Crop yield depends in a large part on stem height and inflorescence structure. Mutations that reduce stem growth have been used widely to improve crop yields but also have undesired side effects, for example during seed formation. In spite of its importance, stem development is poorly understood. Fundamental knowledge of how genes control stem growth is required to develop more precise genetic tools to increase plant productivity by modifying plant height and shape. The ARABIDOPSIS THALIANA HOMEOBOX1 (ATH1) gene inhibits stem development but is rapidly downregulated prior to the floral transition to allow elongation of the inflorescence stem. Data from the host lab indicate that ATH1 integrates two of the key hormonal signals that control stem growth: gibberellin (GA) and brassinosteroid (BR). My objectives are to reveal the mode of action of ATH1 and use this knowledge to develop new ways of modifying plant height with fewer undesired side-effects. I will reveal how ATH1 influences GA and BR signalling, understand molecular mechanisms of ATH1 action and identify cis-regulatory mutations that result in dwarf plants due to persistent ATH1 expression after flowering. Such mutations would be particularly useful for two reasons: first, regulatory mutations have been selected repeatedly in evolution and crop improvement because they allow subtle changes in gene expression, with fewer pleiotropic effects. Second, this type of mutation would be expected to be dominant and especially useful in polyploid crops. In addition to addressing a fundamental problem with practical use, this work will give me cutting edge training in plant developmental genetics and quantitative phenotyping at cellular and macroscopic levels. At the same time, the project will benefit from my knowledge of plant hormonal signalling and extensive experience in genome editing. More broadly, the work will provide me with a valuable network on international contacts and skills for my future career.