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. Involved in SAM development and lateral organ patterning. Essential for proper functioning of stem cells in the SAM. Interacts with REV (PubMed:18055602, PubMed:18408069). Interacts with ATBH-8, ATBH-9, ATB-14 and ATB-15 (PubMed:18408069). Expressed in the adaxial epidermis of the cotyledons and leaves, and in the vascular cylinder of wild-type torpedo stage embryos. Confined in the central zone and the organizing center in the shoot apical meristem. Up-regulated in response to increased HD-ZIPIII activity (PubMed:18055602). Up-regulated by ATHB-14 (PubMed:18408069). Up-regulated by REV (PubMed:22781836). No visible phenotype during the vegetative growth. Disrupted activities of the shoot apical meristem and/or axillary meristems after the transition to reproductive growth. Zpr3 and zpr4 double mutants exhibit homeotic transformation and ectopic meristem activity.