How is ethylene gas produced


reworked by Margret SAUTER - Hamburg

 

Ethylene is an unusual, because it is gaseous, effector with a very simple structure. But ethylene has a number of properties that are quite comparable to those of other hormones. In many cases, ethylene is already effective in nanomolar concentrations.

BLEECKER A.B. and KENDE H (2000) Ethylene: a gaseous signal molecule in plants. Annu. Rev. Cell Dev. Biol. 16: 1-18 (2000) http://cellbio.AnnualReviews.org/cgi/content/full/16/1/1

biosynthesis

In higher plants, ethylene is produced from the amino acid L-methionine. Methionine is activated by the activity of S-adenosylmethionine synthetase (EC 2.5.1.6) with ATP to form S-adenosylmethionine. Two specific steps are involved in ethylene biosynthesis. The cyclic amino acid 1-aminocyclopropane-1-carboxylic acid (ACC), which does not occur in proteins, is formed from S-adenosylmethionine. The reaction is catalyzed by ACC synthase (EC 4.4.1.14), with pyridoxal phosphate acting as a cofactor for this conversion. The formation of ACC is usually a limiting factor for ethylene biosynthesis. The ACC synthase is encoded in plants by a multigene family. Various signals that influence the formation of ethylene lead to an increased expression of individual members of the ACC synthase gene family. The ACC oxidase converts ACC to ethylene. The reaction is dependent on oxygen. Ethylene cannot therefore be synthesized with complete exclusion of oxygen. Fe2+ is a cofactor and ascorbate is a cosubstrate of this reaction. CO2 activates the enzyme. ACC oxidases are also encoded by small multigene families in plants.

The concentration of ethylene depends on the rate of synthesis and on the diffusion of the gas. Ethylene itself is not actively transported and there is no degradation of ethylene. The increase in ethylene synthesis through various signals, such as auxin or wounding, takes place via activation of the ACC synthase and is based on increased gene expression. ACC oxidase is constitutively present in most vegetative tissues. In some cases, however, the ACC oxidase is also formed to a greater extent under the influence of ethylene (positive feeback Regulation).

A stimulation of ethylene production by auxin has been demonstrated in various tissues. In etiolated pea seedlings (Pisum sativum L.) these two hormones are involved in the formation of the apical hypocotyl hook. It is assumed that an uneven distribution of auxin leads to ethylene synthesis being stimulated on the side of the high auxin concentration. The resulting high ethylene concentration leads to unilateral growth inhibition, which leads to a curvature.

In ripening fruits there is an autocatalytic feedback of the ethylene synthesis. This means that ethylene stimulates its own biosynthesis. The self-reinforcing synthesis and diffusion of ethylene ensure synchronization and acceleration of the ripening process.

distribution

Presumably, every plant tissue forms or takes up ethylene during one of its developmental stages and reacts accordingly. The absorption is usually unproblematic, because ethylene can diffuse through membranes without difficulty. The distribution usually takes place via the gas space of the plant tissue (intercellular) and - in dissolved form - from cell to cell. For transport over a longer distance, ACC, the precursor of ethylene, is transported to the site of action via the conductive tissue in the plant, where it is converted into ethylene. For example, if the tomato is waterlogged, it forms ACC in its roots, which is transported to the leaves and converted into ethylene there. There, ethylene causes the epinastic lowering of the petioles.

Ethylene - Perception and Signal Transduction

Many of the effects of ethylene are linked to gene activation. The question is how ethylene is perceived by the cell and how these genes are activated. Much recent research suggests that ethylene is bound by a receptor that exists as a transmembrane protein. The N-terminal domain of the receptor protein is involved in the binding of ethylene. An intracellular domain consists of a protein kinase that is activated when ethylene binds to the N-terminal region. Such a receptor protein kinase is called a two-component regulator. Two-component regulators generally consist of a sensor and a `` response regulator ''. They are also found in bacteria. The first two-component regulator identified from plants was the ethylene receptor ETR1 (ethylene resistant 1) from thale cress (Arabidopsis thaliana L.). The signal transduction then proceeds very likely via a cascade of phosphorylation steps by several protein kinases and possibly via further intermediate steps. At the end of the signal chain there is a transcription factor that is present in the cell and activated. This promotes the transcription of one or more early-induced genes. A known early gene itself codes for a transcription factor that leads to the activation of late-induced genes. The late genes can code, for example, for hydrolytic enzymes that break down the cell wall during fruit ripening or abscission, or for proteins used in pathogen defense (see Effects).

