Ads Below Headers

Ethylene: Production site, Biosynthesis and its factors

Production site: Ethylene is produced in bacteria, fungus, alga, moss, gymnosperms and higher plants. All parts of higher plants produce ethylene but production is higher in the tissues of meristematic, senescence, ripening and wounding regions. The climacteric fruits produce higher concentration of ethylene during their ripening whereas in non-climacteric fruit ethylene production is quite low. 
Biosynthesis: Ethylene is biosynthesized from sulfur containing essential amino acid methionine:

The ethylene biosynthesis process involves three steps:
In first step ATP (AdenosineTri-Phosphate) activates methionine and S-Adenosyl Methionine (SAM) is synthesized from methionine in the process SAM synthetase enzyme involves.
Under second step SAM cleaves into 1-amino cyclopropane-1- carboxylic acid (ACC) and 5-methyl thioadenosine in the presence of catalyzing enzyme ACC synthase that also requires cofactor pyrodoxal phosphate. In the process, ACC also conjugates to form N-malonyl ACC which is storage form of ethylene. The ACC synthesis regulates the ethylene production and the factors which accelerate or slow the ACC synthase activity also affecting the level of ethylene production accordingly. The ACC synthase (ACS) activity is directly proportional to the ethylene production rate. Regulation of ACS activity plays a key role in the rate of ethylene biosynthesis. The malonylation of ACC regulates the level of ACC and thereby production of ethylene.
ethylene biosynthesis, ethylene hormone , ripening hormone, fruit ripening, flower senesence

In the third step, ACC is oxidized into ethylene, Co2, and HCN in the presence of ACC oxidase enzyme which is also called ethylene forming enzyme (EFE). Further, HCN converts into formic acid and ammonia. The ascorbate and Fe++ are required for ACC oxidase activity and the ACC oxidation takes place in the presence of light and O2. The ripening and wounding promote ACC oxidase activity while Co++,Cu++ and Zn++ inhibit the activity. The further synthesis of methionine continues from 5 methyl thioadenosine produced during SAM cleaving.

The ACC synthesis increases with the high concentration of Indole Acetic Acid (IAA) and cytokinins while ACC synthase is inhibited by abscisic acid. Environmental factors like floods, drought, chilling, physical injury and pathogen attack induce ethylene formation in plants. Under floods, roots deprive to sufficient oxygen that results in the synthesis of ACC which is translocated from roots to leaves and oxidized into leaves resulting ethylene biosynthesis. The ethylene synthesis increases with temperature up to 350C and decreases with a decrease in temperature up to 0-20C. Low concentration of O2  also inhibits ethylene synthesis.
Factors affecting biosynthesis
Oxygen: the biosynthesis of ethylene includes oxidation of aminocyclopropane carboxylic acid (ACC) that requires oxygen; therefore the anaerobic conditions inhibit ethylene biosynthesis in plant tissues although methionine a precursor of ethylene is available in the tissues. The wax coating or storage of climacteric fruits into low O2 atmosphere delays the ripening of the fruits because the low availability of O2 prevents the conversion of ACC to ethylene which accelerates fruit ripening and senescence.
Temperature: The optimum range of temperature requires for ethylene biosynthesis but the range varies with plant species. The 300C temperature is optimum in apple for ethylene production. Normally temperature beyond the optimum range increases in the ethylene biosynthesis. The ethylene production increases many folds in the plant which is first exposed to chilling temperature thereafter higher temperature in comparison to the plant exposed constantly to a higher temperature.
Stress: The biotic and abiotic stresses increase ethylene production in plants. The ethylene production increases in the plants under the stress caused by insects and parts damage, bacterial fungal and viral diseases, drought, water logging, higher temperature and physical injury to the plant tissues because under stress the synthesis of enzyme ACC synthase is increased leading more synthesis of ACC that subsequently oxidized to ethylene. The pineapple plants flower early and coleus leaves abscise faster when these plants are placed horizontally than in vertical position because of more ethylene production in a horizontal position.
Light: It is reported that ethylene biosynthesis is increased in dark in the seedlings of a pea, similarly light inhibited the ethylene production in wheat leaves. Light decreases the ACC with a corresponding increase in the MACC (Malonyl ACC) which is biologically inactive. The CO2 increases the ethylene biosynthesis at low concentration whereas it inhibits ethylene action at very higher concentration. In the dark CO2 accumulates in the tissues and promotes ethylene biosynthesis.
Plant Growth regulators: The higher biosynthesis of ethylene in those sites of plants where auxin concentration remains higher indicates the effect of auxin on ethylene production. Auxin increases the ethylene biosynthesis by enhancing the conversion rate of S-adenosylmethionine (SAM) to ACC. Removal of the site of auxin biosynthesis like apical bud or application of TIBA (2,3,5-Tri iodobenzoic acid ) an antagonist of auxin reduces the ethylene production. The synthetic auxins NAA and IBA are effective for a longer period to produce ethylene than natural auxin IAA. Similar to auxin; Gibberellins and Cytokinins are also reported to increase the ethylene biosynthesis. The ethrel or ethaphon when applied releases ethylene without the involvement of enzymes. The ethrel molecule contains CH2=CH2 group and structurally resembles methionine a precursor of ethylene. 

Top Post Ad

Below Post Ad

Ads Area