PAX-regulated self-organizing polarity drives procambial strand formation.

In plants, the vascular network enables the transport of water, nutrients and metabolites. The vascular network formed in leaves is a striking example of tissue patterning, and leaf venation is a defining leaf feature. During patterning, veins arise from aligned, elongated procambial cells derived from undifferentiated ground meristem cells. The mechanisms behind procambial strand formation involving cell selection, polarization, alignment, elongation and division have remained elusive. Here we identify a previously uncharacterized role for the polar protein PAX in establishing coordinated cell and tissue polarity during this process. We uncover a novel, auxin-guided role for the polar PAX protein in establishing cell and tissue polarity that precedes and is independent of the polarization and activity of PIN auxin transporters. Our findings refine and redefine existing models of vascular patterning during vein network formation in plant leaves.