Deciphering hydrogen peroxide-induced signalling towards stress tolerance in plants

Abstract

Plants encounter a variety of adverse environmental conditions, such as high salinity, drought, extreme heat/cold and heavy metals contamination (abiotic stress) or attack of various pathogens (biotic stress). These detrimental environmental factors enhanced the ROS production such as singlet oxygen (¹O₂), superoxide (O^•−₂), hydrogen peroxide (H₂O₂) and hydroxyl radicals (OH^•). ROS are highly reactive and directly target several cellular molecules and metabolites, which lead to severe cellular dysfunction. Plants respond to oxidative damages by activating antioxidant machinery to trigger signalling cascades for stress tolerance. H₂O₂ signalling balances the plant metabolism through cross-talk with other signals and plant hormones during growth, development and stress responses. H₂O₂ facilitates the regulation of different stress-responsive transcription factors (TFs) including NAC, Zinc finger, WRKY, ERF, MYB, DREB and bZIP as both upstream and downstream events during stress signalling. The present review focuses on the biological synthesis of the H₂O₂ and its effect on the upregulation of kinase genes and stress related TFs for imparting stress tolerance.

Introduction

During stress-free conditions, plants’ aerobic metabolic processes such as respiration and photosynthesis produce several reactive oxygen species (ROS; O^•−₂, H₂O₂, OH^• and ¹O₂) continuously as byproducts. The ROS are distributed in plant cellular organelles such as chloroplast, peroxisomes and mitochondria. The electron transport chain (ETC) during the aerobic metabolism and enzymatic catalysis by xanthine oxidase and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase produce O^•−₂ by directly reducing the O₂. Spontaneous or catalytic breakdown of O^•−₂ leads to H₂O₂ production mainly by superoxide dismutase (SOD), an important antioxidative enzyme. SODs are categorized in three groups: copper/zinc SOD (Cu/Zn–SOD); iron SOD (Fe–SOD) and manganese SOD (Mn–SOD) based on the metal cofactors. Cu/Zn-SOD is predominantly localized in cytosol and chloroplast; while Fe–SOD and Mn–SOD are confined to chloroplast and mitochondria, respectively (McKersie et al. 2000; Xing et al. 2013, 2015). H₂O₂ is known for its cytotoxic effects and is controlled by non-enzymatic and enzymatic regulation. Major non-enzymatic antioxidants are ascorbate (vitamin C), glutathione (GSH), tocopherol (vitamin E), flavonoids, alkaloids, and carotenoids in plants. While enzymatic mechanisms include SOD, catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), guaiacol peroxidase (GOPX), and glutathione-S- transferase (GST) (Apel and Hirt 2004). However, H₂O₂ is potentially quite reactive to Fe²⁺ and consequently the Fe²⁺-containing cofactors found in some proteins are oxidized, by means of the Fenton reaction (Becana et al. 1998). The homolysis of H₂O₂ to 2 (OH^•) is the “unpleasant” result of this reaction and ultimately oxidize the polyunsaturated fatty acids (PUFA) of lipid membrane and release water molecule (Fig. 1).

Fig. 1.

Schematic representation of ROS generation in different organelles of plant cell during abiotic and biotic stress, its scavenging through enzymatic and non-enzymatic pathways to pursue oxidative stress tolerance. ROS generation is inevitable by-products of aerobic cell metabolism and dramatically enhance in adverse environmental conditions. Apoplastic ROS is generated through cell membrane-bound NADPH oxidase, cell wall-associated peroxidases and fenton reaction. Singlet oxygen (¹O₂) and hydrogen peroxide (H₂O₂) are produced in high light intensity as well as during photosynthesis and photorespiration in chloroplast. Mitochondrial ETC generates superoxide anion (O^•−₂) in response to stress, which is ultimately converted to H₂O₂ by superoxide dismutase (SOD). In peroxisomes, H₂O₂ is accumulated during glyoxalate synthesis and SOD-mediated dismutation of O^•−₂, produced by Xanthine oxidase (XO). O^•−₂ is dismutated into the H₂O₂ through SOD enzyme. H₂O₂ is catalysed by various enzymes such as catalase (CAT), peroxiredoxins (PRX), ascorbate peroxidase (APx), glutathione peroxidase (GPx) and dehydroascorbate reductase (DHAR). Hydroxyl radicle (OH^•) is reduced non-enzymatically through lipid peroxidation and sugar oxidation. The ¹O₂ is also non-enzymatically processed by donating electron to β-carotene, tocopherol and plastoquinone and confer oxidative stress tolerance. Black arrow (→) indicates ROS generation; Red arrow (→) indicates ROS scavenging

