Introduction

Nitric oxide (NO) is an essential gasotransmitter, which acts as a signaling molecule during plant stress. NO cross-talk with other signaling molecules to transduce stress signals between the cells. These signaling molecules include reactive oxygen species (ROS), phytohormones [Auxin (Aux), gibberellin (GA), cytokinin (CK), ethylene (ET), and abscisic acid (ABA), jasmonic acid (JA)], plant growth regulators’ melatonin (MT), and other signaling molecules. This suggests that two or more biosynthesis pathways share some common path to regulate signals in better ways, also known as crosstalk. Several endogenous growth regulators such as ABA and GA are reported previously for breaking of seed dormancy/inducing seed germination. In recent years, nitrate, nitrite, hydroxylamine, azide, NO, and sodium nitroprusside (SNP) compounds were also identified to regulate seed dormancy and germination processes through phytohormonal cross-talk (Krasuska et al. 2017).

For instance, ET and NO crosstalk with ABA during seed germination and dormancy period counteract the action of ABA (Arc et al. 2013). Similarly, in canola and maize, exogenous application of NO enhances seed germination in a dose-dependent manner (Fan et al. 2013a, b). Nevertheless, different mechanisms stimulate seed germination by light and NO reported (Beligni et al. 2000; Poor et al. 2019). It remains unclear whether the GA- and NO-promoting germination mechanism acts synergistically or antagonistically. CK and NO crosstalk were also reported to regulate the photo-morphogenesis process observed in Arabidopsis, parsley, or tobacco cell (Tun et al. 2001). Exogenous application of NO and CKs inhibit hypocotyl elongation in Arabidopsis and lettuce dark-grown seedlings (Beligni et al. 2000). Recently, Wu et al. (2016b) reported that hydrogen peroxide, NO, and UVR 8 interact with each other and are subjected to anthocyanin accumulation in reddish sprouts. Likewise, NO plays a crucial role in inhibiting primary root growth in Arabidopsis by regulating PHYTOCHROME INTERACTING FACTOR 3 (PIF 3) under light conditions (Bai et al. 2014). There is evidence that IAA and NO regulate the same responses in plants due to sharing some common steps during the signal transduction pathway. For example, growth of maize root segment influenced by NO in a dose-dependent manner similar to indole acetic acid (IAA) (Gouvea et al. 1997).

Studies suggested that NO plays a crucial role in stomatal movement, together with H2O2, abscisic acid (ABA) under water stress (Garcia-Mata et al. 2002; Desikan et al. 2002; Garcia-Mata et al. 2003; Desikan et al. 2004). NO regulate stomatal closure through Ca2 þ–dependent stomatal closure mechanism (Desikan et al. 2001). Synergistic effects of ABA and NO on stomatal closure were observed in Pisum sativum and Vicia faba plants (Neill et al. 2003). Some research also confirmed NO in guard cells (Garcia-Mata et al. 2002), leading to stomata closure through NR activity. Recently, it is suggested that UVR8, H2O2, and NO interact with each other under UV light and close the stomata by regulating the UVR8 pathway (Tossi et al. 2014). NO also increases the chlorophyll content in potato, lettuce, and Arabidopsis (Beligni et al. 2000). NO preserves and increases chlorophyll content similarly to CKs “chlorophyll retention effect” in pea and potato (Leshem and Wills 1998).

Rapid synthesis of NO and a parallel accumulation of ROS are typically observed under biotic and abiotic stresses. Consequently, these adverse responses activate the senescence process, ultimately leading to the death of plant cells. Earlier studies suggest that both NO and ROS play important roles in regulating programmed cell death (PCD) either independently or synergistically (Wang et al. 2013). Therefore, NO plays crucial functions in nutrient homeostasis, ion transport, plastid development, and alleviation of antioxidant genes during normal and unfavorable conditions as signaling compounds. Some of the pivotal roles of NO in plant growth and development are highlighted in Fig. 1.

Fig. 1
figure 1

Schematic illustration of nitric oxide (NO) pools in the cells triggered under stress and their subsequent metabolism. The possible different pathways can lead to the generation of NO pool in the cytosol (enzymatic and non-enzymatic pathways) and apoplast (non-enzymatic pathway). Cytoplasmic organelles such as peroxisome, mitochondria, chloroplast, and acidic vacuoles are the prime compartments for NO biosynthesis. Various effectors regulating (up- or down-regulation) NO pools are phytohormones (ABA, cytokinin, GA, auxin, brassinosteroids), ambient light, transition metals, phenolics, abiotic stressors, and feedback inhibition (NO pool-mediated reduction of NO2− to NO). Solid lines () and dotted lines () are used to avoid the overcrowding and clarity of the figure; ( ) depicts inhibition/inhibitory effect

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Abiotic (drought, salinity, heavy metals, extreme temperature, etc.) stresses are a significant concern for low agricultural production worldwide. They are steadily increasing due to uninvited anthropogenic activities in the natural environment (Asgher et al. 2017). These stresses adversely affect plant growth and development (Khan et al. 2015a; Fancy et al. 2017) by producing ROS (singlet oxygen, hydrogen peroxide, hydroxyl radicals, superoxide radicals, etc.). These are needed for the proper functioning of cells under normal conditions but adversely affect the cell programming system under stressful environments (Gupta et al. 2016; Asgher et al. 2017). The multiple stresses induce modulation of phytohormonal regulation, metabolism, and signaling in plants, which affects the plant defense system through metabolic adjustment, stomatal regulation, and behavioral changes in plant growth and development (Zhang et al. 2006a). The NO has been considered either a protective mediator or stress-inducing agent and plays a crucial role in intracellular redox signaling, ion homeostasis, and activation of antioxidant defense mechanisms (Asgher et al. 2017). Several studies suggested NOs’ role in maintaining pigment composition, stomatal movements, root growth and development, water relations, membrane stability, hormonal balance, osmotic adjustments, and ion channels’ activities in plants under different circumstances through cross-talk with other signaling compounds (Li et al. 2015; Shan et al. 2015; Kaya et al. 2020a, b; Wu et al. 2020; Santos et al. 2020).

