Transcriptional analysis of arogenate dehydratase genes identifies a link between phenylalanine biosynthesis and lignin biosynthesis

Biogenesis of the secondary cell wall in trees involves the massive biosynthesis, of the phenylalanine-derived polymer lignin. Arogenate dehydratase (ADT) catalyzes the last, and rate limiting, step of the main pathway for phenylalanine biosynthesis. In this study, we have found that transcript levels for several members of the large ADT gene family, including ADT-A and ADT-D, were enhanced in compression wood of maritime pine, a xylem tissue enriched in lignin. Transcriptomic analysis of maritime pine silenced for PpMYB8 revealed that it plays a critical role in coordinating the deposition of lignin with the biosynthesis of phenylalanine. Specifically, it was found that ADT-A and ADT-D were strongly downregulated in PpMYB8 silenced plants and that they were transcriptionally regulated through direct interaction of this transcription factor with regulatory elements present in their promoters. Another transcription factor, PpHY5, exhibited an expression profile opposite to that of PpMYB8 and also interacted with specific regulatory elements of ADT-A and ADT-D genes suggesting that it is also involved in transcriptional regulation of phenylalanine biosynthesis. Taken together, the results reveal that PpMYB8 and PpHY5 are involved in the control of phenylalanine formation and its metabolic channeling for lignin biosynthesis and deposition during wood formation in maritime pine.