Probable transcription factor involved in the determination of adaxial-abaxial polarity in ovule primordium. Specifies adaxial leaf fates. Inhibited by ZPR3. Homodimer (PubMed:18408069). Heterodimer with ZPR3 (PubMed:18408069). Interacts with ESR1 and ESR2 (PubMed:17376809). Interacts with ZPR3 (PubMed:18408069). Expressed in the center of the meristem and on the adaxial side of the leaves. Expressed at the youngest stages of ovule development, predominantly to the adaxial side of the ovule primordium. Later, expression becomes restricted to a region in the distal chalaza. When the integuments initiate, expressed in the inner integument, with highest expression in the inner layer of the inner integument. Expressed in the vegetative shoot apical meristem (SAM) and initially throughout the presumptive cotyledons. At the globular stage, just before cotyledon outgrowth, expressed at high levels in cotyledon adaxial domains. Expression is lost from the SAM of torpedo stage embryos but is regained late in embryogenesis, extending to young primordia after germination, and becoming progressively restricted to the adaxial domain and the vasculature. Preferentially expressed in the adaxial domain of the developing leaf. Expressed throughout the plastochron 0 (P0) leaf primordium and expression increases in P1 to become preferentially localized to the adaxial leaf domain by the P2 stage (polar expression). Repressed by miR165 and miR166. The ZIP domain (80-130) is necessary and sufficient for the interaction with ZPR3 (PubMed:18408069). Belongs to the HD-ZIP homeobox family. Class III subfamily.