Exploring the SWEET gene family and its role in root system architecture in Populus tomentosa

Introduction Sugars Will Eventually be Exported Transporters (SWEETs) facilitate sugar translocation and shape plant organ development, yet few studies have characterized their functions in root-related physiological processes in woody perennials. Methods We performed genome-wide screening of the SWEET gene family using the chromosome-scale assembly of Populus tomentosa and conducted a comprehensive set of bioinformatic analyses. The root-preferentially expressed gene PtoSWEET15.1A was selected for functional assays via native promoter-driven overexpression in poplar. Results Forty-three non-redundant SWEET members were identified, which fall into four well-supported phylogenetic clades. Gene duplication analysis indicated that both whole-genome duplication and segmental duplication were the primary drivers of SWEET family expansion in P. tomentosa . Tissue-specific transcript profiling revealed considerable functional divergence among paralogs. Overexpression of PtoSWEET15.1A enhanced adventitious root length, lateral root density and total root biomass. Transgenic root tissues accumulated higher levels of sucrose and fructose, accompanied by elevated transcript levels of key positive regulators of adventitious root organogenesis. Discussion This work systematically characterizes the SWEET repertoire in P. tomentosa and provides experimental evidence supporting PtoSWEET15.1A as a promising candidate gene resource for improving poplar root-related agronomic traits in molecular breeding, expanding our understanding of sugar transporter-mediated carbon allocation in woody plant root systems.