Adverse stress conditions such as salinity, drought, temperature, high light intensity and wounding disturb the equilibrium between production and scavenging of ROS. Gill and Tuteja (2010) reported the enhanced production of ROS in chloroplasts during abiotic stresses such as salt, drought and excess light conditions. Salinity stress damages photosystem and causes the production of O^•−₂ via Mehler reaction by overloading of electron flow in the ETC (Apel and Hirt 2004). During salinity and drought, plants face reduction in water availability and stomatal closure and thus decrease in CO₂ to O₂ ratio in mesophyll cells which finally enhance the photorespiration and production of glycolate in chloroplasts (Miller et al. 2008). Glycolate oxidase-mediated oxidation of glycolate in peroxisomes accounts for the majority of H₂O₂ production during photorespiration (Noctor et al. 2002; Karpinski et al. 2003). In peroxisomes, other major processes such as β-oxidation of fatty acid, the flavin oxidase pathway and the dismutation of O^•−₂ radicals also involved in H₂O₂ production (Corpas et al. 2001; Del Río 2011). Peroxisomes are also source of O^•−₂ production by matrix localised enzyme, xanthine oxidase and NADPH-dependent ETC (Karuppanapandian et al. 2011; Del Río 2011). Under environmental stress, especially in drought and salinity, ROS production has been shown to upsurge in mitochondria (Alscher et al. 1997; Bartoli et al. 2004; Pastore et al. 2007). Over-reduction of the ubiquinone after disruption in mitochondrial ETC function, results in increased ROS production (Rhoads et al. 2006). Abiotic stress also mediate apoplastic ROS production through any of the known four different mechanisms such as NADPH oxidase, peroxidases (O’Brien et al. 2012), oxalate oxidase (Voothuluru and Sharp 2012) and xanthine dehydrogenase (Ma et al. 2016). Plasma membrane located NADPH oxidase-Respiratory burst oxidase homologs (RBOH) protein is the most studied mechanism, which is associated with calcium and ROS signalling during stress and produce superoxide in the apoplast (Gilroy et al. 2016).

ROS mediated cellular damages

Lipid peroxidation

At cellular level, the most damaging consequence of ROS generation and accumulation is lipid peroxidation on both cell and organelle membranes. Beyond the utmost level of ROS accumulation, lipid peroxidation ultimately cause the oxidative stress through production of lipid-derived radicals (Montillet et al. 2005). The membrane PUFAs [linoleic acid (18:2) and linolenic acid (18:3)] are the main targets of O^•−₂ and OH^• and enhance the formation of the polar lipid hydroperoxide complex mixtures, resulting in enhanced membrane fluidity, cytosolic solutes efflux and membrane associated protein functional loss (Moller et al. 2007). As a result of PUFA peroxidation, some aldehydes such as malondialdehyde (MDA) and 4-hydroxy-2-nonenal are created which attack amino acid side chains in proteins and also cause fragmentation of DNA (Gill and Tuteja 2010).

Protein oxidation

The important ROS-mediated post-translational modifications are sulfonylation, carbonylation, glutathionylation and s-nitrosylation. Sulfonylation is the oxidation of sulfhydryl groups, one of the main mechanisms that control the activity of many enzymes and transcription factors (TFs) in plants. H₂O₂ generating sulfenic acid (R-SOH) mainly induces the oxidation that can cause disulfide (S–S) bond formation between cysteine residues and resulted in conformational changes that alter protein/enzyme activity. The oxidation of cysteine residues to sulfenic acid is the first step in the ROS-dependent redox signalling pathway. The process can be relapsed mainly via thioredoxins (Trxs), peroxiredoxins (PRXs) and glutathione (GSH) system, which respond to stress and regulate redox homeostasis.

S-Glutathionylation is the further modification of the sulfenic acid-containing side-chains of proteins through covalent bond formation with low molecular weight thiols (e.g. GSH) and can regulate redox-driven signal transduction cascades and metabolic pathways (Fratelli et al. 2004). Glutathionylation can be reversed through the thiol-disulfide oxidoreductases Grxs activity (also known as thioltransferases; Gallogly and Mieyal 2007).

Another form of protein oxidation is the carbonylation which occurs at arginine, histidine, lysine, proline and threonine residues and considered to be irreversible (Shacter 2000). Indirect reactions of lipoperoxidation products with Cys and His residues can also lead to carbonylation of proteins (Madian and Regnier 2010).

During stress, some proteins get modified by the S-nitrosylation, the covalent binding of nitric oxide (NO) to thiol groups of Cys, modulates the signalling cascades. The different enzymes involved in respiration, antioxidation and photorespiration have been modified via S-nitrosylation during salinity stress (Camejo et al. 2013; Htet Hlaing and Clement 2014). It has been reported that s-nitrosylation also affect DNA binding activity of some TFs as well as inactivate RBOH (Yun et al. 2011). For example, AtMYB30 TF from Arabidopsis thaliana, an essential stress regulatory protein, is negatively regulated by S-nitrosylation (Tavares et al. 2014). H₂O₂ lead to alteration in conformational changes, nucleocytoplasmic shuttling, proteolysis, or interaction with other regulators of some H₂O₂-regulated TFs with redox-sensitive Cys residue (Dietz 2014).

