The photoperiod response of fowering is critical to the survival and successful reproduction in crops. The Hd6 gene, encoding a protein similar to the Arabidopsis a-subunit of CASEIN KINASE2a (CK2a) has been reported to affect flowering time through the photoperiodic pathway in rice. In this study, the maize ortholog, ZmHd6, has been cloned and its role in the photoperiodic pathway was investigated. ZmHd6 encodes 332 amino acids with six distinct domains: two CK2 active domains, a nuclear localization signal sequence, a N terminal domain, an ATP binding site, and an active site of serine/threonine protein kinase. ZmHd6 was constitutively expressed in the leaves and shoot apical meristem (SAM) with the highest expression in SAMs of plants with seven leaves under long and short day conditions. Delayed flowering time under long day (LD) conditions were observed in ZmHd6 overexpression transgenic Arabidopsis plants compared to the wild type. In transgenic Arabidopsis plants, ZmHd6 was expressed in a circadian oscillatory pattern, TOC1 peak expression was delayed, new GI, CO, and FT expressions reached to peak after 3 h while entering illumination period and oscillatory expression patterns of CO and FT were created during the dark phase. These results suggested that ZmHd6 might negatively regulate flowering time under LD conditions by delaying TOC1 peak expression and disrupting CO and FT expression in transgenic Arabidopsis plants.
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