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) have been reported to mediate ovule development. However, it is not clear which intracellular components regulate these TFs during ovule development. After their synthesis, TFs are transported into the nucleus a process regulated by karyopherins commonly known as importin alpha and beta. Around half of Arabidopsis (Arabidopsis thaliana) importin beta-coding genes have been functionally characterized but only two with specific cargos have been identified. We report here that Arabidopsis IMPORTIN beta 4 (IMB4) regulates ovule development through nucleocytoplasmic transport of transcriptional coactivator growth regulating factors-interacting factors (GIFs). Mutations in 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 ovule defects similar to those in imb4 mutants, whereas enhanced expression of GIF1 partially restored the fertility of imb4. The results presented here uncover a novel genetic pathway regulating ovule development and reveal the upstream regulator of GIFs.