Phylogenetic tree of WOX proteins and gene structure of corresponding WOX genes from Populus trichocarpa and Arabidopsis thaliana . Deduced full-length amino acid sequences were aligned using ClustalX 2.0. The phylogenetic tree was constructed using phyML and the maximum likelihood method. The three clades (ancient, intermediate, and modern/WUS) are indicated as different grayscale boxes in the left panel. Empty boxes and black lines in the right panel represent exons and introns respectively. Support values are shown on selected branches.