The mediator subunit PagMED20 is associated with enhanced vegetative growth and photosynthetic performance in poplar

The mediator complex is a central hub in transcriptional regulation in eukaryotes, yet its biological function in woody plants remains largely unexplored. Here, we systematically identified and characterized the Mediator (MED) gene family in the forest model tree Populus alba × P. glandulosa and focused on PagMED20, a conserved subunit of the head module. Genome-wide analysis revealed significant diversity in the evolutionary history, protein structures, and cis-regulatory elements of the PagMED family. Among these subunits, PagMED20 was selected for further study based on its preferential expression in leaves and nuclear localization, suggesting a potential role in leaf-associated processes. Overexpression of PagMED20 in poplar resulted in enhanced vegetative growth, including increases in plant height, stem diameter, and biomass accumulation. Physiological analyses showed that these growth changes were accompanied by increased chlorophyll content, net photosynthetic rate, and stomatal conductance. Cytological observations further revealed increased stomatal density and enhanced secondary xylem development in the overexpression lines. At the molecular level, PagMED20 overexpression was associated with coordinated changes in the expression of genes related to photosystem components, carbon assimilation, and stomatal development. In addition, chlorophyll fluorescence analysis indicated increased electron transport rates and reduced non-photochemical quenching under the tested conditions, suggesting altered photosystem II energy utilization. Together, these results indicate that PagMED20 overexpression is associated with coordinated changes in growth, photosynthetic performance, and stomatal traits in poplar. This study provides a foundation for understanding the potential roles of Mediator subunits in woody plant growth and physiology and identifies PagMED20 as a candidate gene for improving biomass production in forest trees.