The Plant Hormone Ethylene: Stress Acclimation and Agricultural Applications

By Nafees A Khan

The Plant Hormone Ethylene: Stress Acclimation and Agricultural Applications presents current knowledge on our understanding of ethylene perception and signaling, its role in the regulation of plant physiological processes, and its contribution to acclimation in stressful environments.

Plants regularly face environmental constraints due to their immobile nature. In persistently changing environmental conditions, several stress factors influence cellular metabolism, ultimately causing reduced plant growth and development with a significant loss in agricultural productivity. Sustainable agriculture depends on the acclimation of plant processes to the changing environment through altered physiological and molecular responses, which are controlled by plant hormones, including ethylene. Ethylene interacts with other plant hormones and signaling molecules to regulate several cellular processes, plant growth and development, and, ultimately, crop productivity.

This book begins with an introduction to ethylene before providing a detailed study of the latest findings on the role of ethylene in plants, including its role in photosynthetic processes, flower development, leaf senescence, nutrients acquisition, and regulation of abiotic stress responses as well as its application in agriculture. The book is an ideal guide for researchers exploring plant physiology and biochemistry as well as for those investigating the use of ethylene knowledge in agriculture in persistently changing environmental conditions.

• Provides state-of-the art insights into ethylene-regulated photosynthesis, growth, and productivity in crop plants
• Presents regulatory mechanisms of ethylene action
• Assists in developing physiomolecular strategies for augmenting crop performance in persistently changing environmental conditions

• Language: English
• Publisher: Academic Press
• Release date: Dec 5, 2022
• ISBN: 9780323898027

Book preview

Preface

Ethylene was the first identified simple gaseous molecule with biological function and classified as a plant hormone. It was first associated with fruit ripening and the triple response phenomenon observed in dark-grown pea seedlings with reduced stem elongation, increased stem thickening, and horizontal growth habit. Later, ethylene measurements were possible thanks to the advent of gas chromatography and molecular biology brought to success efforts done to explore its biosynthetic and signaling pathways. Significant success has been achieved with the discovery of its signaling components in Arabidopsis thaliana L. as a model system, and acquired knowledge has been applied in different agricultural sectors during the past decades. Ethylene regulates diverse aspects of plant growth and developmental responses in the life cycle of plants, starting from seed germination to senescence. It also plays a very important role in acclimation to stressful conditions. It interacts with other signaling molecules, including other phytohormones and signaling compounds, to influence tolerance to abiotic stresses. The action of ethylene depends on its concentration and sensitivity of plants and plays a major role in the regulation of developmental processes. The modern-omic approaches have resulted in the possibility of elaborating mechanistic hypotheses and unraveling ethylene functions at an unprecedented level of detail in plants growing under both optimal and stressful conditions. This book provides an overview of the most recent studies on the role of ethylene in plants; its role in photosynthetic processes, flower development, leaf senescence, nutrients acquisition, and regulation of abiotic stress responses; and its application in agriculture. The research topics included in the book are an account of the development in ethylene research and may serve as a guideline to further explore the avenues for the use of ethylene knowledge in agriculture in the ongoing changing climatic conditions.

Editors

Nafees A. Khan

Antonio Ferrante

Sergi Munné-Bosch

Chapter 1: Ethylene: A gaseous signaling molecule with diverse roles

Harsha Gautam; Zebus Sehar; Nafees A. Khan    Plant Physiology and Biochemistry Laboratory, Department of Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

Abstract

Ethylene is one of the most important plant hormones of the five classical hormones and has been used in agriculture since ancient times. It has long been recognized as the plant hormone responsible for fruit ripening. Unlike other plant hormones, ethylene may freely diffuse across membranes as a gaseous hormone and is assumed to be generated at or near its site of action. Ethylene has a significant impact on the development and senescence of organisms. It’s important for blooming induction, flower bud opening, and abscission, and it’s also produced in large amounts during leaf senescence and abscission. It increases adventitious root production, stem and petiole elongation, and stomatal aperture opening and closing. However, the multifunctional ability of ethylene in regulating physiological and molecular mechanisms is currently being explored. In this chapter, we briefly covered the significant aspects of ethylene biology in plants: a general account of ethylene, biosynthesis and signaling, approaches to ethylene biosynthesis with and without ethylene modulators in ideal and stressed conditions, a general aspect of ethylene and plant responses, and cross-talk with the other plant hormones.

