In olden days, villagers, even now, used to accelerate the ripening process of banana, mango and other fruits, just before they were taken to market places. The method employed by them was simple. They used to keep the raw and unripened fruits in tightly closed earthern pots and fill the pot with smoke generated by burning cow dung at the base and then seal it. After 12 to 24 hours of this treatment, the fruits would appear yellowish and just started for full ripening and they were ready for marketing. Even today, villagers use this method without knowing why and how ripening is accelerated by the smoke generated by burning the cow dung. But plant physiologists discovered ethylene as the gas that induces and augments fruit ripening. This phenomenon and technology of fruit ripening was known to our village farmers many many centuries back. Even now renowned plant physiologists don't know this. All of us know cow dung produces atmospheric polluting gases such as methane and ethylene. For that matter animal feca and fruit release more pullutants than all the cars (put together) that emit pullutants. Though scientific studies were initiated as early as 1900s, understanding the process of fruit ripening and identification of the causative factor was possible only in 1924. Since then, detailed studies have been made on ethylene and its effect on plants.
Many plant physiologists call ethylene as a plant hormone in gaseous state. But some do not agree with this view instead, they consider ethylene as a byproduct of reactions induced by other phytohormones and not an hormone perse. Two critical enzymes involved are SAM and ACC synthase. Both gene can be used for antisense technology for prolonging commercial fruit ripening, which is help full for farmers for their products will have longer shelf life and this protectiveness ha sno deleterious effect on eaters.
Ethylene is produced in response to exogenous stimuli as shown in the above diagram. In any of the plant developmental processes plant hormones interact with one another.
Ethylene is found in almost all parts of the plant body. But it is found in greater amounts particularly in old and yellowing leaves and ripening fruits. This compound being a gaseous substance diffuses through the intercellular spaces easily and rapidly reaches different regions of the plant body.
Ethylene consists of two CH2 groups held by a common double bond H2C=CH2. The synthesis of ethylene is greatly enhanced by higher concentrations of auxin. Even Gibberellins and cytokinins induce the synthesis of ethylene indirectly. The precursor for ethylene was once believed to be methionine.
But recent investigations, using radioactive isotopes have shown that the precursor for ethylene is 1-amino cyclopropane carboxylic acid and not methionine. ACC acts as the direct and immediate precursor. In fact, higher concentration of auxin induces the synthesis of a group of ethylene synthetase enzymes. These enzymes require FMN, H2O and Cu2+ as the cofactors for their activity. The auxin induced enzymatic activity can be inhibited by actinomycin D and CHI, which suggests that ethylene synthases are inducible enzymes. The site of synthesis of ethylene has been suspected to be chloroplasts and its release is believed to be regulated by phytochromes.
Apart from Auxins, factors like wounding, aging, irritation, light, cold temperature and drought can also induce ethylene synthesis. Most of the above mentioned are stress factors even; ABA is known to induce ethylene
Abscission: Onset of winter, cold treatment, drought and such conditions induce the formation of abscission layer in the stalks of leaves, flowers and fruits. Eventually the said structure separates by death of cells from the plant body and withers, it is also called in Greek language as Apoptosis. The structure and the development of abscission layer has been explained in the chapter ‘Auxin’. To put it in a nut shell, ethylene induces differential gene expression in the region, where the abscission zone develops. As a result, pectinase and cellulase enzymes produced and the same act upon the cell wall and degrade the same. Thus the abscission layer becomes the weak point and the leaves, fruits, etc., fall down by their shear weight.
Fruit Ripening: Once the fruit reaches a particular stage of development, the raw, hard, green colored fruits undergo transformation to produce matured, soft, sweeter and yellow/red/orange/coloured fruits. The repining process requires period of time ranging from 24 hours to a week or so. And ethylene is known to initiate this phenomenon. Once the ripening is on, there is way to stop it.
With the maturity of fruits, the synthesis of ethylene is induced and ethylene induces more ethylene production. During early part of ripening process, ethylene initiates a cascade of events, which follow one after another and end up in a crescendo of biochemical reactions or what is called climacteric state at which all the biochemical reactions are at their maximum efficiency.
