Zinc Finger Homeobox Transcription Factors OsMIF1 and OsMIF2 Regulate Grain Size and Panicle Development in Rice

Mini zinc finger (MIF) proteins are plant-specific zinc finger-homeodomain (ZF-HD) transcription factors lacking a homeodomain, whose biological functions are critical for normal plant development and the response to environmental stress. Here, CRISPR-Cas9 was used to engineer null alleles of rice OsMIF1 and OsMIF2, and the resulting OsMIF1- and OsMIF2-deficient knockout lines were used to identify the biological roles of OsMIF1 and OsMIF2. The results suggest that OsMIF proteins transcriptionally regulate grain size by controlling the size of epidermal cells and the length and branching of rice panicles. RNA-seq analysis of OsMIF-knockout cells revealed altered expression of genes involved in development, the response to environmental stress and grain size. In addition, 10 protein-interacting partners of OsMIF1 were identified using a yeast two-hybrid screen: these proteins play roles in diverse developmental, hormonal, stress response, and metabolic processes, suggesting that OsMIF1 is effectively a regulatory hub, whose role is to integrate signals as they propagate through rice development- and stress response pathways. The results presented here support the conclusion that OsMIF1 and OsMIF2 are master transcription factors that regulate development throughout the adult plant life cycle and contribute significantly to plant resilience in the presence of environmental stressors.