Keywords

Signaling; Development; Physiological response; Ethylene modulators; Cross-talk

Chapter outline

1.Ethylene: A gaseous hormone: A general account

2.Ethylene biosynthesis and signaling

3.Approaches to ethylene action with and without ethylene modulators

4.Ethylene and plant responses: A general aspect

5.Crosstalk of ethylene with other plant hormones

6.Conclusion

References

1: Ethylene: A gaseous hormone: A general account

Ethylene (CH2 glyph_dbnd CH2), the first molecule discovered, is a simple gaseous molecule (28.054 g mol−1) with two C atoms bonded by a double bond but no radical or redox character. Ethylene is the simplest known olefin and, under physiological conditions, is lighter than air. It is highly flammable and easily oxidizes. Ethylene can be produced by almost all portions of higher plants, though the rate of production varies depending on the kind of tissue and growth stage. It is found in all higher plants and is typically associated with fruit ripening and the triple response phenomenon. On dark-grown seedlings, the triple response is decreased stem elongation, increased stem thickness, and horizontal growth habit. Ethylene is a stress-responsive hormone in addition to its role in plant growth and development regulation (Abeles et al., 1992; Khan et al., 2017; Asgher et al., 2018; Gautam et al., 2022).

Observations on the effects of smoke and illuminating gas on plants in the 1800s led to the discovery of ethylene effects on plants. Illuminating gas was originally utilized in the late 1700s (Sedgwick and Schneider, 1911), and by the late 1800s. However, it was widely employed for residential and business illumination, as well as streetlights, in many communities across the world. Coal gas was originally created through destructive distillation of bituminous coal. In 1858, George Fahnestock documented the first incidence of lighting gas damaging plants, noting that leaking pipes caused damage to plants in a Philadelphia greenhouse (Fahnestock, 1858). In 1896, Dimitry Neljubow, a plant physiologist at St. Petersburg University’s Botanical Institute, identified ethylene as the active component in illuminating gas that affects plants (Neljubow, 1901). He demonstrated that ethylene impacted the growth of etiolated pea seedlings, promoting diageotropism and the production of shorter, thicker epicotyls. This was the first time ethylene was demonstrated to have a biological effect. The etiolated seedling triple response (Knight and Crocker, 1913) was named after this response and was employed as a sensitive bioassay for ethylene (Crocker et al., 1913). Several scientists later confirmed and elaborated on Neljubow’s findings (Crocker and Knight, 1908; Knight and Crocker, 1913; Harvey, 1915; Harvey and Rose, 1915; Knight, 1910). Plants could also biosynthesize ethylene, although it took several decades following Neljubow’s discovery to prove it. The first evidence that plants biosynthesize ethylene came from Herbert Cousins’ observation that fungus-damaged oranges emitted a gas that accelerated banana fruit ripening (Cousins, 1910). In fact, it was unclear whether the ethylene produced by the fungus infecting the oranges or by the damaged oranges themselves. When Richard Gane used a quantitative chemical technique to show that apples release ethylene in 1934, he became the first to chemically establish that plants biosynthesized ethylene (Gane, 1934). He eventually proved that other fruits generate ethylene a year later (Gane, 1935). For the next 25 years, however, ethylene was ignored and not recognized as an important plant hormone because many physiologists believed that the effects of ethylene were due to auxin, and auxin is the central plant hormone in plant growth and development, whereas ethylene plays a minor and indirect physiological role. Because ethylene is gaseous in nature, working on it was difficult due to a lack of equipment and techniques for quantification. The importance of ethylene was rediscovered, and its physiological significance as a plant hormone was understood after the invention of gas chromatography in 1959 (Burg and Thimann, 1959). Then, in the field of ethylene detection, a big advance was made when novel detector technologies, such as flame ionization detection and the photoionization detector, became accessible (Lovelock, 1960; McWilliam, 1983), and the detection limit of ethylene enhanced to tens of nanoliters per liter (nL L−1) levels (Bassi and Spencer, 1985).

