A receptor is a protein molecule in a cell to which mobile signaling molecules – such as ABA or pyrabactin, each of which turns on stress-signaling pathways in plants – may attach. Usually at the top of a signaling pathway, the receptor functions like a boss relaying orders to the team below that then proceeds to execute particular decisions in the cell.

Each receptor is equipped with a pocket, akin to a padlock, in which a chemical, like pyrabactin, can dock into, operating like a key. Even though the receptor pockets appear to be fairly similar in structure, subtle differences distinguish a pocket from its peers. The result is that while ABA, a product of evolution, can fit neatly in any of these pockets, pyrabactin is less successful. Still, pyrabactin, by being partially effective (it works better on seeds than on plant parts), serves as a leading molecule for devising new chemicals for controlling stress tolerance in plants.

Cutler explained that each receptor is equipped with a lid that operates like a gate. For the receptor to be activated, the lid must remain closed. Pyrabactin is effective at closing the gate on some receptors, turning them on, but cannot close the gate on others. The researchers have now cracked the molecular basis of this behavior.

“A key insight from the current work is that this difference is controlled by subtle differences between the receptors in their binding pockets,” said Cutler, an associate professor of plant cell biology in the Department of Botany and Plant Sciences and one of the members of the research team.

He explained that in a receptor where the gate closes, pyrabactin fits in snugly to allow the gate to close. In a receptor not activated by pyrabactin, the chemical binds in a way that prevents the gate from closing and activating the receptor.

“These insights suggest new strategies for modifying pyrabactin and related compounds so that they fit properly into the pockets of other receptors,” Cutler said.