Spatiotemporal imaging clarifies leaf primordium patterning

The first step in organ morphogenesis is the subdivision of a primordium into discrete regions by patterning genes. Recently, Burian et al. used live imaging and cell-lineage tracing to illuminate early patterning events during the establishment of leaf primordium adaxial-abaxial (dorsoventral) polarity, which clarifies controversies in the field.