The conserved spliceosomal protein AtSF3B2 controls the floral transition by regulating transcription and splicing in Arabidopsis

Precursor mRNA (pre-mRNA) splicing occurs co-transcriptionally and is coupled to transcription through the coordinated assembly of the splicing and transcription machineries. Splicing factor 3B subunit 2 (SF3B2) plays a critical role in pre-mRNA splicing and facilitates spliceosome assembly in humans, but its function in plants remains unclear. Here, we demonstrate that the Arabidopsis thaliana SF3B2 homolog AtSF3B2, interacts with the splicing factors AtU2AF65B and AtSYF2 via its conserved DUF382 domain. As a negative regulator of floral transition, AtSF3B2 binds to pre-mRNAs and modulates the splicing of its target genes, including the central floral repressor FLOWERING LOCUS C (FLC), its antisense transcript COOLAIR, and their regulator WRKY63. Furthermore, AtSF3B2 promotes the transcription of these genes by interacting with the RNA polymerase II (Pol II) subunit NRPB12, and by binding directly to DNA to influence Pol II enrichment. RNA sequencing analyses reveal that the AtSF3B2 mutation predominantly results in intron retention and exon skipping, especially for shorter exons with a lower GC content. A subset of flowering regulators, including FLM, was identified as an AtSF3B2 target. Additionally, AtSF3B2 functions in high temperature-dependent flowering by modulating transcription and splicing of FLM. Together, our findings reveal transcriptional and post-transcriptional roles for AtSF3B2 in the flowering transition.