ROS cross-talk between biotic and abiotic stresses

ROS and biotic stress

ROS play pivotal role in biotic stress during plant pathogen defence and wounding. ROS are generated by plant cells through the plasma-membrane bound NADPH-oxidases, cell wall-bound peroxidases, oxalate oxidase and xanthine dehydrogenase in the apoplast (Grant and Loake 2000; O’Brien et al. 2012; Voothuluru and Sharp 2012; Ma et al. 2016). Plant PCD is induced by the incompatible plant–pathogen interaction at the infection sites and response known as hypersensitive response (HR; Greenberg and Yao 2004). HR and associated ROS work against hemi-biotrophic and biotrophic pathogens. To avoid the death of the entire plant, HR response may further lead to activation of systemic acquired resistance (SAR). During plant–pathogen interaction, the SA plays an important role for onset of PCD. The coordinated production of ROS (over a certain threshold) and down-regulation of ROS scavenging mechanisms are the absolute requirement for PCD (Delledonne et al. 2001; Mittler et al. 1999). During plant pathogen reactions, a virulent pathogens are recognized by plants via R proteins and generates biphasic ROS (H₂O₂) production; a low-amplitude and transient first phase (ROS burst) followed by an unremitting phase of extreme magnitude (threshold level). Biphasic increases of H₂O₂ also produces during ozone or wounding stress (Lamb and Dixon 1997; Grant and Loake 2000; Joo et al. 2005). Alvarez et al. (1998) reported the oxidative burst in infected leaves of Arabidopsis leading to secondary systemic burst in distal parts and express the defence-related genes to provide plant immunity. Furthermore, AtRbohD also facilitates a long-distance signal for triggering the defence response at distal sites, in response to Pseudomonas syringae pathovar tomato (Pst) DC3000 (Dubiella et al. 2013). In an another study, pathogens Phytophthora cryptogea elevates the ROS generation in chloroplasts, which plays an important role in executing HR-like cell death (Ren et al. 2006).

ROS and abiotic stress

Abiotic stress such as salinity, drought, temperature, flooding generates ROS, which is further scavenged by the enzymatic and non-enzymatic anti-oxidant system. PCD occurs not only after the oxidative burst during pathogen challenge, but also by the exposure of abiotic stress. For example, the ozone-induced oxidative burst causes cell death process that shares similarities with the HR. It is probably due to the initial step of ozone signalling in the regulation of RBOH-dependent ROS production. It was reported that, exposure of acute ozone on A. thaliana induced biphasic ROS generation that is associated with the pathogen response (Joo et al. 2005). The ROS mostly performs opposite role during abiotic stress than it plays during pathogen defence. During abiotic stresses, ROS scavenging mechanisms are induced for the reduction in lethal concentration of intracellular ROS levels. These diversities in the function of ROS between biotic and abiotic stresses could evolve from the cross-talk between different signalling pathways and hormonal actions or from the spatial ROS production and/or accumulation during different stresses.

Stress hormones associated with ROS

Recent studies have shown that abiotic stress also generates a cross-talk among the plant hormones and ROS. Tognetti et al. (2012) have reported mutual interactions between ROS and auxin during abiotic stress causing auxin imbalance. Alteration in the auxin gradient have also been shown by stress-induced ROS production (Xia et al. 2015). Change in auxin equilibrium and perturbed signalling are associated with ROS-mediated auxin degradation (Kawano 2003); auxin conjugation (Tognetti et al. 2010); and auxin bypass through changes in the gene expression encoding auxin transporters (Grunewald and Friml 2010). Zhou et al. (2018) reported that exogenously applied H₂O₂ caused asymmetrical distribution of indole 3-acetic acid (IAA) in the primary root and influenced the gravitropism. During abiotic stress plants synthesisze brassinosteroids (BRs) hormone which helps in acclimation via physiological and biochemical processes (Xia et al. 2009). Nie et al. (2013) have showed correlation of BR synthesis and abiotic stress tolerance in tomato. RBOH transcription and enhanced NADPH oxidase activity were induced by BRs with the concurrent increase in apoplastic H₂O₂. Similarly the effect of abscisic acid (ABA) on RBOH expression and the generation of apoplastic H₂O₂ have been reported (Xia et al. 2015). In a recent study, the H₂O₂-dependent BRASSINAZOLE-RESISTANT1 (BZR1) TF showed regulation of auxin-signalling (AUXIN RESPONSE FACTOR6; ARF6) and light-signalling (PHYTOCHROME INTERACTING FACTOR4; PIF4) pathways during shoot cell elongation. Such significant overlaps of ROS with BR synthesis and auxin-light responses regulate shoot cell elongation through a central module of BZR1 interaction with the PIF4 and ARF6 factors (Tian et al. 2018). During abiotic stress conditions, gibberellins (GAs) promote cell division, cell elongation and plant growth (Colebrook et al. 2014). GAs execute the function under the regulation of the DELLA proteins, which are negative regulators of GA signalling (Achard et al. 2006). Cellular redox steady-state is also controlled via GA signalling and imparts stress tolerance. During drought condition, reduction in leaf GA contents lead to enhanced DELLA activity that resulted in increased ROS quenching capacity in maize and improved survival (Wang et al. 2008). Overexpression of the SUB1A (SUBMERGENCE 1A) gene in rice plants limits the ROS accumulation and oxidative damage during submergence stress by increased expression of the DELLA protein SLR1 and the SLR-like 1 (Fukao et al. 2006, 2011). Salicylic acid (SA) and nitrous oxide (NO) supress the activity of the ROS detoxifying enzymes APX and CAT, thus produce more ROS and activate program cell death (PCD) during the plant pathogen interaction (Klessig et al. 2000). Phytohormone ethylene is also having important role in stress signalling by modulating ethylene response factor (ERF) family TF. H₂O₂ activateds the phosphorylation of ERF6 via MPK6 and in turn bind to ROS-responsive cis-acting element 7 (ROSE7)/GCC box of the promoters of genes up-regulated by ROS. ROS-responsive expression of ERF6 is dependent on the NADPH oxidase (RbohD) and calcium signalling. The erf6 mutant study revealed its possible role in regulation of photooxidative damage as well as in anthocyanin synthesis (Wang et al. 2013; Sewelam et al. 2013). Collectively, all above findings propose the inter-linked role of ROS and phytohormones which regulate growth, development and stress responses in plants.

