Specification of distinct dorsal and ventral domains of polarity is a key process during development of lateral organs. In plants, leaves are derived from the shoot apical meristem (SAM), and require signals from the SAM. In Arabidopsis, the meristem signal may regulate the activity of the class III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIP III) transcription factors to specify the dorsal domain of the leaf and restrict the ventral domain. In Zea mays (maize), dorsoventral patterning depends on coordinate action of several genes. leafbladeless1 (lbl1) is required for dorsal cell fate. rolled leaf1 (rld1) a member of the hd-zip III family, acts downstream of lbl1. rld1 expression is limited to the dorsal side of the leaf due to the microRNA166 mediated repression of rld1 transcripts in the ventral domain. Expression of microRNA166 initiates in a region of the SAM just below the emerging leaf and gradually extends into the ventral domain. The conservation between maize and Arabidopsis of hd-zip III gene function and their regulation by microRNAs, suggests these genes may have evolved an important developmental role during dorsoventral patterning.