PagSPL1B enhances salt tolerance in poplar by coordinating photoprotection, cuticle barrier, and hormone signaling

Elucidating the salt tolerance mechanism in poplar is critically important. Based on WGCNA analysis of the 84 K poplar transcriptome under different NaCl concentrations, this study identified the hub gene PagSPL1B , which is significantly associated with salt tolerance and strongly salt-induced. A total of 55 PagSPLs genes were identified at the whole-genome level in 84 K poplar. The promoter regions of these SPLs genes are enriched with abiotic stress- and hormone-responsive cis -acting elements. Functional experiments confirmed that PagSPL1B overexpression significantly enhanced salt tolerance in 84 K poplar, manifested by alleviated growth inhibition, reduced damage to the photosynthetic apparatus, decreased oxidative stress, and enhanced accumulation of osmotic adjustment substances. Transcriptomic analysis revealed 961 DEGs between WT and PagSPL1B -OE1 under NaCl stress. The upregulated DEGs were significantly enriched in KEGG pathways including “Photosynthesis - antenna proteins”, “Cutin, suberine and wax biosynthesis”, “Brassinosteroid biosynthesis”, and “Carotenoid biosynthesis”. Under salt stress, PagSPL1B alleviated the downregulation of photosynthetic apparatus-related genes and significantly promoted the expression of genes involved in cutin/wax biosynthesis and transport. Additionally, the stress-resistant hormone signaling pathways of abscisic acid (ABA) and brassinosteroids (BRs) were activated. In summary, this study reveals a novel mechanism whereby PagSPL1B synergistically enhances poplar salt stress tolerance by integrating three key pathways: protection of the photosynthetic apparatus, synthesis of cuticular waxes, and hormonal balance. This finding not only confirms a new function of SPLs in abiotic stress adaptation in woody plants, but also provides genetic resources and a theoretical basis for the genetic improvement of stress resistance in forest trees.