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Review
. 2020 Jun 25;10(6):959.
doi: 10.3390/biom10060959.

Ethylene: A Master Regulator of Salinity Stress Tolerance in Plants

Riyazuddin Riyazuddin  1   2 , Radhika Verma  3 , Kalpita Singh  4 , Nisha Nisha  5 , Monika Keisham  6 , Kaushal Kumar Bhati  7 , Sun Tae Kim  8 , Ravi Gupta  9
Affiliations
Review

Ethylene: A Master Regulator of Salinity Stress Tolerance in Plants

Riyazuddin Riyazuddin et al. Biomolecules. .

Abstract

Salinity stress is one of the major threats to agricultural productivity across the globe. Research in the past three decades, therefore, has focused on analyzing the effects of salinity stress on the plants. Evidence gathered over the years supports the role of ethylene as a key regulator of salinity stress tolerance in plants. This gaseous plant hormone regulates many vital cellular processes starting from seed germination to photosynthesis for maintaining the plants' growth and yield under salinity stress. Ethylene modulates salinity stress responses largely via maintaining the homeostasis of Na+/K+, nutrients, and reactive oxygen species (ROS) by inducing antioxidant defense in addition to elevating the assimilation of nitrates and sulfates. Moreover, a cross-talk of ethylene signaling with other phytohormones has also been observed, which collectively regulate the salinity stress responses in plants. The present review provides a comprehensive update on the prospects of ethylene signaling and its cross-talk with other phytohormones to regulate salinity stress tolerance in plants.

Keywords: ROS; antioxidants; ethylene; hormone cross-talk; photosynthesis; programmed cell death; salinity stress; seed germination.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Role of ethylene signaling in seed germination under salinity stress. Ethylene induced by salinity stress activates the signaling pathway by inhibiting the active receptors and by releasing the Constitutive Triple Response1 (CTR1). Salinity-induced ethylene signal is transduced mainly through the classical receptors–CTR1-Ethylene-Insensitive2 (EIN2)-EIN3 pathway to regulate many effectors involved in plant growth and salinity response. EIN3/EIN3-Like (EIL) promotes the entry of Constitutive Photomorphogenesis1 (COP1) into the nucleus and degrades the Elongated Hypocotyl5 (HY5) protein, which inhibits seed germination by upregulating ABI5 gene expression. Degradation of HY5 inhibits the expression of ABI5 and ultimately induced seed germination under salinity stress. On the other hand, EIN3/EIL also directly induces seed germination by scavenging reactive oxygen species (ROS) through upregulating peroxidase activities.
Figure 2
Figure 2
Functions of ethylene in the regulation of photosynthesis under salinity stress. In the absence of ethylene, salinity stress results in an imbalance of Na+/K+ homeostasis, which leads to the production of ROS. This salinity-induced ROS production, in turn, exerts oxidative stress on plants, resulting in stomatal closure and reduced activity of CO2-fixing enzymes, resulting in a decrease in photosynthesis. In the presence of ethylene, Na+/K+ homeostasis and nutrients homeostasis are maintained, and the antioxidant defense mechanism is activated, which limits ROS production, thereby preventing ROS-induced oxidative stress. In the absence of oxidative stress, the rate of photosynthesis is maintained even during salinity stress.
Figure 3
Figure 3
Schematic diagram of hormonal cross-talk under salinity stress. Details of the cross-talk are given in the text. Abbreviations used: BR: Brassinosteroids, IAA: Indole-acetic acid. CK: Cytokinin, ABA: Abscisic acid, GA: Gibberellin, JA: Jasmonic acid.
Figure 4
Figure 4
A schematic representation of ethylene regulation under salinity stress. When plants encounter ionic imbalance, nutrient imbalance, and osmotic stress under salinity stress, activation of upstream mitogen activated protein kinase kinase kinases (MAPKKKs) leads to the gradual phosphorylation and successive activation of downstream mitogen activated protein kinase kinases (MAPKKs) and MAPKs that further phosphorylates ACS2/6. The phosphorylated ACS2/6 stabilizes and thus enhances the production of ethylene through ethylene biosynthesis. Consequently, the downstream signaling activates the transcription of several ethylene-responsive genes. The transcription level of BAG (Bcl-2-associated athanogene) family genes is suppressed by ethylene (1-aminocyclopropane-1-carboxylic acid (ACC)) to inhibit salinity-induced plant cell death (PCD). On the other hand, ethylene enhances ROS signaling to increase calcium ions, which in turn regulate MAPK, salinity overly sensitive (SOS), and Na+/H+ exchangers (NHX) through Na+/K+ homeostasis to provide salinity stress tolerance. Salinity stress reduces the CO2-fixing enzymes in chloroplast that eventually produce ROS. Scavenging of ROS occurs through both enzymatic (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)) and nonenzymatic (ascorbic acid (AsA), glutathione (GSH), α-tocopherol (TOC), flavonoids, and carotenoids) antioxidant defense systems.
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