A brief appraisal of ethylene signaling under abiotic stress in plants

ABSTRACT

For years, ethylene has been known to humankind as the plant hormone responsible for fruit ripening. However, the multitasking aspect of ethylene is still being investigated as ever. It is one of the most diversified signaling molecules which acclimatize plant under adverse conditions. It promotes adventitious root formation, stem and petiole elongation, opening and closing of stomatal aperture, reduces salinity and metal stress, etc. Presence of ethylene checks the production and scavenging of reactive oxygen species by strengthening the antioxidant machinery. Meanwhile, it interacts with other signaling molecules and initiates a cascade of adaptive responses. In the present mini review, the biosynthesis and sources of ethylene production, interaction with other signaling molecules, and its exogenous application under different abiotic stresses have been discussed.

1. Introduction

Plants are widely exposed to direct effect of different environmental factors, which are responsible for alterations in their morphological, anatomical, physiological, biochemical and molecular aspects.¹ It has been reported that nearly 50% of crop loss is due to abiotic stress such as drought, flood, metal, etc. In different abiotic stress responses, plants have developed certain defense mechanisms to cope up with stress for their long survival. In plants under abiotic stress, different phytohormones and their signaling play mitigating roles.²Among various phytohormones, ethylene acts as a stress response hormone and also indulges in various developmental processes.² In the presence of various environmental stresses, ethylene synthesis is triggered in plants.³ Enzyme aminocyclopropane-1-carboxylic acid (ACC) syntheses (ACSs) and ACC oxidases (ACOs) are responsible for ethylene biosynthesis. In plants, ACOs are encoded by poly-gene family.⁴ In Solanum lycopersicum and Arabidopsis, six genes of LeACO and AtACO have been reported.⁵

Under various abiotic stresses, biosynthesis of ethylene is governed by a feedback mechanism, and involves various signaling agents like nitric oxide (NO), hydrogen sulfide (H₂S), and many more (Figure 1). For example, in paddy, it is reported that auxins promote ethylene biosynthesis by regulating transcription of ACOs.⁶ However, Zhu et al.⁷ reported that NO negatively acts upon ethylene biosynthesis. Ethylene helps in making reactive oxygen species (ROS) collection under various abiotic stresses, including metal, drought, high salinity, low temperature, etc.⁸ The ROS synthesis and signaling are primarily regulated by ethylene-responsive transcription factor, i.e., AP2/ERF gene family and link ethylene and ROS signaling under various abiotic stresses.⁹ In Arabidopsis, Ethylene Overproducer 1 (ETO1) acts positively in salt stress by increasing ROS formation, accompanied with Na⁺/K⁺ homeostasis. It was reported that ROS stability is critical for controlling of stress responses and plant development by ethylene.¹⁰ During various stages of different stresses, the reaction of ethylene downstream signaling generally relies upon ROS. For instance, in the early stage of stress, ERF74 increases ROS blast via dictating of gene, expression of RbohD, with subsequent promotion of ROS scavenging genes.¹¹A study on Arabidopsis reveals that under ozone stress, glutathione (GSH) synthesis was enhanced as compared with ethylene-insensitive (ein) plants, and is regulated by ethylene.¹² Role of interaction of ethylene and ROS in paddy under metal stress is reported.¹³ Synthesis of ethylene increased under Cd stress by buildup of ACS2 and ACS6 transcripts.¹⁴ Ethylene is a quite known ruler of flood tolerance; this investigation is well understood in a variety of rice which grows well under slight submergence.¹⁵ It was found out that ethylene synthesis enhanced during re-oxygenation.¹⁶ In the detailed study of Arabidopsis, group VII ERFs reveal their regulatory act in positive response to flooding and hypoxic stress.¹⁷Under oxygen scarcity, ethylene acts primarily to acclimatize plants by adventitious root formation, aerenchyma formation, petiole/stem elongation, and hyponasty response (Figure 1).¹⁸ Under oxygen deficiency due to overwhelming of water, ethylene bio-synthesis does increase as well as various ethylene-receptive genes containing an identified group of ERF/AP2 are expressed in Arabidopsis.¹⁵ In Arabidopsis eto1 mutant, ethylene is involved in both opening and closing of stomatal aperture.¹⁹ However, in another study, it has been shown that ethylene-expressing factors EIN3 and ETR1 are not involved in stomatal closing movement.²⁰ By hindering ROS formation, ethylene controls stomatal closure with the help of flavonol production in guard cell under stress involving an EIN2-mediated pathway.²¹ Increased level of ethylene was reported in many plant species such as faba bean, orange, french bean, and many more species under drought stress conditions.²² Arabidopsis with a constitutive promoter 35 S:ERF1 is a drought-tolerant and ERF protein attached directly to RD29B promoter’s DRE element.²³In view of the above, this review article discusses the possible roles of ethylene and its cross talk with other hormones in plants under abiotic stresses.

Figure 1.

A diagrammatic illustration of interactivity of ethylene sensory system with constitutive triple response1 (CTR1) via ubiquitylation of E1N3 and EIL12 under various abiotic stress conditions. Abiotic stress triggers formation of ethylene in endoplasmic reticulum in the presence of ethylene receptor ETR1. Receptors subsequently activate CTR1 protein. CTR1 mediates proteasomal degradation and represses EIN2. During dephosphorylation, EIN2 releases a C-terminal which moves toward P-body. The F-box proteins target EIN3 and EIN3-LIKE 1 which induce the expression of ethylene response factors (ERFs).

2. Ethylene homeostasis under abiotic stresses

.Ethylene acts wisely in conciliating various targeted growth and developmental process, mainly in retaliation to diverse stress conditions.²⁴ The biogenic and signal transduction mechanisms of ethylene were sharply elaborated (Figure 2).²⁵ After identification of ethylene through endoplasmic reticulum membrane-related receptors, interactivity of ethylene sensory system with CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) has to be detached and ETHYLENE INSENSITIVE 2 (EIN2) will also depart due to phosphorylation. Ethylene is known to induce transcriptional factors like EIN2, which further targets EBF1 mRNA to cytoplasmic processing-body (P-body). Additionally, EIN2-mediated ethylene signaling enforces the suppression of EBF1 and EBF2 (EIN3-BINDING F-BOX 1) mRNA translation. This ethylene signaling is mainly regulated by key signaling transducer, i.e., 3′ UTR mRNA.²⁶ Additionally, in controlling several developmental processes and stress acclimation, APETALA2/ETHYLENE RESPONSE FACTORS (AP2/ERFs) play as a critical role²⁷ and act downstream in ethylene signaling.²⁸

Figure 2.

A simplified overview of synthesis and signaling of ethylene and its interaction with environmental stimuli. Synthesis of the intermediate product 1-aminocyclopropane-1-carboxylic acid (ACC) by ACS enzymes is rate-limiting and controlled by numerous environmental conditions including biotic, osmotic, and drought stress. Ethylene plays differential role in the modulation of different metabolic processes resulting into visible physiological traits of the plant exposed to abiotic stress. ACC, aminocyclopropane-1-carboxylic acid; ACS, aminocyclopropane-1-syntheses and ACOs, aminocyclopropane-1-oxidases.

It was reported that ethylene plays several positive roles in salt-stress tolerance.²⁹ In Arabidopsis, salt tolerance level was decreased by chocking ethylene action.³⁰ Salt-stress tolerance is decreased in ein3 eil1 double mutant and OsEIL1, while OsEIL2 RNAi transgenic plants showed enhanced salt tolerance.³¹ In Arabidopsis, the role of ethylene biosynthesis largely depends on its level in the plant cell. The activity of ethylene also fluctuates with the extent of salt tolerance.³² In Arabidopsis, ETO1 acts positively in salt stress by increasing ROS formation, accompanied with Na^+/K⁺ homeostasis.

In rice, SALT INTOLERANCE 1 (SIT1) acts negatively in salt-stress response by activating MITOGEN-ACTIVATED PROTEIN KINASE 3/6 (MPK3/6) that increases ethylene and ROS overproduction.³³It is reported that ROS stability is critical for controlling stress responses and plant development by ethylene.¹⁰ ROS plays a signaling role to impel downstream metabolic access. ROS also acts vitally in Na⁺/K⁺ equilibrium in Arabidopsis by RESPIRATORY BURST OXIDASE HOMOLOG D (RbohD) and Rbohk.³⁴It is reported that ethylene causes ROS formation through transcriptional controlling on AtRbohF, and magnifies salinity sufferance to the surplus mutant eto1.³⁵ It is observed that ethylene signaling factor EIN3/EIL1 provokes ROS rummage gene utterance to prevent ROS build up and thus increasing salt tolerance.³⁰

