chemical modification. First, NTL6 is proteolytically activated to a mature
nuclear-associated form (Seo et al. 2010), and it is then phosphorylated into a
threonine residue (Thr142) of the NAC domain to facilitate the translocation of
NTL6 into the nucleus to induce a drought-resistant response. The mutation of
this threonine to alanine produced NTL6 proteins that were poorly phosphorylated and failed to move into the nucleus (Kim et al. 2012). The recent results
of Guan et al. (2014) showed that RCF2/CPL1, a positive regulator of heat-stress
responsive gene expression and thermotolerance in Arabidopsis, dephosphorylates
ANAC019 and that both proteins, through protein–protein interactions, induce the
synthesis of chaperones, which are important components of the response to heat
stress (Guan et al. 2014).
5 Outstanding Roles for NAC Proteins
5.1 NAC Proteins Involved in Plant Stress Responses
Increased environmental stresses associated with climate change, particularly high
temperature and prolonged drought, are critical factors that reduce tree growth and
development and, therefore, forest productivity. Accordingly, it is important to understand how forest trees adapt to hostile environmental conditions, so it is necessary to
increase our knowledge of the plant signalling pathways involved in the response to
stress and to identify key regulatory proteins (Harfouche et al. 2014).
Many families of TFs, including DREB, bZIP, MYB, WRKY and NAC, regulate
plant defences and stress responses in herbaceous plants (Seki et al. 2002; Mao et al.
2012; Shen et al. 2012). However, much less information is available about the regulatory proteins involved in the responses of woody plants to environmental stress.
Even less information is available for conifers, which are economically and ecologically important trees in the Northern Hemisphere.
NAC proteins are among the most important families of TFs involved in adaptive
stress response in plants (Wu et al. 2009; Puranik et al. 2012; Pascual et al. 2015). In
addition, one of the putative NAC proteins identified in the group of green algae
Zygnematophyceae presented high similarity with SOG1 of Arabidopsis, a NAC
protein that has been implicated in the DNA damage response in plants (Yoshiyama
2016), suggesting that the evolutionary emergence of these TFs was directly associated with the appearance of novel defence mechanisms against stress.
Most stress-related NAC proteins characterized in Arabidopsis, rice, barley and
woody plants are positive effectors that increase biotic and abiotic stress tolerance. For
instance, the overexpression of ANAC019, ANAC055, ANAC072 (RD26), ATAF1
and ATAF2 in Arabidopsis improved drought and salt tolerance (Tran et al. 2004),
although these transgenic plants also showed increased susceptibility to necrotrophic
pathogens such as Botrytis cinerea and Fusarium oxysporum (Delessert et al. 2005;
Wang et al. 2009; Wu et al. 2009). In addition, these genes showed co-expression
in response to stress hormones (Bu et al. 2008; Wu et al. 2009; Jensen et al. 2010).
206
M. B. Pascual et al.
nuclear-associated form (Seo et al. 2010), and it is then phosphorylated into a
threonine residue (Thr142) of the NAC domain to facilitate the translocation of
NTL6 into the nucleus to induce a drought-resistant response. The mutation of
this threonine to alanine produced NTL6 proteins that were poorly phosphorylated and failed to move into the nucleus (Kim et al. 2012). The recent results
of Guan et al. (2014) showed that RCF2/CPL1, a positive regulator of heat-stress
responsive gene expression and thermotolerance in Arabidopsis, dephosphorylates
ANAC019 and that both proteins, through protein–protein interactions, induce the
synthesis of chaperones, which are important components of the response to heat
stress (Guan et al. 2014).
5 Outstanding Roles for NAC Proteins
5.1 NAC Proteins Involved in Plant Stress Responses
Increased environmental stresses associated with climate change, particularly high
temperature and prolonged drought, are critical factors that reduce tree growth and
development and, therefore, forest productivity. Accordingly, it is important to understand how forest trees adapt to hostile environmental conditions, so it is necessary to
increase our knowledge of the plant signalling pathways involved in the response to
stress and to identify key regulatory proteins (Harfouche et al. 2014).
Many families of TFs, including DREB, bZIP, MYB, WRKY and NAC, regulate
plant defences and stress responses in herbaceous plants (Seki et al. 2002; Mao et al.
2012; Shen et al. 2012). However, much less information is available about the regulatory proteins involved in the responses of woody plants to environmental stress.
Even less information is available for conifers, which are economically and ecologically important trees in the Northern Hemisphere.
NAC proteins are among the most important families of TFs involved in adaptive
stress response in plants (Wu et al. 2009; Puranik et al. 2012; Pascual et al. 2015). In
addition, one of the putative NAC proteins identified in the group of green algae
Zygnematophyceae presented high similarity with SOG1 of Arabidopsis, a NAC
protein that has been implicated in the DNA damage response in plants (Yoshiyama
2016), suggesting that the evolutionary emergence of these TFs was directly associated with the appearance of novel defence mechanisms against stress.
Most stress-related NAC proteins characterized in Arabidopsis, rice, barley and
woody plants are positive effectors that increase biotic and abiotic stress tolerance. For
instance, the overexpression of ANAC019, ANAC055, ANAC072 (RD26), ATAF1
and ATAF2 in Arabidopsis improved drought and salt tolerance (Tran et al. 2004),
although these transgenic plants also showed increased susceptibility to necrotrophic
pathogens such as Botrytis cinerea and Fusarium oxysporum (Delessert et al. 2005;
Wang et al. 2009; Wu et al. 2009). In addition, these genes showed co-expression
in response to stress hormones (Bu et al. 2008; Wu et al. 2009; Jensen et al. 2010).
206
M. B. Pascual et al.