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Maize (Zea mays),as a short day plant,was domesticated from tropical zones,A proper photoperiodic response for flowering is critical for introduction of tropical maize germplasm into temperate zones.In this study,we identified the CIRCADIAN CLOCK ASSOCIATED1 gene of Zea mays (ZmCCA1),which has extensive sequence homology to the clock gene CCA1 in Arabidopsis.Real-time PCR analysis revealed that ZmCCA1 was expressed in leaves and stem apex meristems (SAMs) in a rhythmic pattern under long day and short day conditions,and its peak gene expression appeared during the subjective morning.ZmCCA1 transcripts accumulated in all tissues evaluated,with higher levels in tassels and ears.Additionally,the expression of another photoperiod gene ZmTOC1 peaked 12 h after dawn on long days and at 10 h after dawn on short days.The subcellular localization analysis revealed that the ZmCCA1 protein is directed to the cell nucleus.Overexpression of ZmCCA1 in Arabidopsis reduced levels of downstream genes,including GIGANTEA (G1),CONSTANS (CO),and FLOWERING LOCUST (FT),and resulted in longer hypocotyls and delayed flowering.Taken together,our data suggest that ZmCCA1 may be a core component of the circadian clock in maize.