microRNA164, while the CUC3 genes do not, but this classification is not maintained
in gymnosperms (Larsson et al. 2012). In addition, the miR164 recognition site has not
been found in any NAC gene in P. patens or S. moellendorffii (Axtell et al. 2007),
suggesting that the miR164-mediated post-transcriptional regulation of the CUC genes
was present in a common ancestor of angiosperms and that a duplication event generated two clades in the angiosperm lineage after its divergence from gymnosperms.
The characterization of P. abies PaNAC01 and PaNAC02 showed that PaNAC01
is a functional ortholog of CUC1 and CUC2 in Arabidopsis, thus suggesting an
evolutionary conservation of a regulatory pathway during somatic embryo development (Larsson et al. 2012). Another Arabidopsis NAC gene of this family, NAC1,
and its orthologs in the woody plant Callerya speciosa play an essential role in the
auxin-induced development of lateral roots (Xie et al. 2000; Xu et al. 2016).
5.3 NAC Proteins Involved in Wood Formation
Although the number of NAC TFs characterized in trees is very low, most research
has focused on the NAC proteins involved in secondary cell wall biosynthesis, wood
formation and processes related to improve plant biomass production (Zhong and Ye
2010; Ohtani et al. 2011; Pascual et al. 2017). Woody plant species irreversibly
immobilize large quantities of carbon skeletons in wood during their long life cycles.
Wood is the secondary xylem of vascular plants and is composed of lignin, cellulose
and hemicellulose (Sjostrom 1993), and its properties and composition vary widely
between angiosperms and gymnosperms (Pascual et al. 2016). The secondary xylem
of gymnosperms is composed of single cells known as tracheids that serve as both
water transport and mechanical support, while angiosperm wood is composed
mainly of vessels that are conduits formed of many dead cells to conduct water
and xylem fibres that provide structural support. Few studies on secondary cell wall
biosynthesis and tracheid development have been carried out in conifers (JokipiiLukkari et al. 2017), which limits our current knowledge on the evolutionary origin
and the structural and functional diversification of the angiosperm vessels with
respect to the gymnosperm tracheids or vessel-like water-conducting cells present
in primitive vascular plants (Wan et al. 2018).
The subfamily of NAC genes involved in vascular development is formed by
three types of proteins: (1) VASCULAR-RELATED NAC DOMAIN (VND) proteins, whose expression is associated with developing vascular tissues and might
regulate vessels formation. (2) NAC SECONDARY WALL THICKENING PROMOTING FACTOR 1 (NST1) and NST2/SECONDARY WALL-ASSOCIATED
NAC DOMAIN PROTEIN1 (SND1) working as regulators for fibre cell differentiation (Zhong et al. 2006; Mitsuda et al. 2007). (3) SOMBRERO (SMB), BEARSKINI1
(BRN1) and BRN2 proteins that are expressed in the root cap region, and their
overexpression causes ectopic secondary cell wall deposition (Willemsen et al. 2008;
Bennett et al. 2010).
NAC Transcription Factors in Woody Plants
209
in gymnosperms (Larsson et al. 2012). In addition, the miR164 recognition site has not
been found in any NAC gene in P. patens or S. moellendorffii (Axtell et al. 2007),
suggesting that the miR164-mediated post-transcriptional regulation of the CUC genes
was present in a common ancestor of angiosperms and that a duplication event generated two clades in the angiosperm lineage after its divergence from gymnosperms.
The characterization of P. abies PaNAC01 and PaNAC02 showed that PaNAC01
is a functional ortholog of CUC1 and CUC2 in Arabidopsis, thus suggesting an
evolutionary conservation of a regulatory pathway during somatic embryo development (Larsson et al. 2012). Another Arabidopsis NAC gene of this family, NAC1,
and its orthologs in the woody plant Callerya speciosa play an essential role in the
auxin-induced development of lateral roots (Xie et al. 2000; Xu et al. 2016).
5.3 NAC Proteins Involved in Wood Formation
Although the number of NAC TFs characterized in trees is very low, most research
has focused on the NAC proteins involved in secondary cell wall biosynthesis, wood
formation and processes related to improve plant biomass production (Zhong and Ye
2010; Ohtani et al. 2011; Pascual et al. 2017). Woody plant species irreversibly
immobilize large quantities of carbon skeletons in wood during their long life cycles.
Wood is the secondary xylem of vascular plants and is composed of lignin, cellulose
and hemicellulose (Sjostrom 1993), and its properties and composition vary widely
between angiosperms and gymnosperms (Pascual et al. 2016). The secondary xylem
of gymnosperms is composed of single cells known as tracheids that serve as both
water transport and mechanical support, while angiosperm wood is composed
mainly of vessels that are conduits formed of many dead cells to conduct water
and xylem fibres that provide structural support. Few studies on secondary cell wall
biosynthesis and tracheid development have been carried out in conifers (JokipiiLukkari et al. 2017), which limits our current knowledge on the evolutionary origin
and the structural and functional diversification of the angiosperm vessels with
respect to the gymnosperm tracheids or vessel-like water-conducting cells present
in primitive vascular plants (Wan et al. 2018).
The subfamily of NAC genes involved in vascular development is formed by
three types of proteins: (1) VASCULAR-RELATED NAC DOMAIN (VND) proteins, whose expression is associated with developing vascular tissues and might
regulate vessels formation. (2) NAC SECONDARY WALL THICKENING PROMOTING FACTOR 1 (NST1) and NST2/SECONDARY WALL-ASSOCIATED
NAC DOMAIN PROTEIN1 (SND1) working as regulators for fibre cell differentiation (Zhong et al. 2006; Mitsuda et al. 2007). (3) SOMBRERO (SMB), BEARSKINI1
(BRN1) and BRN2 proteins that are expressed in the root cap region, and their
overexpression causes ectopic secondary cell wall deposition (Willemsen et al. 2008;
Bennett et al. 2010).
NAC Transcription Factors in Woody Plants
209