Vein-Specific Overexpression of Galactinol Synthase Gene Improves Salinity Tolerance in Poplar

Vein vasculature plays a key role in the translocation and metabolism of soluble carbohydrates from source to sink tissues in higher plants. Most prior research has focused on the storage or transport functions of raffinose family oligosaccharides (RFOs) in apoplastic and polymer-trapping plants, yet little is known about vein-localized RFOs in plants with passive symplastic phloem loading. In this study, the transgenic lines of poplar, a model tree with passive symplastic phloem loading, were generated to ectopically express the galactinol synthase gene, which catalyzes the first committed step in RFO biosynthesis, specifically in leaf veins. Under normal growth conditions, the engineered poplars developed a more vigorous root system compared to non-transgenic controls. When exposed to salinity stress, they exhibited significantly enhanced tolerance, which was associated with reduced foliar Na⁺ retention, decreased membrane ion leakage, altered carbohydrate reallocation, and elevated oxidative defence capacity. Gene ontology and Kyoto Encyclopaedia of Genes and Genomes pathway enrichment analyses of differentially expressed genes revealed that multiple biological pathways, including hormone signal transduction, carbohydrate metabolism, and environmental adaptation, contributed to the enhanced salt tolerance in CmGolS1-overexpressing poplars. Collectively, our findings demonstrate that vein-localized overexpression of GolS1 in poplar improves its salt tolerance through a coordinated regulatory mechanism, while also highlighting this vein-targeted biotechnology strategy as a promising tool for improving both stress resilience and agronomic traits in plants.