The majority of reports concerning NAC TFs have indicated that numerous members of the multigene family play roles in the transcriptional reprogramming associated with plant immune responses. This is an active research area that has been extensively reviewed and therefore will only be briefly considered here. To date, it is clear that NAC NFs are central components of many aspects of the plant innate immune system, basal defense, and systemic acquired resistance. There are many examples in which the overexpression or knockdown of NAC gene expression has effects on plant defense, observations that have allowed the resolution of some components of the web of signaling pathways (Figures 3–5; Table 1) (Collinge and Boller, 2001; Delessert et al., 2005; He et al., 2005; Jensen et al., 2007, 2008, 2010).

Sun and co-workers applied Virus-induced gene silencing (VIGS) system to investigate the function of NAC TFs (ONAC122 and ONAC131) in disease resistance against M. grisea (Sun et al., 2013). VIGS is a useful tool for the rapid analysis of gene function in plants (Liu et al., 2002; Purkayastha and Dasgupta, 2009; Scofield and Nelson, 2009). Some VIGS vectors have been developed for dicotyledonous plants among which the tobacco rattle virus (TRV)-based VIGS vector is the most successful example for members of Solanaceae, such as Nicotiana benthamiana and Lycopersicon esculentum (Liu et al., 2002; Chakravarthy et al., 2010). The barley stripe mosaic virus (BSMV)-based VIGS vector was used to characterize multiple genes for their roles in disease resistance in wheat and barley (Hein et al., 2005; Scofield et al., 2005; Zhou et al., 2007; Sindhu et al., 2008). Several scientists have developed a brome mosaic virus (BMV)-based VIGS vector, and this vector was demonstrated to be a versatile tool for rapid gene function analysis in barley, rice, and maize (Ding et al., 2006; Pacak et al., 2010; van der Linde et al., 2011; Biruma et al., 2012). In rice seedlings, 19 and 13 NAC genes were up-regulated after RSV and RTSV infection, respectively, at different days after inoculation (Nuruzzaman et al., 2010). Several NAC proteins can either enhance or inhibit virus multiplication by directly interacting with virus-encoded proteins (Figure 3; Xie et al., 1999; Ren et al., 2000, 2005; Selth et al., 2005; Jeong et al., 2008; Yoshii et al., 2009), and increases in the expression level of NAC genes have been monitored in response to attack by viruses, several fungal elicitors, and bacteria (Figures 3, 4; Xie et al., 1999; Ren et al., 2000; Collinge and Boller, 2001; Mysore et al., 2002; Hegedus et al., 2003; Oh et al., 2005; Selth et al., 2005; Jensen et al., 2007; Lin et al., 2007; Jeong et al., 2008; Wang et al., 2009a,b; Xia et al., 2010a,b). Such dual modulation in plant defense implies the association of NAC proteins with distinct regulatory complexes.

