Sequence alignment of phosphorylation sites and extraction of significantly enriched phosphorylation motifs. (a) Amino acid sequence around the phosphorylated amino acid based on alignment of all phosphorylation sites from the identified DTBs phosphopeptide dataset using Weblogo. (b) Motif-X-extracted motifs from entire phosphopeptide dataset. JGI Populus trichocarpa v1.1 protein database was used as the background database to normalize the score against a random distribution of amino acids. Note that only those phosphorylated amino acids that were confidently identified as the exact site of phosphorylation were used for the analysis (see "Materials and Methods" for detailed description). Motif 1, Pro-directed kinase motif (n = 40); Motif 2, Unknown phosphorylation motifs (n = 20); Motif 3, CKII motif (n = 17); Motif 4, 14-3-3 binding motif (n = 13).