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Background  NAC domain transcription factors initiate secondary cell wall biosynthesis in  Arabidopsis  fibres and vessels by activating numerous transcriptional regulators and biosynthetic genes. NAC family member  SND2  is an indirect target of a principal regulator of fibre secondary cell wall formation, SND1. A previous study showed that overexpression of  SND2  produced a fibre cell-specific increase in secondary cell wall thickness in  Arabidopsis  stems, and that the protein was able to transactivate the  cellulose synthase8  ( CesA8 ) promoter. However, the full repertoire of genes regulated by  SND2  is unknown, and the effect of its overexpression on cell wall chemistry remains unexplored.    Results  We overexpressed  SND2  in  Arabidopsis  and analyzed homozygous lines with regards to stem chemistry, biomass and fibre secondary cell wall thickness. A line showing upregulation of  CesA8  was selected for transcriptome-wide gene expression profiling. We found evidence for upregulation of biosynthetic genes associated with cellulose, xylan, mannan and lignin polymerization in this line, in agreement with significant co-expression of these genes with native  SND2  transcripts according to public microarray repositories. Only minor alterations in cell wall chemistry were detected. Transcription factor  MYB103 , in addition to  SND1 , was upregulated in  SND2 -overexpressing plants, and we detected upregulation of genes encoding components of a signal transduction machinery recently proposed to initiate secondary cell wall formation. Several homozygous T4 and hemizygous T1 transgenic lines with pronounced  SND2  overexpression levels revealed a negative impact on fibre wall deposition, which may be indirectly attributable to excessive overexpression rather than co-suppression. Conversely, overexpression of  SND2  in  Eucalyptus  stems led to increased fibre cross-sectional cell area.    Conclusions  This study supports a function for  SND2  in the regulation of cellulose and hemicellulose biosynthetic genes in addition of those involved in lignin polymerization and signalling. SND2 seems to occupy a subordinate but central tier in the secondary cell wall transcriptional network. Our results reveal phenotypic differences in the effect of  SND2  overexpression between woody and herbaceous stems and emphasize the importance of expression thresholds in transcription factor studies.

bmc plant biology

SND2, a NAC transcription factor gene, regulates genes involved in secondary cell wall development in Arabidopsis fibres and increases fibre cell area in Eucalyptus