VND genes were identified in P. pinaster, each of which is classified in a different
subgroup. PpNAC30 clustered into the VND group with P. glauca PgNAC7 and
PtVNS01 and PtVNS04 from P. trichocarpa, putative orthologs of VND6/7 of
Arabidopsis. The expression of PpNAC30 is preferentially associated with vascular
tissue of cotyledons and roots in pine seedlings (Fig. 3). PpNAC31 belongs to the
SMB group, and its expression is particularly high in developing root tissues
(Fig. 3), similarly to the PgNAC4 of white spruce (Duval et al. 2014). PpNAC1 is
the unique NST gene that has been identified in gymnosperms and whose expression
is clearly predominant in tissues undergoing secondary cell wall biosynthesis
such as secondary xylem and compression wood of adult trees (Pascual et al.
2017). It exhibits an expression pattern consistent with the previously reported for
PtrWNDS in Populus and SND1 and NST1 in Arabidopsis (Zhong et al. 2007,
2010b). Its silencing in maritime pine causes several phenotypes associated with
vascular development and a downregulation of the expression of several genes
involved in cell wall biosynthesis and secondary metabolism (Pascual et al. 2017).
The decrease of its expression was particularly important affecting the level of
expression of certain Myb TF that have been associated with lignin biosynthesis
in other conifers, such as Myb1, Myb4 and Myb8 (Patzlaff et al. 2003a, b; Bomal
et al. 2008; Bedon et al. 2010; Craven-Bartle et al. 2013). Furthermore, the overexpression of this TF in Arabidopsis led to plants with small rosette size and curled
leaves, phenotype characteristic of ectopic lignin deposition and increased secondary
wall thickening (Pascual et al. 2017). These plants also showed the upregulation of
secondary wall biosynthesis genes and secondary cell wall-associated TFs, mainly
Myb TFs directly regulated by SND1, such as Myb46 and Myb83 that have been
described as master switches of secondary wall biosynthesis in Arabidopsis (Zhong
et al. 2007).
These results are consistent with previous studies carried out in poplar (Lin et al.
2013; Wang et al. 2014) and eucalyptus and strongly suggest a functional role of
PpNAC1 as an ortholog of the Arabidopsis SND1 (Zhong et al. 2006) or Populus
PtrWND2B and PtrWND6B (Zhong et al. 2010b, 2011a; Ohtani et al. 2011), supporting the idea that a transcriptional network involved in the regulation of xylem
development could be conserved between angiosperms and gymnosperms.
Several members of the Myb and NAC TF families have been described as
strong candidates participating in the transcriptional regulatory network that governs
the biosynthesis of the main components of the secondary cell wall. More than
6,200 genes, including NAC and MYB transcription factors and genes involved in
lignin and cellulose biosynthesis, exhibited differential expression during xylogenesis in Neolamarckia cadamba, a fast-growing tropical woody tree. The expression of these genes, which corresponds to most of the known transcription factors
involved in regulatory networks in A. thaliana, was significantly higher in both the
middle and basal stem compared to the apical stem. The characterization of genes
and transcription factors involved in wood formation in trees has a great potential for
increasing wood biomass production or modifying wood composition for biofuel
production (Ouyang et al. 2016).
NAC Transcription Factors in Woody Plants
211
subgroup. PpNAC30 clustered into the VND group with P. glauca PgNAC7 and
PtVNS01 and PtVNS04 from P. trichocarpa, putative orthologs of VND6/7 of
Arabidopsis. The expression of PpNAC30 is preferentially associated with vascular
tissue of cotyledons and roots in pine seedlings (Fig. 3). PpNAC31 belongs to the
SMB group, and its expression is particularly high in developing root tissues
(Fig. 3), similarly to the PgNAC4 of white spruce (Duval et al. 2014). PpNAC1 is
the unique NST gene that has been identified in gymnosperms and whose expression
is clearly predominant in tissues undergoing secondary cell wall biosynthesis
such as secondary xylem and compression wood of adult trees (Pascual et al.
2017). It exhibits an expression pattern consistent with the previously reported for
PtrWNDS in Populus and SND1 and NST1 in Arabidopsis (Zhong et al. 2007,
2010b). Its silencing in maritime pine causes several phenotypes associated with
vascular development and a downregulation of the expression of several genes
involved in cell wall biosynthesis and secondary metabolism (Pascual et al. 2017).
The decrease of its expression was particularly important affecting the level of
expression of certain Myb TF that have been associated with lignin biosynthesis
in other conifers, such as Myb1, Myb4 and Myb8 (Patzlaff et al. 2003a, b; Bomal
et al. 2008; Bedon et al. 2010; Craven-Bartle et al. 2013). Furthermore, the overexpression of this TF in Arabidopsis led to plants with small rosette size and curled
leaves, phenotype characteristic of ectopic lignin deposition and increased secondary
wall thickening (Pascual et al. 2017). These plants also showed the upregulation of
secondary wall biosynthesis genes and secondary cell wall-associated TFs, mainly
Myb TFs directly regulated by SND1, such as Myb46 and Myb83 that have been
described as master switches of secondary wall biosynthesis in Arabidopsis (Zhong
et al. 2007).
These results are consistent with previous studies carried out in poplar (Lin et al.
2013; Wang et al. 2014) and eucalyptus and strongly suggest a functional role of
PpNAC1 as an ortholog of the Arabidopsis SND1 (Zhong et al. 2006) or Populus
PtrWND2B and PtrWND6B (Zhong et al. 2010b, 2011a; Ohtani et al. 2011), supporting the idea that a transcriptional network involved in the regulation of xylem
development could be conserved between angiosperms and gymnosperms.
Several members of the Myb and NAC TF families have been described as
strong candidates participating in the transcriptional regulatory network that governs
the biosynthesis of the main components of the secondary cell wall. More than
6,200 genes, including NAC and MYB transcription factors and genes involved in
lignin and cellulose biosynthesis, exhibited differential expression during xylogenesis in Neolamarckia cadamba, a fast-growing tropical woody tree. The expression of these genes, which corresponds to most of the known transcription factors
involved in regulatory networks in A. thaliana, was significantly higher in both the
middle and basal stem compared to the apical stem. The characterization of genes
and transcription factors involved in wood formation in trees has a great potential for
increasing wood biomass production or modifying wood composition for biofuel
production (Ouyang et al. 2016).
NAC Transcription Factors in Woody Plants
211