Ethylene

What is it? Ethylene, or ethene in chemistry circles, is a colorless, flammable gas composed of two carbon atoms joined by a double bond. The chemical formula is C 2 H 4. Discovery as a biological agent… … In the late 19th Century, Dimitry Nikolayevich Neljubov, a young researcher at the Botanical Institute of St Petersburg, identified ethylene as the minor component of illuminating gas that caused his pea seedlings to grow horizontally. This makes ethylene the first plant hormone to be chemically identified. However, it was not until the latter half of the 20th Century, with the advent of gas chromatography, that ethylene achieved respectability as an endogenous regulator of plant growth and development. What does ethylene do? As well as its commonly recognized role in fruit ripening, ethylene influences many developmental processes throughout the life cycles of higher plants. The stimulation of seed germination, the adjustment of seedling growth to soil conditions, the rate and extent of leaf expansion and the timing of vegetative senescence and abscission are a few of the plant processes regulated by ethylene. Ethylene may also mediate responses to environmental challenges such as wounding, pathogen invasion, and water stress. How is ethylene made? Ethylene is synthesized in plants from the common metabolite S-adenosyl-L-methionine (SAM) via an unusual cyclic amino-acid intermediate, 1-aminocyclopropane-1-carboxylic acid (ACC). The first step in this reaction is often rate-limiting and is catalyzed by the enzyme ACC synthase. The second step, oxidation of ACC to ethylene, is catalyzed by ACC oxidase with cyanide as a byproduct. The activities of both ACC synthase and ACC oxidase, and thus the rate of ethylene production, are controlled at the transcriptional level. How does it work? The molecular details of early steps in ethylene signal transduction are becoming firmly established. Many key components have been identified through the study of mutations that affect a broad range of ethylene responses in the model plant Arabidopsis. Ethylene is sensed by a small family of receptors related to bacterial two-component histidine kinases, the ETRs. Members of this family contain a novel membrane-spanning domain that binds ethylene through a copper cofactor. Current evidence indicates that these receptors form a complex with CTR1, a RAF-like Ser/Thr protein kinase. In the absence of ethylene, this complex actively represses ethylene response pathways. Ethylene binding is thought to turn receptors off, resulting in derepression of response pathways. The receptor/CTR1 complex appears to operate by …