Genetic analysis of the control of flowering time in Arabidopsis thaliana
Arabidopsis is a facultative long-day plant, flowering earlier under long than short photoperiods. Many genetic mutations have been identified that affect the timing of the floral transition in Arabidopsis (Redei, 1962; Koornneef et al., 1991; Koornneef et al., 1998b) . These genetic mutations affect genes that are involved in responses to environmental stimuli and the autonomous control of flowering.
Natural genetic variation in flowering
Two methods have been used to identify genes that are involved in the genetics of the transition to flowering. The first approach involves the analysis of natural genetic variation in flowering-time control that occurs between different accessions of Arabidopsis, and the second is based on the isolation of flowering-time genetic mutants after mutagenesis. Both genetic approaches have contributed to the understanding of genes that control the genetical transition to flowering, and have been complimentary to each other. For example the FRIGIDA (FRI) gene was initially identified by analysis of natural variation and not in mutagenesis experiments (Clarke and Dean, 1994) . Homologues of some Arabidopsis flowering-time genes have been found in other species, including rice, barley, wheat, Pharbitis and Brassica napus, demonstrating that parts of the genetic network controlling flowering in Arabidopsis are highly conserved, even in distantly related angiosperms (Takeba, 1966; Vince-Prue, 1985; Lin, 2000c; Mouradov et al., 2002) . This genetic analysis of Arabidopsis mutants and accessions led to the identification of four major flowering-time genetic pathways: Photoperiod; Vernalisation; Gibberellin and Autonomous (Mouradov et al., 2002).
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