The phytohormone abscisic acid (ABA) is induced in response to abiotic stress to mediate plant acclimation to environmental challenge. Key players of the ABA-signaling pathway are the ABA-binding receptors (RCAR/PYR1/PYL), which, together with a plant-specific subclade of protein phosphatase 2C (PP2C), form functional holoreceptors. The Arabidopsis genome encodes nine PP2C coreceptors and 14 different RCARs, which can be divided into three subfamilies. The presence of these gene families in higher plants points to the existence of an intriguing regulatory network and poses questions as to the functional compatibility and specificity of receptor-coreceptor interactions. Here, we analyzed all RCAR-PP2C combinations for their capacity to regulate ABA signaling by transient expression in Arabidopsis protoplasts. Of 126 possible RCAR-PP2C pairings, 113 were found to be functional. The three subfamilies within the RCAR family showed different sensitivities to regulating the ABA response at basal ABA levels when efficiently expressed. At exogenous high ABA levels, the RCARs regulated most PP2Cs and activated the ABA response to a similar extent. The PP2C AHG1 was regulated only by RCAR1/PYL9, RCAR2/PYL7, and RCAR3/PYL8, which are characterized by a unique tyrosine residue. Site-directed mutagenesis of RCAR1 showed that its tyrosine residue is critical for AHG1 interaction and regulation. Furthermore, the PP2Cs HAI1 to HAI3 were regulated by all RCARs, and the ABA receptor RCAR4/PYL10 showed ABA-dependent PP2C regulation. The findings unravel the interaction network of possible RCAR-PP2C pairings and their different potentials to serve a rheostat function for integrating fluctuating hormone levels into the ABA-response pathway.
Keywords: ABA; PP2C; RCAR/PYL; abiotic stress; drought.