The spliceosomal component GAMETOPHYTIC FACTOR 1 (GFA1) regulates a key photoperiodic switch

Plants have evolved sophisticated mechanisms to perceive and interpret daytime in order to flower at an optimal time point. Here we show that the spliceosomal component GAMETOPHYTIC FACTOR 1 (GFA1) constitutes a previously unrecognized key photoperiodic switch, that is essential for flowering in long days. We show that gfa1 hypomorphic (gfa1hyp) plants fail to initiate flowering in long-days (LD), which correlates with ectopic activation of the short-day (SD) flowering repressor ARABIDOPSIS THALIANA CENTRORADIALIS (ATC). Accordingly, flowering is restored upon inactivation of ATC in gfa1hyp mutants. A novel tissue-specific in-planta splice assay and comprehensive RNAseq profiling of gfa1hyp mutants indicate that GFA1 mediated pre-mRNA splicing is substrate specific, as previously suggested for GFA1 orthologs. Furthermore, we show that gfa1hyp mutants accumulate nonsense transcripts of the photoreceptor components PHYB and RRC1, suggesting inappropriate photoreceptor signaling as a potential cause for the ectopic activation of the SD characteristic profile in gfa1hyp. In fact, known downstream targets of the phytochrome system such as RS31, SR34a, SRp30 accumulate reduced amounts of light-dependent splice isoforms. Together, our data reveal a link between spliceosome composition and long-day flowering, based on complex transcriptional readouts in response to day length.