Casein kinase 2 α and β subunits inversely modulate ABA signal output in Arabidopsis protoplasts

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.Electronic supplementary materialThe online version of this article (10.1007/s00425-018-2919-5) contains supplementary material, which is available to authorized users.

Protein kinase CK2α subunits constitutively activate ABA signaling in Arabidopsis

“…These findings support the hypothesis that the positive effect of CK2αs are much smaller than that of core ABA signaling components in ABA responses in the presence of exogenous ABA. Collectively, together with the previous findings that CK2β1 suppressed CK2α-mediated ABRE-dependent gene expression 21 and that CK2αs positively modulate ABRE-dependent gene expression dependently of the core ABA signaling pathway in the presence of exogenous ABA, 21 these data suggest that CK2αs may be implicated in CK2-meidated fine-tuning ABA signaing through the core ABA signaling pathway under stress conditions.

10.1080/15592324.2018.1525998-F0002Figure 2.CK2αs positively modulate ABA signaling during seed germination and ABA-responsive marker genes even under non-stressed conditions. (A) relative expression levels of each CK2α gene in the WT and CK2α4-RNAi in ck2α1/2/3 mutant ( ck2α1/2/3-4i) .

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“…16,17 Recently, we have demonstrated that CK2α and β subunits inversely modulate ABRE-dependent gene expression as ABA signal output in ABA signaling in Arabidopsis cells. 21 Our results indicate that CK2α1 and CK2α2 positively modulate ABRE-dependent gene expression, whereas CK2β1 negatively modulates ABRE-dependent gene expression mediated by AREB-SnRK2 pathway and by CK2αs. 21 However, recent biochemical analyses suggest that CK2 negatively regulates ABA signaling through facilitating both SnRK2 degradation and SnRK2 interaction with PP2C.…”

“…21 Our results indicate that CK2α1 and CK2α2 positively modulate ABRE-dependent gene expression, whereas CK2β1 negatively modulates ABRE-dependent gene expression mediated by AREB-SnRK2 pathway and by CK2αs. 21 However, recent biochemical analyses suggest that CK2 negatively regulates ABA signaling through facilitating both SnRK2 degradation and SnRK2 interaction with PP2C. 22 It thus remains unclear how CK2αs are involved in the positive modulation of ABA signal output.…”

“…8–10,24 We conducted transient expression analyses in Arabidopsis leaf mesophyll protoplasts using a β-glucuronidase ( GUS ) reporter gene under the control of ABRE cis -elements ( RD29B-GUS ) originated from the ABA-responsive RD29B promoter. 21 Transfection of CK2α1 or AREB1 induced a similar level of RD29B-GUS expression in the protoplasts without ABA treatment compared with the vector control (Figure 1). By contrast, the levels of RD29B-GUS expression induced by transfection of AREB1 were more than 7-fold higher than those of CK2α1 with ABA treatment (Figure 1).…”

“…25–27 In addition, CK2α3 (At2g23080) is located adjacent to CK2α4 (At2g23070) on the chromosome. 21 To analyze the role of CK2α in planta , we therefore generated CK2α4-RNAi plants in the ck2α1 ck2α2 ck2α3 triple mutant background (ck2α1/2/3-4i) . Reverse transcription-PCR (RT-PCR) analysis of the ck2α1/2/3-4i plants confirmed that the expression of CK2α1, CK2α2 , and CK2α3 was completely interrupted by the T-DNA insertions and that CK2α4 expression was slightly but significantly reduced compared with the WT plants (Figure 2A).…”