Ethylene Review: Not So Invisible Anymore

A considerable amount of work on ethylene gas has been done to elucidate its role in the regulation of several metabolic activities in plants such as seed germination, fruit ripening, and organ senescence. Commercial fruit growers and biotech companies are particularly interested in this chemical because fruit ripening can be manipulated by its application.

In a review article,  Ethylene: A Gaseous Signal Molecule in Plants, published in the Annual Review of Cell Development Biology (Annual Rev Cell Dev Biol. 2000;16:1-18), A.B. Bleecker and his mentor Hans Kende discuss the present state of knowledge about ethylene’s role in plants. The authors began their review by describing that ethylene is now known to be synthesized in two enzymatic steps in which ACC synthase converts S-adenosyl-L- methionine to1-aminocyclopropane- 1-carboxylic acid (ACC) and ACC is converted to ethylene by ACC oxidase. It is also now known that ethylene biosynthesis is controlled by both external and intracellular signals mediated primarily through differential expression of ACC synthase genes.

The authors then expand on the characterization of  ethylene-response mutants in Arabidopsis.  It is this mutant work, they explain, that has led to the identification of components involved in the ethylene signal transduction pathway.   For example, a study of mutants of the etr1 class, has led to identification of ethylene receptors which have similiarity to the two-component signaling systems1 found in bacteria.  Ethylene insensitivity has been traced to mutations that affect ethylene binding to its receptor.  Constitutive ethylene-response mutants have been associated with a Raf-like Ser/Thr protein kinase called CTR1.  This kinase appears to act downstream of the ethylene receptor and may be part of a MAP kinase2 cascade. Mutations in the genes EIN2 and EIN3, which are epistatic to CTR1, can result in ethylene insensitivity.  Turns out that EIN3 is a putative transcription factor involved in regulating the expression of ethylene- responsive genes.
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1 In bacteria, two protein components perceive and transfer environmental signals. The longer protein is made up of  about 250 amino acids at the C-terminus end and the shorter one of about 120 amino acids at the N-terminal end, respectively.

2 MAP kinase – A family of serine/threonine protein kinases associated with mitogen activation. They have key role in signal transduction pathways. Kinase is an enzyme that phosphorylates one or more amino acids at a certain positions in a protein altering the conformation of the protein.

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