Ethylene regulates plant development throughout its entire life cycle, from early germination to senescence (Yin et al., 2012). The impact of ethylene in plants varies depending on the concentration and sensitivity of the plants to hormone. The ethylene-induced triple reaction was first identified in 1864. A series of ethylene responses were identified in Arabidopsis, which induced accelerated horizontal growth, decreased hypocotyl root extension, and exaggerated apical hook of etiolated seedlings. Plants have been identified with various functions such as regulation of leaf development, root and shoot differentiation, flowering initiation, pollination, anthocyanin synthesis, leaf and fruit abscission, senescence, fruit ripening, stimulation of germination, and defense mechanisms against various biotic and abiotic stresses since its’ discovery around a century ago (Abeles et al., 1992). In addition, ethylene is primarily synthesized in reaction to a wide range of environmental stimuli. It is suggested that ethylene serves as a link between changing environmental conditions and developmental adaptability, as well as allowing plant-plant communication. Ethylene regulates cell division through endogenous signals and external signals, as well as cell-cell progression, and its regulation is organ-dependent. Ethylene promoter has been used in agricultural productivity and is of concern in response to environmental stresses. Endogenous signals generated during development and in response to environmental challenges such as biotic (pathogen attack) and abiotic factors such as chilling, wounding, hypoxia and ozone are primarily responsible for regulating ethylene production (Wang et al., 2002; Geisler-Lee et al., 2010; García et al., 2015). To understand the critical role of ethylene in plants, it is necessary to know how ethylene is synthesized and how its signal is transduced in plants.

2: Ethylene biosynthesis and signaling

In higher plants, the ethylene biosynthesis pathway has been thoroughly studied (Xu and Zhang, 2015). Yang and Hoffman (1984) discovered the ethylene biosynthesis pathway, which consists of three enzymatic reaction steps. The enzyme S-adenosyl methionine synthetase converts methionine to S-adenosyl methionine in the first step (SAM). The enzyme ACC synthase then converts S-Ado-Met directly into ACC, which is a direct precursor of ethylene. Finally, ACC oxidation resulted in the production of ethylene via the enzyme ACO (ACC oxidase) (reviewed in Lin et al., 2009; Khan et al., 2015) and this step requires oxygen as a cosubstrate and activator. In the second phase, methylthioadenosine (MTA) is created as a byproduct, which is recycled into methionine via the Yang cycle, maintaining a methionine pool even while ethylene is rapidly manufactured (Yang and Hoffman, 1984; Kende, 1993; Fig. 1). Polyamines are also derived from decarboxylated S-AdoMet, which is produced by the enzyme S-adenosyl methionine decarboxylase and plays a key role in plant responses to environmental stresses. Because of its rate limiting property, ACS was the initial enzyme in ethylene biosynthesis and is the major target for ethylene production. In Arabidopsis thaliana, there are eight functioning ACS genes, all of which play essential roles in ethylene synthesis (Tsuchisaka and Theologis, 2004b; Tsuchisaka et al., 2009). Each gene has a distinct and important role in plant growth and development. Plant hormones auxin and cytokinin may also increase ethylene production at higher concentrations or through de novo synthesis of ACS, the first enzyme involved in ethylene biosynthesis. However, it is not confirmed that ABA inhibits the production of the ethylene precursor ACC or the conversion of ACC to ethylene. Although the ethylene biosynthesis route is straightforward, its production is tightly controlled at multiple levels. In addition to transcriptional regulation, posttranslational regulation is important for developmental and stress-induced ethylene synthesis in Arabidopsis (Tsuchisaka and Theologis, 2004a,b; Christians et al., 2009; Han et al., 2010; Skottke et al., 2011; Lyzenga et al., 2012).

Fig. 1

Fig. 1 Schematic representation of the ethylene biosynthesis pathway. SAM synthetase converts methionine to S -adenosyl-methionine. ACC synthase then converts SAM to methylthioadenosine (MTA) and 1-aminocyclopropane-1-carboxylic acid (ACC) (ACS). The Yang cycle converts MTA back to methionine. ACC oxidase catalyzes the final step in the ethylene biosynthesis pathway, the production of ethylene from ACC.

The primary components of ethylene signaling include five ethylene receptors, ETR1, ETR2, ERS1, ERS2, EIN4 (Key Positive Regulator) and a negative regulator CTR1 (Constitutive Triple Response 1), primary transcription factors EIN3/EIL1, and ethylene response factors. These receptors phosphorylate EIN2 by activating CTR1. When CTR1 is activated, it is thought to phosphorylate downstream MAPK kinases (MAPKKs), causing MAPK target proteins to be activated (Wurgler-Murphy and Saito, 1997). In the absence of ethylene, CTR1 signals constitutively, causing downstream transcription factors that induce ethylene responses to be down-regulated (Yoo et al., 2009; Wurgler-Murphy and Saito, 1997).

