Importin β4 Mediates Nuclear Import of GRF-Interacting Factors to Control Ovule Development in Arabidopsis

Ovule development is critical for seed development and plant reproduction. Multiple transcription factors (TFs) were reported to mediate ovule development. However, intracellular components regulating these TFs during ovule development are obscure. TFs are transported into nucleus after synthesis for action, a process regulated by karyopherins, or commonly known as importin α and β. Around half of Arabidopsis importin β-coding genes have been functionally characterized but only two with specific cargos identified. We report here that Arabidopsis IMPORTIN β4 regulates ovule development through nucleocytoplasmic transport of transcriptional co-activator GRF-INTERACTING FACTORS (GIFs). Mutations at IMB4 impaired ovule development by affecting integument growth. imb4 mutants were also defective in embryo sac development, leading to partial female sterility. IMB4 directly interacts with GIFs and is critical for the nucleocytoplasmic transport of GIF1. Finally, functional loss of GIFs resulted in similar ovule defects to those of imb4 whereas enhanced expression of GIF1 partially restored the fertility of imb4. Results presented here uncover a novel genetic pathway regulating ovule development and reveal the upstream regulator of GIFs.