Kaneda et al. (2009) reported that OsNAC4 is a key positive regulator of hypersensitive cell death in plants, and hypersensitive cell death is markedly decreased in response to avirulent bacterial strains in OsNAC4-knock-down lines. After induction by an avirulent pathogen recognition signal, OsNAC4 is translocated into the nucleus in a phosphorylation-dependent manner. Conversely, the overexpression of OsNAC6 does not lead to hypersensitive cell death (Kaneda et al., 2009), whereas transgenic rice plants overexpressing OsNAC6 exhibited tolerance to blast disease (Nakashima et al., 2007). ATAF2 overexpression resulted in increased susceptibility toward the necrotrophic fungus Fusarium oxysporum under sterile conditions due to the repression of pathogenesis-related (PR) genes (Delessert et al., 2005) but induced PR genes, reducing tobacco mosaic virus accumulation in a non-sterile environment (Wang et al., 2009b). RNA interference and overexpression studies have also revealed the function of NAC TFs in various plant–pathogen interactions (Figure 4). A number of NAC proteins may positively regulate plant defense responses by activating PR genes, inducing a hypersensitive response (HR), and cell death at the infection site (Figure 4; Jensen et al., 2007, 2008; Kaneda et al., 2009; Seo et al., 2010). ATAF1 and its barley homolog HvNAC6 positively regulate penetration resistance to the biotrophic fungus Blumeria graminis f.sp. hordei (Bgh) (Jensen et al., 2007, 2008) but attenuate the resistance to other pathogens, such as Pseudomonas syringae, Botrytis cinerea, and Alternaria brassicicola (Wang et al., 2009a; Wu et al., 2009). Unlike ATAF2, ATAF1 and HvNAC6 are transcriptional activators and may indirectly regulate the repression of PR genes via a hypothetical negative regulator (Figure 4). Hence, the ATAF subfamily clearly appears to have a conserved but non-redundant function in regulating the responses to different pathogens. The immune response in plants elicited upon pathogen infection is characterized by activation of multiple defense responses including expression of a large set of defense-related genes (van Loon et al., 2006), which are regulated by different types of TFs. Many TFs belonging to the NAC, ERF, and WRKY families have been identified (Eulgem and Somssich, 2007; Gutterson and Reuber, 2004) and revealed to play important roles in regulating expression of defense-related genes.

Arabidopsis stress-responsive NAC genes, such as RD26, respond to JA, a well-described phytohormone that is functionally involved in regulating wounding and biotic stress responses (Fujita et al., 2004, 2006). Hence, it is reasonable to consider that JA-responsive SNAC factors might function in both biotic and abiotic stress responses. In rice, most of the genes in the SNAC group respond to JA. Among them, SNAC1, OsNAC3, OsNAC4, OsNAC5, OsNAC6, and OsNAC10 are present in the same phylogenetic SNAC/(IX) group (Figure 1). In particular, the SNAC group (Figure 1) comprises several genes that regulate disease resistance pathways, as inferred from the increased resistance to pathogens upon overexpression under the control of a constitutive promoter. Data indicate that NAC TFs also have an important role in the regulation of plant defense responses to different pathogens in addition to wounding and insect feeding (Figure 5). The application of exogenous phytohormones, such as JA, SA, and ET, has also been shown to induce NAC genes in several species (Tran et al., 2004; He et al., 2005; Hu et al., 2006; Sindhu et al., 2008; Lu et al., 2007; Nakashima et al., 2007; Yokotani et al., 2009; Zheng et al., 2009; Xia et al., 2010a,b; Yoshii et al., 2010; Nuruzzaman et al., 2012b). Hence, NAC TFs can possibly modulate the phytohormonal regulation of the biotic stress cellular network for convergent and divergent adaptive pathways.

Reactive oxygen species (ROS) is an active molecule in most biotic plant stress. Such ROS as H₂O₂ act as important signal transduction molecules, mediating the acquisition of tolerance to various stresses (Bhattacharjee, 2005; Davletova et al., 2005). In rice, OsNAC6 gene is involved in both response and tolerance to biotic stress (Nakashima et al., 2007). In Arabidopsis, ATAF subfamily (ATAF1, ATAF2, and RD26) is also involved in biotic stress. The expression of RD26 is induced by JA and H₂O₂, and pathogen infections (Fujita et al., 2004; Zimmermann et al., 2004). Large-scale transcriptiome analysis with both types of mutants revealed that RD26-regulated genes are involved in the detoxification of ROS, defense, and senescence (Fujita et al., 2004; Balazadeh et al., 2011). The role of stress-responsive NAC proteins in senescence is poorly understood. Recently, the NTL4, (Lee et al., 2012), MtNAC969 (de Zélicourt et al., 2012), Os07g37920, wheat GPC (Distelfeld et al., 2012) genes were found to be induced senescence in different plants. Leaf senescence is a unique developmental process that is characterized by massive programmed cell death and nutrient recycling. Leaf senescence is induced by pathogen infection (Dhindsa et al., 1981; Buchanan-Wollaston et al., 2003; Gepstein et al., 2003). AtNAP gene, which belongs to the closest NAC subfamily of the ATAF subfamily, has been shown to be involved in senescence (Guo and Gan, 2006). In addition all ATAF subfamily NAC genes, including ATAF1, ATAF2, and RD26, are upregulated during senescence in Arabidopsis leaves (Guo et al., 2004). These findings suggest that RD26 may function at the node of convergence between the pathogen defense and senescence signaling pathways. Taken together, these results support the notion that ROS and senescence may be closely related to NAC-mediated stress responses.