CTR1 is inactivated when ethylene binds to the receptors, allowing for positive control of the ethylene response transcription factor gene expression. In response to the presence of ethylene, the ethylene response transcription factors EIN3 and EIL1 accumulate in the nucleus. While EIN3 increases the expression of genes involved in the ethylene response, it also increases the expression of genes involved in the production of proteins that inhibit EIN3. EIN3 levels are kept from becoming too high and harming the plant by increasing the expression of its own negative regulators, EBF1 and EBF2 9 (Binder et al., 2007; Fig. 2).

Fig. 2

Fig. 2 The ethylene signal transduction pathway. Without ethylene binding, the ethylene receptor complexes (ETR1, ETR2, ERS1, ERS2, and EIN4) function synergistically but differentially to activate CTR1 in the ER. When CTR1 is activated, it phosphorylates a MAPK cascade. The phosphorylation of EIN3 and EIL1 by CTR1 may improve their interaction with the F-box proteins EBF1 and EBF2 to increase protein degradation via the 26S proteasome and decrease ethylene signaling. CTR1 is deactivated when ethylene binds to one of the receptors, and the MAPK cascade is not phosphorylated. As a result, EIN3 and EIL1 are enabled to accumulate in the nucleus, where they express ethylene response factors and induce transcription. EIN3 and EIL1 regulate themselves by increasing the expression of EBF1 and EBF2, which repress EIN3 and EIL1. (Modified from Yoo, S.D., Cho, Y., Sheen, J., 2009. Emerging connections in the ethylene signaling network. Trends Plant Sci. 14, 270–279; Givens, C.S., 2010. A Physiological and Evolutionary Study of the Plant Hormone Ethylene.)

3: Approaches to ethylene action with and without ethylene modulators

Ethylene biosynthesis has been widely investigated in both ideal and stressed conditions. Linolenic acid, propanal, β-alanine, and methionine were thought to be ethylene precursors. However, Lieberman et al. (1966) proposed for the first time that methionine is the significant precursor of ethylene and demonstrated that ethylene was formed from the carbon-3 and carbon-4 of methionine. It was reviewed by Yang and Hoffman (1984). A confirmatory role of methionine as an ethylene precursor was reported by Lieberman et al. (1966), who discovered that labeled methionine was swiftly converted to ethylene in apple, which was then validated by other researchers in apple and other plants. However, the discovery of S-adenosyl-l-methionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC) as ethylene precursors in plants later came (Yang and Hoffman, 1984; Kende, 1993).

Plants synthesize ethylene under optimal conditions via the Yang cycle. The first committed step of ethylene biosynthesis in the Yang cycle is the conversion of SAM to ACC by ACS (Yang and Hoffman, 1984; Kende, 1993). Simultaneously, ACS produces 5′-methylthioadenosine (MTA) for methionine conversion through a modified methionine cycle (Bleecker and Kende, 2000). The methyl group was saved for another cycle of ethylene synthesis, allowing for this rescue mechanism, while simultaneously conserving the S group of methionine to synthesize ethylene continuously without increasing the methionine pool. In the final step, ACO oxidizes ACC to form ethylene. CO2 and cyanide are produced as byproducts during this process; however, they are detoxified to β-cyanoalanine by β-cyanoalanine synthase, preventing toxicity from accumulating cyanide during high rates of ethylene production (Wang et al., 2002). Because methionine and SAM are the major chemical bridges in the production of ethylene, it is estimated that roughly 80% of cellular methionine is converted to SAM at the expense of ATP by SAM synthetase (Ravanel et al., 1998). SAM is a main methyl group donor in plants and serves as a substrate for a variety of metabolic processes, including cell wall synthesis, secondary metabolites, chlorophyll production, DNA replication, glycine betaine, polyamines, and ethylene. Furthermore, SAM is also involved in methylation processes, which affect lipids, proteins, and nucleic acids (Khan et al., 2014).

