Mechanism of Action of Ethylene
Mechanism of Action of Ethylene
Involved of a receptor-
Like most other plant and animal hormones, the action of ethylene is initiated by its binding to a specific receptor site in the target cell. The existence of a receptor site has been demonstrated in various tissues and organs of a variety of plant species. The site is a membrane bound protein perhaps an enzyme containing Cu atom. The ethylene binds to the Cu. The same receptor also recognises CO and CO₂. Carbon mono-oxide causes some of the same physiological effects as ethylene. But CO2 is competitive inhibitor of ethylene action.
Increased membrane permeability-
Ethylene is very soluble in lipids and thus could easily penetrate into cell membranes. Exogenously supplied ethylene has been shown to increase permeability in a variety of plants. In barley aleurone layers, it accelerates the secretion of a-amylase, but not its production. The hormone also causes the swelling of isolated mitochondria, which is taken as an indication of increased permeability. The increase permeability in senescing petals of Tradescantia involves increased RNA and protein synthesis. It has been suggested that ethylene induces phospholipases which degrade phospholipids and cause leakiness of the membrane.
Activation of gene expression-
It is believed that at least some responses of ethylene such as fruit ripening and abscission are due to the activation of gene expression and increased protein synthesis. The nucleic acid and protein synthesis often increase during increased ethylene production in ripening of fruits. Several enzymes such as polygalacturonases, a cell wall softening enzyme, increase due to their activated synthesis. Other enzymes such as phenylalanine ammonia lyase, cellulase, polyphenol oxidase, peroxidase etc. also increase in respone to ethylene. The increase in phenylalanine ammonia lyase is clearly due to its de novo synthesis. This enzyme catalyses the deamination of 1-phenylalanine to transcinnamic acid, which can then be incorporated into numerous phenolic compounds including anthocyanidins. The anthocyanidins impart bright colour to the fruits and their increased production is an important aspect of fruit ripening. Increased cellulase and some related enzyme activities may be the factor involved in softening of the fruits during ripening.
Involvement of auxins-
Auxins induce ethylene production and many responses of ethylene are similar to that of auxins. These include epinasty, inhibition of stem, root and leaf elongation, flower induction in pine apple and mangoes, inhibition of epicotyl or hypocotyl, hook opening in dicot seedlings etc. It is likely that mechanism of action of both hormones is either the same or similar.
All higher plants produce cthylene. By feeding labelled metabolites it has been shown that the primary precursor of ethylene is the amino acid, methionine, Thrid and fourth carbon (C-3 and C-4) of methionine is converted to ethylene.Other eventual products of methionine metabolism are CO₂. formic acid, ammonia, and methyl S-adenosine or some other methyl compound. Both the ammonia and methyl thio group can be metabolised back into methionine thereby conserving nitrogen and sulfur and allowing ethylene synthesis to continue.
In the first step, methionine is activated with ATP and is converted to S-adenosyl methionine (SAM) and pyrophosphate is liberated. This reaction is catalysed by the enzyme SAM synthetase (ATP-methionine S-adenosyl transferase). In SAM, the sulfur atom if methionine is linked to the C-5 of the ribose moiety in adenosine. In many cases, solenomethionine has been found to be better substrate for ethylene biosynthesis than methionine. The enzyme SAM synthetase has a higher affinity for solenomethionine that for methionine.
The next step in the biosynthesis of ethylene is the conversion of SAM to 1-amino cyclopropane carboxylic acid (ACC), which is catalysed by the enzyme ACC synthase. This step is believed to be the rate limiting step in ethylene production, and any factor influencing the activity of ACC synthase influences ethylene production acordingly. Auxins, stress factors such as wounding, chilling, drought or pathogen which induce ethylene production, increase ACC synthase activity. In water logged condition also, more ethylene is produced apparently due to activation of ACC synthase. The chemicals, aminoethoxy vinyl glycine (AVG) and amino oxyacetic acid inhibit ethylene production by inhibiting ACC synthase activity.
ACC synthase was first isolated from tomato pericarp tissues. Since then, it has been isolated, purified and characterised from several tissues. It requires pyridoxal phosphate as a cofactor and SAM as a substrate. Its KM for SAM is 13 to 20 µM. The enzyme is present in low amounts in immature tissues. However, its activity and consequently the ethylene production increases during senescence jand ripening of the fruits.
The final step in the biosynthesis of ethylene is oxidative cleavage of ACC to form ethylene, CO2 and HCN by the enzyme “ethylene forming enzyme”, (EFE), which is also called as ACC oxidase. HCN is ultimately converted to formic acid and ammonia. This reaction requires O2 and is activated by light. The enzyme is induced by ACC in cranation flowers. During senescence also, it increases. The enzyme is thought to be located in membranes, including plasma membranes and tonoplast. The enzyme is thought to be located in membranes, including plasma membrane and tonoplast. The ACC oxidase has been isolated from melon fruits by P. Ververidis and P. John (1991). The enzyme requires Fe²+ and ascorbate for its activity, as well as ACC and O, as substrates. The apparent kM for ACC is 175 µM.The enzyme activity is inhibited by sulfhydryl inhibitors such as CO²+, Cu²+ and Zn. Wounding induces a rapid increase in the activities of ACC synthase and ACC oxidase with a resultant increase in ethylene production. Regarding the site of ethylene biosynthesis, the shoot apex appears
to be the principal site in seedlings. Nodes of dicot seedlings produce more ethylene than intermodes. Roots produce relatively small amounts of ethylene. Leaves, flowers and fruits also produce ethylene, which increases during senescence and ripening.
Plants and micro-organisms are known to metabolise ethylene as well. The product of metabolism appears to be ethylene oxide, which is incorporated into the glyoxylate cycle as ethylene glycol and malonyl ACC. Ethylene may also form a conjugate with glucose.
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