Where is ethylene gas produced in plants

Some isoforms also have a C-terminal receiver domain, which is the second element of the two-component system Fig. In the ethylene receptors, the ethylene-binding domain lies within the ER membrane while the GAF, histidine kinase and receiver domains are in the cytoplasm.

It is unclear why the ethylene receptors reside at the ER membrane, but given the diffusion of ethylene across membranes, there is no obligation for the receptor to be at the cell surface. Ethylene is more soluble in hydrophobic environments, consistent with the location of the ethylene-binding pocket within the membrane.

The five ethylene receptor isoforms in Arabidopsis. The ethylene-binding domain consists of three conserved transmembrane domains at the N-terminus represented by the vertical blue bars. The receptors fall into two subfamilies. Other plant species have similar ethylene receptor isoforms. The ethylene receptors form disulfide-linked dimers, and each dimer is capable of binding a single ethylene molecule [ 18 ] with the help of a copper ion cofactor [ 19 ].

The dimers reside in clusters at the ER membrane where they interact with downstream proteins in the pathway [ 16 , 17 ]. The GAF domain, usually known for binding small molecules, facilitates protein—protein interactions between ethylene receptor monomers as well as between isomers [ 16 , 17 ].

This is a good question. Currently this is best understood at the genetic level. From genetic analyses, we know that the receptors are negative regulators of ethylene responses. In other words, ethylene responses are repressed by ethylene receptor signaling [ 20 ].

CTR1 kinase activity negatively regulates the pathway i. When ethylene binds to the receptors, ethylene receptor signaling ceases.

Consequently, CTR1 is no longer activated and downstream ethylene signaling can proceed Fig. This model is supported by the fact that null mutations in multiple ethylene receptor genes display constitutive ethylene responses similar to ctr1 loss-of-function mutants, whereas dominant, gain-of-function receptor mutations confer ethylene insensitivity [ 20 ]. This is still unresolved. In the canonical two-component system, binding of the ligand either stimulates or inhibits autophosphorylation of a conserved histidine residue followed by transfer of the phosphate to a conserved aspartate in the receiver domain.

Curiously, histidine kinase activity does not appear to play a major role in ethylene receptor signaling [ 16 , 17 ]. In addition, despite hints of two-component signaling elements acting downstream of the receptors, there is strong evidence indicating that this is not the primary mode of ethylene signaling. Instead, the ethylene receptors physically associate with and signal to CTR1 [ 16 ].

Although genetic evidence indicates that ethylene binding inhibits receptor signaling, there is no clear answer to the basic question: Does the binding of ethylene stimulate or inhibit biochemical activity in the receptor? There are data to support each possibility. Although it might be counter-intuitive, a formal possibility is that CTR1 activation occurs by a passive e.

Complete structural data for the receptors, which is not yet available, will hopefully shed light on this question. It is also unclear why plants have multiple ethylene receptor isoforms. Although there is evidence that individual receptors have distinct roles in controlling specific responses, the underlying mechanisms for sub-functionalization are unknown [ 17 ].

Ethylene signaling downstream of CTR1 hinges on the phosphorylation status of EIN2, an enigmatic central regulator of the ethylene-signaling pathway [ 23 ]. EIN2 is tethered to the ER membrane by its N-terminal domain, which has sequence similarity to the widely conserved NRAMP metal ion transporters, but the biochemical function of this domain and its role in ethylene signaling have yet to be determined.

The C-terminal portion C-END of EIN2 consists of a novel plant-specific domain that is cytosolic, and expression of this domain alone is sufficient for the activation of ethylene responses [ 23 , 24 ]. EIN3 was shown to initiate a transcriptional cascade that triggers several dynamic waves of gene expression that include feedback loops and the activation of genes known in numerous other hormone signaling pathways [ 29 ].

These global changes in gene expression result in a diverse array of cellular, metabolic and physiological responses [ 28 ]. Given that plant growth, development and stress responses require the integration of diverse environmental and endogenous signals, there is a growing focus on ethylene cross-talk with other signals [ 31 , 32 ].

Multiple points of ethylene cross-talk have been reported with the plant hormones auxin, gibberellins, brassinosteroids, abscisic acid, cytokinins and jasmonic acid. Cross-talk can occur through the regulation of ethylene biosynthesis, and more work is needed to elucidate such pathways. Conversely, ethylene signaling can induce the biosynthesis of other hormones.

