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 photope riodic 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 rye identified a P. nil ortholog of the Arabi dopsis CONS-TANS (CO) gene, which will be referred to as PnCO. Expression o f PnCO was high after a 14-h night, but low when the dark period was 12 h o r less. Our results indicate that the level of the PnCO transcript is photo periodically regulated. After transfer from continuous light to darkness, P nCO showed a circadian pattern of expression. Expression of the CAB gene, w hich is a molecular marker for the circadian clock, exhibited a different p attern of expression than did PnCO and was not subject to the same photoper iodic control. A major portion of the PnCO transcripts contained an unsplic ed intron. Only the intron-free PnCO was able to complement the co mutant o f Arabidopsis by shortening the time to flower.