Ethylene is a gaseous effector. Though it is rather unusual for the usual hormone concept does it have a number of properties that are quite comparable to that of other hormones. Transport occurs normally through the intracellular space as well as – in a solute state – from cell to cell or via the vascular bundles. In contrast to the other hormones is ethylene also a pheromone, i.e. a (usually) gaseous substance with effects on other individuals. A typical pheromone is the sexual attractant of insects.
As soon as the first half of this century was the stimulating effect of ethylene on fruit abscission and ripening observed, and since 1934 is it known that plants produce it themselves. Most likely produces or reacts every plant tissue to ethylene sometime during its development. Uptake seems to be uncomplicated since ethylene diffuses without problem through membranes.
Higher plants produce ethylene during light exposure from the amino acid L-methionine under use of flavenin adenine mononucleotide (FMN). For its biosynthesis and its effect is oxygen required. Ethylene production is in several tissues stimulated by auxine. The rate of ethylene production in the different parts of a pea shoot is directly proportional to the amount of auxine present. In shoots that are not exposed to light is more ethylene produced (in the subapical region) than in those parts exposed to light.
The concentration of ethylene and its effect is dependent on the rate of synthesis and on transport on one hand, on breakdown, decontamination reactions and diffusion constants on the other hand. Principally is one of these factors enough to balance the ethylene level. The reaction a tissue has on ethylene is usually proportional to the log of the ethylene concentration, and it is mostly reversible. In ripening fruits, for example, is not just the ripening process enhanced, but also the synthesis of ethylene stimulated. The quick spreading of the effect causes a synchronisation of the ripening process.
Among the experience gained by the usage and storage of fruits is that it is not useful to store ripening fruits (like apples) next to late-maturing ones since the pheromone effect is rather strong in closed rooms. Ripening fruits release ethylene and stimulate thus the late-ripening fruits to an premature ripening. The stimulation of the following physiological processes is also caused by ethylene:
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The ripening of fruits. At the beginning of the ripening period rises the concentration of ethylene. The activity of numerous enzymes required for ripening increases. Starch and organic acids, in some cases even oils (like in avocados) are metabolized to sugars. Pectins (of the middle lamina) are broken down, the cells disintegrate and the fruits become soft. Chlorophyll is broken down, and pigments are metabolized. The typical colour of the fruit peel develops. These processes go hand in hand with a high consumption of oxygen. The cells show an intense respiration. |
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In germinating seeds is the concentration of ethylene highest during growth. Here, too, is an increase of the metabolic rate stimulated. |
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During the senescence and abscission of leaves and fruits is the ethylene content especially high in the separating tissues. |
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In contrast to all other plant groups is the flower formation of Bromeliaceae stimulated by ethylene. Usually inhibits ethylene flower formation. Senescence is enhanced in fully differentiated flowers. |
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In addition stimulates ethylene the broadening of the lamina and the curving of leaves. It induces the horizontal growth of the shoot axis. Carbon dioxide offsets the effect of ethylene. Based on the reaction kinetics does it seem likely that both gases compete for the same binding site. |
The following observations seem to contradict the effects of ethylene mentioned up till now: It has been asked rather often in developmental physiology why the submersed parts of semi-aquatic plants develop different from those growing on air. H. KENDE (Michigan State University, 1987) chose rice (Oryza sativa) as his test object, since it grows in many places in deep water. Submerse growth (up to 25 cm/ day) is characterized by growth by division and an elongation that is three times as high as that of the control plants. It is the lower partial pressure of oxygen in water that causes an enhanced ethylene synthesis. The key enzyme 1-aminocyclopropan-1-carboxylate-(ACC)-synthethase is activated in the internodes (but not in leaves!). The high concentration of ethylene induces an increase of the amount of gibberellin (GA3) that is ‘in charge’ of the enhanced elongation. Ethylene does thus act as a link and several observations indicate that it does mainly increase the sensitivity of the internode tissue towards gibberellin.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de