H₂O₂ as a member of ROS family

Even though H₂O₂ is known for its cytotoxic properties, it has also been recognized to play role as an important regulator of signal transduction. The H₂O₂ has very stringent regulatory mechanisms from production to degradation or to maintain ROS homeostasis. The small molecular structure and life longevity (1 ms) of H₂O₂ amongst other ROS make it precise messenger for signalling functions through crossing cellular membranes (Bienert et al. 2006) and acts in both autocrine and paracrine level in plants. H₂O₂ functions in concentration-dependent manner. At a low concentration, H₂O₂ acts as a signalling molecule and at high concentrations provokes cellular death (Gechev and Hille 2005).

Biological sources of H₂O₂

The H₂O₂ production in plant cells occurs during enhanced ETC or redox reactions in chloroplasts or mitochondria, fatty acid oxidation and photorespiration. Oxidation of glycolate by glycolate-oxidase is most significant in the peroxisome during the photosynthetic carbon oxidation cycle for H₂O₂ production (Noctor et al. 2002; Karpinski et al. 2003). The biphasic ROS production or oxidative burst, a part of HR during plant pathogen interaction, is also the cause of rapid increase in H₂O₂ concentration (Miller et al. 2010). The membrane-bound NADPH-dependent oxidases, similar to the RBOH, mostly cooperate in oxidative burst during stress signalling along with other inter-related cell wall-associated enzymes. The spontaneous or catalytic breakdown of superoxide anions, which are produced from oxygen reduction process during aerobic respiration and/or various environmental stresses, is also the predominant source of H₂O₂ generation.

H₂O₂ signalling in plants

Plants developed the mechanism to utilize H₂O₂ as a signalling molecule with remarkable impact on plant growth and behaviour during the stress treatments (Fig. 2). Apoplastic redox status elicits intra- and intercellular signalling. Due to high stability, H₂O₂ can be moved through the plasma membrane by diffusion or by aquaporin channels (Bienert and Chaumont 2014; Tian et al. 2016). H₂O₂ performs as a signalling molecule at both cellular and organ level as an autocrine (includes intracellular) and paracrine (intercellular) signals (Bienert et al. 2006).

• Autocrine signalling

Fig. 2.

Schematic representation of H₂O₂ -mediated transcriptional cascading. Environmental stresses and anthropogenic activities produce stress conditions and lead to accumulation of H₂O₂, which serves as a second messenger and transmits the signal through sensors, including Ca²⁺, MAPK, H₂S and phytohormone. Downstream cascading activates the H₂O₂–responsive transcription factors (TFs), which relays the signal to expression of downstream genes and regulates positively and negatively at transcriptional level for various oxidative stress responses

Autocrine signalling occurs in the same cell through chemical messengers. H₂O₂-mediated stomatal closure in guard cells is an example of autocrine signalling (Pei et al. 2000; Desikan et al. 2004). During water deficit condition, plants produce ABA as a downstream signalling molecule and cause the stomatal closure. NADPH oxidase acts as an important component for this signalling by elevation of apoplastic H₂O₂ levels. H₂O₂ can function directly as an effector molecule as well as a second messenger in signal amplification. It is postulated that redox-sensitive TFs could sense enhanced H₂O₂ levels and regulate downstream cascades. Class A heat shock factors (Hsfs) are the suitable candidates for such TFs, which are revealed as an oxidative stress-responsive both in animals and plants (Miller and Mittler 2006; Kotak et al. 2007). Mitogen-activated protein kinase (MAPK) cascades the external stimuli to cytoplasmic and nuclear responses and involve during osmotic stress (Hirt 1997), ozone (Samuel et al. 2000) and plant–pathogen interactions (Somssich 1997), which are linked to H₂O₂ through the redox network (Kovtun et al. 2000; Matern et al. 2015).

