Ethylene

Nature of Ethylene Ethylene, unlike the rest of the plant hormone compounds is a gaseous hormone. Like abscisic acid, it is the only member of its class. Of all the known plant growth substance, ethylene has the simplest structure. It is produced in all higher plants It is usually associated with fruit ripening and the triple response

Ethylene gas with ripening process The ancient Egyptians : gas stimulate ripening. lemons are often too green to be acceptable in the market  lemon growers used to store newly-harvested lemons in sheds kept warm with kerosene stoves  with a more modern heating system, the no turned yellow on time. the important factor in the ripening process was not heat but the small amount of ethylene gas given off by the burning kerosene.

leaf abscission to the presence of illumination gas (Doubt, 1917; Fahnestock, 1858) In 1864, leaks of gas from street lights showed the triple response : stunting of growth, twisting of plants, and abnormal thickening of stems  inhibition of vegetative tissues (Arteca, 1996; Salisbury and Ross, 1992). illumination gas was responsible for the horizontal growth of etiolated pea seedlings the typical symptoms of ethylene action : inhibition of stem and root growth, leaf abscission, horizontal growth and plant senescence

Figure 1. the ethylene-insensitive mutants etr1-1 and ein2, and the constitutive ethylene-response mutant ctr1-2. Figure 2. The Triple-Response to Ethylene of Dark-Grown Arabidopsis Seedlings. (A) Wild-type seedlings grown in the absence (left) or presence (right) of ethylene. (B) Wild-type seedling grown in the presence of the ethylene precursor ACC. (C) Close-up of the pronounced apical hook found with the triple response to ethylene. (D) Close-up of the shortened root found with the triple response to ethylene.

History of Discovery in Plants the active component was ethylene (Dimitry Neljubow, 1901) plants synthesize ethylene  an endogenous growth regulator. this gas was ethylene was the plant hormone responsible for fruit ripening (Crocker, 1935) plant tissues (ripening apples) naturally produce ethylene (Gane, 1934)

History of Ethylene Research increase in ethylene production was associated with peak in respiration during tomato ripening (Zegzouti, 1997). ethylene is a fruit-ripening agent that acts in very small amounts this endogenous growth regulator as a plant hormone in amounts that can reach 500 nL g-1 h-1 but is active at very low concentrations from 10 to 100 nL L-1 (Abeles et al., 1992) Ethylene is now known to have many other functions.

Biosynthesis and metabolism Ethylene is produced in higher plants (varies with the type of tissue, the plant species, and also the stage of development) More ethylen in meristematic tissues, nodal region, dormant buds, flowers senescence, maturity and ripening fruit Precursor : methionine ethylene is synthesized in response to different type of stress, such as wounding, very low and very high temperatures, flooding or drought, treatments with other hormones, heavy metals and attack by pathogens (Pech et al., 1992).

The ethylene biosynthesis pathway ATP is an essential component in the synthesis of ethylene from methionine. ATP and water are added to methionine resulting in loss of the three phosphates and S-adenosyl methionine (SAM = AdoMet). 1-amino-cyclopropane-1-carboxylic acid synthase (ACC-synthase) facilitates the production of ACC from SAM. Oxygen is then needed in order re oxidize ACC and produce ethylene. This reaction is catalyzed by an oxidative enzyme called ethylene forming enzyme. intermediet prekursor

Figure 3. Ethylene Biosynthetic Pathway Figure 3. Ethylene Biosynthetic Pathway. The enzymes catalyzing each step are shown above the arrows. AdoMet: S-adenyl-methionine; Met: methionine; ACC: 1-aminocyclopropane-1-carboxylic acid; MTA: methylthioadenine.

Functions of Ethylene Stimulates the release of dormancy. Stimulates shoot and root growth and differentiation (triple response) May have a role in adventitious root formation. Stimulates leaf and fruit abscission. Stimulates flower induction. Induction of femaleness in dioecious flowers. Stimulates flower opening. Stimulates flower and leaf senescence. Stimulates fruit ripening.

Ethylene and fruit ripening Fruit ripening involves a series of biochemical and structural changes that make the fruit acceptable for eating. Two major groups of fruits based on the intervention of ethylene during maturation : Non-climacteric fruits Climacteric fruits

fruits Non-climacteric Climacteric are those whose maturation does not depend on ethylene, such as cherry, strawberry and pineapple are characterized by an extraordinary increment in ethylene production which accompanies the respiratory peak during ripening, called the 'climacteric crisis' (Abeles et al., 1992) The sharp increase of ethylene production which occurs at the initiation at the onset of the climacteric phase is the key phenomenon controlling the initiation of changes in color, aromas, texture and flavor tomato, avocado, melon, apple, pear, peach and kiwifruit

Climacteric and non-climacteric fruits Bell pepper Olives Apple Melons Blackberries Orange Apricot Nectarine Blueberries Pineapple Avocado Papaya Cacao Pomegranate Banana Passionfruit Cashew apple Pumpkin Breadfruit Peach Cherry Raspberries Cherimoya Pear Cucumber Strawberries Feijoa Persimmon Eggplant Summer squash Fig Plantain Grape Tart cherries Guanábana Plum Lemon Tree tomato Jackfruit Sapodilla Lime Kiwifruit Sapote Lychee Mango Watermelon Grapefruit Guava Quince Source: Wills, et al., 1982; Kader, 1985

Fruit Ripening Caused by : Ehtylene effect Increase in membrane permeability which releases compartmentalized enzymes Increase in protein (enzyme) synthesis Low temperature in high CO2 can reduce ethylene effects Conversion of starch to sugars via hydrolysis Synthesis of pigments and biochemical involved in flavor Cell wall degradation which lead to softening

Several structural and biochemical changes during fruit maturation organoleptic qualities, such as modifications in the external aspect, texture and flavor of the fruit (Seymour et al., 1993). the change in the color of tomato fruits results from (Gray et al., 1992) : - transformation of chloroplasts into chromoplasts - the degradation of chlorophyll, - the accumulation of pigments such as carotenes and lycopenes, which are responsible for the orange and red color of the fruit alterations in the texture of the fruit, the loss of firmness, due to structural changes in the principal cell wall components (cellulose, hemicellulose and pectin). Finally, the accumulation of sugars such as glucose and fructose and organic acids in vacuoles and the production of complex volatile compounds is responsible for the aroma and flavor of the fruit (Seymour et al., 1993)

Most physical and biochemical changes are associated with : activity of enzymes such as invertase (Iki et al., 1978) and polygalacturonase (Tucker and Grierson, 1982), which increase during the ripening of tomato fruits, or citrate synthase and malate dehydrogenase (Jefferey et al., 1984) which decreases considerably during ripening.

Leaf Epinasty Results When ACC from the Root Is Transported to the Shoot In tomato and other dicots, flooding (waterlogging) or anaerobic conditions around the roots enhances the synthesis of ethylene in the shoot, leading to the epinastic response. Ethylene and high concentrations of auxin induce epinasty, auxin acts indirectly by inducing ethylene production.

Ethylene Biosynthesis in the Abscission Zone Is Regulated by Auxin The shedding of leaves, fruits, flowers, and other plant organs is termed abscission. Abscission place in abscission layers, which become morphologically and biochemically differentiated during organ development.

Weakening of the cell walls at the abscission layer depends on cell wall–degrading enzymes such as cellulase and polygalacturonase Ethylene appears to be the primary regulator of the abscission process, with auxin acting as a suppressor of the ethylene effect

A model of the hormonal control of leaf abscission describes the process in three distinct sequential phases