Protein kinase CK2 alpha subunits constitutively activate ABA signaling in Arabidopsis

Protein kinase CK2 (formerly known as casein kinase II), a Ser/Thr protein kinase highly conserved in eukaryotes, is essential for cell survival by regulating a wide range of plant growth, development, and stress responses. A growing body of evidence has shown a link between CK2 and abscisic acid (ABA) signaling in response to abiotic stress. However, the roles of CK2 subunits in ABA signaling remain unclear in plants. Our recent work in Arabidopsis thaliana has revealed that CK2 alpha and CK2 beta subunits inversely modulate ABA signal output. Here, we examine the roles of CK2 alpha s, by assessing how CK2 alpha s affect ABA signaling. Together with the previous findings, our mutant and transient expression analyses demonstrate that CK2 alpha s positively modulate ABA signaling through the core ABA signaling pathway in the presence of ABA, though the positive effect of CK2 alpha s are much smaller than that of core ABA signaling components in ABA response. In addtion, our current and previous findings also suggest that CK2 alpha s play a role in maintaining constitutively active ABA signaling even in the absence of ABA independently of the core ABA signaling pathway. Thus, we found that CK2 alpha s constitutively activate ABA signaling in the presence or absence of ABA in a different manner in Arabidopsis plants.