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ethylene_T.jpgC’est un gaz. Es un gas. Es ist ein gas. It’s a Gas!

If you are familiar with the gaseous plant hormone ethylene, it’s most likely because of its ability to promote fruit ripening in apples, bananas, tomatoes, etc..

Ethylene also affects many other aspects of a plant’s life cycle, including seed germination, growth and development, responses to physical and biological stress, leaf abscission, and senescence.

It may even be responsible for how some herbicides kill plants.

But how does ethylene work?

That is, how can this simple gas (CH2-CH2) mediate so many different processes in plants? Where and how is this chemical signal produced in plants? And how is this signal received by plant cells and then amplified into the responses we observe?

Making the Signalethylene_T2.jpg

Virtually all the cells in a plant have the potential to make ethylene. Plant cells make ethylene starting from the amino acid methionine (see below).

And it turns out that during the course of a plant’s life – from seed germination to senescence – ethylene may be made not only as part of the developmental program of the plant, but also in response to random events, such as wounding and pathogen attack.

The primary way plants (at least Arabidopsis) regulate the amount of ethylene is through the expression of a family of genes that encode enzymes (the ACS protein family) that catalyze the synthesis of the key metabolic precursor to ethylene, namely, ACC*.

Methionine –> SAM* –ACS proteins –> ACC* + O2 –> Ethylene
* SAM = S-Adenosyl-L-Methionine, ACC = 1-AmminoCyclopropane-Carboxylate

(Please see here for diagram of ethylene biosynthetic pathway in plants.)

Quoting part of a recent report (ref. 1 below) on the genetics of ethylene biosynthesis:

“We view the ACS protein family as a “Symphony Orchestra” (45-member when all nine genes are expressed in a cell) that regulates ethylene-mediated processes by generating appropriate amounts of ACC in the proper spatial and temporal manner through their harmonious interplay. At any given moment, the orchestra is tuned by various inducers to produce ACC sufficient to mediate myriad ethylene responses…”

orchestra.jpg


So, this may explain why the biosynthesis of ethylene in plants can be affected by so many different factors.

Next-time: The Ripe Stuff – Part 3: How Plants Respond to Ethylene

References:

1. Tsuchisaka, A., et al. (November 2009) “A Combinatorial Interplay Among the 1-Aminocyclopropane-1-Carboxylate Isoforms Regulates Ethylene Biosynthesis in Arabidopsis thaliana.” Genetics vol. 183, pp. 979-1003. (PDF)

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