Transcription Factors

Transcriptional factors (TFs) are one of the important regulatory proteins involved in abiotic stress responses. They play essential roles downstream of stress signaling cascades, which could alter the expression of a subset of stress-responsive genes simultaneously and enhance tolerance to environmental stress in plants. Members of AP2/ERF (APETALA2/ethylene response factor), zinc finger, WRKY, bZIP (basic leucine zipper), and NAC (NAM, ATAF, and CUC) families have been characterized with roles in the regulation of plant abiotic stress responses (Yamaguchi-Shinozaki and Shinozaki, 2006; Ariel et al, 2007; Ciftci-Yilmaz and Mittler, 2008; Fang et al, 2008), and some of them have been demonstrated to be involved in ROS homeostasis regulation and abiotic stress resistance in crops.

Proteins containing zinc finger domain(s) were widely reported to be key players in the regulation of ROS-related defense genes in Arabidopsis and other species. For example, the expression of some zinc finger genes in Arabidopsis, ZAT7, Z4T10 and ZAT12, is intensely up-regulated by oxidative stress in At APX1 knock out plants (Miller et al, 2008). Subsequent experiments showed that these zinc finger proteins were involved in ROS regulation and multiple abiotic stresses tolerance (Davletova et al, 2005; Mittler et a/.,;Ciftci-Yilmaz et al, 2007). The zinc finger proteins are divided in to several types, such as C7H,, C-,C?, C,HC, CCCH and C3HC4, based on the number and the location of characteristic residues (Ciftci- Yilmaz and Mittler, 2008). The signaling pathways participating in stomatal movement were well studied in the model plant Arabidopsis, but were largely unknown in crops. Huang et al (2009) identified a drought and salt tolerance (dst) mutant, and the DST was cloned by the map-based cloning. DST encoded a C2H2-type zinc finger transcription factor that negatively regulated stomatal closure by direct regulation of genes related to H-,0-, homeostasis, which identified a novel signaling pathway of DST-mediated H20,-induced stomatal closure (Huang et al, 2009). Loss of DST function increased the accumulation of H202 in guard cell, accordingly, resulted in increased stomatal closure and enhanced drought and salt tolerance in rice. Other two C7H^-type zinc finger proteins, ZFP36 and ZFP179, also play circle role in ROS homeostasis regulation and abiotic stress resistance in rice. ZFP179 encodes a salt-responsive zinc finger protein with two C^H^-type zinc finger motifs (Sun et al, 2010). The ZFP179 transgenic rice plants increased ROS-scavenging ability and expression levels of stress-related genes, and exhibited significantly enhanced tolerance to salt and oxidative stress (Sun et al, 2010). ZFP36 is an ABA and H-,02-responsive C,H2-type zinc finger protein gene, and plays an important role in ABA- induced antioxidant defense and the tolerance of rice to drought and oxidative stresses (Zhang et al, 2014). Moreover, ZFP36 is a major player in the regulation of the cross-talk involving NADPH oxidase, H202, and МАРК in ABA signaling (Zhang et al, 2014). OsTZFl, a CCCH-tandem zinc finger protein, was identified as a negative regulator of leaf senescence in rice under stress conditions (Jan et al, 2013). Meanwhile, OsTZFl confers tolerance to oxidative stress in rice by enhancing the expression of redox homeostasis genes and ROS-scavenging enzymes (Jan et al, 2013). A cotton CCCH-type tandem zinc finger gene, GhTZFl, also serves as a key player in modulating drought stress resistance and subsequent leaf senescence by mediating ROS homeostasis (Zhou et al, 2014b).

Members of AP2/ERF (APETALA2/ethylene response factor) transcription factor family, including DREB/CBF transcription factors, are especially important as they regulate genes involved in multiple abiotic stress responses (Mizoi et al,

