Protein kinase CK2 (formerly known as casein kinase II) is a ubiquitious Ser/Thr kinase present in all eukaryotes. The α (catalytic) and β (regulatory) subunits of CK2 exist both as a tetrameric holoenzyme and as monomers in eukaryotic cells. CK2 has been implicated in multiple developmental and stress-responsive pathways including light signalling and circadian clock in plants. Recent studies using CK2 knockout and dominant negative mutants in Arabidopsis have uncovered new roles for this enzyme. CK2 substrates that have been identified so far are primarily transcription factors or regulatory proteins. CK2-mediated phosphorylation of these factors often results in alteration of the protein function including changes in the DNA-binding affinity, dimerization, stability, protein-protein interactions, and subcellular localization. CK2 has evolved as an essential housekeeping kinase in plants that modifies protein function in a dynamic way. This review summarizes the current knowledge of the role of CK2 in plant development.

). † These authors contributed equally. SUMMARYCasein kinase II (formerly known as CK2), a ubiquitous Ser/Thr kinase, plays critical roles in all higher organisms including plants. The CK2 holoenzyme consists of two catalytic a subunits and two regulatory b subunits. The Arabidopsis genome has four a subunit and four b subunit genes, and members of both the a and b subunit families have been shown to be localized in the cytoplasm, nucleus and also in chloroplasts. However, the biological roles of CK2 subunits have not been fully characterized yet. Here we identified T-DNA insertion mutants in three a subunit genes (a1, a2 and a3) and made double and triple mutants. The CK2 a1a2a3 triple mutants displayed reduced CK2 activity compared with wild-type seedlings. Phenotypic characterization showed that CK2 a1a2a3 triple mutants are late flowering under both long-and short-day conditions. Genes encoding floral integrators are differentially regulated in the triple mutant compared with the wild-type plants. CK2 a1a2a3 triple mutants also displayed reduced hypocotyl growth, smaller cotyledon size and a reduced number of lateral roots compared with wild-type seedlings under light. Abscisic acid-induced blockage of seed germination and cotyledon greening is reduced in CK2 a subunit mutants in an additive manner. Moreover, CK2 a subunit mutants are also hyposensitive to a NaCl-induced blockage of seed germination. Taken together, these data suggest that CK2 a subunits affect diverse developmental and stress responsive pathways in Arabidopsis.

Casein kinase 2 (CK2) is an essential and well-conserved Ser/Thr kinase that regulates proteins in a posttranslational manner. CK2 has been shown to affect a large number of developmental processes across eukaryotes. It is a tetrameric protein composed of a dimer of alpha (catalytic) and beta (regulatory) subunit each. In our previous study we showed that three of the four CK2 α subunits in Arabidopsis act in a functionally redundant manner to regulate various developmental pathways. In this study we constructed two independent CK2 α4 RNAi lines in the CK2 alpha triple mutant background. Through functional characterization of these RNAi lines we show that the fourth α subunit in Arabidopsis also functions redundantly in regulating ABA response, lateral root formation and flowering time. CK2 α4-GFP localizes to the chloroplast in transgenic Arabidopsis seedlings, consistent with the presence of a chloroplast localization signal at the amino-terminus of CK2 α4 subunit. Taken together, our results suggest a functionally overlapping role for the CK2 α4 subunit in regulating various developmental processes in plants.

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