Combinatorial interaction network of abscisic acid receptors and coreceptors from Arabidopsis thaliana

Abstract

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.BA regulates a plethora of responses associated with plant growth and the homeostatic control of water relations. ABA controls root extension and branching, stomatal opening and density, and tolerance to water deficit during seed maturation and drought (1, 2). Core ABA signaling can be considered as a threestep regulatory process involving the receptor complex, protein kinases as intermediate signaling components, and downstream targets such as ion channels and transcription factors (1). The heteromeric receptor complex consists of the ABA-binding RCAR/PYR1/PYL receptor and the PP2C coreceptor. The protein phosphatase activity is regulated by ABA that stabilizes the PP2C-RCAR interaction, which blocks substrate access and thereby inhibits the catalytic activity of the coreceptor (3, 4).The clade A of Arabidopsis PP2Cs comprises nine members and forms a plant-specific subgroup among the large family of PP2Cs, which are Mg 2+ -and Mn 2+-dependent serine-threonine protein phosphatases (5). The ABA-mediated inactivation of PP2C activity releases SNF1-related kinase 2 (SnRK2) from inhibition, and it subsequently targets downstream components such as transcription factors and ion channels (6-12). The 14 different RCARs of Arabidopsis can be divided into three subfamilies according to their sequence hom...

ABA inhibits myristoylation and induces shuttling of the RGLG1 E3 ligase to promote nuclear degradation of PP2CA

“…Currently, at least six Arabidopsis PP2Cs, namely ABI1, ABI2, PP2CA/AHG3, AHG1, HAB1 and HAB2, are known to negatively regulate ABA signaling as well as Highly ABA-Induced 1-3 (HAI1-HAI3) phosphatases, although HAI1-3 have less impact in ABA sensitivity and more in order to regulate particular drought resistance traits Bhaskara et al, 2012;Tischer et al, 2017). PP2CA is a negative regulator of ABA signaling both in seed and vegetative ABA responses, and RGLG1/5 and other E3 ligases regulate PP2CA half-life (Sheen, 1998;Kuhn et al, 2006;Yoshida et al, 2006;Lee et al, 2009;Rubio et al, 2009;Wu et al, 2016).…”

Drought resistance is mediated by divergent strategies in closely related Brassicaceae

“…From these data a picture of the drought signaling system emerges that is differently tuned in the resistant species relative to Ath . Intriguingly, several of the genes that are constitutively expressed at higher levels in the resistant species were shown in Ath to increase WUE and drought resistance (Tischer et al ., ). This opens the possibility that other signaling genes expressed at higher levels in Aly and Esa may have similar beneficial effects.…”

“…It is possible that a smaller stomata aperture affects the water use efficiency (WUE) of the resistant Aly and Esa . Importantly, a recent overexpression screen found that higher levels of the Aly ‐ and Esa ‐elevated ABA receptors increase Ath WUE (Yang et al ., ; Tischer et al ., ), suggesting a causal contribution to Aly and Esa drought resistance. While the functional orthology of the Aly and Esa proteins remains to be shown, this possibly convergent evolution of higher ABA receptor levels in Aly and Esa is consistent with their resistant phenotype.…”

Abscisic acid signalling mediates biomass trade‐off and allocation in poplar

“…In our study, we used poplar lines overexpressing the ABA receptor RCAR1/PYL9 . Poplar RCAR1/PYL9 is induced by ABA and interacts with HAIs (highly ABA-induced, which are members of the PP2C-A clade), demonstrating widely conserved functions in both Arabidopsis and poplar Tischer et al, 2017). Yu et al (2017a,b) observed higher biomass production in RCAR1 overexpressing poplar lines and ascribed this behaviour to hypersensitive stomatal regulation and activation of oxidative stress responses.…”