| Summary: | For floral induction in adult citrus, low temperature is one of the most important environmental factors. <i>FLOWERING LOCUS C</i> (<i>FLC</i>) plays a very important role in low-temperature-induced <i>Arabidopsis</i> flowering by repressed <i>FLC</i> expression under exposure to prolonged low-temperature conditions. However, little is known about the <i>FLC</i> regulation mechanism in perennial woody plants such as citrus. In this study, the functions of citrus <i>FLC</i> homolog (<i>PtFLC</i>) were investigated by ectopic expression in <i>Arabidopsis</i>. Transcription factor of homeodomain leucine zipper I (HD-ZIP I) as an upstream regulator of <i>PtFLC</i> was identified by yeast one-hybrid screen to regulate its transcription. The HD-ZIP I transcription factor was highly homologous to <i>Arabidopsis ATHB13</i> and thus was named <i>PtHB13</i>. Ectopically expressed <i>PtHB13</i> inhibited flowering in transgenic <i>Arabidopsis</i>. Furthermore, the expression of <i>PtFLC</i> and <i>PtHB13</i> showed a seasonal change during the floral induction period and was also affected by low temperature. Thus, we propose that <i>PtHB13</i> binds to <i>PtFLC</i> promoter to regulate its activity during the citrus floral induction process.
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