**4. NO-phytohormone cross talk under salinity stress**

Salinity stress is considered one of the most harmful stresses due to its high magnitude and worldwide distribution [84]. Phytohormones play a key role in enhancing the tolerance and adaptability of plants against salinity stress. Some recent studies suggest that NO acts in concert with signaling pathways of phytohormones to induce tolerance against salt stress [85, 86]. Presumably, plant hormones such as ABA, ET, and AUX are transported from salt-treated roots to leaves to trigger NO synthesis or transport throughout the plant [87]. NO-induced alleviation of oxidative damage in salt-stressed plants is associated with increased antioxidant activities and decreased thiobarbituric acid reactive substances content [69]. ABA stimulates H2 O2 accumulation that results in increased NO generation, leading to the activation of MAPK and up-regulation of genes associated with antioxidant enzymes [17, 18] in plants exposed to abiotic stresses like salinity. However, NO does not always positively interplay with ABA. In cotton, exogenous NO supply (using SNP as NO donor) reduced salt-induced leaf senescence by decreasing ABA content and down regulating the expression of ABA biosynthesis genes (*NCED2* and *NCED9*) [88].

In general, it is believed that ET biosynthesis corresponds to increased damage in plants. However, recent studies indicate ET as a stress-signaling hormone that interacts with signaling cascades of other phytohormones to enhance tolerance against various biotic/abiotic stresses [70, 86]. Studies involving tobacco seedlings showed that transcriptional activation of ethylene response factor (ERF) in ethylene-signaling process improved salt stress tolerance by decreasing ROS accumulation [89]. Treatment of *Arabidopsis* callus with 100 mM NaCl triggered the accumulation of NO that promoted ET emission, resulting in increased expression of the plasma membrane H+ -ATPase genes [90]. Hence, both NO and ET participate in up-regulation of plasma membrane H+ -ATPase that modulates ion homeostasis for improved salt tolerance. NO and ET also cooperate to stimulate the alternative respiratory pathway under salt stress conditions [91, 92]. Contrary reports of Poór and Tari [85] showed antagonistic relationship between NO and ET in tomato cell suspension cultures treated with NaCl (100 and 250 mM). Increased ET synthesis promoted ROS generation leading to high dead cell ratio in salt-stressed cell culture. However, NO generation decreased ROS levels and blocked ET synthesis resulting in lower dead cell ratio. In another study, absence of ET and NO in apical root segments and cell suspension culture, respectively, caused ionic imbalance (Na+ /K+ ) that resulted in increased susceptibility to salinity stress [93]. AUX and NO involvement in inhibition of root meristem growth in salt-stressed *Arabidopsis* was revealed by Liu et al. [94]. They reported that salinity stress repressed root meristem growth by inhibiting the expression of *PINFORMED* (*PIN*) genes, thereby reducing AUX levels. Moreover, stabilization of AUXIN RESISTANT3 (AXR3)/ INDOLE-3-ACETIC ACID17 (IAA17) repressed AUX signaling via NO accumulation.

Participation of both NO and ROS in SA-induced stomatal closure is also reported in literature [95]. Activation of a peroxidase (sensitive to the inhibitor salicylhydroxamic acid) by SA promotes ROS accumulation and NO generation in guard cells, leading to stomatal closure. Experiment with soybean seedlings showed that combined application of SNP (as NO donor) and SA alleviated the toxicity of NaCl-induced salt stress by increased proline accumulation and activation of CAT, APX, and GPX. Similar results were reported by Liu et al. [96] and Dong et al. [97] in *Gossypium hirsutum* and *Fagus sylvatica*, respectively. SA interaction with signaling cascades of NO modulated photosynthetic machinery and reduced H2 O2 accumulation that promoted the influx of H<sup>+</sup> -ATPase to plasma membrane. Moreover, synergistic effect of SA and NO improved Ca2+/Mg2+ absorption and reduced Na2+ uptake under salt stress conditions [97].

**4. NO-phytohormone cross talk under salinity stress**

124 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

lates H2

O2

plasma membrane H+

of plasma membrane H+

synthesis genes (*NCED2* and *NCED9*) [88].