• Paracrine signalling

Transport of H₂O₂ signals between the cells (intercellular), which need to be pass across membrane is called as paracrine signalling. Mittler et al. (2011) have reported the “ROS wave” cascade signal from cell-to-cell after the initial oxidative burst that transmits throughout different tissues and transports the signal over long distances. The “ROS wave model” linked with calcium signalling and other systemic signals such as ABA, hydraulic waves, and electric signals to imparts acclimation during abiotic stresses. Perfect example of paracrine signalling pathway is the H₂O₂-influenced xylem differentiation and lignification. During xylem lignification H₂O₂ lead to the synthesis of lignin biosynthesis enzymes such as phenylalanine ammonia lyase and help in polymerisation of coumaryl, coniferyl and sinapyl alcohols to lignin. In this manner, H₂O₂ instigates the secondary cell wall formation and finally the PCD in differentiating xylem cells (Fukuda 1996; Barcelo 2005).

H₂O₂ and mitogen-activated protein kinases (MAPKs) cascade

In plants, MAPKs are involved in diverse functions such as growth, development and acclimation during environmental changes. The MAPK signalling pathway is composed of three kinases, MAPK (MPK), MAPK kinase (MKK), and MAPK kinase kinase (MAPKKK or MEKK). It is suggested that exogenously applied H₂O₂, its accumulation or H₂O₂-induced oxidative burst induce the activation of MAPK pathway and relays the intracellular H₂O₂ signal. The MAPK cascades the upstream or downstream application of H₂O₂, and reveals its complexity and multi directionality during the stress signalling. H₂O₂ activates the ANP1 (MAPKKK) protein which subsequently induces AtMPK3 and AtMPK6 phosphorylation (Kovtun et al. 2000). While some MAPK pathways such as MEK2 perform upstream of Rboh genes and ultimately produce H₂O₂ (Yoshioka et al. 2003). In Arabidopsis, one of the ROS-mediated MAPK pathway includes the MEKK1–MKK1/2–MPK4 cascading, where MAPK kinase kinase MEKK1 is controlled by H₂O₂ and various stresses in a proteasome-dependent manner (Pitzschke et al. 2009) and activates the downstream kinases MPK3, MPK4 and MPK6. Remarkably, a unique MAPK pathway has been reported (Miao et al. 2007), where MEKK1 directly interacts and phosphorylates the TF WRKY53. Activated WRKY53 subsequently induces stress and defence-related target genes through DNA-binding activity. MAPKs also get activated in a H₂O₂-dependent manner during plant pathogen interaction and disease signals. In Arabidopsis, MPK3 and MPK6 (Nakagami et al. 2005) are associated with H₂O₂, whereas similar regulatory mechanism have been shown by respective homologes SIPK and WIPK kinases in tobacco (Asai and Yoshioka 2008). BRs-mediated apoplastic H₂O₂ accumulation switches on the MPKs and its cascading to impart stress tolerance in plants. In cucumber, BR-induced process upregulate the Rboh, MPK1 and MAPK3 genes (Xia et al. 2009) while in maize, BR-stimulated ZmMPK5 which drives NADPH oxidase-mediated activities of antioxidant enzymes (Zhang et al. 2010). Recently, the calcium-dependent protein kinase CPK27 had shown cross-talk between H₂O₂, NO, and MPK1/2 towards cold acclimation-induced ABA biosynthesis and cold tolerance in tomato (Lv et al. 2018). Similarly, Wang et al. (2017) reported that the H₂O₂ treatment enhanced the expressions of tomato SlMAPK1/2/3 and SlCBF1 genes and imparted the chilling tolerance by modulating the phytohormones concentration. The tolerance towards other stresses such as drought and salinity is also reported by the application of H₂O₂ through MAP kinase pathways (Wang et al. 2010). The cotton MKK1 (GhMKK1) is upregulated by H₂O₂ and showed enhanced stress tolerance in Nicotiana benthamiana transgenics (Lu et al. 2013). The H₂O₂ is also known to activate the rice salt-responsive ERF1 (SERF1) TF, which showed binding to the MAPK kinase kinase6 (MAP3K6) and MAPK5 and involved in initial phase of salt stress and resulted further in salt tolerance (Schmidt et al. 2013). The MAP cascade genes also reported to reduce the H₂O₂ content in the transgenic plants during abiotic stress condition and provides the tolerance. The Raf-like MAPKKK gene, GhRaf19 from the cotton showed chilling tolerance by alleviating the ROS via increased activity of ROS-related enzymes in N. benthamiana (Jia et al. 2016). A poplar PtMKK4 gene showed strong tolerance to drought stress, the transgenics showed significantly reduced level of H₂O₂ and higher antioxidant enzyme activity (Wang et al. 2014). Similarly, the cotton MAPK (GhMPK17) transgenics showed reduced level of H₂O₂ and tolerance to salt and drought tolerance (Zhang et al. 2014).

