Larix gmelinii UGPase enhances vegetative growth in transgenic poplar through an HYH-mediated positive feedback loop

LgHYH targets and upregulates the transcription of LgUGPase1/2, which in turn indirectly activate LgHYH expression through the polysaccharide metabolic pathways, thereby forming a positive feedback loop. Uridine diphosphate glucose pyrophosphorylase (UGPase) catalyzes the conversion of glucose-1-phosphate to uridine diphosphate glucose, a key precursor for cellulose and callose synthesis which is essential for plant cell wall integrity and wood formation. In previous work, we identified two orthologs of UGPase from Larix gmelinii, named LgUGPase1 and LgUGPase2, and demonstrated that their overexpression enhances vegetative growth of transgenic arabidopsis; however, the regulatory mechanisms upstream of LgUGPase transcription remain largely unknown. This study aims to clarify the transcriptional regulatory network of LgUGPase and to characterize its function in woody plants. Expression profiling revealed that LgUGPase1 is broadly expressed in roots, stems, and young leaves, whereas LgUGPase2 is predominantly expressed in apical buds; their expression is induced by phytohormones including gibberellins, methyl jasmonate, and ethylene. Overexpression of LgUGPase in transgenic 84 K poplar significantly elevated contents of soluble sugar, lignin, and cellulose, thereby accelerating vegetative growth. RNA-sequencing analysis showed that differentially expressed genes (DEGs) between non-transgenic and LgUGPase-overexpressing lines were markedly enriched in starch and sucrose metabolism, plant hormone signal transduction, and flavonoid biosynthesis pathways. Notably, Gene Ontology (GO) enrichment analysis revealed a significant alteration in "DNA-binding transcription factor activity," with four PagHY5/HYH homologs exhibiting pronounced upregulation. Further dual-luciferase reporter and electrophoretic mobility shift assays (EMSA) confirmed that LgHYH directly binds to the promoter regions of LgUGPase1/2 and positively regulates their expression, thereby establishing a positive feedback regulatory loop mediated by LgHYH. Collectively, these findings provide potential regulatory targets for future genetic improvement of forest trees and crops.