Abstract
Main conclusionOur transient gene expression analyses in Arabidopsis protoplasts support the view that CK2αs and CK2βs positively and negatively modulate ABRE-dependent gene expression, respectively.The phytohormone abscisic acid (ABA) regulates the expression of thousands of genes via ABA-responsive elements (ABREs), and has a crucial role in abiotic stress response. Casein kinase II (CK2), a conserved Ser/Thr protein kinase in eukaryotes, is essential for plant viability. Although the CK2 has been known as a tetrameric holoenzyme comprised of two catalytic α and two regulatory β subunits, each of the two types of subunits has been proposed to have independent functions. The Arabidopsis genome encodes four α subunits (CK2α1, CK2α2, CK2α3, CK2α4) and four β subunits (CK2β1, CK2β2, CK2β3, CK2β4). There is a growing body of evidence linking CK2 to ABA signaling and abiotic stress responses. However, the roles of each CK2 subunit in ABA signaling remain largely elusive. Using the transient expression system with the core ABA signaling components in Arabidopsis leaf mesophyll protoplasts, we show here that CK2α1 and CK2α2 (CK2α1/2) positively modulate ABRE-dependent gene expression as ABA signal output in ABA signaling, whereas all four CK2βs negatively modulate the ABRE-dependent gene expression mediated by subclass III SnRK2–AREB/ABF pathway and by CK2α1/2. These data indicate that CK2α1/2 and CK2βs positively and negatively modulate ABA signal output, respectively, suggesting that the quantitative balance of CK2 subunits determines the ABA signal output in plants. Given that CK2s act as pleiotropic enzymes involved in multiple developmental and stress–responsive processes, our findings suggest that CK2 subunits may be involved in integration and coordination of ABA-dependent and -independent signaling.
Highlights
The phytohormone abscisic acid (ABA) has crucial roles in a broad range of plant developmental processes and environmental stress responses (Cutler et al 2010; Raghavendra et al 2010; Miyakawa et al 2013; Yoshida et al 2015)
To investigate how the CK2α1/2 positively modulates ABREdependent gene expression, we evaluated the effects of CK2α1 on RD29B-GUS expression in protoplasts derived from three different genotypes: an AREB/ABF subfamily triple mutant areb1/2abf3 (Yoshida et al 2010), a subclass III sucrose nonfermenting related protein kinase 2 (SnRK2) subfamily triple mutant srk2d/e/i
We show that CK2α1/2 positively modulates ABREdependent gene expression dependently and independently of the core ABA signaling pathway in the presence and absence of exogenous ABA in Arabidopsis protoplasts, respectively (Figs. 1, 2)
Summary
The phytohormone abscisic acid (ABA) has crucial roles in a broad range of plant developmental processes and environmental stress responses (Cutler et al 2010; Raghavendra et al 2010; Miyakawa et al 2013; Yoshida et al 2015). In the presence of ABA, the subclass III sucrose nonfermenting 1 related protein kinase 2 (SnRK2) is released from the PP2C-mediated negative regulation and phosphorylates basic leucine zipper (bZIP) transcription factors such as ABRE-binding protein/ ABRE-binding factors (AREB/ABFs), which activate the expression of ABA-responsive genes. Taken together with the recent results from a systemic study of a transcriptional network in response to ABA (Song et al 2016), these findings indicate that ABRE-dependent gene expression constitutes the major ABA-responsive gene expression as ABA signal output in response to dehydration stress
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