During stresses, the reaction of ethylene downstream signaling generally depends upon ROS, i.e., the signaling action of ethylene depends on the concentration of ROS. For instance, in early stage of stress, ERF74 increases ROS explode via dictating gene utterance of RbohD, coincided with expression of ROS scavenging genes.¹¹Yet, TERF1 ethylene-responsive factor provokes stress tolerance by decreasing ROS amount.³⁶ Hence, in plant under salt stress, there is critical relation in ethylene biogenesis and signaling with ROS equilibrium which are related with salt tolerance. Ethylene is the most crucial signaling molecule, across its downstream signaling factors, plays a leading role in salinity tolerance in rice and Arabidopsis thaliana.³⁶ In Arabidopsis thaliana, salt tolerance is enhanced by down streaming signaling to keep ROS under control³⁷ and maintaining Na⁺/K⁺ homeostasis³⁸ by impaled ethylene gathering that may be endogenous in nature³⁵ or may be exogenous application of 1-aminocyclopropane-1-carboxylic acid (ACC). Although³¹ pointed out about optimistic act of ethylene toward salt stress in paddy, in wheat, ethylene concentration is high due to ACC oxidase 1 (ACO1), but the plant is sensitive to the salt even after a high concentration of ethylene.³⁹ On the basis of described reports, it appears that ethylene acts positively as well as negatively in plants under abiotic stress.³⁶ Under abiotic stress, alternative expression of miRNA is observed in various plant species.⁴⁰ The miR319 is essentially involved in various developmental processes, senescence, responses to various stresses and overtone in ethylene signaling, so it is one of the master controllers.³⁶ After 24 hours of exogenous ACC application in Medicago truncatula roots, a latent interplay of miR319 with ethylene was proposed⁴¹ and currently reported.⁴² These studies suggest that miR319 untargeted TCP family gene AtTCP that adversely manages ACS2, an enzyme involved in ethylene biosynthesis in Arabidopsis thaliana.

3. Ethylene and the regulation of metal stress

Under various abiotic stresses, ethylene plays a key role in plant growth and development. Report suggests that the effect of metal stress on ethylene production in plants is metal and concentration-specific.⁴³Among different metals, Cd is thought to be the most phytotoxic inorganic ion which stimulates the production of ethylene in plants.⁴⁴ A study by Masood et al.⁴⁵ on mustard under Cd stress showed that ethylene plays a key role in metal stress tolerance. Ethylene acts critically in enhancing ROS accumulation which plays the role of signaling agent for provoking defense machinery.⁴⁶ During photosynthesis, nicotinamide denine dinucleotide phosphate (reduced) transportation, and finally, transfers its electron to ascorbate-glutathione cycle (AsA-GSH cycle). The role of ethylene in GSH synthesis in de novo was demonstrated.¹² Ethylene directs plant growth and photosynthesis in normal as well as under stress condition. Synthesis of ethylene increases under Cd stress by the buildup of ACS2 and ACS6 transcripts in Arabidopsis¹⁴ but in the double mutant Arabidopsis acs2-1 acs6-1 subjected to Cd have a low level of ethylene followed by positive response on leaf biomass.¹⁴ The negative role of ethylene in alfalfa (Medicago sativa) exposed to mercury by using 1-MCP (ethylene response receptor blocker) was also studied.⁴⁷ Similarly, Cu is also reported to induce expression of ACO1 and ACO3 genes in Nicotiana glutinosa⁴⁸ and it is also speculated that up-regulation of ACO genes aids in ethylene production.^49,50

Ethylene helps in making ROS production under various abiotic stresses. ROS synthesis and signaling are primarily regulated by ethylene-responsive transcription factor, i.e., AP2/ERF gene family and links ethylene and ROS signaling under various abiotic stresses.⁹ The reason behind the elevated ROS level is the downregulation of expression of superoxide dismutase (SOD) and peroxidase (POD) (Figure 3) (ROS scavenger enzymes) by ERF1.⁹ Role of interaction of ethylene and ROS is also reported in paddy under metal stress, i.e., synthesis and signaling of ethylene depends on ROS production as ROS production increases under metal stress.¹³

Figure 3.

A simplified illustration of signaling components involved in the down regulation of antioxidant defense system induced by abiotic stress. The signal is perceived via receptors present on the cell wall. Presence of ethylene triggers various transcription factors and promotes the collection of ROS by promoting ARF genes which down-regulates the antioxidant enzymes of the cell. ERF, ethylene-responsive factor; OEC, oxygen evolving complex, and ARF, auxin response factor.

