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39.5.4: Ethylene

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    Learning Objectives
    • Relate the chemical structure of ethylene to its mode of transport.
    • Identify the locations of synthesis and actions of ethylene.
    • Describe the commercial applications of ethylene.

    Ethylene differs from other plant hormones in that it is a smaller and simpler molecule that is a volatile gas (Figure \(\PageIndex{1}\)). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps. Aging tissues, such as those that are wilting or ripening, and nodes of stems produce ethylene.

    Ethylene consists of two carbon atoms connected by a double bound and four hydrogen atoms.
    Figure \(\PageIndex{1}\): The chemical structure of ethylene. Image by Benjah-bmm27 (public domain).

    Actions of Ethylene

    Ethylene has many functions, and its major functions are associated with senescence, or aging. This includes fruit ripening, flower wilting, and leaf and fruit abscission. Ethylene also promotes germination in some cereals and sprouting of bulbs and potatoes. It is responsible for drooping of leaves and sprouting of potato buds. In monoecious plants, ethylene promotes the production of female flowers where as gibberellic acid promotes male flower production. Ethylene mediates the triple response, which makes the shoots of seedlings that are buried under debris grow short and wide as well as bend horizontally. This makes it possible for the shoot to push through the debris. Ethylene causes stem elongation in rice and other plants that are submerged in water. It promotes the breakdown of abscisic acid (ABA) and thus relieves ABA's inhibition of gibberellic acid.

    Fruit Ripening

    As they approach maturity, many fruits (e.g., apples, oranges, avocados) release ethylene. During fruit ripening, ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocados, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit.

    Abscission

    Ethylene induces the abscission of leaves, fruits, and flower petals. When auxin levels decline, ethylene triggers senescence and ultimately programmed cell death at the site of leaf attachment to the stem. A special layer of cells — the abscission layer (abscission zone) — forms at the base of the petiole or fruit stalk (Figure \(\PageIndex{2}\)). In petioles of some plants, there are two parts of the abscission layer: the more distal separation layer and more proximal protective layer. Before abscission occurs, nutrients are absorbed into the stem so that they are not lost with the leaf. As the separation layer breaks down, the leaf breaks free at this point and leaf falls to the ground in a controlled manner without harming the rest of the plant. The protective layer, which was reinforced with suberin, serves as a seal.

    Leaf abscission is particularly important for temperate deciduous trees in the autumn. This is a vital response to the onset of winter when ground water is frozen - and thus cannot support transpiration - and snow load would threaten to break any branches still in leaf.

    Longitudinal section of a stem and petiole, illustrating the abscission layer.
    Figure \(\PageIndex{2}\): The abscission layer forms at the base of a petiole (as shown) or fruit stalk, ultimately allowing leaves or fruits to fall from the stem. Vascular tissue passes through the center of the stem and petiole. The abscission layer is a dark band of cells at the base of the petiole.

    In drought conditions, the immediate response is closing stomata (see Abscisic Acid). However, because closed stomata prevent gas exchange, plants will die if the stomata remain closed for too long. Thus if a drought persists for too long, the plant will begin sacrificing certain areas by allowing the leaves or stems to die in localized regions. This process may be regulated by ethylene, which can induce localized cell death under certain conditions.

    Mechanism of Ethylene Action

    At a cellular level, ethylene can inhibit or promote cell division. It sometimes inhibits cell expansion. In other circumstances, it stimulates lateral cell expansion. The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of cells. Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.

    Commercial Applications of Ethylene

    Ethylene is widely used in agriculture. Commercial fruit growers can buy equipment to generate ethylene so that their harvest ripens quickly and uniformly. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.

    Attributions

    Curated and authored by Melissa Ha from the following sources:

    This page titled 39.5.4: Ethylene is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .