SummaryA member of the R2R3-MYB family of transcription factors was cloned from a cDNA library constructed from RNA isolated from differentiating pine xylem. This MYB, Pinus taeda MYB4 (PtMYB4), is expressed in cells undergoing ligni®cation, as revealed by in situ RT-PCR. Electrophoretic mobility shift assays (EMSAs) showed that recombinant PtMYB4 protein is able to bind to DNA motifs known as AC elements. AC elements are ubiquitous in the promoters of genes encoding lignin biosynthetic enzymes. Transcriptional activation assays using yeast showed that PtMYB4 could activate transcription in an AC-element-dependent fashion. Overexpression of PtMYB4 in transgenic tobacco plants altered the accumulation of transcripts corresponding to genes encoding lignin biosynthetic enzymes. Lignin deposition increased in transgenic tobacco plants that overexpressed PtMYB4, and extended to cell types that do not normally lignify. Taken together, these ®ndings are consistent with the hypothesis that PtMYB4 is suf®cient to induce ligni®cation, and that it may play this role during wood formation in pine.

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