「植物看得見你」公開課筆記／3.4 Ethylene Influence on Seedling

Ethylene is involved in:

1) aging processes such as fruit ripening

2) leaf abscission leading to the autumn foliage

3) plant responses to environmental stresses like drought and wounding

4) inhibit seedling elongation or cell elongation.

In the laboratory if I grow a plant in darkness but then add ethylene into the air around it, rather than elongating, the seedling will be rather short and fat. Stem elongation is reduced, with a thicker stem bending around to the side. If there's something blocking it, say a rock, there would be a local increase in ethylene, which would then let it bend to get around it.

This is a picture of Arabidopsis, that have been grown in the presence of high ethylene in the dark.

Here we have mutants that were grown under the exact same conditions, and we can see that these mutant Arabidopsis are elongated.

We can think of them as anosmic plants. Anosmia is what happens when we lose our sense of smell. These plants have lost their sense of smell for ethylene, thus lost the ability to respond to ethylene.

There are mutants that make too much ethylene, so that even under normal conditions they think there's ethylene around them all the time. And then there's mutants that think that there's always ethylene being given to them, even when they're not. These mutants are always short and fat and bending over, it doesn't matter what's going on around them.

What is the ethylene receptor? It was the first protein that was really shown to be a receptor of a hormone in plants in 1993. Similar to the way human olfaction receptors work, is found in the plant membrane.

This receptor which is called ETR1 is bound by the plant cell membrane and has part of the protein that goes inside the cell. Such that when the ethylene binds the ethylene receptor, it causes some type of signal to go into the cell. The ethylene receptor is sitting in the cell membrane, communicating information into the cell.

These receptors are active in terms of communicating information, but their activity is something negative. They're inhibiting the ethylene response -- fruit ripening or elongation in the dark.

In the absence of ethylene, the ethylene receptor is turning off the ethylene response pathway. Once ethylene is produced, it is binds to the receptor, and when it binds to the receptor it inactivates it, then stops inhibiting the ethylene response. Then the ethylene response can go forward, whether it's fruit ripening or elongation in the dark.

The ethylene receptor is a repressor with a negative effect. Ethylene molecule, the phytohormone itself comes, binds to the ethylene receptor, on the membrane of the plant cell, turns off the repressor, which then allows of the rest of the ethylene response to continue.

The ethylene is actually going into the middle of this protein, into the middle of the ethylene receptor, and when it binds in the middle of the receptor it's actually changing the shape. Such that in the absence of ethylene, the shape of the receptor allows it to be an active repressor.

Once ethylene has bound to the repressor, and we actually know where it is specifically binding in the protein, it changes the shape of the protein. It's turned off, it can no longer communicate anything. And the rest of the ethylene response can continue.

We now know most of the pathway involved in ethylene signalling. From its reception to transducing this material to other proteins within the cell, down through the cytoplasm until we get into the nucleus. Such that there are specific genes that are being turned on or being turned off, dependent on whether ethylene is present around the cells, or whether it's not.

These experiments showed that a volatile chemical is sensed, Ethylene is smelled by the plant, and this starts a complex signal transduction pathway that regulates plant development. Does this prove that plants communicate with each other through volatile chemicals?