produced plants with enhanced cold and drought tolerance. This study shows that
PbeNAC1 is involved in the cold and drought signalling network, in which NAC
TFs regulate the expression of stress-responsive genes, probably by interacting
with C-repeat binding factor (CBF) genes (Jin et al. 2017a).
Several NAC genes are also upregulated by phytohormones, such as jasmonic acid
(JA), salicylic acid (SA) and ethylene (ET), which play key roles in plant defence
responses to different pathogens, insect feeding and wounding (Nuruzzaman et al.
2013). ATAF1 induces JA-/ET-associated defence signalling genes (Jensen et al.
2008), while ATAF2 controls responses to JA and SA (Delessert et al. 2005). In the
conifer P. pinaster, PpNAC2 and PpNAC3 encode stress-responsive NAC TFs that
are involved in the jasmonate response. Both genes were strongly induced by mechanical wounding, high salinity and methyl jasmonate (MeJA), and the promoter region of
these genes contains, among others, several E-boxes, which are cis-elements commonly found in MeJA-responsive promoters, and binding sites for bHLH TFs. This
study suggests that the jasmonate signalling pathway might be conserved between
angiosperms and gymnosperms (Pascual et al. 2015).
Although a single NAC gene may be mainly associated with a type of stress
response, these NAC TFs do not act alone. Recent research has shown that NAC
proteins interact with other TFs and proteins to exert their biological functions
through complex regulatory networks in plants (Jensen and Skriver 2014; Barah
et al. 2016). For this reason, it is essential to identify the key components of the
signalling networks in which these factors interact with the objective of generating
plants with improved yield and stress tolerance.
5.2 NAC Proteins Involved in Developmental Processes
The first NAC genes identified in a plant belong to the NAM class; the plant was
a petunia mutant that failed to develop the shoot apical meristem (SAM) (Souer et al.
1996). The CUC genes in double mutants of A. thaliana showed a similar phenotype
with no apical meristem and fused cotyledons (Aida et al. 1997). Three CUC genes
have been identified and characterized in Arabidopsis, and they act together with the
class-1 KNOX gene SHOOT MERISTEMLESS (STM) to establish the embryonal
SAM and promote the formation of two separated cotyledons, which is critical for
proper leaf and flower patterning (Aida et al. 1999; Scofield et al. 2013).
Phylogenetic analyses from different species have found the NAM/CUC proteins in the same subfamily, suggesting a functional similarity. Orthologs of these
NAC class genes have been identified in Zea mays (Zimmermann and Werr 2005),
Solanum lycopersicum (Berger et al. 2009), Medicago truncatula (Cheng et al. 2012),
Antirrhinum majus (Weir et al. 2004), P. pinaster (Pascual et al. 2015) and P. abies
(Larsson et al. 2012). Based on the sequence of the NAC domain, the proteins that
belong to this group can be divided into two clades: the NAM clade, which includes
the petunia NAM and Arabidopsis CUC1 and CUC2 proteins, and the CUC3 clade.
In angiosperms, the NAM/CUC1/CUC2 genes contain a binding site for the
208
M. B. Pascual et al.
PbeNAC1 is involved in the cold and drought signalling network, in which NAC
TFs regulate the expression of stress-responsive genes, probably by interacting
with C-repeat binding factor (CBF) genes (Jin et al. 2017a).
Several NAC genes are also upregulated by phytohormones, such as jasmonic acid
(JA), salicylic acid (SA) and ethylene (ET), which play key roles in plant defence
responses to different pathogens, insect feeding and wounding (Nuruzzaman et al.
2013). ATAF1 induces JA-/ET-associated defence signalling genes (Jensen et al.
2008), while ATAF2 controls responses to JA and SA (Delessert et al. 2005). In the
conifer P. pinaster, PpNAC2 and PpNAC3 encode stress-responsive NAC TFs that
are involved in the jasmonate response. Both genes were strongly induced by mechanical wounding, high salinity and methyl jasmonate (MeJA), and the promoter region of
these genes contains, among others, several E-boxes, which are cis-elements commonly found in MeJA-responsive promoters, and binding sites for bHLH TFs. This
study suggests that the jasmonate signalling pathway might be conserved between
angiosperms and gymnosperms (Pascual et al. 2015).
Although a single NAC gene may be mainly associated with a type of stress
response, these NAC TFs do not act alone. Recent research has shown that NAC
proteins interact with other TFs and proteins to exert their biological functions
through complex regulatory networks in plants (Jensen and Skriver 2014; Barah
et al. 2016). For this reason, it is essential to identify the key components of the
signalling networks in which these factors interact with the objective of generating
plants with improved yield and stress tolerance.
5.2 NAC Proteins Involved in Developmental Processes
The first NAC genes identified in a plant belong to the NAM class; the plant was
a petunia mutant that failed to develop the shoot apical meristem (SAM) (Souer et al.
1996). The CUC genes in double mutants of A. thaliana showed a similar phenotype
with no apical meristem and fused cotyledons (Aida et al. 1997). Three CUC genes
have been identified and characterized in Arabidopsis, and they act together with the
class-1 KNOX gene SHOOT MERISTEMLESS (STM) to establish the embryonal
SAM and promote the formation of two separated cotyledons, which is critical for
proper leaf and flower patterning (Aida et al. 1999; Scofield et al. 2013).
Phylogenetic analyses from different species have found the NAM/CUC proteins in the same subfamily, suggesting a functional similarity. Orthologs of these
NAC class genes have been identified in Zea mays (Zimmermann and Werr 2005),
Solanum lycopersicum (Berger et al. 2009), Medicago truncatula (Cheng et al. 2012),
Antirrhinum majus (Weir et al. 2004), P. pinaster (Pascual et al. 2015) and P. abies
(Larsson et al. 2012). Based on the sequence of the NAC domain, the proteins that
belong to this group can be divided into two clades: the NAM clade, which includes
the petunia NAM and Arabidopsis CUC1 and CUC2 proteins, and the CUC3 clade.
In angiosperms, the NAM/CUC1/CUC2 genes contain a binding site for the
208
M. B. Pascual et al.