On recognizing the importance of NO crosstalk in plants under multiple abiotic stresses, in this review, we have explored biosynthesis and metabolism pathways of NO in different cellular sites and their regulating factors. Then, we have discussed the NO cross-talk with other signaling compounds, their regulatory roles, and crucial molecular mechanisms of NO crosstalk under multiple abiotic stresses. This information will help us understand the role of NO crosstalk as a central hub in regulating plant processes under different environmental stresses.

NO Biosynthesis and Metabolism

Nitric Oxide (NO) has multifaceted physiological role in plants as a bioactive gasotransmitter. Eight different enzymatic and non-enzymatic processes that can produce NO in plants have been identified to date. Nitrite (NO2−) or more reduced compounds (L-arginine or hydroxylamine) are produced due to NO generation through oxidation (Mur et al. 2013). Cytoplasm, mitochondria, chloroplast, peroxisome, and apoplast are the major cellular sites for NO2−reduction (Roszer 2012a,b). Reduced NO can be generated through nitrate reductase activity (NR; EC 1.6.6.1 to EC 171) via mitochondrial electron transport chain (mETC) or heme-containing proteins. The oxidative NO can be synthesized through L-arginine and other compounds. In the acidic compartments of plant tissues, non-enzymatic reduction of NO2−/NO can also happen (Roszer 2012a, b; León and Costa‐Broseta 2020). Mechanism of production or synthesis of oxidized and reduced, enzymatic, and non-enzymatic NO are discussed in this section and highlighted in Fig. 2.

Fig. 2
figure 2

Highlights the crucial functions of NO crosstalk in plant growth developments and under stresses. Nitric oxide regulates the crucial function from germination to post-harvesting and regulates critical processes (physiological, biochemical, and molecular) during multiple stresses

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Mechanisms of Reductive Synthesis

By Nitrate Reductase (NR)

Nitrate reductase can reduce NO2− to NO with low efficacy through primary nitrate (NO3−) oxidoreductase activities (Rockel et al. 2002). In cyanobacterium (Anabaena doliolum), green algae, and vascular plants, NR catalytic reductions from NO2− to NO have a crucial role during stress response (Mur et al. 2013; Floryszak-Wieczorek et al. 2016). It indicates one of the oldest forms of NO production mechanisms in plants (Astier et al. 2018). Cytoplasm and chloroplast association are the main pool of NR activity (Kolbert et al. 2019). However, using a reduced cytochrome c as an electron donor, NO2−/NO-reductase (NI-NOR) reduces NO2− to NO. NO NI-NOR generation is similar to NO3−-reduced root-specific NR activity, but NO-NOR may act as a separate protein and needs to be regarded as NR-generated NO (Mohn et al. 2019).

By Mitochondrial ETC (Electron Transport Chain)

Mitochondria can use NO2− as an alternate electron acceptor for ATP synthesis; reduction of NO2− to NO takes place inside complex III (cytochrome bc1) and IV (cytochrome-c oxidase, CCO) (Gupta and Igamberdiev 2011a; Kolbert et al. 2019). The mechanism creates hypoxia in plant cells which results in mitochondrial NO generation. Hypoxia increases NR transcription activities, which converts NO3− to NO2− and results in a cytoplasmic accumulation of NO2−. NO2− reduction is limited in the hypoxic cell, and a continued supply of NO2− for reduced NO synthesis is permitted (Roszer 2012a, b). Therefore, a specific system of O2 transport in plants reduces NO synthesis/mitochondrial NO2− or NO. The NO generated within the mitochondria inhibits the germination of CCO (Gniazdowska et al. 2010a, b), which enhances the energy status of O2-limited cells (Gupta and Igamberdiev 2011b). The reduced mitochondrial NO generation inhibits the photo-respiratory cycle and fermentative metabolism (Oliveira et al. 2013). NO released from mitochondria into the cytosol is oxidized by plant hemoglobin (NO3−) due to hypoxia (Igamberdiev and Hill 2004). This leads to NO/NO2− exchange of mitochondria in the cytoplasm, maintaining a continuous supply of NO2 for ATP synthesis under hypoxia (Gupta and Igamberdiev 2011b). The cytoplasmic conversion NO/ NO3−/NO2 ensures that the low redox level helps adapt to the hypoxic NADH/NADP+ and NADPH/NADP+ ratios (Igamberdiev et al. 2010).

By Heme-Containing Proteins

Plant peroxisomes can produce NO under hypoxic or anoxic conditions by reducing NO2− (Igamberdiev et al. 2010). NO2−/NO may reduce the capacity of deoxygenated heme-containing proteins in the peroxisome matrix, which is the primary production mechanism (Igamberdiev et al. 2010; Sturms et al. 2011). The plant plasma membrane, cytosol, and endoplasmic reticulum have shown a similar reductional NO generation (Igamberdiev et al. 2010). Cyanobacteria (Sturms et al. 2011) and mammalian tissues have also been affected by a reduction in the use of heme-proteins from NO2− to NO (e.g., hemoglobin’s) (Tiso et al. 2011).

Mechanisms of Oxidative NO Synthesis

NOS (EC 1.14.23.29) proteins and NOS encoding genes (Roszer 2010) have been identified in prokaryotes, unicellular eukaryotes, invertebrates, non-mammalian vertebrates, and mammals. However, higher plants lack homologous sequences for known NOS encoding genes (Mur et al. 2013). Oxidative L-arginine synthesis is also present in plants' cells, but the responsible enzyme NO synthase (NOS) has not yet been found. Some of the pathways.

From L-arginine

The chloroplasts and leaf peroxisomes of the vascular plants and the green algae have been identified as a site for enzymatic oxidation of L-arginine to NO and L-citruline (Roszer 2012b). The chloroplastic oxidation of L-arginine to NO requires NADPH and in the absence of Ca2+ (Jasid et al. 2006). In the peroxisomes of leaves, Ca2+, calmodulin, FAD, FMN, and NADPH are required for L-arginine/L-citrulline conversion (del Rio et al. 2003; del Río 2011). It has been recently found that L-arginine-oxylated NO synthesis requires both Ca2+ and NADPH, with tetrahydrobiopterin (BH4) in Ostreococcus green algae species (Foresi et al. 2010). Plant mitochondria also oxidize L-arginine to NO with the help of enzymes available in the matrix or intermembrane space (Guo and Crawford 2005). It is debatable whether plant mitochondria contain a specific NO oxidative synthesis enzyme (Barroso et al. 1999).

