Site-specific phosphorylation of LRP regulates FLC chromatin looping and flowering.

Emerging evidence suggests that multiple RNA recognition motif (RRM)-containing proteins are involved in transcriptional regulation, yet the underlying mechanisms remain largely unclear. Here, we show that the previously uncharacterized LATE-FLOWERING RRM-CONTAINING PROTEIN (LRP) prevents 5'-3' chromatin looping of the key floral repressor gene FLOWERING LOCUS C (FLC) via direct association with its 5' and 3' genomic regions. Loss of LRP function leads to increased 5'-3' gene looping, reduced RNA polymerase II (Pol II) occupancy at the FLC locus, thereby increasing FLC transcription and delaying flowering in Arabidopsis thaliana. Furthermore, site-specific phosphorylation of LRP at serine 17, mediated by the kinase PRP4KA, is essential for LRP protein stability and its function in preventing FLC chromatin looping and Pol II recruitment. Our findings reveal that site-specific phosphorylation of LRP by PRP4KA precludes FLC gene loop formation, constituting a key mechanism for FLC transcriptional repression to ensure timely flowering.