TheDWARF14(D14) gene of rice functions within the signaling pathway of strigolactones, a group of plant hormones that inhibits shoot branching. We isolated a recessive mutant namedsuper apical dormant(sad1-1) from a suppressor screen ofd14-1. The growth of tillers (vegetative shoot branches) is suppressed in both thed14-1 sad1-1double mutant and thesad1-1single mutant. In addition, thesad1-1mutant shows pleiotropic defects throughout development.SAD1encodes an ortholog of RPA34.5, a subunit of RNA polymerase I (Pol I). Consequently, the level of ribosomal RNA (rRNA) is severely reduced in thesad1-1mutant. These results indicate that proper ribosome function is a prerequisite for normal development in plants. The Arabidopsis ortholog of SAD1 was previously isolated as a Mediator-interacting protein. Here we show that SAD1 interacts physically with the Mediator complex through direct binding with OsMED4, a component of the middle module of the Mediator complex in rice. It is known that Mediator interacts with Pol II, which transcribes mRNAs and functions as a central regulator of transcription. This study indicates a novel aspect of Mediator function in Pol I-controlled rRNA transcription. TFIIF2 and RPC53 are the counterparts of RPA34.5 in Pol II and Pol III, respectively. We demonstrate that the rice orthologs of these proteins also interact with OsMED4. Our results suggest that interaction with MED4 in the Mediator complex is a common feature of the three types of RNA polymerases.Significance Statement
This study describes novel features of Pol I function. Our analysis of the sad1 mutant demonstrates that Pol I is involved in various aspects of plant development. Our results therefore indicate a previously unpredicted role for Mediator in the transcription of rRNA by Pol I.