4. Ethylene and the regulation of floods stress

Ethylene is a quite known ruler of flood tolerance, and this investigation is well understood in a variety of rice that grows well under slight submergence.¹⁵ In paddy, water lodging enhances ethylene accumulation, which acts as antagonistic to ABA (antagonistic to gibberellic acid that involves in stem elongation). In semi submergence, ERF minimizes the antagonistic reaction between ABA and GA signaling so it results in stem elongation. By this event, plants are enabled to grow out from water to continue photosynthesis. There are several ERFs and genes which help rice plant to survive in submergence condition such as SNORKELs (SK1 and SK2) which encode VII AP2/ERFs class that adjusts the length of stem in broad water paddy under half submergence by enhancing GA action essential for stem elongation^15,51 and SUBIA, a ERF/AP2 transcription factor is also triggered by ethylene, and thus make survival in fully water lodged condition.⁵²

After running away from flood, aquatic plants are directly exposed to terrestrial environment. Re-oxygenation is critically related to enhanced synthesis of ROS and injurious metabolites. To avoid these deleterious effects in plant, several molecular mechanisms and signaling occur, in which ethylene plays a compact role. It was revealed that ethylene synthesis enhanced during re-oxygenation.¹⁶ Highest ethylene production response in flooding escape plant⁵³ and because of this, uninterrupted shoot elongation occurs even if leaf tip is in underwater. Ethylene plays a critical role in boosting post submerge rehabilitation in non-escaper condition. Enhanced expression of ethylene biosynthetic gene is cognate with re-oxygenation in Arabidopsis.¹⁶ Ethylene-insensitive (ein) mutant ein2-5 and ein3 eil1 relay more accessible to post anoxic stress. SUBIA, an ethylene-producible gene, conciliates rice tolerance to dehydration and oxidative stress by enhancing expression of genes related to ROS refining and readjustment to dehydration.⁵⁴ In the detailed study of Arabidopsis, group VII ERFs reveal their regulatory act in positive response to flooding and hypoxic stress.¹⁷There are five groups of VII ERFs: related to APRTALAT2 12 (RAP2.12), RAP2.2, RAP2.3, hypoxia responsive in Arabidopsis¹⁷out of four in five members (except RAP2.3), dispensable in controlling the expression and survival of hypoxia-responsive gene.⁵⁵For oxygen-dependent degradation by N-end rule way of selected proteolysis of all five Arabidopsis growth VII ERFs have particular N-terminal motif.^15,56

5. Ethylene and the regulation of drought stress

In Arabidopsis eto1 mutant, ethylene is involved in both opening and closing of stomatal aperture.¹⁹ Negative impact of ethylene in abscisic acid induced closing of stomata is also reported. Under drought stress, wild plant shows fast closing movement then ethylene flash flood mutant eto1. Ethylene-expressing factors EIN3 and ETR1 are not involved in stomatal closing movement.²⁰ But with the help of NADPH oxidase, ROS formation increases in guard cells which promotes stomatal closure.⁵⁷ By hindering ROS formation, ethylene controls stomatal closure with the help of flavanol production in guard cell under stress in an EIN2-mediated pathway.²¹

Rice eto1 mutants of rice which have an insensitive ETHYLENE OVERPRODUCER 1-LIKE (OsETOL1) protein with high ethylene level helps in the survival of plants under drought condition.⁵⁸ Rice over expressing OsETOL1 have reduced ethylene due to which it is less fit to drought.⁵⁸ Comparatively to its Arabidopsis equivalent, OsETOL1 reacts with the type II ACS enzyme OsACS2 and diminishes its endeavor.⁵⁸

Under drought stress condition, increased level of ethylene was reported in many plant species such as Vicia faba, orange, French bean, etc. The key enzyme of ethylene biosynthesis is ACS which has different phosphorylation sites on C-terminal region. Based on phosphorylation, ACS is categorized into three types. Type 1 – ACSs 1 have mitogen-activated protein kinase 3/6 (AtMPK-6) and calcium-dependent protein kinase (AtCDPK2, CDPK, or CPK) phosphorylation zone. Type II contains only CPK phosphorylation zone, but in type III ACSs have no phosphorylation zone. In maize, ethylene acts as a leaf growth inhibitor and in drought condition ACC acts as a transduction molecule.⁵⁹Under drought stress, RD29A and RD29B genes are unregulated. During osmotic stress, RD29A promoter has two cis-acting elements; one is activated by dehydrogen responsive element (DRE) and other is ABA-responsive element (ABRE), activated by ABA. RD29B has only ABRE domain and is activated by only ABA.⁶⁰ Arabidopsis with a constitutive promoter 35 S: ERF1 is drought-tolerant and ERF protein attached directly to RD29B promoter’s DRE element.²³A/GCCGAC is a core sequence of DRE element which is a part of cis-acting promoter element and is involved in expression and regulation of gene in drought with high survival rate and low yield.

