In the analysis of the NAC TF family in P. trichocarpa, Arabidopsis and rice
(Hu et al. 2010), an additional subfamily (NAC-j) was identified that contained only
proteins from Populus, suggesting that they may have been acquired in this species
and they could have specialized roles related to the poplar perennial habitat. This
analysis also revealed a subfamily containing only sequences from poplar and
Arabidopsis, suggesting that these NAC genes were likely acquired or differentially
retained in the post-divergence of dicots from monocots. When E. grandis and
Carica papaya sequences were included in the phylogenetic analysis, a subfamily
of NAC genes that contained only P. trichocarpa, E. grandis and C. papaya members was observed (Zhu et al. 2012; Hussey et al. 2015). This suggests that this subfamily may represent tree-specific NAC proteins involved in regulatory mechanisms
controlling specialized aspects of wood formation (Zhu et al. 2012; Hussey et al. 2015).
In summary, the number of subfamilies proposed for the NAC TF family in
different plants is highly variable (Ooka et al. 2003; Shen et al. 2009; Jensen et al.
2010; Zhu et al. 2012; Hussey et al. 2015), and additional subfamilies will likely be
described in the future with increased number of plant genomes sequenced and
analysed. However, most NAC subfamilies have representatives in the genomes
of angiosperms, with some subfamilies restricted to tracheophytes, monocots, dicots
or, exceptionally, the TNACS subfamily that appears restricted to the Solanaceae
(Rushton et al. 2008; Singh et al. 2013).
3.2 Evolutionary Aspects of the NAC Family
All of the NAC TFs characterized so far belong to land plants (Zhu et al. 2012;
Nagata et al. 2015; Pascual et al. 2015). These TFs are involved in many regulatory
processes associated with vegetative and reproductive plant developmental programs (Larsson et al. 2012; Maugarny et al. 2015) or with senescence and biotic
and abiotic stress responses (Puranik et al. 2012; Pascual et al. 2015). NAC proteins
also regulate the formation and differentiation of the xylem and phloem, specialized
tissues for transporting water and organic substances that are essential for land plant
growth. For this reason and the lack of identification of NAC proteins within the
Chlorophyta, the appearance of NAC TFs has always been associated with the emergence of land plants. However, through an analysis of transcriptomic data generated by
the 1,000 Plants projects (www.onekp.com, Wickett et al. 2014) using data from species
ranging from monilophytes to chlorophytes, putative homologous NAC TFs have been
identified in some groups of streptophyte green algae, such as Zygnematophyceae and
Coleochaetales. This analysis also identified NAC-like proteins in organisms belonging
to Klebsormidiales and proteins containing a partial predicted NAC domain in Nitella
mirabilis, a green algae of the Characeae family. However, no sequences corresponding
to NAC proteins were identified in Chlorophyta (Maugarny-Calès et al. 2016).
Zygnematophyceae is the largest group within the streptophyte green algae, and it
includes some species that can grow in terrestrial environments (Ettl and Gärtner 2014).
The NAC proteins identified in Zygnematophyceae are proteins with a NAC domain
NAC Transcription Factors in Woody Plants
203
(Hu et al. 2010), an additional subfamily (NAC-j) was identified that contained only
proteins from Populus, suggesting that they may have been acquired in this species
and they could have specialized roles related to the poplar perennial habitat. This
analysis also revealed a subfamily containing only sequences from poplar and
Arabidopsis, suggesting that these NAC genes were likely acquired or differentially
retained in the post-divergence of dicots from monocots. When E. grandis and
Carica papaya sequences were included in the phylogenetic analysis, a subfamily
of NAC genes that contained only P. trichocarpa, E. grandis and C. papaya members was observed (Zhu et al. 2012; Hussey et al. 2015). This suggests that this subfamily may represent tree-specific NAC proteins involved in regulatory mechanisms
controlling specialized aspects of wood formation (Zhu et al. 2012; Hussey et al. 2015).
In summary, the number of subfamilies proposed for the NAC TF family in
different plants is highly variable (Ooka et al. 2003; Shen et al. 2009; Jensen et al.
2010; Zhu et al. 2012; Hussey et al. 2015), and additional subfamilies will likely be
described in the future with increased number of plant genomes sequenced and
analysed. However, most NAC subfamilies have representatives in the genomes
of angiosperms, with some subfamilies restricted to tracheophytes, monocots, dicots
or, exceptionally, the TNACS subfamily that appears restricted to the Solanaceae
(Rushton et al. 2008; Singh et al. 2013).
3.2 Evolutionary Aspects of the NAC Family
All of the NAC TFs characterized so far belong to land plants (Zhu et al. 2012;
Nagata et al. 2015; Pascual et al. 2015). These TFs are involved in many regulatory
processes associated with vegetative and reproductive plant developmental programs (Larsson et al. 2012; Maugarny et al. 2015) or with senescence and biotic
and abiotic stress responses (Puranik et al. 2012; Pascual et al. 2015). NAC proteins
also regulate the formation and differentiation of the xylem and phloem, specialized
tissues for transporting water and organic substances that are essential for land plant
growth. For this reason and the lack of identification of NAC proteins within the
Chlorophyta, the appearance of NAC TFs has always been associated with the emergence of land plants. However, through an analysis of transcriptomic data generated by
the 1,000 Plants projects (www.onekp.com, Wickett et al. 2014) using data from species
ranging from monilophytes to chlorophytes, putative homologous NAC TFs have been
identified in some groups of streptophyte green algae, such as Zygnematophyceae and
Coleochaetales. This analysis also identified NAC-like proteins in organisms belonging
to Klebsormidiales and proteins containing a partial predicted NAC domain in Nitella
mirabilis, a green algae of the Characeae family. However, no sequences corresponding
to NAC proteins were identified in Chlorophyta (Maugarny-Calès et al. 2016).
Zygnematophyceae is the largest group within the streptophyte green algae, and it
includes some species that can grow in terrestrial environments (Ettl and Gärtner 2014).
The NAC proteins identified in Zygnematophyceae are proteins with a NAC domain
NAC Transcription Factors in Woody Plants
203