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

379759639_4afaae1d71_m.jpgSniffing Out Ethylene

In the previous post I summarized the latest information on how the plant hormone ethylene is made (at least in the model plant Arabidopsis).

Though most people are familiar with the plant hormone ethylene mainly because of its effects on fruit ripening, the list of plant responses to ethylene is long.

How do plants manage to detect (“sniff out”) minute quantities of this gaseous plant hormone?

And how do minute quantities of ethylene, once detected by the plant, elicit such a wide array plant responses, from stimulating seed germination in some plants to promoting aging in others, for example?

Detecting the Ethylene Signal

In order to listen to FM radio, you have to have a FM receiver.

That is, in order to detect FM radio signals (at frequencies invisible to our eyes), you have to have a way to detect such weak signals. (The rest of the radio is for amplifying the signal so you can hear it.)

gymnist.jpgHow do plants detect the weak ethylene signal?

They have ethylene receivers, or in biological terms, ethylene “receptors” that can very specifically discern ethylene from other small molecules. (Hormone receptors are typically proteins because they have the ability to form very complex and precise 3D structures. This allows for their high specificity in recognizing distinct molecular structures.)

It turns out that each ethylene molecule interacts with two receptors, which are paired together to form a pair or “dimer”.

In the photo on the right, the two rings are like the two individual ethylene receptors, and the gymnast is like ethylene.

Interestingly, these ethylene receptors are transmembrane proteins located inside plant cells in the endoplasmic reticulum (ER). (Since ethylene is a relatively small gas molecule, it can easily pass through the plant cell membrane from outside to inside the cells.)

In Arabidopsis there apparently is a 5-member family of these individual ethylene receptors. Each member of the family can pair with another copy of itself to form a “homodimer” functional ethylene receptor or with other members of the family to form a “heterodimer”. (The term “homodimer” is used when the two molecules are identical, e.g. A-A, and “heterodimer” when they are not, e.g. A-B.)

Thus, the ethylene serves as a “key” in the receptors’ “lock”, setting in motion a cascade of events…….

Amplifying the Ethylene Signal
You text a secret to two friends. Then each of your friends texts the secret to two more people. Then each of these people text the secret to two of their friends. And on, and on, and pretty soon it’s no longer a secret.

This “cascade” of events effectively “amplifies” the original text message. The ethylene signal is similarly amplified via a cascade of cellular events. For example, when ethylene binds to its receptors, this causes conformational changes in these receptor proteins, which then “activates” them…somewhat analogous to you using your car key (ethylene) to start your car (receptor proteins).

The “activated” receptor proteins, in turn, initiate an intracellular signal cascade ultimately leading to the expression of an array of genes, which collectively result in the “display” we observe, whether it be inhibition of shoot growth or fruit ripening.

Which genes ethylene “turns on” depends on the biological “context”, that is, what part of the plant?, what stage of the plant’s development?, what plant species?, etc.

Qiagen offers an illustration of “The Big Picture” of the ethylene signaling pathway in Arabidopsis, though it’s somewhat technical.

Bottom line: Decades of scientific research has revealed not only the receptors for ethylene in several plant species but also a complex cascade of cellular events leading to the plants’ responses to ethylene that we can observe in nature.

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