The regulation of flowering time of Arabidopsis in response to daylength

The control of flowering time by photoperiod has been studied genetically in Arabidopsis by several groups. Arabidopsis flowers earlier under long-day (LD) conditions than under short days (SD) and mutations have been identified that specifically either delay flowering under LDs or promote flowering under SDs. The phenotypes of single and double mutants suggest that the mutations that specifically delay LD flowering affect genes in a single genetic pathway. Four of the genes affected by these mutations were cloned by different research groups, and their sequences suggested functions for the gene products. One of the genes,CONSTANS, was proposed to encode a transcription factor, and its mRNA is present at a higher abundance in LDs than SDs suggesting its transcriptional regulation is important in the promotion of flowering. In agreement with this its overexpression in transgenic plants is sufficient to promote early flowering under SDs. The approaches being taken to establish the order with which genes act within the pathway and to identify genes that act afterCONSTANS are described.