It has been known for some time that the hormone abscisic acid (ABA) is central to a plant’s ability to resist a number of stresses, including drought, but until now the mechanism of action was not understood.
Now scientists at the European Molecular Biology Laboratory (EMBL) in Grenoble, France, and the Consejo Superior de Investigaciones Cientificas (CSIC) in Valencia, Spain, have discovered that the key lies in the structure of a protein called PYR1 and how it interacts with the hormone ABA. They say the work could open up new approaches to increasing the ability of crops plants to survive water shortages.
Under normal conditions, a family of proteins called Type 2C phosphatases inhibit the ABA pathway. When a plant is subjected to drought, the concentration of ABA in its cells increases, removing the brake and allowing the signal for drought response to be triggered.
This in turn initiates mechanisms for increasing water uptake and storage, and decreasing water loss.
While it is known that ABA does not interact directly with Type 2C phosphatases, recent studies had indicated that another family of proteins mediate between these two.
Scientists led by José Antonio Márquez from EMBL Grenoble, and Pedro Luis Rodriguez from CSIC, examined one member of this third family of proteins, called PYR1. X-ray crystallography shows that PYR1 has an open structure, like an outstretched hand, in the absence of ABA. When ABA is present it sits in the ‘palm’ of the PYR1 hand, which then closes over the hormone as if holding a ball. This enables a Type 2C phosphatase to sit on top of the folded fingers.
As these features seem to be conserved across most members of this protein family, this confirms PYR1 and others in this protein family as the main ABA receptors. It also demonstrates how the process of stress response starts: by binding to PYR1, ABA causes it to inhibit PP2C molecules, which are therefore not available to block the stress response.
“If you treat plants with ABA before a drought occurs, they take all their water-saving measures before the drought actually hits, so they are more prepared, and more likely to survive that water shortage – they become more tolerant to drought,” said Rodriguez.
ABA is very difficult and expensive to produce. But now this structural biology approach has shown what ABA interacts with and how, it will be possible to develop screens for other molecules which have the same effect.