Characterization of homeodomain-leucine zipper genes in Physcomitrella patens with reference to evolution of homeobox genes in land plants

Land plants have evolved multicellular bodies with specialized cell types in both gametophytes and sporophytes. In the last decade, the molecular mechanisms of sporophyte cell differentiation have been studied extensively, while those in gametophytes are mostly unknown. The moss Physcomitrella (Physcomitrella patens) is a model for studying gametophyte cell differentiation, because functional gene analysis techniques have been established and its gametophytes are easily observed at the cellular level. Although sporophytes and gametophytes acquired multicellular bodies independently, members of a common gene family may regulate cell differentiation in both gametophytes and sporophytes. To study gametophyte cell differentiation, she first screened Physcomitrella gametophytes for homeodomain-leucine zipper (HD-Zip) genes, which are involved in cell differentiation in angiosperm sporophytes. Chapter 1 describes how 10 HD-Zip genes (named Pphbl-10) were isolated from Physcomitrella. Phylogenetic analyses comparing moss and angiosperm HD-Zip genes indicated that all the Pphb genes, except Pphb3, belong to three of the four HD-Zip subfamilies (HD-Zip I, II, and III), and that these subfamilies originated before the divergence of the vascular plant and moss lineages. RNA gel blot analyses showed that eight of the Pphb genes were expressed in gametophytes. One of the genes is further characterized in Chapter 2. Chapter 2 explains how detailed expression analyses and the phenotype of disruptants of Pphb7 showed that this gene is involved in rhizoid cell differentiation. Rhizoids are multicellular filaments that differentiate from epidermal cells of the stem in gametophytes. Pphb7 is expressed in rhizoid initial cells and rhizoids. Pphb7-disrupted rhizoids have defective pigmentation and an increased number and size of chloroplasts, but the position and number of rhizoids do not differ from the wild type. The role of HD-Zip I genes in the diversification of cell types of moss gametophytes is discussed