Fine-tuning of ABA responses by the protein kinase WNK8

Abstract

SUMMARYThe phytohormone abscisic acid (ABA) regulates various growth- and developmental processes in response to limiting water conditions. ABA functions through an established signaling pathway consisting of PYR1/PYL/RCAR ABA receptors that inhibit group A type 2C protein phosphatases (PP2Cs) in an ABA-dependent manner. Inhibition of PP2Cs enables the activation of SnRK2-type protein kinases that phosphorylate downstream targets including transcription factors and ion channels. However, ABA-dependent signals have to be integrated into other growth- and developmental programs to ensure a successful life cycle. Here, we have characterized the role of the protein kinase WNK8 in the ABA signalling pathway. Two T-DNA insertion alleles wnk8-1 and wnk8-4 exhibited contrasting ABA responses during seed germination and young seedling growth. However, reciprocal crossings with wild type plants suggested that wnk8-1 that still expressed the WNK8 kinase domain functioned in a hypermorphic and dominant manner. WNK8 directly interacted with the PP2C PP2CA in planta and was negatively regulated by this phosphatase in vitro. WNK8 also phosphorylated the ABA receptor PYR1 in vitro. Double mutant analyses revealed that the dominant allele wnk8-1 suppressed the ABA- and glucose hypersensitivity of the pp2ca-1 T-DNA allele. In transient protoplast assays WNK8 suppressed ABA-induced reporter gene expression that was dependent on a functional kinase. In summary, we have identified the protein kinase WNK8 as a negative regulator of ABA responses during young seedling establishment through its direct interaction with core ABA signaling components.SIGNIFICANCE STATEMENTThe phytohormone abscisic acid regulates the water household of plants through a defined core signaling pathway. Here we have identified the protein kinase WNK8 as a direct interactor of core abscisic acid signalling components and as a negative modulator of abscisic acid responses during young seedling development in Arabidopsis.

Dry side of the core: a meta-analysis addressing the original nature of the ABA signalosome at the onset of seed imbibition

“…S119 of RCAR12) . The With No lysine (K) protein Kinase 8 (WNK8) was found to interact with RCAR11 in the cytosol and to phosphorylate it at several residues located close to the ABA binding site in vitro (Waadt et al, 2019). The Cytosolic ABA Receptor Kinase 1 (CARK1) interacts with all group III receptors in the cytosol and phosphorylates them in vivo on a conserved Li et al, 2019b).…”

“…Accordingly, xiw1 seeds are less sensitive to ABA during germination. Besides its role in the ESCRT pathway, FYVE1 is phosphorylated upon stress by class III SnRK2s and translocates to the nucleus, where it interacts with the DNA-binding domain of ABI5 and ABF4, thereby inhibiting their transcriptional activities (Li et al, 2019).…”