Structure and function of homodomain-leucine zipper (HD-Zip) proteins

Homeodomain-leucine zipper (HD-Zip) proteins are transcription factors unique to plants and are encoded by more than 25 genes in Arabidopsis thaliana. Based on sequence analyses these proteins have been classified into four distinct groups: HD-Zip Ι-ІV. HD-Zip proteins are characterized by the presence of two functional domains; a homeodomain (HD) responsible for DNA binding and a leucine zipper domain (Zip) located immediately C-terminal to the homeodomain and involved in protein-protein interaction. Despite sequence similarities HD-ZIP proteins participate in a variety of processes during plant growth and development. HD-Zip Ι proteins are generally involved in responses related to abiotic stress, abscisic acid (ABA), blue light, de-etiolation, and embryogenesis. HD-Zip ΙΙ proteins participate in light response, shade avoidance, and auxin signalling. Members of the third group (HD-ZipIII) control embryogenesis, leaf polarity, lateral organ initiation, and meristem function. HD-Zip ΙV proteins play significant roles during anthocyanin accumulation, differentiation of epidermal cells, trichome formation, and root development.