Role of casein kinase 2 (CK2) alpha subunits in the growth and development of Arabidopsis thaliana

Casein kinase 2 (CK2) is a conserved Ser/Thr kinase present in all organisms. It is a tetrameric protein composed of two alpha and two beta subunits. In Arabidopsis, there are 4 genes encoding for each alpha (catalytic) and beta (regulatory) subunit of CK2. The role of CK2 alpha subunits in the growth and development of Arabidopsis was studied through characterization of T-DNA insertion mutants (for α1, α2, α3) and RNAi approach (for α4). Results show that all 4 CK2 alpha subunits positively regulate flowering under both short day and long day conditions, possibly in a functionally redundant manner. Elevated levels of FLC and reduced levels of SOC1 in the CK2 alpha triple mutant compared to wild type (Col-0) suggest that the alpha subunits possibly act through the autonomous pathway to regulate flowering. The alpha subunits also synergistically promote inhibition of seed germination and cotyledon greening in the presence of ABA and salt (NaCl). I also found that the CK2 alpha subunits redundantly control lateral root formation in Arabidopsis. CK2 has been shown to be involved in light signaling and phosphorylates the regulatory proteins HY5, HFR1 and PIF1 in the pathway. PIF1 belongs to a family of bHLH transcription factors (Phytochrome Interacting Factors) which function as repressors of photomorphogenesis. In the presence of light, activated phytochromes (red/far red light photoreceptors) translocate to nucleus and interact with PIFs, which leads to rapid phosphorylation of PIFs followed by their degradation through the 26S proteasome. An in vitro kinase assay was performed to test whether CK2 phosphorylates all the PIF family proteins. Results show that CK2 phosphorylates PIF1, PIF3 and PIF5 but does not phosphorylate PIF4, PIF6 and PIF7. Up to 10 putative CK2 sites in PIF3 protein were mapped and mutated however the mutant protein was still significantly phosphorylated by CK2. The CK2 phospho-deficient mutant of PIF1 (PIF1-7M) showed reduced homo-dimerization capacity than the wild type in yeast two hybrid assays. However, further studies are required to access the functional significance of it. Overall, this research uncovers novel functions for CK2 alpha subunits in the growth and development of Arabidopsis thaliana.