Abiotic stress triggers a wide range of plant responses, from the alteration of gene expression and cellular metabolism to changes in plant growth, development, and crop yield. Thus, understanding the complex mechanism of drought and salinity tolerance is important for agriculture production. Interestingly, many NAC genes have been shown to be involved in plant responses to drought and salinity stress. In transgenic rice, the Os01g66120/OsNAC2/6 and Os11g03300/OsNAC10 genes were found to enhance drought and salt tolerance (Figure 5; Nakashima et al., 2009; Jeong et al., 2010), and Os03g60080/SNAC1 increased grain yield (21–34%) under drought stress (Hu et al., 2006). Udupa et al. (1999) reported that comparative gene expression profiling is an efficient way to identify the pathways and genes regulating a stress response under different stress conditions. The Arabidopsis NAC gene ANAC092 demonstrates an intricate overlap of ANAC092-mediated gene regulatory networks during salt-promoted senescence and seed maturation (Balazadeh et al., 2010). Lan et al. (2005) found that a large portion of the genes regulated by dehydration are also up-regulated by fertilization; indeed, pollen is a major site of variations in the expression levels for many genes (Czechowski et al., 2005). Related conclusions have been drawn from analyses based on promoter-GUS fusions of cold-inducible Os01g66120/SNAC2/6, Os11g03300/OsNAC10, RD29A, COR15A, KIN1, and COR6.6 in rice and Arabidopsis, genes that are regulated during plant development (root, leaf, and pollen) under both stress (drought and cold) and non-stress conditions (Sindhu et al., 2008; Jeong et al., 2010). You et al. (2013) reported that OsOAT is a direct target of the stress-responsive NAC transcription factor SNAC2, and OsOAT overexpression in rice resulted in significantly enhanced drought and osmotic stress tolerance. Plants overexpressing GmNAC085 show enhanced drought tolerance (Le et al., 2011), whereas the overexpression of GmNAC11 led to increased sensitivity to salt and mannitol stresses (Hao et al., 2011). Microarray profiling of the roots and leaves of drought-treated rice revealed the induction of 17 NAC genes by severe or mild drought treatment (Nuruzzaman et al., 2012b). SiNAC is also simultaneously induced by dehydration, salinity, ethephon, and methyl jasmonate treatments (Puranik et al., 2011). Similarly, the expression of DgNAC1, TaNAC2a and EcNAC1 were strongly induced by NaCl and drought stresses in transgenic tobacco plants (Liu et al., 2011; Ramegowda et al., 2012; Tang et al., 2012). Several genes, such as ZmSNAC1 (Lu et al., 2012), TaNAC69 (Xue et al., 2011), CarNAC3 (Peng et al., 2009), miR319, AsNAC60 (Zhou et al., 2013), AhNAC2 (Liu et al., 2011), RhNAC2 or RhEXPA4 (Dai et al., 2012), ClNAC (Huang et al., 2012), CsNAM (Paul et al., 2012), SiNAC (Puranik et al., 2011), HSImyb and HSINAC (Robertson, 2004), and TaNAC2a, TaNAC4a, TaNAC6, and TaNAC4 (Tang et al., 2012; Xia et al., 2010a), were increased by drought and NaCl (Figure 5; Table 2).

