Abstract
Secondary cell wall (SCW) deposition in Arabidopsis is regulated among others by NAC transcription factors, where SND1 chiefly initiates xylem fibre differentiation while VND6 controls metaxylem vessel SCW development, especially programmed cell death and wall patterning. The translational relevance of Arabidopsis SCW regulation theory and the utility of characterized transcription factors as modular synthetic biology tools for improving commercial fibre crops is unclear. We investigated inter-lineage gene activation dynamics for potential fibre and vessel differentiation regulators from the widely grown hardwood Eucalyptus grandis (Myrtales). EgrNAC26, a VND6 homolog, and EgrNAC61, an SND1 homolog, were transiently expressed in Arabidopsis mesophyll protoplasts in parallel to determine early and late (i.e. 7 and 14 hours post-transfection) gene targets. Surprisingly, across the time series EgrNAC26 activated only a subset of SCW-related transcription factors and biosynthetic genes activated by EgrNAC61, specializing instead in targeting vessel-specific wall pit and programmed cell death markers. Promoters of EgrNAC26 and EgrNAC61 both induced reporter gene expression in vessels of young Arabidopsis plants, with EgrNAC61 also conferring xylem- and cork cambium-preferential expression in Populus. Our results demonstrate partial conservation, with notable exceptions, of SND1 and VND6 homologs in Eucalyptus and a first report of cork cambium expression for EgrNAC61.
Highlights
Secondary cell walls (SCWs) equip plants with pathogen resistance, mechanical support and the ability to effectively transport water from the roots to the aerial organs[1,2,3]
Since EgrNAC26 and EgrNAC61 are likely master regulators of SCW deposition in E. grandis based on homology, we tested the ability of these proteins to activate transcription of the His reporter gene in two strains of S. cerevisiae
Supported by the apparently stronger transactivation ability of EgrNAC61 in yeast, our results demonstrate that EgrNAC61 is a more potent transcriptional activator of the Arabidopsis SCW pathway compared to EgrNAC26, while the latter shows a similar trend for programmed cell death (PCD)-associated genes in particular
Summary
Secondary cell walls (SCWs) equip plants with pathogen resistance, mechanical support and the ability to effectively transport water from the roots to the aerial organs[1,2,3]. VND6 is a key regulator of the reticulated and pitted wall patterning observed in secondary xylem vessels, the deposition of which is determined by the bundled microtubule structure of the cytoskeleton[25,26] In woody angiosperms such as Populus the expression of NST and VND clade homologs appear to overlap somewhat, with both the SND1 homolog PtrWND1B (a Populus trichocarpa wood-associated NAC domain protein) and the VND7 homolog PtrWND6A being expressed in xylem and phloem fibres, while vessel-specific differentiation appears to be regulated by the exclusive expression of PtrWND6A in vessels[27,28]. In previous work[62], we identified putative E. grandis orthologs of SND1 (EgrNAC61, orthologous to EgWND1) and VND6 (EgrNAC26) which we hypothesised regulate fibre and vessel SCW development, respectively
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