Competitive inhibitor of the HD-ZIPIII transcription factors in shoot apical meristem (SAM) development. Acts by forming non-functional heterodimers. Part of a negative feedback loop. Essential for proper functioning of stem cells in the SAM. Interacts with REV. Expressed in the adaxial epidermis of the cotyledons and in the vascular cylinder of wild-type torpedo stage embryos. Up-regulated in response to increased HD-ZIPIII activity (PubMed:18055602). Up-regulated by REV (PubMed:22781836). Truncated N-terminus.