Isolation of homeodomain-leucine zipper genes from the moss Physcomitrella patens and the evolution of homeodomain-leucine zipper genes in land plants

Homeobox genes encode transcription factors involved in many aspects of developmental processes. The homeodomain-leucine zipper (HD-Zip) genes, which are characterized by the presence of both a homeodomain and a leucine zipper motif, form a clade within the homeobox superfamily and were previously reported only from vascular plants. Here we report the isolation of 10 HD-Zip genes (named Pphb1-Pphb10) from the moss Physcomitrella patens. Based on a phylogenetic analysis of the 10 Pphb genes and previously reported vascular plant HD-Zip genes, all of the Pphb genes except Pphb3 belong to three of the four HD-Zip subfamilies (HD-Zip I, II, and III), indicating that these subfamilies originated before the divergence of the vascular plant and moss lineages. Pphb3 is sister to the HD-Zip II subfamily and has some distinctive characteristics, including the difference of the a₁ and d₁ sites of its leucine zipper motif, which are well conserved in each HD-Zip subfamily. Comparison of the genetic divergence of representative HD-Zip I and II genes showed that the evolutionary rate of HD-Zip I genes was faster than that of HD-Zip II genes.