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Ethylene is an important plant hormone that regulates many aspects of plant growth, development and senescence. Essentially all parts of higher plants produce ethylene although the rate of production is usually low (Yang & Hoffman, 1984). Ethylene is particularly significant in regulating the ripening process of climacteric fruits and vegetables. The ethylene biosynthesis pathway was discovered by Shang Fa Yang and his group in the 1980s. Then, the first ethylene receptors have been discovered by Bleecker and co-workers in Arabidopsis the 1990s. These breakthrough discoveries have since brought to the revelations of other components and pathways in ethylene biosynthesis and ethylene signalling.
Ethylene biosynthesis comprises three major steps: conversion of methionine to S-adenosyl-L-methionine (SAM); formation of 1 aminocyclopropane-1-carboxylic acid (ACC) from SAM and production of ethylene to ACC (Yang and Hoffman, 1984).
Figure 1: Biosynthesis pathway and regulation of ethylene
The first step, conversion of methionine to SAM is catalysed by SAM synthetase. For every molecule of SAM synthesised, one molecule of ATP is utilised (Yang and Hoffman, 1984; Wang et al, 2007). Next, ACC is generated from SAM via the activity of ACC synthase (ACS). This step is the rate-limiting step of ethylene synthesis as it depends on the amount ACC synthase available. In pre-climacteric fruits, ACC content is usually very low and a substantial increase only occurs at the time when vigorous ethylene production commences (Yang & Hoffman, 1984). Formation of ACC also produces the by-product 5'-methylthioadenosine (MTA). The 5'methyl group of MTA is recycled through the Yang cycle to produce another molecule of methionine (Flores et al, 2006; Wang et al, 2007). Recycling of MTA ensures that a constant concentration of cellular methionine is maintained even during rapid synthesis of ethylene (Barry & Giovannoni, 2007; Wang et al, 2007). The final step, formation of ethylene from ACC is catalysed by ACC oxidase (ACO), is an oxygen-dependent process. The conversion of ACC to ethylene also generates carbon dioxide (CO2 ) and cyanide (HCN) (Chaves & Mello-Farias, 2006; Wang et al, 2007). Ethylene can also regulate its own production (auto-catalytic biosynthesis) through the induction of de novo synthesis of ACCS and ACCO (Wang et al, 2007).
Ethylene receptors are membrane-localised receptors related to the bacterial two-component regulators histidine kinases (Bleecker et al, 1998). In Arabidopsis, there are five ethylene receptors: ETR1, ETR2, ERS1, ERS2 and EIN4. Based on sequence similarity and proteins structures, these five receptors are divided in to two main subfamilies: ETR1-like subfamily and ETR2-like subfamily. ETR1-like subfamily comprises the receptors ETR1 and ERS 1 while ETR2-like subfamily comprises the receptors ETR2, ERS2 and EIN4 (Bleecker et al, 1998; Kevany et al, 2007). In tomatoes, six ethylene receptors have been discovered: LeETR1, LeETR2, LeETR4, LeETR5, LeETR6, and NR. LeETR1, LeETR2, and NR have similar structures to ETR1-like subfamily receptors while LeETR4, LeETR5 and LeETR6 can be classified as ETR2-like subfamily receptors (Barry & Giovannoni, 2007; Kevany et al, 2007).
ETR1-like subfamily receptors are conserved histidine kinases and they contain three hydrophobic subdomians at the N terminus. ETR2-like subfamily receptors have degenerate histidine kinase and instead possess serine kinase domain and possess four hydrophobic subdomains at the N-terminus (Bleecker et al, 1998; Barry & Giovannoni, 2007; Kevany et al, 2007; Wang et al, 2007). All elements necessary and for high affinity ethylene binding are located in the N-terminal sensor domain (Bleecker et al, 1998). The ethylene binding site also possesses a Cu2+ ion, a copper co-factor is essential for the binding of ethylene (Chaves & Mello-Farias, 2006; Wang et al, 2007). In Arabidopsis, ERS1 and ERS2 lacks the receiver domains at their C termini. Apart from NR, all tomato receptors have a receiver domain (Barry & Giovannoni, 2007; Kevany et al, 2007; Wang et al, 2007).
Ethylene transduction pathway
Apart from the Cu2+ containing receptors, the ethylene signalling pathway involves a series of positive and negative regulators- CTR1, EIN2, EIN3; and also the regulation of transcription factors (e.g. ethylene response factors, ERFs) (Chaves & Mello-Farias, 2006).
CTR1 is a protein similar RAF serine/threonine MAP kinase kinase kinase (MAP3K) in mammaliam system. CTR1 works downstream of the ethylene receptors and functions as a negative regulator in the ethylene response pathway (Barry & Giovannoni, 2007; Bleecker et al, 1998; Wang et al, 2002; Kieber et al, 1993) EIN2, which works downstream of CTR1, is membrane transporter-like protein and serves as a positive regulator in the ethylene signalling pathway (Wang et al, 2007). Downstream from EIN2, located in the nucleus is the EIN3 family of transcription factors (Wang et al, 2007) EIN3 also acts as positive regulators of ethylene response (Barry& Giovannoni, 2007).
