Probable transcription factor involved in the negative regulation of cell elongation and specific cell proliferation processes such as lateral root formation and secondary growth of the vascular system. Acts as mediator of the red/far-red light effects on leaf cell expansion in the shading response. Binds to the DNA sequence 5'-CAAT[GC]ATTG-3'. Negatively regulates its own expression. Interacts with DNA as homodimer. Predominantly expressed in leaves and stems. Rapidly and strongly induced by lowering the ratio of red to far-red light. Belongs to the HD-ZIP homeobox family. Class II subfamily.