ABA Signaling Mechanism Decoded

Plants use specialized signals, such as plant hormones, to sense difficult times and adapt to stressful conditions to enhance survival. One plant hormone, abscisic acid (ABA), coordinates responses to stressors such as drought and salinity. ABA regulates various physiological processes such as stomatal closure, bud dormancy and seed germination. Understanding the inner workings of the ABA signaling pathway, scientists say, could help develop crops that thrive in harsh environmental conditions and combat global food shortages.

However, the exact molecular mechanism by which ABA helps plants tolerate extreme conditions remains poorly understood. The hormone receptor has eluded researchers for decades. Earlier this year, Sean Cutler of the University of California Riverside and colleagues identified a family of protein, dubbed PYR/PYL/RCAR, that inhibits the activity of ABA response associated phosphatase enzymes (PP2C). Now six independent groups of researchers have defined the structure and function by which the stress hormone is sensed by PYR/PYL/RCAR proteins.

In the absence of ABA, PP2C inhibits the phosphorylation of a family of kinases (SnRK). ABA enables the receptor proteins PYR/PYL/RCAR to sequester PP2C, therefore ‘liberating' the kinases. These kinases become activated and subsequently activate transcription factors that will initiate the expression of certain genes. Laura Sheard and Ning Zheng, in a synthesis paper published by Nature, summarized the ABA signaling pathway, which they said "is attractive in its simplicity and offers a seamless complement to the known body of ABA literature."

The synthesis article, which provides links to the original research papers, is available to Nature subscribers at http://dx.doi.org/10.1038/462575a