Isolation of a CONSTANS Ortholog from Pharbitis nil and Its Role in Flowering

The short-day plant Pharbitis nil is a model plant for the study of photoperiodic control of floral initiation. Flower formation can be induced at the cotyledon stage by a single long night of at least 14 h in duration. Using differential display of mRNA we identified a P. nil ortholog of the Arabidopsis CONSTANS (CO) gene, which will be referred to as PnCO. Expression of PnCO was high after a 14-h night, but low when the dark period was 12 h or less. Our results indicate that the level of the PnCO transcript is photoperiodically regulated. After transfer from continuous light to darkness, PnCO showed a circadian pattern of expression. Expression of the CAB gene, which is a molecular marker for the circadian clock, exhibited a different pattern of expression than did PnCO and was not subject to the same photoperiodic control. A major portion of the PnCO transcripts contained an unspliced intron. Only the intron-free PnCO was able to complement the co mutant of Arabidopsis by shortening the time to flower.