The phylogenetic and functional characterization of class IV homeodomain leucine zipper genes in streptophytes

Land plants diverged from an aquatic algal ancestor and evolved over time a variety of morphological adaptations providing for the transition of a simple ancestral body form to lineages possessing increasingly complex mechanisms necessary for terrestrial survival. Critical to understanding this transition is the characterization of genetic systems coordinating cellular identity, patterning, and how the components of these systems may have changed over time to either produce new structures or function in a different capacity. To better understand this process, we focused on the class IV homeodomain leucine zipper (class IV HD-Zip) genes---plant-specific transcription factors that, based on phenotypes in Arabidopsis thaliana, seem to play an important role in epidermal development. We sampled all major extant plant lineages and their closest algal relatives for class IV HD-Zip presence and can map their origin to the last common ancestor of land plants and Coleochaetophycean algae. Our analysis suggests a single, ancestral class IV gene duplicated independently within several lineages including mosses, lycophytes, seed plants and most notably, angiosperms.;In light of their phylogenetic distribution, we attempted to characterize the expression and function of class IV HD-Zip genes in Physcomitrella patens, a moss and member of the bryophyte grade of plants that comprise the basal lineages of land plant evolution. Our experiments suggest class IV HD-Zip genes localize to the meristematic regions of the gametophyte and sporophyte phase and may function in the establishment of early cell fate. Sequence analysis and our preliminary functional efforts suggest class IV HD-Zip genes may be functionally redundant.