Arabidopsis Myb46 and Myb83 and regulate secondary wall biosynthesis during
wood formation (Zhong et al. 2013). In P. pinaster, it has been shown that PpNAC1
can activate its own expression and regulate PpMyb4 by binding to the SNBE
elements present in their promoter regions, while PpMyb4 can activate PpMyb8, a
transcriptional regulator of lignin biosynthesis in maritime pine (Craven-Bartle et al.
2013; Pascual et al. 2017). In addition, Zhong et al. (2013) reported that PpMyb4 and
its orthologs, PtMyb4 and EgMyb2, could be involved in the regulation of cellulose
and xylan biosynthesis (Zhong et al. 2013). The cellulose and xylan biosynthetic
pathways have been well described in plants (Mizrachi et al. 2012), but little
information about their regulation in trees has been reported. The overexpression
of SND2 in Arabidopsis showed an upregulation of biosynthetic genes associated
with cellulose, xylan, mannan and lignin polymerization but with minor alteration of
the cell wall chemistry. In addition, the effect of this gene on secondary cell wall
thickness in Arabidopsis is unclear and may be dose-dependent (Hussey et al. 2011;
Zhong et al. 2012). However, overexpression of this gene in eucalyptus stems
significantly increased fibre cell area, and the overexpression of Populus NAC154,
a putative ortholog of SND2, resulted also in an increase of bark production
compared with xylem in poplar trees (Grant et al. 2010). These results show
phenotypic differences of SND2 overexpression between woody and herbaceous
plants that could be attributed to a greater channelling of carbon skeletons toward
wood biosynthesis in woody plants (Hussey et al. 2011).
Duval et al. (2014) reported that the white spruce PgNAC8, a potential ortholog
of SND2/SND3 of Arabidopsis, could regulate cellulose biosynthesis in coordination with PgNAC7 (Lamara et al. 2016). In a study of B. platyphylla, it was observed
that BplNAC1 and BplNAC2 genes, which are closely related to SND2 and NST1 of
Arabidopsis, were upregulated by exogenous gibberellin (GA3) application, a key
signal molecule inducing xylem development and the expression of secondary wall
biosynthesis-related genes such as CESA, PAL and GA oxidase (Guo et al. 2015).
In summary, several studies suggest that, despite the evolutionary distance and
different properties and composition of wood between angiosperms and gymnosperms, many transcriptional regulators involved in the transcriptional network
associated with secondary cell wall biosynthesis could be conserved. In angiosperms, secondary wall biosynthesis in vessels is regulated by VNS genes, with
VND7 being a master regulator of vessel formation in Arabidopsis and VND1–6
being a downstream regulator of VND7, while SND1 is a master regulator of xylem
fibre cells. Some authors have related vessel formation in angiosperms to the
presence of VND7 or VND1–VND3 genes because no orthologs of these genes
have been identified in the genome of plants lacking vessels, such as gymnosperms.
This suggests that the ability to form vessels may have occurred after angiosperms diverged and may be associated with their distinctive growth shapes (Wan
et al. 2018).
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M. B. Pascual et al.
wood formation (Zhong et al. 2013). In P. pinaster, it has been shown that PpNAC1
can activate its own expression and regulate PpMyb4 by binding to the SNBE
elements present in their promoter regions, while PpMyb4 can activate PpMyb8, a
transcriptional regulator of lignin biosynthesis in maritime pine (Craven-Bartle et al.
2013; Pascual et al. 2017). In addition, Zhong et al. (2013) reported that PpMyb4 and
its orthologs, PtMyb4 and EgMyb2, could be involved in the regulation of cellulose
and xylan biosynthesis (Zhong et al. 2013). The cellulose and xylan biosynthetic
pathways have been well described in plants (Mizrachi et al. 2012), but little
information about their regulation in trees has been reported. The overexpression
of SND2 in Arabidopsis showed an upregulation of biosynthetic genes associated
with cellulose, xylan, mannan and lignin polymerization but with minor alteration of
the cell wall chemistry. In addition, the effect of this gene on secondary cell wall
thickness in Arabidopsis is unclear and may be dose-dependent (Hussey et al. 2011;
Zhong et al. 2012). However, overexpression of this gene in eucalyptus stems
significantly increased fibre cell area, and the overexpression of Populus NAC154,
a putative ortholog of SND2, resulted also in an increase of bark production
compared with xylem in poplar trees (Grant et al. 2010). These results show
phenotypic differences of SND2 overexpression between woody and herbaceous
plants that could be attributed to a greater channelling of carbon skeletons toward
wood biosynthesis in woody plants (Hussey et al. 2011).
Duval et al. (2014) reported that the white spruce PgNAC8, a potential ortholog
of SND2/SND3 of Arabidopsis, could regulate cellulose biosynthesis in coordination with PgNAC7 (Lamara et al. 2016). In a study of B. platyphylla, it was observed
that BplNAC1 and BplNAC2 genes, which are closely related to SND2 and NST1 of
Arabidopsis, were upregulated by exogenous gibberellin (GA3) application, a key
signal molecule inducing xylem development and the expression of secondary wall
biosynthesis-related genes such as CESA, PAL and GA oxidase (Guo et al. 2015).
In summary, several studies suggest that, despite the evolutionary distance and
different properties and composition of wood between angiosperms and gymnosperms, many transcriptional regulators involved in the transcriptional network
associated with secondary cell wall biosynthesis could be conserved. In angiosperms, secondary wall biosynthesis in vessels is regulated by VNS genes, with
VND7 being a master regulator of vessel formation in Arabidopsis and VND1–6
being a downstream regulator of VND7, while SND1 is a master regulator of xylem
fibre cells. Some authors have related vessel formation in angiosperms to the
presence of VND7 or VND1–VND3 genes because no orthologs of these genes
have been identified in the genome of plants lacking vessels, such as gymnosperms.
This suggests that the ability to form vessels may have occurred after angiosperms diverged and may be associated with their distinctive growth shapes (Wan
et al. 2018).
214
M. B. Pascual et al.