Positional Cloning of the Responsible Genes for Maturity Loci E1, E2 and E3 in Soybean

The change from vegetative to reproductive growth is a critical developmental transition in the life of plants. Various external cues, such as photoperiod and temperature, are known to initiate plant flowering under the appropriate seasonal conditions. Endogenous cues include a system of juvenile to adult transition that affects competence to flower. To understand the molecular mechanism of flowering, extensive studies have been performed using model plants, Arabidopsis thaliana and rice (Oryza sativa), and these have revealed the numerous regulatory network components associated with flowering (Jung & Muller, 2009; Amasino, 2010). The general concept of the photoperiodic induction of flowering (photoperiodism) and the range of response types among plant species was established by Garner and Allard (1920). Among the external cues, light is the most important, being received by several photoreceptors including phytochromes, cryptochromes and phototropins. The role of phytochromes, that is the R-lightand FR-lightabsorbing photoreceptors, in flowering has been investigated in several plant species. In Arabidopsis, a quantitative long-day (LD) plant, a phyA mutant flowered later in either long-day or short-day (SD) conditions with a night break (Johnson et al., 1994; Reed et al., 1994). In rice, a SD plant, the phyA monogenic mutant exhibited the same flowering time as the wild type under LD conditions, while, in the phyB and phyC mutant backgrounds, the flowering was greatly accelerated relative to phyB and phyC monogenic mutants (Takano et al., 2005). In pea, a LD plant, lossor gainof-function phyA mutants displayed late or early flowering phenotypes, respectively (Weller et al., 1997, 2001). Day length is found to be perceived by leaves by Knott (1934). Because flowering occurs in the shoot apical meristem (SAM), the leaves must transmit a signal to the SAM and this signal is referred to as florigen (Chailakhyan, 1936). In Arabidopsis, three genes, CONSTANS (CO), GIGANTEA (GI) and FLOWERING LOCUS T (FT) were found to be involved in the production of a flowering promoter in LD conditions (Koornneef et al., 1991; Kardailsky et al., 1999). FT protein is now known to be florigen, and CO and GI are key players in the activation of FT expression. CO is a zinc-finger protein that