Negative regulator of cell proliferation in the adaxial side of leaves. Regulates the formation of a symmetric lamina and the establishment of venation. Positively regulates LATERAL ORGAN BOUNDARIES (LOB) within the shoot apex, and the class III HD-ZIP genes REV, PHB, and PHV. Interacts directly with ASYMMETRIC LEAVES 1 (AS1) to repress the knox homeobox genes KNAT1, KNAT2, and KNAT6 and the abaxial determinants ARF3, KAN2 and YAB5. May act in parallel with the RDR6-SGS3-AGO7 pathway, an endogenous RNA silencing pathway, to regulate the leaf morphogenesis (PubMed:11311158, PubMed:12787254, PubMed:12874130, PubMed:14508003, PubMed:16006579, PubMed:16699177, PubMed:17395603, PubMed:17559509). Required for the binding of AS1 to the KNOX genes (PubMed:23271976). Involved in leaf polarity establishment by functioning cooperatively with RH10 or RID2 to repress abaxial genes ARF3, ARF4, KAN1, KAN2, YAB1 and YAB5, and the knox homeobox genes KNAT1, KNAT2, KNAT6, and STM to promote adaxial development in leaf primordia at shoot apical meristems at high temperatures (PubMed:27334696). Homo- and heterodimer with AS1 (PubMed:23271976). Interacts with AS1 (PubMed:12874130, PubMed:23271976). Part of the AS1 repressor complex composed of AS1, LBD6/AS2 and HDA6 (PubMed:23271976, PubMed:12874130). Interacts with LFR (PubMed:29775508). Expressed in young shoots, roots, stems, leaves, flowers and adaxial domains of cotyledonary and leaves primordia. Negatively regulated by SHOOT MERISTEMLESS (STM). Plants are indistinguishable from that of wild-type at 16 degrees Celsius, however they generate a weak phenotype of pointed leaves at 22 degrees Celsius which become narrower at 26 degrees Celsius. In the leaves of plants grown at 26 degrees Celsius, the xylems are located on the adaxial sides and the phloems are on the abaxial sides, similar to those in the wild-type. Plants with double mutations in this protein and in RH10 or AS1 protein have abaxialized filamentous and trumpet-like leaves with loss of the adaxial domain at high temperatures. In double mutants, shapes of epidermal cells of the filamentous leaves are simple and rectangular, similar to those of a petiole, but different from those of flat leaves of wild-type plants. The filamentous leaves of the double mutant at 26 degrees Celsius show primitive or no vascular tissue without apparent xylem cells inside the bundle sheath, suggesting defects in differentiation of xylem cells on the adaxial side. Gain-of-function mutants iso1-D and iso-2D (T-DNA tagging) show completely curled up hyponastic rosette leaves. Belongs to the LOB domain-containing protein family.