Abstract
Poplar (Populus spp.) is a model for tree biology and a platform for engineering woody biomass, biofuels, biomaterials, and bioproducts. Many relevant traits depend on tissue position, developmental stage, and cell type, yet these spatial relationships are difficult to recover from bulk or single–cell transcriptomes. Here, we generated a spatial transcriptome atlas of Populus tremula x P. alba INRA 717–1B4 across the shoot apex, axillary bud, stem, and petiole. After quality control, the atlas retained 29,687 spatial spots from 45 tissue sections and detected 58,748 genes. Histology–guided clustering and marker analysis resolved meristematic, epidermal, cortical, vascular, and organ–specific domains. Cross–organ comparisons assessed whether published markers retained tissue–associated expression across different anatomical contexts and developmental stages, while de novo analysis identified additional domain–enriched candidates. As case studies of the utility of the atlas, we examined the emergence of trichome–associated programs in the shoot apex and adaxial – abaxial expression differences in petioles. A trichome identity score based on poplar markers from the single–cell shoot atlas peaked along the inferred meristem–to–primordium trajectory, revealing spatially localized expression of trichome–associated programs during early leaf development. Petiole expression differences were concentrated in the epidermis and cortex and involved polarity–associated, auxin–responsive, and cell–wall–remodeling genes, with distinct expression profiles across leaf positions. Together, these data provide a spatial reference for investigating tissue differentiation and developmental patterning in a transformable poplar genotype.