Distinct Splicing Complexes Formed by RS2Z32 Isoforms Modulate Alternative Splicing and Flowering

Serine/arginine (SR) splicing factors can alter downstream precursor messenger RNA (pre-mRNA) splicing patterns and influence gene expression. However, the influences of different isoforms generated by alternative splicing of SR proteins themselves on the transcription and splicing of downstream target genes in plants remain to be elucidated. Here, we demonstrate that the interaction between arginine/serine-rich zinc knuckle-containing protein 32 (RS2Z32) splicing isoforms and the Arabidopsis thaliana ortholog of human splicing factor SC35 (SC35) leads to the formation of distinct complexes, through which the recognition of splice sites within intron 3 of the transcription factor circadian clock associated 1 (CCA1) gene is regulated, ultimately resulting in an altered ratio of the CCA1 splice variants. Competitive binding of CCA1α and CCA1β to the flowering locus C (FLC) gene promoter can either promote or inhibit FLC transcription and flowering. This study elucidates the molecular mechanism by which RS2Z32 regulates FLC expression independently of its canonical splicing function, enhancing our understanding of transcriptional regulation mediated by splicing factors in plants.