Abscisic acid receptors.

Abscisic acid (ABA), a sesquiterpenoid phytohormone, was first discovered in the 1960s, and has since been found to be a key regulator of such diverse processes as dormancy, germination, seed development, and responses to biotic and abiotic stress (Cutler et al., 2010). Since the discovery of its structure, advances have been made in understanding ABA metabolism, synthesis, and hormone-responsive genes; however, the understanding of the early stage of ABA perception by the plant cell remained a mystery. During the past decade receptor proteins for the other traditional plant hormones (auxin, gibberellins, cytokinin, and ethylene) were discovered, while the identity of the ABA receptor remained elusive, creating controversy. Finally, in 2009, two research groups converged upon the PYR/PYL/RCAR family of soluble proteins in Arabidopsis (Arabidopsis thaliana), which evidence indicates are one type of ABA-binding receptor-like proteins (Ma et al., 2009; Park et al., 2009). This protein family has 14 members, almost all of which appear capable of forming an ABA-receptor complex that is able to activate the transcription of ABA-responsive genes. The use of multiple biochemical, chemical genetics, and proteomic approaches has provided the evidence to paint a remarkable picture of ABA binding, receptor complex formation, and initial downstream signaling. While the receptors and some important signal mediators have been elucidated, the molecular mechanisms by which they regulate the ABA signaling pathway continue to intrigue researchers and drive the field forward.