The aim of this work was to design strong transcriptional activators that can be used to regulate plant gene expression. The contribution of different components in a transcription factor and target gene system was assayed by measuring transcriptional activation. Each component was optimised to achieve maximal reporter gene expression in transient protoplast transformation assays. The DNA-binding domain of the yeast transcriptional activator GAL4 was studied in the context of fusion proteins with activation domains of the herpes simplex virus protein VP16 or the tomato Myb-like activator THM18. Multimerisation of the activation domain and insertion of a homopolymeric glutamine stretch was used to increase transcription factor potency. Evidence is presented that these modifications can result in even more active transcription factors when they are combined. Finally, it was demonstrated using competition experiments that transcription factors with acidic activation domains can mutually suppress their activation potentials when expressed at high levels.