populus trichocarpa results in phenotypes characteristic of drought tolerance. Here, we confirm that such transgenesis instills an epigenetic path to gene transregulation for drought tolerance. PtrMYB161 binds directly to the MYB-core motif in the promoter of PtrNAC120, a drought response/tolerance gene, to recruit the histone acetyltransferase (HAT) dimer GENERAL CONTROL NON-DEREPRESSIBLE5-1-ALTERATION/DEFICIENCY IN ACTIVATION2b-3 (PtrGCN5-1-PtrADA2b-3), forming the ternary protein complex (PtrMYB161-PtrGCN5-1-PtrADA2b-3). This ternary system enables enhanced acetylation of nucleosome histone 3 lysine-9, -14, and -27 (H3K9, H3K14, and H3K27) for enriched RNA Pol II occupancy in the PtrNAC120 promoter to elevate its expression for drought tolerance. Unlike PtrAREB1-2, an important drought-inducible transcription factor that can also mediate PtrNAC120 transactivation for tolerance, PtrMYB161 expression remains unaffected by drought. However, under drought conditions, induced PtrAREB1-2 could form HAT ternary complexes, PtrAREB1-2-PtrGCN5-1-PtrADA2b-3, and bind to PtrAREB1-binding sites (ABREs) in the PtrNAC120 promoter for PtrNAC120's enhanced H3K acetylation, RNA Pol II occupancy, and transactivation for drought tolerance. PtrMYB161-PtrGCN5-1-PtrADA2b-3-mediated PtrNAC120 transactivation was induced following severe, prolonged drought stress (below 40% Relative Soil Water Content) and PtrAREB1-2-induced PtrNAC120 transactivation. Further loss- and gain-of-function transgenesis experiments in whole plants and stem differentiating xylem protoplasts suggest that, under stress, the PtrAREB1-2 regulatory system activates an ancillary regulation mediated by PtrMYB161. Our findings propose coordinated epigenetic regulations mediated by HAT complexes to jointly sustain drought tolerance in Populus.