Secrets of plant stress hormone revealed

Abscisic acid (ABA) is a common phytohormone that helps govern the reaction of plants to stresses in their environment, thereby influencing growth and reproduction. However, until recently, little was known about the receptors for ABA and how they interact with downstream effectors.

Six new papers, including two that involve researchers from the King Abdullah University of Science and Technology (KAUST) in the Kingdom of Saudi Arabia, have all contributed to clarifying the function and activity of components of the ABA-signalling pathway.

The first of these papers used crystallographic techniques to show how the members of a particular family of proteins (collectively known as PYR/PYL/RCAR) function as ABA receptors. Each member has a binding pocket that is guarded by two protein loops. Once ABA is in the pocket, the loops close to retain the hormone, revealing a new binding site. These bound-receptor complexes are then able to bind to and inhibit the activity of protein phosphatase enzymes (PP2Cs), and are locked in place by an amino acid on the enzyme.

The second paper further elucidated this downstream signalling mechanism. In the absence of ABA, PP2Cs actively prevent autophosphorylation of members of the serine/threonine protein kinase SnRK2 family.

Inhibition of PP2Cs allows this autophosphorylation and, hence, activation of the SnRK2s, which in turn phosphorylate downstream transcription factors and activate stress-response programmes. By reconstituting the ABA-triggered phosphorylation in vitro and testing the results in plant-protoplast assays, the researchers revealed the minimal elements required for this ABA-response cascade. It is a surprisingly simple pathway with only four core components that share many conserved features; complexity is added with multiple members of each protein family.

Understanding the ABA-signalling pathway is an important step towards improving agriculture through modulating plant response to factors such as temperature, water availability and salinity.

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