For instance, in deep water rice, ethylene signaling induces gibberellins, which signal internode elongation, allowing rice plants to escape from complete submergence [ 33 ]. Cross-talk can also occur in the ethylene-signaling pathway. Continued advances in understanding cross-talk and transcriptional networks will lead to a deeper understanding of ethylene-signaling networks that will someday allow for the modeling of specific plant responses.

Applications of such knowledge have tremendous potential for agricultural improvements. Already, our understanding of ethylene biology can provide new strategies for manipulating ethylene responses, particularly by genetic means. Examples include the delay of flower senescence by expressing a dominant mutant etr gene of Arabidopsis [ 36 ] and the breeding of flood-tolerant, high-yield rice plants by introducing an ethylene-inducible ERF transcription factor gene SUB1A [ 33 ].

The days of unknowingly manipulating ethylene biology, e. Ethylene in plant biology. San Diego: Academic Press; Google Scholar. McManus MT. Annual plant reviews vol. Oxford: Wiley-Blackwell; Book Google Scholar.

History of research on the plant hormone ethylene. J Plant Growth Reg. Kende H. Plant biology and the Nobel prize. Van der Straeten D. Plant Sci. Xu J, Zhang S. Ethylene biosynthesis and regulation in plants. In: Wen C-K, editor. Ethylene in plants. Berlin: Springer; Ethylene: an urban air pollutant.

J Air Pollution Control Assoc. Dillard MM. Ethylene--the new general anesthetic. J Natl Med Assoc. The Delphic oracle: a multidisciplinary defense of the gaseous vent theory. J Toxicol Clin Toxicol. Insensitivity to ethylene conferred by a dominant mutation in Arabidopsis thaliana. Exploiting the triple response of Arabidopsis to identify ethylene-related mutants. Plant Cell. Arabidopsis ethylene-response gene ETR1 : similarity of product to two-component regulators.

The first group is called the climacteric fruits, in which ripening are accompanied by a burst of ethylene. These fruits can also respond to external ethylene by increasing their ripening rate. These include fleshy fruits, such as tomato, avocado, apple, melon peach, kiwi, and banana.

The second group is called the non-climacteric fruits, in which ethylene production does not increase during ripening. However, these fruits can still ripen if they are exposed to an external ethylene source, such as a ripening climacteric fruit.

These include strawberry, grape, and citrus fruits [ 3 ]. We will focus on ripening of climacteric fruits that are influenced by ethylene. For climacteric fruit, exposure to an initial, small concentration of ethylene causes the fruit to produce greater quantities of ethylene until a peak concentration is achieved [ 4 ].

The methods we described above for slowing down ripening work in this way, because, in general, low temperatures reduce metabolism in fruit. Controlled atmospheres limit the amount of oxygen around the fruit, and oxygen is needed to make ethylene. Ethylene action is inhibited by carbon dioxide and by 1-MCP. Another method for slowing down ripening is to remove ethylene from the storage environment by using materials that absorb ethylene, such as potassium permanganate.

Once the fruit reaches its destination, it can be ripened by exposure to ethylene gas. The effect of ethylene on ripening is dependent on many factors. The fruits need to be mature enough to be able to respond effectively to ethylene. In highly sensitive species, like cantaloupes or bananas, ripening is immediately stimulated by ethylene, but the more immature the fruit, the greater the concentration of ethylene required to cause ripening.

In the less sensitive species, like tomatoes or apples, ethylene treatment reduces the time before ripening occurs. Some fruits, such as avocados, do not ripen while attached to the tree and gradually increase their sensitivity to ethylene with time after harvest [ 6 ]. All plants produce some ethylene during their life cycle. Ethylene production can increase up to fold or more during particular stages—for instance in response to a wound [ 1 ]. Ancient Egyptians used to cut figs to enhance their ripening, since ethylene produced by the injured fruit tissue triggered the ripening response.

Similarly, the ancient Chinese used to burn incense in closed rooms with stored pears, because ethylene was released as a by-product of the burning incense. Ethylene gas is commercially used to ripen fruits after they have been picked. Fruits, such as tomato, banana, and pear are harvested just before ripening has started typically in a hard, green, but mature stage. This allows time for the fruit to be stored and transported to distant places.

Once the fruit reaches its destination, ripening is conducted under controlled conditions. This is usually carried out in specially constructed ripening rooms, with optimum ripening temperature, humidity, and ethylene concentration. These special conditions cause the fruit to ripen at a consistent rate. By the time the ethylene-treated fruit reaches the consumer, the commercially applied ethylene is gone, and the fruit is producing its own ethylene. Both ethylene and another widely used ripening agent, methyl jasmonate, are reported to be non-toxic to humans; however, they are relatively expensive.

