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HISTORY OF ETHYLENE
• During the 19th century, when coal gas was used as street illumination it was observed that trees in the vicinity of streetlamps defoliated more extensively than other trees.
• In 1901, Dimitry Neljubov observed that dark grown pea seedlings in the laboratory exhibited symptoms that were later termed the triple response.
• The first indication that ethylene is a natural product of plant tissues was published by H. H. Cousins in 1910.
• In 1934, R. Gane and others identified ethylene chemically as a natural product of plant metabolism, and because of its dramatic effects on plant it was classified as a hormone.
OCCURANCE, DISTRIBUTION AND TRANSPORT OF EHYLENE IN PLANTS
• Ethylene can easily be synthesized in all plant organs such as roots, stems, leaves, tubers, fruits and seeds.
• It is highest in senescing tissues and ripening fruits.
• Within the plant organs, ethylene formation is mainly located in peripheral tissues.
• Ethylene is biologically active at low concentration (less than 1 ppm)
• Ethylene can easily pass through plasmamembraneinto the cell, easily diffuse within the plant, and flushed out of plant tissues through intercellular spaces
STRUCTURE AND BIOSYNTHESIS
• Ethylene is the simplest olefin (mol. Wt. 28) .
• It is lighter than water under physiological conditions and readily undergoes oxidation.
CONJUGATION AND CATABOLISM OF ETHYLENE
• Not all the ACC found in the tissue is converted to ethylene. ACC can also be converted to a conjugated form ,N-malonyl ACC which accumulates in the tissue , primarily in the vacuole.
• A minor conjugated form of ACC, 1-(y-L-glutamylamino) cyclopropane-1-carboxylic acid (GACC) has also been identified.
• Carbon dioxide, ethylene oxide, ethylene glycol, and the glucose conjugate of ethylene glycol have been identified as metabolic breakdown products.
FACTORS PROMOTING ETHYLENE BIOSYNTHESIS
• Fruit ripening.
• Stress induced ethylene production,viz
Drought, flooding, chilling, exposure to ozone, and mechanical wounding.
• Auxin induced ethylene production.
• Circadian regulation of ethylene production.
INHIBITORS OF ETHYLENE BIOSYNTHESIS AND ETHYLENE ACTION
• Aminoethoxy-vinylglycine (AVG).
• Aminooxyacetic acid (AOA).
• Silver ions.
• Carbon dioxide at high concentrations (5-10%).
• trans-cyclooctene.
• 1-Methylcyclopropene
PHYSIOLOGICAL EFFECTS OF ETHYLENE
• Fruit ripening.
• Plumular hook formation.
• Triple response.
• Formation of adventitious roots and root hairs.
• Leaf epinasty.
• Senescence.
• Abscission of leaves.
MECHANISM OF ETHYLENE ACTION
• Binding of ethylene to a receptor.
• Activation of one or more signal transduction pathways.
• Modulation of gene expression leading to cellular response.
References
• Taiz L.&Zeiger E.2010Plant Physiology 5th Ed. Sinauer Associates,inc.
• JainVK.1974.fundamentals of Plant physiology 1st
Ed.S.Chand.publishing