Evolution of the AT-hook Motif Containing Nuclear Localized (AHL) Gene Family in Land Plants

Through the evolution of land plants, genes that regulate key biological processes have duplicated, diversified and have often expanded into multiple-member-containing gene families. Members of the land-plant-specific AT-Hook Motif Containing Nuclear Localized (AHL) gene family have been proposed to regulate various aspects of plant biological processes. Our study offers the first attempt to reveal the evolutionary relationships among the AHLs in land plants. We have identified AHL genes from sequenced land plants, ranging from the early diverging moss species Physcomitrella patens to the lycophyte Selaginella to a diversity of monocot and dicot flowering plants. AHL proteins contain one or two AT-hook motifs followed by a Plant and Prokaryote Conserved / Domain of Unknown Function 296 (PPC/DUF296) domain. Aligning the amino acid sequences from identified AHL proteins revealed three types of AHLs (Type-I, -II and –III) classified based on a combination of the number and type of AT-hook motif(s) as well as the type of PPC/DUF296 domain. Our phylogenetic analysis further suggested that Type-I and -II AHLs exist in a one clade (Clade-I), while Type-III AHLs form a second clade (Clade-II). The evolutionary division into these two clades occurred before the separation of the moss Physcomitrella patens from the vascular plant lineage. Our analysis also identified extensive gain, loss, and duplication of AHLs within each species. Results from this study propose an evolutionary scenario for the AHL gene family in land plants. This framework will facilitate new studies probing the biological functions of AHLs in regulating plant growth and development.