H₂O₂-mediated transcription factors

H₂O₂ is reported to induce the signalling cascade by modulating downstream genes expression by switch on or off the transcript expression machinery (Fig. 2). Transcription factor families such as NAC, Zinc finger (ZINC FINGER OF ARABIDOPSIS THALIANA; ZAT), WRKY, ERF, MYB, DEHYDRATION RESPONSIVE ELEMENT BINDING FACTOR (DREB), and BASIC LEUCINE ZIPPER (bZIP) are linked with H₂O₂-induced signalling. Induction of various members of plant TF family and their response in acclimatization of plants under oxidative stress condition after H₂O₂ treatment are mentioned in Table 1.

Table 1.

H₂O₂-induced transcription factors and their downstream cascading in regulation of oxidative stress after H₂O₂ treatment

Sr. no.	Plant	H2O2-induced transcription factor	Regulation of oxidative stress	References
1	Arabidopsis thaliana	ZINC FINGER of ARABIDOPSIS THALIANA 6 (ZAT6)	Activation of anthocyanin biosynthesis	Shi et al. (2018)
2	Arabidopsis thaliana	BRASSINAZOLE-RESISTANT1 (BZR1)	Regulation of cell elongation	Tian et al. (2018)
3	Arabidopsis thaliana	REDOX RESPONSIVE TRANSCRIPTION FACTOR 1 (RRTF1)/ERF109	Generation of an oxidative burst	Heyman et al. (2018)
4	Arabidopsis thaliana	MYB30	Regulation of root elongation and influences plant immunity; Annexin-mediated oxidative stress response through elevation of calcium influx	Mabuchi et al. (2018), Liao et al. (2017)
5	Arabidopsis thaliana	WRKY75	Positive regulator of leaf senescence through repressing the CATALASE2 (CAT2) expression	Guo et al. (2017)
6	Arabidopsis thaliana	Cytokinin Response Factor 6 (CRF6)	Repression of cytokinin-related genes to control the cytokinin-mediated ROS generation and improved oxidative stress	Zwack et al. (2016)
7	Arabidopsis thaliana	Heat shock factor class A (HsfA3)	Increased levels of galactinol and raffinose; Upregulation of APX2 (Ascorbate peroxidase 2) gene	Song et al. (2016), Hwang et al. (2012)
8	Arabidopsis thaliana	ZAT12	Negatively regulate Iron (Fe) uptake; role in oxidative stress signalling	Le et al. (2016), Davletova et al. (2005)
9	Arabidopsis thaliana	Arabidopsis thaliana ACTIVATING FACTOR1 (ATAF1)	Progression of senescence	Garapati et al. (2015)
10	Arabidopsis thaliana	ANAC017	Induction of AOX1a and function in mitochondrial retrograde signalling	Ng et al. (2013)
11	Arabidopsis thaliana	ANAC013	Regulation of Mitochondrial retrograde signalling during oxidative stress	De Clercq et al. (2013)
12	Arabidopsis thaliana	Ethylene response factor6 (ERF6)	Role in the transcriptional regulation of ROS-responsive genes	Wang et al. (2013)
13	Arabidopsis thaliana	JUNGBRUNNEN1 (JUB1)	Delays senescence, reduces intracellular H2O2 levels and enhances abiotic tolerance	Wu et al. (2012)
14	Arabidopsis thaliana	ORESARA1 SISTER1 (ORS1)	Triggers expression of senescence-associated genes and accelerates senescence	Balazadeh et al. (2011)
15	Aeluropus lagopoides	AlNAC4	ROS homeostasis and oxidative stress tolerance in transgenic tobacco	Khedia et al. (2018)
16	Litchi chinensis	LcNAC1	Induction of LcAOX1 gene and regulate ROS production and energy metabolism	Jiang et al. (2017)
17	Lycopersicon esculentum	TOMATO ERF1 (TERF1)	Reduction of ROS accumulation and alleviate H2O2 stress in tobacco transgenics	Zhang et al. (2016)
18	Malus domestica	MdNAC1	Enhances antioxidative enzymes activity (POD, CAT and SOD) as well as their gene expression	Jia et al. (2019)
19	Oriza sativa	Salt-responsive ERF1 (SERF1)	Confer salinity tolerance through MAPK pathway	Schmidt et al. (2013)
20	Poncirus trifoliata	basic/helix-loop-helix (bHLH)	Activation of POD-mediated H2O2 scavenging in transgenic tobacco	Huang et al. (2013)
21	Populus euphratica	PeSTZ1	ROS scavenging through regulation of PeAPX2 and confers cold tolerance	He et al. (2019)
22	Solanum lycopersicum	CBF1 (C-repeat/dehydration-responsive factor)	Enhance chilling stress with phytohormone modulation	Wang et al. (2017)