Understanding the effects of ethylene on fresh produce can be helpful in ripening fruits in our own kitchen. The major drawback of GC was a relatively poor detection limit, i. The TCD measures the difference in thermal conductivity between the sample components in the carrier gas and the pure carrier gas alone as reference.

This difference generates a voltage signal proportional to the concentration of the sample components. Currently, TCDs are considered universal detectors and, in spite of lack of sensitivity, TCDs are non-specific and non-destructive detectors. To overcome the GC's lack of sensitivity, plants were enclosed into a sealed cuvette, allowing ethylene to accumulate for several hours, sometimes as long as 1 d Freebairn and Buddenhagen, ; Abeles, It was a compulsory step, necessary to generate a detectable GC signal, in spite of the risk that the plant was experiencing several environmental stress factors influencing the ethylene production e.

A major breakthrough was achieved in the early s, when novel detector technologies became available Lovelock, ; McWilliam, Flame ionization detectors became the first standard practice in plant-physiology laboratories.

Later on, it was demonstrated that PID had a significant improvement over FID, becoming the most efficient detection system concerning ethylene measurements Bassi and Spencer, The PID is also a popular detector in the field of environmental-pollution and industrial-process monitoring, as it is specifically sensitive to aromatic hydrocarbons and sulfur compounds.

The sampling procedure and subsequently injection of the sample into the GC column are two aspects that require careful consideration Tholl et al. If too much of the sample is injected, the peaks of the chromatogram show a significant tailing, which causes a poorer separation Bassi and Spencer, In the early s, ethylene was sampled from the headspace of a closed cuvette, in which the plant was enclosed for a few hours, and manually injected into the GC column with a gas-tight syringe Abeles et al.

Although this is a simple technique, it was time consuming for the analyst and not very reproducible as the sampled amount could differ slightly; not to mention the induced-physiological impact on the plant material due to the enclosure needed to obtain a high enough ethylene concentration. A solution was quickly developed, with automatic samplers connected to the GC raising the standard GC performance and reliability. In modern chromatography systems, concentric rotary valves allow a discrete sample to be automatically introduced into the column for separation Fig.

These samplers provided accuracy, superior reproducibility, optimum injection flexibility, reliability and were user-friendly. The use of valves to switch between gas streams simplified the analyst's work and made the GC more attractive. Gas chromatography detection. Several configurations have been used with GCs. Both sampling and injection into the column have been improved over the years.

A Manual GC injection was done with syringes. B Auto-injector-facilitated injections brought higher accuracy and could process a large number of samples. Several detection schemes were implemented using a non-destructive, although not sensitive sensor, such as the thermal conductivity detector TCD.

Later on detectors sensitive to hydrocarbons — the flame ionization detector FID — or to aromatic and olefin hydrocarbons — the photoionization detector PID — were developed and became the most commonly adopted detector technologies. The GC systems were coupled with adsorption—thermodesorption devices enabling them to store the emitted ethylene Segal et al. The plants were placed in closed cuvettes and continuously flushed with air, minimizing the accumulation effects.

Ethylene is thus trapped inside a tube containing an appropriate adsorption material e. Several adsorbents have been proposed for efficient ethylene trapping Pham-Tuan et al.

However, thermal desorption suffers from the lack of repeated sample injections and the degradation of the trapping media Clausen and Wolkoff, However, rather than costs, accuracy, speed and repeatability should be the major considerations in the selection of a GC.

For highly sensitivity systems, the sampling is done semi-continuously. The gas sample is periodically introduced into the GC system thanks to a slight overpressure of the input or by pumping the sample directly Gaspar, As the length of the analysis is determined by the RT of different components in the gas mixture, efforts were devoted to reduce the response time of GCs to below 10 min Cramers et al. In this way, the slowest compounds travelling into the GC are not injected into the GC analysis column.

As soon as ethylene enters the analysis column, the flow inside the stripper column is reversed and all the molecules travelling at a lower speed are flushed backwards. Several companies e. Synspec have developed compact and fast GC systems with a response time of a few minutes.

If GCs are considered an efficient tool for biological and chemical analysis, the range of applications in remote, on-site monitoring or monitoring hostile environments is limited due to the nature of the technology. Miniaturization of GCs into a compact, robust and low-power consumption device would enable their range of applications to be extended. The first attempt to develop micro-GC was reported in Terry et al.

