Expression of only a subset of ERFs was altered in response to aminocyclopropane-1-carboxylic acid (ACC) and ethylene treatment (Vahala et al., 2013 Felten et al., 2018). Conversely, though, an inhibitory effect on lignin deposition was observed by heterologous overexpression of the Populus homolog of SHINE2 in tobacco (Liu et al., 2017), calling for a deeper and wider analysis of this large TF family in lignification.Ī genome-wide screen in Populus trichocarpa identified 170 gene models encoding ERFs (Vahala et al., 2013). Another AP2/ERF stimulated lignification of loquat ( Eriobotrea japonica) fruits through interaction with MYB TFs (Zeng et al., 2015) and lignan biosynthesis in Isatis indigotica (Ma et al., 2017). Heterologous overexpression of the Arabidopsis AP2/ERF SHINE in rice resulted in a 45% reduction in lignin content compared with the wild-type (WT) (Ambavaram et al., 2011). Evidence is accumulating, however, that AP2/ERF TFs may play a role in lignin biosynthesis (Ambavaram et al., 2011 Vahala et al., 2013 Taylor-Teeples et al., 2015 Lee et al., 2016). The APETALA2/ethylene response factors (AP2/ERFs) are TFs that have been frequently implicated in regulation of xylem differentiation and wood formation but that are functionally poorly described (Seyfferth et al., 2018). Less is known about the upstream regulators of these key TFs, even though gene regulatory network analyses suggested that the cell cycle regulator E2Fc is a putative direct regulator of vessel differentiation through VND6 and 7 (Taylor-Teeples et al., 2015), whereas LOB DOMAIN-CONTAINING PROTEIN15 (LBD15) was identified as a positive regulator of VND7 (Ohashi-Ito et al., 2018). The direct targets of the NAC domain TFs include SCW biosynthetic, cell wall-modifying, and cell death genes, as well as TFs such as MYBs (Chen et al., 2019). The Arabidopsis NAC domain transcription factors (TFs) VASCULAR-RELATED NAC-DOMAIN6 (VND6) and VND7 regulate xylem vessel differentiation, while NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) have been described as regulators of the transcriptional network underlying SCW deposition in fibers (for a recent review, see Ohtani et al., 2017). SCW biosynthesis is orchestrated by evolutionarily conserved transcriptional mechanisms. The SCWs consist of three major polymers – cellulose, hemicelluloses and lignin – which together maintain the overall structure and strength of the plant (Gorshkova et al., 2015). Wood, or secondary xylem, formation is a developmental program which begins with cell division in the vascular cambium, followed by expansion, secondary cell wall (SCW) deposition and ultimately programmed cell death of the cambial derivatives (Lucas et al., 2013 Smet & De Rybel, 2016). The phenotypes of the transgenic trees and the stem expression profiles of ERF139 potential target genes support the role of ERF139 as a transcriptional regulator of xylem cell expansion and SCW formation, possibly in response to osmotic changes of the cells.Comparative transcriptomics identified genes related to SCW biosynthesis (LAC5, LBD15, MYB86) and salt and drought stress-responsive genes (ANAC002, ABA1) as potential direct targets of ERF139. 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 ERF139 in suppressing the radial expansion of vessel elements and stimulating accumulation of guaiacyl-type lignin and possibly also xylan.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.
The function of Populus ERF139 ( Potri.013G101100) in xylem differentiation was characterized in transgenic overexpression and dominant repressor lines of ERF139 in hybrid aspen ( Populus tremula × tremuloides).Transitions between these phases require strict spatiotemporal control. Differentiation of xylem elements involves cell expansion, secondary cell wall (SCW) deposition and programmed cell death.
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