Probable transcription factor involved in the regulation of vascular development. May promote differentiation of precambial and cambial cells. Interacts with ESR1 and ESR2 (PubMed:17376809). Interacts with ZPR3 (PubMed:18408069). Expressed in the procambial cells and the developing vascular system of embryos, roots and shoots. Expressed during the early stages of revascularization after cutting experiment. By auxin. Repressed by miR165. Belongs to the HD-ZIP homeobox family. Class III subfamily.