The plant vascular system is crucial for water and nutrient transport. Despite its importance, most studies of vascular growth focus on the root whereas other organs remain less well understood. Now, Antia Rodriguez-Villalon and colleagues study the mechanisms of secondary vascular branch formation in embryonic leaves (cotyledons). Cotyledons have a single primary vein (midvein) that connects to the stem vascular system and extends along the length of the organ, from which a pair of secondary veins branch. Through careful observation, genetics and transcriptome analyses, the authors determine two types of branching that occur. In the distal cotyledon, midvein cells act as branching points and distal secondary branches are regulated by polar auxin activity via PIN-FORMED (PIN) proteins. Conversely, proximal branching depends on COTYLEDON VASCULAR PATTERN 2 (CVP2) and CVP2-LIKE 1 (CVL1) in a PIN1-independent manner. In addition, proximal branching is promoted by OCTOPUS. CVP2 and CVL1 are negatively regulated by RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2): rpk2 silencing in cvp2 cvl1 mutants partially rescues proximal branching, whereas RPK2 in wild type establishes a vein boundary that delimits vein extension. Taken together, these data elucidate two different mechanisms of vascular branching in the understudied embryonic cotyledon.
