(Deng et al. 2016). However, several studies have demonstrated the integrative role of brassinosteroids with NO in response to pathogen attack in various species (Hayat et al. 2010; Shi et al. 2015; Zou et al. 2018; Kohli et al. 2019).
1.4 Nitric Oxide’s Role in Abiotic Stress
Existing climatic aberrations cause plants to be exposed to a variety of abiotic stresses such as drought, temperature increases, salinity, and oxidative damage. Because plants are sessile, they must deal with changing environmental conditions throughout their lives. Though plants develop various strategies to cope with adverse conditions, plant scientists and researchers are discovering new methods to learn about plants in stressful environments. In the presence of salt, substantial metals, drought, and oxidative stress, NO plays a critical role in abiotic stress management (Zhao et al. 2001, 2004; Uhida et al. 2002; Kopyra and Gwóźdź 2003). Abiotic stressors are a key stumbling block to agricultural growth while also jeopardizing food security (Figure 1.3). They also jointly influence the metabolism of plants and the defense of inhibitors. Recent NO studies show that they play a serious role in plant growth and mitigate environmental stress (Aroca et al. 2015; Corpas et al. 2019). Drought tolerance of cut leaves and seedlings of wheat was improved by exogenous NO application (Garcia-Mata et al. 2001). NO also mediates drought tolerance by the activation of numerous enzymes and plant metabolism inhibitors (Filippou et al. 2014; Shi et al. 2014). In many species, NO mediates stomatal rotation and defense methods in water stress (Greco et al. 2012; Garcia-Mata and Lamattina 2013; Chen et al. 2016). ROS and NO work synergistically to address the stress in water that causes ABA synthesis in wheat roots (Zhao et al. 2001). As proof of this, ABA accumulation under stress was prevented by the administration of ROS scavengers. The accumulation of NO tested jointly is needed in broad bean throughout ABA-induced stomata closure (Garcia-Mata and Lamattina 2002). Heat treatment of alfalfa cells, for example, has led to an increase in NO synthesis, while the application of exogenous NO improved cold tolerance in tomato, wheat, and maize (Neill et al. 2003). The observed effects were most likely associated with NO’s antioxidative action, which amplifies the negative effects caused by the intensification of peroxidative metabolism in high temperatures (Neill et al. 2002).
Figure 1.3 Schematic illustration of NO signaling in abiotic stress.
Mackerness et al. (2001) demonstrated the involvement of NO in plant response to UV-B radiation, demonstrating poststress induction of chalcone synthase expression, a rise in NOS-type protein activity, and an increase in NO levels. According to the findings of Shi et al. (2005), NO positively shields plants against UV-B radiation, most likely through increased activity of the antioxidative system. NO-donor treatment of potato tubers prior to UV-B irradiation resulted in the development of approximately 50% more healthy leaves than plants not subjected to NO treatment (Neill et al. 2003). Exogenous NO has been shown to reduce the deleterious effects of heavy metals, ethylene, and herbicides on plants in response to alternative abiotic stresses (Kopyra and Gwóźdź 2003). The authors explained the protective effect as a result of NO-donor treatment of plant materials by the effect of NO on the elevation of activity of antioxidative enzymes, particularly SOD (Kopyra and Gwóźdź 2003).
According to the cited authors, such a chain of events could effectively reduce the amount of ROS produced during stress, thereby limiting oxidative stress in plant cells. Similarly to salinity stress, NO-donor treatment of rice seedlings resulted in loss minimization (Uhida et al. 2002). When NO was used, plant growth was increased, along with the maintenance of acceptable photosystem II activity, an increase in antioxidative protein activity, and thus the expression of specific salinity stress resistance genes. On the other hand, prolonged stress situations may result in the production of NO and NO-derived products, resulting in specific responses, referred to as nitrosative stress. Valderrama et al. (2007) found that salinity stress elicited the assembly of RNS, i.e. NO, GSNO, and RSNO, as well as an increase in tyrosine-nitrated proteins, which are sensitive markers of nitrosative stress, in olive leaves. Furthermore, they demonstrated that vascular tissues may play an important role in the distribution of NO-derived forms during nitrosative stress and in signaling processes. Tissue damage, which is usually due to a microorganism invading a cell, frequently results in NO production and H2O2 accumulation (Delledonne et al. 1998). According to Orozco-Cardenas and Ryan (2002), injury does not induce NO synthesis; however, the use of exogenous NO inhibits the method of NO generation and expression of wound-inducing genes.
