Transcriptional Regulation of in the Root Meristem Through an Importin and Its Two Antagonistic Cargos.

Plant roots are sustained through meristem activity at the root tip. Two transcriptional pathways, one mediated by PLETHORAs (PLTs) and the other by SHORTROOT and SCARECROW, play major roles in root meristem development. The role of PLTs during root meristem development requires a concentration gradient, which is not only contributed by posttranslational regulation such as growth dilution and intercellular movement but also likely by a largely unknown fine-tuned transcriptional regulatory mechanism. We report here that Arabidopsis () JANUS positively regulates expression in the root meristem by recruiting RNA polymerase II (Pol II) to and by interacting with PLT1. JANUS-dependent recruitment of Pol II is inhibited through the competitive binding of JANUS by GRF-INTERACTING FACTOR1 (GIF1)/ANGUSTIFOLIA3, a transcriptional cofactor that negatively regulates expression. Finally, GIF1 and JANUS, the antagonistic regulators of , both depend on Arabidopsis IMPORTIN β4 for their nuclear accumulation. The combination of an importin and its two antagonistic cargos in transcription may have logistic benefits in fine-tuning the transcription of in root meristem.