Regulation of Reactive Oxygen Species during Salt Stress in Plants and Their Crosstalk with Other Signaling Molecules-Current Perspectives and Future Directions

Abstract

Salt stress is a severe type of environmental stress. It adversely affects agricultural production worldwide. The overproduction of reactive oxygen species (ROS) is the most frequent phenomenon during salt stress. ROS are extremely reactive and, in high amounts, noxious, leading to destructive processes and causing cellular damage. However, at lower concentrations, ROS function as secondary messengers, playing a critical role as signaling molecules, ensuring regulation of growth and adjustment to multifactorial stresses. Plants contain several enzymatic and non-enzymatic antioxidants that can detoxify ROS. The production of ROS and their scavenging are important aspects of the plant's normal response to adverse conditions. Recently, this field has attracted immense attention from plant scientists; however, ROS-induced signaling pathways during salt stress remain largely unknown. In this review, we will discuss the critical role of different antioxidants in salt stress tolerance. We also summarize the recent advances on the detrimental effects of ROS, on the antioxidant machinery scavenging ROS under salt stress, and on the crosstalk between ROS and other various signaling molecules, including nitric oxide, hydrogen sulfide, calcium, and phytohormones. Moreover, the utilization of "-omic" approaches to improve the ROS-regulating antioxidant system during the adaptation process to salt stress is also described.

Keywords: ascorbate peroxidase; catalase; hydrogen sulfide; nitric oxide; omics; phytohormones; reactive oxygen species; salt stress; superoxide dismutase.

Figure 1

Excessive production of reactive oxygen species (ROS) in plants causes oxidative stress, leading to apoptosis, which severely affects plant growth and productivity. HO•, hydroxyl radical; H₂O₂, hydrogen peroxide; O₂•−, superoxide radical; and ¹O₂, singlet oxygen.

Figure 2

Mechanism and site of production of ROS in plants. SOD, superoxide dismutase; ROS, reactive oxygen species; ETC, electron transport chain; OEC, oxygen-evolving center; H₂O₂, hydrogen peroxide; PS I, photosystem I; PS II, photosystem II; O₂⁻•, superoxide anion; ¹O₂, singlet oxygen; HO•, hydroxyl radical; NADPH, nicotinamide adenine dinucleotide; XOD, xanthine oxidase; and UO, urate oxidase.

Figure 3

Antioxidant defense machinery in plants: (A) different types of antioxidants and (B) intricate regulatory mechanisms of enzymatic and non-enzymatic antioxidants. TRX, thioredoxin; SOD, superoxide dismutase; CAT, catalase; AA, ascorbic acid; AsA, ascorbate; APX, ascorbate peroxidase; DHAR, dehydroascorbate reductase; DHA, dehydroascorbate; GPX, glutathione peroxidase; H₂O₂, hydrogen peroxide; GSH, reduced glutathione; GR, glutathione reductase; GSSG, oxidized glutathione; MDHAR, monodehydroascorbate reductase; MDHA, monodehydroascorbate; GST, glutathione S-transferase; NADPH, nicotinamide adenine dinucleotide phosphate; DHA, dehydroascorbate; AsA, ascorbate; AA, ascorbic acid; AsA-GSH, ascorbate-glutathione; GLA-I, glyoxalase-I; GLA-II, glyoxalase-II; PPO, polyphenol oxidase; POD, peroxidases; PRX, peroxiredoxins; O₂⁻•, superoxide anion; ¹O₂, singlet oxygen; HO•, hydroxyl radical.

Figure 4

The mechanisms by which exogenous and endogenous proline ameliorate salt stress tolerance in plants. ROS, reactive oxygen species; HO•, hydroxyl radical; H₂O₂, hydrogen peroxide; O₂^•−, superoxide radical; ¹O₂, singlet oxygen; AsA-GSH, ascorbate-glutathione cycle.

Figure 5

Overview of crosstalk between the different signaling molecules and phytohormones during salt stress tolerance. ROS, reactive oxygen species; O₂^•−, superoxide anion; ¹O₂, singlet oxygen; HO•, hydroxyl radical; H₂O₂, hydrogen peroxide; NO, nitric oxide; H₂S, hydrogen sulfide; Ca, calcium; SA, salicylic acid; GA, gibberellins; IAA, indole acetic acid; ABA, abscisic acid; SOS, salt overly sensitive; MAPK, mitogen-activated kinase protein.

Figure 6

Utilization of different “-omics” approaches to develop salt-tolerant cultivars for improving the growth and productivity of different plant species.