The J-Domain Protein J3 Mediates the Integration of Flowering Signals in Arabidopsis

The timing of the switch from vegetative to reproductive development in Arabidopsis thaliana is controlled by an intricate network of flowering pathways, which converge on the transcriptional regulation of two floral pathway integrators, FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1). SHORT VEGETATIVE PHASE (SVP) acts as a key flowering regulator that represses the expression of FT and SOC1. Here, we report the identification of another potent flowering promoter, Arabidopsis DNAJ HOMOLOG 3 (J3), which mediates the integration of flowering signals through its interaction with SVP. J3 encodes a type I J-domain protein and is ubiquitously expressed in various plant tissues. J3 expression is regulated by multiple flowering pathways. Loss of function of J3 results in a significant late-flowering phenotype, which is partly due to decreased expression of SOC1 and FT. We further show that J3 interacts directly with SVP in the nucleus and prevents in vivo SVP binding to SOC1 and FT regulatory sequences. Our results suggest a flowering mechanism by which J3 integrates flowering signals from several genetic pathways and acts as a transcriptional regulator to upregulate SOC1 and FT through directly attenuating SVP binding to their regulatory sequences during the floral transition.