Isolation of Homeodomain–Leucine Zipper Genes from the Moss Physcomitrella patens and the Evolution of Homeodomain–Leucine Zipper Genes in Land Plants

Abstract
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 Pphb1Pphb10) 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.