Abstract:
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 coexpression
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 cosuppression.
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.
