An AP2/ERF transcription factor ERF139 coordinates xylem cell expansion and secondary cell wall deposition

Summary Differentiation of xylem elements involves cell expansion, secondary cell wall ( SCW ) deposition and programmed cell death. Transitions between these phases require strict spatiotemporal control. The function of Populus ERF 139 ( Potri.013G101100 ) in xylem differentiation was characterized in transgenic overexpression and dominant repressor lines of ERF 139 in hybrid aspen ( Populus tremula × tremuloides ). Xylem properties, SCW chemistry and downstream targets were analyzed in both types of transgenic trees using microscopy techniques, Fourier transform‐infrared spectroscopy, pyrolysis‐ GC / MS , wet chemistry methods and RNA sequencing. Opposite phenotypes were observed in the secondary xylem vessel sizes and SCW chemistry in the two different types of transgenic trees, supporting the function of ERF 139 in suppressing the radial expansion of vessel elements and stimulating accumulation of guaiacyl‐type lignin and possibly also xylan. Comparative transcriptomics identified genes related to SCW biosynthesis ( LAC 5, LBD 15, MYB 86) and salt and drought stress‐responsive genes ( ANAC 002, ABA 1) as potential direct targets of ERF 139. The phenotypes of the transgenic trees and the stem expression profiles of ERF 139 potential target genes support the role of ERF 139 as a transcriptional regulator of xylem cell expansion and SCW formation, possibly in response to osmotic changes of the cells.