Salinity stress is considered one of the most harmful stresses due to its high magnitude and worldwide distribution [84]. Phytohormones play a key role in enhancing the tolerance and adaptability of plants against salinity stress. Some recent studies suggest that NO acts in concert with signaling pathways of phytohormones to induce tolerance against salt stress [85, 86]. Presumably, plant hormones such as ABA, ET, and AUX are transported from salt-treated roots to leaves to trigger NO synthesis or transport throughout the plant [87]. NO-induced alleviation of oxidative damage in salt-stressed plants is associated with increased antioxidant activities and decreased thiobarbituric acid reactive substances content [69]. ABA stimu-

of MAPK and up-regulation of genes associated with antioxidant enzymes [17, 18] in plants exposed to abiotic stresses like salinity. However, NO does not always positively interplay with ABA. In cotton, exogenous NO supply (using SNP as NO donor) reduced salt-induced leaf senescence by decreasing ABA content and down regulating the expression of ABA bio-

In general, it is believed that ET biosynthesis corresponds to increased damage in plants. However, recent studies indicate ET as a stress-signaling hormone that interacts with signaling cascades of other phytohormones to enhance tolerance against various biotic/abiotic stresses [70, 86]. Studies involving tobacco seedlings showed that transcriptional activation of ethylene response factor (ERF) in ethylene-signaling process improved salt stress tolerance by decreasing ROS accumulation [89]. Treatment of *Arabidopsis* callus with 100 mM NaCl triggered the accumulation of NO that promoted ET emission, resulting in increased expression of the

NO and ET also cooperate to stimulate the alternative respiratory pathway under salt stress conditions [91, 92]. Contrary reports of Poór and Tari [85] showed antagonistic relationship between NO and ET in tomato cell suspension cultures treated with NaCl (100 and 250 mM). Increased ET synthesis promoted ROS generation leading to high dead cell ratio in salt-stressed cell culture. However, NO generation decreased ROS levels and blocked ET synthesis resulting in lower dead cell ratio. In another study, absence of ET and NO in apical root segments and

susceptibility to salinity stress [93]. AUX and NO involvement in inhibition of root meristem growth in salt-stressed *Arabidopsis* was revealed by Liu et al. [94]. They reported that salinity stress repressed root meristem growth by inhibiting the expression of *PINFORMED* (*PIN*) genes, thereby reducing AUX levels. Moreover, stabilization of AUXIN RESISTANT3 (AXR3)/

Participation of both NO and ROS in SA-induced stomatal closure is also reported in literature [95]. Activation of a peroxidase (sensitive to the inhibitor salicylhydroxamic acid) by SA promotes ROS accumulation and NO generation in guard cells, leading to stomatal closure. Experiment with soybean seedlings showed that combined application of SNP (as NO donor) and SA alleviated the toxicity of NaCl-induced salt stress by increased proline accumulation

INDOLE-3-ACETIC ACID17 (IAA17) repressed AUX signaling via NO accumulation.

cell suspension culture, respectively, caused ionic imbalance (Na+

accumulation that results in increased NO generation, leading to the activation



/K+

) that resulted in increased

Sulfur (S) is a major component of metabolites such as reduced glutathione (GSH), coenzyme A, methionine, cysteine (Cys), sulfo-lipids, iron-sulfur (Fe-S) clusters, and thioredoxin system involved in regulation of physiological processes under salt stress conditions [98]. Evidence suggests that NO promotes S-assimilation, which is linked to ET production through Cys synthesis [86]. Hence, it may be speculated that NO and S interact to modulate ABA and ET levels in guard cells that may influence the stomatal and photosynthetic response under salt stress conditions. NO combines with GSH to generate S-nitrosoglutathione (GSNO), leading to enhanced S requirement of plants for improved tolerance under environmental stress conditions [99, 100]. Coordinated effect of NO and S regulated the utilization of S and GSH resulting in improved growth and photosynthetic activity in salt-stressed mustard plants [86]. NO is a key regulatory signal that activates several biochemical processes and interacts with sulfhydryl groups and nitro groups in the process of nitration to enhance tolerance against salt stress [101]. NO also cooperates with other signaling molecules such as H2 S to enhance tolerance against salinity stress in plants. NO and H2 S cross talk helped to maintain low Na+ levels with up-regulation of *HvHA1* and *HvSOS1* and increased plasma membrane H+ -ATPase levels in cytoplast of salt stressed barley seedlings [102]. Moreover, transcriptional activation of vacuolar Na+ /H+ antiporter (*HvVNHX2*) and H+ -ATPase subunit β (*HvVHA-β*) up-regulated the expression of vacuolar Na+ /H+ antiporter (NHE1) that helped to modulate Na+ compartmentation into the vacuoles.

Recently, it has been proposed that NO negatively regulates CK signaling by limiting phosphorelay activity via S-nitrosylation [103]. Contrasting reports of Kong et al. [88] showed that foliar applied SNP (as NO donor) delayed salt-induced leaf senescence in cotton seedlings by up-regulating the expression of CK biosynthesis gene, isopentenyl transferase (*IPT*). NO induced reduction in total free PAs, free Put, spermidine (Spd), and polyamine oxidase (PAO) activity that was reported by Fan et al. [103] in cucumber seedlings exposed to NaCl stress. These reports provide a strong evidence for NO cross talk with plant hormones to induce salt stress tolerance in plants; however, further in depth studies to understand interplay among these plant growth regulators in terms of transcriptional regulation, signal transduction, and ion detoxification are needed.
