The GRF/GIF pathway is emerging as a key target for breeding and biotechnology of cereals. Ectopic expression of GIF1 results in improved yield in maize under drought, natural miRNA-resistant GRF variants increase rice grain size, and expression of a GRF-GIF chimeric protein allows transformation of otherwise recalcitrant wheat varieties. These observations invite further engineering of GRF and GIF proteins tailored to these applications. However, our molecular understanding of these proteins is too limited for rational design. In this project, I aim to generate a panel of GRF and GIF variants with targeted single amino acid changes to find a superactive variant, while simultaneously getting insight in the role of GRF and GIF protein motifs and uncovering possible new levels of GRF-GIF regulation. Promising variants will be overexpressed as transgenes in tissue culture to test their potential to increase maize regeneration, including transformation-recalcitrant varieties and alternative explants. In parallel, I will further develop the CRISPR-mediated base editing toolbox for in planta mutagenesis of maize by expanding the number of available target sites. In an alternative application, superactive GRF or GIF alleles will be reconstructed in planta using base editing and the effect on plant growth under drought examined. This project will improve our understanding of GRF/GIF function, while also providing enhanced biotechnological tools and novel alleles for plant breeding.