Other Forms of Oxidative NO Synthesis

Polyamines and hydroxylamine have recently been shown to increase the synthesis of oxidative NO in plant cells (Wimalasekera et al. 2011). The exact mechanism of polyamines in increasing NO synthesis remains uncertain (Fröhlich and Durner 2011). However, NO cannot mediate the effect of polyamines on plants. Pathways possibly responsible include an interaction between polyamines and NR-catalyzed NO (Rosales et al. 2012) and the indirect impact of polyamine synthesis on L-arginine metabolism (Zhang et al. 2011). Hydroxylamine is an intermediate in the nitrification process and can be oxidized to NO in tobacco cell cultures (Rumer et al. 2009). This mechanism could be a substitute for oxidative NO synthesis via L-arginine. However, the underlying molecular mechanism is still unknown for hydroxylamine's role in NO synthesis (Rumer et al. 2009). The possible contribution to NO plant synthesis for other enzymes needs to be explored.

Non-enzymatic NO Generation

Non-enzymatic NO generation includes release from nitrous acid (HNO2) after protonation. Acidic environments such as apoplast of germinating and hypoxic seeds favor this type of chemical NO release (Yamasaki 2000; Bethke et al. 2004). Consequently, in the aleuronic layer of barley, the NO release from NO2− has been shown (Bethke et al. 2004). Phenolic compounds found in aleuron apoplast and on seed coat increase this non-enzymatic NO release. The NO release in germinating seeds may protect them from soil microorganisms (Bethke et al. 2004). In addition, seed dormancy is interrupted by NO, which suggests that proper germination requires NO2− together with an enzymatic NO synthesis (Roszer 2012b). Together, NO release can synergize with the reduction of the enzyme NO2−/NO to invoke germination NO burst. During germination, NO-mediated programmed cell death occurs when aleuron cells are removed (Lombardi et al. 2010). However, the release of NO from S-nitrosoglutathione (GSNO) (del Río 2011) is another possible, unexplored mechanism for non-enzymatic NO generation. This compound is formed in the oxidative environment of peroxisomes, which allows both GSNO and GSNO to react with glutathione (Barroso et al. 2006). The GSNO is a compound NO-donor and could carry reserve NO distributed in the plant's tissues. GSNO genesis is facilitated by environmentally friendly light and metal transition (Floryszak-Wieczorek et al. 2006). Hydroxylamine is another possible non-synthesis substrate (Rumer et al. 2009), but GSNOR would not support hydroxylamine production.

NO Metabolism

NO metabolism includes a redox range, which displays distinctive properties and reactivity such as nitrosonium (NO+), NO radical (NO*), and nitroxyl anion (NO−) (Gisone et al. 2004). Nitrosation in aqueous phases in organic molecules in –S, –N, –O, and –C centers results in NO+. The biological relevance of NO+ was disputed under slightly acidic or physiologic conditions, but a variety of nitroso-compounds forming effectively under neutral physiological conditions could be interpreted as NO+ reactions (Stamler et al. 1992). These compounds include metal-nitrosyl-complexes, thionitrites (RS-NO), nitrosamines (RNH–NO), alkyl- and aryl-nitrites (RO–NO), and tri- and tetra-oxides (N2O3 and N2O4) of dinitrogen. Numerous nuclear centers in biological systems whose potential nitrosative vulnerability were demonstrated in in vitro studies (Stamler et al. 1992). Dimerization and dehydration quickly convert NO− to the N2O (Basylinski and Hollocher 1985) and reacts with Fe (III) heme (Goretski and Hollocher 1988).

NO* is also reversible in sulfhydryl oxidation, leading to low molecular weight and protein-associated thiols. The transmission of electrons and collisions is standard and generally results in NO radical (NO*) as the main product. S-nitrosothiols are thought to be a (minor) product of the NO disulfide reaction (Stamler et al. 1992). The significant NO reactions are those with O2 and its different redox and transition ions in biological terms. When discussing the chemical and physiological effects, NO is a highly diffused secondary messenger that may generate relative effects far from its production site in plants. Hence, the concentration and the source of NO are the main determinants of its biological effects (Wink and Mitchell 1998). The direct effects of NO are the result of the interaction between NO and metal complexes. NO form complexes, including those found regularly in metalloproteins, of transition metal ions. Heme-containing protein reactions have been studied extensively for NO-complexes.

NO also forms non-heme transition metal complexes, and biochemical focus was given to its responses to the Fe–S center of the proteins, including several mitochondrial electron transportations and enzyme proteins (Henry et al. 1991). NO's reaction to heme-containing proteins includes cytochrome P450 interactions with more considerable physiological consequences (Wink et al. 1993). Tyrosine nitration is also a directly established effect of NO on proteins. Tyrosine nitration is selective and reversible and ONOO− dependent. In vivo nitration pathways were shown to be ONOO− independent (Davis et al. 2001). NO can also stop lipid peroxidation (Rubbo et al. 1995). Nitrosating oxidation or nitration is the indirect effect of NO, generated by the interaction of the NO and O2− (Wink et al. 1993). None of these substances can undergo autoxidation (i.e., reactions to O2) to produce N2O3 in aquatic solutions (Ford et al. 1993). Since NO and O2 are 6–20 times more soluble in lipid layers, the auto-oxidation rate in a lipid phase (Ford et al. 1993) dramatically increases. The primary N2O3 reactions are thought to occur in the membrane fraction.

