Update on Development Photoreceptors and Regulation of Flowering Time

One of the most important environmental factors affecting flowering time is the daily duration of light, the photoperiod, which was first discovered by Garner and Allard in the 1920s (Thomas and Vince-Prue, 1997, and refs. therein). Plants in which flowering occurs or is accelerated in short days (SD) or long days (LD) are known as SD plants or LD plants, respectively. LD plants often flower in later spring or early summer (when the daylength becomes longer) to set seeds in a favorable season. SD plants generally flower in fall (when photoperiods are getting shorter) to finish reproduction before the cold winter arrives. Synchronization of flowering time with a reliable environmental cue such as the photoperiod also increases the chance of out-breeding and genetic recombination. The photoperiodic control of flowering is brought about by the interactions of genes involved in the developmental control of floral initiation, the regulation of the circadian clock, and the signal transduction of photoreceptors (Thomas and Vince-Prue, 1997). Recent molecular genetic studies in a facultative LD plant, Arabidopsis, have made notable progress in identifying genetic pathways and molecular components associated with the control of flowering time and the function of the circadian clock, which have been discussed in two recent Updates (Pineiro and Coupland, 1998; Somers, 1999). This Update focuses on the recent advances in our understanding of plant photoreceptors phytochromes and cryptochromes, and their roles in the regulation of flowering time.