In the absence of an ethylene signal, ethylene receptors bind to CTR1 and activate it. CTR1 negatively regulates ethylene response pathway through a MAP-kinase cascade. (Bleecker et al, 1998) When ethylene is present, it binds to receptors. Ethylene binding inactivates the receptors and leads to the deactivation of CTR1. Suppression of CTR1 on EIN2 is lifted and EIN2 is allowed to send signals downstream to the EIN3. EIN3 binds to the promoter of ERF1 gene and activates its transcription. Transcription factors ERF1 then interacts with the GCC box in the promoter of target genes to induce downstream ethylene responses (such as the ripening response) (Chaves & Mello-Farias, 2006; Wang et al, 2007).
Modified from Alexander & Grierson, (2002).
It was proposed that ethylene degrades the ethylene receptors (Kevany et al, 2007). Ethylene binding probably results in conformational change in the receptors and this modification renders them susceptible to degradation through the action of the 26S proteasome. (Kevany et al, 2007).
Fruits are categorised into climacteric and non-climacteric fruits. Climacteric fruits are characterised by their elevated production of ethylene production at the onset of ripening while non-climacteric fruits maintains relatively constant levels of ethylene throughout their developmental process (Yang & Hoffman, 1984; Barry & Giovannoni, 2007; Kevany et al, 2007).
Ethylene plays an important role especially in the ripening process of climacteric fruits (Yang & Hoffman, 1984). During fruit maturation process, depletion of ethylene receptors leads to gradually increasing hormone sensitivity. Ripening commences when the depletion of ethylene reached a particular threshold (Kevany et al, 2007).
During fruit and vegetable maturation, several structural and biochemical changes occur (Chaves & Mello-Farias, 2006). The commencement of ripening in fruits and vegetables is usually coupled with colour changes, increased sugar metabolism, loss of firmness and changes in texture, production of aroma volatiles and increased vulnerability to attack by pathogen (Barry & Giovannoni, 2007).
Gene regulation during ripening
During ripening, the expression of many genes are induced or upregulated (Alexander & Grierson, 2002). However, it must be noted that both ethylene-independent and ethylene dependent ripening pathways and gene expressions are induced (Pech et al, 2008). Fruit colour change, sugars accumulation and loss of acidity are generally ethylene independent processes. Meanwhile, fruit softening, development of abscission zone and aroma formation are usually totally or partially ethylene-dependent processes (Pech et al, 2008). This is still a gray-shaded area as certain ripening events maybe be ethylene-independent or ethylene dependent in different plant species.
Ethylene and cell wall softening
Many genes encoding for proteins involved in fruit softening are induced by ethylene (total or partial ethylene-dependent) (Pech et al, 2008). The various genes includes those encoding polygalacturonases, pectin methylesterase, xyloglucan endotransglycosylase/hydrolases, B-galactosidaseses and expansin (Pech et al, 2008)
Softening of the fruit and changes in texture is mainly due to the partial disassembly of fruit cell wall during ripening. Ethylene induced pectin methylesterase (PME) de-esterify the highly methyl-esterified polygalacturonas in the cell wall. It was found that esterification of polygalacturonas in mature green fruits decreases from 90% to 35% as the fruit progresses into red ripe stage. This de-esterification of cell walls makes them susceptible to degradation by PG. (Alexander & Grierson, 2002). Meanwhile, expansins disrupts the hydrogen bonds between cellulose microfibrils and matrix polysaccharides, in effect loosening the cell wall (Alexander & Grierson, 2002).
Apart from the increase expression of certain enzyme activities (e.g. those mentioned in the above paragraph), certain enzymes (e.g. citrate synthase and malate dehydrogenase) decreases in response to ethylene (Chaves & Mello-Farias, 2006). Table 3 shows some examples of ethylene induced changes in enzymatic activity induced by ethylene of different plant species.
Source: Chaves & Mello-Farias, 2006).
The effects of ethylene are economically important (Yang and Hoffman, 1984). Ethylene is the key factor in modulating fruit ripening. Proper ripening of fruits and vegetable cannot proceed in the absence of ethylene. However, ethylene also causes rapid ripening (in climacteric fruits) and this may lead to over-ripening of decay of products. Postharvest storage conditions, such as fast shipping, controlled temperature and atmosphere are needed to maintain products in good condition. But all these additional labour and energy uses increase the cost of production (Barry& Giovannoni, 2007). Postharvest and biotechnology can create solutions to his problems. For example, several compounds can be used commercially to inhibit ethylene effects (1-Methylcyclopropne, 1-MCP; chitosan). Production of antisense tomatoes through biotechnology is also another success in delaying fruit ripening. It must also be noted that both ethylene dependent and ethylene independent responses co-exist in climacteric fruits. Therefore, more research has to be done in this area to acquire more knowledge on the gene regulation of ethylene before further innovation of plant biotechnology can proceed.