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. A number of isoforms are produced. According to EST sequences. Up-regulated in response to increased HD-ZIPIII activity (PubMed:18055602). Up-regulated by REV (PubMed:22781836).