The driving idea behind miniaturization was to decrease the analysis time as well as the amount of reagents required for analysis. Since then, several fabrication processes of an integrated GC column have been reported in the literature.

Attempts were made with silicon Reston and Kolesar, ; Lambertus et al. To our knowledge, micro-GC has neither been designed nor applied to ethylene detection. As more compounds could be measured with a single device, the GC is a widely used instrumentation within the plant research community, although other powerful techniques specialized in ethylene monitoring are becoming commercially available.

Requires a pre-concentration step for better sensitivity and optimized plant conditions. Various electrochemical devices, e. Recent advances in electrochemical sensor technology have expanded the application of these devices towards a wide range of compounds, including ethylene.

An electrochemical sensor transforms the concentration of a gas into a detectable physical signal such as: electrical current, resistance, etc. In detail, the target gas undergoes a chemical reaction with the active sensing material, which in the presence of an electrical circuit will generate a change in an electrical parameter Bard and Faulkner, Electrochemical sensors can be classified accordingly to the physical change measured.

Below, these three groups of electrochemical sensors will be discussed as they are the most popularly employed for ethylene monitoring. In its simplest form, an electrochemical sensor consists of a diffusion barrier, a sensing-electrode or anode , a counter-electrode or cathode and often a third electrode or reference electrode , separated by a thin layer of electrolyte usually sulfuric or phosphoric acid Fig. The electrodes are commonly fabricated by fixing a precious metal with a high surface area onto a porous hydrophobic membrane.

For example, currently available amperometric electrochemical sensors use gold anodes. When a voltage is applied between a gold electrode and a reference electrode, ethylene that diffuses into the sensor through the barrier is catalytically oxidized at the surface of the gold electrode Schmidt and Pastor, This results in a current change that is proportional to the concentration of the ethylene gas.

Moreover this sensor was found to be dependent on the ethylene flow rate and suffered from baseline drift one reason could be electrolyte evaporation. Electrochemical sensor. Ethylene diffuses through a barrier into the sensor, which consists of a sensing electrode anode, A , a counter electrode cathode, C and a reference electrode R covered by a thin layer of an electrolytic solution E.

If an electrical potential is applied to the anode most recently made of gold particles ethylene is catalytically oxidized, resulting in a current change proportional to the ethylene concentration. Despite the significant effort dedicated to the design and fabrication of electrodes and electrolytes, the electrochemical sensors have demonstrated that they are not completely gas specific; they are also sensitive to other compounds.

Oxidation of ethylene on a gold—Nafion electrode also generates acetaldehyde, nitrogen oxides and sulfur compounds which are considered interfering gases Pastor and Schmidt, The use of a filter may cause delay in the sensor response which was not the apparent case in this report. It is acknowledged, however, that retention of the interference gases in the trap could shorten its lifetime. As the performances of the amperometric sensors strongly depend of the anode material, much effort over the last few years was devoted to electrode fabrication.

Nanoparticle technology has provided innovative solutions to improve sensor performances Thompson, The nanoporous gold sensor technology has been recently implemented by Fluid Analytics, Inc. An acid electrolyte is necessary to prevent gold oxide formation before the oxidation of ethylene. Research is ongoing to elucidate the role of the composition and strength of the electrolyte in the mechanism of ethylene oxidation. Instead of the acid electrolyte, a non-acidic thin ionic-liquid layer has been proposed recently Zevenbergen et al.

This type of electrolyte is nontoxic and has a low drift since it virtually evaporates. Oxygen is an essential ingredient in the reaction with the gas and in maintaining the generated current when ethylene is present. If the oxygen supply is inadequate, the sensor will not operate properly, thus the sensor cannot be used for certain applications.

Measurements in low-oxygen conditions can be performed if a separate access of air to the counter-electrode is available. Interestingly, electrochemical sensors have been indicated as suitable for post-harvest storage under low-oxygen conditions. Obviously, the main considerations for this choice were their low cost, small size some sensors are portable and low power consumption. The use of a liquid electrolyte makes this sensor sensitive to environmental conditions such as temperature and humidity.

Novel types of electrochemical sensors are currently being developed using carbon nanotubes i. The first type of sensor is a chemoresistance sensor: the active sensing material is a mixture of carbon nanotubes and a copper complex placed between gold electrodes. Ethylene binds to the copper complex known as the electrical conductor , changing the resistance of the nanotubes Esser et al.

