Identification and functional characterization of FLOWERING LOCUS T and TWIN SISTER OF FT floral integrator genes in Pak-choi (Brassica rapa ssp. Chinensis)

FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF), which are key floral integrators expressed in leaves, function at the shoot apical meristem (SAM) in Arabidopsis thaliana. The putative genes BcFT and BcTSF in Pak-choi (Brassica rapa ssp. Chinensis) are homologs of Arabidopsis FT and TSF (AtFT and AtTSF, respectively); however, their specific functions in Pak-choi remain unknown. In this study, we revealed the functional divergence of BcFT and BcTSF and their potential as positive regulators of flowering time in Pak-choi. We observed that the spatiotemporal expression patterns of BcFT and BcTSF were very similar. Subcellular localization assays in protoplasts revealed that the BcFT and BcTSF proteins were present in both the nucleus and cytoplasm. Additionally, both BcFT and BcTSF interacted with Arabidopsis FLOWERING LOCUS D (AtFD), a known partner of AtFT in Arabidopsis. As a complementary approach, we generated transgenic Arabidopsis plants overexpressing BcFT or BcTSF and investigated the effects of BcFT and BcTSF overexpression on flowering. The results indicated that BcFT overexpression failed to rescue the late flowering phenotype in ft-10 mutants. However, BcTSF overexpression partially rescued the flowering phenotype, indicating that BcTSF provided partial functional complementation. These findings suggest that the regulatory mechanisms through which FT and TSF positively regulate flowering time are conserved between Pak-choi and Arabidopsis. Furthermore, the results highlight the important role of BcTSF in the regulation of flowering time in Pak-choi.