In Arabidopsis, many H₂O₂-regulated leaf senescence-associated NAC TFs have been identified, regulating senescence both positively (ATAF1, ANAC092, ANAC059, ANAC029 and ANAC016) and negatively (ANAC042, ANAC083). The H₂O₂ and ABA is reported to activate the Arabidopsis thaliana Activating Factor1 (ATAF1), an upstream regulator of senescence. The ATAF1 induced senescence by activating ANAC092 and repressing Golden 2-like1 (GLK1) genes by shifting the physiological balance (Garapati et al. 2015). Balazadeh et al. (2010a) have reported that H₂O₂-upregulated 15 senescence-associated NAC TFs, among which ANAC032 and ANAC042 TFs showed significantly higher induction. ANAC042 has already been reported in H₂O₂-induced relay cascades via OXI1/MPK3/6 activation (Gechev and Hille 2005). ANAC092/ORE1 have also been proved to play role in the regulation of salt-induced senescence (Balazadeh et al. 2010b) along with the developmental senescence in Arabidopsis (Kim et al. 2009). Some of the NAC TFs are reported to delay the senescence and help in providing the stress tolerance during abiotic stress. JUNGBRUNNEN1 (JUB1) gene, one of the NAC TF family member, showed delayed senescence and conferred abiotic stress tolerance by reducing intracellular H₂O₂ in transgenic Arabidopsis (Wu et al. 2012). The possible mechanism of JUB1 is by activating the DREB2A TF which in turn upregulate several ROS-responsive genes such as glutathione S-transferase. In Litchi chinensis, the enhanced expression of LcNAC1 was observed by ABA and H₂O₂ treatments. LcNAC1 positively activates the LcAOX1a gene and reduces the senescence time by regulating the ROS related genes and energy metabolism, whereas, LcWRKY1 regulates the LcAOX1a antagonistically (Jiang et al. 2017). Recently, it has been reported that overexpression of MusaSNAC1 increased H₂O₂ content in guard cells augmented stomatal closure for facilitating drought tolerance (Negi et al. 2018). Chilling tolerance was found to be enhanced by overexpressing tomato SlNAM1 TF in tobacco, wherein increased H₂O₂ content reduced by antioxidative enzymes (Li et al. 2016). The Aeluropus AlNAC4 TF showed significantly higher expression with H₂O₂ and dehydration treatments. The overexpression of the AlNAC4 TF in the tobacco confers oxidative stress tolerance by reducing H₂O₂ content through ROS homeostasis and regulation of stress-related downstream genes (Khedia et al. 2018). Borgohain et al. (2019) reported the role of tomato SlNAC2 TF in increased abiotic stress tolerance with expression of the important glutathione biosynthetic genes, which showed improved antioxidative response and reduced ROS accumulation (H₂O₂ and O^•−₂) in transgenic plants. Similarly, the apple MdNAC1 exhibited drought tolerance through enhanced activity of antioxidant enzymes such as peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD) and reduced level of H₂O₂ content (Jia et al. 2019).

H₂O₂ also strongly induces the zinc finger TF family, especially various members of the ZINC FINGER OF ARABIDOPSIS THALIANA (ZAT) family at the transcript level (Gadjev et al. 2006). ZAT12 TF is one of the important component of the oxidative stress which transduce the signal to ZAT7 and WRKY25 TFs and express the cytosolic APX1 during elevation of H₂O₂ in Arabidopsis (Rizhsky et al. 2004). The Zat12 mutant plants showed the reduced expression of Zat7 by H₂O₂ treatment, which confirmed that ZAT12 regulate the activity of ZAT7. Miller et al. (2008) have shown the role of AtZAT proteins (AtZAT7, AtZAT10, and AtZAT12) in the ROS signalling and plant stress responses. Shi et al. (2018) showed pivotal role of the AtZAT6 in H₂O₂-mediated secondary metabolite synthesis such as anthocyanin through direct binding to promoters of anthocyanin synthesis genes. Studies have supported the antioxidant role of anthocyanin through reduction in photooxidative damage under high irradiance (Zeng et al. 2010; Zhang et al. 2012; Maruta et al. 2014) and wounding (Gould et al. 2002). Populus euphratica, PeSTZ1, a C2H2-type Zn finger TF showed freezing tolerance through direct regulation of PeAPX2 and modulate ROS scavenging (He et al. 2019).

