Mutations in Arabidopsis SR subfamily genes affect flowering time and the splicing of multiple floral regulators including <i>CONSTANS</i> and <i>FLOWERING LOCUS C</i>

Serine/arginine-rich (SR) proteins are essential splicing factors in animals, where their mutations often cause widespread splicing defects and carcinogenesis. The plant SR subfamily proteins are homologous to the well-studied human serine/arginine-rich splicing factor 1 (SRSF1), but their roles remain unclear. Here, we characterize the Arabidopsis SR subfamily genes: SR30, SR34, SR34a and SR34b. We show that GFP-tagged SR30, SR34 and SR34a co-localized with the spliceosomal protein U1-70K in speckled nuclear structures. To explore their physiological roles, we constructed a series of multiple mutants. Interestingly, the quadruple mutant displayed delayed flowering under long-day conditions but accelerated flowering under short-day conditions. Under long days, SR30, SR34 and SR34a function redundantly, as delayed flowering was observed only when all three were simultaneously disrupted. Under short day, SR34a plays a predominant role, being both necessary and sufficient to maintain normal flowering. RNA-seq and qPCR analysis revealed altered splicing of multiple flowering time regulators, including CONSTANS (CO) and FLOWERING LOCUS C (FLC). Particularly, increased production of an inhibitory CO isoform correlated with delayed flowering under long days, which was rescued by CO.1 overexpression, suggesting the phenotype was linked to CO missplicing. Overall, our findings uncover the roles of SR subfamily genes in floral transition, highlighting the physiological significance of splicing regulation in plants.