The gaseous chemical ethylene signals plants when it’s time for their flowers to fade, their fruits to ripen, or their leaves to fall. A new discovery of how ethylene triggers the changes in gene activity that enable those processes could lead to new ways to stop or slow the ripening of plants harvested for food.
Plants make ethylene as they age, when they are damaged, and in response to changes in temperature, sunlight, and a variety of other factors. Because the hormone is a gas and can travel through the air, it acts not only within the plants that produce it, but also on other nearby plants. Ripe fruit, for example, hastens the ripening of other fruit nearby due to the ethylene it emits.
“If you can manipulate these processes, you can potentially keep things from spoiling.”
Joseph R. Ecker
“If you can manipulate these processes, you can potentially keep things from spoiling,” says Joseph R. Ecker, a Howard Hughes Medical Institute-Gordon and Betty Moore Foundation investigator at the Salk Institute for Biological Studies who led the new study, published August 30, 2012, in the journal Science. That’s more than a matter of convenience, he says, pointing out that a third of the plants produced in the world are lost to spoilage.
Scientists already knew that ethylene binds to its receptor in plant cells and de-represses the activity of a key regulator called EIN2. EIN2 is embedded in the cellular organelle where most protein production happens, the endoplasmic reticulum. And they knew that after exposed to ethylene gas, certain genes in the cell were turned on or off with downstream regulatory proteins called EIN3 family transcription factors. This change in gene expression leads to the end result of riper plants. But they didn’t know how EIN2 caused the activation of genes that were in the cell nucleus, far away from the endoplasmic reticulum.
“The paper describes that the missing link between the key regulators in the ER, and the nuclear-located transcription factors is EIN2 itself,” says Ecker. “We now know how the ethylene signal is transduced from the endoplasmic reticulum to the nucleus.”
The EIN2 protein has a large N-terminus (the end of the protein that is manufactured first) that attaches to the membrane of the endoplasmic reticulum, and a C-terminus (the opposite end) that extends from the ER membrane into the interior of the cell. In their new work, Ecker’s team followed the whole protein as well as the C-terminus of the molecule in biochemical and genetic experiments.
They discovered that phosphorylation of EIN2 depends on an enzyme called CTR1. Their experiments also showed that ethylene treatment causes EIN2 to be dephosphorylated, after which a portion of EIN2’s carboxy-terminal domain is cleaved from the rest of the protein. Freed from its membrane anchor, that piece of the protein moves to the nucleus, where it is predicted to act on transcription factors. The scientists found that they could trigger an ethylene response simply by sending that portion of EIN2 to the cell nucleus —even in the absence of the gas.
The explanation of how EIN2 turns on and off transcription factors could help scientists develop way to stifle a plant’s response to ethylene. In turn, this could have an influence on the aging of fruits, vegetables, and flowers. “What we’ve learned in our experiments using the reference plant Arabidopsis can be applied to many different crops,” Ecker says.
Tweaking the ethylene pathways could have unintended side effects, however. Ethylene doesn’t only affect ripening, but also boosts a plant’s resistance to pathogens. “So you don’t necessarily want to eliminate the entire ethylene response,” says Ecker.
Disease resistance, ripening and other ethylene responses are likely controlled by genetically distinct downstream branches of the same signaling pathway, Ecker says. So his lab’s next goal is to piece apart all the genes affected by the movement of EIN2 into the nucleus and determine which genes are responsible for which effects of ethylene. Then, rather than wipe out the response all together, scientists may have a better idea of how to selectively turn on and off some of the genes under ethylene’s control.
Image: Hong Qiao