According to the authors the choice for copper was inspired by the role of copper as a cofactor for ethylene-binding activity in the ethylene receptors. Chemoresistance sensor. The sensing material is a mixture of single-walled carbon nanotubes SWNTs and copper complex placed between gold electrodes. Ethylene binds to the copper complex, changing the resistance of the nanotubes according to the gas concentration adapted from Esser et al.

The second type of sensor is a capacitor-based sensor at room temperature. The active sensing material is SnO 2 nanoparticles 10—15 nm in diameter used as a dielectric material between two copper electrodes Balachandran et al.

The use of this kind of sensor would be desirable due to its low power consumption and short response times of a few minutes. Moreover the sensor can be integrated with a microstrip patch antenna for passive wireless detection of ethylene gas. The sensor also responds strongly to other, interfering compounds such as ethanol, acetic acid, ammonia, acetone and ethyl acetate and to humidity changes.

In general, the operating lifetime of electrochemical sensors depends on the sensor type, which can vary from 6 months to over 2 years. The lifetime is shortened by a variety of environmental factors, such as low humidity, high temperatures as the electrolyte may dry out and exposure to the target gas and gas interferences consumption of electrolyte. Electrochemical cells are active even when they are stored; therefore, they have a limited lifetime even when not in use.

It is recommended to keep them in a refrigerator when not in use. Electrochemical sensor technology is a proven technology and, like any other technology, it has its advantages and disadvantages. Relatively fast response time to ethylene below 1 min and recovery time of minutes.

Portable and easy to use in laboratory or field conditions some are battery operated up to max. Reduced lifetime when continuously exposed to higher ethylene concentrations. A challenge remains where, on one hand, the electrochemical sensor works better at higher ethylene concentrations by overruling the influence of the interfering gases and, on the other hand, continuous operation over several days exhausts the anode and affects the sensitivity.

When light interacts with ethylene molecules it can be absorbed, emitted or scattered. By knowing the absorption strength of ethylene at a specific IR light frequency, the molecular ethylene concentration can be quantified.

There are several schemes of such sensors and generally they can be classified as dispersive or non-dispersive sensors Fig. Classification of the optical gas sensors. In the non-dispersive sensors the light source used for gas absorption is broadband and multiple narrow-band optical filters are required for detecting ethylene by subtracting the interference gases.

The dispersive broadband sensors have dispersive elements, such as prisms to separate the wavelengths. The laser-based optical sensors are using lasers to selectively excite the ethylene molecules and sensitive detection methods i. Typically, an optical sensor consists of an appropriate light source IR lamp or laser that passes through an absorption cell containing the ethylene sample and reaches an optical detector that measures the light intensity.

Obviously, the light source has to be chosen in the wavelength region where ethylene has absorption features. The measured intensity of the transmitted light is altered due to the light absorption of the ethylene molecules and from this change the ethylene concentration can be determined.

An important element for the NDIR sensors is their band-pass filter. This is necessary to increase the sensor selectivity for ethylene and to attenuate the undesired absorbents.

Like the electrochemical sensors, NDIR devices also suffer from lack of selectivity. Although they are simple and robust, NDIR instruments have limited sensitivity because the measured transmitted light can also be affected by the absorptions of other gases A possible solution to this problem is to use a single source and two identical detectors with two filters: one for measuring ethylene and the other for all also interfering gases.

This is a simple approach in which ethylene released by a biological sample is transported to the absorption cell in this case called the photoacoustic cell. Once inside it absorbs the light of a pre-selected wavelength and converts it into heat. By modulating the light switching the light on and off with a certain frequency with a mechanical chopper, pressure waves i.

The amplitude of the waves is proportional to the concentration of ethylene in the photoacoustic cell Kreuzer, Non-dispersive versus laser-based sensor using photoacoustic spectroscopy. Light is generated by a broadband source passed through a filter wheel A or by a laser B and directed into an absorption cell where it is absorbed by the ethylene molecules and converted into heat.

By switching the light on and off with a mechanical chopper the temperature changes periodically, giving rise to a periodic pressure change, resulting in acoustic energy detected by a miniature microphone.

The intensity of the sound is proportional to the concentration of absorbing gas molecules present in the cell. The laser-based sensors, using a single-frequency light source, are more selective and provide the lowest detection limits so far. These NDIR sensors need multiple narrow-band optical filters that can be mounted in a filter wheel Fig.

The filters are chosen for ethylene as well as for the interfering gases; the data analysis is performed using a mathematical model based on non-linear compensation.