A Cajal body assembly factor regulates cell fate transitions in Arabidopsis

Ribonucleoprotein (RNP) condensates are emerging as key regulators of cell fate transitions, yet their functions have been largely linked to mRNA storage and translational control. Here, we uncover a role for Cajal body (CB)-mediated pre-mRNA splicing in coordinating the transition from stem cell divisions to differentiation in plants. We identify THREE-DIVISION MUTANT 3 (TDM3) as a cell cycle-regulated factor required for post-mitotic CB assembly. Loss of TDM3 or the CB scaffold protein COILIN delays differentiation and prolongs formative cell divisions. Transcriptome analysis revealed that TDM3 and COILIN jointly regulate pre-mRNA splicing, including transcripts controlling cell cycle and fate transitions. These findings establish CB-mediated splicing as a mechanism linking cell cycle progression to cellular differentiation.