WRKY TFs also play important role in biotic, abiotic and oxidative stresses (Eulgem and Somssich 2007). Improved expression of several WRKYs in Arabidopsis have been reported by H₂O₂ treatment (Chen et al. 2010) that regulate the senescence and defence-related gene expression (Besseau et al. 2012). WRKY52 and WRKY70 are widely reported H₂O₂-inducible TFs, from which WRKY52 is a senescence-associated and regulates functions of various stress and defence-responsive genes (Miao et al. 2004). Guo et al. (2017) showed involvement of WRKY75 TF in positive regulation of leaf senescence in the presence of SA and ROS in Arabidopsis. Similarly, WRKY53 TF positively accelerated leaf senescence through SA and JA equilibrium, and negative regulation through interaction with JA-inducible protein EPITHIOSPECIFYING SENESCENCE REGULATOR (ESR/ESP) (Miao and Zentgraf 2007). The jatropha, JcWRKY showed lower accumulation of ROS (H₂O₂ content) in transgenic tobacco compared to wild-type plants during Macrophomina infestation as well as by SA treatment (Agarwal et al. 2018).

Oryza sativa SALT-RESPONSIVE ERF1 (SERF1) a root-specific TF induces during salinity and H₂O₂ treatments. It promotes the expression of the H₂O₂-responsive genes, such as MAPK KINASE KINASE6 (MAP3K6), MAPK KINASE5 (MAPK5), DREB2A, and ZFP179 (Schmidt et al. 2013). Ectopic expression of tomato TERF1 in Nicotiana tabacum showed enhanced H₂O₂ stress tolerance in seedlings as well as activation of downstream genes involved in oxidative responses, including carbonic anhydrase (in hypersensitive defence), catalase, glutathione peroxidase (in antioxidant response) and GDP-d-mannose pyrophosphorylase (in ascorbic acid biosynthesis) (Zhang et al. 2016).

Arabidopsis myb30 mutant showed Ca²⁺ mediated signalling in response to H₂O₂ and heat stress (Liao et al. 2017) and similarly Mabuchi et al. (2018) reported strong response of ROS-induced AtMYB30 in the inhibition of root cell elongation. ZmMYB31 has been reported to play a role in chilling and peroxide stress alleviation in transgenic Arabidopsis through declined photoinhibition and ROS accumulation (Li et al. 2019). SbMYB15 TF from Salicornia brachiata conferred heavy metal (Cd²⁺ and Ni²⁺) tolerance in transgenic tobacco with reduced level of H₂O₂ content and increased ROS scavenging antioxidative enzyme activities (Sapara et al. 2019). SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) TF showed enhanced resistance against P. syringae with higher H₂O₂ accumulation and SA signalling pathway. Foliar H₂O₂ application confirmed the SA-mediated defence response with reduction of miR156 expression, which negatively regulates SPL9 TF (Yin et al. 2019). DREB2A TF is also included in the H₂O₂-induced TFs family (Rizhsky et al. 2003). DREB2A showed salinity and drought tolerance in transgenic tobacco plants by lowering the H₂O₂ content (Gupta et al. 2014). Li et al. (2017) have reported the involvement of bZIP TFs in H₂O₂-regulated family through conferring disease resistance against cassava bacterial blight.

Conclusion and perspectives

Plants being sessile have an innate ability to adapt itself to the various natural and manmade variations through complex stress signalling and leading to rapid alterations in physiology and metabolism pathways. The generation of ROS serves as a key molecular marker for adverse external stimuli and participates both positively and negatively for transcriptional signalling. Understanding the molecular mechanism towards oxidative stress responses of plants is very essential, as it acts as a central dogma towards coordinating the multiple stress responses. The synthesis of ROS-associated phytohormones such as auxin, brassinosteroid, gibberellin, salicylic acid and ethylene cross-talk during stress conditions and facilitates plant acclimation through various physiological and biochemical processes. H₂O₂ has the higher longevity amongst the different ROS family members and function as a secondary messenger during various stress responses. The oxidative burst regulates the ROS scavenging mechanism to maintain the cellular redox status that facilitates the proper enzymatic activation and metabolism. The research on the interaction of ROS production and scavenging for maintain redox status is gaining momentum. The NAC, Zinc finger, WRKY, ERF, MYB, DREB and bZIP are the major H₂O₂-induced TF families involved in oxidative stress responses. It is needed to understand the downstream events leading to specific responses. The dose- age specific ROS responses also is an important aspect of ROS signalling. The integration of the ROS is the central component of the signalling network, however, still the gene network, threshold concentration in different cellular compartments, ROS scavenging and interaction with other signalling components need to be elucidated to gain insights into the biological role of ROS in alleviating stress and also promoting growth and development.

Compliance with ethical standards

Conflict of interest

No conflict of interests exist.