Hierarchical clustering of expression profiles of Populus CCCH genes in different tissues. The expression data were gene-wise normalized and hierarchical clustered based on Pearson correlation. Color scale at the top of each dendrogram represents log2 expression values. The genes highlighted in color (blue and red) were selected for validation by RT-qPCR. Rt, roots; ML, mature leaves; YL, young leaves; FC, female catkins; MC, male catkins; XL, differentiating xylems. IN2, IN3, IN4, IN5 and IN9 represent stem internode 2 to 9 (plastochron index) of an actively growing young tree under field conditions. Among them, the IN3 stem contains well developed primary vascular tissues. IN9 contains well developed secondary xylems.