| Summary: | <i>CONSTANS</i> (CO) and <i>CONSTANS</i>-like (COL) genes play important roles in coalescing signals from photoperiod and temperature pathways. However, the mechanism of CO and COLs involved in regulating the developmental stage transition and photoperiod/temperature senescing remains unclear. In this study, we identified a <i>COL</i> ortholog gene from the Taiwan native orchid <i>Phalaenopsis aphrodite</i>. The <i>Phalaenopsis aphrodite</i> CONSTANS-like 1 (PaCOL1) belongs to the B-box protein family and functions in the nucleus and cytosol. Expression profile analysis of <i>Phalaenopsis aphrodite</i> revealed that <i>PaCOL1</i> was significantly expressed in leaves, but its accumulation was repressed during environmental temperature shifts. We found a differential profile for PaCOL1 accumulation, with peak accumulation at late afternoon and at the middle of the night. <i>Arabidopsis</i> with <i>PaCOL1</i> overexpression showed earlier flowering under short-day (SD) conditions (8 h/23 °C light and 16 h/23 °C dark) but similar flowering time under long-day (LD) conditions (16 h/23 °C light and 8 h/23 °C dark). Transcriptome sequencing revealed several genes upregulated in <i>PaCOL1</i>-overexpressing <i>Arabidopsis</i> plants that were previously involved in flowering regulation of the photoperiod pathway. Yeast two-hybrid (Y2H) analysis and bimolecular fluorescence complementation (BiFC) analysis revealed that PaCOL1 could interact with a crucial clock-associated regulator, AtCCA1, and a flowering repressor, AtFLC. Furthermore, expressing <i>PaCOL1</i> in <i>cca1.lhy</i> partially reversed the mutant flowering time under photoperiod treatment, which confirms the role of PaCOL1 function in the rhythmic associated factors for modulating flowering.
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