Hierarchical clustering of expression profiles of Populus NAC gene across different tissues. The genome-wide microarray data generated by Wilkins and coworker were re-analyzes . The expression data were gene-wise normalized and hierarchical clustered based on Pearson correlation. The genes marked in the same color indicate duplicated gene pairs. The relative expression level of particular gene in each row is normalized against the maximum value. Color scale at the top of each dendrogram represents log2 expression values, green represents low level and red indicates high level of transcript abundances. CL, continuous light-grown seedling; DL, etiolated dark-grown seedling transferred to light for 3h; DS, dark-grown seedlings; YL, young leaf; ML, mature leaf; R, root; DX, differentiating xylem; FC, female catkins; MC, male catkins.