An AP/ERF transcription factor ERF139 coordinates xylem expansion and lignin deposition

Wood formation is a result of massive deposition of secondary cell wall material, a process tightly regulated by the complex interplay of transcription factors. APETALA2/ETHYLENE RESPONSE FACTORs (AP/ERFs) are transcription factors that are often implicated in signaling of ethylene but that have not been investigated in wood formation even though ethylene is clearly important in this process. The function of Populus trichocarpa ERF139 (Potri.013G101100) in wood formation was characterised in overexpression and dominant repression lines of ERF139 created by fusion of ERF139 with the EAR repression domain (SRDX). Pyrolysis-GC/MS analysis revealed opposite phenotypes in the two different types of transgenic trees, supporting the role of ERF139 in altering the flux of lignin biosynthesis towards the guaiacyl (G) type lignin and suppressing it towards the p-hydroxy phenyl (H) type lignin. The results further supported the role of ERF139 in reducing the growth rate of the trees and suppressing the radial expansion of the secondary xylem vessel elements. RNA-Seq based comparative transcriptomics identified genes homologous to master regulators of secondary cell wall deposition (LBD15, ANAC2, MYB86) and genes related to lignin biochemistry (LACCASE5 and UDP-glycosyltransferase 72B1) as potential direct targets of ERF139. These results suggest that ERF139 is a master regulator of transcription factors that form part of regulatory cascades related to wood formation and tree growth.