JULGI coordinates vascular development and leaf patterning through non-cell-autonomous regulation of miR165/166

The interplay between vascular development and leaf development is central to the evolutionary success of vascular plants, yet the underlying mechanisms remain poorly understood. Here, we identify the RNA-binding protein JULGI1 (JUL1) as a key integrator of vascular and leaf development in Arabidopsis. JUL1 is transcribed in leaf veins, while the protein distributes throughout the leaf, where it binds G-rich regions of pri-MIR165a transcripts on the abaxial side. This interaction disrupts hairpin formation and inhibits DICER-LIKE1 (DCL1)-mediated processing, reducing miR165/166 levels. Consequently, transcript levels of their targets, the Class III homeodomain leucine zipper (HD-ZIP III), increase, reinforcing adaxial-abaxial polarity. Our findings reveal a non-cell-autonomous mechanism by which vascular-derived JUL1 modulates miRNA biogenesis to coordinate leaf patterning with vascular development. Leaf and vein development must be precisely coordinated, yet the molecular signals linking these processes have remained unclear. This study identifies JUL1 as a non–cell-autonomous regulator that connects vascular development with the refinement of leaf polarity.