Inhibitors

Inhibitors are often used to study the biosynthesis and effects of ethylene in plants. AVG (Aminoethoxyvinylglycine) and AOA (Aminooxy-acetic acid) are inhibitors of ethylene synthesis. NBD (2,5-norbornadiene) and Ag+ however, inhibit the effect of ethylene by binding to the ethylene receptor. NBD is much more specific than silver ions.

Effects

Already in the first half of the last century it was observed that ethylene promotes the abscission of fruits and the ripening of the fruit. It has been known since 1934 that it is also produced by the plants themselves. Many climacteric fruits such as apples, bananas and tomatoes show a steep increase in ethylene synthesis in the late green stage. As a result, chlorophylls are broken down and other pigments are built up. The typical color of the pericarp arises. The activity of several enzymes required for the ripening process increases. Starch and organic acids, in some cases (e.g. in the case of the avocado fruit) also oils, are metabolized to sugars. Pectins, which make up the main component of the middle lamella, are broken down. The fruits become soft. The conversions are associated with intensive cellular respiration and high oxygen consumption. The ethylene content in separating fabrics is particularly high, which ultimately results in the fruit being absent.

Other processes regulated by ethylene are the aging and fading of flowers, and the abscission of petals and normal leaves. The bloom formation is mostly inhibited by ethylene. In the pineapple family (Bromeliaceae) on the other hand, in contrast to all other plant groups, ethylene is used to promote flower formation.

Ethylene is also the central regulator of cell death programs in plants. If the oxygen supply is poor, cavities, so-called aerenchymes, are formed in the roots and sometimes in other plant organs. This can happen, for example, in damp soils with waterlogging. Lysogenic aerenchymes arise when cells in the cortex of the root undergo a programmed cell death that is triggered by ethers. The programmed cell death that endosperm cells undergo during seed development in cereals is also under ethylene control.

After all, a large number of biotic and abiotic stress factors lead to the formation of ethylene, whereby the physiological effect of stress ethylene can be very different. In some cases, ethylene can exacerbate the symptoms of the disease, weaken them in others, or have no effect.

Ethylene not only influences senescence processes in a broader sense, but also growth such as the asymmetrical formation of the hypocotyl hook of dicotyledons or the horizontal, agravitropic growth of the stem axis. In general, ethylene inhibits the growth of terrestrial plants.

In the case of semi-aquatic plants, on the other hand, a promotion of shoot growth is observed. H. KENDE (Michigan State University, USA; MÉTRAUX and KENDE, 1983; KENDE et al., 1998) chose rice (Oryza sativa L.) as a test object to study this phenomenon, since this cultivated plant in particular grows in deep water in many cultivation areas. Ethylene can only diffuse very slowly in water and therefore accumulates in flooded parts of plants. In addition to the enrichment of ethylene through slowed diffusion, there is an increased synthesis of ethylene. Both enzymes of ethylene synthesis, ACC synthase and ACC oxidase, are induced by the low oxygen partial pressure in the flooded tissue. The high ethylene concentration in turn causes an increase in the amount of gibberellin and an increased sensitivity of the internodial tissue to gibberellin. Gibberellin finally ensures that the shoot axis in the area of ​​the youngest internode begins to grow very quickly (up to 25 cm per day). The division and route growth are each increased three times compared to the control. Ethylene is therefore an intermediate link in the chain of effects that leads from the flood signal to the growth of the shoots.

Applications

Ethylene can accumulate in closed rooms. Ripening fruits release ethylene and thus stimulate the late-ripening fruits to ripen early ("One rotten apple can ruin the whole basketFor the storage of fruits it is desirable to prevent the formation or spread of ethylene. In the fruit industry, therefore, fruits are stored under negative pressure in order to remove released ethylene. Conversely, bananas are transported and stored in the unripe stage and with If required, gassed with ethylene in order to initiate synchronous ripening.

In order to control fruit ripeness and the bleaching of petals, biotechnological approaches have also been pursued. So far, two genetic engineering strategies have been used. On the one hand, attempts were made to inhibit the synthesis of ethylene in tomatoes. This was achieved through counter-strand (Antisense) Expression of ACC synthase or ACC oxidase is achieved. The depletion of the ACC pool through the expression of a bacterial ACC deaminase also led to a reduced formation of ethylene. Second, a mutated and therefore defective ethylene receptor was turned off Arabidopsis introduced into tomato and petunia. As a result, the ethylene signal could no longer be passed on in the plant, resulting in delayed fruit ripening and delayed bleaching of the petals, as well as delayed abscission of the flower.