Regulation of flowering by photoperiod in Arabidopsis

The flowering time of Arabidopsis thaliana is strongly influenced by light quality and photoperiod length. Broadly, two classes of mutations have been characterized that disrupt the response to photoperiod. The first of these, which includes early flowering 3 (elf3) , probably influences flowering time by affecting the endogenous cir‐cadian rhythm that enables the duration of the photoperiod to be measured. The genes affected by the second class of mutations, which includes constans (co) , are required to promote flowering in response to long photoperiods. The CONSTANS gene has been cloned, and analysis of transgenic plants in which the gene is expressed from a strong viral promoter indicates that transcriptional regulation of CO is an important determinant of flowering time in response to photoperiod. I describe recent progress in understanding these two classes of mutations, and how they influence flowering time in response to photoperiod.