A hierarchical auxin signaling cascade from IAA-ARF to GRF transcription factors controls leaf morphogenesis in Populus

As the primary sites of photosynthesis and transpiration, leaves are central to plant growth, productivity, and ecological adaptation. Leaf morphogenesis in plants is precisely regulated by auxin signaling and transcriptional networks; however, the underlying molecular mechanisms in woody perennials remain elusive. Here, we identify PtoIAA3.2 as a key auxin-responsive regulator of leaf development in Populus tomentosa . Functional analyses reveal that the stabilized PtoIAA3.2m protein suppresses both cell proliferation and expansion, resulting in significantly reduced leaf size. Through comprehensive interactome profiling, we demonstrate that PtoIAA3.2 physically interacts with PtoARF5.1, the poplar ortholog of Arabidopsis MONOPTEROS , forming a central regulatory module. Genetic complementation assays show that an interaction-deficient PtoARF5.1 variant fully restores normal leaf development in PtoIAA3.2m -overexpressing lines by reinstating regular cell division and expansion. Integrated transcriptomic and molecular analyses indicate that the PtoIAA3.2-PtoARF5.1 module directly regulates the expression of PtoGRF13 and PtoGRF15 by binding to auxin response elements in their promoters. Crucially, complementation with PtoGRF13m rescues leaf growth in transgenic plants, establishing these GRF transcription factors as essential downstream effectors. Our findings delineate a complete auxin signaling pathway in poplar, spanning from hormone perception through ARF-mediated transcriptional activation to GRF-dependent cellular processes, thereby offering mechanistic insights into leaf development in woody plants and potential targets for genetic improvement of trees. • PtoIAA3.2m suppresses leaf cell proliferation and expansion. • PtoIAA3.2-PtoARF5.1 module controls poplar leaf development. • PtoIAA3.2-PtoARF5.1 module directly activates the expression of PtoGRF13 and PtoGRF15. • The conserved auxin-Aux/IAA-ARF pathway orchestrates leaf development.