The expression of members of the OsNAC gene family under hormone treatment requires extensive cross-talk between the response pathways, and it is likely that substantial physiological connections exist between NAC protein production and phytohormone treatment. Phytohormones are involved in influencing signaling responses by acting in conjunction with or in opposition to each other to maintain cellular homeostasis (Fujita et al., 2006; Miller et al., 2008). The NAC TFs form a complex but interesting group of important arbitrators of this process (Figure 5). ANAC019 and ANAC055 are involved in both ABA- and JA-mediated regulation (Greve et al., 2003; Bu et al., 2008, 2009; Jensen et al., 2010). The ATAF subfamily TFs are another group of NAC proteins that act at the convergence point of biotic and abiotic stress signaling (Delessert et al., 2005; He et al., 2005; Jensen et al., 2007). Because ATAF1 alleles expedite drought perception at the cost of optimal basal defense, ATAF1 acts as a negative regulator of ABA signaling but induces JA/ET-associated defense signaling marker genes (Jensen et al., 2008). Conversely, ATAF2 expression was induced by dehydration, JA, and SA (Figure 5; Delessert et al., 2005). We have proposed the participation of SiNAC in the ABA-independent pathway of abiotic stress and in regulating biotic stress via an antagonistic JA and SA pathway (Puranik et al., 2011). A number of NAC genes (e.g., AtNAC2) in plants are affected by auxin, ethylene (Xie et al., 2000; He et al., 2005), and ABA (e.g., OsNAC5; Sperotto et al., 2009). In Arabidopsis, NAC TF NTL8 regulates GA3-mediated salt signaling in seed germination (Kim et al., 2008). ABA plays a major role in mediating the adaptation of a plant to stress, and this hormone can stimulate root growth in plants that need to increase their ability to extract water from the soil. OsNAC5/ONAC009/ONAC071 and OsNAC6 are homologs that are induced by abiotic stress, such as drought and high salinity, and ABA (Takasaki et al., 2010). AtNAC1 and AtNAC2 are induced by auxin and ABA, respectively, and AtNAC1 mediates auxin signaling to promote lateral root development in Arabidopsis (Xie et al., 2000; He et al., 2005). ABA signaling induces the expression of genes encoding proteins that protect the cells in vegetative tissues from damage when they become dehydrated. These well-known ABA responses are less sensitive to ABA in NPX1-overexpressing plants (Kim et al., 2009). The expression of the RD26 gene is induced by drought and also ABA and high salinity (Fujita et al., 2004). NAC TFs regulate many target genes by binding to the CATGTG motif in the promoter region of the target gene to activate transcription in the response to drought stress (Nakashima et al., 2007), a transcriptional regulatory system that is known as a regulon. ABA is produced under conditions of drought stress and plays a crucial role in drought tolerance in plants (Figure 6; Shinozaki et al., 2003). In addition to NAC and other regulons, OsDREB2 responds to dehydration in rice (Dubouzet et al., 2003); the dehydration-responsive element binding protein 1 (DREB1)/C-repeat-binding factor (CBF) and DREB2 regulons function in ABA-independent gene expression, whereas the ABA-responsive element (ABRE)-binding protein (AREB)/ABRE-binding factor (ABF) regulon functions in ABA-dependent gene expression. ABA-activated OSRK1 protein kinases phosphorylate and activate AREB/ABF-type proteins in rice (Figure 6; Chae et al., 2007). Both ABA-independent and ABA-dependent signal transduction pathways convert the initial stress signal into cellular responses (Figures 5, 6). The TF family members involved in both ABA-independent (AP2/ERF, bHLH, and NAC) and ABA-dependent (MYB, bZIP, and MYC) pathways are up-regulated in rice; the TFs belonging to this family interact with specific cis-elements and/or proteins, and their overexpression confers stress tolerance in heterologous systems (Fujita et al., 2004; Tran et al., 2004; Hu et al., 2006). The expression of OsNAC6 is induced by ABA and abiotic stresses, including cold, drought, and high salinity (Nakashima et al., 2007). Together, these data provide evidence that different NAC genes play differential roles in the specific responses to different phytohormone treatments. Thus, gene expression profiles under both biotic and abiotic stresses to determine the vital role of NAC genes in plant growth and stress responses and the identification of target genes for TFs involved in stress responses are important.