Ethylene to be effective in its action requires a copper containing metallo protein. Strangely, CO2 is known to bind to the same site of the protein at which ethylene binds and thus it competitively inhibits ethylene action. The active ethylene in its complex form first activates respiratory process by which reserve food materials and organic acids if found, are subjected to oxidative and decarboxylation reactions. The increase in respiratory activity is unusually cyanide insensitive, which means that the electron transport chain used in this process appears to be different from usual mitochondrial electron transport chain. Studies in this regard have revealed that the electron transport chain in this process branches of from Cyt.C and bypasses the cyt.a3 oxidase enzyme which is actually the site of cyanide inhibition. Most of the respiratory and other metabolic pathways that are stimulated by ethylene lead to the formation of more and more of sucrose and organic acids.
As the respiratory activity is reaching its climax ethylene simultaneously affects the membrane permeability and also activates a set of genes resulting in the synthesis of specific mRNAs. On translation of these mRNAs, specific proteins such as pectinase and cellulases are produced. The enzymes then act on the middle wall and primary walls to loosen up the cells, thus render the hard fruit into soft fruit. Ethylene induced fruit ripening can be effectively inhibited by actinomycin and CHI, which suggests that ethylene induces differential gene expression.
Ethylene also affects the membrane stability and permeability. As a result, the pigments found in the tonoplast leak out and most of the membrane structures get disturbed. Furthermore, ethylene induces the degradation of chlorophyll by chlorophyllase which is again a product of gene expression. Simultaneously some anthocyanins are also synthesized which develop attractive coloration to the skin and the flesh of the fruit. Thus ethylene ultimately makes the fruit into a softer, sweeter and colorful commodity.
Effect on apical dominance;
Plants which show conical growth, ex., conifers, is known to have a strong apical dominance effect on the lateral buds. This has been attributed to strong influence of IAA present in the apical meristems found in the main axis. Recent investigations, it has been found that IAA induced apical dominance is more due to ethylene production than to auxin itself. Apical meristems of the main axis synthesize auxin and the same is translocated downwards. At the same time, some amount of IAA produced in very young leaves is also translocated towards the stem and more of auxin gets accumulated in the nodal regions. As higher concentration of IAA stimulates the synthesis of ethylene, which on synthesis, diffuses into lateral buds inhibits the growth. So the apical dominance is actually enforced by IAA through its second messenger i.e. ethylene. But the apical dominance can be overcome by the application of cytokinin, which removes the mitotic block imposed by ethylene and activates cell division, so the lateral buds grow into branches.
Effect on Geotropic Movements:
Geotropic responses are explained as due to the sensitivity of stem tip and root tip to different concentrations of auxins. But ethylene, a product induced by higher concentration of auxin brings about a reverse of geotropic curvatures called ageotropic effects, where roots instead of growing downwards into the soil curl upwards. In fact, ethylene treated roots loose their sense of directional growth. Sometimes, the effect of ethylene will be similar to the effects of morphactins. In addition, the other effects induced by ethylene, such as stunting, stem enlargement and prostrate habit by an impaired response to gravity are called ‘Triple response’ to ethylene.
Ethereal: In recent years, ethylene derivatives are sold in the market as ethereal or ethephon, a patented product. This compound is nothing but 2 chloro ethyl phosphonic acid. When these compounds are applied to plants in solution form, they release ethylene which in turn brings about its effects. Application value of this compound in agriculture, pomiculture is very well exploited during harvesting cotton balls and other fruit products. Application of ethereal induces not only ripening in most of the fruits irrespective of the age and degree of ripening, it also induces the abscission layer formation uniformly in stalks, which greatly facilitates harvesting either by mechanical means or manually at any given time.
Ethylene signal transduction pathways:
Ethylene though exists in gaseous form, it diffuses across cells, but when it enters a cell it binds to specific receptor. The signal transduction process are more like RTK but for they have histidine kinase activity. Very often in some cases it looks like an RTK pathway. The ultimate effects are different dpending upon the organ on which it works and the time at which it works. Belwo only the self explanatory diagrams are give for your reference and looking for more information.