ACCUMULATION OF A CLOCK-REGULATED TRANSCRIPT DURING FLOWER-INDUCTIVE DARKNESS IN PHARBITIS-NIL

To clarify the molecular basis of the photoperiodic induction of flowe ring in the short-day plant Pharbitis nil cv Violet, we examined chang es in the level of mRNA in cotyledons during the flower-inductive phot operiod using the technique of differential display by the polymerase chain reaction. A transcript that accumulated during the inductive dar k period was identified and a cDNA corresponding to the transcript, de signated PnC401 (P. nil C401), was isolated. RNA-blot hybridization ve rified that levels of PnC401 mRNA fluctuated with a circadian rhythm, with maxima between 12 and 16 h after the beginning of the dark period ) and minima of approximately 0. This oscillation continued even durin g an extended dark period but was damped under continuous light. Accum ulation of PnC401 mRNA was reduced by a brief exposure to red light at the 8th h of the dark period (night-break treatment) or by exposure t o far-red light at the end of the light period tend-of-day far-red tre atment). These results suggest that fluctuations in levels of PnC401 m RNA are regulated by phytochrome(s) and a circadian clock and that the y are associated with photoperiodic events that include induction of f lowering.