The MOS4-associated complex subunit MAC5A maintains meristem development by regulating transcription elongation in Arabidopsis

The precise coordination of co-transcriptional RNA processing with transcriptional elongation is essential for eukaryotic gene regulation, yet the machineries coupling these processes remain largely elusive. Here, we reveal a role for the spliceosome-associated MOS4-associated complex (MAC) in regulating plant development through the direct regulation of transcriptional elongation. We demonstrate that the MAC subunit MAC5A physically interacts with Elongator Protein 6 (ELP6), a core component of the transcriptional Elongator complex. Genetic analyses reveal that MAC5A and ELP6 function synergistically to regulate apical meristem activity. Importantly, the MAC5A-Elongator module is required for the efficient expression of the critical auxin efflux carrier PIN-FORMED1 (PIN1) by enhancing RNA Polymerase II (RNAPII) occupancy across the PIN1 locus. Furthermore, we show that this elongation-promoting function is not unique to MAC5A, as other core MAC components are similarly required for efficient transcription progression. Together, our findings uncover a splicing-elongation nexus where MAC5A likely acts as a molecular bridge between the spliceosome and the elongation polymerase. This functional coupling ensures the efficient transcription of key developmental regulators, providing a mechanistic framework for the coupling of RNA processing to transcription and suggesting a conserved principle of gene expression control across eukaryotes.