ACS and ACO are the key enzymes catalyzing ethylene synthesis, and they have been well defined and investigated incorporating biochemistry and molecular biology approaches. The molecular cloning of the ACS was the initial step in the ethylene production process (Sato and Theologis, 1989) and (Hamilton et al., 1991; Spanu et al., 1991) genes. This discovery led to the establishment of the multigene family, which is governed by a complex network of developmental and environmental signals that respond to both exogenous and endogenous stimulation (Johnson and Ecker, 1998). In response to external and internal stimuli, the transcriptional and posttranscriptional levels govern the production of ACS and ACO isozymes encoded by multigene families (Dahleen et al., 2012; Yoon and Kieber, 2013). In plants, two systems control ethylene biosynthesis: the first is an auto-inhibitory system for ethylene synthesis: this type of mechanism is seen in plants during normal vegetative growth. Second, when plants require a rapid rise in ethylene production during senescing ethylene-sensitive plant organs and ripening climacteric fruits, a positive feedback mechanism for rapid ethylene biosynthesis is regulated (Nakatsuka et al., 1998; Barry et al., 2000; Kim et al., 2001).

It has been discovered that the rate of transcription of the ACS gene, which stimulates ethylene biosynthesis by stressful situations, regulates and controls de novo synthesis of ACS in ethylene biosynthesis (Hyodo et al., 1991; Abeles et al., 1992). Stress can easily regulate the ACS gene to enhance stress ethylene, according to ACS studies, because ACS is a multigene family with each gene regulated individually (Liang et al., 1992; Zarembinski and Theologis, 1994; Fluhr et al., 1996). Furthermore, ACO has been shown to play a crucial function in the regulation of ethylene biosynthesis (Holdsworth et al., 1987; Kim and Yang, 1994; Barry et al., 1996; Lasserre et al., 1996). SAM synthetase, according to Morgan and Drew (1997), may also be involved in some stress responses. In elicitor-mediated stress, transcripts from its multigene family were shown to be differentially expressed (Kawalleck et al., 1992). Stress induces an oxidative burst within plant cells, stimulating the MAPK cascade and triggering the activation/phosphorylation of ACS, the enzyme responsible for the conversion of SAM to ACC (Liu and Zhang, 2004), subsequently phosphorylated ACS becomes stabilized and increases the production of ethylene. Stearns and Glick (2003) proposed a model. They addressed the contradictory effects of ethylene-stress in plants, emphasizing that stressed plants have an initial modest peak of ethylene near to the commencement of stress, followed by a second, much greater peak later. The plant’s preventive response, such as transcription of pathogen-related genes, is assumed to begin with the first modest peak (Ciardi et al., 2000). According to Robison et al. (2001), ACS gene transcription was stimulated, and more ACC began to accumulate as fuel for the second wave of ethylene production. This is owing to the environmental and developmental cues that regulate ACS genes, as well as the augmentation of its enzymatic activity during stress conditions (Wang et al., 2002). Despite its unspecified reason, stress ethylene is harmful to plants in a variety of ways. Stress ethylene is thought to play a role in wound healing and disease resistance; however, these responses do not appear to be a general phenomenon. Ethylene generation in stressed tissue has been examined in a variety of plant materials, and the methionine-ACC pathway is functional in all cases. Ethylene synthesis and ACC levels are low before stress treatment in each example, but quickly rise following stress. Stress ethylene leads to decrease in photosynthetic characteristics due to decreased accumulation of osmolyte, increased glucose sensitivity and increased thiol synthesis (Khan et al., 2016). The conversion of SAM to ACC is a crucial mechanism determining the formation of stress ethylene, just as it is in auxin-induced ethylene synthesis. As a result, it might be suggested that modulating stress ethylene is an effective technique for reducing stress-related injuries.

The use of ethylene synthesis modulators to control ethylene biosynthesis has been widely acknowledged and implemented in the past. Under stress, plants exhibit a significant increase in ethylene production as well as restricted plant growth and development. Therefore, the importance of necessity of controlling stress ethylene using tools can be understood. Ethephon has been utilized commercially in crop plants to produce ethylene. The ethylene released by ethephon (2-chloroethyl phosphonic acid) has an impact on various cellular developmental and stress response mechanisms related to photosynthesis (Fiorani et al., 2002; Khan et al., 2008; Masood et al., 2012; Khan and Khan, 2014; Gautam et al., 2022). Ethephon is a direct ethylene source for plants, eliciting the same responses as ethylene gas (Cooke and Randall, 1968; Edgerton and Blanpied, 1968). Commercial application of ethephon on vegetative material in cereal management initiatives has been employed in Europe (Hill et al., 1982). It is also applied to Solanum lycopersicum seedlings before shipping to eliminate early flower clusters and harden the plants (Taha et al., 1980). Furthermore, it has been discovered that ethephon increases ethylene release while also controlling endogenous ethylene production and function (Zhang et al., 2010a,b; Iqbal et al., 2012a,b). Many physiological investigations have clearly shown that ethephon has been widely used as a chemical alternative to ethylene therapy (Zhang et al., 2010a,b; Iqbal et al., 2011, 2012a,b).