In its response to O2, NO generates ONOO at a rate near diffusion, which acts as a nitrating agent as well as a powerful oxidant to modify the proteins (nitrotyrosine formation), lipids (lipid oxidation, lipid nitration), and nucleic acids (DNA oxidation and DNA nitration). In short, there are numerous potential reactions of NO depending on the cell milieu facilitating biochemical modifications. The production site, source, and NO concentration collectively determine its effects. In addition, a relative equilibrium exists between oxidative and nitrosating stress. The mechanism of NO biosynthesis and its metabolism are highlighted in Fig. 1.

NO: Plant Signaling Component Hub

Perceiving the cues within cells and outside the environment is vital for the plant life cycle. This perception is accomplished by plant signaling. Plant signaling involves an exchange of information between plant cells from receptors to effector through signaling molecules. Discoveries of molecular components related to signaling provided evidence about the signal response as a cumulative effect of cross-talk between different signaling pathways (Taylor et al. 2004). This cross-talk generally results from pathway integration with the unique signal response as a combination. Such cross-talk involved in physiological processes ranged from development to stress responses (Peck and Mittler 2020). Some of the critical signaling compounds are ROS, PGRs, and signaling peptides discussed in this section in NO as a central signaling molecule. NO orchestrate a plethora of signaling responses in plants. These responses act at inter- and intra-cellular levels to modulate plant growth and development. NO-mediated transcriptional changes or secondary messenger activation regulates these processes (Falak et al. 2021). These processes include photosynthesis, organelles motility, hypersensitive response, programmed cell death, seed germination, cell wall lignification, flowering, pollen tube growth, fruit ripening as well as legume–rhizobium symbiosis, and biotic and abiotic stress (Turkan 2017; Sami et al. 2018; Inmaculada Sánchez-Vicente et al. 2019).

NO signaling operates at various levels, specifically with ROS of the anti-oxidant system (Ma et al. 2016) and affects seed dormancy, plant reproduction mechanisms (Jiménez-Quesada et al. 2016), plant–rhizobia interaction (Damiani et al. 2016), and plant–pathogen interactions (Thalineau et al. 2016). Moreover, higher NO/ROS content correlates with the compromised antioxidant system in plants (Gaupels et al. 2016). This interplay of NO/ROS homeostasis is also vital for N nutrition and plant immunity. These processes are mainly governed by NR activity, which is an essential part of NO signaling after stress induction. Hormonal control on NO/ROS homeostasis is a crucial factor in plant development and stress response, as reported by Sivakumaran et al. (2016). Also, mitochondria play a vital role in the modulation of NO and ROS signaling by changing hypoxic or anoxic conditions (Gupta and Igamberdiev 2016). Other than the direct involvement of NO in ROS production, post-translational modification of NO enzymes is essential for NO/ROS homeostasis, emphasizing the ascorbate–glutathione cycle (Begara-Morales et al. 2016). NO self-regulation also affects ROS levels (Romero-Puertas and Sandalio 2016). This regulation indicates the fine-tuning of NO and ROS as signaling components.

Another facet of NO signaling operates as a secondary messenger in conjugation with other signaling molecules as cytosolic Ca2+ levels, cyclic guanosine 5′-monophosphate (cGMP), cyclic adenosine diphosphate ribose (cADPR), phosphatidic acid, H2O2, JA and SA, and Mitogen-Associated Protein kinases (Santner and Estelle 2009; Foyer and Noctor 2015; Duszyn et al. 2019; Yang et al. 2019). NO- cGMP-dependent pathway in plants opens avenues of NO crosstalk with cGMP signaling (Gross and Durner 2016). While NO-mediated cGMP signaling is well known in mammals, this system is not well defined in plants. However, the identification of enzymes of the cGMP pathway in higher plants supports this hypothesis. This crosstalk provides the molecular basis of physiological and developmental responses generated through NO signaling.

Further, downstream target protein studies give cues about the indirect effect of NO signaling (Simon and Dresselhaus 2015). Other than cross-talk in conventional pathway, NO directly interacts with other molecules to affect the biological processes in the plant, for example, NO–sulfur (Fatma et al. 2016), NO–inositol (Lytvyn et al. 2016), NO–heme oxidase 1 (Wu et al. 2016a), and NO-H2O2 interactions (Molassiotis et al. 2016). Understanding this cross-talk in light of NO response and signaling will provide insights into its mechanism. The NO crosstalk with other crucial signaling compounds is highlighted in Fig. 3 and discussed in this section.

Fig. 3
figure 3

Model highlights the NO crosstalk with PGRs and other signaling compounds in plant growth regulation and stress conditions. NO crosstalk is very complex in nature; it crosstalk with numerous signaling compounds such as H2S, H2O2, Ca, melatonin, ethylene, abscisic acid, and salicylic acid to regulate various homeostasis processes under normal and stress conditions

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Molecular Understanding of NO Crosstalk with Crucial Signaling Compounds

In the signaling cascade, phytohormones are instrumental for orchestrating plant growth, development, and stress responses (Santner and Estelle 2009). NO is an essential cue in signaling cascade interactions with all major hormones and other endogenous molecules (Freschi 2013). Here, NO acts as a secondary messenger for plant hormones involved in stress responses (Saito et al. 2009; Liu et al. 2010). The subsequent section will discuss the NO accumulation in specific tissues to perform particular functions in routes with hormonal regulation.

NO–ABA Crosstalk

Abscisic acid (ABA) is referred to as stress hormone cross-talk with NO during various environmental challenges and activates the antioxidant system (Hancock et al., 2011; Freschi 2013). The ABA-induced response was reduced after the decrease in NO synthesis, which suggests that it is acting downstream of ABA under stress treatments (Tossi et al. 2012; Zhang et al. 2009). On contrary to this, NO counteracts the ABA (Lozano-Juste and Leon 2010a, 2010b). This mechanism operates at cell, tissue, and organ level and indicates the specificity of NO–ABA signaling under specific physiological events. The role of NO–ABA crosstalk was reported in different physiological processes, for example, during germination (Liu et al. 2009) as transcriptional inducer and in the maintenance of seed dormancy (Bethke et al. 2006). Under stress conditions, ROS generation induces the ABA–NO crosstalk by activating antioxidants and transcription factors (Lu et al. 2009; Zhang et al. 2007a). Other signaling molecules such as cGMP MAPK and type 2C protein phosphatases act downstream of NO–ABA interplay and antioxidant system to modulate plant stress response (Desikan et al. 2002; Dubovskaya et al. 2011; Mioto et al. 2013). Mutant studies suggested the role of this cross-talk for salinity stress (Lu et al. 2009; Kong et al. 2016), heat and drought stress (Zandalinas et al. 2016), and thermotolerance of plant calluses (Song et al. 2008).

