Populus ERF85 mediates the transition between xylem cell expansion and secondary cell wall formation in hybrid aspen

Secondary growth relies on precise and specialized transcriptional networks that determine cell division, differentiation and maturation of xylem cells. Here we identify a novel role for the ethylene response factor PtERF85 (Potri.015G023200) as a transition regulator between xylem expansion and secondary cell wall (SCW) formation in hybrid aspen (Populus tremula x tremuloides). Transcript profiling across developing xylem and promoter activity localization showed that PtERF85 expression is high in phloem cells, peaks in the cambium and during expansion of xylem cells, while its low expression in maturing xylem tissue was restricted to ray cells neighboring vessel elements. Extending PtERF85 expression into SCW forming zones through ectopic overexpression, reduced wood density and SCW thickness, but increased cell sizes. Large-scale transcriptome analysis in xylem tissue from PtERF85 overexpressing transgenic trees revealed transcriptional induction of cell expansion-related gene networks, including genes involved in protein translation and cell growth. Genes induced upon overexpression of PtERF85 followed the same stem expression profile as PtERF85, further supporting their importance during xylem expansion. Expression of genes associated with plant vascular development and biosynthesis of SCW chemical components such as xylan and lignin, was suppressed in transgenic trees. Our results suggest that PtERF85 activates gene networks relevant for xylem expansion, while preventing transcriptional activation of gene networks related to SCW formation. The importance of precise spatial expression of PtERF85 during xylogenesis indicates that PtERF85 functions as a switch between developmental phases during xylogenesis.