Conserved CA-rich motifs in gene promoters of Pt×tMYB021-responsive secondary cell wall carbohydrate-active enzymes in Populus

In order to understand gene regulation during wood formation, we cloned a MYB46-like gene in hybrid aspen, Populus tremula × tremuloides, called Pt×tMYB021. Phylogenetic and paired identity analysis of MYB46-like genes in Populus and Arabidopsis reveals relationships between paralogous pairs of Populus MYB46-like proteins and their Arabidopsis counterparts MYB46 and MYB83, and suggest that Pt×tMYB021 is the ortholog of MYB46. Pt×tMYB021 is expressed mainly in xylem tissues, and transiently expressed Pt×tMYB46 transactivates gene promoters of xylan-active CAZymes GT43A, GT43B and Xyn10A. Analysis of conserved motifs within these promoters identify the sequence CCACCAAC, called ACTYP, which is similar to the AC elements mediating transactivation by MYB transcription factors during lignin biosynthesis. Further analysis by Motif Finder identifies four 6 bp CA-rich motifs overlapping ACTYP, and we show that these motifs are enriched in xylem-specific promoters. We propose that AC-type regulatory elements mediate xylem-specific MYB46-dependent expression of secondary cell wall carbohydrate-active enzymes (CAZymes), besides activating gene expression of lignin biosynthesis enzymes.