NO–GA Cross-Talk

Gibberellic acid is a crucial phytohormone associated with seed germination and plant growth. In the signaling cascade, NO promotes the biosynthesis of GA by transcriptional regulation of GA biosynthesis genes (Bethke et al. 2007). NO acts as a balance center for ABA-induced dormancy and GA-stimulated germination. The molecular basis of this balance lies in the activation of the anti-oxidant system along with post-translational modification of other enzymes involved in ethylene synthesis (Gniazdowska et al. 2010a, b; Hebelstrup et al. 2012). GAs have been reported to control hypocotyl growth in coordination with DELLA protein degradation (de Lucas et al. 2008). Interestingly enough, higher NO levels antagonize hypocotyl growth (Beligni and Lamattina 2000). Moreover, NO was also reported in the repression of PIF genes and augmenting DELLA protein content (Lozano-Juste and León 2011). That led to the possibility of NO–GAs–light interplay in the regulation of seed germination events. NO–GAs module also operates at various stress conditions, for example, aluminum toxicity in wheat (He et al. 2012), cadmium toxicity in Arabidopsis (Zhu et al. 2012), and deprived phosphorous condition (Asgher et al. 2017).

NO–Auxins Crosstalk

Auxin is an essential phytohormone associated with cell elongation. NO as a signaling molecule in NO–Auxin crosstalk modulates auxin degradation enzyme activity (Xu et al. 2010), interferes with auxin transport through PIN1 efflux carrier (Fernández-Marcos et al. 2011), and activates auxin signaling by S-nitrosylation of the auxin receptor protein (Terrile et al. 2012). The role of auxin in plant root architecture, lateral root growth, and root hairs is well documented (Overvoorde et al. 2010). Interestingly, most root architecture phenotypes are also influenced by NO as signal molecules (Fernández-Marcos et al. 2011). In vitro cultures suggest that auxin application does not affect NO release (Tun et al. 2001). This advises downstream action of NO in auxin signaling response (Chen et al. 2010). NO–Auxin crosstalk operates from synthesis to perception in response to environmental and developmental cues. This crosstalk was also reported in plant stress responses, for example, iron deficiency (Chen et al. 2010), drought and water stress conditions (Pagnussat et al. 2002; Liao et al. 2012) due to extensive involvement with root architecture regulation, and cadmium toxicity (Yuan et al. 2016; Xu et al. 2010).

NO–Melatonin Crosstalk

Melatonin is the novel amine-derivative hormone class involved in plant growth, development, aging, and stress response. Interaction of NO with melatonin regulates the melatonin synthesis genes and changes the phytohormone level (Zhu et al. 2019). Further, downstream action of NO activates MAPK-associated defense responses. Exogenous application of melatonin induces glycerol, sugar production, ultimately increasing NO and salicylic acid levels. NO–melatonin crosstalk affects several physiological processes like root growth, aging, and iron deficiency alleviation (Zhu et al. 2019; Kaya et al. 2020a).

NO–JA Crosstalk

Jasmonic acid is a fatty acid-derivative phytohormone mainly associated with herbivory and pathogen response. Abiotic stress, such as drought stress, affects the JA-associated signaling genes (Huang et al. 2008). NO treatment induces JA-biosynthesis genes that indicate interplay of NO–JA module (Palmieri et al. 2008). CDPKs are induced by JA, starting the ABA-induced stomatal closure (Munemasa et al. 2007). External treatment with MeJA and ABA increases NO and ROS content in guard cells (Munemasa et al. 2007). Evidence suggests calcium signaling acting downstream of NO–ROS crosstalk. Apart from that, JA associated with NO synthesis increases ROS scavenging enzyme as reported for chilling stress tolerance in Cucumis sativus (Liu et al. 2016).

NO–CK Crosstalk

Cytokinins are a class of phytohormones associated with plant cell division in plant shoot and root. CK–NO module of signaling affects the biosynthesis of nitric oxide; however, peroxynitrite (NO-derived) binds with zeatin to reduce its activity (Liu et al. 2013). Type-A response regulators are a crucial component of CK signaling regulated by NO-mediated S-nitrosylation (Feng et al. 2013). NO–CK crosstalk also operates in different stress responses, such as water stress conditions (Shao et al. 2010) and salt stress conditions. Antagonistic relation of CK on NO levels was also reported in Vicia faba seedlings grown under dark (Song et al. 2011) and leaf development in aging leaves. The molecular basis of this regulation is supposed to be the limitation of phosphorelay activity caused due to S-nitrosylation (Fan et al. 2013a, b).

NO–ET Crosstalk

Ethylene also known as ripening/senescence/stress hormone is important for plant growth regulation. Heavy metal stress often increases the activity of the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) enzyme that is associated with ET (Khan et al. 2015b). Understanding the NO–ET crosstalk provides the operating mechanism of plant stress adaptation mechanism under these stresses. ET–NO crosstalk leads to activation of MAPK cascades and polyamine synthesis during cadmium stress in soybean and pea seedlings (Chmielowska-Bąk et al. 2013; Rodríguez-Serrano et al. 2006). Similarly, treatment of Cd and spermine leads to NO generation in roots in Triticum aestivum seedlings which ultimately inhibits the root growth (Groppa et al. 2008). Fe-deficiency signaling is affected by NO–ET crosstalk with the induction of several genes associated with the iron accumulation and transport (Garcia et al. 2010). Other than heavy metals, NO–ET module works profoundly in salinity stress (Liu et al. 2015).

