PuWRKY46 drives root system architecture and enhances salt tolerance in poplar.

The developmental plasticity of lateral roots (LRs) is an essential adaptive strategy for optimizing root system architecture under salt stress. However, the molecular mechanisms by which salt stress modulates LR plasticity through cell wall remodeling remain largely elusive, particularly in woody species. This study investigates the abscisic acid (ABA)-responsive transcription factor PuWRKY46 as a critical regulator of LR developmental plasticity under salt stress in Populus ussuriensis. This study integrated phenotypic characterization of transgenic overexpression and RNA interference (RNAi) poplar lines, molecular biochemical assays, protein-protein interaction analyses, transcriptomics, DNA affinity purification sequencing (DAP-seq), phosphorylation assays, and cell wall biochemical analyses. Compared with wild-type plants, PuWRKY46 overexpression promotes LR emergence and enhances salt tolerance, whereas PuWRKY46-RNAi lines exhibit pronounced salt sensitivity. Furthermore, the mitogen-activated protein kinase PuMPK15 interacts with PuWRKY46, and the resulting phosphorylation stabilizes PuWRKY46 to enhance salt tolerance. The PuMPK15-PuWRKY46 module targets cell wall-loosening gene EXPANSIN-A11 (PuEXPA11) to promote localized apoplastic acidification and relax hemicellulose cross-linking, thereby driving root cell wall remodeling and facilitating LR development. The PuMPK15-PuWRKY46 signaling cascade regulates salt-adaptive LR architectural plasticity in poplar. This study reveals a conditional-plasticity regulatory mechanism and provides a valuable genetic resource for breeding salt-tolerant woody plants.