The Role of Casein Kinase II in Flowering Time Regulation Has Diversified during Evolution    

Abstract

Casein kinase II (CK2) is a protein kinase with an evolutionarily conserved function as a circadian clock component in several organisms, including the long-day plant Arabidopsis (Arabidopsis thaliana). The circadian clock component CIRCADIAN CLOCK ASSOCIATED1 (CCA1) is a CK2 target in Arabidopsis, where it influences photoperiodic flowering. In rice (Oryza sativa), a shortday plant, Heading date6 (Hd6) encodes a CK2a subunit that delays flowering time under long-day conditions. Here, we demonstrate that control of flowering time in rice by the Hd6 CK2a subunit requires a functional Hd1 gene (an Arabidopsis CONSTANS ortholog) and is independent of the circadian clock mechanism. Our findings from overexpressing the dominantnegative CK2 allele in rice support the independence of CK2 function from the circadian clock. This lack of control of the circadian clock by Hd6 CK2a might be due to the presence of glutamate in OsLHY (a CCA1 ortholog in rice) instead of the serine at the corresponding CK2 target site in CCA1. However, this glutamate is critical for the control of the OsPRR1 gene (a rice ortholog of the Arabidopsis TOC1/PRR1 gene) by OsLHY for regulation of the circadian clock. We also demonstrated that the other conserved CK2 target sites in OsLHY conferred robust rhythmic expression of OsLHY-LUC under diurnal conditions. These findings imply that the role of CK2 in flowering-time regulation in higher plants has diversified during evolution.

The Rice CK2 Kinase Regulates Trafficking of Phosphate Transporters in Response to Phosphate Levels

“…Genes encoding CK2 subunits show a higher degree of redundancy in plants compared with animals (Espunya et al, 2005;Ogiso et al, 2010;Mulekar et al, 2012). Here, we found that the duplication of CK2 subunits was accompanied by functional divergence, as only CK2a3/b3 was active in PT phosphorylation.…”

“…FW, fresh weight. CK2 in many plant species also demonstrates that this kinase is involved in diverse pathways (Plana et al, 1991;Espunya et al, 1999;Ogiso et al, 2010;Mulekar et al, 2012).…”

Arabidopsis casein kinase 1-like 2 involved in abscisic acid signal transduction pathways

“…CK2, which forms a tetrameric complex including two a-subunits and two b-subunits, is a highly conserved protein kinase and could be found in various organisms, such as yeast, plants, and mammals. CK2 can phosphorylate a range of different proteins, including transcription factors, and can modulate DNA-binding ability, intracellular localization, and protein stability by interacting with partner proteins (Ogiso et al 2010).…”

Molecular Regulation of Flowering Time in Grasses

“…The plant circadian system consists of biochemical timing mechanisms that temporarily modulate the function of several signalling pathways to measures changes in day-length and promote suitable timing of flowering to maximize reproductive success [2,3,6,7,9,10,[13][14][15]21,24,26,28,29,31,36,38,41,42,46,48,49,[56][57][58][59][60][61][62][63][64][65][66]. The photoperiod response on flowering time varies among grasses.…”

“…To induce delayed flowering under LD conditions, Hd6 requires the presence of functional Hd1 alleles and plays a critical role in Hd1 activity. Despite this, Hd6 regulation is not mediated by changes in the circadian clock [37,65]. The rice genome contains two important genes for photoperiodic regulation: Ehd1 and Ghd7, specific to grass species such as rice, maize and sorghum but absent in the Arabidopsis genome [44].…”