NO–SA Crosstalk

Salicylic acid is an important plant hormone essential for plant growth, development, and pathological processes. NO–SA interplay regulates plant stress responses; for example, combination of NO and SA prevents nickel toxicity by proline accumulation, reduced lipid peroxidation, and chlorophyll content enhancement in Brassica napus (Kazemi et al. 2010). On contrary to this, NO–SA combination increases the Cd concentration in the cell wall of Arachis hypogaea to prevent organelles from toxic effects (Xu et al. 2015). In addition, ROS also participates with NO in SA-induced closure of stomata (Khokon et al. 2011). Here, SA activates peroxidase enzyme that promotes ROS accumulation, leading to NO generation in guard cells and ultimately stomata closure. Similarly, the combination of NO–SA acts synergistically in alleviating salt stress by improving divalent cations absorption (Dong et al. 2015). Again, pretreatment of SA in Spinacia oleracea modulates the NR activity for improvement in chilling tolerance (Aydin and Nalbantoğlu 2011). This implicates SA interplay in the NO generation pathway that can be used for the future generation of climate-smart crops.

NO–Sulfur Crosstalk

Sulfur (S) is a vital part of essential molecules, such as the thioredoxin system, reduced glutathione (GSH), methionine, and coenzyme A. Under salt stress conditions, NO–S crosstalk changes the ET and ABA levels in guard cells to affect the photosynthetic and stomatal response. NO interacts with GSH and forms S-nitrosoglutathione (GSNO) to impart better stress tolerance (Wang et al. 2015b). Further, NO–sulfur crosstalk is essential for S-assimilation, as shown for Cys synthesis modulation by ET production (Fatma et al. 2016). Interactions of nitro and sulfhydryl groups are crucial during nitration (Leterrier et al. 2011). NO also interacts with H2S to provide salinity stress tolerance by upregulation of salinity stress-induced genes like HvSOS1 and HvHA1 (Chen et al. 2015). This process is mainly governed by transcriptional activation of vacuolar transport and compartmentalization genes where NO acts as a signaling molecule.

NO–BRs Crosstalk

Brassinosteroids (BRs) are the novel class of plant hormones implicated in plant growth, development, and immunity. Recently, reports have suggested NO–BRs interplay in plant root architecture as well as in root development (Tossi et al. 2013). In addition, alleviation of Copper toxicity was mediated by NO–BRs crosstalk in conjunction with ABA in Raphanus sativus seedlings (Choudhary et al. 2012).

These reports suggest precise NO interaction with hormones and other signaling components for fine-tuning the plant growth, development, and stress response. Further experiments on targeted NO homeostasis in controlled induced conditions (Temporal and spatial) will shed light on components of these cross-talks. Direct target identification of NO signaling in biosynthesis, perception, and signal transduction will be important to decipher the underlying regulatory mechanisms.

Molecular Understanding of NO Crosstalk During Plant Stress

Nitric oxide is an essential gasotransmitter with a regulatory role during plant growth and development. These regulatory roles are amplified when NO crosstalk with other signaling molecules or PGRs. The NO crosstalk with other compounds regulate various biosynthetic pathways, signaling processes, and metabolism and ultimately maintains plant growth and development under multiple stresses. Therefore, the mechanism of NO crosstalk under numerous abiotic stress tolerance is highlighted in Fig. 4 and discussed in this section. The NO crosstalk with PGRs and other signaling compounds under multiple stresses and their improved traits for stress tolerance are presented in Table 1.

Fig. 4
figure 4

Illustrated the physiological, biochemical, and molecular mechanisms of NO crosstalk under stress conditions. Under abiotic stresses, plant faces the drastic effects on several physiological, biochemical, and molecular processes, which are balanced by NO crosstalk

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Table 1 Highlights the NO crosstalk with PGRs and other signaling compounds under multiple stresses and their improved traits for stress tolerance
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Drought Stress

It has been well established that NO is required for ABA-induced stomatal closure and provides tolerance to plants under drought stress (Garcia-Mata and Lamattina 2002). Further, stomatal closure is regulated by ABA-induced NO production in Arabidopsis guard cells. Although, Desikan et al. (2002) revealed no stomatal closer in response to ABA in double-mutant nia1 nia2, which are associated with reduced NO production. This suggests the role of other intermediaries in NO–ABA crosstalk. Plants accumulate more ABA in drought stress, leading to activation of NADPH oxidase enzymes such as RBOHF and RBOHD (respiratory burst oxidase homolog F and D), resulting in more superoxide accumulation. This phenomenon is needed for stomatal closure through NO production via NR and activates MAPK signaling cascade (Desikan et al. 2002; Bright et al. 2006; Fency et al. 2017). Several studies showed that the exogenous application of NO could promote the accumulation of ABA in plants under drought stress, which can be reversed by the application of NO scavenger (Zhao et al. 2001; Fency et al. 2017). Thus, there is ambiguity in NO's function in the increased or decreased ABA signaling under water deficit. The NO-mediated S-nitrosylation could be crucial for drought tolerance, as reported in several studies. The central component of ABA signaling is OST1/SnRK2.6 (open stomata 1/sucrose non-fermenting 1-related protein kinase 2.6) induced by the S-nitrosylation process in plants. The protein kinase activity of OST1/SnRK2.6 is inhibited by S-nitrosylation at Cys 137 position. This ABA-induced S-nitrosylation of SnRK2.6 acts as a negative feedback regulator of ABA signaling in plants (Wang et al. 2015a).