Compounds that inhibit ethylene responses have been created as a way to protect plants against ethylene while also prolonging the shelf life of various goods. Ethylene activity inhibitors have been used to regulate plant growth and development. At lower concentrations, some ethylene inhibitor compounds have been observed to retard ethylene production or action. Ethylene inhibitors are divided into two categories: ethylene synthesis inhibitors and ethylene action inhibitors. The ethylene production inhibitors do not protect the crop against external ethylene (Iqbal et al., 2012b). Yang and Hoffman (1984) reviewed these compounds as well as the mechanisms of inhibition. Ethylene synthesis inhibitors interrupt the process by focusing on either ACS or ACO, whereas ethylene action inhibitors occupy ethylene receptors and prevent ethylene action. Aminoethoxyvinylglycine (AVG), an ethylene synthesis inhibitor, reduces ethylene production by inhibiting ACS activity (Yang and Hoffman, 1984). However, Boller et al. (1979) demonstrated that AVG inhibited ethylene biosynthesis by lowering the formation of ACC, a precursor to ethylene. Rajaei and Mohamad (2013) speculated that cobalt chloride (CoCl2), an ethylene synthesis inhibitor reduced seed germination and seedling growth of Brassica napus. Ethylene action inhibitors, on the other hand, are more specific. Inhibitors of ethylene activity bind to a particular receptor (Sisler and Serek, 1997; Hua and Meyerowitz, 1998; Klee, 2004). Ethylene receptor blockers are particularly useful in agricultural applications since they protect tissue from both endogenous and external ethylene (Sisler and Serek, 1997; Feng et al., 2000). In plants, the usage of 1-MCP (1-methyl cyclopropane), an ethylene action blocker, has been extensively studied (Sisler and Serek, 1997, 1999, 2003; Blankenship and Dole, 2003), and several applications in plant stress response amelioration have been documented (Huang and Lin, 2003; Grimmig et al., 2003; Yokotani et al., 2004). Similarly, application of silver thiosulfate is an efficient means of controlling ethylene action (Beyer, 1979; Veen and Overbeek, 1989) by blocking the ethylene receptor in plants (Veen, 1983). Silver ions, which are used for both agronomic and research purposes, block ethylene responses (Beyer, 1976). The Cu-binding position of ethylene receptors is thought to be occupied by silver, which interacts with ethylene while suppressing the ethylene response (Rodrıguez et al., 1999; Zhao et al., 2002; Binder et al., 2007). The compound NBD (norbornadiene) is also very strong tool for ethylene action (Sisler et al., 1986; Sisler and Serek, 1997).

Cyanide is produced as a byproduct in the ethylene biosynthetic pathway in an equal amount as ethylene. Cyanide is a metalloenzyme inhibitor that is very sensitive to cytochrome-c-oxidase and Rubisco (Grossmann, 1996). In a variety of scenarios when ethylene synthesis is greatly elevated, such as herbicide phytotoxicity and necrotic lesions during the hypersensitive response, cyanide has been postulated to have a role in cell death (Grossmann, 1996). The use of chemicals to prevent ethylene activity or to interfere in the ethylene signal transduction pathway make evident that ethylene synthesis per se does not lead to cell harm or death.