2012). During the initial phase of abiotic stresses, elevated ROS levels might act as a vital acclimation signal. But the key regulatory components of ROS-mediated a biotic stress response signaling are largely unknown. Rice salt-and H-,0,- responsive ERF transcription factor, SERF 1, has a critical role in regulating H,Ot mediated molecular signaling cascade during the initial response to salinity in rice (Schmidt et al, 2013). SERF 1 regulates the expression of H-,0-,-responsive genes involved in salt stress responses in roots. SERF1 is also a phosphorylation target of a salt-responsive МАРК (MAPK5), and activation the expression of salt- responsive МАРК cascade genes (MAPK5 and MAPKKK6). well established salt-responsive TF genes (ZFP179 and DREB2A), and itself through direct interaction with the corresponding promoters in plants (Schmidt et al, 2013). The SERF1 is essential for the propagation of the initial ROS signal to mediate salt tolerance. SUB1A, an ERF transcription factor found in limited rice accessions, limits ethylene production and gibberellin responsiveness during submergence, economizing carbohydrate reserves and significantly prolonging endurance (Fukao and Xiong, 2013). After floodwaters subside, submerged plants encounter reexposure to atmospheric oxygen, leading to post anoxic injury and severe leaf desiccation (Setter et al, 2010; Fukao and Xiong, 2013). SUB1A also positively affects post submergence responses by restrained accumulation of ROS in aerial tissue during desubmergence (Fukao et al, 2011). Consistently, SUBlApromptes the expression of ROS scavenging enzyme genes, resulting in enhanced tolerance to oxidative stress. On the other hand, SUB1A improves survival of rapid dehydration following desubmergence and water deficit dining drought by increasing ABA responses, and activating stress- inducible gene expression (Fukao et al, 2011). Ajasmonate and ethylene-responsive ERF gene, JERF3, was isolated from tomato and involved in a ROS-mediated regulatory module in transcriptional networks that govern plant response to stress (Wu et al, 2008). JERF3 modulates the expression of genes involved in osmotic and oxidative stresses responses by binding to the osmotic and oxidative-responsive related ciselements. The expression of these genes leads to reduce accumulation of ROS, resulting in enhanced abiotic stress tolerance in tobacco (Wu et al, 2008).

The WRKY family proteins have one or two conserved WRKY domains comprising a highly conserved WRKYGQK hepta peptide at the N-tenninus and a zinc-finger-like motif at the C-terminus (Eulgem et al, 2000). The conserved

WRKYdomain plays important roles in various physiological processes by binding to the W-box in the promoter regions of target genes (Ulker and Somssich, 2004; Rushton et al, 2010). Wang et <7/.(2015) reported a multiple stress-responsive WRKY gene, GmWRKY27, reduces ROS level and enhances salt and drought tolerance in transgenic soybean hairy roots. GmWRKY27 interacts with GmMYB174, which, in turn, acts in concert to reduce promoter activity and gene expression of GmNAC29 (Wang et al, 2015). Further experiments showed that GmNAC29 is a negative factor of stress tolerance for enhancing the ROS production under abiotic stress by directly activating the expression of the gene encoding ROS production enzyme. In another study, over expression of cotton WRKY gene, Gli WRKY17, reduced transgenic tobacco plants tolerance to drought and salt stress. Subsequent experiments showed that GhWRKY17 involved in stress responses by regulating ABA signaling and cellular levels of ROS (Yan et al, 2014). Sun et al(2015) isolated a WRKY gene, BdWRKY36, from B. distachyon, and found it functions as a positive regulator of drought stress response by controlling ROS homeostasis and regulating transcription of stress related genes.

Members of other TF families also functioned in abiotic str ess response through ROS regulation. ASR proteins are plant specific TFs and considered to be important regulators of plant response to various stresses. Wheat ASR gene, TaASRl, a positive regulator of plant tolerance to drought/osmotic stress, is involved in the modulation of ROS homeostasis by activating antioxidant defense system and transcription of stress responsive genes (Hu et al, 2013). Soybean NACTF, GmNAC2, was identified as a negative regulator during abiotic stress, and participates in ROS signaling pathways through modulation of the expression of genes related to ROS scavenging (Jin et al, 2013). Ramegowda et «/.(2012) isolated a stress-responsive NAG gene, EcNACl, from finger millet (E. coracana). Transgenic tobacco plants expressing EcNACl increased ROS scavenging activity, up-regulated many stress responsive genes and exhibited tolerance to various abiotic stresses and MV-induced oxidativestress (Ramegowda et al, 2012). Recently, a NAG transcription factor gene, SNAC3, functions as a positive regulator under high temperature and drought stress, was identified in rice (Fang et al, 2015). SNAC3 enhances the abiotic stresses tolerance by modulating H20, homeostasis state through controlling the expression of ROS- associated enzyme genes (Fang et al, 2015).

In addition to TFs, transcriptional co-regulator as well as spliceosome component, OsSKIPa, a rice homolog of human Ski-interacting protein (SKIP), has been studied for effects on drought resistance (Hou et al, 2009). OsSKIPa- overexpressing rice exhibited significantly enhanced drought stress tolerance at both the seedling and reproductive stages by increased ROS- scavenging ability and transcript levels of many stress-related genes (Hou et al, 2009).

 
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