6. Ethylene and the regulation of hypoxia and anoxia

In absence of oxygen, ethylene primarily acts to acclimatize plants by increasing the formation of adventitious root, aerenchyma formation, promoting petiole/stem height, and hyponasty response.¹⁸In Arabidopsis, HYPOXIA RESPONSIVE ERS1 and RELATED TO AP2.2 are two transcription factors that are liable for different downstream signaling controlled by ethylene.^61,62 In paddy, ethylene is indulged in the modulation of ERF-like gene, like SUBMERGENCE1-A-1 (SUB1A-1) and SNOKEL1 (SK1) and SK2 in intense water-grown rice to grant submergence lenient.^51,52It is reported that ethylene plays a critical role in governing the homeostasis of hormone gesture, ROS, and metabolic processes during reoxygenation.¹⁶ Even though there is molecular support in relation of ethylene and hypoxia but still the signaling pathway is not much clear. Under oxygen deficiency due to overwhelming of water, ethylene biosynthesis increases as well as various ethylene receptive genes containing an identified group of ERF/AP2 are expressed in Arabidopsis.¹⁵ For the survival of rice in hypoxia, up-regulation of RAP2.2, a group of VII AP2/ERF TF allied to SUBIA by upgrading plant wear out rates in hypoxia, and thus triggering the genes related to ethylene biogenesis and sugar metabolism.⁶¹ ERF73/HRE1 is one more TF related with group VII of AP2/ERF lineage that is tangled in increasing hypoxia tolerance but in ethylene-dependent manner.⁶²

7. Interaction of ethylene with other signaling molecules under abiotic stress

In plants under various abiotic stresses, ethylene acts positively either by direct expression of genes or indirectly through cross-signaling to other molecules. Under metal stress, ethylene interacts with NO to reduce negative effects. The task of NO and ethylene in metal stress has been extensively studied. In pea under cadmium stress, the regulation of ROS and NO is coequal with change in ethylene, JA, and SA.⁶³ In a study, synthesis of ethylene, JA, and SA remarkably enhanced ROS levels and decreased NO level were noticed under abiotic stress and thus, ethylene and NO acts antagonistically in root and leaf of plant under cadmium stress.⁶³ Raised level of both ethylene and nitrate reductase genes in juvenile soybean (Glycine max) was noticed after intermediary cadmium usage.⁶⁴ Under various abiotic stresses like metal and drought, ethylene harmonizes ROS stock.⁴¹ In controlling of synthesis and signal transduction of ROS, ERFs are involved.⁹ Further, under abiotic stress, ROS and ethylene cross talk is mediated by a transcription factor AP2/ERF.⁹ ROS level is enhanced by ERF1 which inhibits expression of ROS scavenger genes during abiotic stress.⁹ The role of ethylene and ROS in rice under metal stress was also reported.¹³ The signaling act of ethylene and ROS in various biological events such as root and root hair growth, as well as different biotic and abiotic response is reported.¹³ However, much more is remaining to decipher crosstalk of ethylene with other signaling molecules under abiotic stress.

7. Conclusions and future perspectives

Thus from the above discussion, it is clear that ethylene plays a very critical role in plant adaptation under abiotic stress. Hence, the concern here is to identify the molecular cocktailing of antagonistic and synergistic role of ethylene with other signaling cues. Though molecular interaction of ethylene and hypoxia is known,t ethylene signaling during stress is still in infancy. Therefore, precise methods for easy and quick detection of proteins modified by ethylene are needed for developing a better understanding of its role. It is also equally important to investigate how the emission of ethylene affects the surrounding flora. This will help the crop in withstanding the harsh climatic condition specifically flood and drought exposed areas of farming. This will allow the farmers to produce better quality crops to feed population.

Funding Statement

This work was supported by the Science and Engineering Research Board [0].

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.