4: Ethylene and plant responses: A general aspect

Ethylene is a small, readily diffusible hormone that plays a role in plant growth and development, including as seed germination, overall growth, sex determination, fruit ripening, abscission, senescence, photosynthesis and cell death (Abeles et al., 1992; Iqbal et al., 2013; Khan and Khan, 2014). It has the potential to influence many aspects of plant metabolism, including antioxidant metabolism, sulfur (S) and nitrogen (N) metabolism, proline metabolism, photosynthesis, and growth (Abeles et al., 1992; Pierik et al., 2006; Iqbal et al., 2011; Khan et al., 2014; Asgher et al., 2014; Gautam et al., 2022). Depending on the concentration of ethylene, it can also impede or enhance developmental processes within the same species (Abeles et al., 1992). Excess ethylene has been shown to reduce the number of nodes, shoot height, leaf area, plant dry mass, and photosynthesis in several crop plants (Kays and Pallas, 1980; Pallas and Kays, 1982; Squier et al., 1985). The environmental stress conditions such as salt (Sehar et al., 2021) flooding (Voesenek et al., 1993), O2 or CO2 levels (Dhawan et al., 1981; Vergara et al., 2012), high temperature (Orihuel-Iranzo et al., 2010; Khan et al., 2014), nutrient availability (Hermans et al., 2010) or heavy metals (Khan and Khan, 2014) enhances ethylene production in plants together with reduced growth and development. Furthermore, ethylene production can differ depending on plant species, organ type (e.g., root, leaf, and flower), and developmental stage of plants. Carbone and Beltrano (1995) discovered that excess ethylene reduced coleoptile and root length in Triticum aestivum. Increased ethylene production reduced grain weight, delayed maturity, induced senescence, and resulted in premature plant mortality in mature Triticum aestivum plants (Beltrano et al., 1999). Similarly, when exposed to water stress, increased ethylene synthesis lowered pigment content and plasma membrane integrity (Beltrano et al., 1999; El-Shintinawy, 2000).

In Cannabis sativa, ethephon treatment enhanced tocopherol accumulation, which is important in protecting lipids and the photosynthetic system from oxidative damage caused by a variety of abiotic stresses (Mansouri et al., 2013). The involvement of ethylene in Solanum lycopersicum organogenesis, on the other hand, results in improved regeneration and shoot development (Trujillo-Moya and Gisbert, 2012). Ethylene enhanced the activity of nitrate reductase (NR) and ATP-sulfurylase (ATP-S), which contributed to higher N and S-assimilation and thus increased stomatal and photosynthetic responses such as net photosynthesis, stomatal conductance and intercellular CO2 concentration (Iqbal et al., 2012a). Whether exogenous or endogenous, ethylene triggers physiological responses related to photosynthesis through its effect on stomatal aperture (Subrahmanyam and Rathore, 1992; Khan, 2004; Desikan et al., 2006), influencing photosynthetic processes (Tholen et al., 2007; Iqbal et al., 2011). Previously, Khan et al. (2008) and Iqbal et al. (2011) demonstrated that ethephon-treated Brassica juncea plants increased NR activity and N-use efficiency and also promoted photosynthesis. Ethylene enhanced plant stomatal conductance, enhancing CO2 diffusion and consequently photosynthesis (Iqbal et al., 2011). Contrarily, it has been shown that ethephon reduced overall dry matter accumulation probably due to lower leaf area and finally reduced capacity or total plant carbon assimilation in Lycopersicon esculentum (Woodrow et al., 1988) and inhibited photosynthesis in Arachis hypogaea, Helianthus annus, and Ipomoea batata (Pallas and Kays, 1982). It has been reported that ethephon at lower concentrations (1.0 μM) increased chlorophyll a, chlorophyll b, and total chlorophyll in comparison to control plants, whereas ethephon at higher concentrations (5.0, 10, and 100 μM) was not different from the control in Cannabis sativa (Mansouri et al., 2013). Ethephon at 1.5 mM increased photosynthetic rate, whereas 3.0 mM ethephon inhibited photosynthetic rate in Brassica juncea and Triticum aestivum (Khan, 2004; Gautam et al., 2022).

It has been observed that ethylene plays a role in regulating N and S uptake and improving photosynthetic characteristics. Exogenously-sourced ethylene boosted stomatal conductance, photosynthesis, and growth under both deficient and sufficient N conditions, according to Iqbal et al. (2011). Under metal stress (Cd, Ni, and Zn), enhanced NR and ATP-S activity has been found (Masood et al., 2012; Khan and Khan, 2014), resulting in more efficient alleviation of Cd, Ni, and Zn-induced oxidative stress. Wangeline et al. (2004) demonstrated that overexpression of ATP-S in Brassica juncea improved metal tolerance. The importance of N and S in enhancing photosynthesis has been well documented (Marschner, 2011; Iqbal et al., 2011, 2012a), and insufficiency of N and S has been shown to cause a significant suppression in plant photosynthetic efficiency (Resurreccion et al., 2001; Lunde et al., 2008) through their influence on Rubisco content. Ethylene-mediated variation in stomatal response (Pallas and Kays, 1982; Madhavan et al., 1983; Merritt et al., 2001; Iqbal et al., 2011) has been shown to influence photosynthesis (Subrahmanyam and Rathore, 1992; Khan, 2004).

