Expression of FRIGIDA in root inhibits flowering in Arabidopsis thaliana.

FRIGIDA (FRI), as the major regulator of flowering time in Arabidopsis accessions, can activate its target FLOWERING LOCUS C (FLC) to delay flowering before vernalization. In addition to FLC, other FRI targets also exist in Arabidopsis. Although leaves sense environmental cues to modulate flowering time, it is not known if roots also regulate the floral transition. In this study, we investigated the spatio-temporal effect of FRI on flowering time. Local expression of FRI in the phloem and leaves activated FLC to delay flowering. Furthermore, we found that local expression of FRI in the roots also delayed flowering by activating other targets, MADS AFFECTING FLOWERING4 (MAF4) and MAF5, in the roots. Graft and genetic experiments revealed that the spatial expression of FRI in the root might generate a mobile signal, which is transmitted from roots to shoot and antagonizes the FT signal to delay flowering. Specifically expressing FRI in the embryo efficiently delayed flowering, even expressing FRI as early as the pro-embryo stage is enough to up-regulate FLC expression to delay flowering. Together, our findings demonstrate the spatio-temporal effect of FRI on delaying flowering, and we propose that root tissue also perceives the flowering signal to fine-tune the flowering time through MAF4/5 as novel targets of FRI.

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