In agriculture, high or low temperature acts as a major negative factor limiting crop production. Indeed, tremendous work has been performed in the past two decades to reveal the complex molecular mechanism in the plant responses to extreme temperature, and there is increasing evidence that NAC proteins are involved in responses to both heat and cold stresses. For example, an NAC TF gene (ONAC063) in rice roots responds to a combination of high-temperature stress (Yokotani et al., 2009). Another example is that transgenic Arabidopsis plants overexpressing ANAC042 show increased tolerance to heat stress when compared to the wild-type plants (Shahnejat-Bushehri et al., 2012). Moreover, the overexpression of ZmSNAC1 enhanced the tolerance to drought and low-temperature stress compared to the control (Lu et al., 2012). The expression of OsNAC10, SNAC2/OsNAC6, TaNAC4,NTL6, TaNAC2a, TaNAC4a, TaNAC6,TaNAC7,TaNAC13, and TaNTL5 is induced by low temperature in plants (Jeong et al., 2010; Xia et al., 2010a; Tang et al., 2012), and a gene expressing a CsNAM-like protein is induced by heat in tea plants (Paul et al., 2012). Northern blot and SNAC2 promoter activity analyses suggest that the SNAC2 gene is induced by low temperature. Additionally, a microarray analysis of rice NAC genes has revealed that 8 of the 14 analyzed OsNAC genes are regulated by severe or mild drought stress (Nuruzzaman et al., 2012b), showing distinct expression patterns upon high-temperature treatment. Yoo et al. (2007) reported that the phenotype resulting from the overexpression of an NAC-domain protein gene (At2g02450) is related to the control of flowering time and cold responses. The importance of NAC proteins in plant development, transcription regulation, and regulatory pathways involving protein–protein interactions is being increasingly recognized. Taken together, NAC proteins function in plants adaptions to temperature variations through the transcriptional reprogramming of downstream stress-related genes.

Various nutrient elements are required for the normal growth and development of plants. Boron (B) is an essential micronutrient for higher plants, but excessive amounts of B inhibit growth (B toxicity). As the optimal range of B concentration in tissues is narrow (Blamey et al., 1997), B toxicity occurs in many plants at levels only slightly above that required for normal growth (Mengel and Kirkby, 2001). The Os04g0477300 gene encodes an NAC-like TF, and the function of the transcript is abolished in B toxicity-tolerant cultivars. Transgenic plants in which the expression of Os04g0477300 is abolished by RNA interference acquire a tolerance to B toxicity (Ochiai et al., 2011). In a transcriptome analysis using Arabidopsis plants under B toxicity, nine genes encoding multidrug and toxic compound extrusion transporters, a zinc-finger family TF, a heat-shock protein-like protein, an NAC-like TF, and unknown proteins were induced (Kasajima and Fujiwara, 2007), though the functions of these proteins are not yet known. A sufficient supply of inorganic phosphate (Pi) is vital to plants, and the low bioavailability of Pi in soils is often a limitation to growth and development. Consequently, plants have evolved a range of regulatory mechanisms to adapt to phosphorus-starvation to optimize the uptake and assimilation of Pi. Recently, significant progress has been achieved in elucidating these mechanisms, revealing that the coordinated expression of a large number of genes is important for many of these adaptations. These studies provide a valuable basis for the identification of new regulatory genes and promoter elements to further the understanding of Pi-dependent gene regulation. With a focus on the Arabidopsis transcriptome, Nilsson et al. (2010) reported common findings that indicate new groups of putative regulators, including the NAC, MYB, and WRKY families. With a number of new discoveries of regulatory elements, a complex regulatory network is emerging. They evaluate the contribution of the regulatory elements to P-responses and present a model comprising the factors directly or indirectly involved in transcriptional regulation. Thus, NAC genes appear to respond to several aspects of nutrient excess and deficiency-induced stresses, implicating their diverse functions in these signaling pathways.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.