and functional redundancy. These duplications also contributed to the evolution of
novel functions, such as adaptation to stress, disease resistance and improvements
in traits of agronomic interest (grain quality and flowering time) (Panchy et al. 2016).
The remarkable amplification of the NAC family across the plant kingdom reflects
their numerous functions, including plant development, hormone signalling, grain
nutrient remobilization, senescence, cell wall biosynthesis and biotic and abiotic
stress responses. It is difficult to elucidate the regulatory role of these TFs since their
response is mediated by interactions with other TFs or other regulatory proteins and
because they show a complex regulation mediated by alternative splicing, miRNA and
post-translational modifications such as phosphorylation and ubiquitination.
NAC proteins have a modular arrangement composed of a conserved N-terminal
DNA-binding domain (DBD), known as the NAC domain, which is responsible for
their oligomerization into dimeric proteins, and of a C-terminal domain (CTD) that
is intrinsically disordered and possesses transcriptional regulatory activity. However,
some variations from this common structure have been described (Puranik et al. 2012).
Several NAC proteins from Arabidopsis have been reported to be involved in
different processes of plant biology; however, the studies performed in forest trees
are much more limited, and the functions of most of these TF remain unknown.
Recent advances in the acquisition of data from different woody plants allow us to
have more information about the putative role of these TFs in trees. This chapter
reviews the structural features, evolution and regulatory role of NAC proteins in key
biological processes in trees, such as the response to biotic and abiotic stresses,
xylogenesis and wood formation.
2 Common Structure of NAC Proteins
2.1 The Conserved N-Terminal NAC Domain
NAC TFs are modular proteins that are mainly formed by one highly conserved
DNA-binding domain (named the NAC domain) in the N-terminal of the protein,
which is responsible for binding to cis-elements of their target genes, and by a highly
divergent transactivation domain placed in the C-terminal region. The NAC domain
consists of approximately 160 amino acid residues and is divided into five subdomains
(A–E), which have been implicated in nuclear localization and the formation of homodimers or heterodimers with other NAC domain proteins (Olsen et al. 2005) (Fig. 2).
Subdomains A, C and D are usually highly conserved in most N-terminal NAC
domains, while subdomains B and E are highly divergent and might confer functional
diversity to NAC proteins (Ernst et al. 2004; Jensen et al. 2008). However, some
NAC proteins have recently been reported that do not share this conserved feature.
In Brachypodium distachyon, one NAC protein lacks the conserved A and B subdomains, and three members of the family do not contain the conserved C, D and E
subdomains (You et al. 2015). P. pinaster NAC proteins belonging to Group II contain
only subdomains A and D and lack subdomains B, C and E; these are replaced by
NAC Transcription Factors in Woody Plants
199
novel functions, such as adaptation to stress, disease resistance and improvements
in traits of agronomic interest (grain quality and flowering time) (Panchy et al. 2016).
The remarkable amplification of the NAC family across the plant kingdom reflects
their numerous functions, including plant development, hormone signalling, grain
nutrient remobilization, senescence, cell wall biosynthesis and biotic and abiotic
stress responses. It is difficult to elucidate the regulatory role of these TFs since their
response is mediated by interactions with other TFs or other regulatory proteins and
because they show a complex regulation mediated by alternative splicing, miRNA and
post-translational modifications such as phosphorylation and ubiquitination.
NAC proteins have a modular arrangement composed of a conserved N-terminal
DNA-binding domain (DBD), known as the NAC domain, which is responsible for
their oligomerization into dimeric proteins, and of a C-terminal domain (CTD) that
is intrinsically disordered and possesses transcriptional regulatory activity. However,
some variations from this common structure have been described (Puranik et al. 2012).
Several NAC proteins from Arabidopsis have been reported to be involved in
different processes of plant biology; however, the studies performed in forest trees
are much more limited, and the functions of most of these TF remain unknown.
Recent advances in the acquisition of data from different woody plants allow us to
have more information about the putative role of these TFs in trees. This chapter
reviews the structural features, evolution and regulatory role of NAC proteins in key
biological processes in trees, such as the response to biotic and abiotic stresses,
xylogenesis and wood formation.
2 Common Structure of NAC Proteins
2.1 The Conserved N-Terminal NAC Domain
NAC TFs are modular proteins that are mainly formed by one highly conserved
DNA-binding domain (named the NAC domain) in the N-terminal of the protein,
which is responsible for binding to cis-elements of their target genes, and by a highly
divergent transactivation domain placed in the C-terminal region. The NAC domain
consists of approximately 160 amino acid residues and is divided into five subdomains
(A–E), which have been implicated in nuclear localization and the formation of homodimers or heterodimers with other NAC domain proteins (Olsen et al. 2005) (Fig. 2).
Subdomains A, C and D are usually highly conserved in most N-terminal NAC
domains, while subdomains B and E are highly divergent and might confer functional
diversity to NAC proteins (Ernst et al. 2004; Jensen et al. 2008). However, some
NAC proteins have recently been reported that do not share this conserved feature.
In Brachypodium distachyon, one NAC protein lacks the conserved A and B subdomains, and three members of the family do not contain the conserved C, D and E
subdomains (You et al. 2015). P. pinaster NAC proteins belonging to Group II contain
only subdomains A and D and lack subdomains B, C and E; these are replaced by
NAC Transcription Factors in Woody Plants
199