There are reports which emphasize the role of transcription factors from MYB family to regulate tolerance mechanism in plants under abiotic stresses. Transcription factor, AtMYB2, is associated with salt, and drought stress tends to inhibit its DNA binding activity after the S-nitrosylation process (Serpa et al. 2007). Another transcription factor, AtMYBB30, has been found to lose its DNA binding activity after S-nitrosylation (Tavares et al. 2014; Fency et al. 2017). Thus, protein kinases and transcriptions factors play a vital role in mitigating plant stress under water deficit. Several recent studies suggested that NO crosstalk is a central player of drought stress tolerance. Wang et al. 2020 reported that crosstalk between NO and H2S mediates priming-induced drought tolerance via accumulation of osmolytes (proline and glycine betaine). Sami et al. (2018) found that NO crosstalk with phytohormones mediates the alteration in plant metabolism, and post-translational modification such as S-nitrosylation confers multiple stress tolerance including drought. Likewise, Shan et al. (2015) reported that NO induced by exogenous application of JA upregulated the AsA–GSH cycle activity and reduced drought stress in wheat crops. Moreover, recent studies suggested that NO crosstalk with other signaling compounds and phytohormones mitigate the drought stress by improving the relative water contents, photosynthetic capacity, antioxidant defense, ionic balance, and other plant growth attributes (Shan et al. 2015; Khan et al. 2017; Kaya et al. 2019). However, the exact mechanism of NO crosstalk under drought tolerance at the molecular level needed to be explored. These studies point out that NO crosstalk plays a crucial role during drought stress tolerance by antioxidant and osmolytes regulation.

Temperature Stress

Plant growth and development are severely affected by low temperature (cold and freezing) and high-temperature stress. Plants have evolved mechanisms during evolution to combat temperature stresses. NO cross-talk plays an essential role in a plant's battle against temperature fluctuations (Majlath et al. 2012; Parankusam et al. 2017; Kolbert et al. 2019). For example, exogenous application of NO induces the expression of MfSAMS1 and thereby increased S-adenosylmethionine (SAM), polyamines (PAs) concentration, and PA oxidation under cold stress in alfalfa (Medicago sativa) (Guo et al. 2014). In this context, SAMs are acting by up-regulating PA oxidation and H2O2-induced antioxidant defense (Guo et al. 2014). There is an antagonistic relationship between NO and ET during fruit ripening in cold stress. For example, Zaharah et al. (2011) studied the different NO levels for fumigation on mango fruits and observed a significant reduction in ET production during fruit ripening. They also found reduced chilling injury, softening, ripening, and delayed fruit color development in mango fruits under cold storage conditions. Thus, cross-talk between NO and ET delays fruit senescence and thereby fruit quality during cold fruit storage.

The crop productivity is adversely affected by heat stress due to adverse effects on photosynthesis, respiration, membrane stability, membrane permeability, and water relations (Kolbert et al. 2019). Heat stress affects cytoskeleton structure, cell metabolism, and membrane fluidity by increasing the accumulation of proteins that affect ROS, NO, and other phytohormones (Wahid et al. 2007). It has been suggested that NO acts via reduction of ROS level through activating antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), and expression of heat shock factor during heat stress in plants (Neill et al. 2002; Song et al. 2006; Wang et al. 2014; Fency et al. 2017). Exogenous application of NO (pre-treatment) increased the survival rate of maize (Zea mays) seedlings and wheat (Triticum aestivum) leaves and reduced heat stress-induced loss in rice (Oryza sativa) seedlings (Lamattina et al. 2001; Uchida et al. 2002). Similarly, crosstalk between NO and H2S regulates the H2O2-induced thermotolerance in maize seedlings. It also affects the Ca and calmodulin levels in tobacco seedlings (Li et al. 2015). These reports suggest that NO crosstalk needed to be further explored for its role during thermotolerance in plants.

Salinity

Soil salinity is one of the main factors for reduced crop production in major food and fodder crops and, by large, emerged due to extensive use of groundwater for irrigation across the world (Slinger et al. 2005). The role of NO to address plant salt tolerance has been extensively studied in various plant species (Zhang et al. 2007b; Hasanuzzaman et al. 2018). For example, artificial application of sodium nitroprusside (SNP, act as NO donor) protects plants against salt stress by altering growth habit and protects from oxidative damage by maintaining plant ion homeostasis (Zhang et al. 2006b). Moreau et al. (2008) studied the effect of NO using Atnoa1 plants (defective in GTPase activity) and concluded a role of NO under salt stress. The S-nitrosylated proteins play an essential role under NaCl stress and negatively affect salt concentration (Tanou et al. 2009). However, exogenous application of NO increased (pre-treatment) the concentration of NaCl-induced S-nitrosylated protein that played a protective role under stress conditions (Tanou et al. 2009). Arora et al. (2016) stated that NO can interact with different metal proteins such as zinc–sulfur clusters, heme–iron, copper, and iron–sulfur clusters and form a stable metal nitrosyl complex that can modify the protein structure as well as function. They also observed the binding of thiols to NO and their role in transporting it to the site of action. Camejo et al. (2013) observed decreased S-nitrosylation of proteins during short-term and long-term salt concentrations. A recent report suggested that pretreatments with CaCl2, H2O2, and SNP improve β-amylase activity, which influences starch breakdown and improved seedling establishments in Chenopodium (Hajihashemi et al. 2020).

Similarly, Singh and Bhatla (2018) reported that NO bind with ACC oxidase and form a ternary complex (ACC–ACC oxidase—NO), which lead to a reduction of ethylene biosynthesis and induce LR formation in sunflower under salt stress conditions. Likewise, Arora and Bhatla (2017) reported that melatonin and NO crosstalk maintain redox homeostasis and differential modulations of SOD isoform in sunflower under salt stress. Moreover, several recent updates on NO crosstalk with other signaling compounds alleviate salinity stress (Fatma et al. 2016; Shi et al. 2017; Kaya et al. 2019). However, there was a significant reduction in S-nitrosylation under long-term salt treatment. Thus, there were inconsistencies between different studies due to differences in plant genotypes/species, tissue-examined, variable NaCl concentration, and duration of time. Further, NO, S-nitrosylation, and associated enzyme GSNOR play an essential role in mitigating salt stress in plants. However, there is a need to focus more on proteomic approaches to identify salt stress signaling components directly and indirectly regulated by redox enzymes and GSNOR.