It is important to note that many types of abiotic stress (cold and heat stress, salt and drought stress) increase polyamine (PA) synthesis (Bouchereau et al. 1999). Tun et al. (2006) discovered that PAs significantly increase NO generation. Work on Arabidopsis seedlings confirms that NO acts as a channel between PA-mediated stress responses and an alternative stress mediator, with NO as a stepping stone.
By scavenging ROS, NO plays a significant role in inhibitor defense against many abiotic stresses. Salinity stress has a negative impact on plant morphological traits and diffusion balance. Additionally, it promotes membrane disintegration, DNA damage, particle discharge, and death. NO has been shown to have a potential effect on diffusion stress tolerance as well as increased spermatophyte growth in rice, lupin, and cucumber when subjected to salt stress (Uchida et al. 2002; Kopyra and Gwóźdź 2003; Fan et al. 2007, 2013; Barakat et al. 2012). Similar evidence has been reported demonstrating the potential effect of NO in mitigating salinity stress in alfalfa, barley, jatropha, chickpea, and sunflower (Nabi et al. 2019). In Oryza sativa, a NO donor SNP inhibited accumulation, reduced ROS generation, and improved root growth (Kushwaha et al. 2019). In various plant species, the potential role of gas in mitigating serious metal toxicity has been investigated (Ahmad et al. 2018; Yuanjie et al. 2019; Bhuyan et al. 2020; Wei et al. 2020; Khator et al. 2021). As a result, it is clear that NO gas could be a potential mitigating agent for abiotic stresses.
1.4.1 Crosstalk of Nitric Oxide with Other Phytohormones in Plants to Confer Abiotic Stress Tolerance
NO is involved in advanced signal mechanisms, as well as synergistic collaboration with phytohormones and alternative secondary signal molecules, to confer stress tolerance in plants. NO has been linked to a variety of phytohormones, including gibberellins, brassinosteroids, and ABA. The interaction of NO with phytohormones has been studied in a variety of ways. NO and plant hormones work together to regulate a wide range of physiological responses in plants. By activating Ca2+ and calcium‐dependent protein kinase via downstream signals, NO and auxin promote root development (Pagnussat et al. 2002). Similarly, the interaction of NO and auxin promotes Cd tolerance in the rosid dicot genus Truncatula by reducing auxin degradation (Xu et al. 2010). Furthermore, there is a growing of evidence pointing to the effect of NO in reducing serious metal toxicity (He et al. 2012; Yuan and Huang 2016; Wei et al. 2020). Iron deficiency, on the other hand, stimulates the assembly of auxin and increases NO levels, thereby upregulating ferric-chelate enzyme activity in Arabidopsis.
Various studies have emphasized the role of cytokinins in plant organic process processes, cellular division, and leaf senescence. Cytokinins and NO interact in a variety of ways, including synergistic and antagonistic responses. NO and cytokinins interact synergistically in response to drought stress causing leaf senescence, cellular division, and photosynthetic activity (Mishina et al. 2007; Shao et al. 2010; Shen et al. 2013). NO signaling via gibberellins induces multiple physiological responses in plants, including seed germination, root growth (Lozano-Juste and León 2011; Sanz et al. 2015), photosynthetic activity, and nutrient use potency. NO and gibberellins also have an antagonistic relationship because NO suppresses gibberellic acid signal events and signal transduction by promoting the accumulation of DELLA proteins (Asgher et al. 2017). Wu et al. (2014) discovered that gibberellins work antagonistically with NO to manage root growth in Arabidopsis at low and high P concentrations. Likewise, NO production is required for ABA-induced stomatal closure in guard cells (Neill et al. 2002). The role of NO–ABA interactions in drought stress and UV-B radiation stress has been well established in controlling stomatal closure and