Depending on the plant’s sensitivity to ethylene, ethephon either increases or lowers photosynthesis (Iqbal et al., 2011; Khan and Khan, 2014; Pallas and Kays, 1982; Khan, 2005). Khan (2004) reported a link between the activity of ACS, a rate-limiting enzyme in ethylene production, and photosynthesis in Brassica juncea cultivars with varying photosynthetic capacities. Tholen et al. (2004) compared photosynthesis and growth on the basis of ethylene sensitivity in genotypes of three species. They discovered that after 14 days of growth in solution culture, the net photosynthesis of wild type (W115) and etr1-1 Petunia (from a different source) was the same, but Arabidopsis thaliana and Nicotiana tabacum (with stronger ethylene insensitivity) had lower net photosynthesis compared to wild-type controls. Furthermore, they determined that the cause of these genotypes reduced photosynthesis was connected with lower N content per unit leaf area associated with higher specific leaf area. Woodrow and Grodzinski (1989) proposed that ethylene-related variations in leaf development and total light interception could affect CO2 assimilation and growth. The researches have shown that the increase in photosynthesis with ethylene-releasing compounds was due to the increase in chlorophyll (Bühler et al., 1978; Grewal and Kolar, 1990; Grewal et al., 1993; Khan et al., 2000; Khan, 2005) and retaining high leaf area in ethephon treated plants also helped in an increase in photosynthesis (Subrahmanyam and Rathore, 1992; Khan et al., 2000; Table 1).

Table 1

5: Crosstalk of ethylene with other plant hormones

Auxin, cytokinin, NAD, 2,4-D, picloram, and other plant hormones can all influence ethylene production (Abeles et al., 1992; Qin et al., 2019). The hormones auxin and cytokinin, in particular, have a greater influence on ethylene evolution than the other hormones. The increase in auxin promotes ethylene evolution via de novo ACS synthesis. Similarly, a high concentration of cytokinins promotes ethylene evolution. When compared to the hormone alone, the addition of auxins and cytokinin increased ethylene synthesis significantly. Regardless, it is unclear whether endogenous abscisic acid (ABA) has the ability to inhibit ethylene synthesis precursor ACC or the conversion of ACC to ethylene. Ethylene inhibits polar auxin transport, despite the fact that the hormone indole acetic acid (IAA) stimulates ethylene synthesis. As a result, ethylene may reduce the amount of active IAA, which may reduce the step of ethylene synthesis. However, the significant collaborative influence of IAA and ethylene may reduce tissue adaptability to ethylene, as suggested by Mathieu et al. (2020) when the substance of ABA and jasmonic acid (JA) was more notable; however, the synthesis of ACC was reduced in chicory plants. Despite the fact that ethylene emission remained consistently high over the course of 5 h, ABA, SA, and JA decreased in Melissa officinalis leaves (Pistelli et al., 2019). The concentrations of ABA, ACC, and JA were significantly higher in leaves, whereas IAA and cytokinins decreased in plants that had previously been exposed to temperature (Prerostova et al., 2020). Ethylene reduces primary root growth in Arabidopsis via activating auxin transport, biosynthesis, and signaling. Cytokinin and ABA suppress primary root growth via modulating ACS at the posttranscriptional level, resulting in increased ethylene production. Higher levels of brassinosteroids enhance ethylene production via increasing ACS, although this is inhibited by ABA (Qin et al., 2019). During cold stress, JAs and ET differentially regulate the C-repeat binding factor (CBF) pathway (Kazan, 2015). ERFs are transcription factors that integrate ethylene and jasmonic acid pathway inputs (Shinshi, 2008). The major ethylene and JA signaling centers, EIN2, EIN3, AP2/ERF and JAZ proteins, CTR1 and MYC2, play a complicated regulatory role during plant adaptation to abiotic stress (Kazan, 2015).

6: Conclusion

To summarize, ethylene is a signaling molecule that has the ability to influence multiple physiological and molecular responses in plants, ranging from plant growth and development processes to responses to changing environmental conditions. It activates a network of signaling pathways and modulates the control of various processes through interactions with other plant hormones. Analyzing how ethylene interacts with other plant hormones to control growth and senescence could result to a potential means for altering the content of these hormones using molecular method to develop specific plant responses. Despite the increasingly rapid speed of scientific research and the development of new research methods, major changes in our understanding of ethylene as a signaling molecule are predicted to continue.

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