Phylogenetic relationships, gene structure and motif compositons of Populus NAC genes. A. Multiple alignments of 163 full-length amino acids of NAC genes from Populus were executed by Clustal X 1.83 and the phylogenetic tree was constructed using MEGA 4.0 by the Neighbor-Joining (NJ) method with 1,000 bootstrap replicates. The percentage bootstrap scores higher than 50% are indicated on the nodes. The ten major phylogenetic subfamilies designated as I to X are marked with different color backgrounds. B. Exon/intron structures of NAC genes from Populus. Exons and introns are represented by green boxes and black lines, respectively. The sizes of exons and introns can be estimated using the scale at bottom. C. Schematic representation of the conserved motifs in the NAC proteins from Populus elucidated by MEME. Each motif is represented by a number in the colored box. The black lines represent the non conserved sequences. Refer to Additional file 7 for the details of individual motif.