**4. Putative pathway of flowering time in soybean**

The responsible gene for the *E4* locus was identified as *GmPhyA2* through the candidate gene approach (Liu et al., 2008). At the *e4* allele, a Ty1/copia-like retrotransposon was inserted in exon 1 of the gene, which resulted in dysfunction of the gene and photoperiod insensitivity. Similarly, natural and artificial mutations of *GmPhyA3* resulted in weak or complete loss of photoperiod sensitivity (Watanabe et al., 2009). The *FT* homologs in soybean have been identified (Kong et al., 2010) and two of them, *GmFT2a* and *GmFT5a*, were highly upregulated under SD conditions and showed diurnal expression patterns with the highest expression 4h after dawn. Under LD conditions, expression of *GmFT2a* and *GmFT5a* was downregulated and did not follow a diurnal pattern. Ectopic expression analysis in *Arabidopsis* confirmed that both *GmFT2a* and *GmFT5a* had the same function as *Arabidopsis FT*. A double-mutant (*e3e3 e4e4*) for *GmPhyA2* and *GmPhyA3* expressed high levels of *GmFT2a* and *GmFT5a* under LD conditions (18-h light) with an R: FR ratio of 1.2, and it flowered slightly earlier under LD than the wild type (*E3E3 E4E4*) grown under SD. The expression levels of *GmFT2a* and *GmFT5a* were regulated by PHYA-mediated photoperiodic regulation system, and the *GmFT5a* expression was also possibly regulated by photoperiod-independent system in LD.

*GI* have the conserved function of controlling the expression of the *FT* gene in *Arabidopsis*, rice and pea (Hayama et al., 2003; Mizoguchi et al., 2005; Hecht et al., 2007). We analyzed the expression of *GmFT2a* and *GmFT5a* at 9:00 a.m. 4 weeks after sowing under natural daylength conditions using *E2* (*FT2*) NILs in which photoperiod changed from LD to SD. A clear association between the *GmFT2a* expression level and early flowering phenotype was observed in both NILs. However, there was no significant difference in the *GmFT5a* expression levels between these NILs. These results suggested that *GmGIa* probably controlled flowering time through the regulation of *GmFT2a*. The recessive alleles of the *E2* (*FT2*) locus were perhaps unable to suppress *GmFT2a* expression and resulted in the early flowering phenotype.

There are strong interaction among the effects of *E1* (*FT1*) and *E2* (*FT2*), *E1* (*FT1*) and *E3* (*FT3*) (Yamanaka et al. 2000; Watanabe et al. 2004). The *e3e3* recessive homozygote can initiate flowering under R-enriched LD, but the *e3e3* genotype is necessary for plants with *e4* mutant allele to flower under FR-enriched LD. In the mapping population with *e3* background, photoperiodic insensitivity could occur in either genotypes of *e1E4*, *E1e4* or *e1e4* (Abe et al., 2003). These results suggest that *E1*, *E2*, *E3* and *E4* might concurrently mediate photoperiodic flowering in a shared pathway. The expression of the candidate gene for the *E1* locus was found to be repressed under SD. Under SD conditions, *E3*/*E4*-mediated photoperiodic regulation system up-regulates the expression of *GmFT2a* and *GmFT5a* possibly through the repression of the *E1* gene (Fig. 16). The *E2* locus also might control the *GmFT2a* expression through the *E1* gene.

Fig. 16. A putative network of flowering time genes in soybean.