Heavy Metal Stress

Heavy metals (HMs) such as mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) have an unknown biological function and are very harmful for plants in higher concentrations. They tend to bio-accumulate (accumulation in plant cell with the time) and non-biodegradable. Plants taking up these HMs through roots from the soil and hyper-accumulation of these HMs bring rapid cellular homeostasis changes (Ghori et al. 2019). Nitric oxide (NO) has a broad spectrum of regulation functions with widespread inter- and intra-cellular messenger activities (Wei et al. 2020). Many enzymatic reactions accelerated through NO, including nitrate reductase and L-Ar-dependent nitric oxide synthase-related reactions, an essential component for HMs tolerance (Wei et al. 2020). Like other stresses, NO also plays a vital role in enhancing antioxidant enzyme activities and alleviates the toxicity of HMs. Rodriguez-Serrano et al. (2009) studied the cadmium (Cd) toxicity effect on nitric oxide (NO) metabolism in pea (Pisum sativum), and results implicated that Cd toxicity inactivated the NO synthase-dependent NO production. Consequently, it leads to calcium (Ca) deficiency in leaves. This suggests that the Cd toxicity effect can be counteracted by calcium (Ca). Exogenous SNP application acts as NO donor to the rice leaves and reduces the Cu and NH4+ accumulation (Mazid et al. 2011). Moreover, Wang et al. (2010) report that NO actions reduce Cu toxicity through antioxidant enzymes, which accelerates the metallothionein and metallothionein. There was an increase in total chlorophyll content and fresh or dry weight of leaves against Cu toxicity in tomato. Also, reports suggested the cross-protection role of putrescine and NO toward Cd toxicity in mung bean seedlings (Nahar et al. 2016). Singh et al. (2008) also found the detoxification and anti-oxidative properties of NO for Cd and Cu toxicity in wheat. Exogenous application of SNP accelerated the ROS scavenging enzymes, which reduced the accumulation of H2O2 and diminished the toxic effect of Cu in tomato (Cui et al. 2009). Similar results were observed in rice against Cd toxicity. The exogenous application of NO ameliorates the tolerance against Cd toxicity by increasing the pectin and hemicelluloses content in the root cell wall (Xiong et al. 2009). In soybean seedlings, the short-term treatment with Cd accelerated the geneS expression of encoding the protein of NO synthesis and ET (Chmielowska-Bak et al. 2013; Kolbert et al. 2019). Likewise, recent studies on the role of NO crosstalk on HMs stress tolerance suggest that it acts via regulating the root growth (biomass, formation, and length), photosynthetic activity, antioxidant defense, accumulation of osmoprotectants, and inhibition of HMs transport to grain and above plant parts (Khan et al. 2020; Kaya et al. 2020a, b; Singh et al. 2020).

Other Stresses

During the stress condition, NO is generated from L-arg-dependent NO synthase. This NO can react with superoxide (O2−) to form ONOO, a powerful oxidant that can lead to tyrosine nitration of proteins. Tyrosine nitration is an indicator of nitrosative stress in plants which acts as the defense system for the plants during stress (Nabi et al. 2019). Recent reports have explained that a wide range of abiotic stresses is leading to NO synthesis and signaling. It is gaining more attention mainly due to its properties like small size, no charge, free radicals, and highly diffusible nature across the cell membranes and many plant physiological functions like growth, development, maturation, and senescence. It is believed that NO signaling is involved in the respiratory electron transport system in mitochondria, where it confers the modulation of ROS and accelerates the antioxidant signaling defense system in the plant, which is exposed to several abiotic factors (Mazid et al. 2011; Santisree et al. 2020). The regulatory function of NO crosstalk is not only limited to drought, cold, heat, cold, and HMs stress but also has a regulatory role during combined stress, nutrient deficiency, and high and low light stress. For example, some studies suggested the NO crosstalk role during the N, P, Mg, and Fe deficient soil and suggested that it regulates the nutrient deficiency by improving root attributes, better translocation of ions, and regulating phytohormones concentration (Yang et al. 2016; Su et al. 2016; Zhu et al. 2017).

Conclusion and Perspectives

NO has gained attention during the last few decades due to its substantial role as a gasotransmitter and defense molecule during numerous environmental stresses. Most of the NO crosstalk functions are associated with redox, oxidative, ion, and hormonal homeostasis through the modulations of downstream genes in the signaling pathway. A large body of research has addressed the elementary mechanism of NO crosstalk regarding plant development and its role as a central hub under abiotic stress tolerance. Broadly, these studies indicate how NO crosstalk with other signaling compounds regulates the cell machinery in optimum ways. Although the mode of NO crosstalk with other signaling compounds is not always synergistic, sometimes antagonist responses also benefit plants under stressful situations. Moreover, the NO crosstalk response under similar stress could vary plant by plant due to the complex nature of signaling compounds and their interacting signals. Components of this crosstalk include genes, transcription factors, and enzymes associated with the NO synthesis and expression during different environmental signals, which need to be more elaborate to understand the exact mechanism of NO crosstalk. However, most studies have shown that the NO crosstalk regulates stress responses via the synthesis and expression of SOD, CAT, APX, MDA, GR, POX, DHAR, and other antioxidant defense enzymes and genes. These factors help in the maintenance of oxidative stress situations at the cell level. Likewise, stress proteins (HSP), phytochelatins, signaling cascades (MAPK, CDPK, GMP), osmoprotectants (sugar, proline), and ion proteins (H+-ATPase) are linked with NO crosstalk. However, the molecular mechanism of NO crosstalk is still unclear and needs to explore more for deep understanding and development of multiple stress tolerance varieties. Most studies focused on single stress conditions, and the mechanism of NO crosstalk under combined and multiple stress still needs to be deciphered. These studies are limited to the germination and vegetative stage. However, the responses of NO crosstalk under the reproductive phase and yield attributing traits are still unclear, which need to be investigated to develop higher yield lines under stress situations. In recent years, integrating omics approaches (integrating genomics, proteomics, metabolomics, and transcriptomics) has further clues on understanding gene–gene, gene–protein, gene–environment interactions and can be a potential approach to understanding the complex NO signaling mechanisms. Further, the integration of omics approaches to next-generation techniques explores the signaling mechanism at molecular levels and insights into full understanding of regulatory pathways and crosstalk mechanism to develop climate-resilient crops. Moreover, the engineering of NO biosynthesis and crosstalk pathways will be crucial for providing novel insights into the crop stress improvements program.