the proximity of the infection, thereby restraining pathogen progression (Heath 2000; Mur et al. 2019; Noman et al. 2020).
2.5 Role of NO in Overcoming Abiotic Stress
Plants are static creatures confined to the particular place where the seed sprouts. During its life cycle, a plant experiences a diverse and changing environment. With time, plants have established various mechanisms to handle the different kinds of stress environments. Although stresses are numerous, plant’s reactions commonly comprise similar components and signaling pathways. NO is an important constituent in numerous plant acclimation responses to biotic and abiotic strains (Figure 2.2) (Verma et al. 2016; Nowicka et al. 2018; Gong et al. 2020). The response mechanisms for NO signaling are generally similar for all types of stresses. Figure 2.3 contains details of the NO signaling pathway to scavenge ROS during different abiotic stresses.
Figure 2.2 General role of nitric oxide molecule in plant growth.
Figure 2.3 Activation of nitric oxide signaling and osmolyte pathways in plant cell under abiotic stress.
2.5.1 Drought and Low Mineral Nutrient Supply
During water deficit, plant functioning (plant growth and photosynthesis) is affected adversely. However, through stomatal closure, plants can overcome a temporary drought. But in long-term drought situations, suppressed leaf expansion, leaf excision, and changes in root morphology have been observed. The vulnerability of different plants to water deficiency is largely dependent upon their structural and physiological modifications to water stress (Santisree et al. 2015).
It has been speculated that slight water deficiencies lead to an increased level of NO production in the roots of cucumber plant. Pretreatment with extrinsic NO, e.g. 100 μM sodium nitroprusside (SNP) and GSNO was capable of neutralizing drought-induced membrane impairment and peroxidation of lipid in water-deficient plants. SNP (200 μM), an NO donor, has also been recognized to employ a shielding effect (improved growth, rise in water content, and less oxidative destruction) in wheat plantlets under polyethylene glycol-induced water stress (Zeppel et al. 2015). The capability of NO to manage drought stress might be linked to its direct effect as an antioxidant, the effects on root physiology, and a role in closure of stomata. In guard cells, NO is involved in the stimulation of intracellular Ca2+release, and regulates Ca2+-sensitive K+ and Cl− channels in the plasma membrane (Simontacchi et al. 2015).
In agriculture, one limitation is the low accessibility of some essential nutrients, which delivers varied surroundings to the plant. Plants have numerous mechanisms to extract minerals from the soil including major changes in the root growth pattern, the improved ability to obtain nutrients from depleted environments, and also the release of different substances that facilitate nutrient availability in the close proximity of roots, thus favoring their accumulation by roots. Recent study indicates that NO is involved in the modulation of these aforementioned mechanisms (Lambers et al. 2011). For example, in white lupin (Lupinus albus) at low phosphorus levels, the contribution of NO in plant responses has been observed and confirmed. Along with this plant, in other dicots and monocots, phosphorus deficiency mediates root branching followed by the release of exudates, comprising organic acids (Wang et al. 2014), which facilitate plants to increase P acquisition. From this perspective, it has been indicated that white lupin roots exposed to P deficiency exhibited heightened accumulation of NO, which was directly linked to the clustering of roots and increased exudation of citrate compounds. Sequentially, the addition of NO donors (e.g. SNP and GSNO) leads to the propagation of cluster roots, whereas NO scavengers (e.g. cPTIO [2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide]) eradicate this progression (Palavan-Unsal and Arisan 2009).
2.5.2 Salinity
In addition to drought and low mineral content, salinity is one of the most prolonged and unfavorable conditions that has a negative effect on plant health. Significantly, not all plants react in a similar manner to those compounds (lethal osmotic constituents) due to the existence of an array of resistance mechanisms that assist the plant to overcome the stress. These adaptive mechanisms might consist of accumulation of organic components, and the regulation of antioxidant machinery (van Zelm et al. 2020). It has been detected that rice plants exposed to SNP (NO donor) had improved overall growth. This shielding influence exerted by exogenous application of NO was related to the preservation of high relative moisture content and chlorophyll, whereas ionic discharge was sustained at a low level. Likewise, a clear effect on the accumulation of Na+ and K+ was also witnessed. These results proposed that NO has a protective effect on plants including managing water levels, sustaining ionic balance, and minimizing the damage imposed during early stages of the salinity response (Suzuki et al. 2014; Bashir et al. 2019; Hernández 2019).
2.5.3 Ultraviolet Radiation
Sunlight is the principal source of energy for plants, and a portion of this energy exists in the form of ultraviolet (UV) radiation. It is typically partitioned into three categories: (i) UV-A (315–400 nm), (ii) UV-B (280–315 nm), and (iii) UV-C (200–280 nm). It is well known that the atmospheric gases completely absorb UV-C radiation, while UV-B rays are moderately absorbed by tropospheric ozone, and only a trivial fraction is transferred to the surface, and UV-A is scarcely absorbed by the ozone. Since the destruction of the ozone layer, the levels of UV-B radiation rise on the Earth’s surface. UV-B has high energy and induces production of ROS that employ harmful morphological and molecular changes in plants (Simontacchi et al. 2015). Chloroplasts are highly affected by UV-B radiation and the light-induced impairment is directed primarily to photosystem II (PSII), with the inhibition of the respiratory chain and oxidative damage to the D1 protein. UV-B treatment increased leakage of ions, hydrogen peroxide levels, and oxidation of thylakoid membrane protein in bean (Phaseolus vulgaris) leaves. It was also observed that the proficiency of PSII photochemistry (F v/F m) and the considerable production of PSII was decreased under UV radiation. But after exposure to NO, carbonyl groups of thylakoid membrane and levels of H2O2 were decreased. Thus, NO could exert a protective role against protein oxidation under stress conditions as was previously reported regarding lipid oxidation (Vanhaelewyn et al. 2016; Mannucci et al. 2020).
UV-B adversely affects the chloroplast and mesophyll cells of a plant. It has been reported that UV-B increases the level of plant hormone ABA, activates the NADPH oxidase, and generates H2O2. Pretreatment with apocynin (an inhibitor of NADPH oxidase), minimizes the UV-B-induced oxidative damage by reducing the breakdown of chlorophyll (Tewari et al. 2013; Bajguz 2014).
Ozone is formed by a photochemical reaction in NOx and volatile organic compounds (VOCs) and enters the plants through stomata where it accumulates in parenchyma tissues. As a pollutant, it affects the plant by generating oxidative stress via releasing ROS, e.g. superoxide O2−, singlet oxygen, hydroxyl radicals, and hydrogen peroxide in the plant cells. This oxidative stress can further damage the DNA, carbohydrate, lipids, and proteins (Vaultier and Jolivet 2015; Li et al. 2018). The contribution of NO in response to UV-B rays acts through ABA-mediated pathways (Xu et al. 2012). It has been proposed that in plant cells, UV-B radiation stress leads to an increase in ABA levels, which stimulates an increase in cytosolic Ca2+ concentration that ultimately starts NO production via NOS and/or NOS-like activities. This increase in NO production increase a plant’s tolerance for higher doses of UV-B indirectly by protecting
